{"gene":"C5AR2","run_date":"2026-04-28T17:12:38","timeline":{"discoveries":[{"year":2001,"finding":"C5L2 (C5AR2) binds C5a with high affinity and has ~10-fold higher affinity for C5a des-Arg74 than CD88/C5aR; it also binds C3a at a distinct site. Unlike CD88, C5L2 does not support degranulation or intracellular Ca2+ increases and is not rapidly internalized, but ligation potentiated IgE receptor-mediated degranulation via a pertussis toxin-sensitive mechanism.","method":"Radioligand binding assays, degranulation assays, Ca2+ flux assays in RBL-2H3 cells transfected with C5L2","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal binding and functional assays in transfected cells; foundational characterization paper","pmids":["11773063"],"is_preprint":false},{"year":2003,"finding":"C5L2 is obligately uncoupled from heterotrimeric G proteins due to a leucine-for-arginine substitution in the DRY motif (end of TM3); it does not induce MAP kinase activation, calcium flux, or chemotaxis following C5a binding. C5L2 has slow ligand on/off rates and does not internalize following C5a binding.","method":"Transfection of human and murine C5L2 into multiple cell lines, G protein coupling assays, MAP kinase assays, Ca2+ flux, chemotaxis, radiolabeled ligand binding kinetics; C5aR-deficient mice bearing only C5L2 showed no transcriptional response to C5a by microarray","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal methods in both transfected cells and genetic mouse models; high citation count indicates foundational study","pmids":["12899627"],"is_preprint":false},{"year":2003,"finding":"C5L2 binds C3a des-Arg77/acylation-stimulating protein (ASP) at a site distinct from the C5a binding site; C5L2 mRNA and protein are expressed in human skin fibroblasts and adipocytes, which are ASP-responsive cells.","method":"Radioligand binding, cross-competition assays, RT-PCR, Western blot in human skin fibroblasts and 3T3-L1 cells","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — binding assays with competition experiments in multiple cell types; single lab","pmids":["12540846"],"is_preprint":false},{"year":2005,"finding":"C5L2 is a functional receptor for acylation-stimulating protein (ASP/C3a des-Arg): stable transfection of C5L2 into HEK293 cells confers ASP-stimulated triglyceride synthesis and glucose transport; antisense knockdown of C5L2 abolishes ASP response; ASP induces beta-arrestin translocation and C5L2 phosphorylation.","method":"Stable transfection, antisense oligonucleotide knockdown, triglyceride synthesis assay, glucose transport assay, DAPI-DAGL enzyme activity, beta-arrestin-GFP translocation, receptor phosphorylation in human skin fibroblasts and 3T3-L1 cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 — gain-of-function transfection plus loss-of-function knockdown with multiple orthogonal functional readouts","pmids":["15833747"],"is_preprint":false},{"year":2005,"finding":"Targeted deletion of C5L2 in mice enhances the biological activity of C5a/C5a desArg both in vivo and in vitro, demonstrating that C5L2 functions as a negative (anti-inflammatory) modulator limiting the pro-inflammatory response to C5a.","method":"C5L2 knockout mice; in vivo C5a challenge; in vitro functional assays with C5L2-deficient cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — genetic KO with in vivo and in vitro functional endpoints; replicated anti-inflammatory role finding","pmids":["16204243"],"is_preprint":false},{"year":2005,"finding":"C5L2 in rat neutrophils (PMN) during sepsis increases over time following CLP, while C5aR decreases; in vitro, C5a (but not fMLP) reduces C5L2 levels on PMN; anti-C5L2 antibody treatment causes ~4-fold increase in IL-6 during sepsis, indicating C5L2 negatively modulates C5a-driven IL-6 production.","method":"Rat CLP sepsis model, cloning of rat C5L2, anti-C5L2 antibody blockade in vivo and in vitro, ELISA for IL-6","journal":"FASEB journal","confidence":"Medium","confidence_rationale":"Tier 2 — in vivo antibody blockade with defined cytokine readout, single lab","pmids":["15784721"],"is_preprint":false},{"year":2006,"finding":"The N-terminal domain of C5L2 contains critical acidic and tyrosine residues for binding C5a des-Arg but not intact C5a; an antibody against the N-terminus of human C5L2 inhibited C5a des-Arg binding but not C5a binding, revealing that C5L2 binds C5a and C5a des-Arg by distinct mechanisms.","method":"Antibody blockade of chimeric/mutant C5L2 receptors, radioligand binding assays, mutagenesis of N-terminal residues","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis combined with ligand binding assays systematically defining binding domains","pmids":["17158873"],"is_preprint":false},{"year":2006,"finding":"C5L2 does not bind C3a or C3a des-Arg77 (ASP), as demonstrated by 125I-ligand binding assays with fluorescently labeled ligands on transfected HEK293 and myeloblastic cell lines, contradicting earlier reports; C5L2 expression on myeloid lines is upregulated by dibutyryl-cAMP and IFN-γ but downregulated by TNF-α on HeLa cells; no C5a-dependent Ca2+ signaling detected from endogenously expressing HeLa cells.","method":"125I-ligand binding assays, flow cytometry with fluorescent ligands, quantitative real-time PCR, Ca2+ flux assay","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 1-2 — direct binding assays with multiple cell types and ligands; contradicts earlier reports, single lab","pmids":["17068344"],"is_preprint":false},{"year":2007,"finding":"C5L2 gene-targeted mice demonstrate that C5L2 is required to facilitate C5a signaling (positive modulator) in neutrophils, macrophages, and fibroblasts in vitro; C5L2 deficiency reduces inflammatory cell infiltration in vivo; C5L2-deficient mice are hypersensitive to LPS-induced septic shock, show reduced OVA-induced airway inflammation, and are mildly delayed in hematopoietic regeneration after irradiation — paralleling C3aR-deficient phenotypes.","method":"Gene targeting (KO mice), in vitro C5a signaling assays in multiple cell types, in vivo models: LPS sepsis, OVA airway challenge, gamma-irradiation","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 — genetic KO with multiple in vivo and in vitro endpoints across multiple disease models; Nature-published foundational study","pmids":["17322907"],"is_preprint":false},{"year":2007,"finding":"C5L2 KO mice on high-fat diet have significantly reduced adipose tissue triglyceride synthesis, lipolysis, and fatty acid re-esterification, with compensatory increases in muscle fatty acid oxidation (elevated UCP3, CD36, cytochrome c, phospho-ACC in skeletal muscle).","method":"C5L2 KO mice, adipose tissue TG synthesis assay, lipolysis assay, indirect calorimetry, Western blotting of muscle metabolic proteins","journal":"The Journal of endocrinology","confidence":"High","confidence_rationale":"Tier 2 — genetic KO with multiple orthogonal metabolic readouts","pmids":["17641279"],"is_preprint":false},{"year":2008,"finding":"Human C5L2 is predominantly intracellular in neutrophils (versus C5aR at the plasma membrane); C5L2 co-localizes with internalized C5aR and beta-arrestin; antibody blockade of C5L2 dramatically increases C5a-mediated chemotaxis and ERK1/2 phosphorylation but not Ca2+ mobilization; C5L2 associates with beta-arrestin by co-immunoprecipitation; C5L2 blockade does not affect C5aR endocytosis — establishing C5L2 as a negative modulator of C5aR signaling through the beta-arrestin pathway.","method":"Confocal microscopy, antibody blockade, chemotaxis assay, ERK1/2 phosphorylation assay, Ca2+ mobilization, co-immunoprecipitation in human neutrophils/PMNs","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP, confocal co-localization, multiple functional assays in primary human cells; highly cited","pmids":["20044484"],"is_preprint":false},{"year":2008,"finding":"Human C5L2 constitutively internalizes ligand via clathrin-dependent endocytosis independently of G-protein coupling; internalized ligand is retained and degraded intracellularly, whereas C5aR releases ligand back extracellularly — supporting C5L2 as a recycling decoy receptor in humans. Engineered chimeric C5L2 with C5aR G-protein coupling sequences failed to restore Ca2+ signaling or beta-arrestin redistribution.","method":"Constitutive ligand-independent internalization assay, clathrin inhibition (dominant-negative), ligand retention/degradation tracking, Ca2+ assay, beta-arrestin redistribution assay in transfected cell lines","journal":"Molecular immunology","confidence":"High","confidence_rationale":"Tier 1-2 — multiple mechanistic assays with pharmacological and molecular perturbations; rigorous mutagenesis-based approach to G-protein coupling","pmids":["19100624"],"is_preprint":false},{"year":2009,"finding":"C5L2 undergoes ligand-dependent (ASP and C5a) endocytosis that is clathrin- and cholesterol-dependent; following endocytosis, C5L2 traffics through early endosomes (Rab5), late endosomes (Rab7), and recycling endosomes (Rab11); the S323I mutation prevents C5L2 from localizing to the cell surface and abolishes ligand-mediated receptor phosphorylation and beta-arrestin-2 translocation, as well as downstream functional responses (glucose transport and TG synthesis).","method":"Beta-arrestin-2-GFP translocation, Rab-GFP co-localization by live imaging, receptor phosphorylation assay, functional glucose transport and TG synthesis assays in stably transfected C5L2-HEK cells; S323I mutant receptor","journal":"Molecular immunology","confidence":"High","confidence_rationale":"Tier 1-2 — mutagenesis with multiple orthogonal assays defining trafficking pathway and functional significance of Ser323","pmids":["19615750"],"is_preprint":false},{"year":2009,"finding":"C5a and C5a-desArg (but not C3a or C3a-desArg) stimulate beta-arrestin2 redistribution to cytoplasmic vesicles via C5L2; direct C5L2-beta-arrestin interaction confirmed by a beta-galactosidase fragment complementation assay; beta-arrestin coupling occurred at subnanomolar C5a concentrations.","method":"GFP-labeled beta-arrestin2 redistribution assay, beta-galactosidase fragment complementation assay in stably transfected cells","journal":"Journal of biomolecular screening","confidence":"High","confidence_rationale":"Tier 1-2 — two orthogonal assays (GFP translocation and protein complementation) confirming direct C5L2-beta-arrestin interaction","pmids":["19641221"],"is_preprint":false},{"year":2010,"finding":"C5L2-deficient mice are protected from OVA- and house dust mite-induced allergic asthma (reduced AHR, Th2 cytokines, eosinophilic inflammation, IgE, mucus); C5L2-deficient myeloid dendritic cells produce IL-23 and drive Th17 responses rather than Th2 responses upon HDM pulsing, indicating C5L2 on mDCs controls the Th1/Th17 versus Th2 balance.","method":"C5L2 KO mice, OVA and HDM asthma models, adoptive transfer of C5L2-deficient mDCs into WT mice, in vitro cytokine profiling of mDCs","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — genetic KO plus adoptive transfer experiment defining cell-type-specific mechanism","pmids":["20974988"],"is_preprint":false},{"year":2011,"finding":"TLR activation induces C5a hypersensitivity by reducing C5L2 activity (not by upregulating C5aR); TLR-induced hypersensitivity was mimicked by C5L2 blockade and absent in C5L2 KO mice, confirming that TLRs amplify C5a responses by targeting C5L2 as a negative modulator of C5aR.","method":"PBMCs and whole blood with TLR ligand pretreatment, C5L2 KO mice, TLR4-deficient mice, C5L2 blocking antibody, C5a-induced cytokine readouts","journal":"European journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — multiple genetic and pharmacological approaches in primary cells and KO mice converging on same mechanism","pmids":["21630250"],"is_preprint":false},{"year":2012,"finding":"C5L2 and C5aR form constitutive homo- and heterodimers in HEK293 cells (by BRET); in adipocytes, ASP or C5a stimulation promotes internalization of C5L2 with increasing perinuclear co-localization with C5aR; C5L2 activation by ASP (but not C5a) increases fatty acid uptake/esterification in adipocytes, while both ligands activate Akt phosphorylation.","method":"BRET (bioluminescent resonance energy transfer) in HEK293 cells, confocal microscopy in 3T3-L1 adipocytes and J774 macrophages, fatty acid uptake assay, Western blotting","journal":"Cellular signalling","confidence":"Medium","confidence_rationale":"Tier 2-3 — BRET for dimerization plus confocal co-localization and functional assays, single lab","pmids":["23268185"],"is_preprint":false},{"year":2013,"finding":"C5L2 blockade with mAb in allergic contact dermatitis in mice dramatically increases inflammation, and this exacerbation is reversed by anti-C5aR mAb, demonstrating that C5L2 suppresses C5aR-mediated beta-arrestin signaling in vivo.","method":"C5L2 KO mice in murine contact sensitivity model, anti-C5aR mAb treatment, inflammatory readouts","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — genetic KO plus antibody rescue experiment defining pathway position in vivo","pmids":["24043888"],"is_preprint":false},{"year":2013,"finding":"Ligation of both C5aR and C5L2 is required for optimal C5a-induced G-CSF production from peritoneal macrophages; C5a-mediated G-CSF induction is associated with PI3K/Akt and MEK1/2 signaling; G-CSF levels during CLP sepsis are reduced in both C5aR- and C5L2-deficient mice.","method":"C5aR KO and C5L2 KO mouse peritoneal macrophages, G-CSF ELISA, PI3K/MEK inhibitors, CLP sepsis model","journal":"European journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — dual KO genetic epistasis with pharmacological pathway dissection in both in vitro and in vivo settings","pmids":["23575697"],"is_preprint":false},{"year":2013,"finding":"C5a, but not C5a-des Arg, induces upregulation of C5aR-C5L2 heteromer formation as measured by BRET; this heteromer formation is inhibited by a C5aR-specific antagonist; the two ligands produce differential IL-10 responses in macrophages, suggesting heteromerization contributes to differential signaling.","method":"BRET in transfected HEK293 cells, wide-field microscopy, ELISA in human monocyte-derived macrophages","journal":"Immunology and cell biology","confidence":"Medium","confidence_rationale":"Tier 2-3 — BRET provides direct protein interaction evidence, but single lab and limited functional follow-up","pmids":["24060963"],"is_preprint":false},{"year":2014,"finding":"C5L2 physically interacts with C5aR and is required for optimal C5a-mediated C5aR internalization through an AP2-dependent, dynamin-dependent mechanism; without C5L2, AP2 recruitment to the receptor complex fails; C5L2-dependent C5aR internalization drives downstream MEK/ERK signaling.","method":"Co-immunoprecipitation, dynamin inhibitor (dynasore), AP2 recruitment assay, ERK phosphorylation in transfected cells; C5L2-deficient mice in acute colitis model","journal":"Cellular signalling","confidence":"High","confidence_rationale":"Tier 1-2 — Co-IP plus pharmacological and genetic loss-of-function approaches defining molecular mechanism of receptor internalization","pmids":["24631530"],"is_preprint":false},{"year":2015,"finding":"In human mast cells (LAD2) expressing surface C5aR2 but not C5aR1, C5a induces ERK phosphorylation, cytokine production (GM-CSF, TNF, CXCL10, CCL2), adhesion, and chemotaxis via C5aR2; these responses depend on beta-arrestin-2 and PI3K; lentiviral shRNA silencing of C5aR2 renders cells unresponsive to all C5a-induced effects.","method":"Lentiviral shRNA knockdown of C5aR2, siRNA against beta-arrestin-2, wortmannin (PI3K inhibitor), ERK phosphorylation, ELISA, adhesion and chemotaxis assays in LAD2 cells","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 1-2 — genetic knockdown plus pharmacological dissection with multiple functional readouts in a cell model with natural expression of only C5aR2","pmids":["26283482"],"is_preprint":false},{"year":2016,"finding":"C5L2-deficient mice (BALB/c) show more severe LPS-induced acute lung injury with greater airway edema, neutrophilia, MPO activity, and pro-inflammatory cytokines/chemokines; anti-C5aR blockade reverses neutrophilic infiltration in C5L2-/- mice, confirming C5L2's anti-inflammatory role is executed through negative modulation of C5aR.","method":"C5L2 KO mice, intranasal LPS model, anti-C5aR blocking antibody, BAL fluid analysis, lung homogenate cytokine measurement","journal":"American journal of respiratory cell and molecular biology","confidence":"High","confidence_rationale":"Tier 2 — genetic KO with antibody rescue experiment in defined in vivo model","pmids":["27285858"],"is_preprint":false},{"year":2016,"finding":"Selective C5aR2 ligands (P32 and P59) selectively recruit beta-arrestin 2 via C5aR2, partially inhibit C5a-induced ERK1/2 activation, and reduce LPS-stimulated IL-6 release from macrophages without directly affecting C5aR1; P32 inhibits C5a-mediated neutrophil mobilization in WT but not C5aR2-/- mice.","method":"Peptide library screen, beta-arrestin 2 recruitment assay, ERK1/2 phosphorylation assay, IL-6 ELISA in human macrophages, in vivo neutrophil mobilization in C5aR2 KO mice","journal":"Immunology and cell biology","confidence":"High","confidence_rationale":"Tier 1-2 — pharmacological tool compounds validated in both in vitro and in vivo KO mouse settings with multiple orthogonal assays","pmids":["27108698"],"is_preprint":false},{"year":2017,"finding":"C5aR2 expression is strongest in brain, bone marrow, and airways; all myeloid-derived cells express C5aR2 with neutrophils showing strong homogeneous expression; T cells are negative; B cells and NK cells express C5aR2. Selective C5aR2 ligation in neutrophils blocks C5a-driven ERK1/2 phosphorylation. Activation of C5aR2 in NK cells suppresses IL-12/IL-18-induced IFN-γ production. IL-33 challenge reduces C5aR2 in pulmonary eosinophils and monocyte-derived DCs.","method":"Floxed tdTomato-C5aR2 knock-in reporter mouse, flow cytometry, ERK1/2 phosphorylation assay in neutrophils, IFN-γ production assay in NK cells, intratracheal IL-33 challenge","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 1-2 — genetic reporter mouse plus functional validation in multiple primary cell types","pmids":["28864475"],"is_preprint":false},{"year":2017,"finding":"C5aR2-deficient mice show increased lesion volumes and worsened recovery from contusive spinal cord injury; acute PMX205 (C5aR1 antagonist) treatment improved outcomes in both WT and fully alleviated worsened recovery in C5aR2-/- mice, demonstrating C5aR2 is neuroprotective by restraining injurious C5aR1 signaling.","method":"C5aR2 KO mice, contusive SCI model, PMX205 C5aR1 antagonist treatment, lesion volume measurement, myelin sparing, behavioral recovery scores","journal":"Journal of neurotrauma","confidence":"High","confidence_rationale":"Tier 2 — genetic KO with pharmacological rescue experiment defining epistatic relationship between C5aR1 and C5aR2","pmids":["28173736"],"is_preprint":false},{"year":2018,"finding":"C5ar2-/- mice show increased skin lesions in a bullous pemphigoid model while C5ar1-/- mice are protected, demonstrating opposing roles for C5aR1 (proinflammatory/pathogenic) and C5aR2 (anti-inflammatory/protective) in autoantibody-mediated tissue damage.","method":"C5ar1 KO and C5ar2 KO mice, anti-Col17 IgG transfer BP model, skin lesion scoring, neutrophil migration assay, PMX53 pharmacological C5aR1 inhibition","journal":"Frontiers in immunology","confidence":"High","confidence_rationale":"Tier 2 — dual KO genetic epistasis with pharmacological validation","pmids":["29599777"],"is_preprint":false},{"year":2018,"finding":"CD10+GPR77+ (C5AR2+) cancer-associated fibroblasts sustain cancer stem cells through NF-κB activation driven by complement signaling via GPR77/C5AR2; GPR77 maintains persistent NF-κB p65 phosphorylation and acetylation; neutralizing anti-GPR77 antibody abolishes tumor formation and restores chemosensitivity.","method":"Patient-derived xenografts, anti-GPR77 neutralizing antibody, NF-κB reporter, p65 phosphorylation/acetylation assays, PDX co-injection experiments","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1-2 — mechanistic pathway defined with multiple orthogonal assays including in vivo PDX experiments and antibody blockade; highly cited","pmids":["29395328"],"is_preprint":false},{"year":2019,"finding":"C5aR2 expressed on endothelial cells acts as an atypical chemoattractant receptor that transports C5a from the perivascular tissue into the vessel lumen (transcytosis); this transported C5a initiates C5aR1-mediated neutrophil arrest, while ACKR1-transported chemokines drive CXCR2-dependent neutrophil transmigration — establishing sequential roles of atypical receptors in neutrophil recruitment.","method":"Intravital microscopy in live mice, immune complex-induced arthritis model, C5aR2-deficient mice, ACKR1-deficient mice, in vivo tracking of C5a transport","journal":"Science immunology","confidence":"High","confidence_rationale":"Tier 1-2 — intravital microscopy with genetic KO models directly demonstrating transcytosis function; high citation, Science Immunology journal","pmids":["31076525"],"is_preprint":false},{"year":2019,"finding":"C5aR2 promotes NLRP3 inflammasome activation and HMGB1 release from macrophages by amplifying dsRNA-dependent PKR expression; PKR upregulation by C5a-C5aR2 interaction depends on MEK/ERK and type I IFN signaling pathways; C5aR2 deficiency restricts NLRP3 activation and HMGB1 release both in vitro and in vivo.","method":"C5aR2-deficient mice and macrophages, siRNA knockdown, immunoblotting, qRT-PCR for PKR expression, MEK inhibitor treatment, NLRP3 inhibition","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 — genetic KO with siRNA, pathway inhibitors, and multiple orthogonal readouts defining molecular mechanism","pmids":["30971430"],"is_preprint":false},{"year":2019,"finding":"C5aR2 is required for C5a-mediated renal inflammation and tissue damage in acute pyelonephritis; C5aR2 signaling activates NLRP3/caspase-1 inflammasome and HMGB1 release in macrophages; blocking HMGB1 reduces C5aR2-mediated NLRP3 activation, placing HMGB1 upstream of NLRP3 in C5aR2 signaling.","method":"C5ar2 KO mice, uropathogenic E. coli bladder inoculation model, C5aR1-deficient macrophages (expressing only C5aR2), HMGB1 blocking antibody, NLRP3/caspase-1/IL-1β measurement","journal":"JCI insight","confidence":"High","confidence_rationale":"Tier 2 — genetic KO plus C5aR1 KO cells plus blocking antibody approach in vivo and in vitro defining pathway order","pmids":["32191644"],"is_preprint":false},{"year":2020,"finding":"Selective C5aR2 agonism (P32) by itself produces no detectable MAPK signaling but significantly dampens C5aR1-, C3aR-, and CMKLR1-mediated ERK signaling and alters intracellular calcium mobilization from these receptors; C5aR2 activation also reduces cytokine production from TLR2, TLR3, TLR4, TLR7, Dectin-1, Dectin-2, Mincle (reducing IL-6/TNF by 80-90%), and STING pathways in primary human macrophages.","method":"C5aR2 agonist P32 treatment of primary human monocyte-derived macrophages, ERK phosphorylation assay, Ca2+ mobilization, ELISA for multiple cytokines","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — selective pharmacological agonist with multiple PRR ligand stimulations and diverse functional readouts in primary human cells","pmids":["32611725"],"is_preprint":false},{"year":2020,"finding":"C5aR2 deficiency in intestinal IR injury worsens intestinal damage due to increased C5aR1-mediated neutrophil infiltration; C5aR1 blockade in C5aR2-/- mice prevents neutrophil accumulation and reduces pathology; C5aR2 deficiency also reduces circulating neutrophil numbers after IR via a C5aR1-independent mechanism, demonstrating distinct and separable functions of C5aR2.","method":"C5aR2 KO mice, intestinal IR model, PMX53 C5aR1 antagonist, neutrophil depletion, tissue damage scoring, cytokine measurement","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — genetic KO with pharmacological rescue dissecting C5aR1-dependent and -independent C5aR2 functions","pmids":["33028618"],"is_preprint":false},{"year":2021,"finding":"SB290157, widely used as a 'C3aR antagonist', is actually a partial agonist at C5aR2, mediating beta-arrestin recruitment at higher doses and significantly dampening C5a-induced ERK signaling in human and mouse primary macrophages via C5aR2.","method":"Beta-arrestin recruitment assay in transfected cells, ERK phosphorylation assay in primary macrophages, pharmacological profiling against C3aR, C5aR1, C5aR2","journal":"Frontiers in pharmacology","confidence":"Medium","confidence_rationale":"Tier 2 — pharmacological characterization with functional readouts in primary cells, single lab","pmids":["33551801"],"is_preprint":false},{"year":2022,"finding":"C5aR2 deficiency in neutrophils reduces intracellular calcium flux, ROS release, and migration capacity when activated with immune complexes or C5a; C5aR2 deficiency lowers the ratio of activating to inhibitory FcγRs on neutrophils; LysM-specific C5aR2 KO recapitulates global C5aR2 KO phenotype in epidermolysis bullosa acquisita.","method":"C5aR2 KO and LysM-Cre/C5aR2-flox mice, antibody transfer EBA model, in vitro Ca2+ flux, ROS assay, migration assay, FcγR expression analysis by flow cytometry","journal":"The Journal of investigative dermatology","confidence":"High","confidence_rationale":"Tier 1-2 — conditional cell-type-specific KO with multiple in vitro mechanistic assays and in vivo disease model","pmids":["35007559"],"is_preprint":false},{"year":2023,"finding":"CRISPR-KO of C5aR2 in macrophages significantly increases STING-mediated IFN-β secretion; STING and IRF3 expression are increased in C5aR2 KO cells; transcriptomic analysis shows upregulation of nucleic acid sensing and antiviral signaling pathways in C5aR2 KO cells, indicating C5aR2 negatively regulates the cGAS-STING-IFN-β axis.","method":"CRISPR-Cas9 KO of C5aR2 in THP-1 cells and primary human monocyte-derived macrophages, ELISA for IFN-β, Western blot for STING/IRF3, RNA-seq","journal":"Cells","confidence":"High","confidence_rationale":"Tier 1-2 — CRISPR KO with transcriptomics and functional cytokine measurements in both cell line and primary cells","pmids":["38067135"],"is_preprint":false},{"year":2025,"finding":"C5aR2 on CD4+ T cells orchestrates Th1 cell contraction via a C5-C5aR2-prostacyclin (PGI2) axis: C5aR2 activation shifts prostaglandin balance from PGE2 to PGI2 production, which signals autocrine through PGI2 receptor to induce IL-1R2 expression; IL-1R2 sequesters Th1-driving intrinsic IL-1β, facilitating Th1 contraction. Disruption of this axis characterizes pathological Th1 persistence in CAPS, Crohn's disease, and RA.","method":"CD4+ T cell-intrinsic genetic and pharmacological dissection, prostaglandin profiling, IL-1R2 expression assay, T cell functional assays, human T cells from CAPS patients with PGE2 synthase inhibitor treatment","journal":"Immunity","confidence":"High","confidence_rationale":"Tier 1-2 — cell-autonomous pathway defined with multiple orthogonal assays and validated in human disease samples","pmids":["40449486"],"is_preprint":false}],"current_model":"C5AR2 (C5L2) is a seven-transmembrane receptor that binds C5a/C5a desArg with high affinity but lacks classical G-protein coupling due to a DRY-motif leucine substitution; instead it signals through β-arrestin recruitment and acts as a pleiotropic immunomodulator — functioning as a decoy/scavenging receptor by internalizing and degrading C5a via clathrin-dependent endocytosis, as a negative modulator of C5aR1 signaling (suppressing ERK, chemotaxis, and cytokine production through β-arrestin-pathway competition and physical C5aR1 heterodimerization), as a positive facilitator of C5aR1 internalization and downstream ERK signaling in some cell contexts, as a transcytotic transporter of C5a across endothelial cells to initiate neutrophil arrest, as a metabolic receptor mediating ASP-driven triglyceride synthesis via serine 323-dependent β-arrestin signaling in adipocytes, as a promoter of NLRP3 inflammasome activation and HMGB1 release through PKR/MEK/IFN-type I signaling in macrophages, as a negative regulator of cGAS-STING-IFN-β signaling, and as a T cell-intrinsic regulator of Th1 contraction through a C5a-C5aR2-prostacyclin-IL-1R2 axis — with its net inflammatory effect (pro- versus anti-inflammatory) being highly context-, cell-type-, and disease-dependent."},"narrative":{"teleology":[{"year":2001,"claim":"Identification of C5L2 as a high-affinity C5a/C5a desArg receptor that does not trigger classical GPCR effector responses (degranulation, Ca²⁺ flux) established it as an atypical complement receptor with undefined signaling capacity.","evidence":"Radioligand binding, Ca²⁺ flux, and degranulation assays in C5L2-transfected RBL-2H3 cells","pmids":["11773063"],"confidence":"High","gaps":["Signaling mechanism downstream of ligand binding undefined","Physiological role in primary cells unknown"]},{"year":2003,"claim":"Demonstration that C5L2 is obligately G-protein-uncoupled due to the DRY→DLC motif substitution, with no MAPK, Ca²⁺, or chemotactic activity, raised the question of whether it functions as a signaling receptor or a decoy/scavenger.","evidence":"Multi-cell-line transfection with G-protein coupling, MAPK, and chemotaxis assays; C5aR-deficient mouse microarray showing no transcriptional response via C5L2 alone","pmids":["12899627"],"confidence":"High","gaps":["Mechanism by which C5L2 modulates C5a biology without G-protein coupling unknown","Potential non-G-protein signaling not yet explored"]},{"year":2005,"claim":"Gain- and loss-of-function experiments established C5L2 as a functional receptor for ASP/C3a desArg that drives triglyceride synthesis via β-arrestin recruitment, revealing the first signaling pathway for this receptor; simultaneously, C5L2-knockout mice showed enhanced C5a responses, defining C5L2 as an anti-inflammatory modulator in vivo.","evidence":"Stable transfection and antisense knockdown in fibroblasts/adipocytes with TG synthesis, glucose transport, β-arrestin-GFP translocation; C5L2 KO mice challenged with C5a in vivo; anti-C5L2 antibody blockade in rat sepsis model","pmids":["15833747","16204243","15784721"],"confidence":"High","gaps":["Whether anti-inflammatory and metabolic functions use the same or distinct β-arrestin pathways unclear","Binding of C3a/C3a desArg disputed by subsequent study"]},{"year":2007,"claim":"Paradoxically, an independent C5L2 KO study found C5L2 facilitates (rather than suppresses) C5a signaling in neutrophils and macrophages, and C5L2 KO mice paralleled C3aR KO phenotypes in asthma and sepsis models, revealing the first evidence that C5L2's net effect is context-dependent.","evidence":"Gene-targeted C5L2 KO mice in LPS sepsis, OVA airway, and irradiation models; in vitro C5a signaling in multiple primary cell types","pmids":["17322907"],"confidence":"High","gaps":["Molecular basis for discrepancy with earlier KO study unresolved","Cell-type-specific roles not yet dissected"]},{"year":2008,"claim":"Discovery that C5L2 is predominantly intracellular in neutrophils, co-localizes with internalized C5aR and β-arrestin, and that its blockade selectively enhances ERK but not Ca²⁺ signaling established the β-arrestin pathway competition model for C5L2's anti-inflammatory activity; separately, C5L2 was shown to constitutively internalize and degrade C5a via clathrin-dependent endocytosis, defining its decoy/scavenging function.","evidence":"Confocal microscopy, Co-IP of C5L2–β-arrestin, antibody blockade with ERK/Ca²⁺ readouts in human neutrophils; clathrin dominant-negative inhibition and ligand degradation tracking in transfected cells","pmids":["20044484","19100624"],"confidence":"High","gaps":["Structural basis for β-arrestin recruitment without G-protein coupling unknown","Relative contributions of decoy versus signaling functions unquantified"]},{"year":2009,"claim":"Detailed trafficking analysis showed C5L2 undergoes ligand-dependent clathrin/cholesterol-dependent endocytosis through Rab5→Rab7→Rab11 compartments, and serine 323 is essential for surface localization, phosphorylation, β-arrestin-2 recruitment, and all downstream metabolic functions.","evidence":"Rab-GFP co-localization, S323I mutagenesis with β-arrestin-2 translocation and TG synthesis assays in stably transfected HEK cells; β-galactosidase complementation confirming direct C5L2–β-arrestin interaction","pmids":["19615750","19641221"],"confidence":"High","gaps":["Whether S323 phosphorylation is mediated by GRKs or other kinases unknown","Crystal/cryo-EM structure of C5L2 absent"]},{"year":2012,"claim":"BRET experiments demonstrated constitutive C5aR-C5L2 homo- and heterodimer formation, with C5a-induced upregulation of heteromers, providing a physical basis for functional crosstalk between the two receptors.","evidence":"BRET in transfected HEK293 cells; confocal co-localization in 3T3-L1 adipocytes and J774 macrophages","pmids":["23268185","24060963"],"confidence":"Medium","gaps":["Heterodimer stoichiometry and structural interface undefined","BRET proximity does not prove functional requirement for dimerization in all contexts","In vivo confirmation of heterodimerization lacking"]},{"year":2013,"claim":"Epistatic studies in allergic contact dermatitis and sepsis models showed C5L2's anti-inflammatory effect is reversed by C5aR1 blockade, while C5L2 and C5aR1 are both required for optimal G-CSF production, demonstrating that C5L2 both restrains and cooperates with C5aR1 depending on downstream pathway.","evidence":"C5L2 KO mice with anti-C5aR mAb rescue in contact sensitivity; dual C5aR1/C5L2 KO macrophages with PI3K/MEK inhibitors in CLP sepsis","pmids":["24043888","23575697"],"confidence":"High","gaps":["Molecular determinants that switch C5L2 from inhibitory to cooperative mode unknown"]},{"year":2014,"claim":"C5L2 was found to physically interact with C5aR and be required for AP2-dependent C5aR internalization and downstream MEK/ERK signaling, defining a positive facilitatory mechanism for C5aR endocytosis.","evidence":"Co-IP, dynasore inhibition, AP2 recruitment assay, ERK phosphorylation in transfected cells; C5L2 KO mice in acute colitis","pmids":["24631530"],"confidence":"High","gaps":["Whether AP2 recruitment requires direct C5L2–AP2 contact or an adaptor protein unknown"]},{"year":2015,"claim":"In LAD2 mast cells expressing C5aR2 but not C5aR1, C5a independently drove ERK phosphorylation, cytokine production, and chemotaxis via β-arrestin-2/PI3K, proving C5aR2 can signal autonomously without C5aR1.","evidence":"Lentiviral shRNA knockdown of C5aR2, β-arrestin-2 siRNA, PI3K inhibitor in LAD2 cells with multiple functional readouts","pmids":["26283482"],"confidence":"High","gaps":["Generalizability to other cell types expressing only C5aR2 not tested","Downstream transcription factor targets of autonomous C5aR2 signaling unknown"]},{"year":2017,"claim":"A genetic reporter mouse mapped C5aR2 expression across immune lineages (strong in neutrophils, myeloid cells, NK cells; absent from T cells), and functional validation showed C5aR2 ligation suppresses C5a-driven ERK in neutrophils and IL-12/IL-18-induced IFN-γ in NK cells, defining cell-type-specific anti-inflammatory functions.","evidence":"Floxed tdTomato-C5aR2 knock-in reporter mouse with flow cytometry; ERK assays in neutrophils; IFN-γ assay in NK cells","pmids":["28864475"],"confidence":"High","gaps":["Expression and function in non-hematopoietic lineages (e.g. neurons, fibroblasts) not fully addressed by reporter"]},{"year":2018,"claim":"CD10⁺GPR77⁺ cancer-associated fibroblasts were shown to sustain cancer stemness via C5aR2-driven NF-κB activation, with neutralizing anti-GPR77 antibody abolishing tumor formation and restoring chemosensitivity, establishing a pro-tumorigenic role for C5aR2 in the tumor microenvironment.","evidence":"Patient-derived xenografts, anti-GPR77 neutralizing antibody, NF-κB reporter and p65 phosphorylation/acetylation assays","pmids":["29395328"],"confidence":"High","gaps":["Ligand driving C5aR2 on CAFs in vivo not definitively identified","Whether NF-κB activation uses β-arrestin or an alternative adaptor unknown"]},{"year":2019,"claim":"C5aR2 on endothelial cells was identified as a transcytotic transporter that shuttles C5a from perivascular tissue to the vascular lumen to initiate C5aR1-dependent neutrophil arrest, establishing a non-signaling transport function for C5aR2 analogous to atypical chemokine receptors.","evidence":"Intravital microscopy in C5aR2 KO and ACKR1 KO mice in immune complex arthritis model","pmids":["31076525"],"confidence":"High","gaps":["Molecular machinery of C5aR2-mediated transcytosis (vesicular carriers, sorting signals) undefined","Whether transcytosis is bidirectional unknown"]},{"year":2019,"claim":"C5aR2 was shown to promote NLRP3 inflammasome activation and HMGB1 release from macrophages by upregulating PKR expression through MEK/ERK and type I IFN signaling, with HMGB1 acting upstream of NLRP3 in this pathway, defining a pro-inflammatory signaling cascade distinct from the anti-inflammatory β-arrestin competition model.","evidence":"C5aR2 KO macrophages, siRNA, MEK inhibitor, HMGB1 blocking antibody; validated in acute pyelonephritis model","pmids":["30971430","32191644"],"confidence":"High","gaps":["How C5aR2 activates MEK/ERK for PKR induction without G-protein coupling is mechanistically unresolved","Whether this pathway operates in non-macrophage cell types unknown"]},{"year":2020,"claim":"Selective C5aR2 agonism broadly dampened signaling from multiple innate receptors (C5aR1, C3aR, CMKLR1, TLR2/3/4/7, Dectin-1/2, Mincle, STING), reducing cytokine output by up to 90%, establishing C5aR2 as a global negative regulator of innate immune receptor signaling.","evidence":"Selective C5aR2 agonist P32 in primary human monocyte-derived macrophages with ERK, Ca²⁺, and multi-cytokine readouts across diverse PRR stimulations; validated with C5aR2 KO mice","pmids":["32611725","27108698"],"confidence":"High","gaps":["Molecular mechanism by which C5aR2 cross-inhibits unrelated PRR pathways not defined","Whether tonic C5aR2 signaling operates without exogenous ligand unknown"]},{"year":2022,"claim":"Conditional (LysM-Cre) C5aR2 deletion showed that myeloid-intrinsic C5aR2 is required for full neutrophil effector function (Ca²⁺ flux, ROS, migration) and modulates activating/inhibitory FcγR ratios, demonstrating a cell-autonomous role in tuning neutrophil responsiveness.","evidence":"LysM-Cre/C5aR2-flox conditional KO and global KO mice in EBA model; in vitro Ca²⁺, ROS, migration, and FcγR flow cytometry","pmids":["35007559"],"confidence":"High","gaps":["Mechanism linking C5aR2 to FcγR expression not elucidated","Whether myeloid-specific deletion fully recapitulates global KO in all disease models untested"]},{"year":2023,"claim":"CRISPR knockout of C5aR2 in macrophages upregulated STING and IRF3 expression and enhanced IFN-β secretion, with transcriptomics revealing de-repression of nucleic acid sensing pathways, establishing C5aR2 as a tonic negative regulator of the cGAS-STING-IFN-β axis.","evidence":"CRISPR-Cas9 KO in THP-1 and primary human monocyte-derived macrophages; IFN-β ELISA, STING/IRF3 Western blot, RNA-seq","pmids":["38067135"],"confidence":"High","gaps":["Whether C5aR2 suppresses STING transcriptionally or post-translationally unclear","Ligand dependence of tonic STING suppression not determined"]},{"year":2025,"claim":"C5aR2 on CD4⁺ T cells was found to orchestrate Th1 contraction through a C5a→C5aR2→PGI₂→IL-1R2 axis that sequesters Th1-promoting IL-1β, with disruption of this pathway linked to pathological Th1 persistence in CAPS, Crohn's disease, and rheumatoid arthritis.","evidence":"T cell-intrinsic genetic/pharmacological dissection, prostaglandin profiling, IL-1R2 expression assay; validated in human CAPS patient T cells treated with PGE2 synthase inhibitor","pmids":["40449486"],"confidence":"High","gaps":["Whether C5aR2 expression on T cells is context-inducible or constitutive across T cell subsets unclear","Source of C5a driving this axis in vivo not defined"]},{"year":null,"claim":"Key unresolved questions include: the structural basis for C5aR2's selective β-arrestin coupling without G-protein engagement; the molecular mechanism by which C5aR2 cross-inhibits diverse unrelated innate receptors (TLRs, STING, Dectins); and the determinants that switch C5aR2 between pro-inflammatory (NLRP3/NF-κB) and anti-inflammatory (ERK suppression/decoy) modes in different cell types.","evidence":"","pmids":[],"confidence":"Low","gaps":["No high-resolution structure of C5aR2 or C5aR2–β-arrestin complex","Mechanism of cross-receptor inhibition undefined","No unified model reconciling pro- and anti-inflammatory functions across cell types"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[4,10,15,17,22,23,25,26,31,32]},{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[0,3,21,27,36]},{"term_id":"GO:0038024","term_label":"cargo receptor activity","supporting_discovery_ids":[11,28]},{"term_id":"GO:0140104","term_label":"molecular carrier activity","supporting_discovery_ids":[28]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,1,6,24]},{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[10,12,13]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[12]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[10]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[4,5,8,10,14,15,17,22,24,25,26,29,30,31,32,34,35,36]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[3,10,13,16,20,21,27,29]},{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[3,9,12]},{"term_id":"R-HSA-5357801","term_label":"Programmed Cell Death","supporting_discovery_ids":[29,30]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[27,36]}],"complexes":["C5aR1-C5aR2 heterodimer"],"partners":["C5AR1","ARRB2","C5","NLRP3","EIF2AK2","AP2"],"other_free_text":[]},"mechanistic_narrative":"C5AR2 (also called C5L2/GPR77) is an atypical seven-transmembrane complement receptor that binds C5a and C5a desArg with high affinity but is uncoupled from heterotrimeric G proteins due to a leucine substitution in its DRY motif, instead signaling through β-arrestin recruitment and clathrin-dependent internalization [PMID:12899627, PMID:19100624, PMID:19641221]. C5AR2 functions as a context-dependent immunomodulator: in neutrophils and macrophages it predominantly restrains C5aR1-mediated pro-inflammatory signaling—suppressing ERK phosphorylation, chemotaxis, and cytokine production via β-arrestin pathway competition and physical heterodimerization with C5aR1—while in other settings it positively facilitates C5aR1 internalization, promotes NLRP3 inflammasome activation and HMGB1 release through PKR/MEK/IFN-I signaling, negatively regulates the cGAS-STING-IFN-β axis, and on endothelial cells transcytoses C5a to initiate neutrophil arrest [PMID:20044484, PMID:30971430, PMID:31076525, PMID:38067135, PMID:24631530]. Beyond immunity, C5AR2 serves as the functional receptor for acylation-stimulating protein (C3a desArg), mediating triglyceride synthesis and glucose transport in adipocytes through serine 323-dependent β-arrestin signaling, and on CD4+ T cells it orchestrates Th1 contraction via a prostacyclin–IL-1R2 axis whose disruption characterizes pathological Th1 persistence in autoinflammatory diseases [PMID:15833747, PMID:19615750, PMID:40449486]."},"prefetch_data":{"uniprot":{"accession":"Q9P296","full_name":"C5a anaphylatoxin chemotactic receptor 2","aliases":["Complement component 5a receptor 2","G-protein coupled receptor 77"],"length_aa":337,"mass_kda":36.1,"function":"Receptor for the chemotactic and inflammatory peptide anaphylatoxin C5a, stimulating chemotaxis, granule enzyme release, intracellular calcium release and superoxide anion production (PubMed:11773063). Also acts as a receptor for dearginated forms of C3a, C4a and C5a anaphylatoxin peptides (ASP/C3adesArg, C4adesArg and C5adesArg, respectively) (PubMed:12540846, PubMed:15833747, PubMed:19615750). Ligand binding causes a conformation change that triggers signaling via guanine nucleotide-binding proteins (G proteins) and modulates the activity of downstream effectors, such as adenylate cyclase (PubMed:12540846). C5AR1 is coupled to G(i)/G(o) (GNAI1 or GNAO1) G alpha proteins and mediates inhibition of adenylate cyclase (PubMed:12540846)","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/Q9P296/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/C5AR2","classification":"Not Classified","n_dependent_lines":1,"n_total_lines":1208,"dependency_fraction":0.0008278145695364238},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/C5AR2","total_profiled":1310},"omim":[{"mim_id":"609949","title":"COMPLEMENT COMPONENT 5a RECEPTOR 2; C5AR2","url":"https://www.omim.org/entry/609949"},{"mim_id":"606416","title":"NLR FAMILY, PYRIN DOMAIN-CONTAINING 3; NLRP3","url":"https://www.omim.org/entry/606416"},{"mim_id":"147720","title":"INTERLEUKIN 1-BETA; IL1B","url":"https://www.omim.org/entry/147720"},{"mim_id":"147678","title":"CASPASE 1, APOPTOSIS-RELATED CYSTEINE PROTEASE; CASP1","url":"https://www.omim.org/entry/147678"},{"mim_id":"147570","title":"INTERFERON, GAMMA; IFNG","url":"https://www.omim.org/entry/147570"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"lymphoid tissue","ntpm":13.2}],"url":"https://www.proteinatlas.org/search/C5AR2"},"hgnc":{"alias_symbol":["C5L2"],"prev_symbol":["GPR77"]},"alphafold":{"accession":"Q9P296","domains":[{"cath_id":"1.20.1070.10","chopping":"79-303","consensus_level":"high","plddt":91.3379,"start":79,"end":303}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9P296","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9P296-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9P296-F1-predicted_aligned_error_v6.png","plddt_mean":86.5},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=C5AR2","jax_strain_url":"https://www.jax.org/strain/search?query=C5AR2"},"sequence":{"accession":"Q9P296","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9P296.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9P296/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9P296"}},"corpus_meta":[{"pmid":"29395328","id":"PMC_29395328","title":"CD10+GPR77+ Cancer-Associated Fibroblasts Promote Cancer Formation and Chemoresistance by Sustaining Cancer Stemness.","date":"2018","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/29395328","citation_count":997,"is_preprint":false},{"pmid":"12899627","id":"PMC_12899627","title":"C5L2, a nonsignaling C5A binding protein.","date":"2003","source":"Biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/12899627","citation_count":211,"is_preprint":false},{"pmid":"20044484","id":"PMC_20044484","title":"The C5a receptor (C5aR) C5L2 is a modulator of C5aR-mediated signal transduction.","date":"2009","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/20044484","citation_count":206,"is_preprint":false},{"pmid":"11773063","id":"PMC_11773063","title":"The orphan receptor C5L2 has high affinity binding sites for complement fragments C5a and C5a des-Arg(74).","date":"2001","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/11773063","citation_count":189,"is_preprint":false},{"pmid":"17322907","id":"PMC_17322907","title":"C5L2 is critical for the biological activities of the anaphylatoxins C5a and C3a.","date":"2007","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/17322907","citation_count":187,"is_preprint":false},{"pmid":"23982144","id":"PMC_23982144","title":"Extracellular histones are essential effectors of C5aR- and C5L2-mediated tissue damage and inflammation in acute lung injury.","date":"2013","source":"FASEB journal : official publication of the Federation of American Societies for Experimental Biology","url":"https://pubmed.ncbi.nlm.nih.gov/23982144","citation_count":181,"is_preprint":false},{"pmid":"23239822","id":"PMC_23239822","title":"C5L2: a controversial receptor of complement anaphylatoxin, C5a.","date":"2012","source":"FASEB journal : official publication of the Federation of American Societies for Experimental Biology","url":"https://pubmed.ncbi.nlm.nih.gov/23239822","citation_count":169,"is_preprint":false},{"pmid":"16204243","id":"PMC_16204243","title":"An anti-inflammatory function for the complement anaphylatoxin C5a-binding protein, C5L2.","date":"2005","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/16204243","citation_count":167,"is_preprint":false},{"pmid":"11090875","id":"PMC_11090875","title":"A putative chemoattractant receptor, C5L2, is expressed in granulocyte and immature dendritic cells, but not in mature dendritic cells.","date":"2000","source":"Molecular immunology","url":"https://pubmed.ncbi.nlm.nih.gov/11090875","citation_count":151,"is_preprint":false},{"pmid":"12540846","id":"PMC_12540846","title":"The chemoattractant receptor-like protein C5L2 binds the C3a des-Arg77/acylation-stimulating protein.","date":"2003","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/12540846","citation_count":140,"is_preprint":false},{"pmid":"15833747","id":"PMC_15833747","title":"C5L2 is a functional receptor for acylation-stimulating protein.","date":"2005","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/15833747","citation_count":138,"is_preprint":false},{"pmid":"18227853","id":"PMC_18227853","title":"Receptors for complement C5a. 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Unlike CD88, C5L2 does not support degranulation or intracellular Ca2+ increases and is not rapidly internalized, but ligation potentiated IgE receptor-mediated degranulation via a pertussis toxin-sensitive mechanism.\",\n      \"method\": \"Radioligand binding assays, degranulation assays, Ca2+ flux assays in RBL-2H3 cells transfected with C5L2\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal binding and functional assays in transfected cells; foundational characterization paper\",\n      \"pmids\": [\"11773063\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"C5L2 is obligately uncoupled from heterotrimeric G proteins due to a leucine-for-arginine substitution in the DRY motif (end of TM3); it does not induce MAP kinase activation, calcium flux, or chemotaxis following C5a binding. C5L2 has slow ligand on/off rates and does not internalize following C5a binding.\",\n      \"method\": \"Transfection of human and murine C5L2 into multiple cell lines, G protein coupling assays, MAP kinase assays, Ca2+ flux, chemotaxis, radiolabeled ligand binding kinetics; C5aR-deficient mice bearing only C5L2 showed no transcriptional response to C5a by microarray\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal methods in both transfected cells and genetic mouse models; high citation count indicates foundational study\",\n      \"pmids\": [\"12899627\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"C5L2 binds C3a des-Arg77/acylation-stimulating protein (ASP) at a site distinct from the C5a binding site; C5L2 mRNA and protein are expressed in human skin fibroblasts and adipocytes, which are ASP-responsive cells.\",\n      \"method\": \"Radioligand binding, cross-competition assays, RT-PCR, Western blot in human skin fibroblasts and 3T3-L1 cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — binding assays with competition experiments in multiple cell types; single lab\",\n      \"pmids\": [\"12540846\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"C5L2 is a functional receptor for acylation-stimulating protein (ASP/C3a des-Arg): stable transfection of C5L2 into HEK293 cells confers ASP-stimulated triglyceride synthesis and glucose transport; antisense knockdown of C5L2 abolishes ASP response; ASP induces beta-arrestin translocation and C5L2 phosphorylation.\",\n      \"method\": \"Stable transfection, antisense oligonucleotide knockdown, triglyceride synthesis assay, glucose transport assay, DAPI-DAGL enzyme activity, beta-arrestin-GFP translocation, receptor phosphorylation in human skin fibroblasts and 3T3-L1 cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — gain-of-function transfection plus loss-of-function knockdown with multiple orthogonal functional readouts\",\n      \"pmids\": [\"15833747\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Targeted deletion of C5L2 in mice enhances the biological activity of C5a/C5a desArg both in vivo and in vitro, demonstrating that C5L2 functions as a negative (anti-inflammatory) modulator limiting the pro-inflammatory response to C5a.\",\n      \"method\": \"C5L2 knockout mice; in vivo C5a challenge; in vitro functional assays with C5L2-deficient cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with in vivo and in vitro functional endpoints; replicated anti-inflammatory role finding\",\n      \"pmids\": [\"16204243\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"C5L2 in rat neutrophils (PMN) during sepsis increases over time following CLP, while C5aR decreases; in vitro, C5a (but not fMLP) reduces C5L2 levels on PMN; anti-C5L2 antibody treatment causes ~4-fold increase in IL-6 during sepsis, indicating C5L2 negatively modulates C5a-driven IL-6 production.\",\n      \"method\": \"Rat CLP sepsis model, cloning of rat C5L2, anti-C5L2 antibody blockade in vivo and in vitro, ELISA for IL-6\",\n      \"journal\": \"FASEB journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vivo antibody blockade with defined cytokine readout, single lab\",\n      \"pmids\": [\"15784721\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"The N-terminal domain of C5L2 contains critical acidic and tyrosine residues for binding C5a des-Arg but not intact C5a; an antibody against the N-terminus of human C5L2 inhibited C5a des-Arg binding but not C5a binding, revealing that C5L2 binds C5a and C5a des-Arg by distinct mechanisms.\",\n      \"method\": \"Antibody blockade of chimeric/mutant C5L2 receptors, radioligand binding assays, mutagenesis of N-terminal residues\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis combined with ligand binding assays systematically defining binding domains\",\n      \"pmids\": [\"17158873\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"C5L2 does not bind C3a or C3a des-Arg77 (ASP), as demonstrated by 125I-ligand binding assays with fluorescently labeled ligands on transfected HEK293 and myeloblastic cell lines, contradicting earlier reports; C5L2 expression on myeloid lines is upregulated by dibutyryl-cAMP and IFN-γ but downregulated by TNF-α on HeLa cells; no C5a-dependent Ca2+ signaling detected from endogenously expressing HeLa cells.\",\n      \"method\": \"125I-ligand binding assays, flow cytometry with fluorescent ligands, quantitative real-time PCR, Ca2+ flux assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1-2 — direct binding assays with multiple cell types and ligands; contradicts earlier reports, single lab\",\n      \"pmids\": [\"17068344\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"C5L2 gene-targeted mice demonstrate that C5L2 is required to facilitate C5a signaling (positive modulator) in neutrophils, macrophages, and fibroblasts in vitro; C5L2 deficiency reduces inflammatory cell infiltration in vivo; C5L2-deficient mice are hypersensitive to LPS-induced septic shock, show reduced OVA-induced airway inflammation, and are mildly delayed in hematopoietic regeneration after irradiation — paralleling C3aR-deficient phenotypes.\",\n      \"method\": \"Gene targeting (KO mice), in vitro C5a signaling assays in multiple cell types, in vivo models: LPS sepsis, OVA airway challenge, gamma-irradiation\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with multiple in vivo and in vitro endpoints across multiple disease models; Nature-published foundational study\",\n      \"pmids\": [\"17322907\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"C5L2 KO mice on high-fat diet have significantly reduced adipose tissue triglyceride synthesis, lipolysis, and fatty acid re-esterification, with compensatory increases in muscle fatty acid oxidation (elevated UCP3, CD36, cytochrome c, phospho-ACC in skeletal muscle).\",\n      \"method\": \"C5L2 KO mice, adipose tissue TG synthesis assay, lipolysis assay, indirect calorimetry, Western blotting of muscle metabolic proteins\",\n      \"journal\": \"The Journal of endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with multiple orthogonal metabolic readouts\",\n      \"pmids\": [\"17641279\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Human C5L2 is predominantly intracellular in neutrophils (versus C5aR at the plasma membrane); C5L2 co-localizes with internalized C5aR and beta-arrestin; antibody blockade of C5L2 dramatically increases C5a-mediated chemotaxis and ERK1/2 phosphorylation but not Ca2+ mobilization; C5L2 associates with beta-arrestin by co-immunoprecipitation; C5L2 blockade does not affect C5aR endocytosis — establishing C5L2 as a negative modulator of C5aR signaling through the beta-arrestin pathway.\",\n      \"method\": \"Confocal microscopy, antibody blockade, chemotaxis assay, ERK1/2 phosphorylation assay, Ca2+ mobilization, co-immunoprecipitation in human neutrophils/PMNs\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP, confocal co-localization, multiple functional assays in primary human cells; highly cited\",\n      \"pmids\": [\"20044484\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Human C5L2 constitutively internalizes ligand via clathrin-dependent endocytosis independently of G-protein coupling; internalized ligand is retained and degraded intracellularly, whereas C5aR releases ligand back extracellularly — supporting C5L2 as a recycling decoy receptor in humans. Engineered chimeric C5L2 with C5aR G-protein coupling sequences failed to restore Ca2+ signaling or beta-arrestin redistribution.\",\n      \"method\": \"Constitutive ligand-independent internalization assay, clathrin inhibition (dominant-negative), ligand retention/degradation tracking, Ca2+ assay, beta-arrestin redistribution assay in transfected cell lines\",\n      \"journal\": \"Molecular immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple mechanistic assays with pharmacological and molecular perturbations; rigorous mutagenesis-based approach to G-protein coupling\",\n      \"pmids\": [\"19100624\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"C5L2 undergoes ligand-dependent (ASP and C5a) endocytosis that is clathrin- and cholesterol-dependent; following endocytosis, C5L2 traffics through early endosomes (Rab5), late endosomes (Rab7), and recycling endosomes (Rab11); the S323I mutation prevents C5L2 from localizing to the cell surface and abolishes ligand-mediated receptor phosphorylation and beta-arrestin-2 translocation, as well as downstream functional responses (glucose transport and TG synthesis).\",\n      \"method\": \"Beta-arrestin-2-GFP translocation, Rab-GFP co-localization by live imaging, receptor phosphorylation assay, functional glucose transport and TG synthesis assays in stably transfected C5L2-HEK cells; S323I mutant receptor\",\n      \"journal\": \"Molecular immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — mutagenesis with multiple orthogonal assays defining trafficking pathway and functional significance of Ser323\",\n      \"pmids\": [\"19615750\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"C5a and C5a-desArg (but not C3a or C3a-desArg) stimulate beta-arrestin2 redistribution to cytoplasmic vesicles via C5L2; direct C5L2-beta-arrestin interaction confirmed by a beta-galactosidase fragment complementation assay; beta-arrestin coupling occurred at subnanomolar C5a concentrations.\",\n      \"method\": \"GFP-labeled beta-arrestin2 redistribution assay, beta-galactosidase fragment complementation assay in stably transfected cells\",\n      \"journal\": \"Journal of biomolecular screening\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — two orthogonal assays (GFP translocation and protein complementation) confirming direct C5L2-beta-arrestin interaction\",\n      \"pmids\": [\"19641221\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"C5L2-deficient mice are protected from OVA- and house dust mite-induced allergic asthma (reduced AHR, Th2 cytokines, eosinophilic inflammation, IgE, mucus); C5L2-deficient myeloid dendritic cells produce IL-23 and drive Th17 responses rather than Th2 responses upon HDM pulsing, indicating C5L2 on mDCs controls the Th1/Th17 versus Th2 balance.\",\n      \"method\": \"C5L2 KO mice, OVA and HDM asthma models, adoptive transfer of C5L2-deficient mDCs into WT mice, in vitro cytokine profiling of mDCs\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO plus adoptive transfer experiment defining cell-type-specific mechanism\",\n      \"pmids\": [\"20974988\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"TLR activation induces C5a hypersensitivity by reducing C5L2 activity (not by upregulating C5aR); TLR-induced hypersensitivity was mimicked by C5L2 blockade and absent in C5L2 KO mice, confirming that TLRs amplify C5a responses by targeting C5L2 as a negative modulator of C5aR.\",\n      \"method\": \"PBMCs and whole blood with TLR ligand pretreatment, C5L2 KO mice, TLR4-deficient mice, C5L2 blocking antibody, C5a-induced cytokine readouts\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple genetic and pharmacological approaches in primary cells and KO mice converging on same mechanism\",\n      \"pmids\": [\"21630250\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"C5L2 and C5aR form constitutive homo- and heterodimers in HEK293 cells (by BRET); in adipocytes, ASP or C5a stimulation promotes internalization of C5L2 with increasing perinuclear co-localization with C5aR; C5L2 activation by ASP (but not C5a) increases fatty acid uptake/esterification in adipocytes, while both ligands activate Akt phosphorylation.\",\n      \"method\": \"BRET (bioluminescent resonance energy transfer) in HEK293 cells, confocal microscopy in 3T3-L1 adipocytes and J774 macrophages, fatty acid uptake assay, Western blotting\",\n      \"journal\": \"Cellular signalling\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — BRET for dimerization plus confocal co-localization and functional assays, single lab\",\n      \"pmids\": [\"23268185\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"C5L2 blockade with mAb in allergic contact dermatitis in mice dramatically increases inflammation, and this exacerbation is reversed by anti-C5aR mAb, demonstrating that C5L2 suppresses C5aR-mediated beta-arrestin signaling in vivo.\",\n      \"method\": \"C5L2 KO mice in murine contact sensitivity model, anti-C5aR mAb treatment, inflammatory readouts\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO plus antibody rescue experiment defining pathway position in vivo\",\n      \"pmids\": [\"24043888\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Ligation of both C5aR and C5L2 is required for optimal C5a-induced G-CSF production from peritoneal macrophages; C5a-mediated G-CSF induction is associated with PI3K/Akt and MEK1/2 signaling; G-CSF levels during CLP sepsis are reduced in both C5aR- and C5L2-deficient mice.\",\n      \"method\": \"C5aR KO and C5L2 KO mouse peritoneal macrophages, G-CSF ELISA, PI3K/MEK inhibitors, CLP sepsis model\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — dual KO genetic epistasis with pharmacological pathway dissection in both in vitro and in vivo settings\",\n      \"pmids\": [\"23575697\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"C5a, but not C5a-des Arg, induces upregulation of C5aR-C5L2 heteromer formation as measured by BRET; this heteromer formation is inhibited by a C5aR-specific antagonist; the two ligands produce differential IL-10 responses in macrophages, suggesting heteromerization contributes to differential signaling.\",\n      \"method\": \"BRET in transfected HEK293 cells, wide-field microscopy, ELISA in human monocyte-derived macrophages\",\n      \"journal\": \"Immunology and cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — BRET provides direct protein interaction evidence, but single lab and limited functional follow-up\",\n      \"pmids\": [\"24060963\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"C5L2 physically interacts with C5aR and is required for optimal C5a-mediated C5aR internalization through an AP2-dependent, dynamin-dependent mechanism; without C5L2, AP2 recruitment to the receptor complex fails; C5L2-dependent C5aR internalization drives downstream MEK/ERK signaling.\",\n      \"method\": \"Co-immunoprecipitation, dynamin inhibitor (dynasore), AP2 recruitment assay, ERK phosphorylation in transfected cells; C5L2-deficient mice in acute colitis model\",\n      \"journal\": \"Cellular signalling\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — Co-IP plus pharmacological and genetic loss-of-function approaches defining molecular mechanism of receptor internalization\",\n      \"pmids\": [\"24631530\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"In human mast cells (LAD2) expressing surface C5aR2 but not C5aR1, C5a induces ERK phosphorylation, cytokine production (GM-CSF, TNF, CXCL10, CCL2), adhesion, and chemotaxis via C5aR2; these responses depend on beta-arrestin-2 and PI3K; lentiviral shRNA silencing of C5aR2 renders cells unresponsive to all C5a-induced effects.\",\n      \"method\": \"Lentiviral shRNA knockdown of C5aR2, siRNA against beta-arrestin-2, wortmannin (PI3K inhibitor), ERK phosphorylation, ELISA, adhesion and chemotaxis assays in LAD2 cells\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — genetic knockdown plus pharmacological dissection with multiple functional readouts in a cell model with natural expression of only C5aR2\",\n      \"pmids\": [\"26283482\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"C5L2-deficient mice (BALB/c) show more severe LPS-induced acute lung injury with greater airway edema, neutrophilia, MPO activity, and pro-inflammatory cytokines/chemokines; anti-C5aR blockade reverses neutrophilic infiltration in C5L2-/- mice, confirming C5L2's anti-inflammatory role is executed through negative modulation of C5aR.\",\n      \"method\": \"C5L2 KO mice, intranasal LPS model, anti-C5aR blocking antibody, BAL fluid analysis, lung homogenate cytokine measurement\",\n      \"journal\": \"American journal of respiratory cell and molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with antibody rescue experiment in defined in vivo model\",\n      \"pmids\": [\"27285858\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Selective C5aR2 ligands (P32 and P59) selectively recruit beta-arrestin 2 via C5aR2, partially inhibit C5a-induced ERK1/2 activation, and reduce LPS-stimulated IL-6 release from macrophages without directly affecting C5aR1; P32 inhibits C5a-mediated neutrophil mobilization in WT but not C5aR2-/- mice.\",\n      \"method\": \"Peptide library screen, beta-arrestin 2 recruitment assay, ERK1/2 phosphorylation assay, IL-6 ELISA in human macrophages, in vivo neutrophil mobilization in C5aR2 KO mice\",\n      \"journal\": \"Immunology and cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — pharmacological tool compounds validated in both in vitro and in vivo KO mouse settings with multiple orthogonal assays\",\n      \"pmids\": [\"27108698\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"C5aR2 expression is strongest in brain, bone marrow, and airways; all myeloid-derived cells express C5aR2 with neutrophils showing strong homogeneous expression; T cells are negative; B cells and NK cells express C5aR2. Selective C5aR2 ligation in neutrophils blocks C5a-driven ERK1/2 phosphorylation. Activation of C5aR2 in NK cells suppresses IL-12/IL-18-induced IFN-γ production. IL-33 challenge reduces C5aR2 in pulmonary eosinophils and monocyte-derived DCs.\",\n      \"method\": \"Floxed tdTomato-C5aR2 knock-in reporter mouse, flow cytometry, ERK1/2 phosphorylation assay in neutrophils, IFN-γ production assay in NK cells, intratracheal IL-33 challenge\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — genetic reporter mouse plus functional validation in multiple primary cell types\",\n      \"pmids\": [\"28864475\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"C5aR2-deficient mice show increased lesion volumes and worsened recovery from contusive spinal cord injury; acute PMX205 (C5aR1 antagonist) treatment improved outcomes in both WT and fully alleviated worsened recovery in C5aR2-/- mice, demonstrating C5aR2 is neuroprotective by restraining injurious C5aR1 signaling.\",\n      \"method\": \"C5aR2 KO mice, contusive SCI model, PMX205 C5aR1 antagonist treatment, lesion volume measurement, myelin sparing, behavioral recovery scores\",\n      \"journal\": \"Journal of neurotrauma\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with pharmacological rescue experiment defining epistatic relationship between C5aR1 and C5aR2\",\n      \"pmids\": [\"28173736\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"C5ar2-/- mice show increased skin lesions in a bullous pemphigoid model while C5ar1-/- mice are protected, demonstrating opposing roles for C5aR1 (proinflammatory/pathogenic) and C5aR2 (anti-inflammatory/protective) in autoantibody-mediated tissue damage.\",\n      \"method\": \"C5ar1 KO and C5ar2 KO mice, anti-Col17 IgG transfer BP model, skin lesion scoring, neutrophil migration assay, PMX53 pharmacological C5aR1 inhibition\",\n      \"journal\": \"Frontiers in immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — dual KO genetic epistasis with pharmacological validation\",\n      \"pmids\": [\"29599777\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"CD10+GPR77+ (C5AR2+) cancer-associated fibroblasts sustain cancer stem cells through NF-κB activation driven by complement signaling via GPR77/C5AR2; GPR77 maintains persistent NF-κB p65 phosphorylation and acetylation; neutralizing anti-GPR77 antibody abolishes tumor formation and restores chemosensitivity.\",\n      \"method\": \"Patient-derived xenografts, anti-GPR77 neutralizing antibody, NF-κB reporter, p65 phosphorylation/acetylation assays, PDX co-injection experiments\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — mechanistic pathway defined with multiple orthogonal assays including in vivo PDX experiments and antibody blockade; highly cited\",\n      \"pmids\": [\"29395328\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"C5aR2 expressed on endothelial cells acts as an atypical chemoattractant receptor that transports C5a from the perivascular tissue into the vessel lumen (transcytosis); this transported C5a initiates C5aR1-mediated neutrophil arrest, while ACKR1-transported chemokines drive CXCR2-dependent neutrophil transmigration — establishing sequential roles of atypical receptors in neutrophil recruitment.\",\n      \"method\": \"Intravital microscopy in live mice, immune complex-induced arthritis model, C5aR2-deficient mice, ACKR1-deficient mice, in vivo tracking of C5a transport\",\n      \"journal\": \"Science immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — intravital microscopy with genetic KO models directly demonstrating transcytosis function; high citation, Science Immunology journal\",\n      \"pmids\": [\"31076525\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"C5aR2 promotes NLRP3 inflammasome activation and HMGB1 release from macrophages by amplifying dsRNA-dependent PKR expression; PKR upregulation by C5a-C5aR2 interaction depends on MEK/ERK and type I IFN signaling pathways; C5aR2 deficiency restricts NLRP3 activation and HMGB1 release both in vitro and in vivo.\",\n      \"method\": \"C5aR2-deficient mice and macrophages, siRNA knockdown, immunoblotting, qRT-PCR for PKR expression, MEK inhibitor treatment, NLRP3 inhibition\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — genetic KO with siRNA, pathway inhibitors, and multiple orthogonal readouts defining molecular mechanism\",\n      \"pmids\": [\"30971430\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"C5aR2 is required for C5a-mediated renal inflammation and tissue damage in acute pyelonephritis; C5aR2 signaling activates NLRP3/caspase-1 inflammasome and HMGB1 release in macrophages; blocking HMGB1 reduces C5aR2-mediated NLRP3 activation, placing HMGB1 upstream of NLRP3 in C5aR2 signaling.\",\n      \"method\": \"C5ar2 KO mice, uropathogenic E. coli bladder inoculation model, C5aR1-deficient macrophages (expressing only C5aR2), HMGB1 blocking antibody, NLRP3/caspase-1/IL-1β measurement\",\n      \"journal\": \"JCI insight\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO plus C5aR1 KO cells plus blocking antibody approach in vivo and in vitro defining pathway order\",\n      \"pmids\": [\"32191644\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Selective C5aR2 agonism (P32) by itself produces no detectable MAPK signaling but significantly dampens C5aR1-, C3aR-, and CMKLR1-mediated ERK signaling and alters intracellular calcium mobilization from these receptors; C5aR2 activation also reduces cytokine production from TLR2, TLR3, TLR4, TLR7, Dectin-1, Dectin-2, Mincle (reducing IL-6/TNF by 80-90%), and STING pathways in primary human macrophages.\",\n      \"method\": \"C5aR2 agonist P32 treatment of primary human monocyte-derived macrophages, ERK phosphorylation assay, Ca2+ mobilization, ELISA for multiple cytokines\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — selective pharmacological agonist with multiple PRR ligand stimulations and diverse functional readouts in primary human cells\",\n      \"pmids\": [\"32611725\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"C5aR2 deficiency in intestinal IR injury worsens intestinal damage due to increased C5aR1-mediated neutrophil infiltration; C5aR1 blockade in C5aR2-/- mice prevents neutrophil accumulation and reduces pathology; C5aR2 deficiency also reduces circulating neutrophil numbers after IR via a C5aR1-independent mechanism, demonstrating distinct and separable functions of C5aR2.\",\n      \"method\": \"C5aR2 KO mice, intestinal IR model, PMX53 C5aR1 antagonist, neutrophil depletion, tissue damage scoring, cytokine measurement\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with pharmacological rescue dissecting C5aR1-dependent and -independent C5aR2 functions\",\n      \"pmids\": [\"33028618\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"SB290157, widely used as a 'C3aR antagonist', is actually a partial agonist at C5aR2, mediating beta-arrestin recruitment at higher doses and significantly dampening C5a-induced ERK signaling in human and mouse primary macrophages via C5aR2.\",\n      \"method\": \"Beta-arrestin recruitment assay in transfected cells, ERK phosphorylation assay in primary macrophages, pharmacological profiling against C3aR, C5aR1, C5aR2\",\n      \"journal\": \"Frontiers in pharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — pharmacological characterization with functional readouts in primary cells, single lab\",\n      \"pmids\": [\"33551801\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"C5aR2 deficiency in neutrophils reduces intracellular calcium flux, ROS release, and migration capacity when activated with immune complexes or C5a; C5aR2 deficiency lowers the ratio of activating to inhibitory FcγRs on neutrophils; LysM-specific C5aR2 KO recapitulates global C5aR2 KO phenotype in epidermolysis bullosa acquisita.\",\n      \"method\": \"C5aR2 KO and LysM-Cre/C5aR2-flox mice, antibody transfer EBA model, in vitro Ca2+ flux, ROS assay, migration assay, FcγR expression analysis by flow cytometry\",\n      \"journal\": \"The Journal of investigative dermatology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — conditional cell-type-specific KO with multiple in vitro mechanistic assays and in vivo disease model\",\n      \"pmids\": [\"35007559\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"CRISPR-KO of C5aR2 in macrophages significantly increases STING-mediated IFN-β secretion; STING and IRF3 expression are increased in C5aR2 KO cells; transcriptomic analysis shows upregulation of nucleic acid sensing and antiviral signaling pathways in C5aR2 KO cells, indicating C5aR2 negatively regulates the cGAS-STING-IFN-β axis.\",\n      \"method\": \"CRISPR-Cas9 KO of C5aR2 in THP-1 cells and primary human monocyte-derived macrophages, ELISA for IFN-β, Western blot for STING/IRF3, RNA-seq\",\n      \"journal\": \"Cells\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — CRISPR KO with transcriptomics and functional cytokine measurements in both cell line and primary cells\",\n      \"pmids\": [\"38067135\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"C5aR2 on CD4+ T cells orchestrates Th1 cell contraction via a C5-C5aR2-prostacyclin (PGI2) axis: C5aR2 activation shifts prostaglandin balance from PGE2 to PGI2 production, which signals autocrine through PGI2 receptor to induce IL-1R2 expression; IL-1R2 sequesters Th1-driving intrinsic IL-1β, facilitating Th1 contraction. Disruption of this axis characterizes pathological Th1 persistence in CAPS, Crohn's disease, and RA.\",\n      \"method\": \"CD4+ T cell-intrinsic genetic and pharmacological dissection, prostaglandin profiling, IL-1R2 expression assay, T cell functional assays, human T cells from CAPS patients with PGE2 synthase inhibitor treatment\",\n      \"journal\": \"Immunity\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — cell-autonomous pathway defined with multiple orthogonal assays and validated in human disease samples\",\n      \"pmids\": [\"40449486\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"C5AR2 (C5L2) is a seven-transmembrane receptor that binds C5a/C5a desArg with high affinity but lacks classical G-protein coupling due to a DRY-motif leucine substitution; instead it signals through β-arrestin recruitment and acts as a pleiotropic immunomodulator — functioning as a decoy/scavenging receptor by internalizing and degrading C5a via clathrin-dependent endocytosis, as a negative modulator of C5aR1 signaling (suppressing ERK, chemotaxis, and cytokine production through β-arrestin-pathway competition and physical C5aR1 heterodimerization), as a positive facilitator of C5aR1 internalization and downstream ERK signaling in some cell contexts, as a transcytotic transporter of C5a across endothelial cells to initiate neutrophil arrest, as a metabolic receptor mediating ASP-driven triglyceride synthesis via serine 323-dependent β-arrestin signaling in adipocytes, as a promoter of NLRP3 inflammasome activation and HMGB1 release through PKR/MEK/IFN-type I signaling in macrophages, as a negative regulator of cGAS-STING-IFN-β signaling, and as a T cell-intrinsic regulator of Th1 contraction through a C5a-C5aR2-prostacyclin-IL-1R2 axis — with its net inflammatory effect (pro- versus anti-inflammatory) being highly context-, cell-type-, and disease-dependent.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"C5AR2 (also called C5L2/GPR77) is an atypical seven-transmembrane complement receptor that binds C5a and C5a desArg with high affinity but is uncoupled from heterotrimeric G proteins due to a leucine substitution in its DRY motif, instead signaling through β-arrestin recruitment and clathrin-dependent internalization [PMID:12899627, PMID:19100624, PMID:19641221]. C5AR2 functions as a context-dependent immunomodulator: in neutrophils and macrophages it predominantly restrains C5aR1-mediated pro-inflammatory signaling—suppressing ERK phosphorylation, chemotaxis, and cytokine production via β-arrestin pathway competition and physical heterodimerization with C5aR1—while in other settings it positively facilitates C5aR1 internalization, promotes NLRP3 inflammasome activation and HMGB1 release through PKR/MEK/IFN-I signaling, negatively regulates the cGAS-STING-IFN-β axis, and on endothelial cells transcytoses C5a to initiate neutrophil arrest [PMID:20044484, PMID:30971430, PMID:31076525, PMID:38067135, PMID:24631530]. Beyond immunity, C5AR2 serves as the functional receptor for acylation-stimulating protein (C3a desArg), mediating triglyceride synthesis and glucose transport in adipocytes through serine 323-dependent β-arrestin signaling, and on CD4+ T cells it orchestrates Th1 contraction via a prostacyclin–IL-1R2 axis whose disruption characterizes pathological Th1 persistence in autoinflammatory diseases [PMID:15833747, PMID:19615750, PMID:40449486].\",\n  \"teleology\": [\n    {\n      \"year\": 2001,\n      \"claim\": \"Identification of C5L2 as a high-affinity C5a/C5a desArg receptor that does not trigger classical GPCR effector responses (degranulation, Ca²⁺ flux) established it as an atypical complement receptor with undefined signaling capacity.\",\n      \"evidence\": \"Radioligand binding, Ca²⁺ flux, and degranulation assays in C5L2-transfected RBL-2H3 cells\",\n      \"pmids\": [\"11773063\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Signaling mechanism downstream of ligand binding undefined\", \"Physiological role in primary cells unknown\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Demonstration that C5L2 is obligately G-protein-uncoupled due to the DRY→DLC motif substitution, with no MAPK, Ca²⁺, or chemotactic activity, raised the question of whether it functions as a signaling receptor or a decoy/scavenger.\",\n      \"evidence\": \"Multi-cell-line transfection with G-protein coupling, MAPK, and chemotaxis assays; C5aR-deficient mouse microarray showing no transcriptional response via C5L2 alone\",\n      \"pmids\": [\"12899627\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which C5L2 modulates C5a biology without G-protein coupling unknown\", \"Potential non-G-protein signaling not yet explored\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Gain- and loss-of-function experiments established C5L2 as a functional receptor for ASP/C3a desArg that drives triglyceride synthesis via β-arrestin recruitment, revealing the first signaling pathway for this receptor; simultaneously, C5L2-knockout mice showed enhanced C5a responses, defining C5L2 as an anti-inflammatory modulator in vivo.\",\n      \"evidence\": \"Stable transfection and antisense knockdown in fibroblasts/adipocytes with TG synthesis, glucose transport, β-arrestin-GFP translocation; C5L2 KO mice challenged with C5a in vivo; anti-C5L2 antibody blockade in rat sepsis model\",\n      \"pmids\": [\"15833747\", \"16204243\", \"15784721\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether anti-inflammatory and metabolic functions use the same or distinct β-arrestin pathways unclear\", \"Binding of C3a/C3a desArg disputed by subsequent study\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Paradoxically, an independent C5L2 KO study found C5L2 facilitates (rather than suppresses) C5a signaling in neutrophils and macrophages, and C5L2 KO mice paralleled C3aR KO phenotypes in asthma and sepsis models, revealing the first evidence that C5L2's net effect is context-dependent.\",\n      \"evidence\": \"Gene-targeted C5L2 KO mice in LPS sepsis, OVA airway, and irradiation models; in vitro C5a signaling in multiple primary cell types\",\n      \"pmids\": [\"17322907\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis for discrepancy with earlier KO study unresolved\", \"Cell-type-specific roles not yet dissected\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Discovery that C5L2 is predominantly intracellular in neutrophils, co-localizes with internalized C5aR and β-arrestin, and that its blockade selectively enhances ERK but not Ca²⁺ signaling established the β-arrestin pathway competition model for C5L2's anti-inflammatory activity; separately, C5L2 was shown to constitutively internalize and degrade C5a via clathrin-dependent endocytosis, defining its decoy/scavenging function.\",\n      \"evidence\": \"Confocal microscopy, Co-IP of C5L2–β-arrestin, antibody blockade with ERK/Ca²⁺ readouts in human neutrophils; clathrin dominant-negative inhibition and ligand degradation tracking in transfected cells\",\n      \"pmids\": [\"20044484\", \"19100624\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis for β-arrestin recruitment without G-protein coupling unknown\", \"Relative contributions of decoy versus signaling functions unquantified\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Detailed trafficking analysis showed C5L2 undergoes ligand-dependent clathrin/cholesterol-dependent endocytosis through Rab5→Rab7→Rab11 compartments, and serine 323 is essential for surface localization, phosphorylation, β-arrestin-2 recruitment, and all downstream metabolic functions.\",\n      \"evidence\": \"Rab-GFP co-localization, S323I mutagenesis with β-arrestin-2 translocation and TG synthesis assays in stably transfected HEK cells; β-galactosidase complementation confirming direct C5L2–β-arrestin interaction\",\n      \"pmids\": [\"19615750\", \"19641221\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether S323 phosphorylation is mediated by GRKs or other kinases unknown\", \"Crystal/cryo-EM structure of C5L2 absent\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"BRET experiments demonstrated constitutive C5aR-C5L2 homo- and heterodimer formation, with C5a-induced upregulation of heteromers, providing a physical basis for functional crosstalk between the two receptors.\",\n      \"evidence\": \"BRET in transfected HEK293 cells; confocal co-localization in 3T3-L1 adipocytes and J774 macrophages\",\n      \"pmids\": [\"23268185\", \"24060963\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Heterodimer stoichiometry and structural interface undefined\", \"BRET proximity does not prove functional requirement for dimerization in all contexts\", \"In vivo confirmation of heterodimerization lacking\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Epistatic studies in allergic contact dermatitis and sepsis models showed C5L2's anti-inflammatory effect is reversed by C5aR1 blockade, while C5L2 and C5aR1 are both required for optimal G-CSF production, demonstrating that C5L2 both restrains and cooperates with C5aR1 depending on downstream pathway.\",\n      \"evidence\": \"C5L2 KO mice with anti-C5aR mAb rescue in contact sensitivity; dual C5aR1/C5L2 KO macrophages with PI3K/MEK inhibitors in CLP sepsis\",\n      \"pmids\": [\"24043888\", \"23575697\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular determinants that switch C5L2 from inhibitory to cooperative mode unknown\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"C5L2 was found to physically interact with C5aR and be required for AP2-dependent C5aR internalization and downstream MEK/ERK signaling, defining a positive facilitatory mechanism for C5aR endocytosis.\",\n      \"evidence\": \"Co-IP, dynasore inhibition, AP2 recruitment assay, ERK phosphorylation in transfected cells; C5L2 KO mice in acute colitis\",\n      \"pmids\": [\"24631530\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether AP2 recruitment requires direct C5L2–AP2 contact or an adaptor protein unknown\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"In LAD2 mast cells expressing C5aR2 but not C5aR1, C5a independently drove ERK phosphorylation, cytokine production, and chemotaxis via β-arrestin-2/PI3K, proving C5aR2 can signal autonomously without C5aR1.\",\n      \"evidence\": \"Lentiviral shRNA knockdown of C5aR2, β-arrestin-2 siRNA, PI3K inhibitor in LAD2 cells with multiple functional readouts\",\n      \"pmids\": [\"26283482\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Generalizability to other cell types expressing only C5aR2 not tested\", \"Downstream transcription factor targets of autonomous C5aR2 signaling unknown\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"A genetic reporter mouse mapped C5aR2 expression across immune lineages (strong in neutrophils, myeloid cells, NK cells; absent from T cells), and functional validation showed C5aR2 ligation suppresses C5a-driven ERK in neutrophils and IL-12/IL-18-induced IFN-γ in NK cells, defining cell-type-specific anti-inflammatory functions.\",\n      \"evidence\": \"Floxed tdTomato-C5aR2 knock-in reporter mouse with flow cytometry; ERK assays in neutrophils; IFN-γ assay in NK cells\",\n      \"pmids\": [\"28864475\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Expression and function in non-hematopoietic lineages (e.g. neurons, fibroblasts) not fully addressed by reporter\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"CD10⁺GPR77⁺ cancer-associated fibroblasts were shown to sustain cancer stemness via C5aR2-driven NF-κB activation, with neutralizing anti-GPR77 antibody abolishing tumor formation and restoring chemosensitivity, establishing a pro-tumorigenic role for C5aR2 in the tumor microenvironment.\",\n      \"evidence\": \"Patient-derived xenografts, anti-GPR77 neutralizing antibody, NF-κB reporter and p65 phosphorylation/acetylation assays\",\n      \"pmids\": [\"29395328\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Ligand driving C5aR2 on CAFs in vivo not definitively identified\", \"Whether NF-κB activation uses β-arrestin or an alternative adaptor unknown\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"C5aR2 on endothelial cells was identified as a transcytotic transporter that shuttles C5a from perivascular tissue to the vascular lumen to initiate C5aR1-dependent neutrophil arrest, establishing a non-signaling transport function for C5aR2 analogous to atypical chemokine receptors.\",\n      \"evidence\": \"Intravital microscopy in C5aR2 KO and ACKR1 KO mice in immune complex arthritis model\",\n      \"pmids\": [\"31076525\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular machinery of C5aR2-mediated transcytosis (vesicular carriers, sorting signals) undefined\", \"Whether transcytosis is bidirectional unknown\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"C5aR2 was shown to promote NLRP3 inflammasome activation and HMGB1 release from macrophages by upregulating PKR expression through MEK/ERK and type I IFN signaling, with HMGB1 acting upstream of NLRP3 in this pathway, defining a pro-inflammatory signaling cascade distinct from the anti-inflammatory β-arrestin competition model.\",\n      \"evidence\": \"C5aR2 KO macrophages, siRNA, MEK inhibitor, HMGB1 blocking antibody; validated in acute pyelonephritis model\",\n      \"pmids\": [\"30971430\", \"32191644\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How C5aR2 activates MEK/ERK for PKR induction without G-protein coupling is mechanistically unresolved\", \"Whether this pathway operates in non-macrophage cell types unknown\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Selective C5aR2 agonism broadly dampened signaling from multiple innate receptors (C5aR1, C3aR, CMKLR1, TLR2/3/4/7, Dectin-1/2, Mincle, STING), reducing cytokine output by up to 90%, establishing C5aR2 as a global negative regulator of innate immune receptor signaling.\",\n      \"evidence\": \"Selective C5aR2 agonist P32 in primary human monocyte-derived macrophages with ERK, Ca²⁺, and multi-cytokine readouts across diverse PRR stimulations; validated with C5aR2 KO mice\",\n      \"pmids\": [\"32611725\", \"27108698\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism by which C5aR2 cross-inhibits unrelated PRR pathways not defined\", \"Whether tonic C5aR2 signaling operates without exogenous ligand unknown\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Conditional (LysM-Cre) C5aR2 deletion showed that myeloid-intrinsic C5aR2 is required for full neutrophil effector function (Ca²⁺ flux, ROS, migration) and modulates activating/inhibitory FcγR ratios, demonstrating a cell-autonomous role in tuning neutrophil responsiveness.\",\n      \"evidence\": \"LysM-Cre/C5aR2-flox conditional KO and global KO mice in EBA model; in vitro Ca²⁺, ROS, migration, and FcγR flow cytometry\",\n      \"pmids\": [\"35007559\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism linking C5aR2 to FcγR expression not elucidated\", \"Whether myeloid-specific deletion fully recapitulates global KO in all disease models untested\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"CRISPR knockout of C5aR2 in macrophages upregulated STING and IRF3 expression and enhanced IFN-β secretion, with transcriptomics revealing de-repression of nucleic acid sensing pathways, establishing C5aR2 as a tonic negative regulator of the cGAS-STING-IFN-β axis.\",\n      \"evidence\": \"CRISPR-Cas9 KO in THP-1 and primary human monocyte-derived macrophages; IFN-β ELISA, STING/IRF3 Western blot, RNA-seq\",\n      \"pmids\": [\"38067135\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether C5aR2 suppresses STING transcriptionally or post-translationally unclear\", \"Ligand dependence of tonic STING suppression not determined\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"C5aR2 on CD4⁺ T cells was found to orchestrate Th1 contraction through a C5a→C5aR2→PGI₂→IL-1R2 axis that sequesters Th1-promoting IL-1β, with disruption of this pathway linked to pathological Th1 persistence in CAPS, Crohn's disease, and rheumatoid arthritis.\",\n      \"evidence\": \"T cell-intrinsic genetic/pharmacological dissection, prostaglandin profiling, IL-1R2 expression assay; validated in human CAPS patient T cells treated with PGE2 synthase inhibitor\",\n      \"pmids\": [\"40449486\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether C5aR2 expression on T cells is context-inducible or constitutive across T cell subsets unclear\", \"Source of C5a driving this axis in vivo not defined\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include: the structural basis for C5aR2's selective β-arrestin coupling without G-protein engagement; the molecular mechanism by which C5aR2 cross-inhibits diverse unrelated innate receptors (TLRs, STING, Dectins); and the determinants that switch C5aR2 between pro-inflammatory (NLRP3/NF-κB) and anti-inflammatory (ERK suppression/decoy) modes in different cell types.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No high-resolution structure of C5aR2 or C5aR2–β-arrestin complex\", \"Mechanism of cross-receptor inhibition undefined\", \"No unified model reconciling pro- and anti-inflammatory functions across cell types\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [4, 10, 15, 17, 22, 23, 25, 26, 31, 32]},\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [0, 3, 21, 27, 36]},\n      {\"term_id\": \"GO:0038024\", \"supporting_discovery_ids\": [11, 28]},\n      {\"term_id\": \"GO:0140104\", \"supporting_discovery_ids\": [28]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 1, 6, 24]},\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [10, 12, 13]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [12]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [10]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [4, 5, 8, 10, 14, 15, 17, 22, 24, 25, 26, 29, 30, 31, 32, 34, 35, 36]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [3, 10, 13, 16, 20, 21, 27, 29]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [3, 9, 12]},\n      {\"term_id\": \"R-HSA-5357801\", \"supporting_discovery_ids\": [29, 30]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [27, 36]}\n    ],\n    \"complexes\": [\n      \"C5aR1-C5aR2 heterodimer\"\n    ],\n    \"partners\": [\n      \"C5AR1\",\n      \"ARRB2\",\n      \"C5\",\n      \"NLRP3\",\n      \"EIF2AK2\",\n      \"AP2\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}