{"gene":"ACKR4","run_date":"2026-06-09T22:02:39","timeline":{"discoveries":[{"year":2006,"finding":"CCX-CKR (ACKR4) mediates rapid CCL19 internalization and progressive sequestration/degradation of large quantities of CCL19. Unlike CCR7, CCX-CKR does not become refractory for CCL19 uptake over time, and its sequestration activity is enhanced with repeated chemokine exposure. CCX-CKR internalization is not critically dependent on β-arrestins or clathrin-coated pits, but caveolin-1 overexpression blocks CCL19 uptake by CCX-CKR.","method":"Transfected HEK293 cells, chemokine internalization/degradation assays, caveolin-1 overexpression, comparison with CCR7-expressing cells","journal":"European journal of immunology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal functional assays in transfected cells, mechanistic dissection of endocytic pathway, single lab but rigorous controls","pmids":["16791897"],"is_preprint":false},{"year":2010,"finding":"CCX-CKR (ACKR4) scavenges CCR7-ligand homeostatic chemokines CCL19 and CCL21 in vivo. CCX-CKR-/- mice show a 5-fold increase in CCL21 protein in blood and 2-3-fold increases in CCL19 and CCL21 in peripheral lymph nodes, confirming scavenger function in vivo. Loss of CCX-CKR skews CD4+ T-cell responses toward Th17 rather than Th1, associated with increased IL-23 in spleen and increased CCL21 in CNS; early disease onset is reversed by anti-CCL21 neutralizing antibody.","method":"CCX-CKR-/- mouse generation, ELISA for chemokine levels, immunization with MOG peptide/CFA, cytokine profiling, anti-CCL21 antibody neutralization","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 / Strong — knockout mouse model with multiple orthogonal readouts, in vivo scavenging confirmed with chemokine quantification, neutralization rescue experiment","pmids":["20562329"],"is_preprint":false},{"year":2013,"finding":"CCX-CKR (ACKR4) recruits β-arrestin2 upon CCL19, CCL21, and CCL25 binding (demonstrated by enzyme-fragment complementation and BRET). Wild-type CCX-CKR and chimeras with substituted intracellular loops induce CRE activity in response to chemokines only in the presence of pertussis toxin (Gi inhibitor), and this signaling requires an intact DRY motif, suggesting inactive Gi proteins impair CCX-CKR signaling to pertussis toxin-insensitive G proteins. CCX-CKR does not activate canonical Gi signaling.","method":"β-arrestin recruitment assays (enzyme-fragment complementation, BRET), CRE-reporter gene assay, pertussis toxin treatment, intracellular loop chimera construction","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (BRET, EFC, reporter assay, chimeric receptors, pharmacological tools), single lab","pmids":["23341447"],"is_preprint":false},{"year":2013,"finding":"Co-expression of human CCX-CKR (ACKR4) completely inhibits CXCR3-induced chemotaxis. CCX-CKR forms heteromeric complexes with CXCR3, and negative binding cooperativity is induced by ligands for both receptors, suggesting heteromerization underlies the inhibition of CXCR3-dependent chemotaxis.","method":"Co-expression in HEK293 and human T cells, chemotaxis assays, co-immunoprecipitation to detect complexes, binding cooperativity assays","journal":"British journal of pharmacology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — co-IP and functional chemotaxis assay, single lab, two complementary methods but limited mechanistic depth on heteromer structure","pmids":["23121557"],"is_preprint":false},{"year":2016,"finding":"ACKR4 on stromal cells (keratinocytes and a subset of dermal lymphatic endothelial cells) scavenges CCL19 in mouse skin to facilitate CCR7-dependent APC egress. During inflammation, ACKR4-mediated scavenging of CCL19 (not CCL21) is critical: aberrant APC trafficking in Ackr4-deficient mice is completely rescued by genetic deletion of Ccl19.","method":"Ackr4-/- mice, Ccl19-/- double knockout rescue experiment, in situ ACKR4-dependent chemokine scavenging assays, cell-type-specific expression analysis","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis with double-knockout rescue, in situ scavenging confirmed, specific chemokine-receptor pathway defined","pmids":["26976955"],"is_preprint":false},{"year":2020,"finding":"ACKR4 recruits GRK3 (and to a lesser extent GRK2) to chemokine-stimulated receptor, and GRK3 recruitment precedes β-arrestin recruitment. GRK2/3 inhibition partially interferes with steady-state interaction and chemokine-driven β-arrestin recruitment. β-arrestins interact with ACKR4 in the steady state and contribute to spontaneous trafficking in the absence of chemokines. Deleting the ACKR4 C-terminus disrupts both β-arrestin interaction and fluorescent chemokine uptake. β-arrestins are dispensable for chemokine scavenging, as β-arrestin-deficient cells retain CCL19 uptake ability.","method":"GRK recruitment assays, β-arrestin recruitment assays, ACKR4 C-terminal deletion mutants, GRK inhibitor treatment, fluorescently labeled chemokine internalization assays, β-arrestin1/2 knockout cell lines","journal":"Frontiers in immunology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal assays (GRK recruitment, β-arrestin assays, mutants, KO cells, fluorescent ligand uptake), single lab but rigorous mechanistic dissection","pmids":["32391018"],"is_preprint":false},{"year":2020,"finding":"ACKR4 inhibits intratumor CD8+ T cell accumulation and activation by regulating intratumor CCL21 abundance. ACKR4 inhibits CD103+ dendritic cell retention in tumors via CCL21 regulation. Non-hematopoietic ACKR4 expression is critical for this effect.","method":"ACKR4-/- mice, tumor growth assays, flow cytometry of intratumoral immune cells, CCL21 quantification, bone marrow chimera experiments to define non-hematopoietic source","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 2 / Strong — KO mouse model with bone marrow chimeras defining cell type, CCL21 quantification, multiple immune readouts, independent from other labs","pmids":["32289156"],"is_preprint":false},{"year":2020,"finding":"Systematic β-arrestin recruitment screening of all 43 human chemokines confirmed CCL19, CCL21, CCL25, and CCL20 as ACKR4 agonists, identified CCL22 as a potent partial agonist of ACKR4, and disproved agonist activity of CXCL13 toward ACKR4.","method":"Highly sensitive β-arrestin recruitment assay (systematic screening of 43 chemokines against ACKR4)","journal":"Journal of leukocyte biology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — systematic single-method screen, replicated CCL20 finding independently but negative result for CXCL13 and new CCL22 finding are single-method","pmids":["32480426"],"is_preprint":false},{"year":2021,"finding":"ACKR4 scavenges two forms of CCL21 in barrier tissues: full-length immobilized CCL21 and cleaved soluble CCL21. Without ACKR4, extracellular CCL21 gradients in barrier sites are saturated and nonfunctional, DCs cannot home directly to lymphatic vessels, and excess soluble CCL21 from peripheral tissues accumulates in downstream lymph nodes.","method":"ACKR4-/- mice, detection and quantification of full-length vs. cleaved CCL21, DC homing assays, intravital microscopy/imaging of CCL21 gradients","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — ACKR4-deficient mouse model, biochemical distinction of CCL21 forms, DC migration assays, resolves prior conflicting data with mechanistic clarity","pmids":["33875601"],"is_preprint":false},{"year":2021,"finding":"ACKR4 promotes IL-6 generation and proliferation of cardiac fibroblasts (CFs). ACKR4 facilitates endothelial-to-mesenchymal transition (EndMT) in endothelial cells through IL-6 paracrine signaling. The p38 MAPK/NF-κB signaling pathway is involved in ACKR4-facilitated IL-6 generation. ACKR4 overexpression in vivo via AAV9 aggravated cardiac functional impairment post-MI, which was abolished by IL-6 neutralizing antibody.","method":"ACKR4 knockout mice, AAV9-mediated ACKR4 overexpression in vivo, IL-6 neutralizing antibody rescue, p38 MAPK/NF-κB pathway inhibition assays, CF proliferation assays, EndMT assays","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo rescue with neutralizing antibody, KO and OE model, pathway inhibition; single lab with multiple orthogonal methods","pmids":["33610913"],"is_preprint":false},{"year":2021,"finding":"ACKR4 is expressed in endothelial cells of a vascular compartment (peri-marginal sinus) in the splenic red pulp. In the absence of ACKR4, T cell homing into the spleen and subsequent migration into T cell areas is impaired, and organization of the marginal zone is severely affected.","method":"ACKR4-GFP reporter mice, immunofluorescence, three-dimensional imaging, T cell homing assays in ACKR4-deficient mice","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 / Moderate — direct localization by reporter mouse with functional consequence in KO, multiple complementary imaging and migration readouts, single lab","pmids":["34260918"],"is_preprint":false},{"year":2022,"finding":"ACKR4 expression in lymphatic collecting vessel endothelial cells is induced by lymph flow (mechanosensitive induction). ACKR4 in collecting vessel endothelium scavenges CCL19 and CCL21, enabling T cell de-adhesion from the vessel wall and passive transport by lymph flow toward draining lymph nodes. In the absence of ACKR4, T cells accumulate in dermal collecting vessel segments and fail to efficiently reach draining lymph nodes.","method":"Intravital microscopy, flow-induced ACKR4 expression assays in lymphatic endothelial cells, ACKR4-/- mice in TPA inflammation model, T cell migration assays","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 / Strong — mechanosensitive expression established by flow assays, KO mouse with specific cellular phenotype (T cell accumulation in collectors), intravital imaging, multiple orthogonal methods","pmids":["35108538"],"is_preprint":false},{"year":2024,"finding":"The C-terminal tip of ACKR4 contains a putative class II PDZ-binding domain critical for receptor function. Addition of a C-terminal tag to ACKR4 significantly augments CCL19 internalization, elevates pre-association of β-arrestins with the plasma membrane, reduces chemokine-driven β-arrestin recruitment, and shifts endocytosis from a β-arrestin-dependent to a β-arrestin-independent pathway. Mutation of the putative PDZ-binding domain at the C-terminal tip recapitulates these effects.","method":"Flow cytometry of fluorescent CCL19 internalization, NanoBiT bystander assays for β-arrestin recruitment, C-terminal tagging and PDZ-domain mutagenesis, β-arrestin1/2-double deficient cell lines, live-cell confocal microscopy","journal":"Cell communication and signaling : CCS","confidence":"High","confidence_rationale":"Tier 2 / Moderate — site-directed mutagenesis, KO cell lines, multiple orthogonal assays (NanoBiT, flow cytometry, live imaging), single lab","pmids":["39623381"],"is_preprint":false},{"year":2026,"finding":"ACKR4 in its apo state constitutively pre-associates with β-arrestins and cycles between the plasma membrane and endosomal compartments. Distinct serine and threonine residues in the ACKR4 C-terminal tail regulate steady-state trafficking and chemokine uptake; a C-terminal serine/threonine cluster is key for ligand-mediated β-arrestin recruitment and efficient chemokine uptake. GRK5/6 primarily phosphorylate ACKR4 in the absence of chemokines; CCL19 stimulation recruits GRK2/3 to enhance ACKR4 phosphorylation at two serine and one threonine residues. Apo ACKR4 forms a ternary complex with GRK2/3 and G protein without activating it. GRK2 plays a leading role in β-arrestin recruitment and CCL19 internalization.","method":"Phosphosite mutagenesis, mass spectrometry phosphoproteomics, NanoBiT/BRET assays for GRK and β-arrestin recruitment, fluorescent CCL19 internalization assays, co-immunoprecipitation of ternary complex","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 / Strong — site-directed mutagenesis combined with MS phosphoproteomics, multiple BRET/NanoBiT assays, co-IP for ternary complex, comprehensive mechanistic dissection in single rigorous study","pmids":["42143096"],"is_preprint":false},{"year":2000,"finding":"CCR11 (ACKR4), the human homolog of bovine PPR1, functions as a high-affinity receptor for MCP family chemokines (CCL13/MCP-4, CCL8/MCP-2, CCL2/MCP-1) mediating chemotaxis in L1.2 cells. Radiolabeled MCP-4 binding revealed a single high-affinity binding site (IC50 = 0.14 nM). Eotaxin and MCP-3 bind with higher affinity than MCP-1 but act as agonists only at 100-fold higher concentrations.","method":"Transfection into murine L1.2 cells, chemotaxis assays, radiolabeled MCP-4 competition binding assays","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional chemotaxis and radioligand binding assays, but this CCR11 ligand profile was later superseded by recognition of ACKR4 as an atypical/scavenger receptor","pmids":["10734104"],"is_preprint":false},{"year":2002,"finding":"Murine CCX-CKR (ACKR4) is a high-affinity receptor for mCCL21, mCCL19, and mCCL25, but unlike most chemokine receptors, is unable to mediate Ca2+ fluxes upon ligand binding when expressed in HEK293 cells.","method":"Receptor expression in HEK293 cells, calcium flux assays, radioligand binding","journal":"European journal of immunology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — functional assays in transfected cells demonstrating absence of canonical signaling and ligand binding specificity, independently replicated across species","pmids":["11981810"],"is_preprint":false},{"year":2012,"finding":"CCX-CKR (ACKR4) deletion in mice results in fewer cortical thymic epithelial cells (cTECs) per thymocyte, accumulation of DN2 cells in the medulla, reduced DN3 thymocyte precursors, and decreased CCL25 within the thymic cortex. cTECs express the highest level of CCX-CKR in the thymus. CCX-CKR on cTECs regulates CCL25 availability, which in turn controls thymocyte distribution and frequency.","method":"CCX-CKR-/- mice, flow cytometric thymocyte subset analysis, CCL25 immunostaining, cell frequency and distribution analysis","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 / Strong — KO mouse model with cell-type-specific expression mapping and CCL25 quantification, multiple thymocyte subset readouts, mechanistic link to chemokine scavenging","pmids":["23152546"],"is_preprint":false},{"year":2018,"finding":"In mouse intestine, ACKR4 expression is restricted to a population of submucosal fibroblasts that form physical interactions with lymphatic endothelial cells and engage in molecular interactions via VEGFD/VEGFR3 and CCL21/ACKR4 pathways. Ackr4 deficiency does not affect dendritic cell abundance in the small intestine and mesenteric lymph nodes under steady state or after R848-induced mobilization.","method":"Ackr4-GFP reporter mice, immunofluorescence, flow cytometry, transcriptional profiling, DC migration assays in Ackr4-/- mice","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct cell-type identification by reporter, transcriptional profiling, physical interaction evidence; single lab","pmids":["29760193"],"is_preprint":false}],"current_model":"ACKR4 is a β-arrestin-biased atypical chemokine receptor that lacks canonical G protein coupling; it binds CCL19, CCL21, CCL25, CCL20, and CCL22 with high affinity, constitutively associates with β-arrestins and cycles between the plasma membrane and endosomes in its apo state, and upon ligand binding recruits GRK2/3 (preceded by GRK5/6 phosphorylation at a C-terminal serine/threonine cluster) to drive β-arrestin-mediated and, to a lesser extent, β-arrestin-independent endocytosis and lysosomal degradation of chemokines, thereby establishing functional CCL19/CCL21 gradients that direct CCR7-dependent trafficking of dendritic cells, Langerhans cells, and T cells through skin, afferent lymphatics, lymph nodes, thymus, and spleen, and can also trans-inhibit CXCR3 signaling via heteromerization."},"narrative":{"mechanistic_narrative":"ACKR4 (CCX-CKR/CCR11) is an atypical, scavenging chemokine receptor that shapes extracellular chemokine gradients to direct leukocyte trafficking through lymphoid and barrier tissues [PMID:20562329, PMID:26976955]. It binds the CCR7 ligands CCL19 and CCL21 plus CCL25 with high affinity, and systematic profiling defines CCL20 as an agonist and CCL22 as a potent partial agonist [PMID:32480426, PMID:11981810]; rather than mediating chemotaxis or canonical signaling, it internalizes and degrades large quantities of bound chemokine, and unlike CCR7 it does not become refractory, sequestering ligand more efficiently with repeated exposure [PMID:16791897]. ACKR4 does not couple to canonical Gi or trigger calcium flux but is a β-arrestin-biased receptor: it constitutively pre-associates with β-arrestins and cycles between plasma membrane and endosomes in its apo state, and chemokine binding recruits β-arrestin2 to drive endocytosis and lysosomal scavenging [PMID:23341447, PMID:42143096, PMID:11981810]. Receptor regulation is ordered through its C-terminal tail, where GRK5/6 phosphorylate the apo receptor and ligand stimulation recruits GRK2/3 to phosphorylate a C-terminal serine/threonine cluster that controls β-arrestin recruitment and chemokine uptake; the apo receptor forms a ternary complex with GRK2/3 and G protein without activating it [PMID:42143096]. β-arrestins are dispensable for scavenging itself, indicating a parallel β-arrestin-independent endocytic route gated by a C-terminal class II PDZ-binding motif [PMID:32391018, PMID:39623381]. By scavenging CCL19/CCL21/CCL25 on stromal cells — keratinocytes, lymphatic and collecting-vessel endothelium induced by lymph flow, splenic sinus endothelium, and cortical thymic epithelial cells — ACKR4 establishes functional gradients that govern CCR7-dependent dendritic cell egress, T cell de-adhesion and lymph-node homing, thymocyte distribution, and splenic marginal-zone organization [PMID:26976955, PMID:33875601, PMID:34260918, PMID:35108538, PMID:23152546]. It can additionally trans-inhibit CXCR3-driven chemotaxis through heteromerization [PMID:23121557].","teleology":[{"year":2000,"claim":"Established the receptor's initial molecular identity as a high-affinity chemokine receptor, originally assigned an MCP-family ligand profile and chemotactic function.","evidence":"Transfection into L1.2 cells with radiolabeled MCP-4 competition binding and chemotaxis assays","pmids":["10734104"],"confidence":"Medium","gaps":["This ligand profile was later superseded by recognition of ACKR4 as a CCR7-ligand scavenger","Did not address signaling bias or absence of canonical coupling"]},{"year":2002,"claim":"Defined the now-canonical high-affinity ligands (CCL19, CCL21, CCL25) and showed the receptor fails to mediate calcium flux, the first hint it was atypical.","evidence":"Murine receptor expression in HEK293 cells with radioligand binding and calcium flux assays","pmids":["11981810"],"confidence":"Medium","gaps":["Did not establish the scavenging mechanism","No explanation for the signaling silence"]},{"year":2006,"claim":"Demonstrated the receptor's defining function as a non-saturating chemokine scavenger, distinguishing its endocytic route from clathrin/β-arrestin-dependent uptake.","evidence":"Chemokine internalization/degradation assays in transfected HEK293 cells with caveolin-1 overexpression and CCR7 comparison","pmids":["16791897"],"confidence":"High","gaps":["Endocytic mechanism left ambiguous (caveolin block vs. β-arrestin independence)","In vivo relevance not addressed"]},{"year":2010,"claim":"Confirmed in vivo scavenging of CCL19/CCL21 and linked gradient regulation to T-helper polarization, establishing physiological consequence.","evidence":"CCX-CKR-/- mice, chemokine ELISA, MOG/CFA immunization, anti-CCL21 neutralization rescue","pmids":["20562329"],"confidence":"High","gaps":["Cellular source of scavenging not resolved","Did not separate CCL19 vs CCL21 contributions"]},{"year":2013,"claim":"Resolved the signaling mode by showing β-arrestin recruitment without canonical Gi activation, defining ACKR4 as β-arrestin-biased and DRY-motif dependent.","evidence":"β-arrestin recruitment (EFC, BRET), CRE-reporter assays with pertussis toxin, intracellular-loop chimeras","pmids":["23341447"],"confidence":"High","gaps":["GRK requirements not defined","Did not establish whether β-arrestins are needed for scavenging"]},{"year":2013,"claim":"Identified a non-scavenging function — heteromerization with CXCR3 to trans-inhibit its chemotaxis — broadening the receptor's regulatory repertoire.","evidence":"Co-expression in HEK293 and T cells, chemotaxis assays, co-IP, binding cooperativity assays","pmids":["23121557"],"confidence":"Medium","gaps":["Heteromer structure and stoichiometry undefined","Single lab; physiological CXCR3 cross-regulation not shown in vivo"]},{"year":2016,"claim":"Pinned scavenging to specific stromal cells and used genetic epistasis to define CCL19 as the critical inflammatory substrate for APC egress.","evidence":"Ackr4-/- and Ccl19-/- double-knockout rescue, in situ scavenging assays, cell-type expression analysis in skin","pmids":["26976955"],"confidence":"High","gaps":["Did not address collecting-vessel or lymph-node roles","Steady-state vs inflammation distinction incomplete"]},{"year":2020,"claim":"Dissected the recruitment hierarchy (GRK3>GRK2 preceding β-arrestin) and the C-terminus requirement, while showing β-arrestins are dispensable for scavenging — implying a parallel uptake route.","evidence":"GRK/β-arrestin recruitment assays, C-terminal deletion mutants, GRK inhibitors, β-arrestin1/2 KO cells, fluorescent chemokine uptake","pmids":["32391018"],"confidence":"High","gaps":["Identity of the β-arrestin-independent endocytic machinery unknown","Phosphosite map not yet defined"]},{"year":2020,"claim":"Established a tumor-immunology role: non-hematopoietic ACKR4 limits CD8+ T cell and CD103+ DC accumulation by regulating intratumoral CCL21.","evidence":"ACKR4-/- mice, tumor growth and flow cytometry, CCL21 quantification, bone-marrow chimeras","pmids":["32289156"],"confidence":"High","gaps":["Specific stromal cell type within tumors not defined","Therapeutic targeting not tested"]},{"year":2020,"claim":"Systematically defined the agonist repertoire, adding CCL20 and CCL22 as agonists and excluding CXCL13.","evidence":"High-sensitivity β-arrestin recruitment screen of all 43 human chemokines","pmids":["32480426"],"confidence":"Medium","gaps":["Single-method screen; CCL22 and CXCL13 results not orthogonally confirmed","In vivo significance of CCL20/CCL22 scavenging unknown"]},{"year":2021,"claim":"Refined the CCL21 substrate to both immobilized full-length and cleaved soluble forms, explaining how ACKR4 keeps barrier-tissue gradients functional for DC homing.","evidence":"ACKR4-/- mice, biochemical discrimination of CCL21 forms, DC homing assays, intravital imaging","pmids":["33875601"],"confidence":"High","gaps":["Protease generating soluble CCL21 not defined here","Mechanism distinguishing immobilized vs soluble uptake unclear"]},{"year":2021,"claim":"Extended the trafficking role to the spleen, where endothelial ACKR4 in the peri-marginal sinus supports T cell homing and marginal-zone organization.","evidence":"ACKR4-GFP reporter mice, immunofluorescence, 3D imaging, T cell homing assays","pmids":["34260918"],"confidence":"High","gaps":["Chemokine substrate driving the splenic phenotype not pinned down","Link to systemic scavenging unresolved"]},{"year":2021,"claim":"Revealed a non-trafficking, pathological function: ACKR4 promotes cardiac fibroblast IL-6 production and EndMT via p38/NF-κB, worsening post-MI dysfunction.","evidence":"ACKR4 KO and AAV9 overexpression mice, IL-6 neutralization rescue, p38/NF-κB inhibition, CF proliferation and EndMT assays","pmids":["33610913"],"confidence":"Medium","gaps":["How a scavenging receptor drives IL-6/p38 signaling mechanistically is unexplained","Reconciliation with β-arrestin-biased silent signaling not addressed"]},{"year":2022,"claim":"Showed ACKR4 expression is mechanosensitively induced by lymph flow in collecting-vessel endothelium and enables T cell de-adhesion for transport to draining nodes.","evidence":"Intravital microscopy, flow-induced expression assays, ACKR4-/- TPA inflammation model, T cell migration assays","pmids":["35108538"],"confidence":"High","gaps":["Mechanotransduction pathway inducing ACKR4 unknown","Molecular link between scavenging and de-adhesion incomplete"]},{"year":2024,"claim":"Identified a C-terminal class II PDZ-binding motif that controls β-arrestin pre-association and switches endocytosis between β-arrestin-dependent and -independent routes.","evidence":"Fluorescent CCL19 internalization, NanoBiT bystander assays, C-terminal tagging and PDZ mutagenesis, β-arrestin DKO cells, live-cell confocal","pmids":["39623381"],"confidence":"High","gaps":["PDZ partner protein not identified","How the motif arbitrates between the two endocytic routes is unresolved"]},{"year":2026,"claim":"Provided a comprehensive phosphoregulatory model: GRK5/6 phosphorylate apo ACKR4, CCL19 recruits GRK2/3 to phosphorylate a C-terminal Ser/Thr cluster, and apo receptor forms a non-activating ternary complex with GRK2/3 and G protein.","evidence":"Phosphosite mutagenesis, MS phosphoproteomics, NanoBiT/BRET GRK and β-arrestin assays, fluorescent CCL19 uptake, ternary-complex co-IP","pmids":["42143096"],"confidence":"High","gaps":["Structural basis of the non-activating ternary complex not solved","Why the receptor sequesters G protein without activating it unexplained"]},{"year":null,"claim":"How ACKR4 reconciles its β-arrestin-independent scavenging route with active signaling outputs (e.g., IL-6/p38 in fibroblasts) and the identity of its PDZ-binding partner remain open.","evidence":"","pmids":[],"confidence":"High","gaps":["No identified PDZ-domain partner protein","No structural model of the apo ternary complex or the β-arrestin-independent endocytic machinery","Mechanism linking a scavenging receptor to active p38/NF-κB signaling is undefined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[2,14,15]},{"term_id":"GO:0038024","term_label":"cargo receptor activity","supporting_discovery_ids":[0,1,4,8]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[3]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[5,13]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[5,13]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[2,13]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[1,4,6,16]},{"term_id":"R-HSA-1500931","term_label":"Cell-Cell communication","supporting_discovery_ids":[4,8,11]}],"complexes":[],"partners":["ARRB2","GRK2","GRK3","GRK5","GRK6","CXCR3"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9NPB9","full_name":"Atypical chemokine receptor 4","aliases":["C-C chemokine receptor type 11","C-C CKR-11","CC-CKR-11","CCR-11","CC chemokine receptor-like 1","CCRL1","CCX CKR"],"length_aa":350,"mass_kda":39.9,"function":"Atypical chemokine receptor that controls chemokine levels and localization via high-affinity chemokine binding that is uncoupled from classic ligand-driven signal transduction cascades, resulting instead in chemokine sequestration, degradation, or transcytosis. Also known as interceptor (internalizing receptor) or chemokine-scavenging receptor or chemokine decoy receptor. Acts as a receptor for chemokines CCL2, CCL8, CCL13, CCL19, CCL21 and CCL25. Chemokine-binding does not activate G-protein-mediated signal transduction but instead induces beta-arrestin recruitment, leading to ligand internalization. Plays an important role in controlling the migration of immune and cancer cells that express chemokine receptors CCR7 and CCR9, by reducing the availability of CCL19, CCL21, and CCL25 through internalization. Negatively regulates CXCR3-induced chemotaxis. Regulates T-cell development in the thymus","subcellular_location":"Early endosome; Recycling endosome; Cell membrane","url":"https://www.uniprot.org/uniprotkb/Q9NPB9/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/ACKR4","classification":"Not Classified","n_dependent_lines":27,"n_total_lines":1208,"dependency_fraction":0.022350993377483443},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/ACKR4","total_profiled":1310},"omim":[{"mim_id":"606065","title":"ATYPICAL CHEMOKINE RECEPTOR 4; ACKR4","url":"https://www.omim.org/entry/606065"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Vesicles","reliability":"Approved"},{"location":"Mitochondria","reliability":"Approved"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"intestine","ntpm":14.8}],"url":"https://www.proteinatlas.org/search/ACKR4"},"hgnc":{"alias_symbol":["CCR11","CCBP2","VSHK1","CCX-CKR","PPR1"],"prev_symbol":["CCRL1"]},"alphafold":{"accession":"Q9NPB9","domains":[{"cath_id":"1.20.1070.10","chopping":"33-144_153-319","consensus_level":"high","plddt":88.0224,"start":33,"end":319}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9NPB9","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9NPB9-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9NPB9-F1-predicted_aligned_error_v6.png","plddt_mean":81.69},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=ACKR4","jax_strain_url":"https://www.jax.org/strain/search?query=ACKR4"},"sequence":{"accession":"Q9NPB9","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9NPB9.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9NPB9/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9NPB9"}},"corpus_meta":[{"pmid":"7958913","id":"PMC_7958913","title":"Crystal structure of a PPR1-DNA complex: DNA recognition by proteins containing a Zn2Cys6 binuclear cluster.","date":"1994","source":"Genes & development","url":"https://pubmed.ncbi.nlm.nih.gov/7958913","citation_count":146,"is_preprint":false},{"pmid":"6096561","id":"PMC_6096561","title":"Yeast regulatory gene PPR1. 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Unlike CCR7, CCX-CKR does not become refractory for CCL19 uptake over time, and its sequestration activity is enhanced with repeated chemokine exposure. CCX-CKR internalization is not critically dependent on β-arrestins or clathrin-coated pits, but caveolin-1 overexpression blocks CCL19 uptake by CCX-CKR.\",\n      \"method\": \"Transfected HEK293 cells, chemokine internalization/degradation assays, caveolin-1 overexpression, comparison with CCR7-expressing cells\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal functional assays in transfected cells, mechanistic dissection of endocytic pathway, single lab but rigorous controls\",\n      \"pmids\": [\"16791897\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"CCX-CKR (ACKR4) scavenges CCR7-ligand homeostatic chemokines CCL19 and CCL21 in vivo. CCX-CKR-/- mice show a 5-fold increase in CCL21 protein in blood and 2-3-fold increases in CCL19 and CCL21 in peripheral lymph nodes, confirming scavenger function in vivo. Loss of CCX-CKR skews CD4+ T-cell responses toward Th17 rather than Th1, associated with increased IL-23 in spleen and increased CCL21 in CNS; early disease onset is reversed by anti-CCL21 neutralizing antibody.\",\n      \"method\": \"CCX-CKR-/- mouse generation, ELISA for chemokine levels, immunization with MOG peptide/CFA, cytokine profiling, anti-CCL21 antibody neutralization\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — knockout mouse model with multiple orthogonal readouts, in vivo scavenging confirmed with chemokine quantification, neutralization rescue experiment\",\n      \"pmids\": [\"20562329\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"CCX-CKR (ACKR4) recruits β-arrestin2 upon CCL19, CCL21, and CCL25 binding (demonstrated by enzyme-fragment complementation and BRET). Wild-type CCX-CKR and chimeras with substituted intracellular loops induce CRE activity in response to chemokines only in the presence of pertussis toxin (Gi inhibitor), and this signaling requires an intact DRY motif, suggesting inactive Gi proteins impair CCX-CKR signaling to pertussis toxin-insensitive G proteins. CCX-CKR does not activate canonical Gi signaling.\",\n      \"method\": \"β-arrestin recruitment assays (enzyme-fragment complementation, BRET), CRE-reporter gene assay, pertussis toxin treatment, intracellular loop chimera construction\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (BRET, EFC, reporter assay, chimeric receptors, pharmacological tools), single lab\",\n      \"pmids\": [\"23341447\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Co-expression of human CCX-CKR (ACKR4) completely inhibits CXCR3-induced chemotaxis. CCX-CKR forms heteromeric complexes with CXCR3, and negative binding cooperativity is induced by ligands for both receptors, suggesting heteromerization underlies the inhibition of CXCR3-dependent chemotaxis.\",\n      \"method\": \"Co-expression in HEK293 and human T cells, chemotaxis assays, co-immunoprecipitation to detect complexes, binding cooperativity assays\",\n      \"journal\": \"British journal of pharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — co-IP and functional chemotaxis assay, single lab, two complementary methods but limited mechanistic depth on heteromer structure\",\n      \"pmids\": [\"23121557\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"ACKR4 on stromal cells (keratinocytes and a subset of dermal lymphatic endothelial cells) scavenges CCL19 in mouse skin to facilitate CCR7-dependent APC egress. During inflammation, ACKR4-mediated scavenging of CCL19 (not CCL21) is critical: aberrant APC trafficking in Ackr4-deficient mice is completely rescued by genetic deletion of Ccl19.\",\n      \"method\": \"Ackr4-/- mice, Ccl19-/- double knockout rescue experiment, in situ ACKR4-dependent chemokine scavenging assays, cell-type-specific expression analysis\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis with double-knockout rescue, in situ scavenging confirmed, specific chemokine-receptor pathway defined\",\n      \"pmids\": [\"26976955\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"ACKR4 recruits GRK3 (and to a lesser extent GRK2) to chemokine-stimulated receptor, and GRK3 recruitment precedes β-arrestin recruitment. GRK2/3 inhibition partially interferes with steady-state interaction and chemokine-driven β-arrestin recruitment. β-arrestins interact with ACKR4 in the steady state and contribute to spontaneous trafficking in the absence of chemokines. Deleting the ACKR4 C-terminus disrupts both β-arrestin interaction and fluorescent chemokine uptake. β-arrestins are dispensable for chemokine scavenging, as β-arrestin-deficient cells retain CCL19 uptake ability.\",\n      \"method\": \"GRK recruitment assays, β-arrestin recruitment assays, ACKR4 C-terminal deletion mutants, GRK inhibitor treatment, fluorescently labeled chemokine internalization assays, β-arrestin1/2 knockout cell lines\",\n      \"journal\": \"Frontiers in immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal assays (GRK recruitment, β-arrestin assays, mutants, KO cells, fluorescent ligand uptake), single lab but rigorous mechanistic dissection\",\n      \"pmids\": [\"32391018\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"ACKR4 inhibits intratumor CD8+ T cell accumulation and activation by regulating intratumor CCL21 abundance. ACKR4 inhibits CD103+ dendritic cell retention in tumors via CCL21 regulation. Non-hematopoietic ACKR4 expression is critical for this effect.\",\n      \"method\": \"ACKR4-/- mice, tumor growth assays, flow cytometry of intratumoral immune cells, CCL21 quantification, bone marrow chimera experiments to define non-hematopoietic source\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — KO mouse model with bone marrow chimeras defining cell type, CCL21 quantification, multiple immune readouts, independent from other labs\",\n      \"pmids\": [\"32289156\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Systematic β-arrestin recruitment screening of all 43 human chemokines confirmed CCL19, CCL21, CCL25, and CCL20 as ACKR4 agonists, identified CCL22 as a potent partial agonist of ACKR4, and disproved agonist activity of CXCL13 toward ACKR4.\",\n      \"method\": \"Highly sensitive β-arrestin recruitment assay (systematic screening of 43 chemokines against ACKR4)\",\n      \"journal\": \"Journal of leukocyte biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — systematic single-method screen, replicated CCL20 finding independently but negative result for CXCL13 and new CCL22 finding are single-method\",\n      \"pmids\": [\"32480426\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"ACKR4 scavenges two forms of CCL21 in barrier tissues: full-length immobilized CCL21 and cleaved soluble CCL21. Without ACKR4, extracellular CCL21 gradients in barrier sites are saturated and nonfunctional, DCs cannot home directly to lymphatic vessels, and excess soluble CCL21 from peripheral tissues accumulates in downstream lymph nodes.\",\n      \"method\": \"ACKR4-/- mice, detection and quantification of full-length vs. cleaved CCL21, DC homing assays, intravital microscopy/imaging of CCL21 gradients\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — ACKR4-deficient mouse model, biochemical distinction of CCL21 forms, DC migration assays, resolves prior conflicting data with mechanistic clarity\",\n      \"pmids\": [\"33875601\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"ACKR4 promotes IL-6 generation and proliferation of cardiac fibroblasts (CFs). ACKR4 facilitates endothelial-to-mesenchymal transition (EndMT) in endothelial cells through IL-6 paracrine signaling. The p38 MAPK/NF-κB signaling pathway is involved in ACKR4-facilitated IL-6 generation. ACKR4 overexpression in vivo via AAV9 aggravated cardiac functional impairment post-MI, which was abolished by IL-6 neutralizing antibody.\",\n      \"method\": \"ACKR4 knockout mice, AAV9-mediated ACKR4 overexpression in vivo, IL-6 neutralizing antibody rescue, p38 MAPK/NF-κB pathway inhibition assays, CF proliferation assays, EndMT assays\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo rescue with neutralizing antibody, KO and OE model, pathway inhibition; single lab with multiple orthogonal methods\",\n      \"pmids\": [\"33610913\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"ACKR4 is expressed in endothelial cells of a vascular compartment (peri-marginal sinus) in the splenic red pulp. In the absence of ACKR4, T cell homing into the spleen and subsequent migration into T cell areas is impaired, and organization of the marginal zone is severely affected.\",\n      \"method\": \"ACKR4-GFP reporter mice, immunofluorescence, three-dimensional imaging, T cell homing assays in ACKR4-deficient mice\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct localization by reporter mouse with functional consequence in KO, multiple complementary imaging and migration readouts, single lab\",\n      \"pmids\": [\"34260918\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"ACKR4 expression in lymphatic collecting vessel endothelial cells is induced by lymph flow (mechanosensitive induction). ACKR4 in collecting vessel endothelium scavenges CCL19 and CCL21, enabling T cell de-adhesion from the vessel wall and passive transport by lymph flow toward draining lymph nodes. In the absence of ACKR4, T cells accumulate in dermal collecting vessel segments and fail to efficiently reach draining lymph nodes.\",\n      \"method\": \"Intravital microscopy, flow-induced ACKR4 expression assays in lymphatic endothelial cells, ACKR4-/- mice in TPA inflammation model, T cell migration assays\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — mechanosensitive expression established by flow assays, KO mouse with specific cellular phenotype (T cell accumulation in collectors), intravital imaging, multiple orthogonal methods\",\n      \"pmids\": [\"35108538\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"The C-terminal tip of ACKR4 contains a putative class II PDZ-binding domain critical for receptor function. Addition of a C-terminal tag to ACKR4 significantly augments CCL19 internalization, elevates pre-association of β-arrestins with the plasma membrane, reduces chemokine-driven β-arrestin recruitment, and shifts endocytosis from a β-arrestin-dependent to a β-arrestin-independent pathway. Mutation of the putative PDZ-binding domain at the C-terminal tip recapitulates these effects.\",\n      \"method\": \"Flow cytometry of fluorescent CCL19 internalization, NanoBiT bystander assays for β-arrestin recruitment, C-terminal tagging and PDZ-domain mutagenesis, β-arrestin1/2-double deficient cell lines, live-cell confocal microscopy\",\n      \"journal\": \"Cell communication and signaling : CCS\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — site-directed mutagenesis, KO cell lines, multiple orthogonal assays (NanoBiT, flow cytometry, live imaging), single lab\",\n      \"pmids\": [\"39623381\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"ACKR4 in its apo state constitutively pre-associates with β-arrestins and cycles between the plasma membrane and endosomal compartments. Distinct serine and threonine residues in the ACKR4 C-terminal tail regulate steady-state trafficking and chemokine uptake; a C-terminal serine/threonine cluster is key for ligand-mediated β-arrestin recruitment and efficient chemokine uptake. GRK5/6 primarily phosphorylate ACKR4 in the absence of chemokines; CCL19 stimulation recruits GRK2/3 to enhance ACKR4 phosphorylation at two serine and one threonine residues. Apo ACKR4 forms a ternary complex with GRK2/3 and G protein without activating it. GRK2 plays a leading role in β-arrestin recruitment and CCL19 internalization.\",\n      \"method\": \"Phosphosite mutagenesis, mass spectrometry phosphoproteomics, NanoBiT/BRET assays for GRK and β-arrestin recruitment, fluorescent CCL19 internalization assays, co-immunoprecipitation of ternary complex\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — site-directed mutagenesis combined with MS phosphoproteomics, multiple BRET/NanoBiT assays, co-IP for ternary complex, comprehensive mechanistic dissection in single rigorous study\",\n      \"pmids\": [\"42143096\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"CCR11 (ACKR4), the human homolog of bovine PPR1, functions as a high-affinity receptor for MCP family chemokines (CCL13/MCP-4, CCL8/MCP-2, CCL2/MCP-1) mediating chemotaxis in L1.2 cells. Radiolabeled MCP-4 binding revealed a single high-affinity binding site (IC50 = 0.14 nM). Eotaxin and MCP-3 bind with higher affinity than MCP-1 but act as agonists only at 100-fold higher concentrations.\",\n      \"method\": \"Transfection into murine L1.2 cells, chemotaxis assays, radiolabeled MCP-4 competition binding assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional chemotaxis and radioligand binding assays, but this CCR11 ligand profile was later superseded by recognition of ACKR4 as an atypical/scavenger receptor\",\n      \"pmids\": [\"10734104\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Murine CCX-CKR (ACKR4) is a high-affinity receptor for mCCL21, mCCL19, and mCCL25, but unlike most chemokine receptors, is unable to mediate Ca2+ fluxes upon ligand binding when expressed in HEK293 cells.\",\n      \"method\": \"Receptor expression in HEK293 cells, calcium flux assays, radioligand binding\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — functional assays in transfected cells demonstrating absence of canonical signaling and ligand binding specificity, independently replicated across species\",\n      \"pmids\": [\"11981810\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"CCX-CKR (ACKR4) deletion in mice results in fewer cortical thymic epithelial cells (cTECs) per thymocyte, accumulation of DN2 cells in the medulla, reduced DN3 thymocyte precursors, and decreased CCL25 within the thymic cortex. cTECs express the highest level of CCX-CKR in the thymus. CCX-CKR on cTECs regulates CCL25 availability, which in turn controls thymocyte distribution and frequency.\",\n      \"method\": \"CCX-CKR-/- mice, flow cytometric thymocyte subset analysis, CCL25 immunostaining, cell frequency and distribution analysis\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — KO mouse model with cell-type-specific expression mapping and CCL25 quantification, multiple thymocyte subset readouts, mechanistic link to chemokine scavenging\",\n      \"pmids\": [\"23152546\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"In mouse intestine, ACKR4 expression is restricted to a population of submucosal fibroblasts that form physical interactions with lymphatic endothelial cells and engage in molecular interactions via VEGFD/VEGFR3 and CCL21/ACKR4 pathways. Ackr4 deficiency does not affect dendritic cell abundance in the small intestine and mesenteric lymph nodes under steady state or after R848-induced mobilization.\",\n      \"method\": \"Ackr4-GFP reporter mice, immunofluorescence, flow cytometry, transcriptional profiling, DC migration assays in Ackr4-/- mice\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct cell-type identification by reporter, transcriptional profiling, physical interaction evidence; single lab\",\n      \"pmids\": [\"29760193\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"ACKR4 is a β-arrestin-biased atypical chemokine receptor that lacks canonical G protein coupling; it binds CCL19, CCL21, CCL25, CCL20, and CCL22 with high affinity, constitutively associates with β-arrestins and cycles between the plasma membrane and endosomes in its apo state, and upon ligand binding recruits GRK2/3 (preceded by GRK5/6 phosphorylation at a C-terminal serine/threonine cluster) to drive β-arrestin-mediated and, to a lesser extent, β-arrestin-independent endocytosis and lysosomal degradation of chemokines, thereby establishing functional CCL19/CCL21 gradients that direct CCR7-dependent trafficking of dendritic cells, Langerhans cells, and T cells through skin, afferent lymphatics, lymph nodes, thymus, and spleen, and can also trans-inhibit CXCR3 signaling via heteromerization.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"ACKR4 (CCX-CKR/CCR11) is an atypical, scavenging chemokine receptor that shapes extracellular chemokine gradients to direct leukocyte trafficking through lymphoid and barrier tissues [#1, #4]. It binds the CCR7 ligands CCL19 and CCL21 plus CCL25 with high affinity, and systematic profiling defines CCL20 as an agonist and CCL22 as a potent partial agonist [#7, #15]; rather than mediating chemotaxis or canonical signaling, it internalizes and degrades large quantities of bound chemokine, and unlike CCR7 it does not become refractory, sequestering ligand more efficiently with repeated exposure [#0]. ACKR4 does not couple to canonical Gi or trigger calcium flux but is a β-arrestin-biased receptor: it constitutively pre-associates with β-arrestins and cycles between plasma membrane and endosomes in its apo state, and chemokine binding recruits β-arrestin2 to drive endocytosis and lysosomal scavenging [#2, #13, #15]. Receptor regulation is ordered through its C-terminal tail, where GRK5/6 phosphorylate the apo receptor and ligand stimulation recruits GRK2/3 to phosphorylate a C-terminal serine/threonine cluster that controls β-arrestin recruitment and chemokine uptake; the apo receptor forms a ternary complex with GRK2/3 and G protein without activating it [#13]. β-arrestins are dispensable for scavenging itself, indicating a parallel β-arrestin-independent endocytic route gated by a C-terminal class II PDZ-binding motif [#5, #12]. By scavenging CCL19/CCL21/CCL25 on stromal cells — keratinocytes, lymphatic and collecting-vessel endothelium induced by lymph flow, splenic sinus endothelium, and cortical thymic epithelial cells — ACKR4 establishes functional gradients that govern CCR7-dependent dendritic cell egress, T cell de-adhesion and lymph-node homing, thymocyte distribution, and splenic marginal-zone organization [#4, #8, #10, #11, #16]. It can additionally trans-inhibit CXCR3-driven chemotaxis through heteromerization [#3].\",\n  \"teleology\": [\n    {\n      \"year\": 2000,\n      \"claim\": \"Established the receptor's initial molecular identity as a high-affinity chemokine receptor, originally assigned an MCP-family ligand profile and chemotactic function.\",\n      \"evidence\": \"Transfection into L1.2 cells with radiolabeled MCP-4 competition binding and chemotaxis assays\",\n      \"pmids\": [\"10734104\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"This ligand profile was later superseded by recognition of ACKR4 as a CCR7-ligand scavenger\", \"Did not address signaling bias or absence of canonical coupling\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Defined the now-canonical high-affinity ligands (CCL19, CCL21, CCL25) and showed the receptor fails to mediate calcium flux, the first hint it was atypical.\",\n      \"evidence\": \"Murine receptor expression in HEK293 cells with radioligand binding and calcium flux assays\",\n      \"pmids\": [\"11981810\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Did not establish the scavenging mechanism\", \"No explanation for the signaling silence\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Demonstrated the receptor's defining function as a non-saturating chemokine scavenger, distinguishing its endocytic route from clathrin/β-arrestin-dependent uptake.\",\n      \"evidence\": \"Chemokine internalization/degradation assays in transfected HEK293 cells with caveolin-1 overexpression and CCR7 comparison\",\n      \"pmids\": [\"16791897\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Endocytic mechanism left ambiguous (caveolin block vs. β-arrestin independence)\", \"In vivo relevance not addressed\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Confirmed in vivo scavenging of CCL19/CCL21 and linked gradient regulation to T-helper polarization, establishing physiological consequence.\",\n      \"evidence\": \"CCX-CKR-/- mice, chemokine ELISA, MOG/CFA immunization, anti-CCL21 neutralization rescue\",\n      \"pmids\": [\"20562329\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cellular source of scavenging not resolved\", \"Did not separate CCL19 vs CCL21 contributions\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Resolved the signaling mode by showing β-arrestin recruitment without canonical Gi activation, defining ACKR4 as β-arrestin-biased and DRY-motif dependent.\",\n      \"evidence\": \"β-arrestin recruitment (EFC, BRET), CRE-reporter assays with pertussis toxin, intracellular-loop chimeras\",\n      \"pmids\": [\"23341447\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"GRK requirements not defined\", \"Did not establish whether β-arrestins are needed for scavenging\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Identified a non-scavenging function — heteromerization with CXCR3 to trans-inhibit its chemotaxis — broadening the receptor's regulatory repertoire.\",\n      \"evidence\": \"Co-expression in HEK293 and T cells, chemotaxis assays, co-IP, binding cooperativity assays\",\n      \"pmids\": [\"23121557\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Heteromer structure and stoichiometry undefined\", \"Single lab; physiological CXCR3 cross-regulation not shown in vivo\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Pinned scavenging to specific stromal cells and used genetic epistasis to define CCL19 as the critical inflammatory substrate for APC egress.\",\n      \"evidence\": \"Ackr4-/- and Ccl19-/- double-knockout rescue, in situ scavenging assays, cell-type expression analysis in skin\",\n      \"pmids\": [\"26976955\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not address collecting-vessel or lymph-node roles\", \"Steady-state vs inflammation distinction incomplete\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Dissected the recruitment hierarchy (GRK3>GRK2 preceding β-arrestin) and the C-terminus requirement, while showing β-arrestins are dispensable for scavenging — implying a parallel uptake route.\",\n      \"evidence\": \"GRK/β-arrestin recruitment assays, C-terminal deletion mutants, GRK inhibitors, β-arrestin1/2 KO cells, fluorescent chemokine uptake\",\n      \"pmids\": [\"32391018\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the β-arrestin-independent endocytic machinery unknown\", \"Phosphosite map not yet defined\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Established a tumor-immunology role: non-hematopoietic ACKR4 limits CD8+ T cell and CD103+ DC accumulation by regulating intratumoral CCL21.\",\n      \"evidence\": \"ACKR4-/- mice, tumor growth and flow cytometry, CCL21 quantification, bone-marrow chimeras\",\n      \"pmids\": [\"32289156\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific stromal cell type within tumors not defined\", \"Therapeutic targeting not tested\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Systematically defined the agonist repertoire, adding CCL20 and CCL22 as agonists and excluding CXCL13.\",\n      \"evidence\": \"High-sensitivity β-arrestin recruitment screen of all 43 human chemokines\",\n      \"pmids\": [\"32480426\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-method screen; CCL22 and CXCL13 results not orthogonally confirmed\", \"In vivo significance of CCL20/CCL22 scavenging unknown\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Refined the CCL21 substrate to both immobilized full-length and cleaved soluble forms, explaining how ACKR4 keeps barrier-tissue gradients functional for DC homing.\",\n      \"evidence\": \"ACKR4-/- mice, biochemical discrimination of CCL21 forms, DC homing assays, intravital imaging\",\n      \"pmids\": [\"33875601\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Protease generating soluble CCL21 not defined here\", \"Mechanism distinguishing immobilized vs soluble uptake unclear\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Extended the trafficking role to the spleen, where endothelial ACKR4 in the peri-marginal sinus supports T cell homing and marginal-zone organization.\",\n      \"evidence\": \"ACKR4-GFP reporter mice, immunofluorescence, 3D imaging, T cell homing assays\",\n      \"pmids\": [\"34260918\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Chemokine substrate driving the splenic phenotype not pinned down\", \"Link to systemic scavenging unresolved\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Revealed a non-trafficking, pathological function: ACKR4 promotes cardiac fibroblast IL-6 production and EndMT via p38/NF-κB, worsening post-MI dysfunction.\",\n      \"evidence\": \"ACKR4 KO and AAV9 overexpression mice, IL-6 neutralization rescue, p38/NF-κB inhibition, CF proliferation and EndMT assays\",\n      \"pmids\": [\"33610913\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"How a scavenging receptor drives IL-6/p38 signaling mechanistically is unexplained\", \"Reconciliation with β-arrestin-biased silent signaling not addressed\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Showed ACKR4 expression is mechanosensitively induced by lymph flow in collecting-vessel endothelium and enables T cell de-adhesion for transport to draining nodes.\",\n      \"evidence\": \"Intravital microscopy, flow-induced expression assays, ACKR4-/- TPA inflammation model, T cell migration assays\",\n      \"pmids\": [\"35108538\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanotransduction pathway inducing ACKR4 unknown\", \"Molecular link between scavenging and de-adhesion incomplete\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Identified a C-terminal class II PDZ-binding motif that controls β-arrestin pre-association and switches endocytosis between β-arrestin-dependent and -independent routes.\",\n      \"evidence\": \"Fluorescent CCL19 internalization, NanoBiT bystander assays, C-terminal tagging and PDZ mutagenesis, β-arrestin DKO cells, live-cell confocal\",\n      \"pmids\": [\"39623381\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"PDZ partner protein not identified\", \"How the motif arbitrates between the two endocytic routes is unresolved\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Provided a comprehensive phosphoregulatory model: GRK5/6 phosphorylate apo ACKR4, CCL19 recruits GRK2/3 to phosphorylate a C-terminal Ser/Thr cluster, and apo receptor forms a non-activating ternary complex with GRK2/3 and G protein.\",\n      \"evidence\": \"Phosphosite mutagenesis, MS phosphoproteomics, NanoBiT/BRET GRK and β-arrestin assays, fluorescent CCL19 uptake, ternary-complex co-IP\",\n      \"pmids\": [\"42143096\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of the non-activating ternary complex not solved\", \"Why the receptor sequesters G protein without activating it unexplained\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How ACKR4 reconciles its β-arrestin-independent scavenging route with active signaling outputs (e.g., IL-6/p38 in fibroblasts) and the identity of its PDZ-binding partner remain open.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No identified PDZ-domain partner protein\", \"No structural model of the apo ternary complex or the β-arrestin-independent endocytic machinery\", \"Mechanism linking a scavenging receptor to active p38/NF-κB signaling is undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [2, 14, 15]},\n      {\"term_id\": \"GO:0038024\", \"supporting_discovery_ids\": [0, 1, 4, 8]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [3]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [5, 13]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [5, 13]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [2, 13]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [1, 4, 6, 16]},\n      {\"term_id\": \"R-HSA-1500931\", \"supporting_discovery_ids\": [4, 8, 11]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"ARRB2\", \"GRK2\", \"GRK3\", \"GRK5\", \"GRK6\", \"CXCR3\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}