{"gene":"CD200","run_date":"2026-06-09T22:57:17","timeline":{"discoveries":[{"year":2003,"finding":"CD200 (OX-2) interacts with CD200R through their N-terminal immunoglobulin-like domains; both human and mouse CD200R were characterized, showing expression restricted predominantly to myeloid cells (macrophages, neutrophils, monocytes, mast cells) and some T lymphocytes. CD200R-related genes (CD200RLa, CD200RLb) pair with the activatory adaptor DAP12 but do not bind CD200, indicating a paired inhibitory/activatory receptor family architecture.","method":"Quantitative RT-PCR, novel monoclonal antibodies, protein binding assays, flow cytometry","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal binding assays, novel mAb panel, protein-level confirmation, replicated across mouse and human systems in one comprehensive study","pmids":["12960329"],"is_preprint":false},{"year":2004,"finding":"CD200 and CD200R interact through their N-terminal Ig-like domains (GFCC' face), spanning four Ig superfamily domains across the cell-cell interface. Site-directed mutagenesis of CD200R confirmed that binding involves its N-terminal domain, mirroring CD200's binding mode, suggesting the interaction evolved from a homotypic interaction.","method":"Site-directed mutagenesis, recombinant protein binding assays","journal":"European journal of immunology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — direct mutagenesis with binding validation, single lab but two orthogonal methods (mutagenesis + recombinant protein binding)","pmids":["15162439"],"is_preprint":false},{"year":2004,"finding":"Engagement of CD200R by CD200 on mouse bone marrow-derived mast cells leads to tyrosine phosphorylation of CD200R, recruitment of adaptor proteins Dok1 and Dok2, subsequent binding of SHIP to phosphorylated Dok1, and recruitment of RasGAP by both Dok1 and Dok2, resulting in inhibition of the Ras/MAPK pathway (ERK, JNK, p38) and suppression of mast cell degranulation and cytokine production.","method":"Co-immunoprecipitation, phosphorylation assays, mast cell degranulation assays, cytokine production assays","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (Co-IP, kinase assays, functional degranulation/cytokine readouts), confirmed in both bone marrow mast cells and peritoneal cells","pmids":["15557172"],"is_preprint":false},{"year":2005,"finding":"Recombinant mouse CD200 binds the inhibitory receptor CD200R with a Kd of ~4 µM but gives negligible binding to activating receptors CD200RLa, CD200RLb, CD200RLc, and CD200RLe, whether assessed as monomeric or dimeric fusion proteins or at the cell surface. This establishes that CD200 is the exclusive ligand for CD200R but not for the activating CD200R-related receptors.","method":"Direct protein binding assays using monomeric and dimeric recombinant fusion proteins, cell surface binding assays","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 1 / Strong — quantitative binding with affinity measurement, multiple protein forms and cell surface confirmation, establishes both positive and negative binding results rigorously","pmids":["16081818"],"is_preprint":false},{"year":2009,"finding":"In human myeloid cells (U937), CD200R inhibitory signaling is mediated via the PTB domain-binding NPLY motif in the cytoplasmic region. Dok2 binds directly to the phosphorylated NPLY motif with ~10-fold higher affinity (Kd ~1 µM) than Dok1, is phosphorylated upon CD200R engagement, and recruits RasGAP. Knockdown of Dok2 and RasGAP by RNAi abolished CD200R-mediated inhibition, while knockdown of Dok1 and SHIP did not affect inhibition.","method":"Mutational analysis of NPLY motif, direct binding affinity measurements, RNA interference knockdown, functional inhibition assays","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — mutational analysis, quantitative direct binding, RNAi knockdown with functional readout, multiple orthogonal methods in human cells","pmids":["19786546"],"is_preprint":false},{"year":2010,"finding":"CD200R-induced phosphorylation of Dok2 precedes that of Dok1; Dok2 recruits RasGAP and the adaptor Nck, while Dok1 recruits CrkL (but not CrkL's relative Crk, which binds Dok1 constitutively). Knockdown of Dok1 or CrkL increases Dok2 phosphorylation and RasGAP recruitment to Dok2, indicating that Dok1 negatively regulates Dok2-mediated CD200R signaling through CrkL recruitment.","method":"Co-immunoprecipitation, RNAi knockdown, phosphorylation assays in U937 cells","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP, RNAi knockdown with functional readout, multiple proteins tested, single lab","pmids":["21078907"],"is_preprint":false},{"year":2013,"finding":"X-ray crystallography structures of CD200R, the activating receptor CD200RLa, and a CD200R-CD200 complex were solved. Both proteins interact through their NH2-terminal Ig domains, consistent with immunological synapse-like cell-cell interactions. The failure of the activating receptor CD200RLa to bind CD200 was mapped to subtle changes around the binding interface. CD200 has been acquired by herpesviruses as a mimic of this host interaction.","method":"X-ray crystallography, structural analysis of CD200-CD200R complex","journal":"Structure","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structures of individual receptors and complex, direct structural validation of binding interface, identifies mechanism for activating receptor's failure to bind CD200","pmids":["23602662"],"is_preprint":false},{"year":2007,"finding":"CD200 is strongly expressed in macrophages at the onset of fusion (not in non-fusing myeloid cells), while CD200R is expressed on osteoclasts and CD4+ T cells. CD200-/- mice have fewer osteoclasts and accumulate more bone. Soluble recombinant CD200 extracellular domain rescued fusion of CD200-/- macrophages and restored downstream RANK signaling (NF-κB and MAP kinase). Conversely, soluble CD200R extracellular domain or shRNA silencing of CD200R prevented macrophage fusion and osteoclastogenesis.","method":"CD200-/- mouse model, recombinant protein rescue experiments, shRNA knockdown, signaling pathway analysis (NF-κB, MAPK), bone histomorphometry","journal":"Proceedings of the National Academy of Sciences","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic KO model with clear phenotype, recombinant protein rescue, shRNA knockdown, downstream signaling assays, multiple orthogonal approaches","pmids":["17726108"],"is_preprint":false},{"year":1984,"finding":"OX-2 (CD200) antigen is localized to the surfaces of neurons in rat brain (all tetanus toxin-positive cells are OX-2-positive) and is absent from GFAP-positive astrocytes, establishing neuronal-specific surface expression in the CNS.","method":"Indirect immunoperoxidase staining, double-immunofluorescence of cerebellar interneuron cultures with OX-2 antibody and tetanus toxin","journal":"Journal of neurochemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct localization by immunofluorescence and immunoperoxidase with cell-type specific markers, single lab, establishes neuronal surface localization","pmids":["6147390"],"is_preprint":false},{"year":1997,"finding":"OX-2 (CD200) expressed on CHO transfectants costimulates murine CD4+ T cell proliferation in both antigen-independent (anti-CD3) and antigen-dependent (peptide) fashions through a non-B7/CD28 pathway. Unlike B7-1, OX-2-mediated costimulation does not induce IL-2, IL-4, or IFN-γ and is not inhibited by CTLA4-Ig, but is blocked by anti-OX-2 mAb.","method":"CHO cell transfection expressing OX-2, T cell proliferation assays, cytokine measurement (IL-2, IL-4, IFN-γ), CTLA4-Ig blocking experiments","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional assay with transfectants, multiple cytokine readouts, blocking antibody confirmation, single lab","pmids":["9144466"],"is_preprint":false},{"year":2004,"finding":"CD200 receptor-like family member CD200RL3 (identified by microarray during helminth infection) is highly expressed on basophils and mast cells, pairs with DAP12 for cell surface expression and signaling (requiring the second Ig-like domain encoded by exon 4 and specific cytoplasmic domain splice variants for efficient DAP12 pairing), and functions as an activating receptor, in contrast to inhibitory CD200R.","method":"Microarray screen, splice variant characterization, DAP12 association assay, cell surface expression analysis","journal":"Journal of Biological Chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct molecular characterization of DAP12 pairing with mutagenesis-like splice variant analysis, single lab","pmids":["15471863"],"is_preprint":false},{"year":2016,"finding":"Shed soluble CD200 (sCD200) retains a biologically active extracellular domain capable of binding to and phosphorylating CD200R1 on CD200R1-expressing HEK293 cells, but lacks the cytoplasmic domain of CD200. CD200 ectodomain shedding is enhanced by PMA stimulation and partially mediated by ADAM28 (and additional metalloproteinases), generating a functionally active immunosuppressive species.","method":"Western blot with domain-specific antibodies, flow cytometry, ADAM28 silencing, CD200R1-expressing HEK293 functional assay, PMA stimulation","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Western blot domain mapping, functional CD200R phosphorylation assay, siRNA knockdown, single lab with multiple orthogonal methods","pmids":["27111430"],"is_preprint":false},{"year":2022,"finding":"In basal cell carcinoma, CD200 undergoes ectodomain shedding by metalloproteinases MMP3 and MMP11, releasing biologically active soluble CD200 into the tumor microenvironment. Soluble CD200 binds CD200R on NK cells to suppress MAPK pathway signaling, blocking IFN-γ release and direct tumor cell killing. Reduced ERK phosphorylation relieves negative regulation of PPARγ-regulated transcription, leading to membrane accumulation of Fas/FADD and FasL, causing activation-induced NK cell apoptosis.","method":"MMP3/MMP11 identification in situ, NK cell signaling assays (ERK phosphorylation), PPARγ pathway analysis, Fas/FADD/FasL expression analysis, CD200 blockade rescue experiments","journal":"Journal of Clinical Investigation","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — identification of specific sheddases, downstream signaling pathway dissection in NK cells, rescue experiments, single lab with multiple methods","pmids":["36074574"],"is_preprint":false},{"year":2018,"finding":"Cell membrane-bound CD200 signals not only extracellularly via CD200R but also intracellularly: γ-secretase cleavage releases the CD200 cytoplasmic tail (CD200C-tail) which translocates to the nucleus, binds DNA (identified by ChIP-seq), and alters expression of transcription factors associated with cell proliferation regulation. CD200C-tail transfection in CD200-negative HEK293 cells reproduced these gene expression changes.","method":"γ-secretase inhibitor experiments, chromatin immunoprecipitation followed by sequencing (ChIP-seq), nuclear translocation assays, CD200C-tail transfection in HEK293 cells, siRNA knockdown of POTEA","journal":"Leukemia research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ChIP-seq, nuclear translocation assay, transfection rescue in non-CD200 cells, single lab","pmids":["29698858"],"is_preprint":false},{"year":2021,"finding":"Mouse ILC2s express CD200R, and CD200R engagement inhibits ILC2 activation, proliferation, and type 2 cytokine production. Mechanistically, CD200R engagement inhibits both canonical and non-canonical NF-κB signaling pathways in activated ILC2s. Human ILC2s also express CD200R, and CD200R engagement ameliorated airway hyperreactivity in humanized mouse models.","method":"Flow cytometry, in vitro ILC2 activation assays, NF-κB pathway analysis, in vivo asthma models (preventative and therapeutic), humanized mouse models","journal":"Nature communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — mechanistic pathway (NF-κB) dissection with functional readout, mouse and human systems, in vivo confirmation, single lab","pmids":["33953190"],"is_preprint":false},{"year":2015,"finding":"CD200-Fc treatment downregulates toll-like receptor 4 (TLR4) expression in macrophages, restraining macrophage phagocytosis of oligodendrocyte precursor cells. In vivo, CD200-Fc treatment after white matter ischemia suppressed TLR4 expression in peripherally circulating macrophages and improved myelination.","method":"Primary cell culture phagocytosis assays, in vivo white matter ischemia model with endothelin-1, TLR4 expression analysis, myelination assessment","journal":"Journal of cerebral blood flow and metabolism","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro and in vivo mechanistic link between CD200 and TLR4 downregulation, functional phagocytosis and myelination readouts, single lab","pmids":["26661156"],"is_preprint":false},{"year":2025,"finding":"CD200R1 inhibits macrophage phagocytosis of tumor cells in a mechanistically distinct manner from SIRPα: unlike SIRPα which uses phosphatases SHP-1 and SHP-2, CD200R1 mediates its inhibitory effect via the kinase Csk. Blocking or removing CD200R1 from macrophages, or CD200 from tumor cells, increases phagocytosis and suppresses tumor growth. Combined CD200R1-CD200 and SIRPα-CD47 blockade further boosts phagocytosis compared to either blockade alone.","method":"CRISPR/genetic KO of CD200R1 and CD200, macrophage phagocytosis assays, signaling pathway analysis (Csk vs SHP-1/SHP-2), tumor growth models, combined blockade experiments","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — direct mechanistic comparison of signaling pathways, genetic KO, in vitro and in vivo tumor models, identification of Csk as CD200R1 effector kinase, multiple orthogonal approaches","pmids":["40461553"],"is_preprint":false},{"year":2014,"finding":"PPAR-γ activation by its endogenous ligand 15d-PGJ2 prevents CD200R1 downregulation and CD200 upregulation in reactive (LPS/IFN-γ stimulated) glial cells. The neuroprotective and anti-inflammatory effects of 15d-PGJ2 in neuron-microglia cocultures depend on the CD200-CD200R1 interaction, placing PPAR-γ upstream of CD200-CD200R1 in the anti-inflammatory pathway.","method":"Mouse primary neuronal and glial cultures, neuron-microglia cocultures, 15d-PGJ2 treatment, LPS/IFN-γ stimulation, CD200-CD200R1 blocking antibodies","journal":"Glia","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — pathway epistasis using blocking antibodies and ligand treatment, single lab, two orthogonal approaches (gene expression + functional neuroprotection readout)","pmids":["24639050"],"is_preprint":false},{"year":2020,"finding":"In vitro cytokine-driven expansion and suppressive activity of human MDSCs was enhanced when cocultured with recombinant CD200 protein, and CD200R expression was elevated on CD11b+CD33+HLA-DRlo/- MDSC populations from PDAC patients. In vivo antibody blockade of CD200 reduced intratumoral MDSC percentage and enhanced efficacy of PD-1 checkpoint antibodies.","method":"Recombinant CD200 MDSC expansion assays, flow cytometry, in vivo antibody blockade in subcutaneous and KPC-Brca2 mouse models, single-cell RNA sequencing","journal":"Journal for immunotherapy of cancer","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct recombinant CD200 functional assay, in vivo mouse tumor models with genetic/pharmacological manipulation, single lab with multiple methods","pmids":["32581043"],"is_preprint":false},{"year":2021,"finding":"CD200 expression on AML cells significantly impairs OXPHOS metabolic activity in T cells from healthy donors, representing a mechanism of T cell suppression. CD200+ leukemia in PBMC-humanized mice showed T cells with an abundance of metabolically quiescent CD8+ central and effector memory cells. A CD200R-CD28 switch receptor on CAR T-cells converted CD200-mediated inhibitory signaling into CD28 costimulation, potently enhancing CAR T-cell polyfunctionality, proliferation, cytotoxicity, and metabolism.","method":"CyTOF, RNA-sequencing, T cell metabolic assays (OXPHOS), PBMC-humanized mouse models, CD200R-CD28 switch receptor engineering, patient-derived xenograft models","journal":"Journal for immunotherapy of cancer","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct mechanistic link between CD200 and T cell OXPHOS impairment, switch receptor functional conversion, in vivo xenograft validation, single lab with multiple methods","pmids":["34326171"],"is_preprint":false},{"year":2021,"finding":"CD200 expression on AML cells impairs cytokine secretion in both innate and adaptive immune cell subsets. Engineered CD200R-CD28 switch receptors on CAR T-cells leverage CD200-CD200R engagement to provide CD28 costimulation, improving CAR T-cell performance, while CD200R knockout was detrimental to CAR T-cell metabolic activity.","method":"Isogenic cell line models, in vitro co-culture with immune cells, CAR T-cell engineering, PBMC-humanized xenograft models, metabolic assays","journal":"Blood","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — CRISPR KO comparison, engineered switch receptor with functional readouts, xenograft validation, single lab","pmids":["37616575"],"is_preprint":false},{"year":2021,"finding":"CD200 expression on AML cells impairs cytokine secretion in both innate and adaptive immune cell subsets. Engineered CD200R-CD28 switch receptors on CAR T-cells leverage CD200-CD200R engagement to provide CD28 costimulation, improving CAR T-cell performance, while CD200R knockout was detrimental to CAR T-cell metabolic activity.","method":"Isogenic cell line models, in vitro co-culture with immune cells, CAR T-cell engineering, PBMC-humanized xenograft models, metabolic assays","journal":"Blood","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — CRISPR KO comparison, engineered switch receptor with functional readouts, xenograft validation, single lab","pmids":["37616575"],"is_preprint":false},{"year":2023,"finding":"CD200-CD200R signaling between amacrine cells and microglia is dysregulated in early diabetic retinopathy (DR). Targeting CD200R attenuates high glucose-induced inflammation and phagocytosis in cultured microglia. In vivo CD200R targeting prevents visual dysfunction, microglia activation, and retinal inflammation in the diabetic mouse.","method":"Type I diabetes mouse model, retinal microglial ablation, flow cytometry (engulfment assays), in vitro high glucose microglial culture with CD200R targeting, in vivo CD200R targeting","journal":"Proceedings of the National Academy of Sciences","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro and in vivo mechanistic link between CD200-CD200R and microglial phagocytosis/inflammation in DR, multiple readouts, single lab","pmids":["37903272"],"is_preprint":false},{"year":2021,"finding":"The CD200-CD200R axis promotes squamous cell carcinoma (cSCC) metastasis via induction of cathepsin K (Ctsk) in CD200R+ infiltrating myeloid cells. CD200-CD200R engagement in a coculture system induced Ctsk expression in CD11b+Cd200R+ myeloid cells. Ctsk inhibition, but not MMP inhibition, blocked cSCC cell migration in vitro, and targeted CD200 disruption plus Ctsk pharmacologic inhibition reduced metastasis in vivo.","method":"RNA sequencing of sorted infiltrating myeloid cells, CD200-CD200R coculture system, Ctsk inhibition assays, in vitro migration assay, in vivo metastasis model with CD200 tumor cell disruption","journal":"Cancer research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — RNA-seq identifies Ctsk, coculture mechanistic confirmation, in vitro and in vivo functional validation, single lab with multiple methods","pmids":["34183355"],"is_preprint":false},{"year":2024,"finding":"CD200+DKK3+ pro-resolving fibroblasts stabilize type 2 innate lymphoid cell (ILC2) phenotype and induce resolution of arthritis via CD200-CD200R1 signaling. Single-cell RNA-sequencing and spatial transcriptomics revealed that pro-resolving CD200+DKK3+ fibroblasts cluster with ILC2s in resolution niches, distinct from pro-inflammatory MMP3+/IL6+ fibroblast clusters.","method":"Biopsy-based single-cell RNA-sequencing, spatial transcriptomics, PET imaging of FAP, functional coculture assays, in vivo experimental arthritis models","journal":"Nature immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — scRNA-seq and spatial transcriptomics identifying specific cell clusters, in vivo functional model, CD200-CD200R1 pathway mechanistically linked to ILC2 phenotype stabilization, single lab","pmids":["38396288"],"is_preprint":false},{"year":2009,"finding":"In rat vascular endothelium, CD200 expression is heterogeneous across vascular beds and regulated by LPS (decreased during endotoxemia). Physical binding between CD200 and CD200R is involved in T-cell adhesion to endothelium, but not in macrophage-endothelium interaction. A CD200 agonist peptide can trigger CD200R signaling in macrophages to suppress their adhesion to endothelium.","method":"Immunoelectron microscopy, in vivo LPS model, T-cell and macrophage adhesion assays, CD200 agonist peptide treatment","journal":"Journal of anatomy","confidence":"Low","confidence_rationale":"Tier 3 / Weak — adhesion assays with agonist peptide and blocking, localization by electron microscopy, single lab, limited mechanistic depth","pmids":["19166481"],"is_preprint":false}],"current_model":"CD200 is a broadly expressed cell-surface glycoprotein (Ig superfamily) that interacts via its N-terminal Ig domain with the structurally related inhibitory receptor CD200R on myeloid cells; upon ligand engagement, CD200R is tyrosine-phosphorylated at its NPLY motif, recruits adaptor Dok2 (and secondarily Dok1), which in turn activates RasGAP to suppress Ras/MAPK signaling—a pathway mechanistically distinct from ITIM-based inhibitory receptors—resulting in suppression of mast cell degranulation, cytokine production, and macrophage/microglial activation; additionally, CD200 undergoes metalloproteinase (MMP3/MMP11/ADAM28)-mediated ectodomain shedding to release biologically active soluble CD200, and γ-secretase cleavage of its cytoplasmic tail allows nuclear translocation and gene regulation; CD200 also regulates osteoclastogenesis by promoting macrophage fusion via RANK downstream signaling, inhibits NK cell and ILC2 activity, and suppresses microglial activation in the CNS through the CD200-CD200R1 axis, which is exploited by tumors and pathogens for immune evasion."},"narrative":{"mechanistic_narrative":"CD200 (OX-2) is a cell-surface immunoglobulin-superfamily glycoprotein that functions as the ligand for the inhibitory receptor CD200R, delivering a tonic immunosuppressive signal that restrains myeloid and lymphoid effector activity [PMID:12960329, PMID:16081818]. CD200 engages CD200R through their N-terminal Ig-like domains in an immunological synapse-like cell-cell contact, a binding mode resolved at atomic resolution; the structurally related activating receptors (CD200RLa and family members that pair with DAP12) fail to bind CD200, defining a paired inhibitory/activatory receptor architecture in which CD200 is the exclusive CD200R ligand [PMID:15162439, PMID:16081818, PMID:23602662, PMID:15471863]. Receptor engagement triggers tyrosine phosphorylation of the CD200R cytoplasmic NPLY motif, which recruits the PTB-domain adaptor Dok2 (with higher affinity than Dok1), and Dok2 in turn recruits RasGAP to suppress Ras/MAPK (ERK, JNK, p38) signaling—a mechanism distinct from ITIM/SHP-based inhibition; Dok1 acting through CrkL negatively tunes this Dok2-RasGAP output [PMID:15557172, PMID:19786546, PMID:21078907]. The downstream consequence is suppression of mast cell degranulation and cytokine production, dampening of NK, ILC2, and macrophage activation, and inhibition of macrophage phagocytosis of tumor cells, the latter mediated by the kinase Csk rather than SHP-1/SHP-2 [PMID:15557172, PMID:33953190, PMID:40461553]. Beyond classical receptor signaling, CD200 generates a biologically active soluble ectodomain via metalloproteinase shedding (ADAM28, MMP3, MMP11) that binds and phosphorylates CD200R at a distance, and γ-secretase cleavage liberates the CD200 cytoplasmic tail, which translocates to the nucleus, binds DNA, and alters expression of proliferation-associated transcription factors [PMID:27111430, PMID:36074574, PMID:29698858]. CD200 also acts developmentally to promote macrophage fusion and osteoclastogenesis through RANK-downstream NF-κB/MAPK signaling, and in the CNS its neuronal expression restrains microglial activation [PMID:17726108, PMID:6147390]. This inhibitory axis is co-opted in disease: tumors exploit it to suppress NK killing, expand MDSCs, and impair T-cell metabolism and cytokine output [PMID:36074574, PMID:32581043, PMID:34326171].","teleology":[{"year":2003,"claim":"Establishing the receptor and its expression pattern was the prerequisite for any mechanism: it defined CD200R as the myeloid-restricted partner of CD200 and revealed a paired inhibitory/activatory receptor family.","evidence":"qRT-PCR, novel monoclonal antibodies, and protein binding assays across human and mouse","pmids":["12960329"],"confidence":"High","gaps":["Did not resolve the molecular signaling cascade downstream of CD200R","Function of DAP12-pairing CD200R-like receptors left undefined"]},{"year":2004,"claim":"Mapping the binding determinants showed the interaction occurs via the N-terminal Ig domains, establishing the structural basis and an evolutionary origin from a homotypic interaction.","evidence":"Site-directed mutagenesis with recombinant protein binding assays","pmids":["15162439"],"confidence":"High","gaps":["Atomic-resolution interface not yet determined","Affinity not quantified"]},{"year":2004,"claim":"Dissecting CD200R signaling answered how an Ig-domain ligand inhibits effector cells: it works through Dok adaptors and RasGAP to suppress Ras/MAPK, a non-ITIM mechanism, with a functional readout in mast cells.","evidence":"Co-IP, phosphorylation assays, and mast cell degranulation/cytokine assays in mouse BMMCs","pmids":["15557172"],"confidence":"High","gaps":["Relative contributions of Dok1 vs Dok2 not yet resolved","Demonstrated in mouse mast cells only"]},{"year":2005,"claim":"Quantifying binding established CD200 as the exclusive ligand of inhibitory CD200R with micromolar affinity and no binding to the activating CD200R-related receptors, ruling out ligand promiscuity.","evidence":"Direct binding assays with monomeric/dimeric fusion proteins and cell-surface binding","pmids":["16081818"],"confidence":"High","gaps":["Endogenous ligand(s) of the activating receptors not identified","Physiological consequence of low (~4 µM) affinity unaddressed"]},{"year":2009,"claim":"Defining the human cytoplasmic motif resolved adaptor hierarchy: the NPLY motif recruits Dok2 (not Dok1 or SHIP) as the essential effector linking to RasGAP, confirmed by loss-of-function.","evidence":"NPLY mutational analysis, affinity measurements, and RNAi knockdown with functional inhibition readouts in U937 cells","pmids":["19786546"],"confidence":"High","gaps":["Identity of the kinase phosphorylating NPLY unspecified","Did not address signaling in primary human myeloid cells"]},{"year":2010,"claim":"Refining the adaptor network showed Dok1 acts as a negative regulator of Dok2-mediated signaling via CrkL, explaining how inhibitory output is tuned.","evidence":"Reciprocal Co-IP and RNAi knockdown with phosphorylation readouts in U937 cells","pmids":["21078907"],"confidence":"High","gaps":["Physiological setting where Dok1 tuning matters not established","Single cell-line system"]},{"year":2013,"claim":"Crystal structures gave the definitive interface and explained, at the residue level, why the activating receptor cannot bind CD200 and how herpesviruses mimic the host ligand.","evidence":"X-ray crystallography of CD200R, CD200RLa, and the CD200R-CD200 complex","pmids":["23602662"],"confidence":"High","gaps":["Does not address signaling dynamics or receptor clustering in cells","Viral mimic functional consequences not tested here"]},{"year":2007,"claim":"A genetic KO revealed a developmental, non-immunosuppressive role: CD200 promotes macrophage fusion and osteoclastogenesis through RANK-downstream signaling, broadening CD200 function beyond inhibition.","evidence":"CD200-/- mice, recombinant protein rescue, shRNA knockdown, and bone histomorphometry with NF-κB/MAPK analysis","pmids":["17726108"],"confidence":"High","gaps":["How a normally inhibitory ligand drives a positive fusion signal mechanistically unresolved","Direct CD200R-to-RANK signaling crosstalk not defined"]},{"year":1984,"claim":"Early localization established neuronal-specific surface expression of CD200 in the CNS, later providing the anatomical basis for its role restraining microglia.","evidence":"Immunoperoxidase and double-immunofluorescence of rat brain and cerebellar cultures with cell-type markers","pmids":["6147390"],"confidence":"Medium","gaps":["No receptor or mechanism identified at the time","Functional consequence of neuronal expression not tested"]},{"year":1997,"claim":"An early functional study placed CD200 in T-cell costimulation through a non-B7/CD28, non-cytokine-inducing pathway, distinguishing it from classical costimulatory ligands.","evidence":"CHO transfectants, T-cell proliferation and cytokine assays, CTLA4-Ig blocking","pmids":["9144466"],"confidence":"Medium","gaps":["Receptor mediating this costimulation not identified","Apparent costimulation contrasts with later inhibitory model and was not reconciled"]},{"year":2004,"claim":"Characterization of an activating CD200R-like receptor (CD200RL3) that pairs with DAP12 fleshed out the paired-receptor family but showed it does not bind CD200.","evidence":"Microarray screen, splice variant and DAP12-association analysis","pmids":["15471863"],"confidence":"Medium","gaps":["Endogenous ligand of the activating receptor unidentified","In vivo function not established"]},{"year":2015,"claim":"A mechanism for CNS immunosuppression was identified: CD200-Fc downregulates macrophage TLR4 to restrain phagocytosis and improve myelination after ischemia.","evidence":"Primary phagocytosis assays and an in vivo white matter ischemia model with TLR4 and myelination readouts","pmids":["26661156"],"confidence":"Medium","gaps":["Link between CD200R signaling and TLR4 downregulation mechanistically incomplete","CD200-Fc may not recapitulate membrane-bound ligand"]},{"year":2016,"claim":"Demonstrating that shed soluble CD200 is functionally active established a paracrine mode: the ectodomain alone binds and phosphorylates CD200R, with ADAM28 implicated as a sheddase.","evidence":"Domain-specific Western blot, flow cytometry, ADAM28 silencing, and CD200R1-HEK293 functional assay with PMA stimulation","pmids":["27111430"],"confidence":"Medium","gaps":["Full set of sheddases not defined","Physiological abundance and reach of soluble CD200 in vivo unquantified"]},{"year":2018,"claim":"An intracellular signaling role was uncovered: γ-secretase releases the CD200 cytoplasmic tail, which enters the nucleus, binds DNA, and reprograms proliferation-associated transcription factors.","evidence":"γ-secretase inhibitor experiments, ChIP-seq, nuclear translocation assays, and CD200C-tail transfection in HEK293","pmids":["29698858"],"confidence":"Medium","gaps":["Direct DNA-binding mode of a non-canonical tail fragment not structurally defined","Physiological relevance versus overexpression artifact in HEK293 untested"]},{"year":2014,"claim":"Pathway epistasis positioned PPAR-γ upstream of CD200-CD200R1, explaining how an anti-inflammatory lipid ligand maintains the axis to protect neurons.","evidence":"Neuron-microglia cocultures with 15d-PGJ2 treatment, LPS/IFN-γ stimulation, and CD200-CD200R1 blocking antibodies","pmids":["24639050"],"confidence":"Medium","gaps":["Direct transcriptional regulation of CD200/CD200R by PPAR-γ not demonstrated","In vivo confirmation lacking"]},{"year":2021,"claim":"ILC2 work extended the inhibitory axis to type 2 immunity, showing CD200R engagement suppresses ILC2 activation via canonical and non-canonical NF-κB, with therapeutic effect on airway hyperreactivity.","evidence":"In vitro ILC2 activation assays, NF-κB analysis, and humanized mouse asthma models","pmids":["33953190"],"confidence":"Medium","gaps":["How CD200R-Dok2-RasGAP signaling intersects NF-κB not reconciled","Single-lab models"]},{"year":2022,"claim":"A tumor-immune-evasion mechanism was dissected: MMP3/MMP11-shed soluble CD200 binds NK-cell CD200R, suppressing MAPK and, via PPARγ-Fas/FADD/FasL, driving activation-induced NK apoptosis.","evidence":"Sheddase identification in basal cell carcinoma, NK ERK/PPARγ/Fas pathway analysis, and CD200 blockade rescue","pmids":["36074574"],"confidence":"Medium","gaps":["Relative contributions of distinct sheddases across tissues unresolved","Generalizability beyond BCC NK cells untested"]},{"year":2025,"claim":"Comparing inhibitory phagocytosis checkpoints showed CD200R1 uses the kinase Csk rather than SHP-1/SHP-2, making CD200-CD200R1 a mechanistically distinct, additive target alongside SIRPα-CD47.","evidence":"CRISPR/genetic KO of CD200R1 and CD200, macrophage phagocytosis assays, Csk vs SHP signaling analysis, and combined-blockade tumor models","pmids":["40461553"],"confidence":"High","gaps":["How Csk integrates with the Dok2-RasGAP module not delineated","Clinical translatability of combined blockade unestablished"]},{"year":2021,"claim":"Cancer-context studies established CD200 as an immunosuppressive driver across tumor types—expanding MDSCs, impairing T-cell metabolism/cytokine output, and inducing cathepsin K in myeloid cells to promote metastasis—and validated CD200R-CD28 switch receptors as a therapeutic conversion strategy.","evidence":"Recombinant CD200 and isogenic models, scRNA-seq/spatial transcriptomics, T-cell metabolic and CAR-T switch-receptor assays, and in vivo blockade/xenograft models","pmids":["32581043","34326171","37616575","34183355","37903272","38396288"],"confidence":"Medium","gaps":["Cell-type-specific downstream effectors (e.g., Ctsk induction) mechanistically incomplete","Many findings are single-lab disease-context observations not 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CD200R-related genes (CD200RLa, CD200RLb) pair with the activatory adaptor DAP12 but do not bind CD200, indicating a paired inhibitory/activatory receptor family architecture.\",\n      \"method\": \"Quantitative RT-PCR, novel monoclonal antibodies, protein binding assays, flow cytometry\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal binding assays, novel mAb panel, protein-level confirmation, replicated across mouse and human systems in one comprehensive study\",\n      \"pmids\": [\"12960329\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"CD200 and CD200R interact through their N-terminal Ig-like domains (GFCC' face), spanning four Ig superfamily domains across the cell-cell interface. Site-directed mutagenesis of CD200R confirmed that binding involves its N-terminal domain, mirroring CD200's binding mode, suggesting the interaction evolved from a homotypic interaction.\",\n      \"method\": \"Site-directed mutagenesis, recombinant protein binding assays\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — direct mutagenesis with binding validation, single lab but two orthogonal methods (mutagenesis + recombinant protein binding)\",\n      \"pmids\": [\"15162439\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Engagement of CD200R by CD200 on mouse bone marrow-derived mast cells leads to tyrosine phosphorylation of CD200R, recruitment of adaptor proteins Dok1 and Dok2, subsequent binding of SHIP to phosphorylated Dok1, and recruitment of RasGAP by both Dok1 and Dok2, resulting in inhibition of the Ras/MAPK pathway (ERK, JNK, p38) and suppression of mast cell degranulation and cytokine production.\",\n      \"method\": \"Co-immunoprecipitation, phosphorylation assays, mast cell degranulation assays, cytokine production assays\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (Co-IP, kinase assays, functional degranulation/cytokine readouts), confirmed in both bone marrow mast cells and peritoneal cells\",\n      \"pmids\": [\"15557172\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Recombinant mouse CD200 binds the inhibitory receptor CD200R with a Kd of ~4 µM but gives negligible binding to activating receptors CD200RLa, CD200RLb, CD200RLc, and CD200RLe, whether assessed as monomeric or dimeric fusion proteins or at the cell surface. This establishes that CD200 is the exclusive ligand for CD200R but not for the activating CD200R-related receptors.\",\n      \"method\": \"Direct protein binding assays using monomeric and dimeric recombinant fusion proteins, cell surface binding assays\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — quantitative binding with affinity measurement, multiple protein forms and cell surface confirmation, establishes both positive and negative binding results rigorously\",\n      \"pmids\": [\"16081818\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"In human myeloid cells (U937), CD200R inhibitory signaling is mediated via the PTB domain-binding NPLY motif in the cytoplasmic region. Dok2 binds directly to the phosphorylated NPLY motif with ~10-fold higher affinity (Kd ~1 µM) than Dok1, is phosphorylated upon CD200R engagement, and recruits RasGAP. Knockdown of Dok2 and RasGAP by RNAi abolished CD200R-mediated inhibition, while knockdown of Dok1 and SHIP did not affect inhibition.\",\n      \"method\": \"Mutational analysis of NPLY motif, direct binding affinity measurements, RNA interference knockdown, functional inhibition assays\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — mutational analysis, quantitative direct binding, RNAi knockdown with functional readout, multiple orthogonal methods in human cells\",\n      \"pmids\": [\"19786546\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"CD200R-induced phosphorylation of Dok2 precedes that of Dok1; Dok2 recruits RasGAP and the adaptor Nck, while Dok1 recruits CrkL (but not CrkL's relative Crk, which binds Dok1 constitutively). Knockdown of Dok1 or CrkL increases Dok2 phosphorylation and RasGAP recruitment to Dok2, indicating that Dok1 negatively regulates Dok2-mediated CD200R signaling through CrkL recruitment.\",\n      \"method\": \"Co-immunoprecipitation, RNAi knockdown, phosphorylation assays in U937 cells\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP, RNAi knockdown with functional readout, multiple proteins tested, single lab\",\n      \"pmids\": [\"21078907\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"X-ray crystallography structures of CD200R, the activating receptor CD200RLa, and a CD200R-CD200 complex were solved. Both proteins interact through their NH2-terminal Ig domains, consistent with immunological synapse-like cell-cell interactions. The failure of the activating receptor CD200RLa to bind CD200 was mapped to subtle changes around the binding interface. CD200 has been acquired by herpesviruses as a mimic of this host interaction.\",\n      \"method\": \"X-ray crystallography, structural analysis of CD200-CD200R complex\",\n      \"journal\": \"Structure\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structures of individual receptors and complex, direct structural validation of binding interface, identifies mechanism for activating receptor's failure to bind CD200\",\n      \"pmids\": [\"23602662\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"CD200 is strongly expressed in macrophages at the onset of fusion (not in non-fusing myeloid cells), while CD200R is expressed on osteoclasts and CD4+ T cells. CD200-/- mice have fewer osteoclasts and accumulate more bone. Soluble recombinant CD200 extracellular domain rescued fusion of CD200-/- macrophages and restored downstream RANK signaling (NF-κB and MAP kinase). Conversely, soluble CD200R extracellular domain or shRNA silencing of CD200R prevented macrophage fusion and osteoclastogenesis.\",\n      \"method\": \"CD200-/- mouse model, recombinant protein rescue experiments, shRNA knockdown, signaling pathway analysis (NF-κB, MAPK), bone histomorphometry\",\n      \"journal\": \"Proceedings of the National Academy of Sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic KO model with clear phenotype, recombinant protein rescue, shRNA knockdown, downstream signaling assays, multiple orthogonal approaches\",\n      \"pmids\": [\"17726108\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1984,\n      \"finding\": \"OX-2 (CD200) antigen is localized to the surfaces of neurons in rat brain (all tetanus toxin-positive cells are OX-2-positive) and is absent from GFAP-positive astrocytes, establishing neuronal-specific surface expression in the CNS.\",\n      \"method\": \"Indirect immunoperoxidase staining, double-immunofluorescence of cerebellar interneuron cultures with OX-2 antibody and tetanus toxin\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct localization by immunofluorescence and immunoperoxidase with cell-type specific markers, single lab, establishes neuronal surface localization\",\n      \"pmids\": [\"6147390\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"OX-2 (CD200) expressed on CHO transfectants costimulates murine CD4+ T cell proliferation in both antigen-independent (anti-CD3) and antigen-dependent (peptide) fashions through a non-B7/CD28 pathway. Unlike B7-1, OX-2-mediated costimulation does not induce IL-2, IL-4, or IFN-γ and is not inhibited by CTLA4-Ig, but is blocked by anti-OX-2 mAb.\",\n      \"method\": \"CHO cell transfection expressing OX-2, T cell proliferation assays, cytokine measurement (IL-2, IL-4, IFN-γ), CTLA4-Ig blocking experiments\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional assay with transfectants, multiple cytokine readouts, blocking antibody confirmation, single lab\",\n      \"pmids\": [\"9144466\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"CD200 receptor-like family member CD200RL3 (identified by microarray during helminth infection) is highly expressed on basophils and mast cells, pairs with DAP12 for cell surface expression and signaling (requiring the second Ig-like domain encoded by exon 4 and specific cytoplasmic domain splice variants for efficient DAP12 pairing), and functions as an activating receptor, in contrast to inhibitory CD200R.\",\n      \"method\": \"Microarray screen, splice variant characterization, DAP12 association assay, cell surface expression analysis\",\n      \"journal\": \"Journal of Biological Chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct molecular characterization of DAP12 pairing with mutagenesis-like splice variant analysis, single lab\",\n      \"pmids\": [\"15471863\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Shed soluble CD200 (sCD200) retains a biologically active extracellular domain capable of binding to and phosphorylating CD200R1 on CD200R1-expressing HEK293 cells, but lacks the cytoplasmic domain of CD200. CD200 ectodomain shedding is enhanced by PMA stimulation and partially mediated by ADAM28 (and additional metalloproteinases), generating a functionally active immunosuppressive species.\",\n      \"method\": \"Western blot with domain-specific antibodies, flow cytometry, ADAM28 silencing, CD200R1-expressing HEK293 functional assay, PMA stimulation\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Western blot domain mapping, functional CD200R phosphorylation assay, siRNA knockdown, single lab with multiple orthogonal methods\",\n      \"pmids\": [\"27111430\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"In basal cell carcinoma, CD200 undergoes ectodomain shedding by metalloproteinases MMP3 and MMP11, releasing biologically active soluble CD200 into the tumor microenvironment. Soluble CD200 binds CD200R on NK cells to suppress MAPK pathway signaling, blocking IFN-γ release and direct tumor cell killing. Reduced ERK phosphorylation relieves negative regulation of PPARγ-regulated transcription, leading to membrane accumulation of Fas/FADD and FasL, causing activation-induced NK cell apoptosis.\",\n      \"method\": \"MMP3/MMP11 identification in situ, NK cell signaling assays (ERK phosphorylation), PPARγ pathway analysis, Fas/FADD/FasL expression analysis, CD200 blockade rescue experiments\",\n      \"journal\": \"Journal of Clinical Investigation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — identification of specific sheddases, downstream signaling pathway dissection in NK cells, rescue experiments, single lab with multiple methods\",\n      \"pmids\": [\"36074574\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Cell membrane-bound CD200 signals not only extracellularly via CD200R but also intracellularly: γ-secretase cleavage releases the CD200 cytoplasmic tail (CD200C-tail) which translocates to the nucleus, binds DNA (identified by ChIP-seq), and alters expression of transcription factors associated with cell proliferation regulation. CD200C-tail transfection in CD200-negative HEK293 cells reproduced these gene expression changes.\",\n      \"method\": \"γ-secretase inhibitor experiments, chromatin immunoprecipitation followed by sequencing (ChIP-seq), nuclear translocation assays, CD200C-tail transfection in HEK293 cells, siRNA knockdown of POTEA\",\n      \"journal\": \"Leukemia research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ChIP-seq, nuclear translocation assay, transfection rescue in non-CD200 cells, single lab\",\n      \"pmids\": [\"29698858\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Mouse ILC2s express CD200R, and CD200R engagement inhibits ILC2 activation, proliferation, and type 2 cytokine production. Mechanistically, CD200R engagement inhibits both canonical and non-canonical NF-κB signaling pathways in activated ILC2s. Human ILC2s also express CD200R, and CD200R engagement ameliorated airway hyperreactivity in humanized mouse models.\",\n      \"method\": \"Flow cytometry, in vitro ILC2 activation assays, NF-κB pathway analysis, in vivo asthma models (preventative and therapeutic), humanized mouse models\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — mechanistic pathway (NF-κB) dissection with functional readout, mouse and human systems, in vivo confirmation, single lab\",\n      \"pmids\": [\"33953190\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"CD200-Fc treatment downregulates toll-like receptor 4 (TLR4) expression in macrophages, restraining macrophage phagocytosis of oligodendrocyte precursor cells. In vivo, CD200-Fc treatment after white matter ischemia suppressed TLR4 expression in peripherally circulating macrophages and improved myelination.\",\n      \"method\": \"Primary cell culture phagocytosis assays, in vivo white matter ischemia model with endothelin-1, TLR4 expression analysis, myelination assessment\",\n      \"journal\": \"Journal of cerebral blood flow and metabolism\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro and in vivo mechanistic link between CD200 and TLR4 downregulation, functional phagocytosis and myelination readouts, single lab\",\n      \"pmids\": [\"26661156\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"CD200R1 inhibits macrophage phagocytosis of tumor cells in a mechanistically distinct manner from SIRPα: unlike SIRPα which uses phosphatases SHP-1 and SHP-2, CD200R1 mediates its inhibitory effect via the kinase Csk. Blocking or removing CD200R1 from macrophages, or CD200 from tumor cells, increases phagocytosis and suppresses tumor growth. Combined CD200R1-CD200 and SIRPα-CD47 blockade further boosts phagocytosis compared to either blockade alone.\",\n      \"method\": \"CRISPR/genetic KO of CD200R1 and CD200, macrophage phagocytosis assays, signaling pathway analysis (Csk vs SHP-1/SHP-2), tumor growth models, combined blockade experiments\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — direct mechanistic comparison of signaling pathways, genetic KO, in vitro and in vivo tumor models, identification of Csk as CD200R1 effector kinase, multiple orthogonal approaches\",\n      \"pmids\": [\"40461553\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"PPAR-γ activation by its endogenous ligand 15d-PGJ2 prevents CD200R1 downregulation and CD200 upregulation in reactive (LPS/IFN-γ stimulated) glial cells. The neuroprotective and anti-inflammatory effects of 15d-PGJ2 in neuron-microglia cocultures depend on the CD200-CD200R1 interaction, placing PPAR-γ upstream of CD200-CD200R1 in the anti-inflammatory pathway.\",\n      \"method\": \"Mouse primary neuronal and glial cultures, neuron-microglia cocultures, 15d-PGJ2 treatment, LPS/IFN-γ stimulation, CD200-CD200R1 blocking antibodies\",\n      \"journal\": \"Glia\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — pathway epistasis using blocking antibodies and ligand treatment, single lab, two orthogonal approaches (gene expression + functional neuroprotection readout)\",\n      \"pmids\": [\"24639050\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"In vitro cytokine-driven expansion and suppressive activity of human MDSCs was enhanced when cocultured with recombinant CD200 protein, and CD200R expression was elevated on CD11b+CD33+HLA-DRlo/- MDSC populations from PDAC patients. In vivo antibody blockade of CD200 reduced intratumoral MDSC percentage and enhanced efficacy of PD-1 checkpoint antibodies.\",\n      \"method\": \"Recombinant CD200 MDSC expansion assays, flow cytometry, in vivo antibody blockade in subcutaneous and KPC-Brca2 mouse models, single-cell RNA sequencing\",\n      \"journal\": \"Journal for immunotherapy of cancer\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct recombinant CD200 functional assay, in vivo mouse tumor models with genetic/pharmacological manipulation, single lab with multiple methods\",\n      \"pmids\": [\"32581043\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"CD200 expression on AML cells significantly impairs OXPHOS metabolic activity in T cells from healthy donors, representing a mechanism of T cell suppression. CD200+ leukemia in PBMC-humanized mice showed T cells with an abundance of metabolically quiescent CD8+ central and effector memory cells. A CD200R-CD28 switch receptor on CAR T-cells converted CD200-mediated inhibitory signaling into CD28 costimulation, potently enhancing CAR T-cell polyfunctionality, proliferation, cytotoxicity, and metabolism.\",\n      \"method\": \"CyTOF, RNA-sequencing, T cell metabolic assays (OXPHOS), PBMC-humanized mouse models, CD200R-CD28 switch receptor engineering, patient-derived xenograft models\",\n      \"journal\": \"Journal for immunotherapy of cancer\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct mechanistic link between CD200 and T cell OXPHOS impairment, switch receptor functional conversion, in vivo xenograft validation, single lab with multiple methods\",\n      \"pmids\": [\"34326171\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"CD200 expression on AML cells impairs cytokine secretion in both innate and adaptive immune cell subsets. Engineered CD200R-CD28 switch receptors on CAR T-cells leverage CD200-CD200R engagement to provide CD28 costimulation, improving CAR T-cell performance, while CD200R knockout was detrimental to CAR T-cell metabolic activity.\",\n      \"method\": \"Isogenic cell line models, in vitro co-culture with immune cells, CAR T-cell engineering, PBMC-humanized xenograft models, metabolic assays\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — CRISPR KO comparison, engineered switch receptor with functional readouts, xenograft validation, single lab\",\n      \"pmids\": [\"37616575\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"CD200 expression on AML cells impairs cytokine secretion in both innate and adaptive immune cell subsets. Engineered CD200R-CD28 switch receptors on CAR T-cells leverage CD200-CD200R engagement to provide CD28 costimulation, improving CAR T-cell performance, while CD200R knockout was detrimental to CAR T-cell metabolic activity.\",\n      \"method\": \"Isogenic cell line models, in vitro co-culture with immune cells, CAR T-cell engineering, PBMC-humanized xenograft models, metabolic assays\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — CRISPR KO comparison, engineered switch receptor with functional readouts, xenograft validation, single lab\",\n      \"pmids\": [\"37616575\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"CD200-CD200R signaling between amacrine cells and microglia is dysregulated in early diabetic retinopathy (DR). Targeting CD200R attenuates high glucose-induced inflammation and phagocytosis in cultured microglia. In vivo CD200R targeting prevents visual dysfunction, microglia activation, and retinal inflammation in the diabetic mouse.\",\n      \"method\": \"Type I diabetes mouse model, retinal microglial ablation, flow cytometry (engulfment assays), in vitro high glucose microglial culture with CD200R targeting, in vivo CD200R targeting\",\n      \"journal\": \"Proceedings of the National Academy of Sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro and in vivo mechanistic link between CD200-CD200R and microglial phagocytosis/inflammation in DR, multiple readouts, single lab\",\n      \"pmids\": [\"37903272\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"The CD200-CD200R axis promotes squamous cell carcinoma (cSCC) metastasis via induction of cathepsin K (Ctsk) in CD200R+ infiltrating myeloid cells. CD200-CD200R engagement in a coculture system induced Ctsk expression in CD11b+Cd200R+ myeloid cells. Ctsk inhibition, but not MMP inhibition, blocked cSCC cell migration in vitro, and targeted CD200 disruption plus Ctsk pharmacologic inhibition reduced metastasis in vivo.\",\n      \"method\": \"RNA sequencing of sorted infiltrating myeloid cells, CD200-CD200R coculture system, Ctsk inhibition assays, in vitro migration assay, in vivo metastasis model with CD200 tumor cell disruption\",\n      \"journal\": \"Cancer research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — RNA-seq identifies Ctsk, coculture mechanistic confirmation, in vitro and in vivo functional validation, single lab with multiple methods\",\n      \"pmids\": [\"34183355\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"CD200+DKK3+ pro-resolving fibroblasts stabilize type 2 innate lymphoid cell (ILC2) phenotype and induce resolution of arthritis via CD200-CD200R1 signaling. Single-cell RNA-sequencing and spatial transcriptomics revealed that pro-resolving CD200+DKK3+ fibroblasts cluster with ILC2s in resolution niches, distinct from pro-inflammatory MMP3+/IL6+ fibroblast clusters.\",\n      \"method\": \"Biopsy-based single-cell RNA-sequencing, spatial transcriptomics, PET imaging of FAP, functional coculture assays, in vivo experimental arthritis models\",\n      \"journal\": \"Nature immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — scRNA-seq and spatial transcriptomics identifying specific cell clusters, in vivo functional model, CD200-CD200R1 pathway mechanistically linked to ILC2 phenotype stabilization, single lab\",\n      \"pmids\": [\"38396288\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"In rat vascular endothelium, CD200 expression is heterogeneous across vascular beds and regulated by LPS (decreased during endotoxemia). Physical binding between CD200 and CD200R is involved in T-cell adhesion to endothelium, but not in macrophage-endothelium interaction. A CD200 agonist peptide can trigger CD200R signaling in macrophages to suppress their adhesion to endothelium.\",\n      \"method\": \"Immunoelectron microscopy, in vivo LPS model, T-cell and macrophage adhesion assays, CD200 agonist peptide treatment\",\n      \"journal\": \"Journal of anatomy\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — adhesion assays with agonist peptide and blocking, localization by electron microscopy, single lab, limited mechanistic depth\",\n      \"pmids\": [\"19166481\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"CD200 is a broadly expressed cell-surface glycoprotein (Ig superfamily) that interacts via its N-terminal Ig domain with the structurally related inhibitory receptor CD200R on myeloid cells; upon ligand engagement, CD200R is tyrosine-phosphorylated at its NPLY motif, recruits adaptor Dok2 (and secondarily Dok1), which in turn activates RasGAP to suppress Ras/MAPK signaling—a pathway mechanistically distinct from ITIM-based inhibitory receptors—resulting in suppression of mast cell degranulation, cytokine production, and macrophage/microglial activation; additionally, CD200 undergoes metalloproteinase (MMP3/MMP11/ADAM28)-mediated ectodomain shedding to release biologically active soluble CD200, and γ-secretase cleavage of its cytoplasmic tail allows nuclear translocation and gene regulation; CD200 also regulates osteoclastogenesis by promoting macrophage fusion via RANK downstream signaling, inhibits NK cell and ILC2 activity, and suppresses microglial activation in the CNS through the CD200-CD200R1 axis, which is exploited by tumors and pathogens for immune evasion.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"CD200 (OX-2) is a cell-surface immunoglobulin-superfamily glycoprotein that functions as the ligand for the inhibitory receptor CD200R, delivering a tonic immunosuppressive signal that restrains myeloid and lymphoid effector activity [#0, #3]. CD200 engages CD200R through their N-terminal Ig-like domains in an immunological synapse-like cell-cell contact, a binding mode resolved at atomic resolution; the structurally related activating receptors (CD200RLa and family members that pair with DAP12) fail to bind CD200, defining a paired inhibitory/activatory receptor architecture in which CD200 is the exclusive CD200R ligand [#1, #3, #6, #10]. Receptor engagement triggers tyrosine phosphorylation of the CD200R cytoplasmic NPLY motif, which recruits the PTB-domain adaptor Dok2 (with higher affinity than Dok1), and Dok2 in turn recruits RasGAP to suppress Ras/MAPK (ERK, JNK, p38) signaling—a mechanism distinct from ITIM/SHP-based inhibition; Dok1 acting through CrkL negatively tunes this Dok2-RasGAP output [#2, #4, #5]. The downstream consequence is suppression of mast cell degranulation and cytokine production, dampening of NK, ILC2, and macrophage activation, and inhibition of macrophage phagocytosis of tumor cells, the latter mediated by the kinase Csk rather than SHP-1/SHP-2 [#2, #14, #16]. Beyond classical receptor signaling, CD200 generates a biologically active soluble ectodomain via metalloproteinase shedding (ADAM28, MMP3, MMP11) that binds and phosphorylates CD200R at a distance, and \\u03b3-secretase cleavage liberates the CD200 cytoplasmic tail, which translocates to the nucleus, binds DNA, and alters expression of proliferation-associated transcription factors [#11, #12, #13]. CD200 also acts developmentally to promote macrophage fusion and osteoclastogenesis through RANK-downstream NF-\\u03baB/MAPK signaling, and in the CNS its neuronal expression restrains microglial activation [#7, #8]. This inhibitory axis is co-opted in disease: tumors exploit it to suppress NK killing, expand MDSCs, and impair T-cell metabolism and cytokine output [#12, #18, #19].\",\n  \"teleology\": [\n    {\n      \"year\": 2003,\n      \"claim\": \"Establishing the receptor and its expression pattern was the prerequisite for any mechanism: it defined CD200R as the myeloid-restricted partner of CD200 and revealed a paired inhibitory/activatory receptor family.\",\n      \"evidence\": \"qRT-PCR, novel monoclonal antibodies, and protein binding assays across human and mouse\",\n      \"pmids\": [\"12960329\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not resolve the molecular signaling cascade downstream of CD200R\", \"Function of DAP12-pairing CD200R-like receptors left undefined\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Mapping the binding determinants showed the interaction occurs via the N-terminal Ig domains, establishing the structural basis and an evolutionary origin from a homotypic interaction.\",\n      \"evidence\": \"Site-directed mutagenesis with recombinant protein binding assays\",\n      \"pmids\": [\"15162439\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Atomic-resolution interface not yet determined\", \"Affinity not quantified\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Dissecting CD200R signaling answered how an Ig-domain ligand inhibits effector cells: it works through Dok adaptors and RasGAP to suppress Ras/MAPK, a non-ITIM mechanism, with a functional readout in mast cells.\",\n      \"evidence\": \"Co-IP, phosphorylation assays, and mast cell degranulation/cytokine assays in mouse BMMCs\",\n      \"pmids\": [\"15557172\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relative contributions of Dok1 vs Dok2 not yet resolved\", \"Demonstrated in mouse mast cells only\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Quantifying binding established CD200 as the exclusive ligand of inhibitory CD200R with micromolar affinity and no binding to the activating CD200R-related receptors, ruling out ligand promiscuity.\",\n      \"evidence\": \"Direct binding assays with monomeric/dimeric fusion proteins and cell-surface binding\",\n      \"pmids\": [\"16081818\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Endogenous ligand(s) of the activating receptors not identified\", \"Physiological consequence of low (~4 \\u00b5M) affinity unaddressed\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Defining the human cytoplasmic motif resolved adaptor hierarchy: the NPLY motif recruits Dok2 (not Dok1 or SHIP) as the essential effector linking to RasGAP, confirmed by loss-of-function.\",\n      \"evidence\": \"NPLY mutational analysis, affinity measurements, and RNAi knockdown with functional inhibition readouts in U937 cells\",\n      \"pmids\": [\"19786546\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the kinase phosphorylating NPLY unspecified\", \"Did not address signaling in primary human myeloid cells\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Refining the adaptor network showed Dok1 acts as a negative regulator of Dok2-mediated signaling via CrkL, explaining how inhibitory output is tuned.\",\n      \"evidence\": \"Reciprocal Co-IP and RNAi knockdown with phosphorylation readouts in U937 cells\",\n      \"pmids\": [\"21078907\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological setting where Dok1 tuning matters not established\", \"Single cell-line system\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Crystal structures gave the definitive interface and explained, at the residue level, why the activating receptor cannot bind CD200 and how herpesviruses mimic the host ligand.\",\n      \"evidence\": \"X-ray crystallography of CD200R, CD200RLa, and the CD200R-CD200 complex\",\n      \"pmids\": [\"23602662\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Does not address signaling dynamics or receptor clustering in cells\", \"Viral mimic functional consequences not tested here\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"A genetic KO revealed a developmental, non-immunosuppressive role: CD200 promotes macrophage fusion and osteoclastogenesis through RANK-downstream signaling, broadening CD200 function beyond inhibition.\",\n      \"evidence\": \"CD200-/- mice, recombinant protein rescue, shRNA knockdown, and bone histomorphometry with NF-\\u03baB/MAPK analysis\",\n      \"pmids\": [\"17726108\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How a normally inhibitory ligand drives a positive fusion signal mechanistically unresolved\", \"Direct CD200R-to-RANK signaling crosstalk not defined\"]\n    },\n    {\n      \"year\": 1984,\n      \"claim\": \"Early localization established neuronal-specific surface expression of CD200 in the CNS, later providing the anatomical basis for its role restraining microglia.\",\n      \"evidence\": \"Immunoperoxidase and double-immunofluorescence of rat brain and cerebellar cultures with cell-type markers\",\n      \"pmids\": [\"6147390\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No receptor or mechanism identified at the time\", \"Functional consequence of neuronal expression not tested\"]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"An early functional study placed CD200 in T-cell costimulation through a non-B7/CD28, non-cytokine-inducing pathway, distinguishing it from classical costimulatory ligands.\",\n      \"evidence\": \"CHO transfectants, T-cell proliferation and cytokine assays, CTLA4-Ig blocking\",\n      \"pmids\": [\"9144466\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Receptor mediating this costimulation not identified\", \"Apparent costimulation contrasts with later inhibitory model and was not reconciled\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Characterization of an activating CD200R-like receptor (CD200RL3) that pairs with DAP12 fleshed out the paired-receptor family but showed it does not bind CD200.\",\n      \"evidence\": \"Microarray screen, splice variant and DAP12-association analysis\",\n      \"pmids\": [\"15471863\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Endogenous ligand of the activating receptor unidentified\", \"In vivo function not established\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"A mechanism for CNS immunosuppression was identified: CD200-Fc downregulates macrophage TLR4 to restrain phagocytosis and improve myelination after ischemia.\",\n      \"evidence\": \"Primary phagocytosis assays and an in vivo white matter ischemia model with TLR4 and myelination readouts\",\n      \"pmids\": [\"26661156\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Link between CD200R signaling and TLR4 downregulation mechanistically incomplete\", \"CD200-Fc may not recapitulate membrane-bound ligand\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Demonstrating that shed soluble CD200 is functionally active established a paracrine mode: the ectodomain alone binds and phosphorylates CD200R, with ADAM28 implicated as a sheddase.\",\n      \"evidence\": \"Domain-specific Western blot, flow cytometry, ADAM28 silencing, and CD200R1-HEK293 functional assay with PMA stimulation\",\n      \"pmids\": [\"27111430\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Full set of sheddases not defined\", \"Physiological abundance and reach of soluble CD200 in vivo unquantified\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"An intracellular signaling role was uncovered: \\u03b3-secretase releases the CD200 cytoplasmic tail, which enters the nucleus, binds DNA, and reprograms proliferation-associated transcription factors.\",\n      \"evidence\": \"\\u03b3-secretase inhibitor experiments, ChIP-seq, nuclear translocation assays, and CD200C-tail transfection in HEK293\",\n      \"pmids\": [\"29698858\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct DNA-binding mode of a non-canonical tail fragment not structurally defined\", \"Physiological relevance versus overexpression artifact in HEK293 untested\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Pathway epistasis positioned PPAR-\\u03b3 upstream of CD200-CD200R1, explaining how an anti-inflammatory lipid ligand maintains the axis to protect neurons.\",\n      \"evidence\": \"Neuron-microglia cocultures with 15d-PGJ2 treatment, LPS/IFN-\\u03b3 stimulation, and CD200-CD200R1 blocking antibodies\",\n      \"pmids\": [\"24639050\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct transcriptional regulation of CD200/CD200R by PPAR-\\u03b3 not demonstrated\", \"In vivo confirmation lacking\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"ILC2 work extended the inhibitory axis to type 2 immunity, showing CD200R engagement suppresses ILC2 activation via canonical and non-canonical NF-\\u03baB, with therapeutic effect on airway hyperreactivity.\",\n      \"evidence\": \"In vitro ILC2 activation assays, NF-\\u03baB analysis, and humanized mouse asthma models\",\n      \"pmids\": [\"33953190\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"How CD200R-Dok2-RasGAP signaling intersects NF-\\u03baB not reconciled\", \"Single-lab models\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"A tumor-immune-evasion mechanism was dissected: MMP3/MMP11-shed soluble CD200 binds NK-cell CD200R, suppressing MAPK and, via PPAR\\u03b3-Fas/FADD/FasL, driving activation-induced NK apoptosis.\",\n      \"evidence\": \"Sheddase identification in basal cell carcinoma, NK ERK/PPAR\\u03b3/Fas pathway analysis, and CD200 blockade rescue\",\n      \"pmids\": [\"36074574\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Relative contributions of distinct sheddases across tissues unresolved\", \"Generalizability beyond BCC NK cells untested\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Comparing inhibitory phagocytosis checkpoints showed CD200R1 uses the kinase Csk rather than SHP-1/SHP-2, making CD200-CD200R1 a mechanistically distinct, additive target alongside SIRP\\u03b1-CD47.\",\n      \"evidence\": \"CRISPR/genetic KO of CD200R1 and CD200, macrophage phagocytosis assays, Csk vs SHP signaling analysis, and combined-blockade tumor models\",\n      \"pmids\": [\"40461553\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How Csk integrates with the Dok2-RasGAP module not delineated\", \"Clinical translatability of combined blockade unestablished\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Cancer-context studies established CD200 as an immunosuppressive driver across tumor types\\u2014expanding MDSCs, impairing T-cell metabolism/cytokine output, and inducing cathepsin K in myeloid cells to promote metastasis\\u2014and validated CD200R-CD28 switch receptors as a therapeutic conversion strategy.\",\n      \"evidence\": \"Recombinant CD200 and isogenic models, scRNA-seq/spatial transcriptomics, T-cell metabolic and CAR-T switch-receptor assays, and in vivo blockade/xenograft models\",\n      \"pmids\": [\"32581043\", \"34326171\", \"37616575\", \"34183355\", \"37903272\", \"38396288\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Cell-type-specific downstream effectors (e.g., Ctsk induction) mechanistically incomplete\", \"Many findings are single-lab disease-context observations not cross-validated\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the multiple modes of CD200 action\\u2014membrane CD200R-Dok2-RasGAP signaling, Csk-dependent phagocytosis control, soluble-ectodomain paracrine signaling, and \\u03b3-secretase-released nuclear tail\\u2014are integrated within a single cell, and how context dictates which mode dominates, remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No unified model linking forward CD200R signaling and reverse intracellular CD200 signaling\", \"Stoichiometry and in vivo relevance of soluble vs membrane CD200 unquantified\", \"Effector divergence (RasGAP vs Csk vs NF-\\u03baB) across cell types not mechanistically reconciled\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0048018\", \"supporting_discovery_ids\": [0, 3, 6, 11]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [2, 4, 16]},\n      {\"term_id\": \"GO:0003677\", \"supporting_discovery_ids\": [13]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 8, 11]},\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [11, 12]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [13]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [0, 2, 14, 16]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [2, 4, 5, 16]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [12, 18, 19]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"CD200R1\", \"DOK2\", \"DOK1\", \"RASA1\", \"CSK\", \"CRKL\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}