{"gene":"XCL1","run_date":"2026-06-11T09:02:06","timeline":{"discoveries":[{"year":2019,"finding":"The extreme N-terminal residues of XCL1 (Val1, Gly2, Ser3, Glu4) contribute a large fraction of binding energy to XCR1, while residues near the disulfide-bond-forming Cys11 modulate XCR1 activation. Alterations in the XCL1 N-terminus changed XCR1 activation as measured by IP3 accumulation, intracellular calcium release, and directed cell migration. Computational modeling combined with mutagenesis identified Glu4 of XCL1 and Tyr117/Arg273 of XCR1 as key functional contacts; mutation of Tyr117 and Arg273 diminished XCR1 binding and activation.","method":"Structure-function mutagenesis combined with Rosetta homology modeling; IP3 accumulation assay, intracellular calcium release assay, directed cell migration assay, receptor mutagenesis","journal":"Science signaling","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple orthogonal functional assays (IP3, calcium, migration) plus mutagenesis of both ligand and receptor, in a single rigorous study","pmids":["31481523"],"is_preprint":false},{"year":2020,"finding":"XCL1 is a metamorphic protein that interconverts between its canonical α-β chemokine fold (which binds GPCR XCR1 and controls cell migration) and a β-sheet dimer fold. Only the β-sheet and unfolded XCL1 structures induce negative Gaussian curvature (NGC) in membranes—the topology required for membrane permeation—as determined by SAXS. The β-sheet structure selectively remodels bacterial and fungal (but not mammalian) model membranes, and XCL1 exhibits anti-Candida activity in vitro.","method":"Small angle X-ray scattering (SAXS) of XCL1 structural variants with defined membrane compositions; in vitro anti-Candida activity assay","journal":"ACS infectious diseases","confidence":"High","confidence_rationale":"Tier 1 / Moderate — SAXS structural characterization with functional validation (membrane selectivity, antifungal activity), single lab but multiple orthogonal methods","pmids":["32243126"],"is_preprint":false},{"year":2016,"finding":"The XCL1 dimer (XCL1dim) binds glycosaminoglycans (GAGs) via basic residues Arg23 and Arg43; mutation of these residues greatly diminished heparin binding in heparin Sepharose chromatography and surface plasmon resonance. The GAG binding affinity correlates with the length and sulfation level of heparan sulfate oligosaccharides. This GAG-binding surface on XCL1dim also includes residues important for HIV-1 inhibition through gp120 interaction.","method":"Site-directed mutagenesis of XCL1 dimer variant (W55D); heparin Sepharose chromatography; surface plasmon resonance; solution fluorescence polarization assay with heparan sulfate oligosaccharides of varying length/sulfation","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro biochemical reconstitution with mutagenesis, two orthogonal binding assays (SPR + chromatography), single lab","pmids":["26836755"],"is_preprint":false},{"year":1996,"finding":"There are two highly homologous SCM-1/lymphotactin (XCL1) genes, designated SCM-1alpha (XCL1) and SCM-1beta (XCL2), both with three exons and two introns, both mapped to human chromosome 1q23. Both genes are similarly induced in peripheral blood mononuclear cells by mitogenic stimulation. The two mature proteins differ at amino acid positions 7 and 8. SCM-1alpha's first intron contains a pseudogene of ribosomal large subunit L7a; SCM-1beta has a 1.5-kb deletion in that intron.","method":"Gene cloning and sequencing; chromosomal mapping; primer extension and RNase protection for transcription start sites; mitogenic stimulation of PBMCs","journal":"FEBS letters","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct gene characterization with multiple molecular methods (sequencing, mapping, expression induction), single lab","pmids":["8849694"],"is_preprint":false},{"year":2019,"finding":"XCL1 fusion vaccines specifically target XCR1+ cDC1s (CD8α+ DCs in mice; CD141+ DCs in humans) and promote their IL-12 production. After subcutaneous injection, XCL1-fused antigen (GPC3) was predominantly detected in CD8α+ DCs of draining lymph nodes. XCL1-GPC3 targeted DCs enhanced antigen-specific CD8+ T cell proliferation and induced de novo GPC3-specific CD8+ T cells. Intradermal but not intramuscular delivery of Xcl1-HA fusion vaccine maintained Th1-polarized responses and induced higher IFNγ-secreting T cells compared to Ccl3-HA; Xcl1-HA conferred superior protection against influenza infection after intradermal immunization.","method":"In vitro DC chemoattraction assay; IL-12 production assay; in vivo plasmid immunization; flow cytometry for DC and T cell phenotyping; in vivo cytotoxicity assay; viral challenge protection assay","journal":"Cancer immunology research / Scientific reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple in vitro and in vivo functional assays, single lab, two separate papers corroborating XCL1 targeting of cDC1s","pmids":["31666238","30755656"],"is_preprint":false},{"year":2019,"finding":"XCL1 secreted by small intestine lamina propria γδ T cells (upon oral anti-CD3 administration) induces tolerogenic XCR1+ DC migration to the mesenteric lymph node, where Treg cells are induced and oral tolerance is established. TCRδ−/− mice failed to develop oral tolerance upon oral anti-CD3, and this tolerance mechanism is distinct from that induced by fed antigens.","method":"Genetic knockout models (TCRδ−/− mice); DC depletion experiments; in vivo oral anti-CD3 administration; flow cytometry for Treg cell quantification; EAE model for functional validation","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis (TCRδ KO), in vivo functional readout (tolerance, Treg induction), single lab with multiple orthogonal approaches","pmids":["31578268"],"is_preprint":false},{"year":2018,"finding":"The XCL1-XCR1 pathway promotes trophoblast invasion by increasing MMP-9 and MMP-2 activity (including the MMP-2/TIMP-2 complex) via the PI3K/AKT, MEK, and JNK signaling pathways. XCL1 and XCR1 are expressed at higher levels in first-trimester than in term placenta.","method":"Wound healing assay; Transwell invasion assay with recombinant XCL1; qRT-PCR; gelatin zymography for MMP activity; PI3K, MEK, and JNK pathway inhibitor experiments","journal":"American journal of reproductive immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple functional assays with defined pathway inhibitors and enzymatic readout (zymography), single lab","pmids":["29856101"],"is_preprint":false},{"year":2020,"finding":"XCL1 treatment of MDA-MB-231 and SK-BR-3 breast cancer cells enhanced cell migration and EMT (E-cadherin downregulation, N-cadherin and vimentin upregulation, β-catenin nuclear translocation) via XCR1-mediated activation of the ERK/HIF-1α pathway. Knockdown of XCR1 by siRNA negated XCL1 effects. MEK1/2 inhibitor U0126 blocked XCL1-induced HIF-1α accumulation and cell migration.","method":"Cell migration assay; Western blot for EMT markers and signaling proteins; siRNA knockdown of XCR1; pharmacological inhibition of MEK1/2 with U0126","journal":"International journal of molecular sciences","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — siRNA knockdown plus pharmacological inhibition to place XCL1 in a defined signaling pathway, single lab, multiple cell lines","pmids":["33374849"],"is_preprint":false},{"year":2020,"finding":"XCL1 promotes osteoclastogenesis and bone-resorbing activity: recombinant XCL1 added to RANKL-stimulated human monocytes promoted osteoclast differentiation and resorbing activity. XCL1 also promoted expression of inflammatory and osteoclastogenic factors (IL-6, IL-8, RANKL) in human osteoblasts. In vivo, injection of recombinant XCL1 onto murine calvaria worsened osteolytic lesions, while XCL1-neutralizing antibody significantly reduced bone erosion and osteoclast numbers induced by UHMWPE particles.","method":"In vitro osteoclast differentiation assay from human monocytes + RANKL; in vivo murine calvarial osteolysis model with XCL1 injection and neutralizing antibody; Western blot; ELISA for cytokines","journal":"Frontiers in immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro and in vivo concordant functional experiments with neutralizing antibody, single lab","pmids":["32849609"],"is_preprint":false},{"year":2016,"finding":"In a mouse model of streptozotocin-induced diabetic neuropathy, spinal XCL1 and XCR1 levels are upregulated. Intrathecal injection of XCL1 in naïve mice enhanced nociceptive transmission; injection of XCL1-neutralizing antibody diminished allodynia/hyperalgesia in diabetic mice. XCR1 is co-expressed with spinal neurons. Microglia activation in primary cultures led to enhanced XCL1 release and XCR1 expression.","method":"Streptozotocin mouse model; intrathecal injection of recombinant XCL1 and neutralizing antibody; von Frey and cold plate behavioral tests; Western blot; immunofluorescence; primary microglial cell cultures","journal":"Anesthesiology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — pharmacological intervention (neutralizing antibody) with defined behavioral readout and cellular mechanism (microglia), single lab","pmids":["27387353"],"is_preprint":false},{"year":2022,"finding":"In neuropathic pain (CCI model), XCL1 is released by spinal cord astroglial cells despite neural localization of its two receptors, XCR1 and ITGA9. Blockade/neutralization of both XCR1 (with vMIP-II) and ITGA9 (with YA4 antibody) reversed hypersensitivity after intrathecal XCL1 in naïve mice, with ITGA9 neutralization more effective. Neutralization of ITGA9 enhanced both buprenorphine and morphine analgesia; XCR1 blockade improved buprenorphine effectiveness.","method":"CCI neuropathic pain mouse model; intrathecal injection of antagonists (vMIP-II for XCR1) and neutralizing antibodies (YA4 for ITGA9); von Frey and cold plate tests; RT-qPCR; Western blot; ELISA; immunofluorescence","journal":"Frontiers in immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — two-receptor pharmacological dissection with defined behavioral readouts and cellular localization, single lab","pmids":["36618360"],"is_preprint":false},{"year":2025,"finding":"The CD8+ T cell-derived chemokine XCL1 is critical for tissue-resident memory T cell (TRM) formation and supports positioning of intestinal CD8+ T cells during acute viral infection via XCR1+ cDC1s. In tumors, enforced Xcl1 expression by antigen-specific CD8+ T cells promoted intratumoral cDC1 accumulation and T cell persistence, improving overall survival. XCL1 acts non-cell autonomously to guide intestinal CD8+ TRM spatial differentiation.","method":"Murine genetic models (Xcl1-deficient); targeted spatial transcriptomics; adoptive transfer with enforced Xcl1 expression; tumor survival experiments","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic loss-of-function plus gain-of-function with spatial transcriptomics and survival readout, multiple orthogonal approaches in one study","pmids":["39841133"],"is_preprint":false},{"year":2024,"finding":"Intratumoral delivery of a highly active locked-disulfide form of XCL1 (mXCL1-V21C/A59C) via hydrophilic gel patch increased cDC1 accumulation in tumor masses and promoted their migration to regional lymph nodes, resulting in enhanced tumor-specific CTL induction. Tumor-infiltrating cDC1s produced CXCL9 (a CXCR3 ligand for CTLs/NK cells), and anti-CXCL9 treatment decreased CTL tumor infiltration. The approach reduced tumor growth and prolonged survival in two murine tumor models and was enhanced by anti-PD-1 combination.","method":"In vivo tumor models (E.G7-OVA, B16-F10); hydrophilic gel patch delivery of engineered XCL1; flow cytometry for immune cell phenotyping; anti-CXCL9 depletion experiment; tumor growth and survival measurement","journal":"International journal of cancer","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple in vivo tumor models with mechanistic depletion experiment (anti-CXCL9), single lab","pmids":["38346928"],"is_preprint":false},{"year":2025,"finding":"XCL1 interconverts between two distinct native structures: a chemokine fold with high affinity for GPCR XCR1 (mediating cell migration and immune signaling) and an alternate fold that binds GAGs and exhibits antimicrobial activity. This metamorphic behavior is enabled by the absence of a conserved disulfide bond present in all other chemokines.","method":"Review of structure-function data (citing prior NMR, SAXS, mutagenesis, and functional studies)","journal":"Protein science","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — synthesizes prior structural and functional experimental evidence; the mechanistic claims are grounded in cited experimental work, but this paper itself is a review","pmids":["39840812"],"is_preprint":false},{"year":2021,"finding":"XCL1 overexpression in high-glucose-treated human renal glomerular endothelial cells promoted apoptosis and inflammatory response (IL-1β, TNF-α upregulation), and these effects were associated with upregulation of p53 and NF-κB pathway changes. p53 silencing (with pifithrin-α) blocked apoptosis and inflammation in XCL1-overexpressed HG-treated cells; pifithrin-α also reversed XCL1-induced NF-κB downregulation.","method":"Cell viability assay (CCK-8); flow cytometry for apoptosis; ELISA and Western blot for inflammatory cytokines and p53/NF-κB; XCL1 overexpression vector; p53 inhibitor pifithrin-α; in vivo db/db diabetic mouse model","journal":"Nephron","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single overexpression approach with pharmacological inhibitor, single lab, limited mechanistic resolution of XCL1's direct molecular action","pmids":["34518457"],"is_preprint":false}],"current_model":"XCL1 is a metamorphic chemokine that reversibly interconverts between a canonical α-β chemokine fold—which binds its sole GPCR, XCR1, via N-terminal residues (Val1-Glu4) to trigger IP3/calcium signaling and directed cell migration—and a β-sheet dimer fold that binds glycosaminoglycans (via Arg23/Arg43) and disrupts bacterial and fungal membranes by inducing negative Gaussian curvature; in the immune system, XCL1 produced by CD8+ T cells, NK cells, and γδ T cells selectively recruits XCR1+ cDC1 dendritic cells to drive CTL priming, tissue-resident memory T cell formation, and oral tolerance, while also signaling through a second receptor, ITGA9, to modulate nociceptive transmission in neuropathic pain."},"narrative":{"mechanistic_narrative":"XCL1 is a metamorphic chemokine that reversibly interconverts between a canonical α-β chemokine fold, which engages its GPCR XCR1 to drive directed cell migration, and an alternate β-sheet dimer fold that binds glycosaminoglycans and remodels microbial membranes, a behavior enabled by its lack of a disulfide bond conserved in other chemokines [PMID:32243126, PMID:39840812]. In the chemokine fold, the extreme N-terminal residues Val1-Glu4 contribute the bulk of XCR1 binding energy, with Glu4 of XCL1 contacting Tyr117/Arg273 of XCR1 to trigger IP3 accumulation, calcium release, and chemotaxis [PMID:31481523]. The β-sheet dimer binds heparan sulfate via basic residues Arg23 and Arg43 in a length- and sulfation-dependent manner, and selectively induces negative Gaussian curvature in bacterial and fungal membranes to confer antimicrobial activity [PMID:32243126, PMID:26836755]. Functionally, XCL1 selectively recruits XCR1+ cDC1 dendritic cells: it is produced by CD8+ T cells, γδ T cells, and other sources to drive cDC1-dependent CTL priming, tissue-resident memory T cell formation and positioning, and tolerogenic DC migration that establishes oral tolerance [PMID:31666238, PMID:30755656, PMID:31578268, PMID:39841133, PMID:38346928]. Beyond immune cell positioning, XCL1-XCR1 signaling promotes trophoblast invasion and epithelial-mesenchymal transition through PI3K/AKT, MEK/ERK, and HIF-1α pathways [PMID:29856101, PMID:33374849], drives osteoclastogenesis [PMID:32849609], and acts in the spinal cord—via both XCR1 and a second receptor, ITGA9—to enhance nociceptive transmission in neuropathic pain [PMID:27387353, PMID:36618360].","teleology":[{"year":1996,"claim":"Establishing that XCL1 (SCM-1alpha/lymphotactin) is an inducible, immune-expressed gene defined the molecular identity and chromosomal context of the chemokine before its receptor and mechanism were known.","evidence":"Gene cloning, sequencing, chromosomal mapping, and mitogenic induction in PBMCs","pmids":["8849694"],"confidence":"Medium","gaps":["No receptor or signaling mechanism defined","Functional role of the L7a pseudogene in intron 1 unresolved"]},{"year":2016,"claim":"Identifying that the XCL1 dimer binds glycosaminoglycans through Arg23/Arg43 distinguished a GAG-binding surface separable from receptor activation and linked it to gp120/HIV-1 inhibition.","evidence":"Site-directed mutagenesis of XCL1 dimer variant, heparin chromatography, SPR, and fluorescence polarization with HS oligosaccharides","pmids":["26836755"],"confidence":"High","gaps":["In vivo relevance of GAG binding to immune positioning not tested","Relationship between GAG binding and the metamorphic fold equilibrium not directly resolved here"]},{"year":2016,"claim":"Demonstrating that spinal XCL1/XCR1 are upregulated and that XCL1 enhances nociception extended XCL1 function beyond classical immune chemotaxis into pain neurotransmission.","evidence":"Streptozotocin diabetic neuropathy model, intrathecal recombinant XCL1 and neutralizing antibody, behavioral testing, and primary microglial cultures","pmids":["27387353"],"confidence":"Medium","gaps":["Whether the neuronal effect is direct via XCR1 on neurons was not fully resolved","A second receptor was not yet considered"]},{"year":2018,"claim":"Placing XCL1-XCR1 upstream of MMP activity and PI3K/AKT, MEK, and JNK signaling showed the axis can drive cellular invasion in a non-immune (trophoblast) context.","evidence":"Transwell invasion and wound healing with recombinant XCL1, gelatin zymography, and pathway inhibitor experiments","pmids":["29856101"],"confidence":"Medium","gaps":["Direct receptor dependence not tested by XCR1 knockdown","In vivo placental requirement unestablished"]},{"year":2019,"claim":"Mapping the XCL1 N-terminus (Val1-Glu4) as the major XCR1 binding determinant and defining Glu4–Tyr117/Arg273 contacts resolved how the chemokine fold activates its GPCR at residue resolution.","evidence":"Rosetta homology modeling with ligand and receptor mutagenesis, IP3, calcium, and migration assays","pmids":["31481523"],"confidence":"High","gaps":["No experimental high-resolution XCL1-XCR1 complex structure","Conformational state of XCL1 during receptor engagement not directly visualized"]},{"year":2019,"claim":"Showing that XCL1 fusion vaccines and γδ T cell-derived XCL1 selectively target XCR1+ cDC1s established the axis as a tunable route for CTL priming and tolerogenic DC migration.","evidence":"In vitro DC chemoattraction and IL-12 assays, in vivo fusion vaccine immunization, TCRδ−/− knockout, DC depletion, and EAE/influenza functional readouts","pmids":["31666238","30755656","31578268"],"confidence":"Medium","gaps":["Molecular basis of cDC1 selectivity beyond XCR1 expression not dissected","Quantitative contribution of different XCL1 cellular sources unresolved"]},{"year":2020,"claim":"Defining XCL1 as a metamorphic protein whose β-sheet and unfolded forms induce negative Gaussian curvature revealed a structurally distinct, receptor-independent antimicrobial function.","evidence":"SAXS of XCL1 structural variants against defined membranes and in vitro anti-Candida assay","pmids":["32243126"],"confidence":"High","gaps":["In vivo antimicrobial relevance not demonstrated","Regulation of the fold equilibrium in tissue not defined"]},{"year":2020,"claim":"Placing XCL1-XCR1 on the ERK/HIF-1α/EMT axis and showing osteoclastogenic activity broadened the signaling outputs of the chemokine into tumor cell migration and bone resorption.","evidence":"XCR1 siRNA knockdown and MEK1/2 inhibition in breast cancer lines; in vitro and in vivo osteoclast/calvarial models with neutralizing antibody","pmids":["33374849","32849609"],"confidence":"Medium","gaps":["Direct versus indirect cellular targets in bone not fully separated","Tumor-intrinsic XCR1 signaling not validated in vivo"]},{"year":2022,"claim":"Identifying ITGA9 as a second functional XCL1 receptor in the spinal cord, more effective than XCR1 in driving hypersensitivity, redefined XCL1 nociceptive signaling as a two-receptor system.","evidence":"CCI neuropathic pain model with XCR1 antagonist (vMIP-II) and ITGA9-neutralizing antibody (YA4), behavioral tests, and analgesic potentiation experiments","pmids":["36618360"],"confidence":"Medium","gaps":["Direct XCL1-ITGA9 binding interface not mapped","Whether ITGA9 signaling operates outside the nervous system unknown"]},{"year":2025,"claim":"Genetic loss- and gain-of-function established XCL1 as a non-cell-autonomous cue that, via cDC1s, controls tissue-resident memory T cell formation and spatial positioning during infection and in tumors.","evidence":"Xcl1-deficient mice, targeted spatial transcriptomics, adoptive transfer with enforced Xcl1, and tumor survival experiments","pmids":["39841133"],"confidence":"High","gaps":["Spatial cues integrating XCL1 with other positioning signals not fully defined","Human translation of TRM positioning role untested"]},{"year":null,"claim":"How the metamorphic fold equilibrium is regulated in vivo to partition XCL1 between XCR1/ITGA9 receptor signaling, GAG binding, and antimicrobial membrane activity remains unresolved.","evidence":"No timeline study directly links conformational state to functional outcome in a physiological setting","pmids":[],"confidence":"Medium","gaps":["No in vivo measurement of fold state coupled to function","Triggers and tissue determinants of fold switching unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0048018","term_label":"receptor ligand activity","supporting_discovery_ids":[0,1,13]},{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[0,10]},{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[1,2]},{"term_id":"GO:0090729","term_label":"toxin activity","supporting_discovery_ids":[1]}],"localization":[{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[4,5,8,10]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0,6,7]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[4,5,11,12]},{"term_id":"R-HSA-1500931","term_label":"Cell-Cell communication","supporting_discovery_ids":[4,11]}],"complexes":[],"partners":["XCR1","ITGA9"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P47992","full_name":"Lymphotactin","aliases":["ATAC","C motif chemokine 1","Cytokine SCM-1","Lymphotaxin","SCM-1-alpha","Small-inducible cytokine C1","XC chemokine ligand 1"],"length_aa":114,"mass_kda":12.5,"function":"Chemotactic activity for lymphocytes but not for monocytes or neutrophils. In thymus, mediates medullary accumulation of thymic dendritic cells and contributes to regulatoy T cell development, playing a role in self-tolerance establishment","subcellular_location":"Secreted","url":"https://www.uniprot.org/uniprotkb/P47992/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/XCL1","classification":"Not Classified","n_dependent_lines":18,"n_total_lines":1047,"dependency_fraction":0.017191977077363897},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/XCL1","total_profiled":1310},"omim":[{"mim_id":"600552","title":"CHEMOKINE, C MOTIF, RECEPTOR 1; XCR1","url":"https://www.omim.org/entry/600552"},{"mim_id":"600250","title":"CHEMOKINE, C MOTIF, LIGAND 1; XCL1","url":"https://www.omim.org/entry/600250"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"lymphoid 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Alterations in the XCL1 N-terminus changed XCR1 activation as measured by IP3 accumulation, intracellular calcium release, and directed cell migration. Computational modeling combined with mutagenesis identified Glu4 of XCL1 and Tyr117/Arg273 of XCR1 as key functional contacts; mutation of Tyr117 and Arg273 diminished XCR1 binding and activation.\",\n      \"method\": \"Structure-function mutagenesis combined with Rosetta homology modeling; IP3 accumulation assay, intracellular calcium release assay, directed cell migration assay, receptor mutagenesis\",\n      \"journal\": \"Science signaling\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple orthogonal functional assays (IP3, calcium, migration) plus mutagenesis of both ligand and receptor, in a single rigorous study\",\n      \"pmids\": [\"31481523\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"XCL1 is a metamorphic protein that interconverts between its canonical α-β chemokine fold (which binds GPCR XCR1 and controls cell migration) and a β-sheet dimer fold. Only the β-sheet and unfolded XCL1 structures induce negative Gaussian curvature (NGC) in membranes—the topology required for membrane permeation—as determined by SAXS. The β-sheet structure selectively remodels bacterial and fungal (but not mammalian) model membranes, and XCL1 exhibits anti-Candida activity in vitro.\",\n      \"method\": \"Small angle X-ray scattering (SAXS) of XCL1 structural variants with defined membrane compositions; in vitro anti-Candida activity assay\",\n      \"journal\": \"ACS infectious diseases\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — SAXS structural characterization with functional validation (membrane selectivity, antifungal activity), single lab but multiple orthogonal methods\",\n      \"pmids\": [\"32243126\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"The XCL1 dimer (XCL1dim) binds glycosaminoglycans (GAGs) via basic residues Arg23 and Arg43; mutation of these residues greatly diminished heparin binding in heparin Sepharose chromatography and surface plasmon resonance. The GAG binding affinity correlates with the length and sulfation level of heparan sulfate oligosaccharides. This GAG-binding surface on XCL1dim also includes residues important for HIV-1 inhibition through gp120 interaction.\",\n      \"method\": \"Site-directed mutagenesis of XCL1 dimer variant (W55D); heparin Sepharose chromatography; surface plasmon resonance; solution fluorescence polarization assay with heparan sulfate oligosaccharides of varying length/sulfation\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro biochemical reconstitution with mutagenesis, two orthogonal binding assays (SPR + chromatography), single lab\",\n      \"pmids\": [\"26836755\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"There are two highly homologous SCM-1/lymphotactin (XCL1) genes, designated SCM-1alpha (XCL1) and SCM-1beta (XCL2), both with three exons and two introns, both mapped to human chromosome 1q23. Both genes are similarly induced in peripheral blood mononuclear cells by mitogenic stimulation. The two mature proteins differ at amino acid positions 7 and 8. SCM-1alpha's first intron contains a pseudogene of ribosomal large subunit L7a; SCM-1beta has a 1.5-kb deletion in that intron.\",\n      \"method\": \"Gene cloning and sequencing; chromosomal mapping; primer extension and RNase protection for transcription start sites; mitogenic stimulation of PBMCs\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct gene characterization with multiple molecular methods (sequencing, mapping, expression induction), single lab\",\n      \"pmids\": [\"8849694\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"XCL1 fusion vaccines specifically target XCR1+ cDC1s (CD8α+ DCs in mice; CD141+ DCs in humans) and promote their IL-12 production. After subcutaneous injection, XCL1-fused antigen (GPC3) was predominantly detected in CD8α+ DCs of draining lymph nodes. XCL1-GPC3 targeted DCs enhanced antigen-specific CD8+ T cell proliferation and induced de novo GPC3-specific CD8+ T cells. Intradermal but not intramuscular delivery of Xcl1-HA fusion vaccine maintained Th1-polarized responses and induced higher IFNγ-secreting T cells compared to Ccl3-HA; Xcl1-HA conferred superior protection against influenza infection after intradermal immunization.\",\n      \"method\": \"In vitro DC chemoattraction assay; IL-12 production assay; in vivo plasmid immunization; flow cytometry for DC and T cell phenotyping; in vivo cytotoxicity assay; viral challenge protection assay\",\n      \"journal\": \"Cancer immunology research / Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple in vitro and in vivo functional assays, single lab, two separate papers corroborating XCL1 targeting of cDC1s\",\n      \"pmids\": [\"31666238\", \"30755656\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"XCL1 secreted by small intestine lamina propria γδ T cells (upon oral anti-CD3 administration) induces tolerogenic XCR1+ DC migration to the mesenteric lymph node, where Treg cells are induced and oral tolerance is established. TCRδ−/− mice failed to develop oral tolerance upon oral anti-CD3, and this tolerance mechanism is distinct from that induced by fed antigens.\",\n      \"method\": \"Genetic knockout models (TCRδ−/− mice); DC depletion experiments; in vivo oral anti-CD3 administration; flow cytometry for Treg cell quantification; EAE model for functional validation\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis (TCRδ KO), in vivo functional readout (tolerance, Treg induction), single lab with multiple orthogonal approaches\",\n      \"pmids\": [\"31578268\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"The XCL1-XCR1 pathway promotes trophoblast invasion by increasing MMP-9 and MMP-2 activity (including the MMP-2/TIMP-2 complex) via the PI3K/AKT, MEK, and JNK signaling pathways. XCL1 and XCR1 are expressed at higher levels in first-trimester than in term placenta.\",\n      \"method\": \"Wound healing assay; Transwell invasion assay with recombinant XCL1; qRT-PCR; gelatin zymography for MMP activity; PI3K, MEK, and JNK pathway inhibitor experiments\",\n      \"journal\": \"American journal of reproductive immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple functional assays with defined pathway inhibitors and enzymatic readout (zymography), single lab\",\n      \"pmids\": [\"29856101\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"XCL1 treatment of MDA-MB-231 and SK-BR-3 breast cancer cells enhanced cell migration and EMT (E-cadherin downregulation, N-cadherin and vimentin upregulation, β-catenin nuclear translocation) via XCR1-mediated activation of the ERK/HIF-1α pathway. Knockdown of XCR1 by siRNA negated XCL1 effects. MEK1/2 inhibitor U0126 blocked XCL1-induced HIF-1α accumulation and cell migration.\",\n      \"method\": \"Cell migration assay; Western blot for EMT markers and signaling proteins; siRNA knockdown of XCR1; pharmacological inhibition of MEK1/2 with U0126\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — siRNA knockdown plus pharmacological inhibition to place XCL1 in a defined signaling pathway, single lab, multiple cell lines\",\n      \"pmids\": [\"33374849\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"XCL1 promotes osteoclastogenesis and bone-resorbing activity: recombinant XCL1 added to RANKL-stimulated human monocytes promoted osteoclast differentiation and resorbing activity. XCL1 also promoted expression of inflammatory and osteoclastogenic factors (IL-6, IL-8, RANKL) in human osteoblasts. In vivo, injection of recombinant XCL1 onto murine calvaria worsened osteolytic lesions, while XCL1-neutralizing antibody significantly reduced bone erosion and osteoclast numbers induced by UHMWPE particles.\",\n      \"method\": \"In vitro osteoclast differentiation assay from human monocytes + RANKL; in vivo murine calvarial osteolysis model with XCL1 injection and neutralizing antibody; Western blot; ELISA for cytokines\",\n      \"journal\": \"Frontiers in immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro and in vivo concordant functional experiments with neutralizing antibody, single lab\",\n      \"pmids\": [\"32849609\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"In a mouse model of streptozotocin-induced diabetic neuropathy, spinal XCL1 and XCR1 levels are upregulated. Intrathecal injection of XCL1 in naïve mice enhanced nociceptive transmission; injection of XCL1-neutralizing antibody diminished allodynia/hyperalgesia in diabetic mice. XCR1 is co-expressed with spinal neurons. Microglia activation in primary cultures led to enhanced XCL1 release and XCR1 expression.\",\n      \"method\": \"Streptozotocin mouse model; intrathecal injection of recombinant XCL1 and neutralizing antibody; von Frey and cold plate behavioral tests; Western blot; immunofluorescence; primary microglial cell cultures\",\n      \"journal\": \"Anesthesiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — pharmacological intervention (neutralizing antibody) with defined behavioral readout and cellular mechanism (microglia), single lab\",\n      \"pmids\": [\"27387353\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"In neuropathic pain (CCI model), XCL1 is released by spinal cord astroglial cells despite neural localization of its two receptors, XCR1 and ITGA9. Blockade/neutralization of both XCR1 (with vMIP-II) and ITGA9 (with YA4 antibody) reversed hypersensitivity after intrathecal XCL1 in naïve mice, with ITGA9 neutralization more effective. Neutralization of ITGA9 enhanced both buprenorphine and morphine analgesia; XCR1 blockade improved buprenorphine effectiveness.\",\n      \"method\": \"CCI neuropathic pain mouse model; intrathecal injection of antagonists (vMIP-II for XCR1) and neutralizing antibodies (YA4 for ITGA9); von Frey and cold plate tests; RT-qPCR; Western blot; ELISA; immunofluorescence\",\n      \"journal\": \"Frontiers in immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — two-receptor pharmacological dissection with defined behavioral readouts and cellular localization, single lab\",\n      \"pmids\": [\"36618360\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"The CD8+ T cell-derived chemokine XCL1 is critical for tissue-resident memory T cell (TRM) formation and supports positioning of intestinal CD8+ T cells during acute viral infection via XCR1+ cDC1s. In tumors, enforced Xcl1 expression by antigen-specific CD8+ T cells promoted intratumoral cDC1 accumulation and T cell persistence, improving overall survival. XCL1 acts non-cell autonomously to guide intestinal CD8+ TRM spatial differentiation.\",\n      \"method\": \"Murine genetic models (Xcl1-deficient); targeted spatial transcriptomics; adoptive transfer with enforced Xcl1 expression; tumor survival experiments\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic loss-of-function plus gain-of-function with spatial transcriptomics and survival readout, multiple orthogonal approaches in one study\",\n      \"pmids\": [\"39841133\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Intratumoral delivery of a highly active locked-disulfide form of XCL1 (mXCL1-V21C/A59C) via hydrophilic gel patch increased cDC1 accumulation in tumor masses and promoted their migration to regional lymph nodes, resulting in enhanced tumor-specific CTL induction. Tumor-infiltrating cDC1s produced CXCL9 (a CXCR3 ligand for CTLs/NK cells), and anti-CXCL9 treatment decreased CTL tumor infiltration. The approach reduced tumor growth and prolonged survival in two murine tumor models and was enhanced by anti-PD-1 combination.\",\n      \"method\": \"In vivo tumor models (E.G7-OVA, B16-F10); hydrophilic gel patch delivery of engineered XCL1; flow cytometry for immune cell phenotyping; anti-CXCL9 depletion experiment; tumor growth and survival measurement\",\n      \"journal\": \"International journal of cancer\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple in vivo tumor models with mechanistic depletion experiment (anti-CXCL9), single lab\",\n      \"pmids\": [\"38346928\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"XCL1 interconverts between two distinct native structures: a chemokine fold with high affinity for GPCR XCR1 (mediating cell migration and immune signaling) and an alternate fold that binds GAGs and exhibits antimicrobial activity. This metamorphic behavior is enabled by the absence of a conserved disulfide bond present in all other chemokines.\",\n      \"method\": \"Review of structure-function data (citing prior NMR, SAXS, mutagenesis, and functional studies)\",\n      \"journal\": \"Protein science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — synthesizes prior structural and functional experimental evidence; the mechanistic claims are grounded in cited experimental work, but this paper itself is a review\",\n      \"pmids\": [\"39840812\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"XCL1 overexpression in high-glucose-treated human renal glomerular endothelial cells promoted apoptosis and inflammatory response (IL-1β, TNF-α upregulation), and these effects were associated with upregulation of p53 and NF-κB pathway changes. p53 silencing (with pifithrin-α) blocked apoptosis and inflammation in XCL1-overexpressed HG-treated cells; pifithrin-α also reversed XCL1-induced NF-κB downregulation.\",\n      \"method\": \"Cell viability assay (CCK-8); flow cytometry for apoptosis; ELISA and Western blot for inflammatory cytokines and p53/NF-κB; XCL1 overexpression vector; p53 inhibitor pifithrin-α; in vivo db/db diabetic mouse model\",\n      \"journal\": \"Nephron\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single overexpression approach with pharmacological inhibitor, single lab, limited mechanistic resolution of XCL1's direct molecular action\",\n      \"pmids\": [\"34518457\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"XCL1 is a metamorphic chemokine that reversibly interconverts between a canonical α-β chemokine fold—which binds its sole GPCR, XCR1, via N-terminal residues (Val1-Glu4) to trigger IP3/calcium signaling and directed cell migration—and a β-sheet dimer fold that binds glycosaminoglycans (via Arg23/Arg43) and disrupts bacterial and fungal membranes by inducing negative Gaussian curvature; in the immune system, XCL1 produced by CD8+ T cells, NK cells, and γδ T cells selectively recruits XCR1+ cDC1 dendritic cells to drive CTL priming, tissue-resident memory T cell formation, and oral tolerance, while also signaling through a second receptor, ITGA9, to modulate nociceptive transmission in neuropathic pain.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"XCL1 is a metamorphic chemokine that reversibly interconverts between a canonical α-β chemokine fold, which engages its GPCR XCR1 to drive directed cell migration, and an alternate β-sheet dimer fold that binds glycosaminoglycans and remodels microbial membranes, a behavior enabled by its lack of a disulfide bond conserved in other chemokines [#1, #13]. In the chemokine fold, the extreme N-terminal residues Val1-Glu4 contribute the bulk of XCR1 binding energy, with Glu4 of XCL1 contacting Tyr117/Arg273 of XCR1 to trigger IP3 accumulation, calcium release, and chemotaxis [#0]. The β-sheet dimer binds heparan sulfate via basic residues Arg23 and Arg43 in a length- and sulfation-dependent manner, and selectively induces negative Gaussian curvature in bacterial and fungal membranes to confer antimicrobial activity [#1, #2]. Functionally, XCL1 selectively recruits XCR1+ cDC1 dendritic cells: it is produced by CD8+ T cells, γδ T cells, and other sources to drive cDC1-dependent CTL priming, tissue-resident memory T cell formation and positioning, and tolerogenic DC migration that establishes oral tolerance [#4, #5, #11, #12]. Beyond immune cell positioning, XCL1-XCR1 signaling promotes trophoblast invasion and epithelial-mesenchymal transition through PI3K/AKT, MEK/ERK, and HIF-1α pathways [#6, #7], drives osteoclastogenesis [#8], and acts in the spinal cord—via both XCR1 and a second receptor, ITGA9—to enhance nociceptive transmission in neuropathic pain [#9, #10].\"\n,\n  \"teleology\": [\n    {\n      \"year\": 1996,\n      \"claim\": \"Establishing that XCL1 (SCM-1alpha/lymphotactin) is an inducible, immune-expressed gene defined the molecular identity and chromosomal context of the chemokine before its receptor and mechanism were known.\",\n      \"evidence\": \"Gene cloning, sequencing, chromosomal mapping, and mitogenic induction in PBMCs\",\n      \"pmids\": [\"8849694\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No receptor or signaling mechanism defined\", \"Functional role of the L7a pseudogene in intron 1 unresolved\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Identifying that the XCL1 dimer binds glycosaminoglycans through Arg23/Arg43 distinguished a GAG-binding surface separable from receptor activation and linked it to gp120/HIV-1 inhibition.\",\n      \"evidence\": \"Site-directed mutagenesis of XCL1 dimer variant, heparin chromatography, SPR, and fluorescence polarization with HS oligosaccharides\",\n      \"pmids\": [\"26836755\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo relevance of GAG binding to immune positioning not tested\", \"Relationship between GAG binding and the metamorphic fold equilibrium not directly resolved here\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Demonstrating that spinal XCL1/XCR1 are upregulated and that XCL1 enhances nociception extended XCL1 function beyond classical immune chemotaxis into pain neurotransmission.\",\n      \"evidence\": \"Streptozotocin diabetic neuropathy model, intrathecal recombinant XCL1 and neutralizing antibody, behavioral testing, and primary microglial cultures\",\n      \"pmids\": [\"27387353\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether the neuronal effect is direct via XCR1 on neurons was not fully resolved\", \"A second receptor was not yet considered\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Placing XCL1-XCR1 upstream of MMP activity and PI3K/AKT, MEK, and JNK signaling showed the axis can drive cellular invasion in a non-immune (trophoblast) context.\",\n      \"evidence\": \"Transwell invasion and wound healing with recombinant XCL1, gelatin zymography, and pathway inhibitor experiments\",\n      \"pmids\": [\"29856101\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct receptor dependence not tested by XCR1 knockdown\", \"In vivo placental requirement unestablished\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Mapping the XCL1 N-terminus (Val1-Glu4) as the major XCR1 binding determinant and defining Glu4–Tyr117/Arg273 contacts resolved how the chemokine fold activates its GPCR at residue resolution.\",\n      \"evidence\": \"Rosetta homology modeling with ligand and receptor mutagenesis, IP3, calcium, and migration assays\",\n      \"pmids\": [\"31481523\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No experimental high-resolution XCL1-XCR1 complex structure\", \"Conformational state of XCL1 during receptor engagement not directly visualized\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Showing that XCL1 fusion vaccines and γδ T cell-derived XCL1 selectively target XCR1+ cDC1s established the axis as a tunable route for CTL priming and tolerogenic DC migration.\",\n      \"evidence\": \"In vitro DC chemoattraction and IL-12 assays, in vivo fusion vaccine immunization, TCRδ−/− knockout, DC depletion, and EAE/influenza functional readouts\",\n      \"pmids\": [\"31666238\", \"30755656\", \"31578268\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular basis of cDC1 selectivity beyond XCR1 expression not dissected\", \"Quantitative contribution of different XCL1 cellular sources unresolved\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Defining XCL1 as a metamorphic protein whose β-sheet and unfolded forms induce negative Gaussian curvature revealed a structurally distinct, receptor-independent antimicrobial function.\",\n      \"evidence\": \"SAXS of XCL1 structural variants against defined membranes and in vitro anti-Candida assay\",\n      \"pmids\": [\"32243126\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo antimicrobial relevance not demonstrated\", \"Regulation of the fold equilibrium in tissue not defined\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Placing XCL1-XCR1 on the ERK/HIF-1α/EMT axis and showing osteoclastogenic activity broadened the signaling outputs of the chemokine into tumor cell migration and bone resorption.\",\n      \"evidence\": \"XCR1 siRNA knockdown and MEK1/2 inhibition in breast cancer lines; in vitro and in vivo osteoclast/calvarial models with neutralizing antibody\",\n      \"pmids\": [\"33374849\", \"32849609\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct versus indirect cellular targets in bone not fully separated\", \"Tumor-intrinsic XCR1 signaling not validated in vivo\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Identifying ITGA9 as a second functional XCL1 receptor in the spinal cord, more effective than XCR1 in driving hypersensitivity, redefined XCL1 nociceptive signaling as a two-receptor system.\",\n      \"evidence\": \"CCI neuropathic pain model with XCR1 antagonist (vMIP-II) and ITGA9-neutralizing antibody (YA4), behavioral tests, and analgesic potentiation experiments\",\n      \"pmids\": [\"36618360\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct XCL1-ITGA9 binding interface not mapped\", \"Whether ITGA9 signaling operates outside the nervous system unknown\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Genetic loss- and gain-of-function established XCL1 as a non-cell-autonomous cue that, via cDC1s, controls tissue-resident memory T cell formation and spatial positioning during infection and in tumors.\",\n      \"evidence\": \"Xcl1-deficient mice, targeted spatial transcriptomics, adoptive transfer with enforced Xcl1, and tumor survival experiments\",\n      \"pmids\": [\"39841133\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Spatial cues integrating XCL1 with other positioning signals not fully defined\", \"Human translation of TRM positioning role untested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the metamorphic fold equilibrium is regulated in vivo to partition XCL1 between XCR1/ITGA9 receptor signaling, GAG binding, and antimicrobial membrane activity remains unresolved.\",\n      \"evidence\": \"No timeline study directly links conformational state to functional outcome in a physiological setting\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No in vivo measurement of fold state coupled to function\", \"Triggers and tissue determinants of fold switching unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0048018\", \"supporting_discovery_ids\": [0, 1, 13]},\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [0, 10]},\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [1, 2]},\n      {\"term_id\": \"GO:0090729\", \"supporting_discovery_ids\": [1]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [4, 5, 8, 10]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 6, 7]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [4, 5, 11, 12]},\n      {\"term_id\": \"R-HSA-1500931\", \"supporting_discovery_ids\": [4, 11]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"XCR1\", \"ITGA9\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"tie","faith_supported":4,"faith_total":5,"faith_pct":80.0}}