{"gene":"SIGLEC15","run_date":"2026-04-28T20:42:07","timeline":{"discoveries":[{"year":2007,"finding":"Siglec-15 is a type-I transmembrane protein with two Ig-like domains that preferentially recognizes the Neu5Acα2-6GalNAcα- (sialyl-Tn) structure and associates with activating adaptor proteins DAP12 and DAP10 via a lysine residue in its transmembrane domain, implying activating signaling function.","method":"Recombinant protein binding assays, co-immunoprecipitation, domain analysis","journal":"Glycobiology","confidence":"High","confidence_rationale":"Tier 1-2 — biochemical binding assay plus Co-IP identifying transmembrane lysine as DAP12/DAP10 association determinant, foundational paper","pmids":["17483134"],"is_preprint":false},{"year":2011,"finding":"Siglec-15 mRNA and protein increase during osteoclast differentiation stimulated by RANKL or TNF-α, and antibody blockade of Siglec-15 markedly inhibits osteoclast differentiation in primary mouse bone marrow macrophages and human osteoclast precursors.","method":"In vitro osteoclast differentiation assay with polyclonal antibody blockade, TRAP staining","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 — clean antibody KD with defined cellular phenotype, single lab","pmids":["21586272"],"is_preprint":false},{"year":2012,"finding":"Siglec-15 is expressed on tumor-associated macrophages and, upon recognition of sialyl-Tn (sTn) antigen on tumor cells, transduces a signal through DAP12 to Syk kinase, leading to enhanced TGF-β secretion from monocytes/macrophages; this requires the DAP12-binding lysine (Lys274) in the transmembrane domain, and is blocked by Syk inhibitors or K274A mutation.","method":"Co-culture model, Syk inhibitor treatment, site-directed mutagenesis (K274A), ELISA for TGF-β","journal":"Glycobiology","confidence":"High","confidence_rationale":"Tier 1-2 — mutagenesis of critical residue + pharmacological inhibition + functional readout, multiple orthogonal methods in one study","pmids":["23035012"],"is_preprint":false},{"year":2012,"finding":"Siglec-15 is NFAT2/NFATc1-inducible in osteoclasts and links RANKL-RANK-NFAT2 signaling to DAP12; both sialylated glycan recognition by the V-set domain and association with DAP12 via Lys-272 are essential for functional osteoclast formation. Siglec-15 knockdown reduces multinucleated cell formation and bone resorption. Siglec-15 forms complexes with Syk through DAP12 in response to vitronectin.","method":"siRNA knockdown, retroviral transduction of mutants (K272A), bone resorption assay, Co-IP (Siglec-15/DAP12/Syk complex)","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 — reconstitution with mutants, Co-IP, functional bone resorption assay, multiple orthogonal methods","pmids":["22451653"],"is_preprint":false},{"year":2013,"finding":"Siglec-15 regulates osteoclast development by modulating RANKL-induced PI3K/Akt and Erk signaling pathways in association with DAP12. Siglec-15-deficient mice show mild osteopetrosis due to impaired osteoclast development. The association of Siglec-15 with DAP12 is required for downstream RANK signal transduction, and OSCAR/FcRγ signaling can compensate for Siglec-15 loss in the primary spongiosa.","method":"Siglec-15 knockout mice, retroviral transduction of wild-type and mutant Siglec-15, Western blotting for PI3K/Akt and Erk, histomorphometry, in vitro osteoclast assays","journal":"Journal of bone and mineral research","confidence":"High","confidence_rationale":"Tier 1-2 — genetic KO mouse plus retroviral rescue with mutants plus signaling pathway analysis, strong mechanistic evidence","pmids":["23677868"],"is_preprint":false},{"year":2014,"finding":"Siglec-15 is localized to the plasma membrane where it interacts with DAP12 and induces Akt activation when clustered on the osteoclast cell surface. Monoclonal antibodies targeting Siglec-15 induce rapid internalization, lysosomal targeting, and degradation of Siglec-15 via receptor dimerization, and antibody treatment increases bone mineral density in mice.","method":"Monoclonal antibody treatment, in vitro osteoclast differentiation assay, mouse bone density measurement (DXA), Western blot for Akt activation, immunofluorescence for localization/internalization","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal methods including in vitro signaling, internalization mechanism, and in vivo bone density, single rigorous study","pmids":["24446437"],"is_preprint":false},{"year":2014,"finding":"Siglec-15-deficient mice are resistant to estrogen deficiency-induced bone loss after ovariectomy. Siglec-15-null osteoclasts are small and fail to spread on bone surfaces, indicating impaired cytoskeletal organization. The Siglec-15/DAP12 pathway is specifically important for cytoskeletal organization in both RANKL- and TNF-α-induced osteoclastogenesis.","method":"Ovariectomy mouse model with Siglec-15 KO mice, histomorphometry, in vitro osteoclast assays, TRAP staining","journal":"Bone","confidence":"High","confidence_rationale":"Tier 2 — genetic KO with defined skeletal phenotype replicated in vivo and in vitro","pmids":["25460183"],"is_preprint":false},{"year":2015,"finding":"Siglec-15 mediates periarticular bone loss but not joint destruction or cartilage damage in antigen-induced arthritis. Siglec-15-deficient mice show significantly lower periarticular bone loss with impaired mature multinucleated osteoclast formation in periarticular regions.","method":"Murine antigen-induced arthritis model, histological analysis, Siglec-15 KO mice","journal":"Bone","confidence":"High","confidence_rationale":"Tier 2 — genetic KO in disease model with specific histological phenotype readout","pmids":["26027508"],"is_preprint":false},{"year":2019,"finding":"Siglec-15 is a critical immune suppressor in the tumor microenvironment: it suppresses antigen-specific T cell responses in vitro and in vivo. Its expression is induced by M-CSF and downregulated by IFN-γ, and it is mutually exclusive with B7-H1/PD-L1 expression. Genetic ablation or antibody blockade of Siglec-15 amplifies anti-tumor immunity and inhibits tumor growth.","method":"Genome-scale T cell activity array, in vitro T cell suppression assays, genetic ablation, antibody blockade, mouse tumor models","journal":"Nature medicine","confidence":"High","confidence_rationale":"Tier 2 — genome-scale screen plus in vitro and in vivo functional validation, multiple orthogonal methods, highly cited foundational paper","pmids":["30833750"],"is_preprint":false},{"year":2019,"finding":"A SIGLEC15 polymorphism associated with recurrent vulvovaginal candidiasis leads to an altered cytokine profile after Candida stimulation in PBMCs, increased IL-1B and NLRP3 expression in HeLa cells, and in vivo silencing of Siglec15 at the vaginal surface increases fungal burden while increasing polymorphonuclear leukocytes.","method":"GWAS/genomic integration, PBMC stimulation assays, in vivo siRNA silencing in mouse vaginal infection model, flow cytometry","journal":"Science translational medicine","confidence":"Medium","confidence_rationale":"Tier 2 — in vivo silencing with defined infectious phenotype, supported by cytokine profiling in human cells","pmids":["31189718"],"is_preprint":false},{"year":2020,"finding":"Siglec-15 is crucial for bone erosion in a serum-transfer arthritis model; Siglec-15 knockout mice show significantly reduced bone erosion area and osteoclast numbers in arthritic joints, while inflammation and cartilage destruction are unchanged.","method":"K/BxN serum-transfer arthritis model, Siglec-15 KO mice, histological analysis","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — genetic KO in established disease model with specific histological outcome measures","pmids":["33020147"],"is_preprint":false},{"year":2020,"finding":"N-glycosylation of Siglec-15 stabilizes the protein by decreasing its lysosome-dependent degradation and promotes Siglec-15 transportation to the cell membrane; Siglec-15 is completely N-glycosylated in vitro and in vivo, and this is regulated by glucose uptake.","method":"Glycosidase and glycosylation inhibitor treatment, immunofluorescence for subcellular localization, Western blot for protein stability","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 — biochemical demonstration of N-glycosylation effect on stability and localization, single lab","pmids":["32921411"],"is_preprint":false},{"year":2020,"finding":"LINC00973 positively regulates Siglec-15 expression at the transcriptional level by sponging miR-7109-3p (competing endogenous RNA mechanism), thereby controlling Siglec-15 cell surface abundance and contributing to cancer immune suppression.","method":"lncRNA knockdown/overexpression, luciferase reporter assay, miRNA target validation, co-culture immune activation assay","journal":"Cancer science","confidence":"Medium","confidence_rationale":"Tier 3 — ceRNA mechanism with functional immune readout, single lab","pmids":["32780490"],"is_preprint":false},{"year":2021,"finding":"Siglec-15 promotes migration of hepatoma cells by interacting with CD44 (which is modified by α2,6-linked sialic acids on N-glycans); CD44 sialylation is required for the Siglec-15/CD44 interaction, and Siglec-15 prevents lysosomal-mediated degradation of CD44 to promote its stability.","method":"Co-immunoprecipitation, sialidase treatment, lysosomal inhibitor assay, migration assay, Western blot","journal":"FEBS letters","confidence":"Medium","confidence_rationale":"Tier 2-3 — Co-IP plus mechanistic validation of glycan dependency and lysosomal protection, single lab","pmids":["34328657"],"is_preprint":false},{"year":2021,"finding":"Siglec-15 binds with higher avidity to α2,3- and α2,6-linked sialoglycans (not exclusively sialyl-Tn) as shown by glycan microarray; antibody cross-linking of Siglec-15 activates SYK/MAPK signaling in monocytic cells. Enhanced TGF-β secretion following co-culture with sTn-expressing tumor cells could not be reproduced.","method":"Glycan microarray, antibody cross-linking, Western blot for SYK/MAPK activation, co-culture with tumor cells","journal":"Glycobiology","confidence":"Medium","confidence_rationale":"Tier 2 — glycan array defines binding specificity, signaling activation confirmed biochemically","pmids":["32501471"],"is_preprint":false},{"year":2021,"finding":"Glycosylation of Siglec-15 at N172 (N173 in mouse) is required for its immunosuppressive function; the N172Q glycosylation-deficient mutant reduced tumor growth in immunocompetent (C57BL/6) but not nude mice, indicating glycosylation is necessary for immune evasion function.","method":"Mass spectrometry, site-directed mutagenesis (N172Q), xenograft models in immunocompetent vs. nude mice","journal":"American journal of cancer research","confidence":"High","confidence_rationale":"Tier 1-2 — MS site identification plus mutagenesis plus in vivo functional validation in immunocompetent vs. immunodeficient mice","pmids":["34094685"],"is_preprint":false},{"year":2021,"finding":"Siglec-15 promotes OS cell proliferation, migration, and invasion through activation of DUSP1 (MKP1), which mediates suppression of p38/MAPK and JNK/MAPK; Siglec-15 silencing downregulates DUSP1 and DUSP1 overexpression rescues the phenotype of Siglec-15-knockdown cells.","method":"siRNA knockdown, RNA-Seq, DUSP1 rescue overexpression, Western blot for MAPK pathways, xenograft model","journal":"Frontiers in oncology","confidence":"Medium","confidence_rationale":"Tier 2 — epistasis by rescue experiment plus pathway analysis, single lab","pmids":["34336699"],"is_preprint":false},{"year":2022,"finding":"SIGLEC15 on tumor-associated macrophages in pancreatic cancer interacts with α-2,3-linked sialic acids on PDAC cells to stimulate SYK phosphorylation in TAMs, which promotes M2-like polarization and immunosuppressive cytokine/chemokine production. This is blocked by SYK inhibitor in vivo.","method":"Co-culture assay, Western blot for SYK phosphorylation, sialic acid binding assay, SYK inhibitor treatment in vivo, flow cytometry","journal":"Cancer letters","confidence":"High","confidence_rationale":"Tier 2 — biochemical signaling validated with pharmacological inhibition both in vitro and in vivo","pmids":["35077803"],"is_preprint":false},{"year":2022,"finding":"Siglec-15 promotes M2-macrophage polarization in pancreatic cancer partly by interacting with Glut1 to upregulate glycolysis, and this regulation is also dependent on the cGAS-STING pathway; Siglec-15 knockout in macrophages reduces M2 markers (Arg1, CD206) and inhibits tumor growth in vivo.","method":"Siglec-15 KO macrophages, Co-IP (Siglec-15/Glut1), subcutaneous tumor model, Western blot, flow cytometry","journal":"Oxidative medicine and cellular longevity","confidence":"Low","confidence_rationale":"Tier 3 — Co-IP and KO phenotype but paper was subsequently retracted (PMID 37388523); findings uncertain","pmids":["36105484"],"is_preprint":false},{"year":2023,"finding":"Crystal structure of Siglec-15 was solved and its binding epitope determined via co-crystallization with a blocking antibody. STD-NMR and molecular dynamics reveal binding mode to α2,3- and α2,6-linked sialic acids and STn glycoform. Siglec-15 binding to T cells (which lack STn) depends on α2,3- and α2,6-linked sialoglycans, and the leukocyte integrin CD11b was identified as a Siglec-15 binding partner on human T cells.","method":"X-ray crystallography, STD-NMR, molecular dynamics simulation, Co-IP/pulldown for CD11b interaction, T cell binding assays","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 — crystal structure plus NMR binding mode plus identification of CD11b as binding partner, multiple orthogonal methods","pmids":["37311743"],"is_preprint":false},{"year":2023,"finding":"Osteoclast-derived apoptotic bodies bearing Siglec-15 on their membranes bind sialylated TLR2 on naive CD8+ T cells, blocking downstream co-stimulatory signaling and inhibiting T cell activation; Siglec-15 neutralizing antibodies reduce secondary breast cancer bone metastasis.","method":"AB-null MRL/lpr mice, flow cytometry, Co-IP (Siglec-15/sialylated TLR2), T cell activation assays, neutralizing antibody treatment in vivo","journal":"Cell reports. Medicine","confidence":"Medium","confidence_rationale":"Tier 2-3 — Co-IP identifies TLR2 as binding partner, functional T cell and in vivo data support mechanism","pmids":["37607544"],"is_preprint":false},{"year":2023,"finding":"Transcription factors ETS-1 and ETS-2 bind to the Siglec-15 promoter to enhance transcription in hepatocellular carcinoma cells; TGF-β1 activates the Ras/C-Raf/MEK/ERK1/2 pathway to phosphorylate ETS-1 and ETS-2, which upregulates Siglec-15 transcription.","method":"ChIP assay (ETS-1/ETS-2 promoter binding), luciferase reporter assay, TGF-β1 stimulation, ERK1/2 phosphorylation Western blot, siRNA knockdown","journal":"International journal of molecular sciences","confidence":"Medium","confidence_rationale":"Tier 2 — ChIP plus reporter plus signaling pathway validation, single lab","pmids":["36614238"],"is_preprint":false},{"year":2023,"finding":"Siglec-15 expression in B-ALL is driven by NFκB activation, which also increases surface localization of Siglec-15. Soluble/secreted Siglec-15 circulates at elevated levels in children with B-ALL and correlates with an immunosuppressive cytokine milieu. Genetic inhibition of Siglec-15 in murine B-ALL promotes immune clearance with expanded CD8+ T cells and reduced immunosuppressive cytokines.","method":"NFκB inhibition, Western blot, flow cytometry, mouse B-ALL model with Siglec-15 KO, cytokine measurement (ELISA)","journal":"Cancer research communications","confidence":"Medium","confidence_rationale":"Tier 2 — genetic KO with defined immune phenotype plus mechanistic NFκB driver identification, single lab","pmids":["37465593"],"is_preprint":false},{"year":2023,"finding":"Siglec-15 inhibits T cell activation in anaplastic thyroid cancer by reducing NFAT1, NFAT2, and NF-κB signals. Anti-Siglec-15 antibody increases IFN-γ and IL-2 secretion and enhances CD8+ T cell cytotoxicity in co-culture and in immunocompetent mouse ATC models.","method":"Anti-Siglec-15 antibody co-culture, zebrafish xenograft model, immunocompetent mouse tumor model, Western blot for NFAT/NF-κB, flow cytometry","journal":"International immunopharmacology","confidence":"Medium","confidence_rationale":"Tier 2 — mechanistic signaling readout plus in vivo tumor model, single lab","pmids":["38652971"],"is_preprint":false},{"year":2023,"finding":"Siglec-15 promotes migration of thyroid carcinoma cells by binding EGFR in a sialic acid-dependent manner and increasing EGFR protein stability; removal of sialic acid residues disrupts the Siglec-15/EGFR interaction, and EGFR pathway inhibition blocks the Siglec-15-mediated migratory effect.","method":"Pull-down assay (Siglec-15/EGFR), cycloheximide protein stability assay, sialidase treatment, wound-healing and transwell migration assay, EGFR inhibitor treatment","journal":"Glycobiology","confidence":"Medium","confidence_rationale":"Tier 2 — pulldown identifies EGFR as binding partner, sialic-acid dependency and stability confirmed biochemically","pmids":["37129515"],"is_preprint":false},{"year":2021,"finding":"SIGLEC15 expression in PBMCs and mouse vaginal tissue is upregulated by Aspergillus fumigatus stimulation; SIGLEC15 silencing decreases PBMC killing capacity of A. fumigatus, indicating a role in antifungal defense.","method":"In vitro PBMC stimulation, siRNA silencing, killing assay","journal":"Cell reports. Medicine","confidence":"Medium","confidence_rationale":"Tier 2 — functional KD with defined killing phenotype in human cells and in vivo context","pmids":["34095887"],"is_preprint":false},{"year":2024,"finding":"Siglec-15 activates M-CSF-induced RAP1/Rac1 cytoskeletal remodeling in osteoclasts through formation of a complex with p130CAS and CrkII; Siglec-15-deficient osteoclasts fail to form actin rings, and Siglec-15/FcRγ double-deficient mice develop severe osteopetrosis.","method":"Siglec-15 KO mice, Siglec-15/FcRγ double KO mice, biochemical complex analysis (co-immunoprecipitation of p130CAS/CrkII), actin ring assay, bone mass analysis","journal":"Bone research","confidence":"High","confidence_rationale":"Tier 1-2 — genetic evidence from double KO plus Co-IP defining novel complex, multiple orthogonal methods","pmids":["38849345"],"is_preprint":false},{"year":2024,"finding":"The chromatin remodeling factor Arid1a cooperates with transcription factors Jun/Fos to increase chromatin accessibility at the Siglec15 promoter, thereby upregulating Siglec15 and promoting osteoclast differentiation; BMDM-specific Arid1a KO reduces Siglec15 expression, impairs osteoclast fusion, and increases bone mass.","method":"Conditional Arid1a KO in BMDMs, ATAC-seq/chromatin accessibility assay, ChIP for Jun/Fos at Siglec15 promoter, in vivo bone phenotype analysis, ovariectomy model","journal":"Journal of bone and mineral research","confidence":"High","confidence_rationale":"Tier 1-2 — ChIP plus chromatin accessibility plus in vivo genetic rescue, multiple orthogonal methods","pmids":["38477755"],"is_preprint":false},{"year":2024,"finding":"CA72-4 derived from synovial cells binds to Siglec-15 on macrophages (demonstrated by Co-IP) and through this interaction activates TIGIT/SHP-1 signaling to inhibit MSU-induced macrophage M1 polarization; Siglec-15 knockdown weakens TIGIT/SHP-1 activation and the inhibitory effect on M1 polarization.","method":"Co-immunoprecipitation (CA72-4/Siglec-15), Siglec-15 siRNA knockdown, ELISA for cytokines, Western blot for SHP-1","journal":"Autoimmunity","confidence":"Medium","confidence_rationale":"Tier 3 — Co-IP plus functional KD, single lab, single method for binding partner","pmids":["41920724"],"is_preprint":false},{"year":2026,"finding":"Genome-wide CRISPR knockout screening identifies St3gal4 (producing α2-3-linked sialic acid on N-glycans) and LRP1 as key contributors to Siglec-15 ligand expression on osteoclast precursors. LRP1 is identified as a direct Siglec-15 counter-receptor; retriever complex-mediated endosome-to-plasma membrane recycling maintains LRP1 and thus Siglec-15 ligand surface expression. St3gal4, Lrp1, and Vps35l deficiency each impair osteoclast differentiation.","method":"Genome-wide CRISPR KO screen, Siglec-15 binding assay, osteoclast differentiation assay, LRP1 expression analysis in retriever-deficient cells","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 1-2 — genome-wide unbiased screen plus functional validation of identified counterreceptor and glycan epitope","pmids":["41569849"],"is_preprint":false}],"current_model":"Siglec-15 is a conserved type-I transmembrane sialic acid-binding lectin that associates via its transmembrane lysine with DAP12 (and DAP10), coupling glycan recognition (preferring α2,3- and α2,6-linked sialoglycans, including sialyl-Tn, on counter-receptors such as LRP1 and CD11b) to downstream Syk/PI3K-Akt/Erk/RAP1-Rac1 signaling; in osteoclasts this cascade drives NFATc1-dependent differentiation, actin ring formation, and bone resorption, while in the tumor microenvironment Siglec-15 on macrophages and cancer cells suppresses CD8+ T cell responses (partly by engaging sialylated TLR2 and CD11b), promotes M2-like polarization via SYK/MAPK signaling, and is transcriptionally regulated by ETS-1/ETS-2 (downstream of TGF-β/ERK) and Arid1a/Jun-Fos chromatin remodeling, with its surface stability and immunosuppressive function dependent on N-glycosylation at N172."},"narrative":{"teleology":[{"year":2007,"claim":"Identifying Siglec-15 as a DAP12/DAP10-associated sialic acid-binding receptor with sialyl-Tn preference established it as a potential activating signaling lectin, resolving the question of whether this orphan Siglec had signaling capacity.","evidence":"Recombinant protein binding assays and co-immunoprecipitation with transmembrane domain analysis","pmids":["17483134"],"confidence":"High","gaps":["Physiological ligands on cells unknown","Downstream signaling pathways not mapped","In vivo function uncharacterized"]},{"year":2012,"claim":"Demonstrating that Siglec-15 signals through a DAP12-Syk complex to drive osteoclast formation and TGF-β secretion upon tumor sialyl-Tn recognition established dual roles in bone biology and tumor-immune crosstalk, answering what the DAP12 coupling achieves functionally.","evidence":"siRNA knockdown, K272A/K274A mutagenesis, Syk inhibitor treatment, bone resorption assay, and co-culture TGF-β ELISA","pmids":["22451653","23035012"],"confidence":"High","gaps":["In vivo skeletal phenotype not yet established","Whether TGF-β induction is reproducible across labs (later contested)","Downstream transcriptional targets unresolved"]},{"year":2013,"claim":"Generation of Siglec-15 knockout mice revealed mild osteopetrosis and impaired PI3K/Akt and Erk signaling, establishing Siglec-15 as a non-redundant co-stimulatory receptor for osteoclast differentiation in vivo while showing that OSCAR/FcRγ can partially compensate.","evidence":"Siglec-15 KO mice with histomorphometry, retroviral rescue of mutants, Western blot for PI3K/Akt/Erk","pmids":["23677868"],"confidence":"High","gaps":["Role in pathological bone loss models untested","Mechanism of cytoskeletal defects unclear","Compensatory pathways not fully defined"]},{"year":2014,"claim":"Showing that Siglec-15-null mice resist ovariectomy-induced bone loss and that anti-Siglec-15 antibodies increase bone density by inducing receptor internalization and degradation established Siglec-15 as a therapeutic target for osteoporosis.","evidence":"Ovariectomy model in KO mice, monoclonal antibody-induced internalization/degradation, DXA bone density measurement","pmids":["25460183","24446437"],"confidence":"High","gaps":["Antibody mechanism of action in immune cells unknown","Whether surface removal affects immune functions untested","Cytoskeletal signaling intermediates unidentified"]},{"year":2019,"claim":"A genome-scale T cell activity screen identified Siglec-15 as a major immune suppressor mutually exclusive with PD-L1, fundamentally expanding its biology from bone to cancer immunology and answering whether Siglec-15 has non-redundant immunoregulatory function.","evidence":"Genome-scale screen, in vitro T cell suppression, genetic ablation and antibody blockade in mouse tumor models","pmids":["30833750"],"confidence":"High","gaps":["Receptor on T cells not identified","Molecular mechanism of T cell suppression unknown","Relevance to human clinical responses unproven"]},{"year":2019,"claim":"Linking a SIGLEC15 polymorphism to recurrent vulvovaginal candidiasis revealed a role in mucosal antifungal immunity beyond bone and cancer, showing that Siglec-15 regulates inflammatory cytokine responses and fungal clearance.","evidence":"GWAS integration, PBMC stimulation, in vivo siRNA silencing in mouse vaginal infection model","pmids":["31189718"],"confidence":"Medium","gaps":["Precise ligand on Candida or host cells at mucosal surface unknown","Mechanism connecting Siglec-15 to NLRP3/IL-1β not established","Not replicated in independent cohorts"]},{"year":2021,"claim":"Broadening glycan specificity from sialyl-Tn alone to general α2,3- and α2,6-sialoglycans, and showing that N-glycosylation at N172 is required for protein stability and immunosuppressive function, resolved the ligand selectivity question and identified a key post-translational regulatory mechanism.","evidence":"Glycan microarray, site-directed mutagenesis (N172Q), tumor models in immunocompetent vs. nude mice, glycosidase treatment","pmids":["32501471","34094685","32921411"],"confidence":"High","gaps":["Structural basis for glycan selectivity not resolved at atomic level","Glycosylation-dependent interactome changes uncharacterized","TGF-β secretion finding from 2012 could not be reproduced"]},{"year":2022,"claim":"Demonstrating that Siglec-15 on tumor-associated macrophages engages α2,3-sialylated PDAC cells to activate SYK and drive M2-like polarization established the immunosuppressive signaling axis in solid tumors and showed it is pharmacologically reversible with SYK inhibitors.","evidence":"Co-culture with SYK phosphorylation readout, sialic acid binding assay, SYK inhibitor treatment in vivo","pmids":["35077803"],"confidence":"High","gaps":["Specific sialylated glycoprotein counter-receptor on tumor cells not identified in this context","Whether SYK inhibition affects bone-side Siglec-15 function simultaneously","Human TAM relevance limited to correlative data"]},{"year":2023,"claim":"Crystal structure determination and identification of CD11b and sialylated TLR2 as functional binding partners on T cells and osteoclast-derived apoptotic bodies, respectively, answered what the molecular targets of Siglec-15 are on immune cells and revealed the structural basis for antibody blockade.","evidence":"X-ray crystallography, STD-NMR, molecular dynamics, Co-IP for CD11b and TLR2, T cell binding assays, neutralizing antibody in bone metastasis model","pmids":["37311743","37607544"],"confidence":"High","gaps":["Whether CD11b and TLR2 binding are mutually exclusive or cooperative unknown","How Siglec-15 engagement of CD11b mechanistically suppresses T cell activation not detailed","Relative contributions of different counter-receptors in vivo unquantified"]},{"year":2023,"claim":"Identification of ETS-1/ETS-2 downstream of TGF-β/ERK and NFκB as transcriptional drivers of SIGLEC15, and Arid1a/Jun-Fos chromatin remodeling as an epigenetic regulator, established how Siglec-15 expression is controlled in disease contexts.","evidence":"ChIP for ETS-1/ETS-2 and Jun/Fos at SIGLEC15 promoter, ATAC-seq, NFκB inhibition, luciferase reporters, conditional Arid1a KO","pmids":["36614238","37465593","38477755"],"confidence":"Medium","gaps":["Integration of NFκB vs. ETS-1/2 vs. Arid1a regulatory inputs unclear","Whether these transcriptional mechanisms operate in immune cells vs. tumor cells specifically","Post-transcriptional regulation (e.g., miR-7109-3p axis) not connected to chromatin-level control"]},{"year":2024,"claim":"A genome-wide CRISPR screen identified LRP1 as a direct Siglec-15 counter-receptor on osteoclast precursors and St3gal4 as the glycosyltransferase generating its α2,3-sialylated ligand, while biochemical work revealed Siglec-15 activates RAP1/Rac1 via a p130CAS/CrkII complex for actin ring formation; Siglec-15/FcRγ double KO causes severe osteopetrosis.","evidence":"Genome-wide CRISPR KO screen, Siglec-15 binding validation, Co-IP of p130CAS/CrkII, double KO mice with bone analysis","pmids":["41569849","38849345"],"confidence":"High","gaps":["Whether LRP1 is also a Siglec-15 counter-receptor in the immune context","Structural basis for Siglec-15/LRP1 interaction unresolved","How p130CAS/CrkII complex is recruited to the DAP12/Syk axis not fully delineated"]},{"year":null,"claim":"Key open questions include how Siglec-15 engagement of distinct counter-receptors (LRP1, CD11b, TLR2) leads to context-specific outcomes in bone vs. immune cells, what determines the switch between DAP12-Syk and alternative downstream pathways, and whether Siglec-15 blockade will prove effective in human cancer immunotherapy.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No human clinical efficacy data for anti-Siglec-15 therapy reported in the timeline","Context-specific signaling downstream of different counter-receptors not compared systematically","Relative contribution of soluble vs. membrane-bound Siglec-15 in immunosuppression unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[0,2,3,14]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[8,17,20,23]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[5,11,15]},{"term_id":"GO:0005764","term_label":"lysosome","supporting_discovery_ids":[5,11]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[8,17,20,22,23]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[2,3,4,14,17]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[1,3,4,6,26,27]}],"complexes":["Siglec-15/DAP12/Syk","Siglec-15/p130CAS/CrkII"],"partners":["DAP12","DAP10","SYK","CD11B","LRP1","TLR2","CD44","EGFR"],"other_free_text":[]},"mechanistic_narrative":"Siglec-15 is a conserved sialic acid-binding immunoglobulin-like lectin that couples glycan recognition to intracellular signaling through the DAP12/Syk axis, functioning as a critical regulator of both osteoclast differentiation and immune suppression. In osteoclasts, Siglec-15 associates via its transmembrane lysine with DAP12 to activate PI3K/Akt, Erk, and RAP1/Rac1 pathways (through a p130CAS/CrkII complex), driving NFATc1-dependent differentiation, actin ring formation, and bone resorption; Siglec-15-deficient mice exhibit osteopetrosis and resistance to estrogen-deficiency-induced bone loss [PMID:22451653, PMID:23677868, PMID:25460183, PMID:38849345]. Siglec-15 recognizes α2,3- and α2,6-linked sialoglycans on counter-receptors including LRP1 and CD11b, and its surface stability and immunosuppressive activity depend on N-glycosylation at N172 [PMID:37311743, PMID:41569849, PMID:34094685]. In the tumor microenvironment, Siglec-15 expressed on macrophages and tumor cells suppresses CD8+ T cell responses—in a manner mutually exclusive with PD-L1—by engaging sialylated ligands such as TLR2 and CD11b, and promotes M2-like macrophage polarization via SYK/MAPK signaling; genetic ablation or antibody blockade of Siglec-15 amplifies anti-tumor immunity across multiple tumor models [PMID:30833750, PMID:35077803, PMID:37607544]."},"prefetch_data":{"uniprot":{"accession":"Q6ZMC9","full_name":"Sialic acid-binding Ig-like lectin 15","aliases":["CD33 antigen-like 3"],"length_aa":328,"mass_kda":35.7,"function":"Binds sialylated glycoproteins","subcellular_location":"Membrane","url":"https://www.uniprot.org/uniprotkb/Q6ZMC9/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/SIGLEC15","classification":"Not 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immunopharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/41260165","citation_count":0,"is_preprint":false},{"pmid":"41349947","id":"PMC_41349947","title":"Investigating the role and mechanism of methionine in different types of skeletal fluorosis based on Siglec-15 methylation.","date":"2025","source":"Environmental pollution (Barking, Essex : 1987)","url":"https://pubmed.ncbi.nlm.nih.gov/41349947","citation_count":0,"is_preprint":false},{"pmid":"41569849","id":"PMC_41569849","title":"Comprehensive identification of the genes involved in the expression of Siglec-15 ligands on osteoclast precursors.","date":"2026","source":"Cell reports","url":"https://pubmed.ncbi.nlm.nih.gov/41569849","citation_count":0,"is_preprint":false},{"pmid":"39880295","id":"PMC_39880295","title":"Screening and preparation of nanobodies for SIGLEC-15 detection.","date":"2025","source":"Protein expression and purification","url":"https://pubmed.ncbi.nlm.nih.gov/39880295","citation_count":0,"is_preprint":false},{"pmid":"41920724","id":"PMC_41920724","title":"CA72-4 derived from synovial cells inhibits monosodium urate-induced macrophage M1 polarization by activating the TIGIT/SHP-1 axis through binding to Siglec-15.","date":"2026","source":"Autoimmunity","url":"https://pubmed.ncbi.nlm.nih.gov/41920724","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":42114,"output_tokens":7445,"usd":0.119009},"stage2":{"model":"claude-opus-4-6","input_tokens":11223,"output_tokens":3489,"usd":0.21501},"total_usd":0.334019,"stage1_batch_id":"msgbatch_011rkcQdVV1RaFq9XhcG6df6","stage2_batch_id":"msgbatch_01RzgrJafdF4EWHhj5mNi6dh","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2007,\n      \"finding\": \"Siglec-15 is a type-I transmembrane protein with two Ig-like domains that preferentially recognizes the Neu5Acα2-6GalNAcα- (sialyl-Tn) structure and associates with activating adaptor proteins DAP12 and DAP10 via a lysine residue in its transmembrane domain, implying activating signaling function.\",\n      \"method\": \"Recombinant protein binding assays, co-immunoprecipitation, domain analysis\",\n      \"journal\": \"Glycobiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — biochemical binding assay plus Co-IP identifying transmembrane lysine as DAP12/DAP10 association determinant, foundational paper\",\n      \"pmids\": [\"17483134\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Siglec-15 mRNA and protein increase during osteoclast differentiation stimulated by RANKL or TNF-α, and antibody blockade of Siglec-15 markedly inhibits osteoclast differentiation in primary mouse bone marrow macrophages and human osteoclast precursors.\",\n      \"method\": \"In vitro osteoclast differentiation assay with polyclonal antibody blockade, TRAP staining\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean antibody KD with defined cellular phenotype, single lab\",\n      \"pmids\": [\"21586272\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Siglec-15 is expressed on tumor-associated macrophages and, upon recognition of sialyl-Tn (sTn) antigen on tumor cells, transduces a signal through DAP12 to Syk kinase, leading to enhanced TGF-β secretion from monocytes/macrophages; this requires the DAP12-binding lysine (Lys274) in the transmembrane domain, and is blocked by Syk inhibitors or K274A mutation.\",\n      \"method\": \"Co-culture model, Syk inhibitor treatment, site-directed mutagenesis (K274A), ELISA for TGF-β\",\n      \"journal\": \"Glycobiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — mutagenesis of critical residue + pharmacological inhibition + functional readout, multiple orthogonal methods in one study\",\n      \"pmids\": [\"23035012\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Siglec-15 is NFAT2/NFATc1-inducible in osteoclasts and links RANKL-RANK-NFAT2 signaling to DAP12; both sialylated glycan recognition by the V-set domain and association with DAP12 via Lys-272 are essential for functional osteoclast formation. Siglec-15 knockdown reduces multinucleated cell formation and bone resorption. Siglec-15 forms complexes with Syk through DAP12 in response to vitronectin.\",\n      \"method\": \"siRNA knockdown, retroviral transduction of mutants (K272A), bone resorption assay, Co-IP (Siglec-15/DAP12/Syk complex)\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — reconstitution with mutants, Co-IP, functional bone resorption assay, multiple orthogonal methods\",\n      \"pmids\": [\"22451653\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Siglec-15 regulates osteoclast development by modulating RANKL-induced PI3K/Akt and Erk signaling pathways in association with DAP12. Siglec-15-deficient mice show mild osteopetrosis due to impaired osteoclast development. The association of Siglec-15 with DAP12 is required for downstream RANK signal transduction, and OSCAR/FcRγ signaling can compensate for Siglec-15 loss in the primary spongiosa.\",\n      \"method\": \"Siglec-15 knockout mice, retroviral transduction of wild-type and mutant Siglec-15, Western blotting for PI3K/Akt and Erk, histomorphometry, in vitro osteoclast assays\",\n      \"journal\": \"Journal of bone and mineral research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — genetic KO mouse plus retroviral rescue with mutants plus signaling pathway analysis, strong mechanistic evidence\",\n      \"pmids\": [\"23677868\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Siglec-15 is localized to the plasma membrane where it interacts with DAP12 and induces Akt activation when clustered on the osteoclast cell surface. Monoclonal antibodies targeting Siglec-15 induce rapid internalization, lysosomal targeting, and degradation of Siglec-15 via receptor dimerization, and antibody treatment increases bone mineral density in mice.\",\n      \"method\": \"Monoclonal antibody treatment, in vitro osteoclast differentiation assay, mouse bone density measurement (DXA), Western blot for Akt activation, immunofluorescence for localization/internalization\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal methods including in vitro signaling, internalization mechanism, and in vivo bone density, single rigorous study\",\n      \"pmids\": [\"24446437\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Siglec-15-deficient mice are resistant to estrogen deficiency-induced bone loss after ovariectomy. Siglec-15-null osteoclasts are small and fail to spread on bone surfaces, indicating impaired cytoskeletal organization. The Siglec-15/DAP12 pathway is specifically important for cytoskeletal organization in both RANKL- and TNF-α-induced osteoclastogenesis.\",\n      \"method\": \"Ovariectomy mouse model with Siglec-15 KO mice, histomorphometry, in vitro osteoclast assays, TRAP staining\",\n      \"journal\": \"Bone\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with defined skeletal phenotype replicated in vivo and in vitro\",\n      \"pmids\": [\"25460183\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Siglec-15 mediates periarticular bone loss but not joint destruction or cartilage damage in antigen-induced arthritis. Siglec-15-deficient mice show significantly lower periarticular bone loss with impaired mature multinucleated osteoclast formation in periarticular regions.\",\n      \"method\": \"Murine antigen-induced arthritis model, histological analysis, Siglec-15 KO mice\",\n      \"journal\": \"Bone\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO in disease model with specific histological phenotype readout\",\n      \"pmids\": [\"26027508\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Siglec-15 is a critical immune suppressor in the tumor microenvironment: it suppresses antigen-specific T cell responses in vitro and in vivo. Its expression is induced by M-CSF and downregulated by IFN-γ, and it is mutually exclusive with B7-H1/PD-L1 expression. Genetic ablation or antibody blockade of Siglec-15 amplifies anti-tumor immunity and inhibits tumor growth.\",\n      \"method\": \"Genome-scale T cell activity array, in vitro T cell suppression assays, genetic ablation, antibody blockade, mouse tumor models\",\n      \"journal\": \"Nature medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genome-scale screen plus in vitro and in vivo functional validation, multiple orthogonal methods, highly cited foundational paper\",\n      \"pmids\": [\"30833750\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"A SIGLEC15 polymorphism associated with recurrent vulvovaginal candidiasis leads to an altered cytokine profile after Candida stimulation in PBMCs, increased IL-1B and NLRP3 expression in HeLa cells, and in vivo silencing of Siglec15 at the vaginal surface increases fungal burden while increasing polymorphonuclear leukocytes.\",\n      \"method\": \"GWAS/genomic integration, PBMC stimulation assays, in vivo siRNA silencing in mouse vaginal infection model, flow cytometry\",\n      \"journal\": \"Science translational medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vivo silencing with defined infectious phenotype, supported by cytokine profiling in human cells\",\n      \"pmids\": [\"31189718\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Siglec-15 is crucial for bone erosion in a serum-transfer arthritis model; Siglec-15 knockout mice show significantly reduced bone erosion area and osteoclast numbers in arthritic joints, while inflammation and cartilage destruction are unchanged.\",\n      \"method\": \"K/BxN serum-transfer arthritis model, Siglec-15 KO mice, histological analysis\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO in established disease model with specific histological outcome measures\",\n      \"pmids\": [\"33020147\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"N-glycosylation of Siglec-15 stabilizes the protein by decreasing its lysosome-dependent degradation and promotes Siglec-15 transportation to the cell membrane; Siglec-15 is completely N-glycosylated in vitro and in vivo, and this is regulated by glucose uptake.\",\n      \"method\": \"Glycosidase and glycosylation inhibitor treatment, immunofluorescence for subcellular localization, Western blot for protein stability\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — biochemical demonstration of N-glycosylation effect on stability and localization, single lab\",\n      \"pmids\": [\"32921411\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"LINC00973 positively regulates Siglec-15 expression at the transcriptional level by sponging miR-7109-3p (competing endogenous RNA mechanism), thereby controlling Siglec-15 cell surface abundance and contributing to cancer immune suppression.\",\n      \"method\": \"lncRNA knockdown/overexpression, luciferase reporter assay, miRNA target validation, co-culture immune activation assay\",\n      \"journal\": \"Cancer science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — ceRNA mechanism with functional immune readout, single lab\",\n      \"pmids\": [\"32780490\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Siglec-15 promotes migration of hepatoma cells by interacting with CD44 (which is modified by α2,6-linked sialic acids on N-glycans); CD44 sialylation is required for the Siglec-15/CD44 interaction, and Siglec-15 prevents lysosomal-mediated degradation of CD44 to promote its stability.\",\n      \"method\": \"Co-immunoprecipitation, sialidase treatment, lysosomal inhibitor assay, migration assay, Western blot\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — Co-IP plus mechanistic validation of glycan dependency and lysosomal protection, single lab\",\n      \"pmids\": [\"34328657\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Siglec-15 binds with higher avidity to α2,3- and α2,6-linked sialoglycans (not exclusively sialyl-Tn) as shown by glycan microarray; antibody cross-linking of Siglec-15 activates SYK/MAPK signaling in monocytic cells. Enhanced TGF-β secretion following co-culture with sTn-expressing tumor cells could not be reproduced.\",\n      \"method\": \"Glycan microarray, antibody cross-linking, Western blot for SYK/MAPK activation, co-culture with tumor cells\",\n      \"journal\": \"Glycobiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — glycan array defines binding specificity, signaling activation confirmed biochemically\",\n      \"pmids\": [\"32501471\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Glycosylation of Siglec-15 at N172 (N173 in mouse) is required for its immunosuppressive function; the N172Q glycosylation-deficient mutant reduced tumor growth in immunocompetent (C57BL/6) but not nude mice, indicating glycosylation is necessary for immune evasion function.\",\n      \"method\": \"Mass spectrometry, site-directed mutagenesis (N172Q), xenograft models in immunocompetent vs. nude mice\",\n      \"journal\": \"American journal of cancer research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — MS site identification plus mutagenesis plus in vivo functional validation in immunocompetent vs. immunodeficient mice\",\n      \"pmids\": [\"34094685\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Siglec-15 promotes OS cell proliferation, migration, and invasion through activation of DUSP1 (MKP1), which mediates suppression of p38/MAPK and JNK/MAPK; Siglec-15 silencing downregulates DUSP1 and DUSP1 overexpression rescues the phenotype of Siglec-15-knockdown cells.\",\n      \"method\": \"siRNA knockdown, RNA-Seq, DUSP1 rescue overexpression, Western blot for MAPK pathways, xenograft model\",\n      \"journal\": \"Frontiers in oncology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — epistasis by rescue experiment plus pathway analysis, single lab\",\n      \"pmids\": [\"34336699\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"SIGLEC15 on tumor-associated macrophages in pancreatic cancer interacts with α-2,3-linked sialic acids on PDAC cells to stimulate SYK phosphorylation in TAMs, which promotes M2-like polarization and immunosuppressive cytokine/chemokine production. This is blocked by SYK inhibitor in vivo.\",\n      \"method\": \"Co-culture assay, Western blot for SYK phosphorylation, sialic acid binding assay, SYK inhibitor treatment in vivo, flow cytometry\",\n      \"journal\": \"Cancer letters\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — biochemical signaling validated with pharmacological inhibition both in vitro and in vivo\",\n      \"pmids\": [\"35077803\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Siglec-15 promotes M2-macrophage polarization in pancreatic cancer partly by interacting with Glut1 to upregulate glycolysis, and this regulation is also dependent on the cGAS-STING pathway; Siglec-15 knockout in macrophages reduces M2 markers (Arg1, CD206) and inhibits tumor growth in vivo.\",\n      \"method\": \"Siglec-15 KO macrophages, Co-IP (Siglec-15/Glut1), subcutaneous tumor model, Western blot, flow cytometry\",\n      \"journal\": \"Oxidative medicine and cellular longevity\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — Co-IP and KO phenotype but paper was subsequently retracted (PMID 37388523); findings uncertain\",\n      \"pmids\": [\"36105484\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Crystal structure of Siglec-15 was solved and its binding epitope determined via co-crystallization with a blocking antibody. STD-NMR and molecular dynamics reveal binding mode to α2,3- and α2,6-linked sialic acids and STn glycoform. Siglec-15 binding to T cells (which lack STn) depends on α2,3- and α2,6-linked sialoglycans, and the leukocyte integrin CD11b was identified as a Siglec-15 binding partner on human T cells.\",\n      \"method\": \"X-ray crystallography, STD-NMR, molecular dynamics simulation, Co-IP/pulldown for CD11b interaction, T cell binding assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure plus NMR binding mode plus identification of CD11b as binding partner, multiple orthogonal methods\",\n      \"pmids\": [\"37311743\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Osteoclast-derived apoptotic bodies bearing Siglec-15 on their membranes bind sialylated TLR2 on naive CD8+ T cells, blocking downstream co-stimulatory signaling and inhibiting T cell activation; Siglec-15 neutralizing antibodies reduce secondary breast cancer bone metastasis.\",\n      \"method\": \"AB-null MRL/lpr mice, flow cytometry, Co-IP (Siglec-15/sialylated TLR2), T cell activation assays, neutralizing antibody treatment in vivo\",\n      \"journal\": \"Cell reports. Medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — Co-IP identifies TLR2 as binding partner, functional T cell and in vivo data support mechanism\",\n      \"pmids\": [\"37607544\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Transcription factors ETS-1 and ETS-2 bind to the Siglec-15 promoter to enhance transcription in hepatocellular carcinoma cells; TGF-β1 activates the Ras/C-Raf/MEK/ERK1/2 pathway to phosphorylate ETS-1 and ETS-2, which upregulates Siglec-15 transcription.\",\n      \"method\": \"ChIP assay (ETS-1/ETS-2 promoter binding), luciferase reporter assay, TGF-β1 stimulation, ERK1/2 phosphorylation Western blot, siRNA knockdown\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — ChIP plus reporter plus signaling pathway validation, single lab\",\n      \"pmids\": [\"36614238\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Siglec-15 expression in B-ALL is driven by NFκB activation, which also increases surface localization of Siglec-15. Soluble/secreted Siglec-15 circulates at elevated levels in children with B-ALL and correlates with an immunosuppressive cytokine milieu. Genetic inhibition of Siglec-15 in murine B-ALL promotes immune clearance with expanded CD8+ T cells and reduced immunosuppressive cytokines.\",\n      \"method\": \"NFκB inhibition, Western blot, flow cytometry, mouse B-ALL model with Siglec-15 KO, cytokine measurement (ELISA)\",\n      \"journal\": \"Cancer research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with defined immune phenotype plus mechanistic NFκB driver identification, single lab\",\n      \"pmids\": [\"37465593\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Siglec-15 inhibits T cell activation in anaplastic thyroid cancer by reducing NFAT1, NFAT2, and NF-κB signals. Anti-Siglec-15 antibody increases IFN-γ and IL-2 secretion and enhances CD8+ T cell cytotoxicity in co-culture and in immunocompetent mouse ATC models.\",\n      \"method\": \"Anti-Siglec-15 antibody co-culture, zebrafish xenograft model, immunocompetent mouse tumor model, Western blot for NFAT/NF-κB, flow cytometry\",\n      \"journal\": \"International immunopharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — mechanistic signaling readout plus in vivo tumor model, single lab\",\n      \"pmids\": [\"38652971\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Siglec-15 promotes migration of thyroid carcinoma cells by binding EGFR in a sialic acid-dependent manner and increasing EGFR protein stability; removal of sialic acid residues disrupts the Siglec-15/EGFR interaction, and EGFR pathway inhibition blocks the Siglec-15-mediated migratory effect.\",\n      \"method\": \"Pull-down assay (Siglec-15/EGFR), cycloheximide protein stability assay, sialidase treatment, wound-healing and transwell migration assay, EGFR inhibitor treatment\",\n      \"journal\": \"Glycobiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — pulldown identifies EGFR as binding partner, sialic-acid dependency and stability confirmed biochemically\",\n      \"pmids\": [\"37129515\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"SIGLEC15 expression in PBMCs and mouse vaginal tissue is upregulated by Aspergillus fumigatus stimulation; SIGLEC15 silencing decreases PBMC killing capacity of A. fumigatus, indicating a role in antifungal defense.\",\n      \"method\": \"In vitro PBMC stimulation, siRNA silencing, killing assay\",\n      \"journal\": \"Cell reports. Medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional KD with defined killing phenotype in human cells and in vivo context\",\n      \"pmids\": [\"34095887\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Siglec-15 activates M-CSF-induced RAP1/Rac1 cytoskeletal remodeling in osteoclasts through formation of a complex with p130CAS and CrkII; Siglec-15-deficient osteoclasts fail to form actin rings, and Siglec-15/FcRγ double-deficient mice develop severe osteopetrosis.\",\n      \"method\": \"Siglec-15 KO mice, Siglec-15/FcRγ double KO mice, biochemical complex analysis (co-immunoprecipitation of p130CAS/CrkII), actin ring assay, bone mass analysis\",\n      \"journal\": \"Bone research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — genetic evidence from double KO plus Co-IP defining novel complex, multiple orthogonal methods\",\n      \"pmids\": [\"38849345\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"The chromatin remodeling factor Arid1a cooperates with transcription factors Jun/Fos to increase chromatin accessibility at the Siglec15 promoter, thereby upregulating Siglec15 and promoting osteoclast differentiation; BMDM-specific Arid1a KO reduces Siglec15 expression, impairs osteoclast fusion, and increases bone mass.\",\n      \"method\": \"Conditional Arid1a KO in BMDMs, ATAC-seq/chromatin accessibility assay, ChIP for Jun/Fos at Siglec15 promoter, in vivo bone phenotype analysis, ovariectomy model\",\n      \"journal\": \"Journal of bone and mineral research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — ChIP plus chromatin accessibility plus in vivo genetic rescue, multiple orthogonal methods\",\n      \"pmids\": [\"38477755\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"CA72-4 derived from synovial cells binds to Siglec-15 on macrophages (demonstrated by Co-IP) and through this interaction activates TIGIT/SHP-1 signaling to inhibit MSU-induced macrophage M1 polarization; Siglec-15 knockdown weakens TIGIT/SHP-1 activation and the inhibitory effect on M1 polarization.\",\n      \"method\": \"Co-immunoprecipitation (CA72-4/Siglec-15), Siglec-15 siRNA knockdown, ELISA for cytokines, Western blot for SHP-1\",\n      \"journal\": \"Autoimmunity\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — Co-IP plus functional KD, single lab, single method for binding partner\",\n      \"pmids\": [\"41920724\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"Genome-wide CRISPR knockout screening identifies St3gal4 (producing α2-3-linked sialic acid on N-glycans) and LRP1 as key contributors to Siglec-15 ligand expression on osteoclast precursors. LRP1 is identified as a direct Siglec-15 counter-receptor; retriever complex-mediated endosome-to-plasma membrane recycling maintains LRP1 and thus Siglec-15 ligand surface expression. St3gal4, Lrp1, and Vps35l deficiency each impair osteoclast differentiation.\",\n      \"method\": \"Genome-wide CRISPR KO screen, Siglec-15 binding assay, osteoclast differentiation assay, LRP1 expression analysis in retriever-deficient cells\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — genome-wide unbiased screen plus functional validation of identified counterreceptor and glycan epitope\",\n      \"pmids\": [\"41569849\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"Siglec-15 is a conserved type-I transmembrane sialic acid-binding lectin that associates via its transmembrane lysine with DAP12 (and DAP10), coupling glycan recognition (preferring α2,3- and α2,6-linked sialoglycans, including sialyl-Tn, on counter-receptors such as LRP1 and CD11b) to downstream Syk/PI3K-Akt/Erk/RAP1-Rac1 signaling; in osteoclasts this cascade drives NFATc1-dependent differentiation, actin ring formation, and bone resorption, while in the tumor microenvironment Siglec-15 on macrophages and cancer cells suppresses CD8+ T cell responses (partly by engaging sialylated TLR2 and CD11b), promotes M2-like polarization via SYK/MAPK signaling, and is transcriptionally regulated by ETS-1/ETS-2 (downstream of TGF-β/ERK) and Arid1a/Jun-Fos chromatin remodeling, with its surface stability and immunosuppressive function dependent on N-glycosylation at N172.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"Siglec-15 is a conserved sialic acid-binding immunoglobulin-like lectin that couples glycan recognition to intracellular signaling through the DAP12/Syk axis, functioning as a critical regulator of both osteoclast differentiation and immune suppression. In osteoclasts, Siglec-15 associates via its transmembrane lysine with DAP12 to activate PI3K/Akt, Erk, and RAP1/Rac1 pathways (through a p130CAS/CrkII complex), driving NFATc1-dependent differentiation, actin ring formation, and bone resorption; Siglec-15-deficient mice exhibit osteopetrosis and resistance to estrogen-deficiency-induced bone loss [PMID:22451653, PMID:23677868, PMID:25460183, PMID:38849345]. Siglec-15 recognizes α2,3- and α2,6-linked sialoglycans on counter-receptors including LRP1 and CD11b, and its surface stability and immunosuppressive activity depend on N-glycosylation at N172 [PMID:37311743, PMID:41569849, PMID:34094685]. In the tumor microenvironment, Siglec-15 expressed on macrophages and tumor cells suppresses CD8+ T cell responses—in a manner mutually exclusive with PD-L1—by engaging sialylated ligands such as TLR2 and CD11b, and promotes M2-like macrophage polarization via SYK/MAPK signaling; genetic ablation or antibody blockade of Siglec-15 amplifies anti-tumor immunity across multiple tumor models [PMID:30833750, PMID:35077803, PMID:37607544].\",\n  \"teleology\": [\n    {\n      \"year\": 2007,\n      \"claim\": \"Identifying Siglec-15 as a DAP12/DAP10-associated sialic acid-binding receptor with sialyl-Tn preference established it as a potential activating signaling lectin, resolving the question of whether this orphan Siglec had signaling capacity.\",\n      \"evidence\": \"Recombinant protein binding assays and co-immunoprecipitation with transmembrane domain analysis\",\n      \"pmids\": [\"17483134\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological ligands on cells unknown\", \"Downstream signaling pathways not mapped\", \"In vivo function uncharacterized\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Demonstrating that Siglec-15 signals through a DAP12-Syk complex to drive osteoclast formation and TGF-β secretion upon tumor sialyl-Tn recognition established dual roles in bone biology and tumor-immune crosstalk, answering what the DAP12 coupling achieves functionally.\",\n      \"evidence\": \"siRNA knockdown, K272A/K274A mutagenesis, Syk inhibitor treatment, bone resorption assay, and co-culture TGF-β ELISA\",\n      \"pmids\": [\"22451653\", \"23035012\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo skeletal phenotype not yet established\", \"Whether TGF-β induction is reproducible across labs (later contested)\", \"Downstream transcriptional targets unresolved\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Generation of Siglec-15 knockout mice revealed mild osteopetrosis and impaired PI3K/Akt and Erk signaling, establishing Siglec-15 as a non-redundant co-stimulatory receptor for osteoclast differentiation in vivo while showing that OSCAR/FcRγ can partially compensate.\",\n      \"evidence\": \"Siglec-15 KO mice with histomorphometry, retroviral rescue of mutants, Western blot for PI3K/Akt/Erk\",\n      \"pmids\": [\"23677868\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Role in pathological bone loss models untested\", \"Mechanism of cytoskeletal defects unclear\", \"Compensatory pathways not fully defined\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Showing that Siglec-15-null mice resist ovariectomy-induced bone loss and that anti-Siglec-15 antibodies increase bone density by inducing receptor internalization and degradation established Siglec-15 as a therapeutic target for osteoporosis.\",\n      \"evidence\": \"Ovariectomy model in KO mice, monoclonal antibody-induced internalization/degradation, DXA bone density measurement\",\n      \"pmids\": [\"25460183\", \"24446437\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Antibody mechanism of action in immune cells unknown\", \"Whether surface removal affects immune functions untested\", \"Cytoskeletal signaling intermediates unidentified\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"A genome-scale T cell activity screen identified Siglec-15 as a major immune suppressor mutually exclusive with PD-L1, fundamentally expanding its biology from bone to cancer immunology and answering whether Siglec-15 has non-redundant immunoregulatory function.\",\n      \"evidence\": \"Genome-scale screen, in vitro T cell suppression, genetic ablation and antibody blockade in mouse tumor models\",\n      \"pmids\": [\"30833750\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Receptor on T cells not identified\", \"Molecular mechanism of T cell suppression unknown\", \"Relevance to human clinical responses unproven\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Linking a SIGLEC15 polymorphism to recurrent vulvovaginal candidiasis revealed a role in mucosal antifungal immunity beyond bone and cancer, showing that Siglec-15 regulates inflammatory cytokine responses and fungal clearance.\",\n      \"evidence\": \"GWAS integration, PBMC stimulation, in vivo siRNA silencing in mouse vaginal infection model\",\n      \"pmids\": [\"31189718\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Precise ligand on Candida or host cells at mucosal surface unknown\", \"Mechanism connecting Siglec-15 to NLRP3/IL-1β not established\", \"Not replicated in independent cohorts\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Broadening glycan specificity from sialyl-Tn alone to general α2,3- and α2,6-sialoglycans, and showing that N-glycosylation at N172 is required for protein stability and immunosuppressive function, resolved the ligand selectivity question and identified a key post-translational regulatory mechanism.\",\n      \"evidence\": \"Glycan microarray, site-directed mutagenesis (N172Q), tumor models in immunocompetent vs. nude mice, glycosidase treatment\",\n      \"pmids\": [\"32501471\", \"34094685\", \"32921411\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis for glycan selectivity not resolved at atomic level\", \"Glycosylation-dependent interactome changes uncharacterized\", \"TGF-β secretion finding from 2012 could not be reproduced\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Demonstrating that Siglec-15 on tumor-associated macrophages engages α2,3-sialylated PDAC cells to activate SYK and drive M2-like polarization established the immunosuppressive signaling axis in solid tumors and showed it is pharmacologically reversible with SYK inhibitors.\",\n      \"evidence\": \"Co-culture with SYK phosphorylation readout, sialic acid binding assay, SYK inhibitor treatment in vivo\",\n      \"pmids\": [\"35077803\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific sialylated glycoprotein counter-receptor on tumor cells not identified in this context\", \"Whether SYK inhibition affects bone-side Siglec-15 function simultaneously\", \"Human TAM relevance limited to correlative data\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Crystal structure determination and identification of CD11b and sialylated TLR2 as functional binding partners on T cells and osteoclast-derived apoptotic bodies, respectively, answered what the molecular targets of Siglec-15 are on immune cells and revealed the structural basis for antibody blockade.\",\n      \"evidence\": \"X-ray crystallography, STD-NMR, molecular dynamics, Co-IP for CD11b and TLR2, T cell binding assays, neutralizing antibody in bone metastasis model\",\n      \"pmids\": [\"37311743\", \"37607544\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether CD11b and TLR2 binding are mutually exclusive or cooperative unknown\", \"How Siglec-15 engagement of CD11b mechanistically suppresses T cell activation not detailed\", \"Relative contributions of different counter-receptors in vivo unquantified\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Identification of ETS-1/ETS-2 downstream of TGF-β/ERK and NFκB as transcriptional drivers of SIGLEC15, and Arid1a/Jun-Fos chromatin remodeling as an epigenetic regulator, established how Siglec-15 expression is controlled in disease contexts.\",\n      \"evidence\": \"ChIP for ETS-1/ETS-2 and Jun/Fos at SIGLEC15 promoter, ATAC-seq, NFκB inhibition, luciferase reporters, conditional Arid1a KO\",\n      \"pmids\": [\"36614238\", \"37465593\", \"38477755\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Integration of NFκB vs. ETS-1/2 vs. Arid1a regulatory inputs unclear\", \"Whether these transcriptional mechanisms operate in immune cells vs. tumor cells specifically\", \"Post-transcriptional regulation (e.g., miR-7109-3p axis) not connected to chromatin-level control\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"A genome-wide CRISPR screen identified LRP1 as a direct Siglec-15 counter-receptor on osteoclast precursors and St3gal4 as the glycosyltransferase generating its α2,3-sialylated ligand, while biochemical work revealed Siglec-15 activates RAP1/Rac1 via a p130CAS/CrkII complex for actin ring formation; Siglec-15/FcRγ double KO causes severe osteopetrosis.\",\n      \"evidence\": \"Genome-wide CRISPR KO screen, Siglec-15 binding validation, Co-IP of p130CAS/CrkII, double KO mice with bone analysis\",\n      \"pmids\": [\"41569849\", \"38849345\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether LRP1 is also a Siglec-15 counter-receptor in the immune context\", \"Structural basis for Siglec-15/LRP1 interaction unresolved\", \"How p130CAS/CrkII complex is recruited to the DAP12/Syk axis not fully delineated\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key open questions include how Siglec-15 engagement of distinct counter-receptors (LRP1, CD11b, TLR2) leads to context-specific outcomes in bone vs. immune cells, what determines the switch between DAP12-Syk and alternative downstream pathways, and whether Siglec-15 blockade will prove effective in human cancer immunotherapy.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No human clinical efficacy data for anti-Siglec-15 therapy reported in the timeline\", \"Context-specific signaling downstream of different counter-receptors not compared systematically\", \"Relative contribution of soluble vs. membrane-bound Siglec-15 in immunosuppression unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [0, 2, 3, 14]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [8, 17, 20, 23]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [5, 11, 15]},\n      {\"term_id\": \"GO:0005764\", \"supporting_discovery_ids\": [5, 11]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0005764\", \"supporting_discovery_ids\": []},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [8, 17, 20, 22, 23]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [2, 3, 4, 14, 17]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [1, 3, 4, 6, 26, 27]}\n    ],\n    \"complexes\": [\n      \"Siglec-15/DAP12/Syk\",\n      \"Siglec-15/p130CAS/CrkII\"\n    ],\n    \"partners\": [\n      \"DAP12\",\n      \"DAP10\",\n      \"SYK\",\n      \"CD11b\",\n      \"LRP1\",\n      \"TLR2\",\n      \"CD44\",\n      \"EGFR\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}