{"gene":"SIGLEC1","run_date":"2026-04-28T20:42:07","timeline":{"discoveries":[{"year":2012,"finding":"Siglec-1 (CD169) on mature dendritic cells specifically binds HIV-1 and vesicles carrying sialyllactose-containing gangliosides (GM3) on the viral membrane, mediating HIV-1 capture and trans-infection of CD4+ T cells through a glycoprotein-independent, ganglioside-dependent pathway.","method":"Selective siRNA knockdown of Siglec-1, neutralizing antibodies, exogenous Siglec-1 expression in naive cells, GSL depletion from virions; co-localization imaging of HIV-1 with CD169 on DC surface and in non-lysosomal compartments","journal":"PLoS biology","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal methods (KD, antibody blocking, rescue by exogenous expression, lipid depletion) in single lab with strong mechanistic resolution; replicated across multiple labs","pmids":["23271952"],"is_preprint":false},{"year":2013,"finding":"CD169/Siglec-1 is required for the capture of B cell-derived exosomes via α2,3-linked sialic acids on their surface; CD169-expressing macrophages in the marginal zone of spleen and subcapsular sinus of lymph node capture and retain exosomes, with CD169-/- mice showing altered exosomal distribution and enhanced immune response to exosomal antigen.","method":"In vitro binding assays on tissue sections, CD169-/- knockout mice, in vivo exosome tracking","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — knockout mouse model with mechanistic receptor identification and in vitro confirmation; moderate evidence from single lab with multiple methods","pmids":["24255917"],"is_preprint":false},{"year":2013,"finding":"Type I IFN-inducible Siglec-1/CD169 on dendritic cells captures HIV-1 in a GM3 ganglioside-dependent manner; captured virus co-localizes with CD169 on DC surface and within non-lysosomal compartments that redistribute to DC-T cell infectious synapses, enabling trans-infection of CD4+ T cells.","method":"Selective CD169 downregulation, neutralizing antibodies, exogenous CD169 expression in receptor-naive cells, GSL depletion, co-localization imaging","journal":"PLoS pathogens","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal methods (KD, antibody block, rescue, lipid depletion, imaging) with strong mechanistic resolution; independently replicated","pmids":["23593001"],"is_preprint":false},{"year":2015,"finding":"Siglec1 associates with DAP12 to recruit and activate SHP2, which then recruits E3 ubiquitin ligase TRIM27; TRIM27 induces K48-linked ubiquitination of TBK1 at Lys251 and Lys372, leading to TBK1 degradation and suppression of type I IFN production in macrophages during viral infection.","method":"Co-immunoprecipitation, siRNA knockdown, ubiquitination assays, site-directed mutagenesis of TBK1 ubiquitination sites, overexpression studies","journal":"Cell research","confidence":"High","confidence_rationale":"Tier 1-2 — reconstitution of signaling complex with Co-IP, mutagenesis of specific ubiquitination sites, and functional readout; moderate evidence from single lab with multiple orthogonal methods","pmids":["26358190"],"is_preprint":false},{"year":2015,"finding":"CD169/Siglec-1 on sinus-lining macrophages captures retroviruses (MLV and HIV) via recognition of gangliosides; MLV-laden CD169+ macrophages form long-lived synaptic contacts with B-1 cells to trans-infect them; infected B-1 cells then migrate into lymph nodes to spread infection via virological synapses. Robust lymph node and spleen infection requires CD169.","method":"Intravital microscopy in living mice, CD169-/- knockout mice, retroviral infection models","journal":"Science (New York, N.Y.)","confidence":"High","confidence_rationale":"Tier 1-2 — direct in vivo visualization of mechanism by intravital microscopy combined with CD169-/- genetic loss-of-function; strong evidence with multiple methods","pmids":["26429886"],"is_preprint":false},{"year":2017,"finding":"Siglec-1 on macrophages initiates formation of the virus-containing compartment (VCC) by capturing ganglioside-containing viral particles from the plasma membrane; Siglec-1 depletion or ganglioside depletion from viral particles prevents VCC formation and substantially reduces VCC volume, and Siglec-1-mediated VCC formation is required for efficient trans-infection of autologous T cells.","method":"Siglec-1 depletion (siRNA), non-infectious VLP addition to macrophage cultures, ganglioside depletion, volumetric imaging of VCC","journal":"PLoS pathogens","confidence":"High","confidence_rationale":"Tier 2 — KD and lipid depletion with quantitative VCC volumetric readout and functional trans-infection assay; moderate evidence with multiple methods","pmids":["28129379"],"is_preprint":false},{"year":2015,"finding":"CD169/Siglec-1-mediated HIV-1 trafficking to deep plasma membrane invaginations (VCCs) protects HIV-1 from antibody-mediated neutralization; the cytoplasmic tail of CD169 is dispensable for HIV-1 trafficking and retention within VCCs, but introduction of a di-aromatic endocytic motif in the tail causes endocytosis and suppresses trans-infection.","method":"Super-resolution microscopy, cytoplasmic tail mutagenesis, broadly neutralizing antibody treatment, trans-infection assays","journal":"PLoS pathogens","confidence":"High","confidence_rationale":"Tier 1-2 — structure-function mutagenesis with super-resolution imaging and functional neutralization assay; moderate evidence","pmids":["25760631"],"is_preprint":false},{"year":2014,"finding":"GM3-CD169 binding is a gp120-independent signal mediating HIV-1 sequestration and preservation of infectivity in non-lysosomal tetraspanin-positive compartments in dendritic cells; this was demonstrated using artificial virus nanoparticles (AVNs) free of viral glycoproteins containing defined GM3 composition.","method":"Artificial virus nanoparticles with defined lipid composition, CD169-expressing HeLa cells and mature DCs, compartment co-localization imaging","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 — reconstituted system with artificial nanoparticles lacking viral glycoproteins; clean mechanistic dissection of GM3-CD169 interaction","pmids":["24947940"],"is_preprint":false},{"year":2011,"finding":"Porcine sialoadhesin (CD169) functions as an endocytic receptor; antibody-triggered internalization is clathrin- and Eps15-dependent and targets cargo to early endosomes but not lysosomes.","method":"Chemical inhibitors of endocytic pathways, dominant-negative constructs, double immunofluorescence staining, primary porcine macrophages and recombinant CD169-expressing cells","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 1-2 — dominant-negative constructs and chemical inhibitors with pathway-specific readout; moderate evidence","pmids":["21359217"],"is_preprint":false},{"year":2012,"finding":"Sialoadhesin (CD169)/Sn-targeted liposomes decorated with high-affinity glycan ligands are selectively internalized by Sn-expressing cells via Sn-mediated endocytosis; uptake is absent in Sn-/- macrophages, and internalized antigen is presented to T cells, demonstrating CD169 as a functional antigen-delivery endocytic receptor.","method":"Liposomal nanoparticle binding/uptake assays, Sn-/- knockout macrophages, T cell proliferation assays","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 2 — specific knockout controls and functional antigen presentation readout; moderate evidence","pmids":["22723922"],"is_preprint":false},{"year":2013,"finding":"CD169/Sn-mediated endocytosis of lipid antigens in CD169+ macrophages enables robust iNKT cell activation in liver and spleen; activation is abrogated in Cd169-/- mice and is macrophage-dependent, demonstrating that targeting CD169+ macrophages is sufficient for systemic iNKT cell activation.","method":"Cd169-/- knockout mice, targeted liposome delivery, macrophage depletion, in vivo iNKT activation readout","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — knockout and depletion models with in vivo functional readout; strong evidence replicated in mouse and human systems","pmids":["23610394"],"is_preprint":false},{"year":2015,"finding":"Mouse Siglec-1 expressed on primary macrophages in an IFN-α-responsive manner captures MLV particles via sialylated gangliosides and mediates trans-infection of B cells more efficiently than T cells; the N-acyl side chain of sialic acid is a critical determinant, with N-butanoyl, N-isobutanoyl, N-glycolyl, or N-pentanoyl modifications reducing virus capture and trans-infection by up to 92% and 80% respectively.","method":"Primary macrophage cultures, biosynthetic sialic acid analog incorporation, MLV capture assays, trans-infection assays, molecular modeling","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — submolecular dissection of sialic acid N-acyl side chain requirements with six analogs and molecular modeling; rigorous in vitro mechanism","pmids":["26370074"],"is_preprint":false},{"year":2019,"finding":"Siglec-1 on dendritic cells recognizes sialylated gangliosides anchored to Ebola virus membranes, mediating viral uptake and cytoplasmic entry; anti-Siglec-1 monoclonal antibodies block Ebola virus uptake and entry, and provide cross-protection against other ganglioside-containing viruses including HIV-1.","method":"Anti-Siglec-1 blocking antibodies, viral uptake and entry assays in activated DCs, cross-protection experiments","journal":"Nature microbiology","confidence":"High","confidence_rationale":"Tier 2 — antibody blocking with functional viral entry readout; strong mechanistic evidence","pmids":["31160823"],"is_preprint":false},{"year":2016,"finding":"Siglec-1 induction in macrophages by endotoxin tolerance suppresses innate immune responses by promoting TGF-β1 production via association with adaptor protein DAP12 and transduction of a signal through Syk; knockdown of Siglec-1 inhibits TGF-β1 production via ubiquitin-dependent degradation of Syk.","method":"Siglec-1 knockdown in RAW 264.7 cells, Syk inhibitor, TGF-β1 measurement, co-association studies with DAP12","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2-3 — KD with functional readout and signaling pathway identification; single lab, moderate methods","pmids":["27129263"],"is_preprint":false},{"year":2018,"finding":"CD169 on splenic macrophages mediates cell-cell contact with BATF3-dependent CD8α+ dendritic cells via its sialic acid-binding capacity, enabling antigen transfer to CD8α+ DCs and subsequent CD8+ T cell cross-priming; this interaction and cross-priming is dependent on the functional sialic acid-binding domain of CD169.","method":"CD169 sialic acid-binding mutants, BATF3-/- mice, DNGR-1-/- mice, antigen targeting assays, T cell activation readout","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 — functional CD169 binding mutants combined with knockout mice and defined cellular phenotype; moderate evidence with multiple methods","pmids":["29425504"],"is_preprint":false},{"year":2020,"finding":"Siglec-1 localizes mainly on microtubule-containing tunneling nanotubes (TNTs) in macrophages; Siglec-1 expression is induced by Mycobacterium tuberculosis-driven type I IFN production, and Siglec-1 depletion decreases TNT length, diminishes HIV-1 capture and cell-to-cell transfer via TNTs, and abrogates the exacerbation of HIV-1 infection induced by Mtb.","method":"Siglec-1 depletion, transcriptomic analysis, TNT imaging, HIV-1 transfer assays, non-human primate co-infection model","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 — depletion with multiple mechanistic readouts (TNT length, viral capture, cell-to-cell transfer) and in vivo NHP validation; moderate-to-strong evidence","pmids":["32223897"],"is_preprint":false},{"year":2022,"finding":"CD169 facilitates ACE2-independent SARS-CoV-2 fusion and entry into macrophages; CD169-mediated SARS-CoV-2 entry results in expression of viral genomic and subgenomic RNAs with minimal viral protein expression and no infectious particle release (post-entry restriction), and restricted viral RNA expression elicits pro-inflammatory cytokine production (TNFα, IL-6, IL-1β) in a RIG-I/MDA-5/MAVS-dependent manner.","method":"CD169 expression/knockdown in macrophages, exogenous ACE2 rescue, viral entry assays, remdesivir treatment, RIG-I/MDA-5/MAVS pathway inhibition","journal":"PLoS pathogens","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal interventions (KD, rescue, pathway inhibitors, antiviral treatment) with defined mechanistic readouts; moderate evidence","pmids":["36279285"],"is_preprint":false},{"year":2017,"finding":"IFN-α-treated myeloid cells (DCs, macrophages) upregulate CD169, which enhances HIV-1 attachment and entry (virus entry step), offsetting the antiviral effects of type I IFN; in DC-T cell cocultures, CD169 on IFNα-treated DCs rescues trans-infection of CD4+ T cells despite IFN-α-induced restriction in T cells.","method":"CD169 KD in THP-1 and primary MDMs, IFN-α treatment, HIV-1 infection assays, DC-T cell coculture trans-infection, SIV-infected macaque lymph node immunohistochemistry","journal":"Journal of virology","confidence":"High","confidence_rationale":"Tier 2 — KD with mechanistic entry-step readout, in vitro and in vivo (NHP) evidence; moderate-to-strong evidence","pmids":["28794041"],"is_preprint":false},{"year":2019,"finding":"Siglec-1-expressing subcapsular sinus macrophages provide anchorage to pioneer metastatic melanoma cells via interactions with hypersialylated cancer cells; Siglec-1-cancer cell interaction drives proliferation (enrichment of cell cycle progression genes in Siglec-1-interacting cells), and knockout of St3gal3 sialyltransferase reduces α-2,3-linked sialylation and impairs metastatic efficiency.","method":"In vitro co-culture, GFP-melanoma mouse model, transcriptome profiling of Siglec1-interacting vs non-interacting cancer cells, St3gal3 knockout","journal":"eLife","confidence":"Medium","confidence_rationale":"Tier 2-3 — in vitro co-culture with transcriptomics and sialyltransferase KO functional validation; moderate evidence from single lab","pmids":["31872800"],"is_preprint":false},{"year":2019,"finding":"Siglec-1 on dendritic cells in the cervical mucosa (lamina propria of ectocervix and endocervix) mediates HIV-1 capture and trans-infection; type I IFN environment enhances viral capture via Siglec-1, and anti-Siglec-1 antibodies prevent HIV-1 transfer.","method":"Ex vivo cervical biopsy, anti-Siglec-1 blocking antibodies, in vivo detection of Siglec-1+ cells harboring HIV-1 compartments in viremic patient biopsy","journal":"Frontiers in immunology","confidence":"Medium","confidence_rationale":"Tier 2-3 — antibody blocking with ex vivo and in vivo confirmation; moderate evidence","pmids":["31114569"],"is_preprint":false},{"year":2019,"finding":"Pre-DC precursors constitutively express Siglec-1, which promotes attachment and fusion of HIV-1 viral particles and enables productive infection of pre-DCs; Siglec-1 does not counteract restriction factors like SAMHD1 but acts at the viral attachment/fusion step, and infected pre-DCs accumulate new particles in intracellular compartments resembling macrophage VCCs.","method":"Siglec-1-dependent infection assays in pre-DCs, HIV-2/SIVmac Vpx rescue, viral particle accumulation imaging","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 — Siglec-1-dependent infection in a defined cell type with mechanistic step identification; moderate evidence, single lab","pmids":["31591213"],"is_preprint":false},{"year":2000,"finding":"Sialoadhesin (Siglec-1) is selectively expressed on the plasma membrane of macrophage subsets, particularly at areas of intimate contact with lymphocytes, and is also detected in intracellular vesicles taken up by macrophages, suggesting roles in cell-cell adhesion and endocytosis.","method":"Immunoelectron microscopy on lymph node and spleen sections of rodents","journal":"Immunobiology","confidence":"Medium","confidence_rationale":"Tier 3 — electron microscopy localization study providing subcellular detail; single method but high resolution","pmids":["11131149"],"is_preprint":false},{"year":2006,"finding":"Sialoadhesin (Siglec-1) expressed on peripheral nerve macrophages promotes CD8+ T lymphocyte accumulation and macrophage activation in demyelinating neuropathy; Siglec-1-deficient P0+/- mice show reduced CD8+ T lymphocytes and macrophages in peripheral nerves and less severe demyelination with improved nerve conduction.","method":"Siglec-1/Sn-deficient mouse crossed with P0+/- demyelination model, nerve conduction measurements, flow cytometry and histology","journal":"Molecular and cellular neurosciences","confidence":"Medium","confidence_rationale":"Tier 2 — double-mutant genetic model with defined neurological phenotype; moderate evidence","pmids":["16458537"],"is_preprint":false},{"year":2022,"finding":"CD169+ macrophage differentiation in lymph node and spleen requires dual signals from LTβR and RANK receptors; loss of either receptor perturbs differentiation, and combined haploinsufficiency reveals equal contributions; splenic RANKL originates from marginal zone stromal cells; loss of CD169+ macrophages (MMMs) compromises viral capture and CD8+ T cell expansion.","method":"Conditional receptor ablation (Cd169-directed), RANKL reporter mouse, double haploinsufficiency models, viral infection challenge","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis with multiple conditional knockout models and defined functional consequences; strong evidence","pmids":["35031565"],"is_preprint":false},{"year":2018,"finding":"CD169+ macrophage depletion in murine Friend virus complex infection reduces viral capture at lymph nodes but paradoxically limits systemic spread to spleen; CD169-expressing macrophages capture blood-borne retroviruses and activate cDC1s, promoting cytotoxic CD8+ T cell responses and efficient clearing of infected cells. CD169 blockade leads to higher viral loads and accelerated death.","method":"CD169-/- mice, CD169 blockade, intravital imaging, cDC1 activation assays, viral load measurement","journal":"Cell host & microbe","confidence":"High","confidence_rationale":"Tier 2 — CD169-/- and blockade with defined in vivo viral dissemination and immune readouts; strong evidence","pmids":["30595553"],"is_preprint":false},{"year":2016,"finding":"SIGLEC1 expression on macrophages is upregulated by viral infection through the IFN/JAK/STAT1 signaling pathway; miR-27a directly targets Siglec1 mRNA, and type I IFN-induced downregulation of miR-27a leads to upregulation of Siglec1 and TRIM27, which feedback inhibits type I IFN production.","method":"miRNA screening in macrophages, miR-27a sponge transgenic mice, luciferase reporter assays for direct targeting, viral infection models (VSV)","journal":"Journal of immunology (Baltimore, Md. : 1950)","confidence":"High","confidence_rationale":"Tier 1-2 — direct miRNA target validation (reporter assay) plus in vivo sponge transgenic mice confirming feedback loop; moderate-strong evidence","pmids":["26700765"],"is_preprint":false},{"year":2016,"finding":"SIGLEC1 loss-of-function variant (Glu88Ter) in humans results in cells that are functionally null or haploinsufficient for Siglec-1 activity in HIV-1 capture and trans-infection ex vivo, confirming the essential role of Siglec-1 in these processes at a molecular level.","method":"Human exome analysis, direct genotyping of 4,233 HIV-infected individuals, ex vivo HIV-1 capture and trans-infection assays on cells from loss-of-function carriers","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 — natural human loss-of-function variant with ex vivo functional validation; strong mechanistic confirmation","pmids":["27510803"],"is_preprint":false},{"year":2020,"finding":"CD169 expression on monocytes is directly induced by type I IFNs (but not type II IFN/IFNγ) via the JAK/STAT signaling pathway within 6-8 hours; IFNα and other type I IFNs specifically upregulate CD169, whereas IFNγ preferentially upregulates CD64, and the two IFN types partially inhibit each other's effects.","method":"Whole blood stimulation with purified IFNs, flow cytometry over defined kinetics, JAK/STAT pathway analysis","journal":"Immunity, inflammation and disease","confidence":"Medium","confidence_rationale":"Tier 2-3 — defined cytokine stimulation with kinetic readout; moderate evidence, single lab","pmids":["32031762"],"is_preprint":false},{"year":2010,"finding":"Sialoadhesin (CD169) on rhinovirus-activated dendritic cells, together with B7-H1 (CD274), delivers a combinatorial signal to T cells that induces IL-35 production and suppressor function; blocking both CD169 and B7-H1 with antibodies prevents induction of IL-35+ regulatory T cells.","method":"Blocking antibodies against CD169 and B7-H1 on DC, IL-35 production assay, T cell suppressor function assay","journal":"European journal of immunology","confidence":"Medium","confidence_rationale":"Tier 3 — dual antibody blocking with functional immune readout; moderate evidence, single lab","pmids":["19950173"],"is_preprint":false},{"year":2020,"finding":"Siglec-1 knockdown in COPD macrophages decreases phagocytosis of non-typeable Haemophilus influenzae (NTHi), demonstrating that Siglec-1 functions as a phagocytic receptor for bacteria on alveolar macrophages.","method":"Anti-Siglec-1 blocking antibody on human alveolar macrophages, phagocytosis assay with fluorescence-tagged bacteria","journal":"Respiratory research","confidence":"Medium","confidence_rationale":"Tier 3 — antibody blocking with defined phagocytosis readout; moderate evidence, single lab","pmids":["31992280"],"is_preprint":false},{"year":2016,"finding":"Knockdown of Siglec-1 in apoE-deficient mice attenuates atherosclerotic lesion formation by suppressing monocyte-endothelial cell adhesion, decreasing chemokine secretion (MCP-1, CXCL2) from aortas and CCR2/CXCR2 expression on monocytes, and reducing macrophage accumulation and oxLDL uptake.","method":"Lentiviral shRNA knockdown in Apoe-/- mice, plaque area measurement, cytokine/chemokine measurement, oxLDL uptake assay","journal":"Clinical immunology (Orlando, Fla.)","confidence":"Medium","confidence_rationale":"Tier 2-3 — in vivo KD with multiple mechanistic readouts; moderate evidence, single lab","pmids":["27871915"],"is_preprint":false}],"current_model":"SIGLEC1/CD169 is a macrophage- and DC-expressed I-type lectin that recognizes α2,3-linked sialic acids and gangliosides (particularly GM3) on pathogens, extracellular vesicles, and cancer cells; it functions as a clathrin-dependent endocytic receptor mediating antigen internalization to early endosomes (not lysosomes) and antigen transfer to DCs and T cells, captures and trans-infects retroviruses (HIV-1, MLV, EBOV) by sequestering ganglioside-bearing viral particles into non-lysosomal virus-containing compartments (VCCs) formed at plasma membrane invaginations for subsequent synaptic transfer, is transcriptionally induced by type I IFN via JAK/STAT1 (and feedback-regulated by miR-27a/TRIM27/TBK1 ubiquitination), signals through DAP12/SHP2/Syk to modulate innate cytokine production, and collaborates with BATF3-dependent cDC1s via sialic acid-dependent cell contacts to drive CD8+ T cell cross-priming."},"narrative":{"teleology":[{"year":2000,"claim":"Ultrastructural localization established that sialoadhesin resides at macrophage–lymphocyte contact zones and in intracellular vesicles, providing the first evidence for dual roles in cell adhesion and endocytosis.","evidence":"Immunoelectron microscopy on rodent lymph node and spleen sections","pmids":["11131149"],"confidence":"Medium","gaps":["Single imaging method without functional confirmation","Molecular mechanism of internalization unknown","Ligands on contacting lymphocytes uncharacterized"]},{"year":2011,"claim":"The endocytic mechanism of CD169 was resolved: internalization is clathrin- and Eps15-dependent and routes cargo to early endosomes rather than lysosomes, explaining how captured material avoids degradation.","evidence":"Chemical inhibitors, dominant-negative constructs, and immunofluorescence in porcine macrophages and recombinant cells","pmids":["21359217"],"confidence":"High","gaps":["Porcine system; pathway conservation in human cells not fully demonstrated","Cytoplasmic tail motifs mediating sorting uncharacterized"]},{"year":2012,"claim":"CD169 was identified as the key receptor mediating HIV-1 capture on dendritic cells through recognition of GM3 ganglioside on viral membranes rather than viral glycoproteins, establishing a glycolipid-dependent, gp120-independent capture mechanism and trans-infection pathway.","evidence":"siRNA knockdown, neutralizing antibodies, exogenous expression rescue, ganglioside depletion from virions, and co-localization imaging","pmids":["23271952","23593001"],"confidence":"High","gaps":["Structural basis of GM3-CD169 interaction at atomic level unresolved","Relative contribution of CD169 versus DC-SIGN in physiological settings uncertain"]},{"year":2012,"claim":"CD169 was validated as a functional antigen-delivery receptor: sialic acid-decorated liposomes are internalized exclusively by Sn-expressing macrophages and processed for T cell antigen presentation, absent in Sn-knockout cells.","evidence":"Liposomal nanoparticle uptake assays with Sn-/- macrophages and T cell proliferation readout","pmids":["22723922"],"confidence":"High","gaps":["Antigen processing pathway downstream of early endosomal delivery not mapped","Whether MHC-I or MHC-II presentation predominates undetermined"]},{"year":2013,"claim":"CD169+ macrophages were shown to capture B cell-derived exosomes in vivo via α2,3-sialic acids, controlling exosome distribution in lymphoid tissues; CD169 knockout altered exosomal distribution and augmented immune responses to exosomal antigen.","evidence":"In vitro binding assays, CD169-/- knockout mice, in vivo exosome tracking in spleen and lymph node","pmids":["24255917"],"confidence":"High","gaps":["Whether exosome capture is tolerogenic or immunogenic context-dependent not resolved","Fate of captured exosomes (degraded vs. transferred) unclear"]},{"year":2013,"claim":"CD169-mediated lipid antigen delivery was shown to be sufficient for systemic iNKT cell activation, establishing CD169+ macrophages as a critical antigen-presenting niche for lipid immunity.","evidence":"Cd169-/- mice, targeted liposome delivery, macrophage depletion, in vivo iNKT activation readout","pmids":["23610394"],"confidence":"High","gaps":["CD1d loading mechanism downstream of CD169-mediated uptake uncharacterized","Whether this pathway operates for endogenous lipid antigens unclear"]},{"year":2014,"claim":"Using artificial virus nanoparticles lacking all viral proteins, the GM3–CD169 interaction alone was shown to be sufficient for sequestration of particles into non-lysosomal tetraspanin-positive compartments, definitively separating the ganglioside recognition mechanism from any viral glycoprotein contribution.","evidence":"Artificial virus nanoparticles with defined lipid composition in CD169-expressing HeLa cells and mature DCs","pmids":["24947940"],"confidence":"High","gaps":["Tetraspanin identity and role in compartment formation not determined","Whether compartment fate differs between reconstituted and physiological systems"]},{"year":2015,"claim":"Three advances defined the VCC biology and signaling axis: (1) CD169 drives formation of virus-containing compartments (VCCs) from plasma membrane invaginations, protecting virus from neutralizing antibodies; (2) the cytoplasmic tail is dispensable for VCC trafficking but an endocytic motif redirects cargo and suppresses trans-infection; (3) CD169 signals through DAP12/SHP2 to recruit TRIM27, which K48-ubiquitinates TBK1 at Lys251/Lys372, degrading it and suppressing type I IFN production.","evidence":"Super-resolution microscopy and tail mutagenesis (VCC); Co-IP, mutagenesis, ubiquitination assays (signaling); intravital microscopy in CD169-/- mice (retroviral spread); sialic acid analog dissection (ligand specificity)","pmids":["25760631","26358190","26429886","26370074"],"confidence":"High","gaps":["How VCC membrane invagination is mechanically generated unknown","Whether DAP12-dependent signaling and endocytic/VCC functions are coupled or independent","TRIM27 recruitment mechanism downstream of SHP2 not structurally resolved"]},{"year":2016,"claim":"Transcriptional and post-transcriptional regulation was mapped: type I IFN induces SIGLEC1 via JAK/STAT1 while simultaneously downregulating miR-27a (which directly targets SIGLEC1 mRNA), creating a feed-forward loop; a human loss-of-function variant (Glu88Ter) confirmed SIGLEC1 is essential for HIV-1 capture and trans-infection ex vivo.","evidence":"miRNA sponge transgenic mice, luciferase reporter assays, human exome analysis of 4,233 HIV-infected individuals with ex vivo functional validation","pmids":["26700765","27510803"],"confidence":"High","gaps":["Whether Glu88Ter homozygosity has clinical consequences beyond HIV capture unknown","Additional transcription factors beyond STAT1 not characterized"]},{"year":2016,"claim":"CD169's DAP12/Syk signaling axis was linked to TGF-β1 production during endotoxin tolerance, and separately, CD169 knockdown in atherosclerosis-prone mice attenuated lesion formation by reducing monocyte adhesion and oxLDL uptake, extending CD169's functional repertoire beyond infectious disease.","evidence":"Syk inhibitor and KD in RAW264.7 cells (tolerance); lentiviral shRNA in ApoE-/- mice with plaque and chemokine readouts (atherosclerosis)","pmids":["27129263","27871915"],"confidence":"Medium","gaps":["Syk-TGF-β1 link demonstrated in cell line only, not primary macrophages","Atherosclerosis phenotype from a single lab with shRNA (not genetic KO)","Direct ligand on endothelial cells or oxLDL mediating these effects not identified"]},{"year":2018,"claim":"CD169's sialic acid-binding domain was shown to mediate direct cell-cell contact with BATF3-dependent CD8α+ dendritic cells, enabling antigen transfer and CD8+ T cell cross-priming — resolving how macrophage-captured antigens reach the cross-presentation pathway.","evidence":"CD169 sialic acid-binding mutants, BATF3-/- and DNGR-1-/- mice, antigen targeting with T cell activation readout","pmids":["29425504"],"confidence":"High","gaps":["Identity of the sialylated ligand on cDC1 surface not determined","Whether antigen is transferred as intact protein, peptide-MHC, or membrane fragment unknown"]},{"year":2019,"claim":"CD169's pathogen capture role was extended to Ebola virus (ganglioside-dependent uptake blocked by anti-Siglec-1 antibodies), and separately, Siglec-1 on subcapsular sinus macrophages was found to anchor hypersialylated metastatic melanoma cells, promoting their proliferation.","evidence":"Anti-Siglec-1 blocking antibodies with viral entry assays in activated DCs (Ebola); co-culture, GFP-melanoma model, St3gal3 KO (metastasis)","pmids":["31160823","31872800"],"confidence":"High","gaps":["Whether anti-Siglec-1 antibodies are protective in vivo against Ebola not tested","Proliferative signal delivered by Siglec-1 to cancer cells not identified","Whether Siglec-1-mediated tumor cell anchorage is α2,3-sialic acid-exclusive"]},{"year":2020,"claim":"CD169 was localized to tunneling nanotubes (TNTs) in macrophages, where it enhances HIV-1 capture and cell-to-cell transfer; Mycobacterium tuberculosis co-infection amplifies this through type I IFN-driven CD169 induction, explaining TB-HIV synergy.","evidence":"Siglec-1 depletion, TNT imaging, HIV-1 transfer assays, non-human primate co-infection model","pmids":["32223897"],"confidence":"High","gaps":["Mechanism by which CD169 promotes TNT elongation not resolved","Whether TNT-mediated transfer dominates over VCC-synapse transfer quantitatively unclear"]},{"year":2022,"claim":"CD169 mediates ACE2-independent SARS-CoV-2 entry into macrophages leading to abortive infection with restricted viral RNA expression that triggers RIG-I/MDA-5/MAVS-dependent pro-inflammatory cytokine production, linking CD169 to COVID-19 macrophage hyperactivation.","evidence":"CD169 expression/knockdown, ACE2 rescue, remdesivir treatment, RIG-I/MDA-5/MAVS pathway inhibition in macrophages","pmids":["36279285"],"confidence":"High","gaps":["Whether CD169-dependent SARS-CoV-2 entry contributes to disease severity in vivo untested","Mechanism of post-entry restriction in macrophages not identified"]},{"year":2022,"claim":"The differentiation signals for CD169+ macrophage subsets were resolved: dual LTβR and RANK signaling from marginal zone stromal cells is required, and loss of these macrophages compromises viral capture and CD8+ T cell expansion.","evidence":"Conditional receptor ablation (Cd169-directed), RANKL reporter mouse, double haploinsufficiency models, viral challenge","pmids":["35031565"],"confidence":"High","gaps":["Downstream transcription factors linking LTβR/RANK to CD169 expression not identified","Whether human CD169+ macrophage differentiation follows the same dual-signal requirement unknown"]},{"year":null,"claim":"Key unresolved questions include the structural basis of GM3–CD169 recognition at atomic resolution, how VCC membrane invaginations are mechanically generated, the identity of sialylated ligands on cDC1s mediating antigen transfer, and whether the DAP12/Syk signaling and endocytic/VCC functions are mechanistically coupled or independent.","evidence":"","pmids":[],"confidence":"Low","gaps":["No crystal structure of full-length CD169 with GM3 ligand","VCC biogenesis mechanism unknown","Sialylated cDC1 counter-receptor unidentified","Coupling between signaling and trafficking functions unexplored"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098631","term_label":"cell adhesion mediator activity","supporting_discovery_ids":[0,1,14,18,21]},{"term_id":"GO:0038024","term_label":"cargo receptor activity","supporting_discovery_ids":[8,9,10]},{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[3,13]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,2,5,6,21]},{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[5,7,8,21]},{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[1,14]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[3,10,13,14,24,25]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[3,13,25]},{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[5,8,9]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[0,2,4,12,16,17]}],"complexes":[],"partners":["DAP12","SHP2","TRIM27","TBK1","SYK"],"other_free_text":[]},"mechanistic_narrative":"SIGLEC1 (CD169/sialoadhesin) is a macrophage- and dendritic cell-restricted I-type lectin that recognizes α2,3-linked sialic acids on gangliosides (particularly GM3), functioning as an endocytic and adhesion receptor that captures extracellular vesicles, pathogens, and sialylated cells to orchestrate innate and adaptive immune responses. SIGLEC1 mediates clathrin-dependent endocytosis targeting cargo to early endosomes, and captures ganglioside-bearing enveloped viruses (HIV-1, MLV, Ebola, SARS-CoV-2) into non-lysosomal virus-containing compartments formed at plasma membrane invaginations, enabling trans-infection of lymphocytes via virological synapses [PMID:23271952, PMID:28129379, PMID:36279285, PMID:31160823]. Transcriptionally induced by type I interferons through JAK/STAT1 signaling and post-transcriptionally regulated by miR-27a, SIGLEC1 signals via the adaptor DAP12 to recruit SHP2 and Syk, and promotes TRIM27-mediated K48-linked ubiquitination and degradation of TBK1, thereby feedback-suppressing type I IFN production [PMID:26358190, PMID:26700765, PMID:27129263]. On subcapsular sinus and marginal zone macrophages, SIGLEC1 captures blood-borne antigens and exosomes via sialic acid recognition, facilitates antigen transfer to BATF3-dependent cDC1s for CD8+ T cell cross-priming, enables iNKT cell activation by lipid antigens, and provides anchorage to hypersialylated metastatic tumor cells [PMID:24255917, PMID:29425504, PMID:23610394, PMID:31872800]."},"prefetch_data":{"uniprot":{"accession":"Q9BZZ2","full_name":"Sialoadhesin","aliases":["Sialic acid-binding Ig-like lectin 1","Siglec-1"],"length_aa":1709,"mass_kda":182.6,"function":"Macrophage-restricted adhesion molecule that mediates sialic-acid dependent binding to lymphocytes, including granulocytes, monocytes, natural killer cells, B-cells and CD8 T-cells. Plays a crucial role in limiting bacterial dissemination by engaging sialylated bacteria to promote effective phagocytosis and antigen presentation for the adaptive immune response (PubMed:12940982, PubMed:33489013). Mediates the uptake of various enveloped viruses via sialic acid recognition and subsequently induces the formation of intracellular compartments filled with virions (VCCs) (PubMed:28129379). In turn, enhances macrophage-to-T-cell transmission of several viruses including HIV-1 or SARS-CoV-2 (PubMed:28129379, PubMed:34782760). Acts as an endocytic receptor mediating clathrin dependent endocytosis. Preferentially binds to alpha-2,3-linked sialic acid (PubMed:12940982). Binds to SPN/CD43 on T-cells (By similarity). May play a role in hemopoiesis. Plays a role in the inhibition of antiviral innate immune by promoting TBK1 degradation via TYROBP and TRIM27-mediated ubiquitination (PubMed:26358190) (Microbial infection) Facilitates viral cytoplasmic entry into activated dendritic cells via recognition of sialylated gangliosides pesent on viral membrane","subcellular_location":"Secreted","url":"https://www.uniprot.org/uniprotkb/Q9BZZ2/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/SIGLEC1","classification":"Not Classified","n_dependent_lines":1,"n_total_lines":1208,"dependency_fraction":0.0008278145695364238},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/SIGLEC1","total_profiled":1310},"omim":[{"mim_id":"613381","title":"CYSTATHIONINE BETA-SYNTHASE; CBS","url":"https://www.omim.org/entry/613381"},{"mim_id":"606094","title":"SIALIC ACID-BINDING IMMUNOGLOBULIN-LIKE LECTIN 12; SIGLEC12","url":"https://www.omim.org/entry/606094"},{"mim_id":"600751","title":"SIALIC ACID-BINDING IMMUNOGLOBULIN-LIKE LECTIN 1; SIGLEC1","url":"https://www.omim.org/entry/600751"},{"mim_id":"182160","title":"SIALOPHORIN; SPN","url":"https://www.omim.org/entry/182160"},{"mim_id":"107266","title":"CD22 ANTIGEN; CD22","url":"https://www.omim.org/entry/107266"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in many","driving_tissues":[],"url":"https://www.proteinatlas.org/search/SIGLEC1"},"hgnc":{"alias_symbol":["SIGLEC-1","CD169","FLJ00051","FLJ00055","FLJ00073","FLJ32150","dJ1009E24.1"],"prev_symbol":["SN"]},"alphafold":{"accession":"Q9BZZ2","domains":[{"cath_id":"2.60.40.10","chopping":"23-215","consensus_level":"medium","plddt":85.6568,"start":23,"end":215},{"cath_id":"2.60.40.10","chopping":"240-322","consensus_level":"high","plddt":85.5081,"start":240,"end":322},{"cath_id":"2.60.40.10","chopping":"338-407","consensus_level":"medium","plddt":79.2916,"start":338,"end":407},{"cath_id":"2.60.40.10","chopping":"410-508_924-975","consensus_level":"medium","plddt":77.6404,"start":410,"end":975},{"cath_id":"2.60.40.10","chopping":"512-596","consensus_level":"medium","plddt":79.7429,"start":512,"end":596},{"cath_id":"2.60.40.10","chopping":"719-732_741-786","consensus_level":"high","plddt":78.452,"start":719,"end":786},{"cath_id":"2.60.40.10","chopping":"794-892","consensus_level":"medium","plddt":76.4071,"start":794,"end":892},{"cath_id":"2.60.40.10","chopping":"1171-1260","consensus_level":"medium","plddt":73.6606,"start":1171,"end":1260},{"cath_id":"2.60.40.10","chopping":"1269-1337","consensus_level":"medium","plddt":78.7371,"start":1269,"end":1337},{"cath_id":"2.60.40.10","chopping":"1347-1390_1399-1531","consensus_level":"medium","plddt":73.8628,"start":1347,"end":1531},{"cath_id":"2.60.40.10","chopping":"1538-1634","consensus_level":"high","plddt":75.5265,"start":1538,"end":1634}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BZZ2","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BZZ2-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BZZ2-F1-predicted_aligned_error_v6.png","plddt_mean":76.62},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SIGLEC1","jax_strain_url":"https://www.jax.org/strain/search?query=SIGLEC1"},"sequence":{"accession":"Q9BZZ2","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9BZZ2.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9BZZ2/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BZZ2"}},"corpus_meta":[{"pmid":"21282381","id":"PMC_21282381","title":"Bone 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co-localization imaging of HIV-1 with CD169 on DC surface and in non-lysosomal compartments\",\n      \"journal\": \"PLoS biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal methods (KD, antibody blocking, rescue by exogenous expression, lipid depletion) in single lab with strong mechanistic resolution; replicated across multiple labs\",\n      \"pmids\": [\"23271952\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"CD169/Siglec-1 is required for the capture of B cell-derived exosomes via α2,3-linked sialic acids on their surface; CD169-expressing macrophages in the marginal zone of spleen and subcapsular sinus of lymph node capture and retain exosomes, with CD169-/- mice showing altered exosomal distribution and enhanced immune response to exosomal antigen.\",\n      \"method\": \"In vitro binding assays on tissue sections, CD169-/- knockout mice, in vivo exosome tracking\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — knockout mouse model with mechanistic receptor identification and in vitro confirmation; moderate evidence from single lab with multiple methods\",\n      \"pmids\": [\"24255917\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Type I IFN-inducible Siglec-1/CD169 on dendritic cells captures HIV-1 in a GM3 ganglioside-dependent manner; captured virus co-localizes with CD169 on DC surface and within non-lysosomal compartments that redistribute to DC-T cell infectious synapses, enabling trans-infection of CD4+ T cells.\",\n      \"method\": \"Selective CD169 downregulation, neutralizing antibodies, exogenous CD169 expression in receptor-naive cells, GSL depletion, co-localization imaging\",\n      \"journal\": \"PLoS pathogens\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal methods (KD, antibody block, rescue, lipid depletion, imaging) with strong mechanistic resolution; independently replicated\",\n      \"pmids\": [\"23593001\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Siglec1 associates with DAP12 to recruit and activate SHP2, which then recruits E3 ubiquitin ligase TRIM27; TRIM27 induces K48-linked ubiquitination of TBK1 at Lys251 and Lys372, leading to TBK1 degradation and suppression of type I IFN production in macrophages during viral infection.\",\n      \"method\": \"Co-immunoprecipitation, siRNA knockdown, ubiquitination assays, site-directed mutagenesis of TBK1 ubiquitination sites, overexpression studies\",\n      \"journal\": \"Cell research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — reconstitution of signaling complex with Co-IP, mutagenesis of specific ubiquitination sites, and functional readout; moderate evidence from single lab with multiple orthogonal methods\",\n      \"pmids\": [\"26358190\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"CD169/Siglec-1 on sinus-lining macrophages captures retroviruses (MLV and HIV) via recognition of gangliosides; MLV-laden CD169+ macrophages form long-lived synaptic contacts with B-1 cells to trans-infect them; infected B-1 cells then migrate into lymph nodes to spread infection via virological synapses. Robust lymph node and spleen infection requires CD169.\",\n      \"method\": \"Intravital microscopy in living mice, CD169-/- knockout mice, retroviral infection models\",\n      \"journal\": \"Science (New York, N.Y.)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — direct in vivo visualization of mechanism by intravital microscopy combined with CD169-/- genetic loss-of-function; strong evidence with multiple methods\",\n      \"pmids\": [\"26429886\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Siglec-1 on macrophages initiates formation of the virus-containing compartment (VCC) by capturing ganglioside-containing viral particles from the plasma membrane; Siglec-1 depletion or ganglioside depletion from viral particles prevents VCC formation and substantially reduces VCC volume, and Siglec-1-mediated VCC formation is required for efficient trans-infection of autologous T cells.\",\n      \"method\": \"Siglec-1 depletion (siRNA), non-infectious VLP addition to macrophage cultures, ganglioside depletion, volumetric imaging of VCC\",\n      \"journal\": \"PLoS pathogens\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KD and lipid depletion with quantitative VCC volumetric readout and functional trans-infection assay; moderate evidence with multiple methods\",\n      \"pmids\": [\"28129379\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"CD169/Siglec-1-mediated HIV-1 trafficking to deep plasma membrane invaginations (VCCs) protects HIV-1 from antibody-mediated neutralization; the cytoplasmic tail of CD169 is dispensable for HIV-1 trafficking and retention within VCCs, but introduction of a di-aromatic endocytic motif in the tail causes endocytosis and suppresses trans-infection.\",\n      \"method\": \"Super-resolution microscopy, cytoplasmic tail mutagenesis, broadly neutralizing antibody treatment, trans-infection assays\",\n      \"journal\": \"PLoS pathogens\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — structure-function mutagenesis with super-resolution imaging and functional neutralization assay; moderate evidence\",\n      \"pmids\": [\"25760631\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"GM3-CD169 binding is a gp120-independent signal mediating HIV-1 sequestration and preservation of infectivity in non-lysosomal tetraspanin-positive compartments in dendritic cells; this was demonstrated using artificial virus nanoparticles (AVNs) free of viral glycoproteins containing defined GM3 composition.\",\n      \"method\": \"Artificial virus nanoparticles with defined lipid composition, CD169-expressing HeLa cells and mature DCs, compartment co-localization imaging\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted system with artificial nanoparticles lacking viral glycoproteins; clean mechanistic dissection of GM3-CD169 interaction\",\n      \"pmids\": [\"24947940\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Porcine sialoadhesin (CD169) functions as an endocytic receptor; antibody-triggered internalization is clathrin- and Eps15-dependent and targets cargo to early endosomes but not lysosomes.\",\n      \"method\": \"Chemical inhibitors of endocytic pathways, dominant-negative constructs, double immunofluorescence staining, primary porcine macrophages and recombinant CD169-expressing cells\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — dominant-negative constructs and chemical inhibitors with pathway-specific readout; moderate evidence\",\n      \"pmids\": [\"21359217\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Sialoadhesin (CD169)/Sn-targeted liposomes decorated with high-affinity glycan ligands are selectively internalized by Sn-expressing cells via Sn-mediated endocytosis; uptake is absent in Sn-/- macrophages, and internalized antigen is presented to T cells, demonstrating CD169 as a functional antigen-delivery endocytic receptor.\",\n      \"method\": \"Liposomal nanoparticle binding/uptake assays, Sn-/- knockout macrophages, T cell proliferation assays\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — specific knockout controls and functional antigen presentation readout; moderate evidence\",\n      \"pmids\": [\"22723922\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"CD169/Sn-mediated endocytosis of lipid antigens in CD169+ macrophages enables robust iNKT cell activation in liver and spleen; activation is abrogated in Cd169-/- mice and is macrophage-dependent, demonstrating that targeting CD169+ macrophages is sufficient for systemic iNKT cell activation.\",\n      \"method\": \"Cd169-/- knockout mice, targeted liposome delivery, macrophage depletion, in vivo iNKT activation readout\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — knockout and depletion models with in vivo functional readout; strong evidence replicated in mouse and human systems\",\n      \"pmids\": [\"23610394\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Mouse Siglec-1 expressed on primary macrophages in an IFN-α-responsive manner captures MLV particles via sialylated gangliosides and mediates trans-infection of B cells more efficiently than T cells; the N-acyl side chain of sialic acid is a critical determinant, with N-butanoyl, N-isobutanoyl, N-glycolyl, or N-pentanoyl modifications reducing virus capture and trans-infection by up to 92% and 80% respectively.\",\n      \"method\": \"Primary macrophage cultures, biosynthetic sialic acid analog incorporation, MLV capture assays, trans-infection assays, molecular modeling\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — submolecular dissection of sialic acid N-acyl side chain requirements with six analogs and molecular modeling; rigorous in vitro mechanism\",\n      \"pmids\": [\"26370074\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Siglec-1 on dendritic cells recognizes sialylated gangliosides anchored to Ebola virus membranes, mediating viral uptake and cytoplasmic entry; anti-Siglec-1 monoclonal antibodies block Ebola virus uptake and entry, and provide cross-protection against other ganglioside-containing viruses including HIV-1.\",\n      \"method\": \"Anti-Siglec-1 blocking antibodies, viral uptake and entry assays in activated DCs, cross-protection experiments\",\n      \"journal\": \"Nature microbiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — antibody blocking with functional viral entry readout; strong mechanistic evidence\",\n      \"pmids\": [\"31160823\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Siglec-1 induction in macrophages by endotoxin tolerance suppresses innate immune responses by promoting TGF-β1 production via association with adaptor protein DAP12 and transduction of a signal through Syk; knockdown of Siglec-1 inhibits TGF-β1 production via ubiquitin-dependent degradation of Syk.\",\n      \"method\": \"Siglec-1 knockdown in RAW 264.7 cells, Syk inhibitor, TGF-β1 measurement, co-association studies with DAP12\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — KD with functional readout and signaling pathway identification; single lab, moderate methods\",\n      \"pmids\": [\"27129263\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"CD169 on splenic macrophages mediates cell-cell contact with BATF3-dependent CD8α+ dendritic cells via its sialic acid-binding capacity, enabling antigen transfer to CD8α+ DCs and subsequent CD8+ T cell cross-priming; this interaction and cross-priming is dependent on the functional sialic acid-binding domain of CD169.\",\n      \"method\": \"CD169 sialic acid-binding mutants, BATF3-/- mice, DNGR-1-/- mice, antigen targeting assays, T cell activation readout\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — functional CD169 binding mutants combined with knockout mice and defined cellular phenotype; moderate evidence with multiple methods\",\n      \"pmids\": [\"29425504\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Siglec-1 localizes mainly on microtubule-containing tunneling nanotubes (TNTs) in macrophages; Siglec-1 expression is induced by Mycobacterium tuberculosis-driven type I IFN production, and Siglec-1 depletion decreases TNT length, diminishes HIV-1 capture and cell-to-cell transfer via TNTs, and abrogates the exacerbation of HIV-1 infection induced by Mtb.\",\n      \"method\": \"Siglec-1 depletion, transcriptomic analysis, TNT imaging, HIV-1 transfer assays, non-human primate co-infection model\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — depletion with multiple mechanistic readouts (TNT length, viral capture, cell-to-cell transfer) and in vivo NHP validation; moderate-to-strong evidence\",\n      \"pmids\": [\"32223897\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"CD169 facilitates ACE2-independent SARS-CoV-2 fusion and entry into macrophages; CD169-mediated SARS-CoV-2 entry results in expression of viral genomic and subgenomic RNAs with minimal viral protein expression and no infectious particle release (post-entry restriction), and restricted viral RNA expression elicits pro-inflammatory cytokine production (TNFα, IL-6, IL-1β) in a RIG-I/MDA-5/MAVS-dependent manner.\",\n      \"method\": \"CD169 expression/knockdown in macrophages, exogenous ACE2 rescue, viral entry assays, remdesivir treatment, RIG-I/MDA-5/MAVS pathway inhibition\",\n      \"journal\": \"PLoS pathogens\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal interventions (KD, rescue, pathway inhibitors, antiviral treatment) with defined mechanistic readouts; moderate evidence\",\n      \"pmids\": [\"36279285\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"IFN-α-treated myeloid cells (DCs, macrophages) upregulate CD169, which enhances HIV-1 attachment and entry (virus entry step), offsetting the antiviral effects of type I IFN; in DC-T cell cocultures, CD169 on IFNα-treated DCs rescues trans-infection of CD4+ T cells despite IFN-α-induced restriction in T cells.\",\n      \"method\": \"CD169 KD in THP-1 and primary MDMs, IFN-α treatment, HIV-1 infection assays, DC-T cell coculture trans-infection, SIV-infected macaque lymph node immunohistochemistry\",\n      \"journal\": \"Journal of virology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KD with mechanistic entry-step readout, in vitro and in vivo (NHP) evidence; moderate-to-strong evidence\",\n      \"pmids\": [\"28794041\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Siglec-1-expressing subcapsular sinus macrophages provide anchorage to pioneer metastatic melanoma cells via interactions with hypersialylated cancer cells; Siglec-1-cancer cell interaction drives proliferation (enrichment of cell cycle progression genes in Siglec-1-interacting cells), and knockout of St3gal3 sialyltransferase reduces α-2,3-linked sialylation and impairs metastatic efficiency.\",\n      \"method\": \"In vitro co-culture, GFP-melanoma mouse model, transcriptome profiling of Siglec1-interacting vs non-interacting cancer cells, St3gal3 knockout\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — in vitro co-culture with transcriptomics and sialyltransferase KO functional validation; moderate evidence from single lab\",\n      \"pmids\": [\"31872800\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Siglec-1 on dendritic cells in the cervical mucosa (lamina propria of ectocervix and endocervix) mediates HIV-1 capture and trans-infection; type I IFN environment enhances viral capture via Siglec-1, and anti-Siglec-1 antibodies prevent HIV-1 transfer.\",\n      \"method\": \"Ex vivo cervical biopsy, anti-Siglec-1 blocking antibodies, in vivo detection of Siglec-1+ cells harboring HIV-1 compartments in viremic patient biopsy\",\n      \"journal\": \"Frontiers in immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — antibody blocking with ex vivo and in vivo confirmation; moderate evidence\",\n      \"pmids\": [\"31114569\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Pre-DC precursors constitutively express Siglec-1, which promotes attachment and fusion of HIV-1 viral particles and enables productive infection of pre-DCs; Siglec-1 does not counteract restriction factors like SAMHD1 but acts at the viral attachment/fusion step, and infected pre-DCs accumulate new particles in intracellular compartments resembling macrophage VCCs.\",\n      \"method\": \"Siglec-1-dependent infection assays in pre-DCs, HIV-2/SIVmac Vpx rescue, viral particle accumulation imaging\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — Siglec-1-dependent infection in a defined cell type with mechanistic step identification; moderate evidence, single lab\",\n      \"pmids\": [\"31591213\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Sialoadhesin (Siglec-1) is selectively expressed on the plasma membrane of macrophage subsets, particularly at areas of intimate contact with lymphocytes, and is also detected in intracellular vesicles taken up by macrophages, suggesting roles in cell-cell adhesion and endocytosis.\",\n      \"method\": \"Immunoelectron microscopy on lymph node and spleen sections of rodents\",\n      \"journal\": \"Immunobiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — electron microscopy localization study providing subcellular detail; single method but high resolution\",\n      \"pmids\": [\"11131149\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Sialoadhesin (Siglec-1) expressed on peripheral nerve macrophages promotes CD8+ T lymphocyte accumulation and macrophage activation in demyelinating neuropathy; Siglec-1-deficient P0+/- mice show reduced CD8+ T lymphocytes and macrophages in peripheral nerves and less severe demyelination with improved nerve conduction.\",\n      \"method\": \"Siglec-1/Sn-deficient mouse crossed with P0+/- demyelination model, nerve conduction measurements, flow cytometry and histology\",\n      \"journal\": \"Molecular and cellular neurosciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — double-mutant genetic model with defined neurological phenotype; moderate evidence\",\n      \"pmids\": [\"16458537\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"CD169+ macrophage differentiation in lymph node and spleen requires dual signals from LTβR and RANK receptors; loss of either receptor perturbs differentiation, and combined haploinsufficiency reveals equal contributions; splenic RANKL originates from marginal zone stromal cells; loss of CD169+ macrophages (MMMs) compromises viral capture and CD8+ T cell expansion.\",\n      \"method\": \"Conditional receptor ablation (Cd169-directed), RANKL reporter mouse, double haploinsufficiency models, viral infection challenge\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with multiple conditional knockout models and defined functional consequences; strong evidence\",\n      \"pmids\": [\"35031565\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"CD169+ macrophage depletion in murine Friend virus complex infection reduces viral capture at lymph nodes but paradoxically limits systemic spread to spleen; CD169-expressing macrophages capture blood-borne retroviruses and activate cDC1s, promoting cytotoxic CD8+ T cell responses and efficient clearing of infected cells. CD169 blockade leads to higher viral loads and accelerated death.\",\n      \"method\": \"CD169-/- mice, CD169 blockade, intravital imaging, cDC1 activation assays, viral load measurement\",\n      \"journal\": \"Cell host & microbe\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — CD169-/- and blockade with defined in vivo viral dissemination and immune readouts; strong evidence\",\n      \"pmids\": [\"30595553\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"SIGLEC1 expression on macrophages is upregulated by viral infection through the IFN/JAK/STAT1 signaling pathway; miR-27a directly targets Siglec1 mRNA, and type I IFN-induced downregulation of miR-27a leads to upregulation of Siglec1 and TRIM27, which feedback inhibits type I IFN production.\",\n      \"method\": \"miRNA screening in macrophages, miR-27a sponge transgenic mice, luciferase reporter assays for direct targeting, viral infection models (VSV)\",\n      \"journal\": \"Journal of immunology (Baltimore, Md. : 1950)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — direct miRNA target validation (reporter assay) plus in vivo sponge transgenic mice confirming feedback loop; moderate-strong evidence\",\n      \"pmids\": [\"26700765\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"SIGLEC1 loss-of-function variant (Glu88Ter) in humans results in cells that are functionally null or haploinsufficient for Siglec-1 activity in HIV-1 capture and trans-infection ex vivo, confirming the essential role of Siglec-1 in these processes at a molecular level.\",\n      \"method\": \"Human exome analysis, direct genotyping of 4,233 HIV-infected individuals, ex vivo HIV-1 capture and trans-infection assays on cells from loss-of-function carriers\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — natural human loss-of-function variant with ex vivo functional validation; strong mechanistic confirmation\",\n      \"pmids\": [\"27510803\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"CD169 expression on monocytes is directly induced by type I IFNs (but not type II IFN/IFNγ) via the JAK/STAT signaling pathway within 6-8 hours; IFNα and other type I IFNs specifically upregulate CD169, whereas IFNγ preferentially upregulates CD64, and the two IFN types partially inhibit each other's effects.\",\n      \"method\": \"Whole blood stimulation with purified IFNs, flow cytometry over defined kinetics, JAK/STAT pathway analysis\",\n      \"journal\": \"Immunity, inflammation and disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — defined cytokine stimulation with kinetic readout; moderate evidence, single lab\",\n      \"pmids\": [\"32031762\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Sialoadhesin (CD169) on rhinovirus-activated dendritic cells, together with B7-H1 (CD274), delivers a combinatorial signal to T cells that induces IL-35 production and suppressor function; blocking both CD169 and B7-H1 with antibodies prevents induction of IL-35+ regulatory T cells.\",\n      \"method\": \"Blocking antibodies against CD169 and B7-H1 on DC, IL-35 production assay, T cell suppressor function assay\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — dual antibody blocking with functional immune readout; moderate evidence, single lab\",\n      \"pmids\": [\"19950173\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Siglec-1 knockdown in COPD macrophages decreases phagocytosis of non-typeable Haemophilus influenzae (NTHi), demonstrating that Siglec-1 functions as a phagocytic receptor for bacteria on alveolar macrophages.\",\n      \"method\": \"Anti-Siglec-1 blocking antibody on human alveolar macrophages, phagocytosis assay with fluorescence-tagged bacteria\",\n      \"journal\": \"Respiratory research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — antibody blocking with defined phagocytosis readout; moderate evidence, single lab\",\n      \"pmids\": [\"31992280\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Knockdown of Siglec-1 in apoE-deficient mice attenuates atherosclerotic lesion formation by suppressing monocyte-endothelial cell adhesion, decreasing chemokine secretion (MCP-1, CXCL2) from aortas and CCR2/CXCR2 expression on monocytes, and reducing macrophage accumulation and oxLDL uptake.\",\n      \"method\": \"Lentiviral shRNA knockdown in Apoe-/- mice, plaque area measurement, cytokine/chemokine measurement, oxLDL uptake assay\",\n      \"journal\": \"Clinical immunology (Orlando, Fla.)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — in vivo KD with multiple mechanistic readouts; moderate evidence, single lab\",\n      \"pmids\": [\"27871915\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SIGLEC1/CD169 is a macrophage- and DC-expressed I-type lectin that recognizes α2,3-linked sialic acids and gangliosides (particularly GM3) on pathogens, extracellular vesicles, and cancer cells; it functions as a clathrin-dependent endocytic receptor mediating antigen internalization to early endosomes (not lysosomes) and antigen transfer to DCs and T cells, captures and trans-infects retroviruses (HIV-1, MLV, EBOV) by sequestering ganglioside-bearing viral particles into non-lysosomal virus-containing compartments (VCCs) formed at plasma membrane invaginations for subsequent synaptic transfer, is transcriptionally induced by type I IFN via JAK/STAT1 (and feedback-regulated by miR-27a/TRIM27/TBK1 ubiquitination), signals through DAP12/SHP2/Syk to modulate innate cytokine production, and collaborates with BATF3-dependent cDC1s via sialic acid-dependent cell contacts to drive CD8+ T cell cross-priming.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"SIGLEC1 (CD169/sialoadhesin) is a macrophage- and dendritic cell-restricted I-type lectin that recognizes α2,3-linked sialic acids on gangliosides (particularly GM3), functioning as an endocytic and adhesion receptor that captures extracellular vesicles, pathogens, and sialylated cells to orchestrate innate and adaptive immune responses. SIGLEC1 mediates clathrin-dependent endocytosis targeting cargo to early endosomes, and captures ganglioside-bearing enveloped viruses (HIV-1, MLV, Ebola, SARS-CoV-2) into non-lysosomal virus-containing compartments formed at plasma membrane invaginations, enabling trans-infection of lymphocytes via virological synapses [PMID:23271952, PMID:28129379, PMID:36279285, PMID:31160823]. Transcriptionally induced by type I interferons through JAK/STAT1 signaling and post-transcriptionally regulated by miR-27a, SIGLEC1 signals via the adaptor DAP12 to recruit SHP2 and Syk, and promotes TRIM27-mediated K48-linked ubiquitination and degradation of TBK1, thereby feedback-suppressing type I IFN production [PMID:26358190, PMID:26700765, PMID:27129263]. On subcapsular sinus and marginal zone macrophages, SIGLEC1 captures blood-borne antigens and exosomes via sialic acid recognition, facilitates antigen transfer to BATF3-dependent cDC1s for CD8+ T cell cross-priming, enables iNKT cell activation by lipid antigens, and provides anchorage to hypersialylated metastatic tumor cells [PMID:24255917, PMID:29425504, PMID:23610394, PMID:31872800].\",\n  \"teleology\": [\n    {\n      \"year\": 2000,\n      \"claim\": \"Ultrastructural localization established that sialoadhesin resides at macrophage–lymphocyte contact zones and in intracellular vesicles, providing the first evidence for dual roles in cell adhesion and endocytosis.\",\n      \"evidence\": \"Immunoelectron microscopy on rodent lymph node and spleen sections\",\n      \"pmids\": [\"11131149\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single imaging method without functional confirmation\", \"Molecular mechanism of internalization unknown\", \"Ligands on contacting lymphocytes uncharacterized\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"The endocytic mechanism of CD169 was resolved: internalization is clathrin- and Eps15-dependent and routes cargo to early endosomes rather than lysosomes, explaining how captured material avoids degradation.\",\n      \"evidence\": \"Chemical inhibitors, dominant-negative constructs, and immunofluorescence in porcine macrophages and recombinant cells\",\n      \"pmids\": [\"21359217\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Porcine system; pathway conservation in human cells not fully demonstrated\", \"Cytoplasmic tail motifs mediating sorting uncharacterized\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"CD169 was identified as the key receptor mediating HIV-1 capture on dendritic cells through recognition of GM3 ganglioside on viral membranes rather than viral glycoproteins, establishing a glycolipid-dependent, gp120-independent capture mechanism and trans-infection pathway.\",\n      \"evidence\": \"siRNA knockdown, neutralizing antibodies, exogenous expression rescue, ganglioside depletion from virions, and co-localization imaging\",\n      \"pmids\": [\"23271952\", \"23593001\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of GM3-CD169 interaction at atomic level unresolved\", \"Relative contribution of CD169 versus DC-SIGN in physiological settings uncertain\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"CD169 was validated as a functional antigen-delivery receptor: sialic acid-decorated liposomes are internalized exclusively by Sn-expressing macrophages and processed for T cell antigen presentation, absent in Sn-knockout cells.\",\n      \"evidence\": \"Liposomal nanoparticle uptake assays with Sn-/- macrophages and T cell proliferation readout\",\n      \"pmids\": [\"22723922\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Antigen processing pathway downstream of early endosomal delivery not mapped\", \"Whether MHC-I or MHC-II presentation predominates undetermined\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"CD169+ macrophages were shown to capture B cell-derived exosomes in vivo via α2,3-sialic acids, controlling exosome distribution in lymphoid tissues; CD169 knockout altered exosomal distribution and augmented immune responses to exosomal antigen.\",\n      \"evidence\": \"In vitro binding assays, CD169-/- knockout mice, in vivo exosome tracking in spleen and lymph node\",\n      \"pmids\": [\"24255917\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether exosome capture is tolerogenic or immunogenic context-dependent not resolved\", \"Fate of captured exosomes (degraded vs. transferred) unclear\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"CD169-mediated lipid antigen delivery was shown to be sufficient for systemic iNKT cell activation, establishing CD169+ macrophages as a critical antigen-presenting niche for lipid immunity.\",\n      \"evidence\": \"Cd169-/- mice, targeted liposome delivery, macrophage depletion, in vivo iNKT activation readout\",\n      \"pmids\": [\"23610394\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"CD1d loading mechanism downstream of CD169-mediated uptake uncharacterized\", \"Whether this pathway operates for endogenous lipid antigens unclear\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Using artificial virus nanoparticles lacking all viral proteins, the GM3–CD169 interaction alone was shown to be sufficient for sequestration of particles into non-lysosomal tetraspanin-positive compartments, definitively separating the ganglioside recognition mechanism from any viral glycoprotein contribution.\",\n      \"evidence\": \"Artificial virus nanoparticles with defined lipid composition in CD169-expressing HeLa cells and mature DCs\",\n      \"pmids\": [\"24947940\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Tetraspanin identity and role in compartment formation not determined\", \"Whether compartment fate differs between reconstituted and physiological systems\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Three advances defined the VCC biology and signaling axis: (1) CD169 drives formation of virus-containing compartments (VCCs) from plasma membrane invaginations, protecting virus from neutralizing antibodies; (2) the cytoplasmic tail is dispensable for VCC trafficking but an endocytic motif redirects cargo and suppresses trans-infection; (3) CD169 signals through DAP12/SHP2 to recruit TRIM27, which K48-ubiquitinates TBK1 at Lys251/Lys372, degrading it and suppressing type I IFN production.\",\n      \"evidence\": \"Super-resolution microscopy and tail mutagenesis (VCC); Co-IP, mutagenesis, ubiquitination assays (signaling); intravital microscopy in CD169-/- mice (retroviral spread); sialic acid analog dissection (ligand specificity)\",\n      \"pmids\": [\"25760631\", \"26358190\", \"26429886\", \"26370074\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How VCC membrane invagination is mechanically generated unknown\", \"Whether DAP12-dependent signaling and endocytic/VCC functions are coupled or independent\", \"TRIM27 recruitment mechanism downstream of SHP2 not structurally resolved\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Transcriptional and post-transcriptional regulation was mapped: type I IFN induces SIGLEC1 via JAK/STAT1 while simultaneously downregulating miR-27a (which directly targets SIGLEC1 mRNA), creating a feed-forward loop; a human loss-of-function variant (Glu88Ter) confirmed SIGLEC1 is essential for HIV-1 capture and trans-infection ex vivo.\",\n      \"evidence\": \"miRNA sponge transgenic mice, luciferase reporter assays, human exome analysis of 4,233 HIV-infected individuals with ex vivo functional validation\",\n      \"pmids\": [\"26700765\", \"27510803\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether Glu88Ter homozygosity has clinical consequences beyond HIV capture unknown\", \"Additional transcription factors beyond STAT1 not characterized\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"CD169's DAP12/Syk signaling axis was linked to TGF-β1 production during endotoxin tolerance, and separately, CD169 knockdown in atherosclerosis-prone mice attenuated lesion formation by reducing monocyte adhesion and oxLDL uptake, extending CD169's functional repertoire beyond infectious disease.\",\n      \"evidence\": \"Syk inhibitor and KD in RAW264.7 cells (tolerance); lentiviral shRNA in ApoE-/- mice with plaque and chemokine readouts (atherosclerosis)\",\n      \"pmids\": [\"27129263\", \"27871915\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Syk-TGF-β1 link demonstrated in cell line only, not primary macrophages\", \"Atherosclerosis phenotype from a single lab with shRNA (not genetic KO)\", \"Direct ligand on endothelial cells or oxLDL mediating these effects not identified\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"CD169's sialic acid-binding domain was shown to mediate direct cell-cell contact with BATF3-dependent CD8α+ dendritic cells, enabling antigen transfer and CD8+ T cell cross-priming — resolving how macrophage-captured antigens reach the cross-presentation pathway.\",\n      \"evidence\": \"CD169 sialic acid-binding mutants, BATF3-/- and DNGR-1-/- mice, antigen targeting with T cell activation readout\",\n      \"pmids\": [\"29425504\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the sialylated ligand on cDC1 surface not determined\", \"Whether antigen is transferred as intact protein, peptide-MHC, or membrane fragment unknown\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"CD169's pathogen capture role was extended to Ebola virus (ganglioside-dependent uptake blocked by anti-Siglec-1 antibodies), and separately, Siglec-1 on subcapsular sinus macrophages was found to anchor hypersialylated metastatic melanoma cells, promoting their proliferation.\",\n      \"evidence\": \"Anti-Siglec-1 blocking antibodies with viral entry assays in activated DCs (Ebola); co-culture, GFP-melanoma model, St3gal3 KO (metastasis)\",\n      \"pmids\": [\"31160823\", \"31872800\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether anti-Siglec-1 antibodies are protective in vivo against Ebola not tested\", \"Proliferative signal delivered by Siglec-1 to cancer cells not identified\", \"Whether Siglec-1-mediated tumor cell anchorage is α2,3-sialic acid-exclusive\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"CD169 was localized to tunneling nanotubes (TNTs) in macrophages, where it enhances HIV-1 capture and cell-to-cell transfer; Mycobacterium tuberculosis co-infection amplifies this through type I IFN-driven CD169 induction, explaining TB-HIV synergy.\",\n      \"evidence\": \"Siglec-1 depletion, TNT imaging, HIV-1 transfer assays, non-human primate co-infection model\",\n      \"pmids\": [\"32223897\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which CD169 promotes TNT elongation not resolved\", \"Whether TNT-mediated transfer dominates over VCC-synapse transfer quantitatively unclear\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"CD169 mediates ACE2-independent SARS-CoV-2 entry into macrophages leading to abortive infection with restricted viral RNA expression that triggers RIG-I/MDA-5/MAVS-dependent pro-inflammatory cytokine production, linking CD169 to COVID-19 macrophage hyperactivation.\",\n      \"evidence\": \"CD169 expression/knockdown, ACE2 rescue, remdesivir treatment, RIG-I/MDA-5/MAVS pathway inhibition in macrophages\",\n      \"pmids\": [\"36279285\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether CD169-dependent SARS-CoV-2 entry contributes to disease severity in vivo untested\", \"Mechanism of post-entry restriction in macrophages not identified\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"The differentiation signals for CD169+ macrophage subsets were resolved: dual LTβR and RANK signaling from marginal zone stromal cells is required, and loss of these macrophages compromises viral capture and CD8+ T cell expansion.\",\n      \"evidence\": \"Conditional receptor ablation (Cd169-directed), RANKL reporter mouse, double haploinsufficiency models, viral challenge\",\n      \"pmids\": [\"35031565\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream transcription factors linking LTβR/RANK to CD169 expression not identified\", \"Whether human CD169+ macrophage differentiation follows the same dual-signal requirement unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the structural basis of GM3–CD169 recognition at atomic resolution, how VCC membrane invaginations are mechanically generated, the identity of sialylated ligands on cDC1s mediating antigen transfer, and whether the DAP12/Syk signaling and endocytic/VCC functions are mechanistically coupled or independent.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No crystal structure of full-length CD169 with GM3 ligand\", \"VCC biogenesis mechanism unknown\", \"Sialylated cDC1 counter-receptor unidentified\", \"Coupling between signaling and trafficking functions unexplored\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098631\", \"supporting_discovery_ids\": [0, 1, 14, 18, 21]},\n      {\"term_id\": \"GO:0038024\", \"supporting_discovery_ids\": [8, 9, 10]},\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [3, 13]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 2, 5, 6, 21]},\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [5, 7, 8, 21]},\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [1, 14]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [3, 10, 13, 14, 24, 25]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [3, 13, 25]},\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [5, 8, 9]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [0, 2, 4, 12, 16, 17]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"DAP12\", \"SHP2\", \"TRIM27\", \"TBK1\", \"SYK\"],\n    \"other_free_text\": []\n  }\n}\n```"}