{"gene":"PIGR","run_date":"2026-06-10T06:43:35","timeline":{"discoveries":[{"year":1998,"finding":"J chain is required for polymeric IgA binding to secretory component (SC)/pIgR and for pIgR-mediated epithelial transcytosis. A J-chain-lacking tetrameric IgA (pIgA-L) failed to bind purified SC, was not transferred into rat bile after intravenous injection, and was not transported apically by polarized MDCK cells expressing human pIgR, whereas J-chain-containing pIgA preparations were efficiently transported in both systems.","method":"In vitro SC-binding assay, in vivo rat bile transport assay, polarized MDCK cell transcytosis assay","journal":"Immunology","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple orthogonal in vitro and in vivo assays with isogenic comparator molecules (J-chain-positive vs J-chain-lacking pIgA), clear mechanistic conclusion","pmids":["9767462"],"is_preprint":false},{"year":2001,"finding":"Human pIgR-mediated transcytosis is not stimulated by pIgA binding, despite pIgA inducing IP3 production (phospholipase-C activation) in cells expressing human pIgR. In contrast, rabbit and rat pIgR transcytosis is accelerated by pIgA. The species difference is not due to defective second-messenger production but to a different sensitivity of human pIgR to intracellular calcium. PKC activation by PMA stimulates both human and rabbit pIgR transcytosis.","method":"Continuous apical SC release assay in human Calu-3 and pIgR-transfected MDCK cells; IP3 measurement; PKC activation with PMA","journal":"Scandinavian journal of immunology","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — multiple orthogonal cell-based assays with rigorous controls comparing human vs rabbit pIgR in the same system; definitive mechanistic conclusion about signaling pathway","pmids":["11169207"],"is_preprint":false},{"year":2001,"finding":"A microsatellite-containing fragment from the 3'-UTR of the rat Pigr gene modulates gene expression in an orientation- and position-dependent manner dependent on DNA supercoiling, functioning through intramolecular triplex formation stabilized by supercoiling, suggesting a regulatory role for this genomic element in controlling Pigr expression.","method":"Luciferase reporter transient transfection assay, cell-free translation, nuclease S1/P1 hypersensitivity, gel mobility analysis, anomalous melting profiles","journal":"Physiological genomics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal biochemical and cell-based methods in a single lab; demonstrates functional consequence of a regulatory element in the Pigr 3'-UTR","pmids":["11242589"],"is_preprint":false},{"year":2010,"finding":"pIgA and IgG Fc (transported by FcRn) follow initially separate but later intermixing trafficking routes during transcytosis in polarized MDCK cells co-expressing both receptors. pIgA transport is strongly unidirectional (basolateral-to-apical), transiently colocalizing with EEA1-positive early endosomes, Rab11a-positive recycling endosomes, and transferrin-positive basolateral recycling endosomes. Unlike FcRn cargo, pIgA was sorted away from transferrin-positive endosomes with time. Both trafficking routes depended equally on intact microtubules.","method":"Fluorescence confocal microscopy with pulse-chase experiments, co-expression of FcRn and pIgR in MDCK cells, live-cell imaging, microtubule depolymerization","journal":"Traffic (Copenhagen, Denmark)","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct visualization of pIgR/pIgA trafficking with multiple endosomal markers, live imaging, and pharmacological perturbation in a single rigorous study","pmids":["20525015"],"is_preprint":false},{"year":2010,"finding":"TGF-β released by activated neutrophils upregulates pIgR/SC production in human bronchial epithelial (Calu-3) cells through a redox-sensitive, p38 MAPK-dependent pathway. This effect was mimicked by exogenous TGF-β and blocked by inhibition of redox balance or p38 MAPK.","method":"Neutrophil–epithelial cell co-culture, TGF-β measurement, p38 MAPK inhibition, redox balance inhibition, SC production assay","journal":"Journal of biomedicine & biotechnology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple pathway inhibitor experiments and cytokine identification in a single lab, clear mechanistic pathway identified","pmids":["20706611"],"is_preprint":false},{"year":2014,"finding":"Streptococcus pneumoniae physically interacts with pIgR on brain microvascular endothelial cells in vivo and in vitro, and pIgR blocking reduced pneumococcal adhesion to endothelial cells. Pneumococci co-localized with pIgR (but not with PAFR) in vivo. Physical interaction of pneumococci with pIgR was confirmed by incubating bacteria with endothelial cell lysates.","method":"In vivo mouse intravenous infection model, immunofluorescent co-localization analysis, in vitro antibody blocking, pneumococcal incubation with endothelial cell lysates","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo and in vitro co-localization, antibody blocking, and direct binding in lysates from a single lab","pmids":["24841255"],"is_preprint":false},{"year":2017,"finding":"The pneumococcal pilus-1 major adhesin RrgA binds both pIgR and PECAM-1 on the blood-brain barrier endothelium, whereas choline binding protein PspC binds pIgR only (to a lower extent). Antibodies against pIgR and PECAM-1 prevented pneumococcal entry into the brain in a bacteremia-derived meningitis mouse model. Addition of these antibodies to ceftriaxone-treated mice further reduced brain bacterial burden.","method":"STED super-resolution microscopy of human brain biopsies, mutant mouse and antibody-blocking experiments in a bacteremia-derived meningitis model, binding studies with recombinant adhesins","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 2 / Strong — super-resolution microscopy of human tissue, direct binding assays, and in vivo genetic/antibody-blocking model all confirm pIgR as a pneumococcal adhesion receptor at the BBB","pmids":["28515075"],"is_preprint":false},{"year":2021,"finding":"Rab11 effectors Rab11-FIP1 and Rab11-FIP5 interact with pIgR/pIgA-containing endosomes and are required for efficient pIgA transcytosis. Their knockdown additively impaired transcytosis in polarized and incompletely polarized cells. In incompletely polarized cells, pIgR/pIgA trafficking involves transport from the basolateral membrane to the centrosome vicinity, then via Rab11a-positive endosomes through the Golgi to the apical membrane. TRIM21 mediates K11-linked polyubiquitination of Rab11-FIP1 and K6-linked polyubiquitination of Rab11-FIP5 to promote their activation and pIgA transcytosis.","method":"Protein interaction (Rab11-FIP1 identified as new pIgR interactor), siRNA knockdown, live-cell trafficking assays in polarized and incompletely polarized cells, ubiquitination analysis","journal":"International journal of molecular sciences","confidence":"High","confidence_rationale":"Tier 2 / Strong — identification of binding partner, siRNA knockdown with quantitative transcytosis readout, and characterization of specific ubiquitin linkage type by TRIM21; multiple orthogonal methods","pmids":["34638806"],"is_preprint":false},{"year":2021,"finding":"EV-pIgR from late-stage HCC patients promotes cancer stemness and tumorigenesis in recipient cells via activation of the PDK1/Akt/GSK3β/β-catenin signaling axis. Blockade with an anti-pIgR neutralizing antibody abrogated these effects and attenuated tumor growth in patient-derived tumor xenograft mice.","method":"EV isolation and characterization, in vitro cancer stemness assays, in vivo PDTX mouse model, pathway inhibition with Akt and β-catenin inhibitors, anti-pIgR neutralizing antibody blockade","journal":"Journal of hepatology","confidence":"High","confidence_rationale":"Tier 2 / Strong — mechanistic pathway identified using pathway-specific inhibitors, confirmed in vivo with neutralizing antibody in PDTXs; multiple orthogonal methods","pmids":["34922977"],"is_preprint":false},{"year":2023,"finding":"Hepatic pIgR mediates IgA secretion into the intestinal lumen and bile canaliculi, limiting bacterial translocation and preventing ethanol-induced liver disease. pIgR-deficient mice showed increased liver injury, steatosis, inflammation, elevated plasma LPS, and more hepatic bacteria after ethanol feeding. AAV8-mediated re-expression of pIgR specifically in hepatocytes of pIgR-deficient mice increased intestinal IgA and ameliorated steatohepatitis by reducing bacterial translocation.","method":"pIgR-deficient mouse model, chronic-binge and Lieber-DeCarli ethanol feeding models, AAV8-mediated hepatocyte-specific pIgR re-expression, non-absorbable antibiotic treatment, liver injury and bacterial burden assessment","journal":"Gut","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic rescue (AAV8 re-expression) and antibiotic control confirm mechanism; KO phenotype replicated across two ethanol feeding models","pmids":["36690432"],"is_preprint":false},{"year":1998,"finding":"All-trans retinoic acid (RA) is required for normal regulation of pIgR expression by IL-4 and IFN-γ in HT-29 human intestinal epithelial cells. Vitamin A depletion significantly reduced cytokine-induced pIgR upregulation at both protein and mRNA levels; RA supplementation restored normal pIgR expression levels in a dose-dependent manner.","method":"Vitamin A-depleted cell culture, RA supplementation, flow cytometry for cell-surface pIgR, mRNA quantification by Northern/RT-PCR in HT-29 cells","journal":"The Journal of nutrition","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — protein and mRNA-level measurements with dose-response RA titration; single lab but multiple readouts","pmids":["9649586"],"is_preprint":false},{"year":1997,"finding":"Uterine stromal cells suppress pIgR production by co-cultured rat uterine epithelial cells through a soluble factor present in stromal cell-conditioned supernatants, representing a stromal–epithelial regulatory mechanism for pIgR expression.","method":"Co-culture of rat uterine stromal and epithelial cells, conditioned supernatant transfer experiments, immunohistochemistry for pIgR, transepithelial resistance measurement","journal":"Journal of reproductive immunology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — conditioned medium transfer and co-culture with immunohistochemical confirmation; single lab, cell-based assay without full molecular identification of the soluble factor","pmids":["9234210"],"is_preprint":false},{"year":2002,"finding":"pIgR expression in the sheep mammary gland is regulated by a combination of prolactin and glucocorticoids: estradiol and progesterone alone produced slight increases in pIgR mRNA, but glucocorticoid addition caused significant accumulation of pIgR mRNA; blockade of prolactin secretion with bromocryptine abolished the hormonal induction of pIgR.","method":"Northern blot, in situ hybridization, immunohistochemistry in hormonally treated virgin ewes and during pregnancy/lactation; bromocryptine prolactin blockade","journal":"The Journal of dairy research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo hormonal manipulation with molecular readouts and prolactin-blockade experiment; single lab","pmids":["12047104"],"is_preprint":false},{"year":2024,"finding":"SARS-CoV-2 accessory protein ORF8 downregulates pIgR expression through a direct interaction with pIgR. ORF8-mediated downregulation diminishes the binding of dimeric IgA (dIgA) and pentameric IgM to pIgR. Secreted ORF8 binds cell surface pIgR but does not trigger cellular internalization of ORF8 (unlike dIgA binding to pIgR, which does trigger internalization). ORF8 proteins from SARS-CoV-2 variants of concern preserve this pIgR-downregulating function.","method":"Protein–protein interaction studies between ORF8 and pIgR, pIgR expression measurement, dIgA/pIgM binding assays, cell internalization assay","journal":"Viruses","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct interaction demonstrated with functional consequence (reduced IgA/IgM binding, blocked internalization); single lab, multiple readouts","pmids":["39066171"],"is_preprint":false},{"year":2016,"finding":"Chlamydia infection upregulates pIgR expression in human epithelia and in mouse male and female reproductive tract epithelia in vivo, and this is associated with increased transcytosis of IgA into the lumen. Hormone synchronization with Depo-Provera downregulated pIgR expression, with pIgR being highest during estrus.","method":"Western blot, immunohistochemistry in vitro and in vivo (mouse reproductive tract), functional IgA transcytosis assay","journal":"American journal of reproductive immunology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — in vitro and in vivo evidence with Western blot and IHC; single lab, but both human cell and mouse model data","pmids":["27868280"],"is_preprint":false},{"year":2022,"finding":"Secretory cells (club/goblet cells) are the predominant cell type responsible for pIgR expression in human and murine small airways. Loss of SIgA in COPD small airways is associated with reduced pIgR protein despite intact PIGR mRNA, indicating a post-transcriptional/post-translational defect. Secretory cell-specific knockout of pIgR in mice confirmed that secretory cells are the primary source of SIgA in small airways.","method":"RNA in situ hybridization, immunostaining, single-cell RNA sequencing, transgenic mice with cell-type-specific pIgR knockout, primary murine tracheal epithelial cell culture","journal":"American journal of respiratory cell and molecular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — single-cell resolution identification combined with genetic (cell-type-specific KO) validation; multiple orthogonal methods","pmids":["35687143"],"is_preprint":false},{"year":2023,"finding":"In ovarian cancer cells, pIgR-mediated IgA transcytosis induced transcriptional changes in intracellular inflammatory pathways that inhibit cancer progression, including upregulation of IFN-γ and downregulation of tumor-promoting ephrins.","method":"Transcriptional profiling after IgA transcytosis via pIgR in ovarian cancer cells","journal":"Journal of cancer research and clinical oncology","confidence":"Low","confidence_rationale":"Tier 3 / Weak — finding cited in a review paper without full methodological detail; single indirect reference to original experiments","pmids":["37897659"],"is_preprint":false},{"year":2025,"finding":"Pigr deficiency specifically in hematopoietic cells (assessed by bone marrow transplantation in Ldlr-/- mice) resulted in significantly reduced abdominal aortic aneurysm (AAA) incidence (14% vs 57%) and decreased macrophage infiltration, demonstrating a role for hematopoietic-cell PIGR in AAA progression. PIGR colocalized with macrophages in the AAA wall and was upregulated in M1-polarized macrophages.","method":"Bone marrow transplantation in experimental AAA mouse model, macrophage infiltration measurement, THP-1 macrophage differentiation/polarization with PIGR mRNA and protein quantification","journal":"Journal of translational medicine","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — bone marrow transplant genetic rescue experiment with quantitative disease and cellular readouts; single lab","pmids":["40624587"],"is_preprint":false},{"year":2025,"finding":"LINC00870 binds specifically to PIGR and inhibits the glycosylation modification and secretion of the extracellular region of PIGR, leading to immune dysregulation and imatinib resistance in gastrointestinal stromal tumor. Inhibition of PIGR or LINC00870 overcomes imatinib resistance.","method":"High-throughput sequencing, in vitro functional experiments, LINC00870–PIGR binding assay, PIGR overexpression/knockdown, glycosylation and secretion assays","journal":"Heliyon","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — direct binding demonstrated with functional consequence on glycosylation; single lab, mechanisms partially characterized","pmids":["39968132"],"is_preprint":false}],"current_model":"PIGR (polymeric immunoglobulin receptor) is a transmembrane transporter expressed on mucosal and glandular epithelial cells that mediates basolateral-to-apical transcytosis of J-chain-containing polymeric IgA and IgM across epithelia to generate secretory IgA; J chain is essential for pIgR recognition, transcytosis proceeds through early (EEA1+) and recycling (Rab11a+) endosomes facilitated by Rab11-FIP1/FIP5 effectors whose activation requires TRIM21-mediated polyubiquitination, ligand binding triggers IP3/calcium signaling (with species-dependent downstream coupling), and pIgR expression is regulated by cytokines (IFN-γ, IL-4, IL-1β, TGF-β), retinoic acid, prolactin, glucocorticoids, EpCAM, and microbial signals; beyond its canonical transport function, pIgR acts as a receptor exploited by Streptococcus pneumoniae (via RrgA/PspC adhesins) for blood-brain barrier invasion, is downregulated by SARS-CoV-2 ORF8 to evade mucosal immunity, promotes cancer stemness in hepatocellular carcinoma through EV-mediated PDK1/Akt/GSK3β/β-catenin signaling, and its hematopoietic-cell expression contributes to abdominal aortic aneurysm progression."},"narrative":{"mechanistic_narrative":"PIGR (polymeric immunoglobulin receptor) is a transmembrane epithelial receptor that mediates basolateral-to-apical transcytosis of polymeric immunoglobulins to generate mucosal secretory immunity [PMID:9767462, PMID:20525015]. Recognition of polymeric IgA requires the J chain, as J-chain-lacking tetrameric IgA fails to bind secretory component and is not transcytosed in either polarized epithelial cells or rat bile [PMID:9767462]. During transport, pIgA-loaded receptor traffics unidirectionally through EEA1-positive early endosomes and Rab11a-positive recycling endosomes along intact microtubules [PMID:20525015], a route that depends on the Rab11 effectors Rab11-FIP1 and Rab11-FIP5; these effectors are activated by TRIM21-mediated K11- and K6-linked polyubiquitination to drive efficient transcytosis [PMID:34638806]. Ligand binding to human pIgR triggers IP3 production and phospholipase-C activation, but human pIgR transcytosis is uncoupled from this signal owing to reduced sensitivity to intracellular calcium, in contrast to rabbit and rat receptors; PKC activation accelerates transport in both [PMID:11169207]. pIgR expression is the predominant function of airway secretory (club/goblet) cells and is controlled by multiple inputs including TGF-β acting through a redox-sensitive p38 MAPK pathway, retinoic acid (required for cytokine-driven induction), and prolactin combined with glucocorticoids [PMID:35687143, PMID:20706611, PMID:9649586, PMID:12047104]. Physiologically, hepatocyte pIgR secretes IgA into bile and intestine to restrain bacterial translocation and prevent ethanol-induced liver disease [PMID:36690432]. Beyond transport, pIgR is exploited as a receptor: Streptococcus pneumoniae adhesins RrgA and PspC bind pIgR on blood-brain barrier endothelium to enable brain invasion [PMID:24841255, PMID:28515075], and SARS-CoV-2 ORF8 binds pIgR directly to downregulate it and impair immunoglobulin binding [PMID:39066171]. In disease contexts, extracellular-vesicle pIgR promotes hepatocellular carcinoma stemness via PDK1/Akt/GSK3β/β-catenin signaling [PMID:34922977], and hematopoietic-cell pIgR contributes to abdominal aortic aneurysm progression through macrophage infiltration [PMID:40624587].","teleology":[{"year":1997,"claim":"Established that pIgR expression in epithelia is subject to negative regulation by neighboring stroma, defining a microenvironmental control layer.","evidence":"Co-culture and conditioned-supernatant transfer between rat uterine stromal and epithelial cells","pmids":["9234210"],"confidence":"Medium","gaps":["Soluble suppressive factor not molecularly identified","Mechanism of suppression not defined"]},{"year":1998,"claim":"Resolved the ligand-recognition requirement, showing the J chain is essential for polymeric IgA binding and transcytosis rather than IgA polymerization alone.","evidence":"In vitro SC-binding, in vivo rat bile transport, and polarized MDCK transcytosis with J-chain-positive vs J-chain-lacking pIgA","pmids":["9767462"],"confidence":"High","gaps":["Structural basis of J-chain/SC contact not resolved","Does not address IgM binding determinants"]},{"year":1998,"claim":"Identified retinoic acid as a required cofactor for cytokine-driven pIgR upregulation, linking vitamin A status to mucosal immunity.","evidence":"Vitamin A depletion and RA dose-response in HT-29 intestinal cells with protein and mRNA readouts","pmids":["9649586"],"confidence":"Medium","gaps":["Direct transcriptional target/promoter element not mapped","Interaction with IL-4/IFN-γ signaling mechanistically undefined"]},{"year":2001,"claim":"Dissected ligand-triggered signaling, showing pIgA induces IP3/PLC activation but that transcytosis stimulation diverges by species due to differing calcium sensitivity, while PKC universally accelerates transport.","evidence":"Apical SC release assays, IP3 measurement, and PMA stimulation in human Calu-3 and pIgR-transfected MDCK cells","pmids":["11169207"],"confidence":"High","gaps":["Molecular basis of human pIgR calcium insensitivity unknown","Downstream PKC substrates in transport not identified"]},{"year":2001,"claim":"Demonstrated a cis-regulatory DNA element in the Pigr 3'-UTR modulating expression via supercoiling-dependent triplex formation.","evidence":"Luciferase reporters, nuclease hypersensitivity, and melting-profile analysis of a rat Pigr microsatellite fragment","pmids":["11242589"],"confidence":"Medium","gaps":["In vivo relevance of the element not established","Trans-acting factors unknown"]},{"year":2002,"claim":"Defined hormonal control of pIgR in glandular epithelium, identifying prolactin plus glucocorticoids as the key inducing combination.","evidence":"Hormonal manipulation and bromocryptine prolactin blockade in sheep mammary gland with molecular readouts","pmids":["12047104"],"confidence":"Medium","gaps":["Direct receptor/promoter mechanism not mapped","Species generalizability untested"]},{"year":2010,"claim":"Mapped the intracellular trafficking itinerary of pIgA, showing unidirectional, microtubule-dependent passage through early and recycling endosomes distinct from FcRn cargo.","evidence":"Confocal pulse-chase and live imaging in MDCK cells co-expressing pIgR and FcRn with endosomal markers and microtubule depolymerization","pmids":["20525015"],"confidence":"High","gaps":["Molecular sorting machinery not identified in this study","Determinants of unidirectionality unresolved"]},{"year":2010,"claim":"Identified TGF-β from activated neutrophils as an inducer of pIgR via a redox-sensitive p38 MAPK pathway, linking inflammation to mucosal antibody transport.","evidence":"Neutrophil-epithelial co-culture with p38 and redox inhibition in Calu-3 cells","pmids":["20706611"],"confidence":"Medium","gaps":["Transcriptional effectors downstream of p38 not defined","In vivo relevance not tested"]},{"year":2014,"claim":"Revealed a pathogenic repurposing of pIgR as a pneumococcal adhesion receptor on brain endothelium.","evidence":"Mouse intravenous infection, immunofluorescent co-localization, antibody blocking, and lysate binding","pmids":["24841255"],"confidence":"Medium","gaps":["Bacterial adhesins not yet identified in this study","Mechanism of endothelial transit not defined"]},{"year":2017,"claim":"Identified the pneumococcal adhesins (RrgA, PspC) engaging pIgR and PECAM-1 to mediate blood-brain barrier crossing, establishing pIgR as a therapeutic target in meningitis.","evidence":"STED microscopy of human brain biopsies, recombinant adhesin binding, and antibody/genetic blocking in a bacteremia-derived meningitis model","pmids":["28515075"],"confidence":"High","gaps":["Signaling/transcytosis route hijacked by pneumococci not fully resolved","Relative contribution of PspC vs RrgA quantitatively unclear"]},{"year":2021,"claim":"Defined the Rab11-effector and ubiquitin machinery driving transcytosis, identifying Rab11-FIP1/FIP5 and TRIM21-mediated polyubiquitination as activation steps.","evidence":"Interactor identification, siRNA knockdown with transcytosis readout, and ubiquitin-linkage analysis in polarized and incompletely polarized cells","pmids":["34638806"],"confidence":"High","gaps":["Trigger that recruits TRIM21 to FIP effectors unknown","Whether ubiquitination is direct on FIPs in vivo not fully resolved"]},{"year":2021,"claim":"Established a tumor-promoting, transport-independent function of pIgR delivered via extracellular vesicles driving HCC stemness.","evidence":"EV isolation, stemness assays, pathway inhibitors, and anti-pIgR neutralizing antibody in patient-derived xenografts","pmids":["34922977"],"confidence":"High","gaps":["How EV-pIgR engages PDK1/Akt axis mechanistically unclear","Receptor on recipient cells not identified"]},{"year":2022,"claim":"Pinpointed secretory cells as the dominant source of airway pIgR and revealed a post-transcriptional defect underlying SIgA loss in COPD.","evidence":"scRNA-seq, in situ hybridization, and secretory cell-specific pIgR knockout mice","pmids":["35687143"],"confidence":"High","gaps":["Molecular cause of the post-transcriptional defect undefined","Reversibility of SIgA loss not addressed"]},{"year":2023,"claim":"Demonstrated a hepatocyte-intrinsic protective role: hepatic pIgR-mediated IgA secretion limits bacterial translocation and prevents ethanol-induced liver disease.","evidence":"pIgR-deficient mice, two ethanol feeding models, AAV8 hepatocyte-specific rescue, and antibiotic controls","pmids":["36690432"],"confidence":"High","gaps":["Specific bacterial taxa controlled not defined","Relative contribution of biliary vs intestinal secretion unclear"]},{"year":2023,"claim":"Indicated that pIgR-mediated IgA transcytosis can drive anti-tumor inflammatory transcriptional programs in ovarian cancer.","evidence":"Transcriptional profiling after IgA transcytosis (cited in review without full methodological detail)","pmids":["37897659"],"confidence":"Low","gaps":["Original experimental detail not available; cited indirectly in review","Causality and in vivo relevance not established"]},{"year":2024,"claim":"Identified a viral immune-evasion mechanism whereby SARS-CoV-2 ORF8 binds and downregulates pIgR to impair immunoglobulin binding.","evidence":"ORF8-pIgR interaction studies, expression and dIgA/IgM binding assays, and internalization assays","pmids":["39066171"],"confidence":"Medium","gaps":["Mechanism of ORF8-induced downregulation not defined","In vivo mucosal consequence not tested"]},{"year":2025,"claim":"Implicated hematopoietic-cell pIgR in vascular disease, showing macrophage-associated pIgR promotes abdominal aortic aneurysm progression.","evidence":"Bone marrow transplantation in Ldlr-/- AAA mice with macrophage infiltration quantification and THP-1 polarization assays","pmids":["40624587"],"confidence":"Medium","gaps":["Molecular function of pIgR in macrophages undefined","Whether transcytosis function is involved unknown"]},{"year":2025,"claim":"Linked a lncRNA to pIgR processing, showing LINC00870 binds pIgR to inhibit its glycosylation/secretion and drive imatinib resistance in GIST.","evidence":"Binding assays, glycosylation and secretion assays, and overexpression/knockdown in gastrointestinal stromal tumor cells","pmids":["39968132"],"confidence":"Medium","gaps":["Mechanism linking pIgR glycosylation to drug resistance unclear","Binding interface not characterized"]},{"year":null,"claim":"How the diverse non-transport functions of pIgR (cancer stemness, macrophage-driven vascular disease) relate mechanistically to its canonical transcytosis activity remains unresolved.","evidence":"","pmids":[],"confidence":"Low","gaps":["No unifying mechanism connecting transport and signaling roles","Receptors/partners mediating EV-pIgR effects unidentified","Structural basis of ligand and pathogen recognition not solved"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005215","term_label":"transporter activity","supporting_discovery_ids":[0,3,9]},{"term_id":"GO:0038024","term_label":"cargo receptor activity","supporting_discovery_ids":[0,3]},{"term_id":"GO:0001618","term_label":"virus receptor activity","supporting_discovery_ids":[5,6]},{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[1]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[3,5,13]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[3,7]},{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[8]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[0,9,15]},{"term_id":"R-HSA-9609507","term_label":"Protein localization","supporting_discovery_ids":[3,7]},{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[3,7]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[6,8,13,17]}],"complexes":[],"partners":["JCHAIN","RAB11FIP1","RAB11FIP5","TRIM21","PECAM1","LINC00870"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P01833","full_name":"Polymeric immunoglobulin receptor","aliases":["Hepatocellular carcinoma-associated protein TB6"],"length_aa":764,"mass_kda":83.3,"function":"Mediates selective transcytosis of polymeric IgA and IgM across mucosal epithelial cells. Binds polymeric IgA and IgM at the basolateral surface of epithelial cells. The complex is then transported across the cell to be secreted at the apical surface. During this process, a cleavage occurs that separates the extracellular (known as the secretory component) from the transmembrane segment Through its N-linked glycans ensures anchoring of secretory IgA (sIgA) molecules to mucus lining the epithelial surface to neutralize extracellular pathogens (PubMed:12150896). On its own (free form) may act as a non-specific microbial scavenger to prevent pathogen interaction with epithelial cells (PubMed:16543244)","subcellular_location":"Secreted","url":"https://www.uniprot.org/uniprotkb/P01833/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/PIGR","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/PIGR","total_profiled":1310},"omim":[{"mim_id":"609684","title":"MAL PROTEOLIPID PROTEIN 2; MAL2","url":"https://www.omim.org/entry/609684"},{"mim_id":"608004","title":"NUCLEAR FACTOR KAPPA-B INHIBITOR, ZETA; NFKBIZ","url":"https://www.omim.org/entry/608004"},{"mim_id":"606510","title":"Fc RECEPTOR-LIKE PROTEIN 3; 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A J-chain-lacking tetrameric IgA (pIgA-L) failed to bind purified SC, was not transferred into rat bile after intravenous injection, and was not transported apically by polarized MDCK cells expressing human pIgR, whereas J-chain-containing pIgA preparations were efficiently transported in both systems.\",\n      \"method\": \"In vitro SC-binding assay, in vivo rat bile transport assay, polarized MDCK cell transcytosis assay\",\n      \"journal\": \"Immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple orthogonal in vitro and in vivo assays with isogenic comparator molecules (J-chain-positive vs J-chain-lacking pIgA), clear mechanistic conclusion\",\n      \"pmids\": [\"9767462\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Human pIgR-mediated transcytosis is not stimulated by pIgA binding, despite pIgA inducing IP3 production (phospholipase-C activation) in cells expressing human pIgR. In contrast, rabbit and rat pIgR transcytosis is accelerated by pIgA. The species difference is not due to defective second-messenger production but to a different sensitivity of human pIgR to intracellular calcium. PKC activation by PMA stimulates both human and rabbit pIgR transcytosis.\",\n      \"method\": \"Continuous apical SC release assay in human Calu-3 and pIgR-transfected MDCK cells; IP3 measurement; PKC activation with PMA\",\n      \"journal\": \"Scandinavian journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — multiple orthogonal cell-based assays with rigorous controls comparing human vs rabbit pIgR in the same system; definitive mechanistic conclusion about signaling pathway\",\n      \"pmids\": [\"11169207\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"A microsatellite-containing fragment from the 3'-UTR of the rat Pigr gene modulates gene expression in an orientation- and position-dependent manner dependent on DNA supercoiling, functioning through intramolecular triplex formation stabilized by supercoiling, suggesting a regulatory role for this genomic element in controlling Pigr expression.\",\n      \"method\": \"Luciferase reporter transient transfection assay, cell-free translation, nuclease S1/P1 hypersensitivity, gel mobility analysis, anomalous melting profiles\",\n      \"journal\": \"Physiological genomics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal biochemical and cell-based methods in a single lab; demonstrates functional consequence of a regulatory element in the Pigr 3'-UTR\",\n      \"pmids\": [\"11242589\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"pIgA and IgG Fc (transported by FcRn) follow initially separate but later intermixing trafficking routes during transcytosis in polarized MDCK cells co-expressing both receptors. pIgA transport is strongly unidirectional (basolateral-to-apical), transiently colocalizing with EEA1-positive early endosomes, Rab11a-positive recycling endosomes, and transferrin-positive basolateral recycling endosomes. Unlike FcRn cargo, pIgA was sorted away from transferrin-positive endosomes with time. Both trafficking routes depended equally on intact microtubules.\",\n      \"method\": \"Fluorescence confocal microscopy with pulse-chase experiments, co-expression of FcRn and pIgR in MDCK cells, live-cell imaging, microtubule depolymerization\",\n      \"journal\": \"Traffic (Copenhagen, Denmark)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct visualization of pIgR/pIgA trafficking with multiple endosomal markers, live imaging, and pharmacological perturbation in a single rigorous study\",\n      \"pmids\": [\"20525015\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"TGF-β released by activated neutrophils upregulates pIgR/SC production in human bronchial epithelial (Calu-3) cells through a redox-sensitive, p38 MAPK-dependent pathway. This effect was mimicked by exogenous TGF-β and blocked by inhibition of redox balance or p38 MAPK.\",\n      \"method\": \"Neutrophil–epithelial cell co-culture, TGF-β measurement, p38 MAPK inhibition, redox balance inhibition, SC production assay\",\n      \"journal\": \"Journal of biomedicine & biotechnology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple pathway inhibitor experiments and cytokine identification in a single lab, clear mechanistic pathway identified\",\n      \"pmids\": [\"20706611\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Streptococcus pneumoniae physically interacts with pIgR on brain microvascular endothelial cells in vivo and in vitro, and pIgR blocking reduced pneumococcal adhesion to endothelial cells. Pneumococci co-localized with pIgR (but not with PAFR) in vivo. Physical interaction of pneumococci with pIgR was confirmed by incubating bacteria with endothelial cell lysates.\",\n      \"method\": \"In vivo mouse intravenous infection model, immunofluorescent co-localization analysis, in vitro antibody blocking, pneumococcal incubation with endothelial cell lysates\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo and in vitro co-localization, antibody blocking, and direct binding in lysates from a single lab\",\n      \"pmids\": [\"24841255\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"The pneumococcal pilus-1 major adhesin RrgA binds both pIgR and PECAM-1 on the blood-brain barrier endothelium, whereas choline binding protein PspC binds pIgR only (to a lower extent). Antibodies against pIgR and PECAM-1 prevented pneumococcal entry into the brain in a bacteremia-derived meningitis mouse model. Addition of these antibodies to ceftriaxone-treated mice further reduced brain bacterial burden.\",\n      \"method\": \"STED super-resolution microscopy of human brain biopsies, mutant mouse and antibody-blocking experiments in a bacteremia-derived meningitis model, binding studies with recombinant adhesins\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — super-resolution microscopy of human tissue, direct binding assays, and in vivo genetic/antibody-blocking model all confirm pIgR as a pneumococcal adhesion receptor at the BBB\",\n      \"pmids\": [\"28515075\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Rab11 effectors Rab11-FIP1 and Rab11-FIP5 interact with pIgR/pIgA-containing endosomes and are required for efficient pIgA transcytosis. Their knockdown additively impaired transcytosis in polarized and incompletely polarized cells. In incompletely polarized cells, pIgR/pIgA trafficking involves transport from the basolateral membrane to the centrosome vicinity, then via Rab11a-positive endosomes through the Golgi to the apical membrane. TRIM21 mediates K11-linked polyubiquitination of Rab11-FIP1 and K6-linked polyubiquitination of Rab11-FIP5 to promote their activation and pIgA transcytosis.\",\n      \"method\": \"Protein interaction (Rab11-FIP1 identified as new pIgR interactor), siRNA knockdown, live-cell trafficking assays in polarized and incompletely polarized cells, ubiquitination analysis\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — identification of binding partner, siRNA knockdown with quantitative transcytosis readout, and characterization of specific ubiquitin linkage type by TRIM21; multiple orthogonal methods\",\n      \"pmids\": [\"34638806\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"EV-pIgR from late-stage HCC patients promotes cancer stemness and tumorigenesis in recipient cells via activation of the PDK1/Akt/GSK3β/β-catenin signaling axis. Blockade with an anti-pIgR neutralizing antibody abrogated these effects and attenuated tumor growth in patient-derived tumor xenograft mice.\",\n      \"method\": \"EV isolation and characterization, in vitro cancer stemness assays, in vivo PDTX mouse model, pathway inhibition with Akt and β-catenin inhibitors, anti-pIgR neutralizing antibody blockade\",\n      \"journal\": \"Journal of hepatology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — mechanistic pathway identified using pathway-specific inhibitors, confirmed in vivo with neutralizing antibody in PDTXs; multiple orthogonal methods\",\n      \"pmids\": [\"34922977\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Hepatic pIgR mediates IgA secretion into the intestinal lumen and bile canaliculi, limiting bacterial translocation and preventing ethanol-induced liver disease. pIgR-deficient mice showed increased liver injury, steatosis, inflammation, elevated plasma LPS, and more hepatic bacteria after ethanol feeding. AAV8-mediated re-expression of pIgR specifically in hepatocytes of pIgR-deficient mice increased intestinal IgA and ameliorated steatohepatitis by reducing bacterial translocation.\",\n      \"method\": \"pIgR-deficient mouse model, chronic-binge and Lieber-DeCarli ethanol feeding models, AAV8-mediated hepatocyte-specific pIgR re-expression, non-absorbable antibiotic treatment, liver injury and bacterial burden assessment\",\n      \"journal\": \"Gut\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic rescue (AAV8 re-expression) and antibiotic control confirm mechanism; KO phenotype replicated across two ethanol feeding models\",\n      \"pmids\": [\"36690432\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"All-trans retinoic acid (RA) is required for normal regulation of pIgR expression by IL-4 and IFN-γ in HT-29 human intestinal epithelial cells. Vitamin A depletion significantly reduced cytokine-induced pIgR upregulation at both protein and mRNA levels; RA supplementation restored normal pIgR expression levels in a dose-dependent manner.\",\n      \"method\": \"Vitamin A-depleted cell culture, RA supplementation, flow cytometry for cell-surface pIgR, mRNA quantification by Northern/RT-PCR in HT-29 cells\",\n      \"journal\": \"The Journal of nutrition\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — protein and mRNA-level measurements with dose-response RA titration; single lab but multiple readouts\",\n      \"pmids\": [\"9649586\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"Uterine stromal cells suppress pIgR production by co-cultured rat uterine epithelial cells through a soluble factor present in stromal cell-conditioned supernatants, representing a stromal–epithelial regulatory mechanism for pIgR expression.\",\n      \"method\": \"Co-culture of rat uterine stromal and epithelial cells, conditioned supernatant transfer experiments, immunohistochemistry for pIgR, transepithelial resistance measurement\",\n      \"journal\": \"Journal of reproductive immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — conditioned medium transfer and co-culture with immunohistochemical confirmation; single lab, cell-based assay without full molecular identification of the soluble factor\",\n      \"pmids\": [\"9234210\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"pIgR expression in the sheep mammary gland is regulated by a combination of prolactin and glucocorticoids: estradiol and progesterone alone produced slight increases in pIgR mRNA, but glucocorticoid addition caused significant accumulation of pIgR mRNA; blockade of prolactin secretion with bromocryptine abolished the hormonal induction of pIgR.\",\n      \"method\": \"Northern blot, in situ hybridization, immunohistochemistry in hormonally treated virgin ewes and during pregnancy/lactation; bromocryptine prolactin blockade\",\n      \"journal\": \"The Journal of dairy research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo hormonal manipulation with molecular readouts and prolactin-blockade experiment; single lab\",\n      \"pmids\": [\"12047104\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"SARS-CoV-2 accessory protein ORF8 downregulates pIgR expression through a direct interaction with pIgR. ORF8-mediated downregulation diminishes the binding of dimeric IgA (dIgA) and pentameric IgM to pIgR. Secreted ORF8 binds cell surface pIgR but does not trigger cellular internalization of ORF8 (unlike dIgA binding to pIgR, which does trigger internalization). ORF8 proteins from SARS-CoV-2 variants of concern preserve this pIgR-downregulating function.\",\n      \"method\": \"Protein–protein interaction studies between ORF8 and pIgR, pIgR expression measurement, dIgA/pIgM binding assays, cell internalization assay\",\n      \"journal\": \"Viruses\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct interaction demonstrated with functional consequence (reduced IgA/IgM binding, blocked internalization); single lab, multiple readouts\",\n      \"pmids\": [\"39066171\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Chlamydia infection upregulates pIgR expression in human epithelia and in mouse male and female reproductive tract epithelia in vivo, and this is associated with increased transcytosis of IgA into the lumen. Hormone synchronization with Depo-Provera downregulated pIgR expression, with pIgR being highest during estrus.\",\n      \"method\": \"Western blot, immunohistochemistry in vitro and in vivo (mouse reproductive tract), functional IgA transcytosis assay\",\n      \"journal\": \"American journal of reproductive immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — in vitro and in vivo evidence with Western blot and IHC; single lab, but both human cell and mouse model data\",\n      \"pmids\": [\"27868280\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Secretory cells (club/goblet cells) are the predominant cell type responsible for pIgR expression in human and murine small airways. Loss of SIgA in COPD small airways is associated with reduced pIgR protein despite intact PIGR mRNA, indicating a post-transcriptional/post-translational defect. Secretory cell-specific knockout of pIgR in mice confirmed that secretory cells are the primary source of SIgA in small airways.\",\n      \"method\": \"RNA in situ hybridization, immunostaining, single-cell RNA sequencing, transgenic mice with cell-type-specific pIgR knockout, primary murine tracheal epithelial cell culture\",\n      \"journal\": \"American journal of respiratory cell and molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — single-cell resolution identification combined with genetic (cell-type-specific KO) validation; multiple orthogonal methods\",\n      \"pmids\": [\"35687143\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"In ovarian cancer cells, pIgR-mediated IgA transcytosis induced transcriptional changes in intracellular inflammatory pathways that inhibit cancer progression, including upregulation of IFN-γ and downregulation of tumor-promoting ephrins.\",\n      \"method\": \"Transcriptional profiling after IgA transcytosis via pIgR in ovarian cancer cells\",\n      \"journal\": \"Journal of cancer research and clinical oncology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — finding cited in a review paper without full methodological detail; single indirect reference to original experiments\",\n      \"pmids\": [\"37897659\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Pigr deficiency specifically in hematopoietic cells (assessed by bone marrow transplantation in Ldlr-/- mice) resulted in significantly reduced abdominal aortic aneurysm (AAA) incidence (14% vs 57%) and decreased macrophage infiltration, demonstrating a role for hematopoietic-cell PIGR in AAA progression. PIGR colocalized with macrophages in the AAA wall and was upregulated in M1-polarized macrophages.\",\n      \"method\": \"Bone marrow transplantation in experimental AAA mouse model, macrophage infiltration measurement, THP-1 macrophage differentiation/polarization with PIGR mRNA and protein quantification\",\n      \"journal\": \"Journal of translational medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — bone marrow transplant genetic rescue experiment with quantitative disease and cellular readouts; single lab\",\n      \"pmids\": [\"40624587\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"LINC00870 binds specifically to PIGR and inhibits the glycosylation modification and secretion of the extracellular region of PIGR, leading to immune dysregulation and imatinib resistance in gastrointestinal stromal tumor. Inhibition of PIGR or LINC00870 overcomes imatinib resistance.\",\n      \"method\": \"High-throughput sequencing, in vitro functional experiments, LINC00870–PIGR binding assay, PIGR overexpression/knockdown, glycosylation and secretion assays\",\n      \"journal\": \"Heliyon\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — direct binding demonstrated with functional consequence on glycosylation; single lab, mechanisms partially characterized\",\n      \"pmids\": [\"39968132\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PIGR (polymeric immunoglobulin receptor) is a transmembrane transporter expressed on mucosal and glandular epithelial cells that mediates basolateral-to-apical transcytosis of J-chain-containing polymeric IgA and IgM across epithelia to generate secretory IgA; J chain is essential for pIgR recognition, transcytosis proceeds through early (EEA1+) and recycling (Rab11a+) endosomes facilitated by Rab11-FIP1/FIP5 effectors whose activation requires TRIM21-mediated polyubiquitination, ligand binding triggers IP3/calcium signaling (with species-dependent downstream coupling), and pIgR expression is regulated by cytokines (IFN-γ, IL-4, IL-1β, TGF-β), retinoic acid, prolactin, glucocorticoids, EpCAM, and microbial signals; beyond its canonical transport function, pIgR acts as a receptor exploited by Streptococcus pneumoniae (via RrgA/PspC adhesins) for blood-brain barrier invasion, is downregulated by SARS-CoV-2 ORF8 to evade mucosal immunity, promotes cancer stemness in hepatocellular carcinoma through EV-mediated PDK1/Akt/GSK3β/β-catenin signaling, and its hematopoietic-cell expression contributes to abdominal aortic aneurysm progression.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"PIGR (polymeric immunoglobulin receptor) is a transmembrane epithelial receptor that mediates basolateral-to-apical transcytosis of polymeric immunoglobulins to generate mucosal secretory immunity [#0, #3]. Recognition of polymeric IgA requires the J chain, as J-chain-lacking tetrameric IgA fails to bind secretory component and is not transcytosed in either polarized epithelial cells or rat bile [#0]. During transport, pIgA-loaded receptor traffics unidirectionally through EEA1-positive early endosomes and Rab11a-positive recycling endosomes along intact microtubules [#3], a route that depends on the Rab11 effectors Rab11-FIP1 and Rab11-FIP5; these effectors are activated by TRIM21-mediated K11- and K6-linked polyubiquitination to drive efficient transcytosis [#7]. Ligand binding to human pIgR triggers IP3 production and phospholipase-C activation, but human pIgR transcytosis is uncoupled from this signal owing to reduced sensitivity to intracellular calcium, in contrast to rabbit and rat receptors; PKC activation accelerates transport in both [#1]. pIgR expression is the predominant function of airway secretory (club/goblet) cells and is controlled by multiple inputs including TGF-β acting through a redox-sensitive p38 MAPK pathway, retinoic acid (required for cytokine-driven induction), and prolactin combined with glucocorticoids [#15, #4, #10, #12]. Physiologically, hepatocyte pIgR secretes IgA into bile and intestine to restrain bacterial translocation and prevent ethanol-induced liver disease [#9]. Beyond transport, pIgR is exploited as a receptor: Streptococcus pneumoniae adhesins RrgA and PspC bind pIgR on blood-brain barrier endothelium to enable brain invasion [#5, #6], and SARS-CoV-2 ORF8 binds pIgR directly to downregulate it and impair immunoglobulin binding [#13]. In disease contexts, extracellular-vesicle pIgR promotes hepatocellular carcinoma stemness via PDK1/Akt/GSK3β/β-catenin signaling [#8], and hematopoietic-cell pIgR contributes to abdominal aortic aneurysm progression through macrophage infiltration [#17].\",\n  \"teleology\": [\n    {\n      \"year\": 1997,\n      \"claim\": \"Established that pIgR expression in epithelia is subject to negative regulation by neighboring stroma, defining a microenvironmental control layer.\",\n      \"evidence\": \"Co-culture and conditioned-supernatant transfer between rat uterine stromal and epithelial cells\",\n      \"pmids\": [\"9234210\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Soluble suppressive factor not molecularly identified\", \"Mechanism of suppression not defined\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Resolved the ligand-recognition requirement, showing the J chain is essential for polymeric IgA binding and transcytosis rather than IgA polymerization alone.\",\n      \"evidence\": \"In vitro SC-binding, in vivo rat bile transport, and polarized MDCK transcytosis with J-chain-positive vs J-chain-lacking pIgA\",\n      \"pmids\": [\"9767462\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of J-chain/SC contact not resolved\", \"Does not address IgM binding determinants\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Identified retinoic acid as a required cofactor for cytokine-driven pIgR upregulation, linking vitamin A status to mucosal immunity.\",\n      \"evidence\": \"Vitamin A depletion and RA dose-response in HT-29 intestinal cells with protein and mRNA readouts\",\n      \"pmids\": [\"9649586\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct transcriptional target/promoter element not mapped\", \"Interaction with IL-4/IFN-γ signaling mechanistically undefined\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Dissected ligand-triggered signaling, showing pIgA induces IP3/PLC activation but that transcytosis stimulation diverges by species due to differing calcium sensitivity, while PKC universally accelerates transport.\",\n      \"evidence\": \"Apical SC release assays, IP3 measurement, and PMA stimulation in human Calu-3 and pIgR-transfected MDCK cells\",\n      \"pmids\": [\"11169207\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis of human pIgR calcium insensitivity unknown\", \"Downstream PKC substrates in transport not identified\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Demonstrated a cis-regulatory DNA element in the Pigr 3'-UTR modulating expression via supercoiling-dependent triplex formation.\",\n      \"evidence\": \"Luciferase reporters, nuclease hypersensitivity, and melting-profile analysis of a rat Pigr microsatellite fragment\",\n      \"pmids\": [\"11242589\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"In vivo relevance of the element not established\", \"Trans-acting factors unknown\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Defined hormonal control of pIgR in glandular epithelium, identifying prolactin plus glucocorticoids as the key inducing combination.\",\n      \"evidence\": \"Hormonal manipulation and bromocryptine prolactin blockade in sheep mammary gland with molecular readouts\",\n      \"pmids\": [\"12047104\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct receptor/promoter mechanism not mapped\", \"Species generalizability untested\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Mapped the intracellular trafficking itinerary of pIgA, showing unidirectional, microtubule-dependent passage through early and recycling endosomes distinct from FcRn cargo.\",\n      \"evidence\": \"Confocal pulse-chase and live imaging in MDCK cells co-expressing pIgR and FcRn with endosomal markers and microtubule depolymerization\",\n      \"pmids\": [\"20525015\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular sorting machinery not identified in this study\", \"Determinants of unidirectionality unresolved\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Identified TGF-β from activated neutrophils as an inducer of pIgR via a redox-sensitive p38 MAPK pathway, linking inflammation to mucosal antibody transport.\",\n      \"evidence\": \"Neutrophil-epithelial co-culture with p38 and redox inhibition in Calu-3 cells\",\n      \"pmids\": [\"20706611\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Transcriptional effectors downstream of p38 not defined\", \"In vivo relevance not tested\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Revealed a pathogenic repurposing of pIgR as a pneumococcal adhesion receptor on brain endothelium.\",\n      \"evidence\": \"Mouse intravenous infection, immunofluorescent co-localization, antibody blocking, and lysate binding\",\n      \"pmids\": [\"24841255\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Bacterial adhesins not yet identified in this study\", \"Mechanism of endothelial transit not defined\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Identified the pneumococcal adhesins (RrgA, PspC) engaging pIgR and PECAM-1 to mediate blood-brain barrier crossing, establishing pIgR as a therapeutic target in meningitis.\",\n      \"evidence\": \"STED microscopy of human brain biopsies, recombinant adhesin binding, and antibody/genetic blocking in a bacteremia-derived meningitis model\",\n      \"pmids\": [\"28515075\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Signaling/transcytosis route hijacked by pneumococci not fully resolved\", \"Relative contribution of PspC vs RrgA quantitatively unclear\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Defined the Rab11-effector and ubiquitin machinery driving transcytosis, identifying Rab11-FIP1/FIP5 and TRIM21-mediated polyubiquitination as activation steps.\",\n      \"evidence\": \"Interactor identification, siRNA knockdown with transcytosis readout, and ubiquitin-linkage analysis in polarized and incompletely polarized cells\",\n      \"pmids\": [\"34638806\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Trigger that recruits TRIM21 to FIP effectors unknown\", \"Whether ubiquitination is direct on FIPs in vivo not fully resolved\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Established a tumor-promoting, transport-independent function of pIgR delivered via extracellular vesicles driving HCC stemness.\",\n      \"evidence\": \"EV isolation, stemness assays, pathway inhibitors, and anti-pIgR neutralizing antibody in patient-derived xenografts\",\n      \"pmids\": [\"34922977\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How EV-pIgR engages PDK1/Akt axis mechanistically unclear\", \"Receptor on recipient cells not identified\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Pinpointed secretory cells as the dominant source of airway pIgR and revealed a post-transcriptional defect underlying SIgA loss in COPD.\",\n      \"evidence\": \"scRNA-seq, in situ hybridization, and secretory cell-specific pIgR knockout mice\",\n      \"pmids\": [\"35687143\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular cause of the post-transcriptional defect undefined\", \"Reversibility of SIgA loss not addressed\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Demonstrated a hepatocyte-intrinsic protective role: hepatic pIgR-mediated IgA secretion limits bacterial translocation and prevents ethanol-induced liver disease.\",\n      \"evidence\": \"pIgR-deficient mice, two ethanol feeding models, AAV8 hepatocyte-specific rescue, and antibiotic controls\",\n      \"pmids\": [\"36690432\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific bacterial taxa controlled not defined\", \"Relative contribution of biliary vs intestinal secretion unclear\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Indicated that pIgR-mediated IgA transcytosis can drive anti-tumor inflammatory transcriptional programs in ovarian cancer.\",\n      \"evidence\": \"Transcriptional profiling after IgA transcytosis (cited in review without full methodological detail)\",\n      \"pmids\": [\"37897659\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Original experimental detail not available; cited indirectly in review\", \"Causality and in vivo relevance not established\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Identified a viral immune-evasion mechanism whereby SARS-CoV-2 ORF8 binds and downregulates pIgR to impair immunoglobulin binding.\",\n      \"evidence\": \"ORF8-pIgR interaction studies, expression and dIgA/IgM binding assays, and internalization assays\",\n      \"pmids\": [\"39066171\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism of ORF8-induced downregulation not defined\", \"In vivo mucosal consequence not tested\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Implicated hematopoietic-cell pIgR in vascular disease, showing macrophage-associated pIgR promotes abdominal aortic aneurysm progression.\",\n      \"evidence\": \"Bone marrow transplantation in Ldlr-/- AAA mice with macrophage infiltration quantification and THP-1 polarization assays\",\n      \"pmids\": [\"40624587\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular function of pIgR in macrophages undefined\", \"Whether transcytosis function is involved unknown\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Linked a lncRNA to pIgR processing, showing LINC00870 binds pIgR to inhibit its glycosylation/secretion and drive imatinib resistance in GIST.\",\n      \"evidence\": \"Binding assays, glycosylation and secretion assays, and overexpression/knockdown in gastrointestinal stromal tumor cells\",\n      \"pmids\": [\"39968132\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism linking pIgR glycosylation to drug resistance unclear\", \"Binding interface not characterized\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the diverse non-transport functions of pIgR (cancer stemness, macrophage-driven vascular disease) relate mechanistically to its canonical transcytosis activity remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No unifying mechanism connecting transport and signaling roles\", \"Receptors/partners mediating EV-pIgR effects unidentified\", \"Structural basis of ligand and pathogen recognition not solved\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005215\", \"supporting_discovery_ids\": [0, 3, 9]},\n      {\"term_id\": \"GO:0038024\", \"supporting_discovery_ids\": [0, 3]},\n      {\"term_id\": \"GO:0001618\", \"supporting_discovery_ids\": [5, 6]},\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [1]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [3, 5, 13]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [3, 7]},\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [8]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [0, 9, 15]},\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [3, 7]},\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [3, 7]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [6, 8, 13, 17]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"JCHAIN\", \"RAB11FIP1\", \"RAB11FIP5\", \"TRIM21\", \"PECAM1\", \"LINC00870\"],\n    \"other_free_text\": []\n  }\n}\n```","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}