{"gene":"PIGR","run_date":"2026-04-28T19:45:44","timeline":{"discoveries":[{"year":1998,"finding":"J chain is required for pIgR-mediated transcytosis of polymeric IgA: J chain-lacking pIgA (pIgA-L) failed to bind secretory component (SC) in vitro, was not transported into rat bile after intravenous injection, and was not transcytosed 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 hepatocyte transcytosis (bile transport after IV injection), polarized MDCK cell monolayer transcytosis assay","journal":"Immunology","confidence":"High","confidence_rationale":"Tier 1 — reconstitution/in vivo assay with multiple orthogonal systems; replicated across in vitro and in vivo models","pmids":["9767462"],"is_preprint":false},{"year":2001,"finding":"Binding of polymeric IgA to human pIgR induces IP3 production (phospholipase C activation) but does not stimulate transcytosis, in contrast to rabbit pIgR where pIgA binding accelerates transcytosis. PKC activator (PMA) accelerates transcytosis of both species' pIgR, but ionomycin (mimicking calcium increase) stimulates only rabbit pIgR transcytosis, indicating that the species difference is due to differential sensitivity of pIgR to intracellular calcium rather than defective second messenger production.","method":"Continuous apical SC release assay in polarized MDCK and Calu-3 cells expressing human or rabbit pIgR, IP3 production measurement, pharmacological manipulation (PMA, ionomycin)","journal":"Scandinavian journal of immunology","confidence":"High","confidence_rationale":"Tier 1 — functional in vitro assay with mutagenic/pharmacological dissection in multiple cell lines","pmids":["11169207"],"is_preprint":false},{"year":2010,"finding":"pIgR-mediated transcytosis of pIgA (basolateral-to-apical) is strongly unidirectional; pIgA traffics through early (EEA1-positive) and recycling (Rab11a-positive) endosomes and is sorted away from transferrin-positive common/basolateral recycling endosomes at later time points. Microtubule depolymerization equivalently impairs both pIgR/pIgA and FcRn/IgG trafficking routes.","method":"Fluorescence confocal microscopy, pulse-chase experiments, live-cell imaging, microtubule depolymerization, co-expression of pIgR and FcRn in MDCK cells","journal":"Traffic (Copenhagen, Denmark)","confidence":"High","confidence_rationale":"Tier 2 — direct live imaging with endosomal marker co-localization and functional perturbation in polarized epithelial cells","pmids":["20525015"],"is_preprint":false},{"year":2017,"finding":"The pneumococcal pilus-1 adhesin RrgA binds both pIgR and PECAM-1 on blood-brain barrier endothelium, while choline-binding protein PspC binds pIgR to a lesser extent. Antibodies against pIgR and PECAM-1 prevent pneumococcal entry into the brain and meningitis development in a bacteremia-derived meningitis mouse model.","method":"STED super-resolution microscopy of brain biopsies, in vitro antibody blocking, bacteremia-derived meningitis mouse model with mutant mice, incubation of pneumococci with endothelial cell lysates (pulldown)","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods including super-resolution microscopy, in vivo mouse model, and direct binding assays; replicated in vitro and in vivo","pmids":["28515075"],"is_preprint":false},{"year":2014,"finding":"S. pneumoniae co-localizes with and physically interacts with pIgR on brain microvascular endothelial cells (BBB) in vivo. Blocking pIgR with antibodies reduces pneumococcal adhesion to endothelial cells in vitro; bacteria physically interact with pIgR in endothelial cell lysates. S. pneumoniae does not co-localize with PAFR.","method":"Immunofluorescent analysis of mouse brain tissue in vivo (IV infection model), in vitro antibody blocking, incubation of pneumococci with endothelial cell lysates","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 — in vivo co-localization plus direct binding in cell lysates; single lab","pmids":["24841255"],"is_preprint":false},{"year":2021,"finding":"Rab11-FIP1 is a novel pIgR interacting protein; Rab11-FIP1 and Rab11-FIP5 cooperatively facilitate pIgR/pIgA transcytosis. TRIM21 mediates K11-linked polyubiquitination of Rab11-FIP1 and K6-linked polyubiquitination of Rab11-FIP5 to promote their activation and pIgA transcytosis. In incompletely polarized cells, endocytosed pIgR/pIgA is transported from basolateral membrane to the vicinity of the centrosome where Rab11-FIP1 and Rab11-FIP5 bind it, before transport to apical membrane via Golgi apparatus.","method":"Co-immunoprecipitation (pulldown), siRNA knockdown of Rab11-FIP1/FIP5, pIgA transcytosis assay in polarized and incompletely polarized cells, ubiquitination assays","journal":"International journal of molecular sciences","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP plus functional KD with specific transcytosis readout; multiple orthogonal methods","pmids":["34638806"],"is_preprint":false},{"year":2021,"finding":"EV-pIgR (pIgR enriched in extracellular vesicles from late-stage HCC patients) promotes cancer stemness and tumorigenesis in recipient cells via activation of the PDK1/Akt/GSK3β/β-catenin signaling axis. This effect is abrogated by Akt and β-catenin inhibitors and by anti-pIgR neutralizing antibody.","method":"EV isolation and characterization, in vitro functional assays, in vivo patient-derived tumor xenografts (PDTX), Akt/β-catenin inhibitor treatment, anti-pIgR neutralizing antibody blockade, proteomic analysis","journal":"Journal of hepatology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal in vitro and in vivo methods with specific pathway inhibitors and mechanistic rescue","pmids":["34922977"],"is_preprint":false},{"year":2023,"finding":"Hepatic pIgR transports IgA across the epithelial barrier into intestinal lumen and hepatic canaliculi, thereby 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 hepatic re-expression of pIgR in pIgR-deficient mice increased intestinal IgA levels and ameliorated steatohepatitis by reducing bacterial translocation.","method":"pIgR-deficient mouse model, chronic-binge ethanol feeding model (NIAAA), AAV8-mediated hepatic pIgR re-expression, non-absorbable antibiotic treatment, IgA ELISA, LPS measurement, histology","journal":"Gut","confidence":"High","confidence_rationale":"Tier 2 — genetic KO with defined phenotype, mechanistic rescue via AAV re-expression, multiple readouts in vivo","pmids":["36690432"],"is_preprint":false},{"year":1998,"finding":"Vitamin A metabolite all-trans retinoic acid (RA) is required for regulation of pIgR expression by IL-4 and IFN-γ in human intestinal epithelial cells. Vitamin A-depleted conditions significantly reduced the upregulation of pIgR by cytokines; RA restored pIgR mRNA and cell-surface protein expression in a dose-dependent manner.","method":"Vitamin A-depleted cell culture, flow cytometry for cell-surface pIgR, RT-PCR for pIgR mRNA in HT-29 cells, dose-response with retinoic acid","journal":"The Journal of nutrition","confidence":"Medium","confidence_rationale":"Tier 2 — direct functional assay in human cell line with multiple readouts; single lab","pmids":["9649586"],"is_preprint":false},{"year":2010,"finding":"Neutrophils upregulate epithelial pIgR/secretory component (SC) production through a TGF-β-mediated pathway dependent on redox balance and p38 MAP kinase activation; elastase inhibitor SLPI favors this upregulation. This is counterbalanced by neutrophil-mediated SC degradation, and an imbalance between these two mechanisms may lead to net downregulation as seen in severe COPD.","method":"Co-culture of neutrophils with Calu-3 bronchial epithelial cells, measurement of SC production, TGF-β neutralization, p38 MAPK inhibitor, redox perturbation, SLPI treatment","journal":"Journal of biomedicine & biotechnology","confidence":"Medium","confidence_rationale":"Tier 2 — functional co-culture with specific signaling pathway inhibitors; single lab","pmids":["20706611"],"is_preprint":false},{"year":2022,"finding":"Secretory cells (club cells) are the predominant cell type responsible for pIgR expression in human and murine small airways, established by RNA in situ hybridization, immunostaining, single-cell RNA sequencing, and transgenic mice with secretory or ciliated cell-specific knockout of pIgR. Loss of SIgA in small airways of COPD patients is not due to loss of secretory cells but to reduced pIgR protein despite intact PIGR mRNA.","method":"RNA in situ hybridization, immunostaining, single-cell RNA sequencing, transgenic mice with cell-specific pIgR knockout, primary murine tracheal epithelial cell culture","journal":"American journal of respiratory cell and molecular biology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods including cell-type specific KO in transgenic mice with functional consequence","pmids":["35687143"],"is_preprint":false},{"year":2022,"finding":"M1 macrophage-secreted IL-1β is the major cytokine driving increased PIGR expression in MDA-MB468 breast cancer cells. This was confirmed by IL-1 receptor blockade, which abrogated the M1 macrophage conditioned media-induced PIGR upregulation.","method":"M1/M2 macrophage conditioned media treatment, recombinant cytokine treatment, IL-1 receptor blockade, RT-qPCR, Western blotting in breast cancer cell lines","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 2 — receptor blockade confirming specific cytokine-receptor mechanism; single lab","pmids":["36207349"],"is_preprint":false},{"year":2024,"finding":"SARS-CoV-2 accessory protein ORF8 interacts with pIgR and potently downregulates pIgR expression, thereby diminishing binding of dimeric IgA (dIgA) and pentameric IgM to pIgR. Secreted ORF8 binds cell-surface pIgR but does not trigger cellular internalization of ORF8 (which requires dIgA binding to pIgR). ORF8 proteins from SARS-CoV-2 variants of concern preserve this pIgR downregulation activity.","method":"Interaction studies between ORF8 and pIgR, pIgR expression assay after ORF8 expression, dIgA/pIgM binding assays, cell internalization assay","journal":"Viruses","confidence":"Medium","confidence_rationale":"Tier 2 — direct protein interaction and functional consequence established; single lab, moderate mechanistic depth","pmids":["39066171"],"is_preprint":false},{"year":2001,"finding":"A microsatellite-containing fragment from the 3'-UTR of the rat Pigr gene functions as a position- and orientation-dependent regulator of gene expression. In supercoiled plasmids, the fragment attenuates reporter gene expression when in 3'-UTR or 3'-flanking position and stimulates expression in 5'-flanking position. The effects depend on DNA supercoiling and correlate with altered mRNA levels (5' and 3' flanking) or control mRNA with altered translation efficiency (3'-UTR position). The fragment shows intramolecular triplex-forming properties.","method":"Transient transfection luciferase reporter assay, cell-free translation, nuclease S1/P1 hypersensitivity, gel mobility assays, plasmid linearization","journal":"Physiological genomics","confidence":"Medium","confidence_rationale":"Tier 1 — in vitro reconstitution with multiple reporter assays; single lab","pmids":["11242589"],"is_preprint":false},{"year":2016,"finding":"Chlamydial infection of human epithelial cells upregulates pIgR expression and enhances transcytosis of IgA into the lumen in both male and female reproductive tracts in vivo and in vitro. Hormone cycling affects pIgR expression, with highest expression during estrus and lowest during diestrus or after hormonal synchronization with Depo-Provera.","method":"Western blot, immunohistochemistry, in vitro infection of human epithelia, in vivo mouse reproductive tract infection model","journal":"American journal of reproductive immunology","confidence":"Medium","confidence_rationale":"Tier 2 — in vitro and in vivo concordant results with direct functional readout (IgA transcytosis); single lab","pmids":["27868280"],"is_preprint":false},{"year":2002,"finding":"pIgR expression in sheep mammary gland is regulated by prolactin and glucocorticoids. Hormonal treatment with estradiol, progesterone, and glucocorticoids increases pIgR mRNA levels, and blocking prolactin secretion with bromocryptine abolishes the increase in pIgR mRNA during the hormonal induction protocol.","method":"Northern blot, in situ hybridization, immunohistochemistry, hormonal treatment, bromocryptine-mediated prolactin blockade in sheep","journal":"The Journal of dairy research","confidence":"Medium","confidence_rationale":"Tier 2 — in vivo hormonal manipulation with mechanistic rescue/inhibition; single lab","pmids":["12047104"],"is_preprint":false},{"year":1997,"finding":"Uterine stromal cells suppress pIgR production by uterine epithelial cells through soluble factors: co-culture with stromal cells or stromal cell-conditioned supernatants decreased epithelial pIgR production as shown by immunohistochemistry, while epithelial cell viability was maintained.","method":"Co-culture assay (stromal and epithelial cells), conditioned supernatant treatment, immunohistochemistry, transepithelial resistance measurement","journal":"Journal of reproductive immunology","confidence":"Medium","confidence_rationale":"Tier 3 — functional co-culture with specific cellular readout; single lab, moderate mechanistic depth","pmids":["9234210"],"is_preprint":false},{"year":2025,"finding":"PIGR deficiency in hematopoietic cells reduces abdominal aortic aneurysm (AAA) incidence and decreases macrophage infiltration in the AAA wall, as demonstrated by bone marrow transplantation experiments in an experimental mouse AAA model. PIGR mRNA is expressed by macrophages and is upregulated in M1-polarized macrophages compared to M2 macrophages.","method":"Bone marrow transplantation in Ldlr-/- mice with angiotensin II-induced AAA, Q-PCR, ELISA, macrophage polarization in THP-1 cells, immunohistochemistry","journal":"Journal of translational medicine","confidence":"Medium","confidence_rationale":"Tier 2 — genetic epistasis via bone marrow transplant with defined phenotypic readout; single study","pmids":["40624587"],"is_preprint":false},{"year":2025,"finding":"LINC00870 binds specifically to PIGR and inhibits glycosylation modification and secretion of the extracellular region of PIGR, leading to immune dysregulation and imatinib resistance in gastrointestinal stromal tumor. Inhibition of either PIGR or LINC00870 overcomes imatinib resistance.","method":"RNA-protein binding assay, overexpression/knockdown functional assays, glycosylation analysis, imatinib resistance assays in GIST cell lines","journal":"Heliyon","confidence":"Low","confidence_rationale":"Tier 3 — single lab, binding and functional assays but limited mechanistic detail on glycosylation mechanism","pmids":["39968132"],"is_preprint":false},{"year":2023,"finding":"In ovarian cancer cells, pIgR-mediated transcytosis of IgA is accompanied by upregulation of IFN-gamma and downregulation of tumor-promoting ephrins, indicating that both pIgR levels and transcytosed IgA affect intracellular inflammatory signaling pathways in cancer cells.","method":"Transcriptional analysis of ovarian cancer cells following IgA transcytosis (cited as findings within the review paper, referencing prior studies)","journal":"Journal of cancer research and clinical oncology","confidence":"Low","confidence_rationale":"Tier 3 — mechanistic claim is stated in a review context without direct experimental detail","pmids":["37897659"],"is_preprint":false}],"current_model":"PIGR (polymeric immunoglobulin receptor) is a transmembrane epithelial receptor that mediates basolateral-to-apical transcytosis of J chain-containing polymeric IgA and IgM across mucosal epithelia and hepatocytes, trafficking through EEA1+ early and Rab11a+ recycling endosomes with the help of TRIM21-ubiquitinated Rab11-FIP1/FIP5; ligand binding triggers IP3/PKC-dependent intracellular signaling but species-specific calcium sensitivity determines whether pIgA accelerates transcytosis; pIgR expression is regulated by cytokines (IFN-γ, IL-4, IL-1β, TGF-β), hormones (prolactin, glucocorticoids, retinoic acid), microbial stimuli (via TLR4/NF-κB), and stromal cell-derived signals; at the blood-brain barrier endothelium, pIgR also serves as an adhesion receptor for pneumococcal adhesins RrgA and PspC; in the extracellular vesicle context, EV-pIgR activates PDK1/Akt/GSK3β/β-catenin signaling to promote cancer stemness; and SARS-CoV-2 ORF8 interacts with and downregulates pIgR to evade mucosal immunity."},"narrative":{"teleology":[{"year":1997,"claim":"Establishing that pIgR expression is not constitutive but subject to paracrine regulation by neighboring cells showed that mucosal IgA transport capacity is dynamically tuned by the tissue microenvironment.","evidence":"Co-culture of uterine stromal and epithelial cells with conditioned-medium transfer and immunohistochemistry","pmids":["9234210"],"confidence":"Medium","gaps":["Identity of soluble suppressive factor(s) not determined","Signaling pathway mediating suppression unknown"]},{"year":1998,"claim":"Demonstrating that J chain is absolutely required for pIgR binding and transcytosis resolved a longstanding question about what distinguishes pIgA competent for mucosal transport from that which is not.","evidence":"SC-binding assay, rat bile transport in vivo, and polarized MDCK transcytosis for J chain-positive versus J chain-negative pIgA","pmids":["9767462"],"confidence":"High","gaps":["Structural basis of J chain-dependent recognition not resolved","Stoichiometry of J chain per polymeric IgA needed for binding not tested"]},{"year":1998,"claim":"Identifying retinoic acid as a necessary cofactor for cytokine-induced PIGR upregulation linked nutritional (vitamin A) status to mucosal immune defense capacity.","evidence":"Vitamin A-depleted HT-29 cultures with IL-4/IFN-γ stimulation, dose-response retinoic acid rescue, RT-PCR and flow cytometry","pmids":["9649586"],"confidence":"Medium","gaps":["Whether retinoic acid acts through RAR/RXR elements in the PIGR promoter was not tested","In vivo relevance in vitamin A deficiency not established"]},{"year":2001,"claim":"Showing that pIgA binding triggers IP3/PKC signaling at the human pIgR yet fails to accelerate human transcytosis — unlike rabbit pIgR — pinpointed species-specific calcium sensitivity as the divergent regulatory step in receptor trafficking.","evidence":"Continuous SC-release assay in MDCK/Calu-3 cells expressing human or rabbit pIgR, IP3 measurement, PMA and ionomycin pharmacology","pmids":["11169207"],"confidence":"High","gaps":["Molecular determinant in human pIgR cytoplasmic tail conferring calcium insensitivity not mapped","Physiological significance of species difference in vivo unclear"]},{"year":2002,"claim":"Demonstrating prolactin and glucocorticoid dependence of PIGR mRNA in mammary gland extended the regulatory framework beyond cytokines to endocrine control during lactation.","evidence":"Bromocryptine-mediated prolactin blockade and hormonal induction in sheep mammary gland, Northern blot and in situ hybridization","pmids":["12047104"],"confidence":"Medium","gaps":["Prolactin-responsive cis-elements in PIGR promoter not identified","Whether prolactin regulation is conserved in human mammary epithelium not tested"]},{"year":2010,"claim":"Live imaging of pIgR/pIgA transit through EEA1+ early and Rab11a+ recycling endosomes, with sorting away from basolateral recycling endosomes, defined the intracellular itinerary that ensures unidirectional transcytosis.","evidence":"Confocal live-cell imaging, pulse-chase co-localization with endosomal markers, microtubule depolymerization in MDCK cells co-expressing pIgR and FcRn","pmids":["20525015"],"confidence":"High","gaps":["Sorting signal on pIgR cytoplasmic tail directing exit from common endosomes not mapped","Role of adaptor proteins at endosomal sorting step not determined"]},{"year":2010,"claim":"Identifying TGF-β/p38 MAPK as the neutrophil-derived pathway that upregulates pIgR revealed how innate immune cells amplify mucosal antibody transport, and that loss of this balance contributes to COPD pathology.","evidence":"Neutrophil-epithelial co-culture with TGF-β neutralization and p38 MAPK inhibition, SC secretion measurement in Calu-3 cells","pmids":["20706611"],"confidence":"Medium","gaps":["Whether neutrophil-derived TGF-β acts directly on PIGR promoter or indirectly not resolved","In vivo validation in COPD patient tissue limited"]},{"year":2014,"claim":"Discovering that S. pneumoniae co-localizes with and physically interacts with pIgR on brain microvascular endothelial cells revealed an unexpected non-canonical role for pIgR as a pathogen adhesion receptor at the blood–brain barrier.","evidence":"Immunofluorescence of mouse brain after IV pneumococcal infection, antibody blocking of adhesion in vitro, pulldown from endothelial lysates","pmids":["24841255"],"confidence":"Medium","gaps":["Specific pneumococcal ligand mediating pIgR binding not identified in this study","Whether pIgR engagement triggers endothelial signaling or passive internalization unknown"]},{"year":2017,"claim":"Identifying RrgA (pilus adhesin) and PspC as the specific pneumococcal ligands for pIgR on BBB endothelium and showing that anti-pIgR antibodies prevent meningitis established pIgR as a druggable entry receptor for CNS invasion.","evidence":"STED super-resolution microscopy, anti-pIgR/PECAM-1 antibody blocking in bacteremia-derived meningitis mouse model, pulldown with endothelial lysates","pmids":["28515075"],"confidence":"High","gaps":["Structural basis of RrgA–pIgR interaction not determined","Whether pIgR mediates actual transcytosis of bacteria across BBB endothelium versus adhesion alone is unclear"]},{"year":2021,"claim":"Identifying Rab11-FIP1 as a pIgR interactor and showing that TRIM21-mediated K11/K6-linked polyubiquitination of FIP1/FIP5 promotes pIgA transcytosis provided the first mechanistic link between ubiquitin signaling and the transcytotic sorting machinery.","evidence":"Reciprocal co-immunoprecipitation, siRNA knockdown of FIP1/FIP5, ubiquitination assays, transcytosis in polarized and incompletely polarized cells","pmids":["34638806"],"confidence":"High","gaps":["Whether TRIM21 ubiquitinates FIP1/FIP5 constitutively or in response to pIgA binding is unclear","Deubiquitinase counteracting TRIM21 not identified"]},{"year":2021,"claim":"Demonstrating that extracellular vesicle-associated pIgR activates PDK1/Akt/GSK3β/β-catenin signaling to promote cancer stemness uncovered a gain-of-function oncogenic role for pIgR outside its canonical transcytosis function.","evidence":"EV isolation from HCC patients, patient-derived tumor xenografts, Akt/β-catenin inhibitor rescue, anti-pIgR neutralizing antibody blockade","pmids":["34922977"],"confidence":"High","gaps":["Mechanism by which pIgR on EV surface activates PDK1 not established","Whether EV-pIgR requires a ligand or signals constitutively is unknown"]},{"year":2022,"claim":"Cell-type-specific knockout established club (secretory) cells as the predominant pIgR-producing cell in small airways and showed that COPD-associated SIgA loss results from post-transcriptional pIgR downregulation rather than cell loss.","evidence":"scRNA-seq, RNA ISH, immunostaining, secretory and ciliated cell-specific pIgR knockout mice","pmids":["35687143"],"confidence":"High","gaps":["Post-transcriptional mechanism reducing pIgR protein in COPD not identified","Whether this applies to large airways or other mucosal sites unknown"]},{"year":2022,"claim":"Identifying M1 macrophage-derived IL-1β as a major driver of PIGR upregulation in breast cancer cells connected innate inflammatory signaling to pIgR-dependent IgA transport in the tumor microenvironment.","evidence":"M1/M2 conditioned media, recombinant cytokines, IL-1 receptor blockade in MDA-MB468 cells","pmids":["36207349"],"confidence":"Medium","gaps":["Downstream transcription factor linking IL-1R to PIGR promoter not determined","Functional consequence for anti-tumor immunity not tested in vivo"]},{"year":2023,"claim":"Demonstrating that hepatic pIgR transports IgA to limit bacterial translocation and that AAV-mediated re-expression rescues ethanol-induced liver disease established pIgR as a non-redundant guardian of the gut–liver immune axis.","evidence":"pIgR-KO mice with chronic-binge ethanol model, AAV8-mediated hepatic rescue, LPS/bacterial burden measurement","pmids":["36690432"],"confidence":"High","gaps":["Whether hepatic pIgR directly transports IgA into bile canaliculi or acts indirectly not fully dissected","Contribution of secretory component cleavage versus intact receptor in hepatocyte transcytosis not addressed"]},{"year":2024,"claim":"Showing that SARS-CoV-2 ORF8 binds and downregulates pIgR, reducing dIgA and IgM binding, identified a specific viral immune evasion mechanism targeting mucosal antibody transport.","evidence":"ORF8–pIgR interaction studies, pIgR expression and dIgA/pIgM binding assays, variant ORF8 comparison","pmids":["39066171"],"confidence":"Medium","gaps":["Mechanism of ORF8-induced pIgR downregulation (degradation vs. transcriptional) not determined","In vivo relevance in SARS-CoV-2 infection not demonstrated"]},{"year":2025,"claim":"Bone marrow transplant showing that hematopoietic PIGR deficiency reduces AAA incidence revealed an unexpected function for pIgR in macrophage-driven vascular inflammation.","evidence":"Bone marrow transplant into Ldlr−/− mice with angiotensin II-induced AAA, macrophage polarization assays","pmids":["40624587"],"confidence":"Medium","gaps":["Ligand and signaling mechanism of pIgR in macrophages unknown","Whether macrophage pIgR functions in transcytosis or as a signaling receptor not addressed"]},{"year":null,"claim":"The structural basis of pIgR's interaction with J chain-containing polymeric Ig, the mechanism by which pIgR is sorted from recycling to apical endosomes, and the non-canonical signaling roles of pIgR in cancer and vascular inflammation remain incompletely understood.","evidence":"","pmids":[],"confidence":"Low","gaps":["No high-resolution structure of pIgR–pIgA–J chain complex available","Post-transcriptional mechanism of pIgR downregulation in COPD undefined","Ligand and function of pIgR in macrophages and other non-epithelial cells uncharacterized"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0038024","term_label":"cargo receptor activity","supporting_discovery_ids":[0,2,5,7]},{"term_id":"GO:0098631","term_label":"cell adhesion mediator activity","supporting_discovery_ids":[3,4]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,1,3,12]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[2]},{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[2,5,6]},{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[6,7]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[0,7,12]},{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[2,5]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[1,6]}],"complexes":[],"partners":["RAB11FIP1","RAB11FIP5","TRIM21","RAB11A","PECAM1"],"other_free_text":[]},"mechanistic_narrative":"PIGR encodes the polymeric immunoglobulin receptor, a transmembrane glycoprotein expressed on mucosal epithelial cells that mediates unidirectional basolateral-to-apical transcytosis of J chain-containing polymeric IgA and IgM, thereby delivering secretory immunoglobulins to mucosal surfaces and maintaining barrier immunity [PMID:9767462, PMID:20525015, PMID:36690432]. Transcytosis proceeds through EEA1-positive early endosomes and Rab11a-positive recycling endosomes, with TRIM21-dependent polyubiquitination of Rab11-FIP1 and Rab11-FIP5 promoting cargo delivery to the apical membrane; ligand binding activates phospholipase C/IP3 signaling, though species-specific calcium sensitivity determines whether pIgA accelerates transit [PMID:34638806, PMID:11169207, PMID:20525015]. PIGR expression is regulated by cytokines (IL-4, IFN-γ, IL-1β, TGF-β), hormones (prolactin, glucocorticoids), and retinoic acid, and is predominantly produced by secretory (club) cells in small airways [PMID:9649586, PMID:20706611, PMID:35687143, PMID:12047104]. Beyond canonical mucosal immunity, pIgR serves as an endothelial adhesion receptor exploited by Streptococcus pneumoniae for blood–brain barrier invasion, is targeted for downregulation by SARS-CoV-2 ORF8 to evade mucosal immunity, and when carried on extracellular vesicles activates PDK1/Akt/GSK3β/β-catenin signaling to promote hepatocellular carcinoma stemness [PMID:28515075, PMID:39066171, PMID:34922977]."},"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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transition","date":"2024-12-05","source":"bioRxiv","url":"https://doi.org/10.1101/2024.12.02.626361","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":43566,"output_tokens":5442,"usd":0.106164},"stage2":{"model":"claude-opus-4-6","input_tokens":8984,"output_tokens":4052,"usd":0.21933},"total_usd":0.325494,"stage1_batch_id":"msgbatch_01SUR8NDzVrNmyszWyJeCct7","stage2_batch_id":"msgbatch_01G7Yfutko3cDWzejrwH5AMx","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1998,\n      \"finding\": \"J chain is required for pIgR-mediated transcytosis of polymeric IgA: J chain-lacking pIgA (pIgA-L) failed to bind secretory component (SC) in vitro, was not transported into rat bile after intravenous injection, and was not transcytosed 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 hepatocyte transcytosis (bile transport after IV injection), polarized MDCK cell monolayer transcytosis assay\",\n      \"journal\": \"Immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution/in vivo assay with multiple orthogonal systems; replicated across in vitro and in vivo models\",\n      \"pmids\": [\"9767462\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Binding of polymeric IgA to human pIgR induces IP3 production (phospholipase C activation) but does not stimulate transcytosis, in contrast to rabbit pIgR where pIgA binding accelerates transcytosis. PKC activator (PMA) accelerates transcytosis of both species' pIgR, but ionomycin (mimicking calcium increase) stimulates only rabbit pIgR transcytosis, indicating that the species difference is due to differential sensitivity of pIgR to intracellular calcium rather than defective second messenger production.\",\n      \"method\": \"Continuous apical SC release assay in polarized MDCK and Calu-3 cells expressing human or rabbit pIgR, IP3 production measurement, pharmacological manipulation (PMA, ionomycin)\",\n      \"journal\": \"Scandinavian journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — functional in vitro assay with mutagenic/pharmacological dissection in multiple cell lines\",\n      \"pmids\": [\"11169207\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"pIgR-mediated transcytosis of pIgA (basolateral-to-apical) is strongly unidirectional; pIgA traffics through early (EEA1-positive) and recycling (Rab11a-positive) endosomes and is sorted away from transferrin-positive common/basolateral recycling endosomes at later time points. Microtubule depolymerization equivalently impairs both pIgR/pIgA and FcRn/IgG trafficking routes.\",\n      \"method\": \"Fluorescence confocal microscopy, pulse-chase experiments, live-cell imaging, microtubule depolymerization, co-expression of pIgR and FcRn in MDCK cells\",\n      \"journal\": \"Traffic (Copenhagen, Denmark)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct live imaging with endosomal marker co-localization and functional perturbation in polarized epithelial cells\",\n      \"pmids\": [\"20525015\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"The pneumococcal pilus-1 adhesin RrgA binds both pIgR and PECAM-1 on blood-brain barrier endothelium, while choline-binding protein PspC binds pIgR to a lesser extent. Antibodies against pIgR and PECAM-1 prevent pneumococcal entry into the brain and meningitis development in a bacteremia-derived meningitis mouse model.\",\n      \"method\": \"STED super-resolution microscopy of brain biopsies, in vitro antibody blocking, bacteremia-derived meningitis mouse model with mutant mice, incubation of pneumococci with endothelial cell lysates (pulldown)\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods including super-resolution microscopy, in vivo mouse model, and direct binding assays; replicated in vitro and in vivo\",\n      \"pmids\": [\"28515075\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"S. pneumoniae co-localizes with and physically interacts with pIgR on brain microvascular endothelial cells (BBB) in vivo. Blocking pIgR with antibodies reduces pneumococcal adhesion to endothelial cells in vitro; bacteria physically interact with pIgR in endothelial cell lysates. S. pneumoniae does not co-localize with PAFR.\",\n      \"method\": \"Immunofluorescent analysis of mouse brain tissue in vivo (IV infection model), in vitro antibody blocking, incubation of pneumococci with endothelial cell lysates\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vivo co-localization plus direct binding in cell lysates; single lab\",\n      \"pmids\": [\"24841255\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Rab11-FIP1 is a novel pIgR interacting protein; Rab11-FIP1 and Rab11-FIP5 cooperatively facilitate pIgR/pIgA transcytosis. TRIM21 mediates K11-linked polyubiquitination of Rab11-FIP1 and K6-linked polyubiquitination of Rab11-FIP5 to promote their activation and pIgA transcytosis. In incompletely polarized cells, endocytosed pIgR/pIgA is transported from basolateral membrane to the vicinity of the centrosome where Rab11-FIP1 and Rab11-FIP5 bind it, before transport to apical membrane via Golgi apparatus.\",\n      \"method\": \"Co-immunoprecipitation (pulldown), siRNA knockdown of Rab11-FIP1/FIP5, pIgA transcytosis assay in polarized and incompletely polarized cells, ubiquitination assays\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP plus functional KD with specific transcytosis readout; multiple orthogonal methods\",\n      \"pmids\": [\"34638806\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"EV-pIgR (pIgR enriched in extracellular vesicles from late-stage HCC patients) promotes cancer stemness and tumorigenesis in recipient cells via activation of the PDK1/Akt/GSK3β/β-catenin signaling axis. This effect is abrogated by Akt and β-catenin inhibitors and by anti-pIgR neutralizing antibody.\",\n      \"method\": \"EV isolation and characterization, in vitro functional assays, in vivo patient-derived tumor xenografts (PDTX), Akt/β-catenin inhibitor treatment, anti-pIgR neutralizing antibody blockade, proteomic analysis\",\n      \"journal\": \"Journal of hepatology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal in vitro and in vivo methods with specific pathway inhibitors and mechanistic rescue\",\n      \"pmids\": [\"34922977\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Hepatic pIgR transports IgA across the epithelial barrier into intestinal lumen and hepatic canaliculi, thereby 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 hepatic re-expression of pIgR in pIgR-deficient mice increased intestinal IgA levels and ameliorated steatohepatitis by reducing bacterial translocation.\",\n      \"method\": \"pIgR-deficient mouse model, chronic-binge ethanol feeding model (NIAAA), AAV8-mediated hepatic pIgR re-expression, non-absorbable antibiotic treatment, IgA ELISA, LPS measurement, histology\",\n      \"journal\": \"Gut\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with defined phenotype, mechanistic rescue via AAV re-expression, multiple readouts in vivo\",\n      \"pmids\": [\"36690432\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Vitamin A metabolite all-trans retinoic acid (RA) is required for regulation of pIgR expression by IL-4 and IFN-γ in human intestinal epithelial cells. Vitamin A-depleted conditions significantly reduced the upregulation of pIgR by cytokines; RA restored pIgR mRNA and cell-surface protein expression in a dose-dependent manner.\",\n      \"method\": \"Vitamin A-depleted cell culture, flow cytometry for cell-surface pIgR, RT-PCR for pIgR mRNA in HT-29 cells, dose-response with retinoic acid\",\n      \"journal\": \"The Journal of nutrition\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct functional assay in human cell line with multiple readouts; single lab\",\n      \"pmids\": [\"9649586\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Neutrophils upregulate epithelial pIgR/secretory component (SC) production through a TGF-β-mediated pathway dependent on redox balance and p38 MAP kinase activation; elastase inhibitor SLPI favors this upregulation. This is counterbalanced by neutrophil-mediated SC degradation, and an imbalance between these two mechanisms may lead to net downregulation as seen in severe COPD.\",\n      \"method\": \"Co-culture of neutrophils with Calu-3 bronchial epithelial cells, measurement of SC production, TGF-β neutralization, p38 MAPK inhibitor, redox perturbation, SLPI treatment\",\n      \"journal\": \"Journal of biomedicine & biotechnology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional co-culture with specific signaling pathway inhibitors; single lab\",\n      \"pmids\": [\"20706611\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Secretory cells (club cells) are the predominant cell type responsible for pIgR expression in human and murine small airways, established by RNA in situ hybridization, immunostaining, single-cell RNA sequencing, and transgenic mice with secretory or ciliated cell-specific knockout of pIgR. Loss of SIgA in small airways of COPD patients is not due to loss of secretory cells but to reduced pIgR protein despite intact PIGR mRNA.\",\n      \"method\": \"RNA in situ hybridization, immunostaining, single-cell RNA sequencing, transgenic mice with cell-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 — multiple orthogonal methods including cell-type specific KO in transgenic mice with functional consequence\",\n      \"pmids\": [\"35687143\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"M1 macrophage-secreted IL-1β is the major cytokine driving increased PIGR expression in MDA-MB468 breast cancer cells. This was confirmed by IL-1 receptor blockade, which abrogated the M1 macrophage conditioned media-induced PIGR upregulation.\",\n      \"method\": \"M1/M2 macrophage conditioned media treatment, recombinant cytokine treatment, IL-1 receptor blockade, RT-qPCR, Western blotting in breast cancer cell lines\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — receptor blockade confirming specific cytokine-receptor mechanism; single lab\",\n      \"pmids\": [\"36207349\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"SARS-CoV-2 accessory protein ORF8 interacts with pIgR and potently downregulates pIgR expression, thereby diminishing binding of dimeric IgA (dIgA) and pentameric IgM to pIgR. Secreted ORF8 binds cell-surface pIgR but does not trigger cellular internalization of ORF8 (which requires dIgA binding to pIgR). ORF8 proteins from SARS-CoV-2 variants of concern preserve this pIgR downregulation activity.\",\n      \"method\": \"Interaction studies between ORF8 and pIgR, pIgR expression assay after ORF8 expression, dIgA/pIgM binding assays, cell internalization assay\",\n      \"journal\": \"Viruses\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct protein interaction and functional consequence established; single lab, moderate mechanistic depth\",\n      \"pmids\": [\"39066171\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"A microsatellite-containing fragment from the 3'-UTR of the rat Pigr gene functions as a position- and orientation-dependent regulator of gene expression. In supercoiled plasmids, the fragment attenuates reporter gene expression when in 3'-UTR or 3'-flanking position and stimulates expression in 5'-flanking position. The effects depend on DNA supercoiling and correlate with altered mRNA levels (5' and 3' flanking) or control mRNA with altered translation efficiency (3'-UTR position). The fragment shows intramolecular triplex-forming properties.\",\n      \"method\": \"Transient transfection luciferase reporter assay, cell-free translation, nuclease S1/P1 hypersensitivity, gel mobility assays, plasmid linearization\",\n      \"journal\": \"Physiological genomics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with multiple reporter assays; single lab\",\n      \"pmids\": [\"11242589\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Chlamydial infection of human epithelial cells upregulates pIgR expression and enhances transcytosis of IgA into the lumen in both male and female reproductive tracts in vivo and in vitro. Hormone cycling affects pIgR expression, with highest expression during estrus and lowest during diestrus or after hormonal synchronization with Depo-Provera.\",\n      \"method\": \"Western blot, immunohistochemistry, in vitro infection of human epithelia, in vivo mouse reproductive tract infection model\",\n      \"journal\": \"American journal of reproductive immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vitro and in vivo concordant results with direct functional readout (IgA transcytosis); single lab\",\n      \"pmids\": [\"27868280\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"pIgR expression in sheep mammary gland is regulated by prolactin and glucocorticoids. Hormonal treatment with estradiol, progesterone, and glucocorticoids increases pIgR mRNA levels, and blocking prolactin secretion with bromocryptine abolishes the increase in pIgR mRNA during the hormonal induction protocol.\",\n      \"method\": \"Northern blot, in situ hybridization, immunohistochemistry, hormonal treatment, bromocryptine-mediated prolactin blockade in sheep\",\n      \"journal\": \"The Journal of dairy research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vivo hormonal manipulation with mechanistic rescue/inhibition; single lab\",\n      \"pmids\": [\"12047104\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"Uterine stromal cells suppress pIgR production by uterine epithelial cells through soluble factors: co-culture with stromal cells or stromal cell-conditioned supernatants decreased epithelial pIgR production as shown by immunohistochemistry, while epithelial cell viability was maintained.\",\n      \"method\": \"Co-culture assay (stromal and epithelial cells), conditioned supernatant treatment, immunohistochemistry, transepithelial resistance measurement\",\n      \"journal\": \"Journal of reproductive immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — functional co-culture with specific cellular readout; single lab, moderate mechanistic depth\",\n      \"pmids\": [\"9234210\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"PIGR deficiency in hematopoietic cells reduces abdominal aortic aneurysm (AAA) incidence and decreases macrophage infiltration in the AAA wall, as demonstrated by bone marrow transplantation experiments in an experimental mouse AAA model. PIGR mRNA is expressed by macrophages and is upregulated in M1-polarized macrophages compared to M2 macrophages.\",\n      \"method\": \"Bone marrow transplantation in Ldlr-/- mice with angiotensin II-induced AAA, Q-PCR, ELISA, macrophage polarization in THP-1 cells, immunohistochemistry\",\n      \"journal\": \"Journal of translational medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis via bone marrow transplant with defined phenotypic readout; single study\",\n      \"pmids\": [\"40624587\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"LINC00870 binds specifically to PIGR and inhibits glycosylation modification and secretion of the extracellular region of PIGR, leading to immune dysregulation and imatinib resistance in gastrointestinal stromal tumor. Inhibition of either PIGR or LINC00870 overcomes imatinib resistance.\",\n      \"method\": \"RNA-protein binding assay, overexpression/knockdown functional assays, glycosylation analysis, imatinib resistance assays in GIST cell lines\",\n      \"journal\": \"Heliyon\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single lab, binding and functional assays but limited mechanistic detail on glycosylation mechanism\",\n      \"pmids\": [\"39968132\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"In ovarian cancer cells, pIgR-mediated transcytosis of IgA is accompanied by upregulation of IFN-gamma and downregulation of tumor-promoting ephrins, indicating that both pIgR levels and transcytosed IgA affect intracellular inflammatory signaling pathways in cancer cells.\",\n      \"method\": \"Transcriptional analysis of ovarian cancer cells following IgA transcytosis (cited as findings within the review paper, referencing prior studies)\",\n      \"journal\": \"Journal of cancer research and clinical oncology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — mechanistic claim is stated in a review context without direct experimental detail\",\n      \"pmids\": [\"37897659\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PIGR (polymeric immunoglobulin receptor) is a transmembrane epithelial receptor that mediates basolateral-to-apical transcytosis of J chain-containing polymeric IgA and IgM across mucosal epithelia and hepatocytes, trafficking through EEA1+ early and Rab11a+ recycling endosomes with the help of TRIM21-ubiquitinated Rab11-FIP1/FIP5; ligand binding triggers IP3/PKC-dependent intracellular signaling but species-specific calcium sensitivity determines whether pIgA accelerates transcytosis; pIgR expression is regulated by cytokines (IFN-γ, IL-4, IL-1β, TGF-β), hormones (prolactin, glucocorticoids, retinoic acid), microbial stimuli (via TLR4/NF-κB), and stromal cell-derived signals; at the blood-brain barrier endothelium, pIgR also serves as an adhesion receptor for pneumococcal adhesins RrgA and PspC; in the extracellular vesicle context, EV-pIgR activates PDK1/Akt/GSK3β/β-catenin signaling to promote cancer stemness; and SARS-CoV-2 ORF8 interacts with and downregulates pIgR to evade mucosal immunity.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"PIGR encodes the polymeric immunoglobulin receptor, a transmembrane glycoprotein expressed on mucosal epithelial cells that mediates unidirectional basolateral-to-apical transcytosis of J chain-containing polymeric IgA and IgM, thereby delivering secretory immunoglobulins to mucosal surfaces and maintaining barrier immunity [PMID:9767462, PMID:20525015, PMID:36690432]. Transcytosis proceeds through EEA1-positive early endosomes and Rab11a-positive recycling endosomes, with TRIM21-dependent polyubiquitination of Rab11-FIP1 and Rab11-FIP5 promoting cargo delivery to the apical membrane; ligand binding activates phospholipase C/IP3 signaling, though species-specific calcium sensitivity determines whether pIgA accelerates transit [PMID:34638806, PMID:11169207, PMID:20525015]. PIGR expression is regulated by cytokines (IL-4, IFN-γ, IL-1β, TGF-β), hormones (prolactin, glucocorticoids), and retinoic acid, and is predominantly produced by secretory (club) cells in small airways [PMID:9649586, PMID:20706611, PMID:35687143, PMID:12047104]. Beyond canonical mucosal immunity, pIgR serves as an endothelial adhesion receptor exploited by Streptococcus pneumoniae for blood–brain barrier invasion, is targeted for downregulation by SARS-CoV-2 ORF8 to evade mucosal immunity, and when carried on extracellular vesicles activates PDK1/Akt/GSK3β/β-catenin signaling to promote hepatocellular carcinoma stemness [PMID:28515075, PMID:39066171, PMID:34922977].\",\n  \"teleology\": [\n    {\n      \"year\": 1997,\n      \"claim\": \"Establishing that pIgR expression is not constitutive but subject to paracrine regulation by neighboring cells showed that mucosal IgA transport capacity is dynamically tuned by the tissue microenvironment.\",\n      \"evidence\": \"Co-culture of uterine stromal and epithelial cells with conditioned-medium transfer and immunohistochemistry\",\n      \"pmids\": [\"9234210\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Identity of soluble suppressive factor(s) not determined\", \"Signaling pathway mediating suppression unknown\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Demonstrating that J chain is absolutely required for pIgR binding and transcytosis resolved a longstanding question about what distinguishes pIgA competent for mucosal transport from that which is not.\",\n      \"evidence\": \"SC-binding assay, rat bile transport in vivo, and polarized MDCK transcytosis for J chain-positive versus J chain-negative pIgA\",\n      \"pmids\": [\"9767462\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of J chain-dependent recognition not resolved\", \"Stoichiometry of J chain per polymeric IgA needed for binding not tested\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Identifying retinoic acid as a necessary cofactor for cytokine-induced PIGR upregulation linked nutritional (vitamin A) status to mucosal immune defense capacity.\",\n      \"evidence\": \"Vitamin A-depleted HT-29 cultures with IL-4/IFN-γ stimulation, dose-response retinoic acid rescue, RT-PCR and flow cytometry\",\n      \"pmids\": [\"9649586\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether retinoic acid acts through RAR/RXR elements in the PIGR promoter was not tested\", \"In vivo relevance in vitamin A deficiency not established\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Showing that pIgA binding triggers IP3/PKC signaling at the human pIgR yet fails to accelerate human transcytosis — unlike rabbit pIgR — pinpointed species-specific calcium sensitivity as the divergent regulatory step in receptor trafficking.\",\n      \"evidence\": \"Continuous SC-release assay in MDCK/Calu-3 cells expressing human or rabbit pIgR, IP3 measurement, PMA and ionomycin pharmacology\",\n      \"pmids\": [\"11169207\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular determinant in human pIgR cytoplasmic tail conferring calcium insensitivity not mapped\", \"Physiological significance of species difference in vivo unclear\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Demonstrating prolactin and glucocorticoid dependence of PIGR mRNA in mammary gland extended the regulatory framework beyond cytokines to endocrine control during lactation.\",\n      \"evidence\": \"Bromocryptine-mediated prolactin blockade and hormonal induction in sheep mammary gland, Northern blot and in situ hybridization\",\n      \"pmids\": [\"12047104\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Prolactin-responsive cis-elements in PIGR promoter not identified\", \"Whether prolactin regulation is conserved in human mammary epithelium not tested\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Live imaging of pIgR/pIgA transit through EEA1+ early and Rab11a+ recycling endosomes, with sorting away from basolateral recycling endosomes, defined the intracellular itinerary that ensures unidirectional transcytosis.\",\n      \"evidence\": \"Confocal live-cell imaging, pulse-chase co-localization with endosomal markers, microtubule depolymerization in MDCK cells co-expressing pIgR and FcRn\",\n      \"pmids\": [\"20525015\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Sorting signal on pIgR cytoplasmic tail directing exit from common endosomes not mapped\", \"Role of adaptor proteins at endosomal sorting step not determined\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Identifying TGF-β/p38 MAPK as the neutrophil-derived pathway that upregulates pIgR revealed how innate immune cells amplify mucosal antibody transport, and that loss of this balance contributes to COPD pathology.\",\n      \"evidence\": \"Neutrophil-epithelial co-culture with TGF-β neutralization and p38 MAPK inhibition, SC secretion measurement in Calu-3 cells\",\n      \"pmids\": [\"20706611\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether neutrophil-derived TGF-β acts directly on PIGR promoter or indirectly not resolved\", \"In vivo validation in COPD patient tissue limited\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Discovering that S. pneumoniae co-localizes with and physically interacts with pIgR on brain microvascular endothelial cells revealed an unexpected non-canonical role for pIgR as a pathogen adhesion receptor at the blood–brain barrier.\",\n      \"evidence\": \"Immunofluorescence of mouse brain after IV pneumococcal infection, antibody blocking of adhesion in vitro, pulldown from endothelial lysates\",\n      \"pmids\": [\"24841255\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Specific pneumococcal ligand mediating pIgR binding not identified in this study\", \"Whether pIgR engagement triggers endothelial signaling or passive internalization unknown\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Identifying RrgA (pilus adhesin) and PspC as the specific pneumococcal ligands for pIgR on BBB endothelium and showing that anti-pIgR antibodies prevent meningitis established pIgR as a druggable entry receptor for CNS invasion.\",\n      \"evidence\": \"STED super-resolution microscopy, anti-pIgR/PECAM-1 antibody blocking in bacteremia-derived meningitis mouse model, pulldown with endothelial lysates\",\n      \"pmids\": [\"28515075\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of RrgA–pIgR interaction not determined\", \"Whether pIgR mediates actual transcytosis of bacteria across BBB endothelium versus adhesion alone is unclear\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Identifying Rab11-FIP1 as a pIgR interactor and showing that TRIM21-mediated K11/K6-linked polyubiquitination of FIP1/FIP5 promotes pIgA transcytosis provided the first mechanistic link between ubiquitin signaling and the transcytotic sorting machinery.\",\n      \"evidence\": \"Reciprocal co-immunoprecipitation, siRNA knockdown of FIP1/FIP5, ubiquitination assays, transcytosis in polarized and incompletely polarized cells\",\n      \"pmids\": [\"34638806\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether TRIM21 ubiquitinates FIP1/FIP5 constitutively or in response to pIgA binding is unclear\", \"Deubiquitinase counteracting TRIM21 not identified\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Demonstrating that extracellular vesicle-associated pIgR activates PDK1/Akt/GSK3β/β-catenin signaling to promote cancer stemness uncovered a gain-of-function oncogenic role for pIgR outside its canonical transcytosis function.\",\n      \"evidence\": \"EV isolation from HCC patients, patient-derived tumor xenografts, Akt/β-catenin inhibitor rescue, anti-pIgR neutralizing antibody blockade\",\n      \"pmids\": [\"34922977\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which pIgR on EV surface activates PDK1 not established\", \"Whether EV-pIgR requires a ligand or signals constitutively is unknown\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Cell-type-specific knockout established club (secretory) cells as the predominant pIgR-producing cell in small airways and showed that COPD-associated SIgA loss results from post-transcriptional pIgR downregulation rather than cell loss.\",\n      \"evidence\": \"scRNA-seq, RNA ISH, immunostaining, secretory and ciliated cell-specific pIgR knockout mice\",\n      \"pmids\": [\"35687143\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Post-transcriptional mechanism reducing pIgR protein in COPD not identified\", \"Whether this applies to large airways or other mucosal sites unknown\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Identifying M1 macrophage-derived IL-1β as a major driver of PIGR upregulation in breast cancer cells connected innate inflammatory signaling to pIgR-dependent IgA transport in the tumor microenvironment.\",\n      \"evidence\": \"M1/M2 conditioned media, recombinant cytokines, IL-1 receptor blockade in MDA-MB468 cells\",\n      \"pmids\": [\"36207349\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Downstream transcription factor linking IL-1R to PIGR promoter not determined\", \"Functional consequence for anti-tumor immunity not tested in vivo\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Demonstrating that hepatic pIgR transports IgA to limit bacterial translocation and that AAV-mediated re-expression rescues ethanol-induced liver disease established pIgR as a non-redundant guardian of the gut–liver immune axis.\",\n      \"evidence\": \"pIgR-KO mice with chronic-binge ethanol model, AAV8-mediated hepatic rescue, LPS/bacterial burden measurement\",\n      \"pmids\": [\"36690432\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether hepatic pIgR directly transports IgA into bile canaliculi or acts indirectly not fully dissected\", \"Contribution of secretory component cleavage versus intact receptor in hepatocyte transcytosis not addressed\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Showing that SARS-CoV-2 ORF8 binds and downregulates pIgR, reducing dIgA and IgM binding, identified a specific viral immune evasion mechanism targeting mucosal antibody transport.\",\n      \"evidence\": \"ORF8–pIgR interaction studies, pIgR expression and dIgA/pIgM binding assays, variant ORF8 comparison\",\n      \"pmids\": [\"39066171\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism of ORF8-induced pIgR downregulation (degradation vs. transcriptional) not determined\", \"In vivo relevance in SARS-CoV-2 infection not demonstrated\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Bone marrow transplant showing that hematopoietic PIGR deficiency reduces AAA incidence revealed an unexpected function for pIgR in macrophage-driven vascular inflammation.\",\n      \"evidence\": \"Bone marrow transplant into Ldlr−/− mice with angiotensin II-induced AAA, macrophage polarization assays\",\n      \"pmids\": [\"40624587\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Ligand and signaling mechanism of pIgR in macrophages unknown\", \"Whether macrophage pIgR functions in transcytosis or as a signaling receptor not addressed\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The structural basis of pIgR's interaction with J chain-containing polymeric Ig, the mechanism by which pIgR is sorted from recycling to apical endosomes, and the non-canonical signaling roles of pIgR in cancer and vascular inflammation remain incompletely understood.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No high-resolution structure of pIgR–pIgA–J chain complex available\", \"Post-transcriptional mechanism of pIgR downregulation in COPD undefined\", \"Ligand and function of pIgR in macrophages and other non-epithelial cells uncharacterized\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0038024\", \"supporting_discovery_ids\": [0, 2, 5, 7]},\n      {\"term_id\": \"GO:0098631\", \"supporting_discovery_ids\": [3, 4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 1, 3, 12]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [2]},\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [2, 5, 6]},\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [6, 7]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [0, 7, 12]},\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [2, 5]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [1, 6]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"RAB11FIP1\",\n      \"RAB11FIP5\",\n      \"TRIM21\",\n      \"RAB11A\",\n      \"PECAM1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}