The polymeric immunoglobulin receptor (pIgR) on mucosal epitheial cells binds dimeric IgA (dIgA) on the baolateral surface and mates transport of dIgA to the apical surface. Using Madin-Darby canine kidney epithelial cells stably transfected with pIgR cDNA, we found that soluble immune complexes (ICs) of 125I-labeled rat monoclonal antidinitrophenyl (DNP) dIgA ("I-dIgA) and DNP/biot-bovine serum albumin were transported from the basolateral to the apical surface and then r dl. Monomeric IgA ICs were not transported, consnt with the specificity of pIgR for polymeric Immunogbulins. Essentially all the '25I-dIgA in apical culture supernatants was streptavidin precipitable, iditig that dIgA remained bound to antigen during ytoss.

M cells represent the primary route by which mucosal Ags are transported across the intestinal epithelium and delivered to underlying gut-associated lymphoid tissues. In rodents and rabbits, Peyer’s patch M cells selectively bind and endocytose secretory IgA (SIgA) Abs. Neither the nature of the M cell IgR nor the domains of SIgA involved in this interaction are known. Using a mouse ligated ileal loop assay, we found that monoclonal IgA Abs with or without secretory component, but not IgG or IgM Abs, bound to the apical surfaces of Peyer’s patch M cells, indicating that the receptor is specific for the IgA isotype. Human serum IgA and colostral SIgA also bound to mouse M cells. The asialoglycoprotein receptor or other lectin-like receptors were not detected on the apical surfaces of M cells. We used recombinant human IgA1 and human IgA2 Abs and domain swapped IgA/IgG chimeras to determine that both domains Cα1 and Cα2 are required for IgA adherence to mouse Peyer’s patch M cells. This distinguishes the M cell IgA receptor from CD89 (FcαI), which binds domains Cα2-Cα3. Finally, we observed by immunofluorescence microscopy that some M cells in the human ileum are coated with IgA. Together these data suggest that mouse, and possibly human, M cells express an IgA-specific receptor on their apical surfaces that mediates the transepithelial transport of SIgA from the intestinal lumen to underlying gut-associated organized lymphoid tissues.

Transferrin receptor (TfR) has been identified as a candidate IgA1 receptor expressed on human mesangial cells (HMC). TfR binds IgA1 but not IgA2, co-localizes with mesangial IgA1 deposits, and is overexpressed in patients with IgA nephropathy (IgAN). Here, structural requirements of IgA1 for its interaction with mesangial TfR were analyzed. Polymeric but not monomeric IgA1 interacted with TfR on cultured HMC and mediates internalization. IgA1 binding was significantly inhibited (>50%) by soluble forms of both TfR1 and TfR2, confirming that TfR serves as mesangial IgA1 receptor. Hypogalactosylated serum IgA1 from patients with IgAN bound TfR more efficiently than IgA1 from healthy individuals. Serum IgA immune complexes from patients with IgAN containing aberrantly glycosylated IgA1 bound more avidly to TfR than those from normal individuals. This binding was significantly inhibited by soluble TfR, highlighting the role of TfR in mesangial IgA1 deposition. For addressing the potential role of glycosylation sites in IgA1-TfR interaction, a variety of recombinant dimeric IgA1 molecules were used in binding studies on TfR with Daudi cells that express only TfR as IgA receptor. Deletion of either N- or O-linked glycosylation sites abrogated IgA1 binding to TfR, suggesting that sugars are essential for IgA1 binding. However, sialidase and beta-galactosidase treatment of IgA1 significantly enhanced IgA1/TfR interaction. These results indicate that aberrant glycosylation of IgA1 as well as immune complex formation constitute essential factors favoring mesangial TfR-IgA1 interaction as initial steps in IgAN pathogenesis.

Human immunoglobulin (Ig)A exists in blood as two isotypes, IgA1 and IgA2, with IgA2 present as three allotypes: IgA2m(1), IgA2m(2), and IgA2m(n). We now demonstrate that recombinant, chimeric IgA1 and IgA2 differ in their pharmacokinetic properties. The major pathway for the clearance of all IgA2 allotypes is the liver. Liver-mediated uptake is through the asialoglycoprotein receptor (ASGR), since clearance can be blocked by injection of excess galactose-Ficoll ligand and suppressed in ASGR-deficient mice. In contrast, only a small percentage of IgA1 is cleared through this pathway. The clearance of IgA1 lacking the hinge region with its associated O-linked carbohydrate was more rapid than that of wild-type IgA1. IgA1 and IgA2 that are not rapidly eliminated by the ASGR are both removed through an undefined ASGR-independent pathway with half-lives of 14 and 10 h, respectively. The rapid clearance of IgA2 but not IgA1 through the liver may in part explain why the serum levels of IgA1 are greater than those of IgA2. In addition, dysfunction of the ASGR or altered N-linked glycosylation, but not O-glycans, that affects recognition by this receptor may account for the elevated serum IgA seen in liver disease and IgA nephropathy.

No abstract

Secretory IgA (sIgA) plays a critical role in providing protection against infection at the mucosal surfaces. Normally, sIgA is the product of two different cell types with heavy, light, and J chains produced by the plasma cells, whereas secretory component (SC), a cleavage product of the polymeric immunoglobulin receptor (pIgR), is added during the transit of dimeric IgA through the epithelial cell layer. In the current study, by introducing a gene for the processed form of SC into a cell line that produces dimeric IgA, we have succeeded in creating a single cell that is able to produce and secrete covalently joined sIgA. To our knowledge, this is the first time it has been possible to efficiently produce large quantities of sIgA of defined specificity in mammalian cells. The sIgA made using this approach has great potential as an immunotherapeutic.Secretory IgA (sIgA) provides the first line of immune defense at the mucosal surfaces of the gastrointestinal, respiratory, and genitourinary tracts, where more than 95% of infections are initiated. In vivo, sIgA is the product of two different cell types, the plasma cell and the epithelial cell (1, 2). Plasma cells synthesize and assemble ␣ H chains and L chains with J chain into polymeric IgA. The polymeric IgA secreted by the plasma cell binds to the polymeric Ig receptor (pIgR) expressed on the basolateral surface of the mucosal epithelium. The IgA-pIgR complex is transcytosed to the apical surface; during transit a disulfide bond is formed between the IgA and the pIgR. At the apical surface, an unknown enzyme cleaves between the ectoplasmic domain [also known as secretory component (SC)] and the transmembrane domain, releasing the IgA-SC complex into external secretions.

IFN-α, a cytokine crucial for the innate immune response, also demonstrates antitumor activity. However, use of IFN-α as an anticancer drug is hampered by its short half-life and toxicity. One approach to improving IFN-α’s therapeutic index is to increase its half-life and tumor localization by fusing it to a tumor-specific Ab. In the present study, we constructed a fusion protein consisting of anti-HER2/neu-IgG3 and IFN-α (anti-HER2/neu-IgG3-IFN-α) and investigated its effect on a murine B cell lymphoma, 38C13, expressing human HER2/neu. Anti-HER2/neu-IgG3-IFN-α exhibited potent inhibition of 38C13/HER2 tumor growth in vivo. Administration of three daily 1-μg doses of anti-HER2/neu-IgG3-IFN-α beginning 1 day after tumor challenge resulted in 88% of the mice remaining tumor free. Remarkably, anti-HER2/neu-IgG3-IFN-α demonstrated potent activity against established 38C13/HER2 tumors, with complete tumor remission observed in 38% of the mice treated with three daily doses of 5 μg of the fusion protein (p = 0.0001). Ab-mediated targeting of IFN-α induced growth arrest and apoptosis of lymphoma cells contributing to the antitumor effect. The fusion protein also had a longer in vivo half-life than rIFN-α. These results suggest that IFN-α Ab fusion proteins may be effective in the treatment of B cell lymphoma.

Both IgM and IgA exist as polymeric immunoglobulins. IgM is assembled into pentamers with J chain and hexamers lacking J chain. In contrast, polymeric IgA exists mostly as dimers with J chain. Both IgM and IgA possess an 18-amino acid extension of the C terminus (the tail-piece (tp)) that participates in polymerization through a penultimate cysteine residue. The IgM (tp) and IgA (␣tp) tail-pieces differ at seven amino acid positions. However, the tail-pieces by themselves do not determine the extent of polymerization. We now show that the restriction of polymerization to dimers requires both C ␣ 3 and ␣tp and that more efficient dimer assembly occurs when C ␣ 2 is also present; the dimers contain J chain. Formation of pentamers containing J chain requires C 3, C 4, and the tp. IgM-␣tp is present mainly as hexamers lacking J chain, and ␥-tp forms tetramers and hexamers lacking J chain, whereas IgA-tp is present as high order polymers containing J chain. In addition, there is heterogeneous processing of the Nlinked carbohydrate on IgA-tp, with some remaining in the high mannose state. These data suggest that in addition to the tail-piece, structural motifs in the constant region domains are critical for polymer assembly and J chain incorporation.