Ig ISOTYPE BINDING BY POLYMERIC IG RECEPTORS (plgRs) IN XENOPUS
Emily Flowers, University of Maryland
Flowers E.M.1, Neely H.R.2, Ohta Y.1, and Flajnik M.F.1
1. Department of Microbiology and Immunology, University of Maryland Baltimore School of Medicine, Baltimore, MD, USA.
2. Department of Microbiology and Immunobiology, Harvard Medical School, Boston, MA, USA.
A single population of antigen presenting cells (APCs) called XL (Xenopus laevis) cells has been described in the X. laevis spleen. After immunization, XL cells migrate to the internal perimeter of the B cell zone and bear native antigen on their surface, as well as IgM, the IgA orthologue IgX, and the IgG orthologue IgY, presumably in complex with the antigen. It is unknown which Fc Receptors (FcR) are involved in immune complex acquisition by XL cells. Two copies of polymeric Ig receptor (pIgR) family, xlPIGR and xlPIGR2, are found in the Xenopus genome. In general, pIgR is the oldest identifiable FcR and transports mammalian IgM and IgA acrossmucosal epithelial cells. xlPIGR is the orthologue of mammalian pIgR, is expressed at high levels in intestine and lung, and the encoded receptor was shown previously to interact with Xenopus J-chain. We found that xlPIGR2 is highly expressed by XL cells and thus it could be involved in antigen acquisition, processing, and presentation. Using stably transfected 293 cells we investigated Ig isotype binding by xlPIGR and xlPIGR2. Consistent with prior work, we showed that xlPIGR bound IgM, and further demonstrated its failure to bind to IgX or IgY. IgX is a polymeric mucosal antibody, but it seems to assemble without J-chain, which may explain its lack of binding to xlPIGR. xlPIGR2 bound IgM and IgX, but not IgY, suggesting that xlPIGR2 binding is J chain-independent and that it may be involved in immune complex binding and antigen handling by XL cells.