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Secretory Antibody Formation: Conserved Binding Interactions between J Chain and Polymeric Ig Receptor from Humans and Amphibians¹

Ranveig Braathen^*, Valerie S. Hohman, Per Brandtzaeg^* and Finn-Eirik Johansen^2,*

^* Laboratory for Immunohistochemistry and Immunopathology, Institute of Pathology, University of Oslo, Rikshospitalet-Radiumhospitalet Medical Center, Oslo, Norway; and Department of Biology, University of San Diego, San Diego, CA 92110

Abs of the secretory Ig (SIg) system reinforce numerous innate defense mechanisms to protect the mucosal surfaces against microbial penetration. SIgs are generated by a unique cooperation between two distinct cell types: plasma cells that produce polymers of IgA or IgM (collectively called pIgs) and polymeric Ig receptor (pIgR)-expressing secretory epithelial cells that mediate export of the pIgs to the lumen. Apical delivery of SIgs occurs by cleavage of the pIgR to release its extracellular part as a pIg-bound secretory component, whereas free secretory components are derived from an unoccupied receptor. The joining chain (J chain) is crucial in pIg/SIg formation because it serves to polymerize Igs and endows them with a binding site for the pIgR. In this study, we show that the J chain from divergent tetrapods including mammals, birds, and amphibians efficiently induced polymerization of human IgA, whereas the J chain from nurse shark (a lower vertebrate) did not. Correctly assembled polymers showed high affinity to human pIgR. Sequence analysis of the J chain identified two regions, conserved only in tetrapods, which by mutational analysis were found essential for pIgA-pIgR complexing. Furthermore, we isolated and characterized pIgR from the amphibian Xenopus laevis and demonstrated that its pIg binding domain showed high affinity to human pIgA. These results showed that the functional site of interaction between pIgR, J chain and Ig H chains is conserved in these species and suggests that SIgs originated in an ancestor common to tetrapods.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ Funding for this project has been provided by the Research Council of Norway.

² Address correspondence and reprint requests to Dr. Finn-Eirik Johansen, Laboratory for Immunohistochemistry and Immunopathology, Institute of Pathology, University of Oslo, Rikshospitalet-Radiumhospitalet Medical Center, N-0027 Oslo, Norway. E-mail address: f.e.johansen{at}medisin.uio.no

³ Abbreviations used in this paper: SIgA, secretory IgA; CHO, Chinese hamster ovary; D1, domain 1; D2, domain 2; J chain, joining chain; NIP, -iodo-4-hydroxy-2-nitrophenylacetyl; pIgA, polymeric IgA; pIgR, polymeric Ig receptor; SC, secretory component.