Secretory Component The Polymeric Ig Receptor

Per Brandtzaeg, Finn-Eirik Johansen, Peter Krajci and Inger B Natvig, Laboratory of Immunohistochemistry and Immunopathology (LIIPAT), University of Oslo, The National Hospital, Rikshospitalet, Oslo, Norway

Copyright © 1998 Elsevier Ltd. All Rights Reserved.

The existence of an adaptive mucosal immune system was proposed in 1919 by Besredka when he demonstrated that rabbits, after oral immunization with killed Shigella, were protected against fatal dysentery irrespective of the serum antibody titer. In 1922 Dav-ies was able to detect antibodies against the dysentery bacillus in stools from infected patients several days before such activity was present in serum. The interest in local immunity was significantly revived after 1965 when it was reported by Tomasi's group and also other laboratories that the predominating immunoglobulin (Ig) in external body fluids was IgA; this secretory IgA (S-IgA) was shown to have unique immunochemical and physicochemical properties because of its polymeric nature and association with an epithelial glycoprotein called 'secretory piece' or 'secretory component' (SC). At the same time Crabbc and coworkers demonstrated a class distribution of intestinal Ig-producing plasma cells remarkably different from that in peripheral lymphoid tissues; mucosal IgA immunocytes were strikingly predominant - being more than 20 times as numerous as IgG-producing cells. This finding has been repeatedly verified for most categories of normal exocrine tissues. Moreover, in the early 1970s Brandtzaeg and coworkers obtained evidence suggesting that the mucosal IgA immunocytes produce dimers or larger polymers (collectively called plgA) rather than monomers.

The latter workers also pointed out that local production alone does not explain the preferential appearance of plgA in exocrine fluids. They showed that serum IgG is abundantly represented in interstitial tissue fluid but is not internalized by secretory epithelia, whereas pentameric IgM (plgM) is trans-epithelially translocated to the lumen in a preferential manner like plgA. A common receptor-mediated external transport mechanism for S-IgA and secretory IgM (S-IgM) was therefore proposed by Brandtzaeg in 1973-74. The SC was suggested to be the transmembrane receptor responsible for the epithelial binding and uptake of the pig ligands. The J chain was taken to be a key factor in the initial noncovalent binding process. This polypeptide was in 1971 shown by Mestecky and coworkers to be a common disulfide-linked component of plgA and plgM; it is known to be selectively incorporated into these polymers within the Ig-producing plasma cells. Secretory immunity thus depends on a fascinating cooperation between the mucosal B cell system and the secretory epithelia.

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