Expression of the polymeric Immunoglobulin Receptor (pIgR) in mucosal tissues of common carp (Cyprinus carpio L.).

The mucosal immune system seems to be an important defence mechanism for fish but the binding of IgM in mucosal organs is poorly described in fish. In this study the gene encoding the polymeric Immunoglobulin Receptor (pIgR) in carp has been isolated and sequenced from a liver cDNA-library and aligned with other species. The pIgR of carp consists of 2 Ig domains, a transmembrane and an intracellular region, together 327 amino acids. In situ hybridisations with sense and anti-sense DIG-labelled pIgR RNA probes were performed on liver, gut and skin of common carp (Cyprinus carpio L.) and in these organs only anti-sense probes were found to hybridise. In liver the majority of hepatocytes was stained around the nucleus. In gut and skin, staining could be detected around the nucleus of the epithelial cells, but in gut also a subpopulation of lymphoid cells was stained in epithelium and lamina propria. The specific in situ hybridisation of the epithelia and hepatocytes coincides with the in situ binding of FITC-labelled carp IgM to the same cells. RT-PCR results indicate the expression of the pIgR gene in all lymphoid organs of carp, but not in muscle. Macrophages/neutrophils enriched by adherence or sorted B cells (MACS) did not show expression of the pIgR gene and are excluded as the pIgR expressing lymphoid cells in the intestine. The relevance of pIgR staining and gene expression in mucosal organs is discussed.