The teleost polymeric Ig receptor counterpart in ballan wrasse (Labrus bergylta) differs from pIgR in higher vertebrates

As mucosal barriers in fish are the main sites where pathogens are encountered, mucosal immunity is crucial to avoid infection in the aquatic environment. In teleost fish, immunoglobulins are present in gut, gill and skin mucus, although not in the same amounts as in higher vertebrates. In mammals, the poly-Ig receptor (pIgR) is synthesized in epithelial cells and mediates the active transport of poly-immunoglobulins (pIgs) across the epithelium. During transport, a component of the pIgR, the secretory component (SC), is covalently bound to pIgs secreted into the mucus providing protection against proteases and avoiding degradation. The teleost pIgR gene does not show synteny to higher vertebrates, the overall structure of the protein is different (comprising two Ig domains) and its functional mechanisms remain unclear. The J-chain which is essential for pIgR-mediated transport of IgA and IgM in higher vertebrates is absent in teleost fish. The aim of the present study was to characterize the ballan wrasse (Labrus bergylta) pIgR and use it as a marker for further studies of mucosal immunity in this species. The pIgR gene was unambiguously identified. Unexpectedly, reverse transcription real time PCR (RT-qPCR) revealed highest abundance of pIgR mRNA in liver and significantly lower expression in mucosal organs such as foregut, hindgut, and skin. In situ hybridization showed pIgR-positive cells dispersed in the lamina propria while it was undetectable in epithelial cells of foregut and hindgut of ballan wrasse. A similar pattern was observed in Atlantic salmon. Liquid Chromatography-Mass Spectrometry (LC-MS/MS) analysis of IgM enriched mucus samples from gut, gill, skin, and bile gave relatively few matches to wrasse pIgR. Notably, the matching peptides were from the transmembrane (TM) and cytoplasmatic (Cy) region as well as the putative SC, indicating leakage from lysed cells rather than covalent bonds between IgM and SC. Altogether, the results indicate that pIgR has another (or at least an additional) function in wrasse. Another pIgR-like molecule (pIgRL) in ballan wrasse (comprising three Ig domains) was analyzed to see if this could be an alternative functional pIgR homolog. However, the presence of pIgRL mRNA in blood leukocytes and a relatively high expression in immune organs like spleen and head kidney pointed to a receptor function on a circulating leukocyte population. As significant amounts of IgM were found in bile of ballan wrasse further studies should consider the hepato-biliary route regarding IgM delivery to the gut lumen