Polymeric immunoglobulin receptor (pIgR) in ray-finned fish (Actinopterygii)

The mammalian polymeric immunoglobulin receptor (pIgR) is a glycosylated type I membrane protein. It is a receptor with dual specificity for dimeric IgA (dIgA), and pentameric IgM (pIgM) produced by local lamina propria (LP) plasma cells and present only on epithelial cells. This receptor binds dIgA or pIgM and, in the process of transcytosis by mucosal epithelial cells, transports them, in a J-chain-dependent mechanism, towards the apical surface of epithelial cells. The effect of this process is to detach the ectodomain from pIgR and release it from the mucosa into the intestinal lumen in the form of a complex with dIgA or pIgM, i.e., in the form of secretory antibodies. The pIgRs in Actinopterygii are deemed to be functionally equivalent receptors to pIgRs in mammals. This paper introduces pIgRs in Actinopterygii (Act-pIgRs), discusses their structure, natural and provoked expression, and the transcriptional regulation and functional properties of these proteins, highlighting issues that need clarification and further research.