Characteristics of the polymeric immunoglobulin receptor (pIgR) of commercial grass carp and the immune response of pIgR and immunoglobulin to Flavobacterium columnare

The transcytosis of polymeric immunoglobulins is primarily regulated by the polymeric immunoglobulin (pIg) receptor (pIgR), which can activate the host immune system and protect against pathogen invasion. To improve understanding of the immune defense of teleost fish, in this study for the first time the full-length complementary DNA (cDNA) sequence of the pIgR in grass carp Ctenopharyngodon idellus was cloned and characterized; its messenger RNA (mRNA) expression and molecular functions were also investigated. We determined that the complete sequence of the pIgR cDNA has 1640 nucleotides, which includes a 1011-base pair (bp) open reading frame, a 68-bp 5 untranslated region (UTR), and a 561-bp 3 UTR. Similar to other teleost homologs, the grass carp pIgR gene encodes a 336-amino acid transmembrane protein that contains an extracellular region with two immunoglobulin-like domains, a transmembrane region, and an intracellular region. Reverse transcription-polymerase chain reaction revealed the presence of the pIgR in all tested tissues of grass carp, with the highest levels in skin, gill, intestine, and liver. In addition, the immune response to Flavobacterium columnare of the pIgR and its two ligands (IgM and IgZ) was investigated. The patterns of increase in IgM and IgZ mRNA translation were similar to that of the pIgR. These results should aid the development of novel strategies against fish diseases.