Molecular cloning and expression analysis of major histocompatibility complex class I, IIA and IIB genes of blunt snout bream (Megalobrama amblycephala)

Major histocompatibility complex (MHC) plays an important role in the immune response of vertebrates. In this study, we isolated MHC class IIA and IIB genes from blunt snout bream (Megalobrama amblycephala) by rapid amplification of cDNA ends polymerase chain reaction (RACE-PCR). In order to study the function of the MHC genes in M. amblycephala, tissue distribution and immune response of the MHC genes to bacterial challenge were analyzed. All the characteristic features of MHC class II chain structure could be identified in the deduced amino sequences of MHC IIA and IIB, including the leader peptide, α1/β1 and α2/β2 domains, connecting peptide and transmembrane and cytoplasmic regions, as well as conserved cysteines and N-glycosylation site. The deduced amino acid sequence of the MHC IIA and IIB molecules shared from 48% to 88% and from 65% to 77% similarity with those of other teleosts, respectively. Quantitative real-time PCR (qRT-PCR) demonstrated that MHC I and II genes were ubiquitously expressed in ten tissues, with high level in immune related tissues, including kidney, intestine, gill and spleen. Challenge of M. amblycephala with the extracellular pathogen, Aeromonas hydrophila, resulted in a significant increase in the expression of MHC I, MHC IIA and IIB mRNA within 72 h after infection in gill, kidney, intestine and liver, followed by a recovery to normal level after 120 h. The changes of expression levels for MHC IIA and IIB in most tissues were significantly higher than that of MHC I in the corresponding tissues at most time points (P < 0.05). These results demonstrated the MHC genes played an important role in response to bacterial infection in M. amblycephala; however, MHC class I and II genes showed different functional activity, which need be further investigated in teleost.