Molecular characterization and expression analysis of MHC class II α and β genes in large yellow croaker (Pseudosciaena crocea)

MHC class II molecules play an important role in the activation of CD4+ T cells, which are the central orchestrating cells of an immune response. Here, we report the cloning of MHC class II α and β cDNAs from large yellow croaker (Pscr-DAAs and Pscr-DAB) by expressed sequence tags analysis and RACE-PCR techniques. Three different class II α and two class II β sequences were obtained from spleens of two individual fish. Each of the three class II α sequences encodes a polypeptide of 239 amino acids while the two class II β cDNA sequences encode for a protein of 249 aa. All the characteristic features of MHC class II chain structure could be identified in the deduced proteins of three class II α and two class II β sequences, 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. RT-PCR analysis showed that MHC class II α and β mRNAs were broadly expressed in various tissues examined, although at different levels. Upon stimulation with inactivated trivalent bacterial vaccine or polyinosinic polycytidylic acid (poly(I:C)), the expression levels of both α and β genes were obviously up-regulated in intestine, kidney and spleen. Real-time PCR analysis demonstrated that the expression levels of class II α and β were quickly up-regulated in spleen, kidney, and intestine at 12 h after induction with poly(I:C), while their expression levels significantly increased at 48 h upon immunization with bacterial vaccine, indicating that the up-regulation of both class II α and β expression was induced by bacterial vaccine or poly(I:C) at the early phase of induction, and that class II α and β transcripts were quicker up-regulated by poly I:C than by bacterial vaccine.