Characterization, expression, and polymorphism of MHC II α and MHC II β in Sichuan taimen (Hucho bleekeri)

The major histocompatibility complex (MHC) is involved in antigen presentation and plays an essential role in regulating immune function. In the present study, we identified two MHC class II genes and investigated their potential roles in Hucho bleekeri. The MHC II α and MHC II β of H. bleekeri had typical leading peptides, extracellular domains, connecting peptides, transmembrane region, and cytoplasmic region. Amino acid sequence comparison revealed that MHC II of H. bleekeri has high homology with other vertebrates, among which homology with salmonid fish was the highest. Phylogenetic analysis showed that H. bleekeri MHC II clustered with salmonid fish; moreover they clustered with orthologous genes of other fish, whereas mammalian MHC II clustered into a separate branch. Tissue distribution analysis revealed MHC II was widely expressed in all tested tissues, with both MHC II α and MHC II β highly expressed in the spleen, gill, kidney, and hindgut. After lipopolysaccharide (LPS) and polyinosinic:polycytidylic acid (poly(I:C)) stimulation, the expression of MHC II in the head kidney and spleen of H. bleekeri was significantly upregulated. Compared with MHC II α, MHC II β acted faster in response to the stimulation. Polymorphism analysis of MHC II revealed that all the different alleles belonged to the same major type, and very limited polymorphisms were found in H. bleekeri MHC II α and II β. Selection pressure analysis showed signs of weak and non-significant positive selection in the MHC II α and MHC II β extracellular region. Our study reveals the potential role of MHC II in the immune response of H. bleekeri and provides a reference for studying the evolutionary model of teleost MHC II.