Major histocompatibility complex class IIA and IIB genes of the spotted halibut Verasper variegatus: genomic structure, molecular polymorphism, and expression analysis

The major histocompatibility complex (MHC) is a large genomic region characterized by extremely high polymorphism and its association with resistance/susceptibility to disease in vertebrates. In this study, the full lengths of MHC IIA and IIB cDNA were obtained from spotted halibut (Verasper variegates) by expressed sequence tag (EST) and rapid amplification of cDNA ends (RACE) approaches. The genomic structure, molecular polymorphism, and expression patterns were examined to study MHC II gene functions in fish. As in other teleosts, the genomic structure of the spotted halibut MHC IIA contained 4 exons and 3 introns. The deduced amino acid sequence of the class IIA molecule shared 28-79% similarity with those of teleosts and mammals. Nine class IIA alleles were identified from five individuals. Three alleles originating from a single individual suggested the existence of at least two class IIA loci in the genome. Six exons and 5 introns were identified from spotted halibut MHC IIB, and the deduced amino acid sequence shared 33-79% similarity with those of teleosts and mammals. Twelve alleles were identified, among which five were observed in a single individual, which suggested at least three class IIB loci. Quantitative real-time PCR analysis revealed the presence of class IIA and IIB transcripts in nine normal tissues with high expression level in kidney and gill. Furthermore, MHC IIA and IIB are probably two candidates of immune molecules involved in the acute-phase response in spotted halibut, because their transcriptional levels were significantly up-regulated in blood and liver after bacterial challenge.