The major histocompatibility complex (MHC) class Ⅱβplay an important role in the immune response of teleost. In this study

full-length MHC Ⅱβ cDNA and genomic sequences were obtained from Nile tilapia (Oreochromis niloticus) by rapid amplification of cDNA ends polymerase chain reaction (RACE-PCR).The full length of cDNA was 1 142 bp and the open reading frame (ORF) was 750 bp encoding 249 amino acid. The genomic structure of MHC Ⅱβ genes was similar with other teleost species including 5 exons and 4 introns. By homology comparison and phylogenetic analysis

MHCⅡβ genes of Nile tilapia had a higher identity and a closer evolutionary relationship with perciform fishes than other fishes. The putative protein had 30.6%~78.6% identity with those of other teleosts and mammals. Ten MHC Ⅱβ alleles were obtained from the fifty positive clones of ORF from 25 individuals. The amino acid sequence analysis showed that polymorphism was mostly focused on exon 2.PCR-SSCP and sequencing results revealed one to six different alleles for two to twelve bands in susceptibile or resistant fishes that after Streptococcus iniae challenge

which suggested there were at least 3 loci.qPCR indicated that high expression of MHC Ⅱβ was detected in gill

heart

spleen

and lower expression in kidney

muscle

brain and the lowest expression in skin

intestine and liver. After Streptococcus iniae challenge

the expression presented the tendency of the down-up-down in head and kidney

which demonstrated MHC Ⅱβ involved in tilapia immune response.