The Asian seabass (Lates calcarifer) is of importance both as farmed and wild animals. However, with the emergence of infectious diseases, there is a need to understand the immune system and its diversity within the species. The highly polymorphic MHC class I (MHC-I) molecules are essential for antigen presentation to eliciate adaptive immunity. In teleost fish, six MHC-I lineages U, Z, S, L, P and H have been described, with only the U lineage being well studied at the molecular and functional level due to its peptide binding abilities similar to the classical human leukocyte antigen. Currently, other than the MHC-I sequences that were obtained from a single Asian seabass, there is no information on its diversity. Therefore, in the present study, we sequenced and characterized the peptide binding regions α1 and α2 domains of a single MHC-I gene in Asian seabass originated from Singapore and Australia.
Pairwise comparisons of the Asian seabass MHC-I α1 and α2 domain sequences show an overall similarity within Singapore and Australia -derived Asian seabass, with more conserved residues in α1 domain as compared to α2 domain (Fig. a). This is supported by variability metric (V) analysis where 4 polymorphic sites with V > 1 in the α1 domain and 16 polymorphic sites in the α2 domain were identified (Fig b). Phylogenetic tree analysis revealed that the sequences belong to the U lineage, forming a single cluster (Fig c). Mapping conserved binding residues positions (red) on human HLA-A2 and grass carp crystal structure showed a high degree of similarity (Fig d).
In conclusion, the availability of the U lineage MHC-I α1 and α2 sequences enhances the quality of MHC class I genetic information in Asian seabass and that the critical amino acid residues correspond closely to that even of humans. The sequences will provide new tools to analyse fish immune responses to pathogen infections and will be applicable in the study of the phylogeny and evolution of antigen-specific receptors.