High allelic polymorphism, moderate sequence diversity and diversifying selection for B-NK but not B-lec, the pair of lectin-like receptor genes in the chicken MHC

We previously characterised the C-type lectin-like receptor genes B-NK and B-lec, located next to each other in opposite orientations in the chicken major histocompatibility complex (MHC). We showed that B-NK is an inhibitory receptor expressed on natural killer cells, whereas B-lec is an activation-induced receptor with a broader expression pattern. It is interesting to note that the chicken MHC has been linked with resistance or susceptibility to Marek’s disease virus (MDV), an oncogenic herpes virus. Recent reports show that the C-type lectin-like receptors in mouse and rat (Ly49H, NKR-P1 and Clr) are associated with resistance to another herpesvirus, cytomegalovirus (CMV). Therefore, B-NK and B-lec are potential candidate genes for the MHC-mediated resistance to MDV. In this paper, we report that both genes encode glycosylated type II membrane proteins that form disulphide-linked homodimers. The gene sequences from nine lines of domestic chicken representing seven haplotypes show that B-lec is well conserved between the different haplotypes, apparently under purifying selection. In contrast, B-NK has high allelic polymorphism and moderate sequence diversity, with 21 nucleotide changes in the complementary deoxyribonucleic acids (cDNAs) resulting in 20 amino acid substitutions. The allelic variations include substitutions, an indel and loss/gain of three predicted N-linked glycosylation sites. Strikingly, there is as much as 7% divergence between protein sequences of B-NK from different haplotypes, greater than the difference observed between the highly polymorphic human KIR NK receptors. Analysis of ds and dn reveal evidence of strong positive selection for B-NK to be polymorphic at the protein level, and modelling demonstrates significant variation between haplotypes in the predicted ligand binding face of B-NK.