B-haplotype control of CD4/CD8 subsets and TCR V beta usage in chicken T lymphocytes

The major histocompatibility (B) complex of the chicken contains genes similar to Class I (B-F) and Class II (B-L beta) genes in mammals, as well as a highly-polymorphic gene family (B-G) whose exact function is not known. Specific B-haplotypes are strongly associated with resistance to a number of infectious diseases, and with immune responses to soluble and cellular antigens. In mammals, Class I and Class II molecules control development of the T cell repertoire, including selection of CD4+ and CD8+ T cells. One study of chickens reported that low CD4:CD8 ratio was associated with the B4 haplotype, which shares expressed B-F/B-L genes with the B13 haplotype. In studies reported here, chickens of two haplotypes carried in the Auburn R line, B302 and B305 (which is B13-related), were evaluated for percentages of T cells expressing the CD4, CD8, CD3, TCR1, TCR2 and TCR3 antigens in peripheral blood lymphocytes (PBL), thymus, and spleen. These two haplotypes were chosen for comparison because they differ in resistance to Marek's disease (MD) and are closely-related in B-F and B-L genes by restriction fragment length polymorphism analyses. Homozygous birds of each B haplotype were produced from crosses of (B302 x B305)F1 sires and dams. PBL, thymocytes, and splenocytes from B302 homozygotes had higher CD4:CD8 ratios than B305 homozygotes. However, CD4:CD8 ratio differences could not be attributed to haplotype-controlled differences in V beta usage within CD4/CD8 subsets, as has been described for certain V beta families in mice and humans. These results indicate that thymic selection events involving CD4 and CD8 subsets and TCR V beta usage are controlled by a gene or genes closely-linked to the B-complex, which may or may not be Class I or Class II genes.