VARIATION IN EXPRESSION OF MHC CLASS-II ANTIGENS ON HORSE LYMPHOCYTESDETERMINED BY MHC HAPLOTYPE

A panel of monoclonal antibodies was used to characterize the expressi on of equine Major Histocompatibility Complex (MHC) class II antigens on lymphocytes of horses of different MHC types. MHC class II antigen expression was compared between adult horses and foals, and the level of expression of MHC class II antigens on horse T cell subpopulations was also determined. Peripheral blood lymphocytes (PBL) from young and adult healthy horses of different MHC haplotypes were labeled with th e antibodies and assayed by single- and two-color immunofluorescence f low cytometry. A variation in the expression of equine MHC class II an tigens on PBL was identified that was determined by MHC haplotype. Bot h young and old animals carrying the MHC class II haplotype D3 express ed lower levels of MHC class II antigens on their PBL than did horses of other MHC class II types. D3 heterozygotes had an intermediate leve l of expression. Neonates and foals of any MHC haplotype had lower lev els of expression of MHC class II antigens than did adults. MHC class II antigens were identified on surface immunoglobulin positive (sIg+) B cells as well as on T cells, with the B cells staining brightest. Du al staining for MHC class II antigens and markers for equine T cell su bsets demonstrated that both EqCD4+ and EqCD8+ T cells expressed MHC c lass II molecules. These results suggest that the relatively low level s of MHC class II antigen expression on foal and neonate lymphocytes m ay be developmentally regulated, and not determined by environmental f actors. The cause of the low expression of MHC class II antigens linke d to the MHC class II haplotype D3 is not known, but it appears to ref lect a decreased expression by all lymphocyte subpopulations.