Differential expression of bovine MHC class II antigens identified by monoclonal antibodies

Four monoclonal antibodies (mAbs), UC‐A4, UC‐D3, UC‐H9, and IL‐A21, specific for bovine major histocompatibility complex class II proteins are described. Sequential immunoprecipitation experiments using biotin‐labeled peripheral blood mononuclear cells suggested, but did not conclusively establish, that each of these antibodies recognized a different epitope. The epitope identified by IL‐A21 appeared to be common to all of the class II proteins precipitated by the four mAbs, and UG‐D3 and UC‐H9 each appeared to react with distinct epitopes on separate subsets of these class II proteins. Monoclonal antibody UC‐A4 appeared to identify an epitope on a subset of the class II molecules identified by UC‐H9. Differences found in the expression by lymphoid cells of class II proteins identified by the four mAbs were indicative of each mAb recognizing a different epitope. UC‐H9 and IL‐A21 class II proteins were detected on all surface immunoglobulin (S'lg) positive cells in peripheral blood, but UC‐A4 and UC‐D3 class II proteins were not. Expression of UC‐A4 class II proteins, detected at low density on a strikingly reduced number of S'lg + cells from the blood of some bovine leukosis virus‐infected cattle, could be increased by culturing these B cells with lipopolysaccharide. All peripheral blood monocytes expressed UG‐H9 and IL‐A21 class II proteins, but only a proportion of monocytes expressed detectable UC‐A4 and UG‐D3 class II proteins. Almost all mitogen‐stimulated BoCD4 + and BoCD8 + T cells expressed UC‐H9 and IL‐ A21 class II proteins, whereas fewer stimulated T cells of both subsets expressed UC‐A4 and UC‐D3 class II proteins. All gamma/delta receptor (γ/δ R) T cells expressed UC‐D3, UG‐H9, and IL‐A21 class II proteins, but no cells (of γ/δ R + or CD4 + /CD2 + phenotype) from γ/δ R + T cell‐enriched cultures expressed UC‐A4 class II proteins.