Monoclonal antibody BLT-1, specific for the bovine homologue of CD5, reacts with the majority of mature T cells, a subpopulation of B cells and stimulates T cell proliferation.

Monoclonal antibody (MAb) BLT-1, an IgG2a with kappa light chains, reacted strongly with 21% of bovine thymocytes, weakly with 15% of thymocytes, with a subpopulation of peripheral blood B cells that also expressed CD20 and with peripheral blood T cells. Practically all of the reactive thymocytes were of a large cell subpopulation. By immunoprecipitation, BLT-1 was shown to recognize a membrane molecule with a molecular mass of 67 kDa. In competitive assays for lymphocyte surface binding, BLT-1 and MAb CC-29 (which had been shown previously to react with bovine CD5) blocked one another, indicating that the epitopes recognized were identical or extensively overlapping. In contrast, another CD-5-reactive MAb, CC-17, did not block BLT-1 reactivity with lymphocytes although the reactivity of CC-17 was blocked by BLT-1, suggesting partial overlap of the epitopes or steric hindrance by BLT-1 but not by CC-17. BLT-1 was able to induce proliferation of bovine lymphocytes in culture alone if monocytes were present or in the absence of monocytes synergized with PMA. The results indicate that BLT-1 recognizes an epitope of the bovine homologue of CD5 and that perturbation of the epitope by MAb binding results in signal transduction to bovine lymphocytes.