Characterization of monoclonal antibodies specific for equine homologues of CD3 and CD5.

Two monoclonal antibodies (mAb), UC F6G-3 and UC F13C-5, were characterized as being specific for the apparent equine homologues of CD3 and CD5, respectively. Both antibodies exhibited characteristics of pan-T-lymphocyte markers based upon immunohistology and two-colour flow cytometry. UC F6G-3 precipitated a complex of proteins (up to seven) with molecular weights ranging from 18,000 to 42,000, similar to the human and murine CD3 complex. Upon further dissociation of the precipitated complex, two proteins were identified with molecular weights of 22,000 and 27,000. Immobilized UC F6G-3 was effective at inducing interleukin-2 receptor (IL-2R) expression on T lymphocytes, a feature consistent with antibodies specific for the epsilon chain of human and murine CD3. Three populations of cells in the thymus were distinguishable by UC F6G-3 target antigen density, suggesting increasing stages of T-cell maturation. UC F13C-5 precipitated a 67,000 MW protein, consistent with reported values for CD5 in multiple species. While this antibody exhibited characteristics of a pan-T-cell marker, low numbers of B lymphocytes also expressed the target antigen. Phorbol esters induced variable increases in target antigen density on B lymphocytes. These two antibodies, taken together with the few equine CD markers currently available, represent a substantial resource for further defining the equine immune system in health and disease.