CHARACTERIZATION OF MONOCLONAL-ANTIBODIES SPECIFIC FOR EQUINE HOMOLOGS OF CD3 AND CD5

Two monoclonal antibodies (mAb), UC F6G-3 and UC F13C-5, were characte rized 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 cyto metry. 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 interleuki n-2 receptor (IL-2R) expression on T lymphocytes, a feature consistent with antibodies specific for the epsilon chain of human and murine CD 3. 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 wit h reported values for CD5 in multiple species. While this antibody exh ibited characteristics of a pan-T-cell marker, low numbers of B lympho cytes also expressed the target antigen. Phorbol esters induced variab le increases in target antigen density on B lymphocytes. These two ant ibodies, taken together with the few equine CD markers currently avail able, represent a substantial resource for further defining the equine immune system in health and disease.