Distinct epitopes on the T cell antigen receptor of HPB-ALL tumor cells identified by monoclonal antibodies

The T cell antigen receptor is a ~90,000 dalton disulfide linked heterodimer that is non-covalently associated with the CD3 complex. Prior studies have demonstrated that anti-CD3 or -Ti antibodies can mimic antigen and induce cellular proliferation and the secretion of lymphokines. An early event in activation via CD3/Ti is a rapid increase in concentration of intracellular Ca²⁺ levels. In the present studies, we have produced a panel of monoclonal antibodies (MAb) against the Ti expressed on HPB-ALL tumor cells. All MAb immunoprecipitate a ~90,000 dalton disulfide linked heterodimer and induced co-modulation of Ti and CD3. On the basis of competitive binding studies, four distinct epitopes on the Ti of HPB-ALL were identified with MAb L38, L39, L41, and L42. These epitopes were additionally discriminated on the basis of reactivity with normal polyclonal T cell populations and functional effects on HPB-ALL. L39 reacted with a monomorphic epitope present on ~2 to 5% of peripheral blood T lymphocytes from all donors examined and was specifically mitogenic for peripheral blood T cells expressing this epitope. L39⁺ T cells in blood included both CD4⁺ and CD8⁺ lymphocytes. In contrast, L28, L41, and L42 failed to react with peripheral blood T cells and were not mitogenic for peripheral blood lymphocytes. Anti-Leu-4, L38, L39, and L41 MAb all induced a rapid increase in (Ca²⁺)(i) in HPB-ALL tumor cells, similar to previous findings with anti-CD3 and anti-Ti MAb against various tumor cells and peripheral blood T cells. In contrast, L42 MAb did not induce a substantial increase in (Ca²⁺)(i). Failure of L42 to induce a substantial increased (Ca²⁺)(i) could not be attributed to the apparent titer, avidity, or isotype of the antibody. These findings suggest that induction of increased (Ca²⁺)(i) upon binding of Ti is epitope dependent. Furthermore, these data demonstrate that several distinct public and private epitopes can be identified on the T cell antigen receptor.