Differential activation requirements associated with stimulation of T cells via different epitopes of CD3

A panel of monoclonal antibodies directed to different epitopes of porcine CD3 were employed to investigate stimulation requirements of porcine T lymphocytes. It was found that epitope specificity was an important property of the anti-CD3 antibodies that determined the requirements for T-cell proliferation. Thus, T-cell proliferation induced by triggering different CD3 epitopes showed three different requirements: (a) proliferation induced by the most insensitive epitope required both epitope ligation and some unknown additional signal(s); (b) proliferation induced by the most common epitopes only required epitope ligation, either by monocytes or by immobilization; (c) proliferation induced by the most sensitive epitope required neither epitope ligation nor participation of antigen-presenting cells (APC). These findings may help to explain the previous confusion over the requirements for T-cell activation through the CD3 pathway. Finally, the above conclusions apply only to alpha beta T cells, as porcine gamma delta T cells, either in bulk culture or isolated, did not proliferate in response to anti-CD3 stimulation. Therefore, the mechanism underlying gamma delta T-cell activation may be different from that of alpha beta T cells.