Heterogeneity in the activation requirements of T cells stimulated by phytohaemagglutinin.

Data are presented showing that resting T cells proliferated in response to phytohaemagglutinin (PHA) provided that either sheep erythrocytes (SRC) or interleukin-2(IL-2) was added. Proliferation requiring PHA and SRC was inhibited by anti-CD2 monoclonal antibody (OKT11) and anti-CD3 monoclonal antibody (OKT3). These monoclonal antibodies only partially inhibited IL-2 production stimulated by PHA and SRC, and added IL-2 did not restore proliferation in these cultures. Low concentrations of cyclosporine inhibited both proliferation and IL-2 production (ED50 = 12 and 9 ng/ml, respectively) and this inhibition was partially relieved in the presence of added IL-2 (ED50 = 110 ng/ml). In contrast, proliferation stimulated by PHA with exogenous IL-2 was not inhibited in the presence of OKT11, and moderate concentrations of cyclosporine were required to inhibit proliferation (ED50 = 49 ng/ml). Evidence was obtained, from analysis of cultures at limiting dilution, to suggest that the different requirements for stimulation of PHA-responsive T cells reflected different T-cell subpopulations. The majority of PHA-responsive cells proliferated in the presence of PHA and SRC, whereas only a minority proliferated in the presence of PHA and exogenous IL-2.