Requirements for phytohaemagglutinin activation of resting pure CD4+ and CD8+ T cells

We have utilized a new method for obtaining highly purified cells using positive selection by immunomagnetic separation to study the conditions required for phytohaemagglutinin (PHA) activation of pure T4 and T8 cells. In the presence of accessory cells (AC), a comparable proliferative response was obtained in the two subsets. In the absence of AC, PHA induced low levels of interleukin 2 (IL-2) receptor expression as well as responsiveness to IL-2 in both T4 and T8 cells. If AC or 12-O-tetradecanoyl-phorbol-13-acetate (TPA) were also present, IL-2 production and DNA synthesis were seen in both subsets. A short preincubation with PHA 'primed' T cells for subsequent responsiveness to IL-2 or TPA, while preincubation with TPA did not induce response to PHA. Thus, PHA alone is sufficient for the first step of T cell activation leading to IL-2 receptor expression. The second step leading to IL-2 production, is dependent on direct interaction with AC in the presence of PHA. While T8 cells are dependent on help by T4 cells for proliferation to occur during stimulation with antigen, in PHA stimulation the requirements for activation and proliferation seem to be identical for T4 and T8 cells.