Signals involved in T cell activation. II. Distinct roles of intact accessory cells, phorbol esters, and interleukin 1 in activation and cell cycle progression of resting T-lymphocytes

The signals involved in the initiation of mitogen-induced activation of resting guinea pig T cells were examined. The combination of phytohemagglutinin (PHA) and 4β-phorbol 12-myristate 13-acetate (PMA) stimulated DNA synthesis by accessory cell (AC)-depleted T cells cultured at high density, but the use of low density cultures indicated that intact AC were absolutely necessary for PHA-stimulated T cell DNA synthesis even in the presence of PMA, interleukin 1 (IL 1), or interleukin 2 (IL 2). In contrast, AC-depleted T cells were able to respond to the combination of the calcium ionophore, ionomycin, and PMA regardless of the cell density at which they were cultured. Cell cycle analysis by acridine orange staining indicated that neither PHA nor ionomycin, in the absence of AC, activated resting T cells. PMA in the absence of all AC, supported cell cycle entry and progression to the DNA synthetic phase of the majority of ionomycin-stimulated T cells, but permitted only a small number of PHA-triggered T cells to enter the initial stage of the cell cycle (G(1a)) characterized by a modest increase in cellular RNA content. Although PMA permitted some PHA-stimulated T cells to enter the cell cycle, most required intact AC to enter G₁, and all required intact AC to progress through G₁ and synthesize maximal amounts of RNA. No PHA-stimulated cells reached the S phase without intact AC. In PHA-stimulated cultures containing intact AC, PMA increased the number of cells entering the cell cycle and increased the rate of their progress to the DNA synthetic phase. IL 1 also augmented PHA-stimulated AC-dependent T cell DNA synthesis in the presence or absence of PMA, but appeared to be most active during the later stage of the first cell cycle, augmenting the number of activated cells that entered the S phase of the cell cycle. These results support the conclusion that intact AC, IL 1, and PMA-like signal play distinct roles in the progression of mitogen-stimulated T cells through the first round of the cell cycle.