Signals involved in T cell activation. I. Phorbol esters enhance responsiveness but cannot replace intact accessory cells in the induction of mitogen-stimulated T cell proliferation

The role of accessory cells (AC) in the initiation of mitogen-induced T cell proliferation was examined by comparing the effect of intact macrophages (MΦ) with that of 4-β-phorbol 12-myristate 13-acetate (PMA). In high-density cultures, purified guinea pig T cells failed to proliferate in response to stimulation with phytohemagglutinin (PHA), concanavalin A (Con A), or PMA alone. The addition of MΦ to PHA or Con A but not PMA-stimulated cultures restored T cell proliferation. The addition of PMA to high-density T cell cultures stimulated with PHA or Con A also permitted [³H]thymidine incorporation, but was less effective than intact MΦ in this regard. This action of PMA was dependent on the small number of AC contaminating the T cell cultures as evidenced by the finding that PMA could not support mitogen responsiveness of T cells that had been depleted of Ia-bearing cells by panning, even when these cells were cultured at high density. When PMA was added to T cell cultures supported by optimal numbers of MΦ, catalase-reversible suppression of responses was noted. Even in cultures containing catalase, PMA failed to enhance responsiveness above that supported by optimal numbers of MΦ. A low-density culture system was used to examine in greater detail the possibility that PMA could completely substitute for MΦ in promoting T cells activation. In low-density cultures, mitogen-induced T cell proliferation required intact MΦ. PMA could not suppport responses even in cultures supplemented with interleukin 1-containing MΦ supernatants or purified interleukin 2 alone or in combination. Similar results were found in high-density cultures of T cells depleted of Ia-bearing cells. These results support a model of T cell activation in which AC play at least two distinct roles. The initiation of the response requires a signal conveyed by an intact MΦ, which cannot be provided by either a MΦ supernatant factor or PMA. The response can be amplified by additional MΦ or MΦ supernatant factors. PMA can substitute for MΦ in this regard and can provide the signal necessary for amplification of T cell proliferation suppported by small numbers of intact AC.