Cooperative pathway induction of T lymphocyte mitogen stimulation.

Isolated peritoneal mouse macrophages pretreated with the mitogenic enzyme combination neuraminidase (EC 3.2.1.18) plus galactose oxidase (EC 1.1.3.9.) (NAGO), or with NaIO4, stimulate macrophage-depleted lymphocytes mitogenically by a macrophage-derived signal, different from the originally used mitogen. Polyethylene glycol (PEG) treatment of the cultures, although itself nonmitogenic, strongly enhances the mitogenic response of the lymphocytes. Under culture conditions the macrophage-derived signal is transmitted to lymphocytes by direct cell contact, a finding which explains the need of a critical cell density for T lymphocyte stimulation. In the absence of macrophages, lysates from mitogen-preteated macrophages stimulate column-purified lymphocytes in the presence of PEG. Our results indicate that mitogenic activation of lymphocytes is mediated through two sequential triggering events, induction (by mitogen treatment) of a macrophage-derived signal and commitment (by nonmitogenic PEG treatment) of lymphocytes to react to the signal. Reconstitution of the mitogenic response can be achieved by a sequential induction of both these events.