Phorbol ester circumvents the need for macrophages as well as for mitogenic lectins in the stimulation of lymphocytes with wheat germ agglutinin or the calcium ionophores A23187 or ionomycin.

Numerous biochemical events precede the proliferation of primary lymphocytes stimulated by mitogenic lectins in the presence of macrophages. Various compounds can activate parts of this response. Specifically the tumor-promoting phorbol ester, 12-O-tetradecanoylphorbol-13-acetate, (TPA), can replace the requirement for macrophages, apparently by mimicking the macrophage product interleukin 1 (IL1). Wheat germ agglutinin (WGA), itself a non-mitogenic lectin, is reported to cause a calcium flux, phosphatidylinositol turnover, and enhance interleukin 2 (IL2) synthesis. In spite of these positive responses, WGA inhibits DNA synthesis caused by mitogenic lectins. Nevertheless, in this study, we tested the possibility that together TPA and WGA could complement and bring about DNA synthesis. This prediction turned out to be true. The combination of two non-mitogenic compounds resulted in lymphocyte proliferation. The TPA overcame the inhibitory effects of WGA. Moreover, macrophages were not required. The TPA also synergized with the calcium ionophores A23187 or ionomycin to cause lymphocyte proliferation in the absence of macrophages. WGA and the ionophores together did not cause proliferation, a finding which suggested that they fulfill the same roles. These observations led us to conclude that at least two signals were required for lymphocyte stimulation. One signal caused the mobilization of calcium and the other signal circumvented the need for macrophages or macrophage products possibly by mimicking diacylglycerol, the activator of protein kinase C.