IL-6 is an accessory signal in the alternative CD2-mediated pathway of T cell activation.

Recent studies have demonstrated that IL-1 and IL-6 are synergistic accessory signals for activation of T cells. In this study, highly purified human T cells were cultured with either a stimulating pair of anti-CD2 mAb or with immobilized anti-CD3 mAb. Monocytes, a cellfree monocyte culture supernatant or IL-1 were required for anti-CD2-stimulated T cell proliferation, and they each strongly enhanced anti-CD3-induced T cell growth. IL-6 was synergistic with IL-1 as a helper factor for T cell growth after activation via CD2, but we could not demonstrate any effect of IL-6 in the CD3 pathway. The mechanism of the synergistic helper activity of IL-1 and IL-6 on T cell activation in the CD2 pathway was further examined. IL-1 (but not IL-6) was required for induction of IL-2 production. Both IL-1 and IL-6 enhanced IL-2R (p55) expression and the proliferative response to IL-2. T cell proliferation after stimulation with anti-CD2 and IL-1 or IL-1/IL-6 proceeded through an autocrine IL-2-dependent pathway. Moreover we found that, in the absence of IL-1, IL-6 still supported a transient and limited proliferation of anti-CD2- (but not of anti-CD3-) stimulated T cells, which apparently was independent of the autocrine growth factors IL-2 or IL-4. Our data suggest that IL-6 is important as an accessory signal for T cell growth in the CD2 pathway of T cell activation.