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Activation signals via CD2 molecule and interleukin 2 receptor act in synergy for helper function induction

The membrane CD2 molecule appears to play an important role in T cell activation. Indeed, T cell stimulation by some combinations of anti‐CD2 monoclonal antibodies (mAb) can result in antigen‐independent expression of helper function as assessed by proliferation and lymphokine secretion. We report here that T cell stimulation by a combination of two anti‐CD2 mAb recognizing GT2 and T111 epitopes, respectively, cannot alone induce T helper clones to proliferate when preincubated in culture medium devoid of exogenous interleukin 2 (IL2). The concerted action of both anti‐GT2 + T111 mAb and exogenous recombinant IL2 is required to induce cloned helper T cells to produce IL 2 and interferon‐γ to significantly increase IL 2 receptors (IL2R) and finally to divide by an autocrine mechanism, whereas each signal alone has no effect. This therefore suggests that, under some conditions of CD2 stimulation, two minimal signals may be delivered through CD2 and IL 2R and act synergistically to achieve a complete expression of T helper cell functions. Moreover, analysis of phosphatidylinositol and phosphatidic acid metabolic changes mediated by each signal separately or together suggests that, in this model, IL2 increases the phosphoinositid turnover induced by anti‐CD2 antibodies up to a level required for helper function acquisition.