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The benefits of immunotherapy by regulatory T (Treg) cells are unpredictable partially due to the uncertainty of their suppressive mechanism. In fact, various suppressive mechanisms have been proposed but each remains controversial. To better understand Treg-mediated suppression, we have investigated factors which may influence the suppressive effects. In an in vitro suppression assay, over-expression of anti-apoptotic Bcl2 enhancing survival of conventional T responder cells (Tconvs) did not subvert Treg-mediated suppression. In contrast, enhancing activation of Tconvs by increasing the potency of calcium signals completely abrogated Treg-mediated suppression. While Tregs were incapable of suppressing already activated Tconvs, they prevented expression of activation markers on naïve Tconvs during activation, thereby indicating that Tregs mediate suppression through controlling early activation stage …