Defective cell cycle induction by IL-2 in naive T-cells antigen stimulated in the presence of refractory T-lymphocytes.

CD4+ T cells enter a transient refractory period after stimulation. Upon re-stimulation the refractory cells produce little IL-2 and show diminished proliferation. We previously demonstrated that refractory T cells can also, like anergic and CD4+CD25+ regulatory T cells, suppress in trans the proliferation of antigen-stimulated naive T cells. The suppressed T cells up-regulate high-affinity IL-2R but do not produce IL-2. This IL-2 deficit could potentially explain the proliferation failure, but does not appear to do so. Supplementation of refractory-naive co-cultures with exogenous IL-2 fails to alleviate both the proliferation suppression and IL-2 production defects. This does not result from a failure of IL-2 to stimulate its receptor. Proximal IL-2 signaling into suppressed T cells through STAT5 and Akt is intact. However, refractory cell-co-cultured T cells fail to up-regulate cyclins and c-myc and incompletely down-regulate p27kip1 in response to IL-2, and the downstream consequences of this signaling are therefore dissociated. IL-2 signaling is not fully disabled as IL-2 up-regulates the anti-apoptotic protein Bcl-xL to control levels. This up-regulation correlates with enhanced survival of refractory cell-co-cultured T cells placed in IL-2 when compared with cells cultured without IL-2. Thus, refractory T cells are able to suppress naive T-cell proliferative responses in part by blocking both IL-2 production and the mitogenic but not anti-apoptotic effects of IL-2. These results have implications for how activation-refractory T cells may influence nascent immune responses.