Defective Cell Cycle Induction by IL-2 in Naïve 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 fail to produce 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 naïve T-cells. The suppressed T-cells upregulate high affinity IL-2R but do not produce IL-2, potentially explaining the proliferation failure. We further analyze here suppression mediated by refractory T-cells and find that it cannot be exclusively explained by IL-2 deprivation. Depletion of IL-2 from stimulated naïve T-cells with a blocking antibody results in a pattern of proliferation suppression and CD25 upregulation distinct from that caused by refractory T-cells. Further, supplementation of refractory-naïve cell co-cultures with exogenous IL-2 fails to alleviate refractory cell-mediated suppression, demonstrating that IL-2 deficiency is not exclusively responsible for it. Proximal IL-2 signaling through STAT5 and AKT is intact. However, refractory cell-co-cultured T-cells fail to upregulate cyclins and c-myc and incompletely downregulate p27^kip1 in response to IL-2. IL-2 signaling is not fully disabled as IL-2 upregulates the anti-apoptotic protein Bcl-x_L to control levels. This upregulation correlates with enhanced survival of co-cultured T-cells placed in IL-2 when compared with cells cultured without IL-2. Thus refractory T-cells are able to suppress naïve T-cell proliferative responses both by blocking IL-2 production and by blocking 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.