IL-2-deprivation and TGF-beta are two non-redundant suppressor mechanisms of CD4+CD25+ regulatory T cell which jointly restrain CD4+CD25- cell activation.

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. Interestingly, IL-2 deprivation or TGF-beta, two suppressive mechanisms, did not effectively inhibit Tconv activation and proliferation when applied alone. In contrast, IL-2 deprivation combined with TGF-beta suppressed Tconv activation as potently as Tregs. More importantly, in the transwell system, that separates Tregs from Tconvs, TGF-beta contributed to Treg suppression under IL-2 depriving condition. In conclusion, these two suppressive mechanisms acting in concert may be necessary to effectively restrain the early activation of Tconvs.

IL-7, but not thymic stromal lymphopoietin (TSLP), during priming enhances the generation of memory CD4+ T cells.

Multiple activation signals (including antigen, co-stimulation, and cytokines) during T-cell priming affect the subsequent generation of memory T cells, whose survival is maintained by IL-7 and IL-15. Since the IL-7 receptor is highly expressed not only on the surface of memory T cells but also on naïve T cells, we propose that early exposure to IL-7 during priming of naïve T cells may promote their survival, and thus enhances the generation of memory cells. To test this hypothesis, TCR transgenic OT-II CD4(+) T cells were stimulated in vitro with OVA(323-339) peptide presented by syngeneic antigen-presenting cells (APCs). IL-7 or an IL-7 like cytokine, thymic stromal lymphopoietin (TSLP), was added at the initial 2-day cultivation stage. We found that a short exposure to IL-7 or TSLP during priming did not affect activation, proliferation, and glucose uptake by CD4(+) T cells compared to controls when examined on culture day 6. However, the same 6-day cultures showed that IL-7 (but not TSLP) significantly decreased the frequency of apoptotic CD4(+) T cells compared to controls. More importantly, an adoptive transfer of the 6-day primed OT-II T cells into CD45.1(+) congenic mice demonstrated that IL-7 (but not TSLP) elevated by 3- and 4-fold the number of transferred CD4(+) T cells in spleen (p<0.05) and lymph nodes (p<0.05), respectively, compared to controls. Almost all transferred CD4(+) T populations displayed phenotypes of effector (CD44(+)CD62L(-)) or central (CD44(+)CD62L(+)) memory T cells. We thus conclude that exposure of CD4(+) T cells to IL-7 during priming results in an increased frequency of CD4(+) memory T cells.