STAT3 governs hyporesponsiveness and granzyme B-dependent suppressive capacity in human CD4+ T cells.

Signal transducer and activator of transcription 3 (STAT3) integrates key signals of cell surface immune receptors, yet its precise role in cluster of differentiation (CD)4(+) T cells is not well-established. Current research has indicated T-helper cell 17-inducing roles but also tolerogenic roles. To address this issue, human T cells were transduced with the constitutively active STAT3 mutant STAT3C. Following stimulation, STAT3C(+) T cells up-regulated IL-10 (4.1 ± 0.5-fold; P < 0.001) and granzyme B (2.5 ± 1.2, P < 0.05) secretion, combined with significantly reduced IFN-γ (35 ± 5%), IL-2 (57 ± 4%), TNF-α (64 ± 8%), and IL-13 (89 ± 3%) secretion (P < 0.001). CD3/CD2- or CD3/CD28-activated STAT3C(+) T cells revealed reduced proliferation (53.4 ± 23.5% and 70.5 ± 10.4%, respectively), which was independent of IL-10 production and significantly suppressed effector T cell proliferation by 68.7 ± 10.6% and 65.9 ± 2.6%, respectively (P < 0.001). Phenotypically, STAT3C-transgenic CD4(+) T cells resembled effector T cells regarding expression of T regulatory cell markers, but up-regulated granzyme B expression levels by 2.4-fold (P < 0.05). Suppression was cell contact dependent and mediated by granzyme B-induced cell death, but was independent of IL-10 and TGF-β. Notably, peripheral blood CD4(+)CD45RA(-)lymphocyte activation gene-3(+)CD49(+) type 1 regulatory T cells revealed activation-induced hyperphosphorylation of STAT3. In agreement, pharmacological inhibition of STAT3 activation partially reverted hyporesponsiveness of peripheral type 1 regulatory T cells (increasing their division index from 0.46 ± 0.11 to 0.89 ± 0.04; P < 0.01). These observations indicate a clear-cut relation between activation of STAT3 and the acquisition of a tolerogenic program, which is also used by peripheral blood type 1 regulatory T cells.