Human resting CD4+ T cells are constitutively inhibited by TGFβ under steady-state conditions

Based on studies in knockout mice, several inhibitory factors such as TGF beta, IL-10, or CTLA-4 have been implicated as gate keepers of adaptive immune responses. Lack of these inhibitory molecules leads to massive inflammatory responses mainly mediated by activated T cells. In humans, the integration of these inhibitory signals for keeping T cells at a resting state is less well understood. To elucidate this regulatory network, we assessed early genome-wide transcriptional changes during serum deprivation in human mature CD4(+) T cells. The most striking observation was a "TGF beta loss signature" defined by down-regulation of many known TGF beta target genes. Moreover, numerous novel TGF beta target genes were identified that are under the suppressive control of TGF beta. Expression of these genes was up-regulated once TGF beta signaling was lost during serum deprivation and again suppressed upon TGF beta reconstitution. Constitutive TGF beta signaling was corroborated by demonstrating phosphorylated SMAD2/3 in resting human CD4(+) T cells in situ, which were dephosphorylated during serum deprivation and rephosphorylated by minute amounts of TGF beta. Loss of TGF beta signaling was particularly important for T cell proliferation induced by low-level TCR and costimulatory signals. We suggest TGF beta to be the most prominent factor actively keeping human CD4(+) T cells at a resting state.