Human Resting CD4⁺ T Cells Are Constitutively Inhibited by TGFβ under Steady-State Conditions

Abstract

Based on studies in knockout mice, several inhibitory factors such as TGFβ, 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 “TGFβ loss signature” defined by down-regulation of many known TGFβ target genes. Moreover, numerous novel TGFβ target genes were identified that are under the suppressive control of TGFβ. Expression of these genes was up-regulated once TGFβ signaling was lost during serum deprivation and again suppressed upon TGFβ reconstitution. Constitutive TGFβ 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 TGFβ. Loss of TGFβ signaling was particularly important for T cell proliferation induced by low-level TCR and costimulatory signals. We suggest TGFβ to be the most prominent factor actively keeping human CD4⁺ T cells at a resting state.