Tumor-derived TGF-β inhibits mitochondrial respiration to suppress IFN-γ production by human CD4+ T cells

Transforming growth factor-beta (TGF-beta) is produced by tumors, and increased amounts of this cytokine in the tumor microenvironment and serum are associated with poor patient survival. TGF-beta-mediated suppression of antitumor T cell responses contributes to tumor growth and survival. However, TGF-beta also has tumorsuppressive activity; thus, dissecting cell type-specific molecular effects may inform therapeutic strategies targeting this cytokine. Here, using human peripheral and tumor-associated lymphocytes, we investigated how tumor-derived TGF-beta suppresses a key antitumor function of CD4(+) T cells, interferon-gamma (IFN-gamma) production. Suppression required the expression and phosphorylation of Smad proteins in the TGF-beta signaling pathway, but not their nuclear translocation, and depended on oxygen availability, suggesting a metabolic basis for these effects. Smad proteins were detected in the mitochondria of CD4(+) T cells, where they were phosphorylated upon treatment with TGF-beta. Phosphorylated Smad proteins were also detected in the mitochondria of isolated tumor-associated lymphocytes. TGF-beta substantially impaired the ATP-coupled respiration of CD4(+) T cells and specifically inhibited mitochondrial complex V (ATP synthase) activity. Last, inhibition of ATP synthase alone was sufficient to impair IFN-gamma production by CD4(+) T cells. These results, which have implications for human antitumor immunity, suggest that TGF-beta targets T cell metabolism directly, thus diminishing T cell function through metabolic paralysis.