TCR signaling intensity controls CD8+ T cell responsiveness to TGF-β

DGK-zeta is a negative regulator of TCR signaling that causes degradation of the second messenger DAG, terminating DAG-mediated activation of Ras and PKC theta. Cytotoxic T cells deficient in DGK-zeta demonstrate enhanced effector functions in vitro and antitumor activity in vivo, perhaps because of insensitivity to inhibitory cytokines. We sought to determine whether the enhanced responsiveness of DGK-zeta-deficient T cells renders them insensitive to the inhibitory cytokine TGF-beta and to determine how the loss of DGK-zeta facilitates this insensitivity. We identified decreased transcriptional and functional responses to TGF-beta in CD8(+) DGK-zeta(-/-) T cells but preserved TGF-beta-mediated conversion of naive DGK-zeta(-/-) CD4(+) T cells to a regulatory T cell phenotype. Decreased CD8(+) T cell responsiveness to TGF-beta did not result from impaired canonical TGF-beta signal transduction, because similar levels of TGF-beta-R and intracellular Smad components were identified in WT and DGK-zeta(-/-) CD8(+) T cells, and TGF-beta-mediated activation of Smad2 was unchanged. Instead, an enhanced TCR signal strength was responsible for TGF-beta insensitivity, because (i) loss of DGK-zeta conferred resistance to TGF-beta-mediated inhibition of Erk phosphorylation, (ii) TGF-beta insensitivity could be recapitulated by exogenous addition of the DAG analog PMA, and (iii) TGF-beta sensitivity could be observed in DGK-zeta-deficient T cells at limiting dilutions of TCR stimulation. These data indicate that enhanced TCR signal transduction in the absence of DGK-zeta makes T cells relatively insensitive to TGF-beta, in a manner independent of Smads, a finding with practical implications in the development of immunotherapies that target TGF-beta.