TGF-β1 enhances T-cell PD-1 expression through a Smad3-dependant increase in transcription. (IRM4P.499)

Programmed cell death-1 (PD-1) is an inhibitory receptor highly expressed on T cells specific for viral and tumor antigens. TGF-β1 affects the tumor microenvironment and high serum levels are associated with both malignancy and pathogenesis in some chronic viral infections. However, the effect of TGF-β1 on PD-1 expression is unknown. PD-1 expression requires nuclear factor of activated T cells (NFATc1) activation through T cell receptor (TCR) signaling. We found that TGF-β1 signaling increases NFATc1-induced pdcd-1 mRNA and PD-1 expression on human peripheral CD4+ and CD8+ T cells. Smad2 and Smad3 are phosphorylated following TGF-β Receptor I binding and ultimately mediate gene transcription. We found that inhibition of TGFβRI kinase activity or Smad3 phosphorylation blocked TGF-β1-dependent PD-1 enhancement on T cells. Similarly, when compared to TCR stimulation alone, TGF-β1 increased PD-1 expression on murine wild-type (WT) but not on Smad3 knock-out (KO) CD4+ T cells. In contrast, Smad2 KO CD4+ T cells maintain increased PD-1 expression in the presence of TGF-β1, demonstrating a specific role for Smad3. We show that Smad3 increases pdcd-1 transcription by directly binding to a proximal region of the human PD-1 promoter and stabilizing NFATc1 binding. In addition to TGF-β1’s previously known effects on T cell function, our findings suggest that TGF-β1 mediates suppression via PD-1 upregulation. Smad3 may represent an additional target in therapeutic modulation of PD-1.