Transmembrane TNF-α Promotes Suppressive Activities of Myeloid-Derived Suppressor Cells via TNFR2

It has been reported that TNFR2 is involved in regulatory T cell induction and myeloid-derived suppressor cell (MDSC) accumulation, two kinds of immunosuppressive cells contributing to tumor immune evasion. Because transmembrane TNF-alpha (tmTNF-alpha) is the primary ligand for TNFR2, we hypothesized that tmTNF-alpha is mainly responsible for the activation of MDSCs. Indeed, we found that tmTNF-alpha, rather than secretory TNF-alpha (sTNF-alpha), activated MDSCs with enhanced suppressive activities, including upregulating arginase-1 and inducible NO synthase transcription, promoting secretion of NO, reactive oxygen species, IL-10, and TGF-beta, and enhancing inhibition of lymphocyte proliferation. This effect of tmTNF-alpha was mediated by TNFR2, as TNFR2 deficiency significantly impaired tmTNF-alpha-induced release of IL-10 and NO and inhibition of T cell proliferation by MDSC supernatant. Furthermore, tmTNF-alpha caused p38 phosphorylation and NF-kappa B activation, whereas inhibition of NF-kappa B or p38 with an inhibitor pyrrolidine dithiocarbamate or SB203580 abrogated tmTNF-alpha-mediated increased suppression of lymphocyte proliferation by MDSCs. Consistently, our in vivo study showed that ectopic expression of uncleavable tmTNF-alpha mutant by 4T1 cells significantly promoted tumor progression and angiogenesis, accompanied with more accumulation of MDSCs and regulatory T cells in the tumor site, increased production of NO, IL-10, and TGF-beta, as well as poor lymphocyte infiltration. In contrast, enforced expression of sTNF-a mutant by 4T1 cells that only released sTNF-alpha without expression of surface tmTNF-alpha markedly reduced MDSC accumulation and induced more lymphocyte infiltration instead, showing obvious tumor regression. Our data suggest that tmTNF-alpha acts as a potent activator of MDSCs via TNFR2 and reveals another novel immunosuppressive effect of this membrane molecule that promotes tumor immune escape.