The role of vascular endothelial growth factor (VEGF) in tumor angiogenesis is well characterized; nevertheless, it is also a key element in promoting tumor evasion of the immune system by downregulating dendritic cell maturation and thus T cell activation. We sought to investigate the possible direct effect of VEGF on T cell activation and through which type of VEGF receptor (VEGFR) it exerts this effect. Circulating T cells from healthy donors and ovarian cancer patients were expanded in cultures with anti-CD3 and IL-2 with or without VEGF for 14 days, and the number of T cells was assessed. Cultured T cells were also tested for their cytotoxic activity in a standard 4-hr (51) Cr-release assay, and the expression of VEGFRs 1, 2 and 3 was assayed by flow cytometry, immunocytochemistry and Western blotting. To assess the ability of activated T cells to secrete VEGF, levels in culture supernatants were measured by enzyme linked immunosorbent assay. The addition of VEGF in cultures significantly reduced T cell proliferation in a dose-dependent manner. Protein expression studies demonstrated that CD3(+) T cells express VEGFR-2 on their surface upon activation. Experiments with anti-VEGFR-2 antibodies showed that the direct suppressive effect of VEGF on T cell proliferation is mediated by VEGFR-2. We also showed that VEGF significantly reduced the cytotoxic activity of T cells and that activated T cells secrete VEGF in the culture environment. Overall, our study shows that T cells secret VEGF and expresses VEGFR-2 upon activation. VEGF directly suppresses T cell activation via VEGF receptor type 2.
Copyright © 2011 UICC.