The effect of VEGF on T cells from ovarian cancer patients and healthy individuals via VEGF receptor type 2.

Abstract

e15552 Background: 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 down-regulating dendritic cell maturation and thus T cell activation. We investigated the possible direct effect of VEGF on T cell activation and through which type of VEGF receptor (VEGFR) it exerts this effect. Methods: Circulating T cells from healthy donors (n=9) and ovarian cancer patients (n=5) 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 against K562 and Daudi cell lines, in a standard 4-h 51Cr-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 ELISA. Results: The addition of VEGF in cultures showed that concentrations above 1 ng/ml significantly reduced T cell proliferation (p=0.043 for comaprison with controls). The suppressive effect was dose-dependent from 1 to 10 ng/ml, after which there was a plateau of the inhibitory effect. Protein expression studies demonstrated that CD3+ T cells express VEGFR-2 on their surface upon activation. The direct suppressive effect of VEGF on T cell proliferation was reversed by anti-VEGFR-2 antibodies. This finding proves that the suppressive effect of VEGF on T cells is mediated by VEGFR-2. We also showed that VEGF at 5 ng/ml significantly reduced the cytotoxic activity of T cells against K562 and Daudi cells (p=0.042 for comparison with controls). Activated T cells secreted VEGF in the culture environment (mean value: 192 pg/ml). Nevertheless, these levels are well below those necessary to produce T cell suppression in culture (1 ng/ml). Conclusions: This study shows that VEGF directly suppresses T cell activation via VEGF receptor type 2. The levels shown to produce suppression in vitro (>= 1 ng/ml) are in concert to the level of VEGF in ascites, which predicted for a poor outcome (1.9 ng/ml) in a previous study (Bamias et al, Gynecol Oncol 2008).