T-cell apoptosis and suppression of T-cell receptor/CD3-zeta by Fas ligand-containing membrane vesicles shed from ovarian tumors.

Abstract

The accumulation of shed plasma membrane vesicles in the peripheral circulation is unique to cancer. Because these membrane fragments (MFs) express biologically active components, such as Fas ligand (FasL), the objective of this study was to define the link between the presence of shed membrane vesicles, apoptosis, and suppression of T-cell receptor/CD3-zeta expression in T lymphocytes of patients with ovarian cancer. MF shedding was measured chromatographically in sera from women with ovarian cancer (n = 11) and, as controls, non-cancer-bearing females (n = 9) and women with benign ovarian disease (n = 4). FasL associated with these shed fragments was assayed by Western immunoblots, whereas HLA class I expression was defined by slot-blotting. The effect of shed MFs on CD3-zeta expression was evaluated using a T-cell bioassay, and apoptosis of circulating T cells was measured by a cell-death ELISA and electrophoretic analysis of caspase-3. MFs were undetectable in control sera, and their levels were significantly elevated in sera from women with ovarian cancer. These tumor-derived MFs expressed 41-kDa FasL and HLA class I antigens. In co-incubation experiments, dose-dependent suppression of T-cell receptor/CD3-zeta expression by MFs was observed. Decreases in zeta expression correlated with the level of FasL in MFs but not with the level of HLA. The suppression of CD3-zeta by MFs appeared to be linked to the induction of apoptosis and caspase-3 within T cells. Our results suggest that FasL associated with tumor-derived MFs is responsible for apoptosis of T lymphocytes and a concomitant loss of zeta-chain expression in patients with ovarian carcinoma.