Objective: In patients with oral squamous cell carcinoma, a high proportion of T cells in the tumor undergo apoptosis, which correlates with Fas ligand (FasL) expression on tumor cells. The present study was done to identify mechanisms responsible for apoptosis of T cells seen in the peripheral circulation of these patients.
Methods: Sera of 27 patients, normal donor sera, and supernatants of cultured normal or tumor cells were fractionated by size exclusion chromatography and ultracentrifugation to isolate microvesicles. The presence of microvesicle-associated FasL was studied by Western blots, blocking with anti-Fas reagents, and immunoelectron microscopy. Biological activities of microvesicles were tested including the ability to induce apoptosis of Jurkat and T-cell blasts. Semiquantitative analysis of FasL in microvesicles was correlated with caspase-3 activity, DNA fragmentation, cytochrome c release, loss of mitochondrial membrane potential, and TCR-zeta chain expression in lymphocytes.
Results: FasL-positive (FasL+) microvesicles were detected in sera of 21 of 27 patients. Microvesicles contained 42 kDa FasL. These microvesicles induced caspase-3 cleavage, cytochrome c release, loss of mitochondrial membrane potential, and reduced TCR-zeta chain expression in target lymphocytes. Biological activity of the FasL+ microvesicles was partially blocked by ZB4 anti-Fas monoclonal antibody. Microvesicle-associated FasL levels correlated with the patients' tumor burden and nodal involvement.
Conclusion: Sera of patients with active oral squamous cell carcinoma contain FasL+ microvesicles, which induce the receptor and mitochondrial apoptotic pathways in Jurkat and activated T cells.