Maximal Proliferation of Cytotoxic T Lymphocytes Requires Reverse Signaling through Fas Ligand

Fas ligand (FasL/CD95L) is best known for its role in delivering apoptotic signals through its receptor, Fas (APO-1/CD95). In this study, we present evidence that FasL has a second role as a signaling receptor. Alloantigen-specific proliferation by multiple FasL⁻ murine CTL lines is depressed compared to that of FasL⁺ CTL lines. FasL⁻ CTLs kill efficiently on a per recovered cell basis and can achieve wild-type levels of proliferation upon stimulation by optimal doses of anti-CD3, suggesting the lack of a costimulatory signal during antigen stimulation. To test this hypothesis directly, soluble FasIgG, a fusion protein of murine Fas and human IgG₁, was added to FasL⁺ CTLs to demonstrate that blocking cell surface Fas–FasL interactions mimics the depression observed for FasL⁻ CTLs. In addition, plate-bound FasIgG in conjunction with suboptimal anti-CD3 stimulation augments proliferative signals in FasL⁺ but not FasL⁻ CTLs. In contrast to these results with CD8⁺ T cells, alloantigen-stimulated FasL⁻ CD4⁺ T cells proliferate vigorously compared to FasL⁺ cells. These data demonstrate that reverse signaling through FasL is required for CTLs to achieve maximal proliferation and may provide clues to differences in the homeostatic regulation of activated CD4⁺ and CD8⁺ T cells during an immune response.