Effects of Activation Cytokines on Fas Ligand Storage and Use in Cytotoxic T Lymphocytes

CD8+ Cytotoxic T lymphocytes (CTL) use a repertoire of non-redundant effector mechanisms to kill infected cells. One mechanism involves the interaction of Fas Ligand (FasL) on the CTL with the Fas receptor on the target cells. However, the role of FasL in the CTL response following infection has not been well-characterized. It is known that the unique cytokine milieu during activation affects the differentiation of CTL, in addition to their ability to utilize effector mechanisms. We hypothesize that activation in the presence of cytokines that enhance the differentiation to effector cells (IL-2, IL-12, Type I IFN) should increase stores of FasL and the FasL response upon restimulation. Differentiated Effector CTL have stronger cytolytic ability than Memory CTL, so we expect that Effector CTL are associated with the strongest FasL response. We activated ex vivo naive murine CD8+ T cells and examined FasL storage and use for up to 13 days, as detected by flow cytometry. Compared to cells activated in the absence of cytokines, addition of Interleukin 12 (IL-12), type I IFN, and high levels of IL-2 independently increase stored FasL, as well as surface FasL following restimulation. While inclusion of IL‑12 only transiently increases stored FasL, Type I IFN sustains high stored FasL for an additional 3 days. Cells with the most stored FasL appear to belong to a subset of memory cells, Central Memory cells. Although the effects of cytokines on FasL are consistent with previous findings on the effect of IL-2 and inflammatory cytokines on other effector mechanisms, the characterization of Central Memory cells as storing high levels of FasL suggests that FasL, different from other mechanisms, may be more broadly employed by a diversity of CTL phenotypes following activation. Further studies should confirm the responding cell populations employing FasL following in vivo infection.