Interferon-γ Induces Apoptosis and Augments the Expression of Fas and Fas Ligand by Microglia in Vitro

Abstract

Activation of microglia by interferon-γ (IFN-γ) has been implicated in a number of central nervous system (CNS) inflammatory disease processes. Because IFN-γ has also been shown to play a role in programmed cell death, we investigated its cytotoxicity and its effect on the Fas apoptotic pathway in microglia. Flow cytometry was used to quantify the IFN-γ-mediated apoptotic response and Fas and Fas ligand (FasL) expression in two well-characterized murine microglia cell lines (BV-2 and N9). Nuclear fragmentation, suggestive of apoptosis, was noted within 24 h of incubation of microglia with IFN-γ (10 U/ml). After a 72-h incubation, almost every BV-2 and N9 microglia, but not GL261 glioma cells, underwent cell death and detached from the culture plates. This cytotoxicity occurred even at low IFN-γ concentrations (1 U/ml) and was inhibited by BAF, a pan-caspase inhibitor. Incubation of BV-2 and N9 microglia, but not GL261 glioma cells, with IFN-γ also potentiated the expression of Fas and FasL in a similar dose–response and time-course manner, as seen for the apoptotic response. Whereas Fas expression increased by 100% in both microglia cells, FasL upregulation was more pronounced and increased by as much as 200% in the N9 cells. These findings suggest that in addition to its role as a microglia activator, IFN-γ may also induce apoptosis of microglia, possibly through simultaneous upregulation of Fas and FasL. Interferon-γ modulation of the Fas pathway and apoptosis in microglia may be important in the pathogenesis of inflammatory CNS disease processes.