Microglia are more susceptible than macrophages to apoptosis in the central nervous system in experimental autoimmune encephalomyelitis through a mechanism not involving Fas (CD95)

Morphological studies have shown that macrophages and microglia undergo apoptosis in the central nervous system (CNS) in acute experimental autoimmune encephalomyelitis (EAE) in the Lewis rat. To assess the relative levels of macrophage and microglial apoptosis, and the molecular mechanisms involved in this process, we used three-colour flow cytometry to identify CD45lowCD11b/c+ microglial cells and CD45highCD11b/c+ macrophages in the inflammatory cells isolated from the spinal cords of Lewis rats 13 days after immunization with myelin basic protein (MBP) and complete Freund's adjuvant. Simultaneously, we analyzed the DNA content of these cell populations to assess the proportions of cells undergoing apoptosis and in different stages of the cell cycle or examined their expression of three apoptosis-regulating proteins, i.e. Fas (CD95), Fas ligand (FasL) and Bcl-2. Microglia were highly vulnerable to apoptosis and were over-represented in the apoptotic population. Macrophages were less susceptible to apoptosis than microglia and underwent mitosis more frequently than microglia. The different susceptibilities of microglia and macrophages to apoptosis did not appear to be due to variations in Fas, FasL or Bcl-2 expression, as the proportions of microglia and macrophages expressing these proteins were similar, and were relatively high. Furthermore, in contrast to T cell apoptosis, apoptosis of microglia/macrophages did not occur more frequently in cells expressing Fas or FasL, or less frequently in cells expressing Bcl-2. These results indicate that the apoptosis of microglia and CNS macrophages in EAE is not mediated through the Fas pathway, and that Bcl-2 expression does not protect them from apoptosis. Expression of FasL by macrophages and microglia may contribute to the pathogenesis and immunoregulation of EAE through interactions with Fas+ oligodendrocytes and Fas+ T cells. The high level of microglial apoptosis in EAE indicates that microglial apoptosis may be an important homeostatic mechanism for controlling the number of microglia in the CNS following microglial activation and proliferation.