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 undergoapoptosis in the central nervous system (CNS) in acute experimentalautoimmune encephalomyelitis (EAE) in the Lewis rat. To assess the relativelevels of macrophage and microglial apoptosis, and the molecular mechanismsinvolved in this process, we used three-colour flow cytometry to identifyCD45lowCD11b/c+ microglial cells and CD45highCD11b/c+ macrophages in theinflammatory cells isolated from the spinal cords of Lewis rats 13 daysafter immunization with myelin basic protein (MBP) and complete Freund'sadjuvant. Simultaneously, we analyzed the DNA content of these cellpopulations to assess the proportions of cells undergoing apoptosis and indifferent stages of the cell cycle or examined their expression of threeapoptosis- regulating proteins, i.e. Fas (CD95), Fas ligand (FasL) andBcl-2. Microglia were highly vulnerable to apoptosis and wereover-represented in the apoptotic population. Macrophages were lesssusceptible to apoptosis than microglia and underwent mitosis morefrequently than microglia. The different susceptibilities of microglia andmacrophages to apoptosis did not appear to be due to variations in Fas,FasL or Bcl- 2 expression, as the proportions of microglia and macrophagesexpressing these proteins were similar, and were relatively high.Furthermore, in contrast to T cell apoptosis, apoptosis ofmicroglia/macrophages did not occur more frequently in cells expressing Fasor FasL, or less frequently in cells expressing Bcl-2. These resultsindicate that the apoptosis of microglia and CNS macrophages in EAE is notmediated through the Fas pathway, and that Bcl-2 expression does notprotect them from apoptosis. Expression of FasL by macrophages andmicroglia may contribute to the pathogenesis and immunoregulation of EAEthrough interactions with Fas+ oligodendrocytes and Fas+ T cells. The highlevel of microglial apoptosis in EAE indicates that microglial apoptosismay be an important homeostatic mechanism for controlling the number ofmicroglia in the CNS following microglial activation and proliferation.