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 underg o apoptosis in the central nervous system (CNS) in acute experimental autoimmune encephalomyelitis (EAE) in the Lewis rat. To assess the rel ative levels of macrophage and microglial apoptosis, and the molecular mechanisms involved in this process, we used three-colour flow cytome try to identify CD45(low)CD11b/c(+) microglial cells and CD45(high)CD1 1b/c(+) macrophages in the inflammatory cells isolated from the spinal cords of Lewis rats 13 days after immunization with myelin basic prot ein (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 vul nerable to apoptosis and were over-represented in the apoptotic popula tion. Macrophages were less susceptible to apoptosis than microglia an d underwent mitosis more frequently than microglia, The different susc eptibilities of microglia and macrophages to apoptosis did not appear to be due to variations in Fas, Fast or Bcl-2 expression, as the propo rtions of microglia and macrophages expressing these proteins were sim ilar, and were relatively high. Furthermore, in contrast to T cell apo ptosis, apoptosis of microglia/macrophages did not occur more frequent ly in cells expressing Fas or Fast, or less frequently in cells expres sing Bcl-2, These results indicate that the apoptosis of microglia and CNS macrophages in EAE is not mediated through the Fas pathway, and t hat Bcl-2 expression does not protect them from apoptosis, Expression of Fast by macrophages and microglia may contribute to the pathogenesi s and immunoregulation of EAE through interactions with Fas(+) oligode ndrocytes and Fas(+) T cells, The high level of microglial apoptosis i n EAE indicates that microglial apoptosis may be an important homeosta tic mechanism for controlling the number of microglia in the CNS follo wing microglial activation and proliferation.