In addition to the major population of infiltrating leukocytes recovered from inflamed rat central nervous system (CNS), all of which expressed high levels of leukocyte common antigen CD45, many cells were coisolated that were MRC OX42+ (complement receptor 3/CD1lb) but expressed low-to-moderate levels of CD45 and major histocompatibility complex (MHC) class I molecules. Most cells from normal CNS, in contrast, lay within this latter, CD45'1 population.From previous in sit immunohistochemical studies, the fortuitously isolated CD45"w cells were probably resident (ramified) microglia. Using irradiation chimeras, we show that resident microglia respond to inflammation by upregulating CD45, CD4, and MHC class I molecules with a minority of these cells increasing their expression of MHC class H molecules. A 3-to 4-fold increase in the number ofmicroglia isolated from inflamed CNS provided indirect evidence that the cells had proliferated. In normal CNS, a very small population of blood-derived CD451fth-expressing cells are present; most MHC class II expression is associated with these few cells and not with the resident microglia.

Recent studies have shown that gamma-interferon (IFN-gamma) induces the expression of Ia antigen on astrocytes. This observation is of immunological significance because such activated astrocytes can act as antigen-presenting cells, as demonstrated with myelin basic protein for antigen-specific encephalitogenic T-cell lines. However, the lack of lymphatic drainage in brain and the presence of the so-called blood-brain barrier restricting traffic of cells and macromolecules suggests that IFN-gamma may not be readily available, at least during the initial phases of viral infections. The question therefore arises as to whether astrocytes can be induced to express Ia antigens by other signals directly related to viral infection and possibly independent of IFN-gamma. In the present report we demonstrate that a neurotropic murine hepatitis virus induces expression of Ia antigen on astrocytes in tissue culture without infection, rendering these brain cells competent to participate directly in the immune response to a viral infection.

SUMMARYA panel of hybridoma antibodies that react with the surface peplomer glycoprotein (E2) of the murine coronavirus JHM were produced to characterize major antigenic domains associated with functions related to virulence. Three groups of hybridoma antibodies were differentiated by immunoprecipitation of lysates from JHM-infected cells. One group precipitated the virion structural proteins gpl70 and gp98 together with the intracellular form of E2, gpl50. A second group reacted with gp98 and gpl50, and a third group precipitated gpl50 only. Competition assays with biotinylated hybridoma antibodies allowed the definition of at least six different epitope groups. Only those antibodies which immunoprecipitated both gpl70 and gp98 neutralized infectivity, inhibited cell fusion and protected infected rats against acute disease. Another class of antibodies binding to gpl70 and gp98 also neutralized JHM virus, but did not inhibit fusion and did not protect against disease. Antibodies that immunoprecipitated gp 150 and gp98 revealed only weak neutralization and did not inhibit cell fusion or protect animals. Four epitopes were defined by antibodies that immunoprecipitated gp 150, but revealed no biological activity. These data indicate that the site responsible for cell fusion is associated with an epitope group carried by gpl70 and gp98. Neutralizing antibodies bind to this and another epitope. Furthermore, protection of JHM-infected rats against acute disease requires both inhibition of cell fusion and neutralization of virus.

No abstract