RAT MICROGLIAL INTERLEUKIN-3

Interleukin-3 (IL-3, multi-CSF) is a growth factor for a variety of he matopoietic progenitor cells. Recently, microglial cells, the resident macrophages of the central nervous system (CNS) have been shown to pr oliferate in the presence of IL-3 both in vivo and in culture. Data ob tained from cultured astrocytes gave rise to the hypothesis that astro cytes synthesize the microglial growth factor. This is the first repor t identifying rat microglial cells themselves as a source of IL-3. Cul ture media conditioned by isolated microglia enhanced microglial proli feration above fresh media controls. IL-3 polypeptide was detected in both conditioned media (CM) and in microglial cells by Western blottin g and immunoprecipitation. Furthermore, anti-IL-3 antibodies were able to inhibit microglial proliferation induced by conditioned media. mRN A(IL-3) was present in single microglial cells as revealed by in situ hybridization. Total RNA prepared from purified microglia yielded a si ngle PCR amplification product. Identity of the PCR product was confir med by Southern blot hybridization using a cDNA(IL-3) probe and by DNA sequencing. Expression of mRNA(IL-3) was observed in both absence and presence of lipopolysaccharide, a bacterial endotoxin, that commonly induces expression of inflammatory cytokines and inhibits microglial p roliferation. It is concluded that IL-3 expression in microglial cells is an early marker of inflammatory events in the brain preceding the expression of other cytokines and most likely ensuring the recruitment of enhanced numbers of immunocompetent cells at sites of lesion. In t he light of weak immune reactions in the brain, it is hypothesized tha t the expression of a characteristic T cell feature in monocyte-derive d microglia may be a partial compensation of T cell functions in brain lesions.