Cytokine Production by Human Fetal Microglia and Astrocytes: Differential Induction by Lipopolysaccharide and IL-1β

As part of a study on the role of cytokines in central nervous system development and dysfunction, we determined the pattern of cytokine production in highly purified cultures of microglia and astrocytes isolated from second-trimester human fetal brains. Levels of TNF-α, IL-1β, and IL-6 mRNA and protein were determined by Northern blot analysis and ELISA before and after stimulation with LPS, TNF-α, or IL-1β. In microglia, LPS induced mRNA for all three cytokines. High protein levels of IL-6 and TNF-α were also found in the medium, whereas IL-1β protein was mostly cell associated. IL-1β also induced message for all three cytokines, in the rank order of IL-1β > IL-6 > TNF-α. TNF-α induced mRNA and protein for IL-1β but not for TNF-α or IL-6. In contrast, LPS failed to stimulate either mRNA or protein expression for any of the three cytokines in astrocytes. On the other hand, IL-1β provided a strong stimulus for astrocytes. IL-1β induced mRNA and protein for both TNF-α and IL-6, but the kinetics of the response differed for the two cytokines. TNF-α mRNA and protein levels peaked early (at 4 h and 16 h, respectively) and were undetectable by 72 h, whereas IL-6 mRNA peaked later (at 16 h) and protein levels continued to accumulate in the medium through 72 h. IL-1β did not induce IL-1β mRNA or protein in astrocytes. TNF-α did not induce expression of any of the cytokines in astrocytes. In conclusion, our results demonstrate that cytokine production can be induced in human fetal microglia and astrocytes but that the stimuli for induction differed significantly for the two cell types. Whereas LPS was a potent stimulus for microglia, astrocytes primarily responded to IL-1β. The data further suggest that microglia may be key regulators of astrocyte response, working primarily through the expression of cell-associated IL-1β.