Amplification and propagation of interleukin-1β signaling by murine brain endothelial and glial cells

Background: During acute infections and chronic illnesses, the pro-inflammatory cytokine interleukin-1 beta (IL-1 beta) acts within the brain to elicit metabolic derangements and sickness behaviors. It is unknown which cells in the brain are the proximal targets for IL-1 beta with respect to the generation of these illness responses. We performed a series of in vitro experiments to (1) investigate which brain cell populations exhibit inflammatory responses to IL-1 beta and (2) examine the interactions between different IL-1 beta-responsive cell types in various co-culture combinations. Methods: We treated primary cultures of murine brain microvessel endothelial cells (BMEC), astrocytes, and microglia with PBS or IL-1 beta, and then performed qPCR to measure inflammatory gene expression or immunocytochemistry to evaluate nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kappa B) activation. To evaluate whether astrocytes and/or BMEC propagate inflammatory signals to microglia, we exposed microglia to astrocyte-conditioned media and co-cultured endothelial cells and glia in transwells. Treatment groups were compared by Student's t tests or by ANOVA followed by Bonferroni-corrected t tests. Results: IL-1 beta increased inflammatory gene expression and NF-kappa B activation in primary murine-mixed glia, enriched astrocyte, and BMEC cultures. Although IL-1 beta elicited minimal changes in inflammatory gene expression and did not induce the nuclear translocation of NF-kappa B in isolated microglia, these cells were more robustly activated by IL-1 beta when co-cultured with astrocytes and/or BMEC. We observed a polarized endothelial response to IL-1 beta, because the application of IL-1 beta to the abluminal endothelial surface produced a more complex microglial inflammatory response than that which occurred following luminal IL-1 beta exposure. Conclusions: Inflammatory signals are detected, amplified, and propagated through the CNS via a sequential and reverberating signaling cascade involving communication between brain endothelial cells and glia. We propose that the brain's innate immune response differs depending upon which side of the blood-brain barrier the inflammatory stimulus arises, thus allowing the brain to respond differently to central vs. peripheral inflammatory insults.