Infiltrating macrophages contribute to age-related neuroinflammation in C57/BL6 mice

The recognized role of neuroinflammation in the age-related deterioration of neuronal function highlights the importance of understanding the factors that control microglial activation. Microglia, as the immune cells of the brain, are the arbiters of the inflammatory profile in the brain. Normally they are maintained in a quiescent state by means of ligand-receptor interactions with neurons, within a prevailing anti-inflammatory microenvironment. The evidence indicates that, as the ageing process continues, microglia become activated, shift towards an inflammatory phenotype and alter the milieu in the brain. Although there has been progress in identifying factors that contribute to age-related microglial activation, our understanding remains incomplete. Here we report that there was an age-related increase in circulating inflammatory cytokines, accompanied by microglial activation. Neutrophils, and to a greater extent, macrophages, infiltrate the brain with age, perhaps as a result of increased chemokine expression in the brain, specifically CXCL1 and CCL2. We sought to determine whether macrophages might trigger microglial activation and the evidence shows that conditioned medium obtained from interferon-γ (IFNγ)-stimulated macrophages potently activated microglia. The data suggest that infiltrating macrophages may be one factor that contributes to age-related microglial activation.