Autocrine and paracrine IL-6 is necessary to induce phagocytosis of viable neurons by brucella abortus-activated microglia

Abstract

Central nervous system infection by bacteria of the genus Brucella results in an inflammatory disorder called neurobrucellosis. We have previously demonstrated that soluble mediators released by B. abortus-infected astrocytes induce inflammatory activation of microglia and phagocytosis of viable neurons. We have also demonstrated that neutralization of IL-6 in neurons/microglia co-cultures treated with supernatants from B. abortus-infected astrocytes inhibits neuronal death, and this effect is caused by a decrease in the phagocytic activity of microglia. Considering that both astrocytes and microglia are capable of secrete IL-6, we aimed to investigate the contribution of each cell type in this phenomenon. Astrocytes from wild type (WT) and IL-6 KO mice were infected or not with B. abortus for 24 h. After that, cell-free culture supernatants were used to stimulate primary murine co-cultures of WT and IL-6 KO microglia with neurons during 48 h. Neuronal density was evaluated by fluorescence microscopy. Treatment of WT co-cultures with supernatants from IL-6 KO infected astrocytes caused a partial inhibition of neuronal death (p<0.05). Similar results were obtained when neurons/IL-6 KO microglia co-cultures were treated with supernatants from WT- infected astrocytes (p<0.05). Furthermore, neuronal loss was totally prevented in co-cultures of neurons/IL-6 KO microglia treated with IL-6 KO infected astrocytes (p<0.005). Moreover, B. abortus-activated microglia from IL-6 KO mice were unable to induce neuronal death (p<0.05).To evaluate the role of IL-6 in neuronal loss induced by microglia directly activated by B. abortus, we stimulate co-cultures of WT and IL-6 KO microglia with heat-killed B. abortus (HKBA). Activated IL-6 KO microglia were unable to induce neuronal death (p<0.05). These results indicate that both paracrine and autocrine IL-6 signaling in microglia can be sufficient to induce phagocytosis of viable neurons in the context of a B. abortus infection, and could highlight the relevance of this cytokine in neuropathological mechanisms caused by Brucella spp.