Abstract
Astrocytes participate in CNS innate immune responses as evident by their ability to produce a wide array of inflammatory mediators upon exposure to diverse stimuli. Although we have established that astrocytes use TLR2 to signal inflammatory mediator production in response to Staphylococcus aureus, a common etiological agent of CNS infections, the signal transduction pathways triggered by this pathogen and how TLR2 expression is regulated remain undefined. Three disparate inhibitors that block distinct steps in the NF-kappaB pathway, namely SC-514, BAY 11-7082, and caffeic acid phenethyl ester, attenuated NO, TNF-alpha, and CXCL2 release from S. aureus-activated astrocytes. Among these proinflammatory mediators, autocrine/paracrine TNF-alpha was pivotal for augmenting TLR2 expression, since receptor levels were not elevated in astrocytes isolated from TNF-alpha knockout mice upon bacterial exposure. Since TLR2 is critical for signaling astrocytic cytokine production in response to S. aureus, we evaluated the effect of TNF-alpha loss on proinflammatory mediator release. Interestingly, among the molecules assayed, only NO production was significantly attenuated in TNF-alpha knockout astrocytes compared with wild-type cells. Similar results were obtained following LPS treatment, suggesting that TNF-alpha is an important regulator of astrocytic TLR2 expression and NO release in response to diverse microbial stimuli. In addition, NF-kappaB inhibitors attenuated TNF-alpha-induced TLR2 expression in astrocytes. Overall, this study suggests that two important anti-bacterial effector molecules, TLR2 and NO, are regulated, in part, by NF-kappaB-dependent autocrine/paracrine effects of TNF-alpha in astrocytes.