Microglia are one of the resident mononuclear phagocyte populations within the central nervous system (CNS). These cells share many phenotypical and functional characteristics with macrophages, indicating that microglia participate in innate immune responses in the brain. We have recently demonstrated that microglia are capable of recognizing S. aureus and respond by elaborating numerous inflammatory mediators and exhibit bactericidal activity. As such, microglia are uniquely poised to provide an initial line of defense against invading microorganisms into the CNS prior to leukocyte infiltration. However, the receptor(s) responsible for mediating microglial activation in response to S. aureus have not been identified. The pattern recognition receptors (PRRs) Toll-like receptor 2 (TLR2) and CD 14 play a pivotal role in macrophage activation in response to the gram-positive cell wall products peptidoglycan (PGN) and lipoteichoic acid (LTA). We have recently revealed that microglia express both of these PRRs which may be responsible for mediating cell activation in response to S. aureus. With the goal of defining the role(s) of TLR2 and CD14 in microglial responses to pyrogenic bacteria in the CNS, the following specific aims are proposed: I) To characterize the response of microglia to intact S. aureus organisms, PGN, LTA, and secreted virulence factors in terms of inflammatory mediator production; II) To examine the expression and regulation of TLR2 and CD14 on microglia; III) To delineate the functional significance of TLR2 and CD14 expression on microglial activation using microglia from receptor knockout mice, receptor blocking antibodies, and transient transduction of dominant negative receptor constructs; and IV) To examine the importance of microglial TLR2 and CD14 expression in the pathogenesis of S. aureus-induced brain abscesses using receptor knockout mice and radiation bone marrow chimeras. Although we are examining microglial activation in response to S. aureus, it is likely that our findings will extend to other gram-positive organisms by virtue of their conserved structural components. Understanding the mechanisms by which microglia recognize and respond to microbial products could have a significant impact on a broad range of bacterial infectious diseases in the CNS.
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