Role of the inflammasome in the pathogenesis and immune responses to Staphylococcus aureus infection in the central nervous system

Staphylococcus aureus (S. aureus) is a leading cause of community-acquired and hospital-associated infections. With the emergence of methicillin-resistant S. aureus (MRSA), there is an urgent need to identify therapeutic options that lie beyond the scope of conventional antibiotics. The prevalence of MRSA infections within the central nervous system (CNS) has steadily climbed in recent years. S. aureus colonization of the CNS can lead to brain abscess formation, typified by widespread inflammation and tissue damage. Previous work in our laboratory has identified interleukin-1 (IL-1) as a pivotal cytokine for eliciting protective anti-bacterial immunity during the acute phase of brain abscess development. However, the molecular machinery for IL-1 production in the CNS remains poorly understood. The Nod-like receptor protein 3 (NLRP3) inflammasome is a protein complex responsible for processing pro-IL-1beta and pro-IL-18 into their active forms following exposure to both pathogen- and danger-associated molecular patterns (PAM Ps and DAMPs, respectively). The NLRP3 inflammasome consists of the NLRP3 protein, the adapter protein ASC (apoptosis-associated speck-like caspase-1 recruiting domain-containing protein), and pro-caspase-1. The overall goal of this project was to investigate the functional importance of NLRP3 inflammasome activation during brain abscess pathogenesis. Primary microglia isolated from NLRP3 knockout (KO), ASC KO and C57BL/6 WT mice were challenged with live MRSA strain USA300, in vitro. Interestingly, IL-1beta, but not IL-18 production, was significantly attenuated in both NLRP3 and ASC KO microglia following live S. aureus exposure. IL-1beta, but not IL-18, secretion was partially dependent on autocrine/paracrine ATP release, caspase-1 activation, cathepsin B and alpha- and gammahemolysins produced by live bacteria. We next investigated the functional significance of the NLRP3 inflammasome in the context of CNS S. aureus infection in NLRP3 KO, ASC KO and WT mice. Approximately 50% of ASC KO animals succumbed within 24 h post-infection; whereas, the survival rate of NLRP3 KO animals was similar to WT. Susceptibility of ASC KO mice to CNS S. aureus infection was partially dependent on the actions of caspase-1 and a-hemolysin, whereas caspase-7 was protective. Microarray analysis revealed several changes in the expression of neuron-specific genes, including synapse formation and signal transduction as well as ubiquitin pathway-related genes. Collectively, these studies demonstrate an important role for the NLRP3 inflammasome in regulating IL-1beta secretion during CNS S. aureus infection both in vivo and in vitro, as well as additional novel functions of ASC that may be inflammasome-independent.