Neutrophil Accumulation in Bacterial Pneumonia

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DESCRIPTION (provided by applicant): Lung infections are a leading cause of morbidity, mortality and health care costs worldwide. Rapid clearance of pathogens from the respiratory tract is dependent on effective innate immune responses in the lung. Understanding the innate defense mechanisms in the lower respiratory tract is critical for the development of novel treatment and/or prevention strategies to reduce this burden of disease. The signaling cascades triggering innate immune responses consist of a delicate balance between pro- inflammatory responses that facilitate pathogen clearance, and counteracting anti-inflammatory responses that control excessive systemic inflammatory host responses. In general, it is poorly understood how these pathways converge to regulate host defense while minimizing inflammatory tissue injury. The long-term goal of this research is to understand how multiple innate immune events are integrated into effective antimicrobial resistance. As a model to elucidate the basic host defense mechanisms, we have focused on a primary pathogen, Klebsiella pneumoniae because this extracellular Gram-negative bacterium causes severe pneumonia; and the extensive spread of multiple drug-resistant K. pneumoniae strains worldwide. Although K. pneumoniae signals via Toll-like receptor (TLR)-4 and 9, a few studies have also indicated the involvement of NOD-like receptors (NLRs) as cytosolic immune sensors. Some NLRs termed "inflammasomes" can activate caspase-1 in order to cleave IL-1¿ and IL-18. We have previously shown the NLR family CARD domain containing 4 (NLRC4; IPAF) as a new sensor to K. pneumoniae in human and mouse macrophages that regulates caspase-1 dependent maturation of the inflammatory cytokines, IL1¿ and IL18. IL-1R1 knockout (KO) mice show greater susceptibility than NLRC4 KO mice to intrapulmonary K. pneumoniae infection, suggesting the involvement of other more prominent NLR proteins. Our preliminary data demonstrate that (1) human lungs with bacterial pneumonia show higher expression of NLR family pyrin domain containing 6 (NLRP6) inflammasome; (2) neutrophils show the highest expression of NLRP6 during resting stage and upon bacterial infection; (3) as compared with other NLRs (NOD1, NOD2, NLRP3 and NLRC4), NLRP6 is most prominent for bacterial clearance in the lungs during Klebsiella pneumonia; (4) neutrophils produce IL-17A and IL-17F in the lungs during bacterial pneumonia; and (5) NLRP6 knockdown human alveolar macrophages produce attenuated levels of IL-1¿ and IL-18 proteins following K. pneumoniae LPS challenge. Our key findings support a critical yet unrecognized function for NLRP6 during Klebsiella infection. This renewal proposal seeks to address the central hypothesis that NLRP6 is a key mediator of host defense during gram-negative bacterial pneumonia via induction of IL-17; thus, NLRP6 is a potential therapeutic target that could augment host defenses to acute respiratory infections. The Aims are: (1) Determine the effects of NLRP6 on bacterial clearance and neutrophil function following K. pneumoniae challenge.; (2) Determine the effects of NLRP6-dependent IL-17 production on host resistance to bacterial pneumonia.; (3) Identify functional alterations in alveolar macrophages caused by NLRP6 disruption with bacterial pneumonia.; and (4) Explore if manipulation of NLRP6 signaling can augment host resistance during Klebsiella pneumonia. A unique combination of in vivo and in vitro systems, including conditional KO mice, lentiviral transduction, adoptive transfer, and cytokine restoration strategies will be employed to address these Aims. Proving that the specific inflammasome plays a critical role in lung inflammation and host defense will lead to a major paradigm shift and ultimately lead to new therapeutic and prevention strategies of the treatment of ALI and ARDS in bacterial pneumonia because prior studies of the role of NLRs and other cytosolic sensors have been exclusively confined to in vitro studies and systemic Infection models.