Mechanisms of Innate Immunity in Polymicrobial Sepsis

Abstract

Severe bacterial sepsis leads to a pro-inflammatory condition that can manifest as septic shock, multiple organ failure, and death. Neutrophils are critical for the rapid elimination of bacteria, however, the role of neutrophil chemoattractant CXCL1, pattern recognition receptors (PRRs)- NLR protein 3 (NLRP3) and alcohol in bacterial clearance during sepsis remains elusive. We demonstrate that CXCL1 plays a pivotal role in mediating host defense to polymicrobial sepsis following cecal ligation and puncture (CLP) in gene-deficient mice. CXCL1 appears to be essential for restricting bacterial outgrowth and preventing multiple organ failure and death in mice. Moreover, CXCL1 is essential for neutrophil migration, expression of pro-inflammatory mediators, Recombinant interleukin 17 (IL-17) rescued impaired host defenses in cxcl1−/− mice. CXCL1 is important for IL-17A production via Th17 differentiation. CXCL1 is essential for reactive oxygen species production and neutrophil extracellular trap (NET) formation. This study reveals a novel role for CXCL1 in neutrophil recruitment via modulating T cell function and neutrophil-related bactericidal functions. These studies suggest that modulation of CXCL1 levels could reduce bacterial burden and excessive inflammatory injury in sepsis. NLRP3-/- mice or mice treated with NLRP3 inhibitor were protected in response to polymicrobial sepsis. NLRP3-/- mice showed reduced bacterial burden and production of proinflammatory cytokines. Intriguingly, neutrophils obtained from NLRP3-/- or NLRP3-inhibited mice display impaired critical functions of neutrophils, including phagocytosis, bacterial killing, NET formation, autophagy, chemotaxis, and cell death. These unique and novel findings position NLRP3 as a critical linker between neutrophil function and bacterial clearance, highlighting NLRP3 as a therapeutic target to control infection in polymicrobial sepsis. Alcoholics are more susceptible to bacterial sepsis and thus have higher mortality rate as compared to non-alcoholics. In this study, acute alcohol intoxication prior to the induction of polymicrobial sepsis show reduced NETosis. Diminished NETosis was consistent with attenuated ROS production and bacterial clearance in alcohol-challenged CLP-induced mice. Our findings demonstrate that alcohol-suppressed NETosis and NET-mediated extracellular killing of bacteria contribute to the pathogenesis of polymicrobial sepsis, and thus, furthers our understanding on alcohol-induced immune defect during bacterial infection.