Research Article: NLRP6 negatively regulates pulmonary host defense in Gram-positive bacterial infection through modulating neutrophil recruitment and function

Date Published: September 24, 2018

Publisher: Public Library of Science

Author(s): Laxman Ghimire, Sagar Paudel, Liliang Jin, Pankaj Baral, Shanshan Cai, Samithamby Jeyaseelan, Dana J. Philpott.

http://doi.org/10.1371/journal.ppat.1007308

Abstract

Gram-positive bacteria, including Staphylococcus aureus are endemic in the U.S., which cause life-threatening necrotizing pneumonia. Neutrophils are known to be critical for clearance of S. aureus infection from the lungs and extrapulmonary organs. Therefore, we investigated whether the NLRP6 inflammasome regulates neutrophil-dependent host immunity during pulmonary S. aureus infection. Unlike their wild-type (WT) counterparts, NLRP6 knockout (KO) mice were protected against pulmonary S. aureus infection as evidenced by their higher survival rate and lower bacterial burden in the lungs and extrapulmonary organs. In addition, NLRP6 KO mice displayed increased neutrophil recruitment following infection, and when neutrophils were depleted the protective effect was lost. Furthermore, neutrophils from the KO mice demonstrated enhanced intracellular bacterial killing and increased NADPH oxidase-dependent ROS production. Intriguingly, we found higher NK cell-mediated IFN-γ production in KO mouse lungs, and treatment with IFN-γ was found to enhance the bactericidal ability of WT and KO neutrophils. The NLRP6 KO mice also displayed decreased pyroptosis and necroptosis in the lungs following infection. Blocking of pyroptosis and necroptosis in WT mice resulted in increased survival, reduced bacterial burden in the lungs, and attenuated cytokine production. Taken together, these novel findings show that NLRP6 serves as a negative regulator of neutrophil-mediated host defense during Gram-positive bacterial infection in the lungs through regulating both neutrophil influx and function. These results also suggest that blocking NLRP6 to augment neutrophil-associated bacterial clearance should be considered as a potential therapeutic intervention strategy for treatment of S. aureus pneumonia.

Partial Text

Acute pneumonia is a leading cause of childhood mortality (<5 years of age) accounting for the death of 920,136 children annually [1], and methicillin-resistant Staphylococcus aureus (MRSA) has been implicated in severe life-threatening infections, including necrotizing pneumonia and sepsis [2]. In addition, S. aureus infection is also one of the major causes of secondary pneumonia following influenza infection [3]. Furthermore, S. aureus has developed resistance to multiple antibiotics and effective treatment strategies against this bacterium are limited [2, 4]. Therefore, S. aureus is a serious threat to human health and novel therapeutic strategies are needed. Lung diseases induced by Gram-positive pathogens are an important cause of morbidity and mortality in both immunocompetent and immunocompromised individuals [13, 33]. Although antibiotics decrease the mortality rates of bacterial pneumonia, the efficacy is somewhat limited due to the substantial number of immunocompromised individuals, growing number of elderly patients, and the rise of multi-antibiotic resistant bacterial strains. Thus, alternative therapeutic approaches, including the manipulation of host signaling events, are needed. However, detailed understanding of the host innate immune response is critical for the design of potential therapeutic interventions. Because the lung is continuously exposed to pathogens and their virulence factors, this organ possesses a multifaceted host defense system. Moreover, a successful immune response in the lung is critical for efficient clearance of microbial pathogens and therefore, the innate immune system possess germline-encoded pattern-recognition receptors.   Source: http://doi.org/10.1371/journal.ppat.1007308