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

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.

BCL6 modulates tissue neutrophil survival and exacerbates pulmonary inflammation following influenza virus infection

“…Our data and the findings reported by Brandes et al (8) support the view that early engagement of neutrophils instigates a damaging feedforward innate inflammatory circuit responsible for severe disease development following IAV infection. Neutrophils are the first responders to infiltrate the sites of infection to clear bacterial infections (37,38). In this regard, enhanced tissue neutrophil apoptosis caused by BCL6 deficiency may increase pulmonary bacterial burden and worsen outcome during certain respiratory bacterial infections, such as Staphylococcus pneumonia infection (37,38).…”

“…Neutrophils are the first responders to infiltrate the sites of infection to clear bacterial infections (37,38). In this regard, enhanced tissue neutrophil apoptosis caused by BCL6 deficiency may increase pulmonary bacterial burden and worsen outcome during certain respiratory bacterial infections, such as Staphylococcus pneumonia infection (37,38). Conversely, excessive neutrophil recruitment is a hallmark of acute susceptibility to Mycobacterium tuberculosis and is associated with overexuberant inflammatory response and disease progression (39).…”

The NLRP6 inflammasome in health and disease

“…Thus, it is unique in the NLR family as it can perform functions of both NLRs and TLRs. 10 This notion is supported by other studies proposing a wide scope of physiological functions of Nlrp6 ranging from modulation of the host-microbial interface 11 and host defense against pathogens 12,13 to inhibition of carcinogenesis 14 and neuroinflammation. 15 However, Nlrp6 has also been shown to negatively regulate NF-κB signaling in mouse model.…”

“…Recently, we 12 and others 18 reported that the Nlrp6 inflammasome is activated during bacterial infections. In our study, Nlrp6 −/ − bone marrow-derived macrophages (BMDMs) showed reduced caspase-1 processing and IL-β secretion compared to that of WT BMDMs when infected with methicillin-resistant Staphylococcus aureus (MRSA).…”

“…As reported by Grenier et al 10 and Levy et al, 17 we found that Nlrp6 co-localizes with ASC in BMDMs after infection. 12 Similarly, an excellent study from Hara et al 18 reported that Grampositive bacteria, such as Listeria monocytogenes, activate the Nlrp6 inflammasome. More importantly, the authors demonstrated that lipoteichoic acid (LTA) from L. monocytogenes upregulates the expression of Nlrp6 and caspase-11 via type 1 interferon (IFN) signaling, which could be regarded as the first signal.…”