SPLUNC1 Promotes Lung Innate Defense Against Mycoplasma pneumoniae Infection in Mice

Abstract

Short palate, lung, and nasal epithelium clone 1 (SPLUNC1) protein is highly expressed in normal airways, but is dramatically decreased in allergic and cigarette smoke exposure settings. We have previously demonstrated SPLUNC1 in vitro antibacterial property against Mycoplasma pneumoniae (Mp). However, its in vivo biological functions remain unclear. The objectives of this study were to determine the in vivo functions of SPLUNC1 following bacterial (eg, Mp) infection, and to examine the underlying mechanisms. We generated SPLUNC1-deficient mice and utilized transgenic mice overexpressing human SPLUNC1 exclusively within the airway epithelium. These mice were infected with Mp and, twenty-four hours post infection, their host defense responses were compared to littermate controls. Mp levels and inflammatory cells increased in the lungs of SPLUNC1 ؊/؊ mice as compared to wild type controls. SPLUNC1 deficiency was shown to contribute to impaired neutrophil activation. In contrast, mice overexpressing hSPLUNC1 exclusively in airway epithelial cells demonstrated lower Mp levels. Furthermore, neutrophil elastase activity was significantly increased in mice overexpressing hSPLUNC1. Lastly, we demonstrated that SPLUNC1 enhanced Mp-induced human neutrophil elastase (HNE) activity, and HNE directly inhibited the growth of Mp. Our findings demonstrate a critical in vivo role of SPLUNC1 in host defense against bacterial infection, and likely provide a novel therapeutic approach to restore impaired lung innate immune responses to bacteria in patients with chronic lung diseases.

Short Palate, Lung, and Nasal Epithelial Clone–1 Is a Tightly Regulated Airway Sensor in Innate and Adaptive Immunity

“…SPLUNC1 has well-defined antimicrobial properties (16)(17)(18)(19). It blocks microbial invasion in the respiratory tract by inhibiting pathogen growth, and through its surfactant properties inhibits biofilm development (16,19,20,24).…”

“…It was initially hypothesized to express antimicrobial functions, and has been shown to inhibit the growth of multiple pathogens (16)(17)(18)(19), inhibit biofilm formation, and possess surfactant properties (20,21). Although most studies of SPLUNC1 have focused on these antimicrobial effects, SPLUNC1 also regulates the epithelial sodium channel (ENaC) to maintain airway surface liquid depth, affects mucociliary clearance, and regulates inflammation, thereby limiting allergic airway inflammation (10,(22)(23)(24).…”

SPLUNC1 Deficiency Enhances Airway Eosinophilic Inflammation in Mice

“…We previously demonstrated that the Th2 cytokine IL-13 significantly reduces SPLUNC1 expression and secretion from well-differentiated human primary airway epithelial cells, rendering a host susceptible to bacterial infections (10). Recent studies by our group and others demonstrated that SPLUNC1 is critical for clearing respiratory pathogens such as Mycoplasma pneumoniae and Pseudomonas aeruginosa from murine lungs (10)(11)(12). In addition to its antimicrobial function, we have shown that murine recombinant SPLUNC1 protein inhibits the Toll-like receptor 2 (TLR2) agonist-induced production of IL-8 in human airway epithelial cells (10).…”

SPLUNC1 Is a Significant Marker in Pleural Effusion from Lung Cancer Compared to Tuberculosis

“…(17) SPLUNC1 has also been shown to have an anti-inflammatory function in exogenous microbe-induced respiratory infection, such as K. pneumoniae, (18) Mycoplasma pneumoniae, (19) P. aeruginosa, and Epstein-Barr virus. (20) Additionally, SPLUNC1 could also regulate ion/mucus transport and hydration levels in the lung, (21) lower surface tension by its surfactant-like properties, (22) and act as an inflammatory mediator.…”