Relationship between type 2 cytokine and inflammasome responses in obesity-associated asthma

BACKGROUND

Obesity is a risk factor for asthma, and obese asthmatic individuals are more likely to have severe, steroid-insensitive disease. How obesity affects the pathogenesis and severity of asthma is poorly understood. Roles for increased inflammasome-mediated neutrophilic responses, type 2 immunity, and eosinophilic inflammation have been described.

OBJECTIVE

We investigated how obesity affects the pathogenesis and severity of asthma and identified effective therapies for obesity-associated disease.

METHODS

We assessed associations between body mass index and inflammasome responses with type 2 (T2) immune responses in the sputum of 25 subjects with asthma. Functional roles for NLR family, pyrin domain-containing (NLRP) 3 inflammasome and T2 cytokine responses in driving key features of disease were examined in experimental high-fat diet-induced obesity and asthma.

RESULTS

Body mass index and inflammasome responses positively correlated with increased IL-5 and IL-13 expression as well as C-C chemokine receptor type 3 expression in the sputum of subjects with asthma. High-fat diet-induced obesity resulted in steroid-insensitive airway hyperresponsiveness in both the presence and absence of experimental asthma. High-fat diet-induced obesity was also associated with increased NLRP3 inflammasome responses and eosinophilic inflammation in airway tissue, but not lumen, in experimental asthma. Inhibition of NLRP3 inflammasome responses reduced steroid-insensitive airway hyperresponsiveness but had no effect on IL-5 or IL-13 responses in experimental asthma. Depletion of IL-5 and IL-13 reduced obesity-induced NLRP3 inflammasome responses and steroid-insensitive airway hyperresponsiveness in experimental asthma.

CONCLUSION

We found a relationship between T2 cytokine and NLRP3 inflammasome responses in obesity-associated asthma, highlighting the potential utility of T2 cytokine-targeted biologics and inflammasome inhibitors.

Impaired induction of Slc26a4 promotes respiratory acidosis and severe, steroid-resistant asthma

CO2 produced by systemic cellular respiration is hydrated into carbonic acid (H2CO3) that dissociates into H+ and HCO3−. These are transported in the plasma to the lungs where HCO3− is converted back into H2CO3 and CO2 that are expelled through breathing. Evidence suggests that dysfunction of the mechanisms that govern these processes may result in the development of respiratory acidosis (RA) and the cardinal features of severe, steroid-resistant (SSR) asthma. Reduced lung function, which occurs in SSR asthma, impairs removal of volatile H2CO3 and CO2, resulting in acid accumulation and increased arterial PaCO2. Patients with severe asthma often develop complications from increased PaCO2, which skews the PaCO2/HCO3− ratio resulting in increased H+ concentration and reduced pH. We developed three mouse models of respiratory infection and ovalbumin-induced SSR allergic airways disease (SSRAAD) that are highly representative of SSR asthma in humans. We used these models to show a role for impaired homeostatic acid-base balance in SSR asthma. All three infections suppress the induction of the expression of the chloride (Cl−)/HCO− pump, Slc26a4, in the airway mucosa in AAD. Importantly, SSRAAD is associated with increased levels of free H+ ions in bronchoalveolar lavage fluid. Administration of Slc26a4-specific siRNA in steroid-sensitive AAD, which mimics the effect of decreased Slc26a4 responses in SSRAAD, induced RA and steroid-resistant airway inflammation and AHR. Importantly, treatment of RA with NaHCO3 during infection-induced SSRAAD suppressed steroid-resistant AHR. Thus, we have identified a previously unrecognised role for deficient Slc26a4 responses that result in the development of RA and the pathogenesis of SSRAAD.