CC16 Deficiency in the Context of Early-Life Mycoplasma pneumoniae Infection Results in Augmented Airway Responses in Adult Mice.

Studies have shown that club cell secretory protein (CC16) plays important protective roles in the lungs, yet its complete biological functions are unclear. We devised a translational mouse model in order to investigate the impact of early life infections, in the context of CC16 deficiency, on lung function in adult mice. CC16 sufficient (WT) and deficient (CC16-/-) mice were infected with Mycoplasma pneumoniae (Mp) as weanlings and assessed as adults (early life infection model; ELIM) and compared to adult mice infected for only 3 days (adult infection model; AIM). CC16-/- Mp-infected mice had significantly increased airway hyperresponsiveness (AHR) in both models compared to WT mice. However, CC16-/- mice infected in early life (ELIM) displayed significantly increased AHR compared to CC16-/- mice infected in adulthood (AIM). In stark contrast, lung function in ELIM WT mice returned to levels similar to saline-treated controls. While WT mice cleared Mp infection in the ELIM, CC16-/- mice remained colonized with Mp throughout the model, which likely contributed to increased airway remodeling and persistence of Muc5ac expression. When CC16-/- mouse tracheal epithelial cells (MTECs) were infected with Mp, increased Mp colonization and collagen gene expression were also detected compared to WT cells, suggesting that CC16 plays a protective role during Mp infection, in part through epithelial-driven host defense mechanisms.

Surfactant Protein-A Protects against IL-13-Induced Inflammation in Asthma.

The lung surfactant proteins are recognized as critical not only for their role in lowering lung surface tension but also in innate host defense. Reports have shown that some asthmatic patients have decreased levels of one member of this protein family in particular, surfactant protein-A (SP-A). Our studies set out to determine the contribution of SP-A to the response of a key effector cytokine in asthma, IL-13. Our studies employ both animal models sufficient and deficient in SP-A challenged with IL-13 and primary epithelial cells from participants with asthma that are exogenously treated with SP-A in the context of IL-13 challenge. The inflammatory response and mucin production were assessed in both model systems. As compared with WT mice, we show that the activity of IL-13 is dramatically augmented in SP-A-/- mice, which have significantly increased neutrophil and eosinophil recruitment, mucin production and asthma-associated cytokines in the bronchoalveolar lavage fluid. In parallel, we show asthma-associated factors are attenuated in human cells from asthma subjects when exogenous SP-A is added during IL-13 challenge. Although many of these phenotypes have previously been associated with STAT6 signaling, SP-A inhibited IL-13-induced STAT3 phosphorylation in mice and in human epithelial cells while having little effect on STAT6 phosphorylation. In addition, when either STAT3 or IL-6 were inhibited in mice, the phenotypes observed in SP-A-/- mice were significantly attenuated. These studies suggest a novel mechanism for SP-A in asthma as a modulator of IL-13-induced inflammation via mediating downstream IL-6/STAT3 signaling.

Genetic variation in SP-A2 leads to differential binding to Mycoplasma pneumoniae membranes and regulation of host responses.

Mycoplasma pneumoniae is an extracellular pathogen that colonizes mucosal surfaces of the respiratory tract and is associated with asthma exacerbations. Previous reports demonstrate that surfactant protein-A (SP-A) binds live M. pneumoniae and mycoplasma membrane fractions (MMF) with high affinity. Humans express a repertoire of single-amino acid genetic variants of SP-A that may be associated with lung disease, and our findings demonstrate that allelic differences in SP-A2 (Gln223Lys) affect the binding to MMF. We show that SP-A(-/-) mice are more susceptible to MMF exposure and have significant increases in mucin production and neutrophil recruitment. Novel humanized SP-A2-transgenic mice harboring the hSP-A2 223K allele exhibit reduced neutrophil influx and mucin production in the lungs when challenged with MMF compared with SP-A(-/-) mice. Conversely, mice expressing hSP-A2 223Q have increased neutrophil influx and mucin production that are similar to SP-A(-/-) mice. Using tracheal epithelial cell cultures, we show that enhanced mucin production to MMF occurs in the absence of SP-A and is not dependent upon neutrophil recruitment. Increased phosphorylation of the epidermal growth factor receptor (EGFR) was evident in the lungs of MMF-challenged mice when SP-A was absent. Pharmacologic inhibition of EGFR prior to MMF challenge dramatically reduced mucin production in SP-A(-/-) mice. These findings suggest a protective role for SP-A in limiting MMF-stimulated mucin production that occurs through interference with EGFR-mediated signaling. SP-A interaction with the EGFR signaling pathway appears to occur in an allele-specific manner that may have important implications for SP-A polymorphisms in human diseases.