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.

Mast cell TNF receptors regulate responses to Mycoplasma pneumoniae in surfactant protein A (SP-A)-/- mice.

BACKGROUND: Mycoplasma pneumoniae (Mp) frequently colonizes the airways of patients with chronic asthma and likely contributes to asthma exacerbations. We previously reported that mice lacking surfactant protein A (SP-A) have increased airway hyperresponsiveness (AHR) during M pneumoniae infection versus wild-type mice mediated by TNF-α. Mast cells (MCs) have been implicated in AHR in asthma models and produce and respond to TNF-α. OBJECTIVE: Determine the contribution of MC/TNF interactions to AHR in airways lacking functional SP-A during Mp infection. METHODS: Bronchoalveolar lavage fluid was collected from healthy and asthmatic subjects to examine TNF-α levels and M pneumoniae positivity. To determine how SP-A interactions with MCs regulate airway homeostasis, we generated mice lacking both SP-A and MCs (SP-A(-/-)Kit(W-sh/W-sh)) and infected them with M pneumoniae. RESULTS: Our findings indicate that high TNF-α levels correlate with M pneumoniae positivity in human asthmatic patients and that human SP-A inhibits M pneumoniae-stimulated transcription and release of TNF-α by MCs, implicating a protective role for SP-A. MC numbers increase in M pneumoniae-infected lungs, and airway reactivity is dramatically attenuated when MCs are absent. Using SP-A(-/-)Kit(W-sh/W-sh) mice engrafted with TNF-α(-/-) or TNF receptor (TNF-R)(-/-) MCs, we found that TNF-α activation of MCs through the TNF-R, but not MC-derived TNF-α, leads to augmented AHR during M pneumoniae infection when SP-A is absent. Additionally, M pneumoniae-infected SP-A(-/-)Kit(W-sh/W-sh) mice engrafted with TNF-α(-/-) or TNF-R(-/-) MCs have decreased mucus production compared with that seen in mice engrafted with wild-type MCs, whereas burden was unaffected. CONCLUSION: Our data highlight a previously unappreciated but vital role for MCs as secondary responders to TNF-α during the host response to pathogen infection.