In situ hybridization of SP-A mRNA in adult human conducting airways.

In our study, surfactant protein (SP)-A was characterized in adult human trachea and bronchi. SP-A mRNA and protein were localized to serous cells in submucosal gland by in situ hybridization and immunohistochemistry, respectively. A 2.2 kb SP-A mRNA transcript was detected in tracheal tissues by Northern blot analysis. Primer extension analysis and gene-specific reverse transcriptase polymerase chain reaction (RT-PCR) revealed the predominance of SP-A2 mRNA. However, using nested PCR, we also detected low amounts of SP-A1 mRNA in the tracheal tissues. A approximately 35 kDa SP-A immunoreactive protein was detected in the tracheal tissues by immunoblot analysis and was shown to be modified by the addition of N-linked oligosaccharides. We conclude that submucosal glands in the conducting airways produce a novel SP-A protein with a molecular weight and post-translational modification similar to the SP-A produced in the distal lung. We speculate that this SP-A2 protein, like other serous secretions from airway submucosal glands, functions in local antimicrobial host defense mechanisms in the conducting airways.