In our study, surfactant protein (SP)-A was characterized in adult human tr
achea and bronchi. SP-A mRNA and protein were localized to serous cells in
submucosal glands by in situ hybridization and immunohistochemistry, respec
tively. A 2.2 kb SP-A mRNA transcript was detected in tracheal tissues by N
orthern 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 S
P-A1 mRNA in the tracheal tissues. A similar to 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 conc
lude 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 prot
ein, like other serous secretions from airway submucosal glands, functions
in local antimicrobial host defense mechanisms in the conducting airways.