Surfactant proteins A and D in Eustachian tube epithelium

Surfactant protein (SP) A and SP-D are collectins that have roles in host d efense. The Eustachian tube (ET) maintains the patency between the upper ai rways and the middle ear. Dysfunction of local mucosal immunity in ET may p redispose infants to recurrent otitis media. We recently described prelimin ary evidence of the expression of SP-A and SP-D in the ET. Our present aim was to establish the sites of SP-A and SP-D expression within the epitheliu m of the ET in vivo. With in situ hybridization, electron microscopy, and i mmunoelectron microscopy, the cells responsible for SP-A and SP-D expressio n and storage were identified. SP-A expression was localized within the ET epithelium, and the protein was found in the electron-dense granules of mic rovillar epithelial cells. Being concentrated in the epithelial lining, onl y a few cells revealed intracellular SP-D, and it was not associated with g ranules. The SP-A and SP-D immunoreactivities in ET lavage fluid, as shown by Western blot analyses, were similar to those in bronchoalveolar lavage f luid. We propose that there are specialized cells in the ET epithelium expr essing and secreting SP-A and SP-D. SP-A and SP-D may be important for anti body-independent protection of the middle ear against infections.