Immunohistochemical localization of surfactant apoproteins in usual interstitial pneumonia associated with pulmonary carcinoma.

Surfactant apoproteins A and B (SP-A and SP-B) are antigenic determinants of pulmonary surfactant complexes. The role and functional significance of these proteins are largely unknown and the pattern of expression is probably related to the functional maturation of type II pneumocytes. Differential expression of SP-A and SP-B was reported in the developing human lung but little is known of their expression in the chronic injury. We studied 5 surgical cases of usual interstitial pneumonia (UIP) associated with carcinoma to evaluate the expression of pulmonary surfactant apoproteins. These cases were immunohistochemically examined by the streptavidin-biotin complex method using monoclonal antibodies HS-1 and HS-2 against pulmonary surfactant apoprotein A (SP-A) and B (SP-B), respectively. In UIP, SP-B was expressed strongly in type II pneumocytes and Clara cells but bronchiolar epithelium and metaplastic squamous cell lines in the honeycomb lesion were non-reactive. SP-A showed a similar pattern but much weaker reactivity when compared to that of SP-B. Type II pneumocytes in normal lung tissue exhibited weak immunoreactivity and no difference in the intensity of staining between SP-A and SP-B. Neither carcinomatous area nor metaplastic lining cells at honeycomb lesion show immunoreactivity to SP-A and SP-B. These results suggest that type II pneumocytes in the UIP are functionally immature in their expression of the apoprotein types and the metaplastic squamous cells or neoplastic transformed cells do not have molecular characteristics of type II pneumocytes.