HUMAN LUNG SURFACTANT PROTEIN D (SP-D) IS LOCATED IN THE GASTROINTESTINAL TRACT AND IN SALIVA.

Abstract

Aims: Lung surfactant-associated protein D (SP-D) is an oligomeric molecule with lectin (carbohydrate-binding) domains and collagen regions which has been demonstrated in lung washings. SP-D is believed to be of importance in innate immunity primarily through lectin-like activity and to participate in the formation of lung surfactant and mucus. SP-D is synthesized by pulmonary alveolar type II cells and non-ciliated bronchial cells. Recently, SP-D was demonstrated in the gastric mucosa (Fisher and Mason, 1995). The aim of this study was to investigate the distribution of SP-D throughout the gastrointestinal tract. Methods: Monoclonal antibodies were raised against SP-D and their specificity was confirmed by Western blotting. One antibody was subsequently used for immunohistochemistry. Reverse transcriptase PCR with an antisense SP-D primer and appropriate controls was used to look for SP-D mRNA. The presence of SP-D in saliva was investigated using affinity purification on maltose-agarose columns followed by separation on polyacryl amide gel (SDS-PAGE), Western blotting and detection with antibody. Results: Immunohistochemical localization of SP-D was observed within pulmonary alveolar type II cells, alveolar macrophages and in parotid and submandibular glands. Furthermore, SP-D was found in normal gastric mucosal lining cells and in epithelial cells of small intestinal and colonic tissue (8 subjects). By reverse transcriptase PCR products of approx. 300 base pairs identical to the finding in pulmonary tissue were detected in tissue from parotid gland, stomach, small intestine, colon and pancreas. SP-D was observed in saliva with a molecular mass of 43 kDa under reducing conditions, identical to the findings of SP-D from lung washings. Conclusions: These data indicate that the occurrence of SP-D is not restricted to pulmonary cells, but is found widely distributed at mucosal surfaces. SP-D may play a significant role in mucosal immunity of the gastrointestinal tract.