Localization of lung surfactant protein D on mucosal surfaces in human tissues

Lung surfactant protein-D (SP-D), a collectin mainly produced by alveolar t ype II cells, initiates the effector mechanisms of innate immunity on bindi ng to microbial carbohydrates. A panel of mRNAs from human tissues was scre ened for SP-D mRNA by RT-PCR, The lung was the main site of synthesis, but transcripts were readily amplified from trachea, brain, testis, salivary gl and, heart, prostate gland, kidney, and pancreas. Minor sites of synthesis were uterus, small intestine, placenta, mammary gland, and stomach. The seq uence of SP-D derived from parotid gland mRNA was identical with that of pu lmonary SP-D. mAbs were raised against SP-D, and one was used to locate SP- D in cells and tissues by immunohistochemistry. SP-D immunoreactivity was f ound in alveolar type II cells, Clara cells, on and within alveolar macroph ages, in epithelial cells of large and small ducts of the parotid gland, sw eat glands, and lachrymal glands, in epithelial cells of the gall bladder a nd intrahepatic bile ducts, and in exocrine pancreatic ducts. SP-D was also present in epithelial cells of the skin, esophagus, small intestine, and u rinary tract, as well as in the collecting ducts of the kidney. SP-D is gen erally present on mucosal surfaces and not restricted to a subset of cells in the lung. The localization and functions of SP-D indicate that this coll ectin is the counterpart in the innate immune system of IgA in the adaptive immune system.