Porcine lung surfactant protein D: complementary DNA cloning, chromosomal localization, and tissue distribution

Porcine organs and lung surfactant have medically important applications in both xenotransplantation and therapy. We have started to characterize porcine lung surfactant by cloning the cDNA of porcine surfactant protein D (SP-D). SP-D and SP-A are important mediators in innate immune defense for the lung and possibly other mucosal surfaces. Porcine SP-D will also be an important reagent for use in existing porcine animal models for human lung infections. The complete cDNA sequence of porcine SP-D, including the 5' and 3' untranslated regions, was determined from two overlapping bacteriophage clones and by PCR cloning. Three unique features were revealed from the porcine sequence in comparison to SP-D from other previously characterized species, making porcine SP-D an intriguing species addition to the SP-D/collectin family. The collagen region contains an extra cysteine residue, which may have important structural consequences. The other two differences, a potential glycosylation site and an insertion of three amino acids, lie in the loop regions of the carbohydrate recognition domain, close to the carbohydrate binding region and thus may have functional implications. These variations were ruled out as polymorphisms or mutations by confirming the sequence at the genomic level in four different pig breeds. Porcine SP-D was shown to localize primarily to the lung and with less abundance to the duodenum, jejunum, and ileum. The genes for SP-D and SP-A were also shown to colocalize to a region of porcine chromosome 14 that is syntenic with the human and murine collectin loci.