Surfactant protein D (SP-D) is an innate immune molecule that was originally discovered in the lungs as a part of pulmonary surfactant system. Later on, it became evident that SP-D has extensive tissue distribution that lent its credentials as a versatile innate immune molecule of mucosal system. In addition to its ability to protect against pathogens and allergens, and to modulate inflammatory reactions, SP-D has emerged as an immune surveillance molecule against cancer. SP-D can induce apoptosis in a variety of cancer cell lines and primary cancer cells derived from patients, including lung, pancreatic, prostate, ovarian and breast cancers. The apoptotic mechanisms including pathways, signaling and key mediators involved, have been delineated. The most striking feature of recent studies is the demonstration that a recombinant form of human SP-D (rfhSP-D) composed of homotrimeric C-type lectin domains can bring about these anti-tumor effects, raising the possibility of a therapeutic development. In addition to apoptosis induction, rfhSP-D can also interfere with epithelial-to-mesenchymal (EMT) transition in pancreatic cancer. In view of the above-mentioned in vitro studies, a recent bioinformatics analysis has examined if SP-D can serve as a potential prognostic marker for human lung cancer. It appears that compared to their normal tissue counterparts, there is a lower expression of SP-D in lung, gastric, and breast cancers, as opposed to ovarian cancer. In the lung cancer, the existence of SP-D is likely to be associated with a favourable prognosis.
