Surfactant protein D: prognostic insights and emerging therapeutic roles in cancer

Surfactant protein D (SP-D), a C-type lectin found at the mucosal surfaces, plays crucial roles in innate immunity and homeostasis of the lung. In addition to the lungs, extra-pulmonary expression of SP-D has been reported in several tissues. Differential expression of SP-D in lung adenocarcinoma and metastases implicated its importance in diagnosis and prognosis. Serendipitous discovery of the direct anti-tumor effect of SP-D in human eosinophilic leukemia and breast cancer cells opened the field for exploring its therapeutic potential in various cancers and its contribution to immunomodulatory mechanisms. The current review discusses the role of SP-D as a prognostic marker in lung, breast, gastrointestinal, pancreatic, ovarian and prostate cancer. Additionally, levels of SP-D in serum and bronchoalveolar lavage enable diagnosis and assessment of the risk of developing lung cancer. The molecular interaction of SP-D with Epidermal Growth Factor Receptor and Glucose-Related Protein 78, induction of cell cycle arrest and activation of the intrinsic apoptosis pathways are some of the salient mechanisms contributing to the immune surveillance of tumor cells by SP-D. The review also discusses SP-D interactions with oncogenic pathogens and therapeutic potential of recombinant SP-D.