Odyssey of Surfactant Proteins SP-A and SP-D: Innate Immune Surveillance Molecules

Lung surfactant proteins SP-A and SP-D were discovered as hydrophilic protein constituents of the pulmonary surfactant secreted by alveolar epithelial cells and were initially purified from lung lavage and amniotic fluid. Once their oligomeric structure was revealed, they caught immediate attention of immunologists, as both these C-type lectins were quite similar to C1q, the recognition subcomponent of the complement classical pathway. Though neither of them had any role in complement cascade, it was discovered that their collagen domains can interact with macrophages, through C1q receptor, though later multiple receptors were to be discovered. Important roles of SP-A and SP-D in host defence were highlighted by the fact that they can recognise a range of viral, bacterial and fungal pathogens through their carbohydrate recognition domain and activate the innate immune system. Regulation of the adaptive immune system became evident by inhibition of the both IL-2 dependent and independent T lymphocyte proliferation and inhibition of specific IgE binding with the glycosylated allergens. With their recombinant forms, studies in animal models confirmed their translational potential as well as provided an insight to their integral role in immune homeostasis. Discovery of anti-neoplastic role of SP-D was serendipity during a study investigating its interaction with eosinophils wherein an eosinophilic leukemic cell line was used. Magnanimity accrued to SP-A and SP-D when a series of studies unravelled their presence in several extra pulmonary tissues and thus, highlighted their multifaceted roles in various organ systems including male and female reproductive tract. This Topic welcomes manuscripts that show important roles of SP-A or SP-D in immune surveillance in health and disease.