Biological activities of SP-A and SP-D against extracellular and intracellular pathogens

Surfactant proteins, SP-A and SP-D, are hydrophillic C-type lectins that are involved in innate immune responses at the mucosal surfaces, especially in the lungs, against various pathogens including viruses, bacteria and fungi. Both proteins have characteristic N-terminal triple-helical collagen region followed by trimeric neck and C-type lectin domains. Several in vivo and in vitro studies have shown that SP-A and SP-D can bind, mostly via their C-type lectin domains, to a range of pathogens displaying different carbohydrate molecular patterns; they show greater affinity towards microbe-associated ligands such as mannose and glucose. Oligomerizations of these collectins have been found to be a critical feature of its function for efficient host defence such as pathogen neutralization and opsonisation. SP-A and SP-D can also bind and activate macrophages for enhanced phagocytosis and reactive oxygen species-mediated destruction of pathogens, and influence pro-inflammatory cytokine production. Studies on gene knock-out mice and patient samples have revealed that SP-A or SP-D levels in the lungs can render susceptibility or resistance to microbial challenge and subsequent pathogenesis. This present chapter summarises the biological activities of SP-A and SP-D in preventing dissemination of infectious microbes inside the host.