Collectins and pentraxins

Innate immunity is the first line of defense against pathogens. It plays a key role in the activation and orientation of adaptive immunity. Recognition and elimination of pathogens and of dying cells is mediated by pattern recognition receptors (PPRs) that have both cellular and humoral components. Cellular PPRs such as Toll-like receptors, scavenger and C-type lectin receptors are present on the antigen presenting cells. The humoral innate immune molecules are multifunctional and diverse and include soluble factors such as C1q, the collectins (mannose-binding lectin, surfactant protein A and D), the ficolins, and the pentraxins. This chapter discusses the roles of collectins and pentraxins in defense against fungal pathogens. Collectins have been shown to be involved in a range of immune functions including viral neutralisation, clearance of bacteria, fungi and apoptotic and necrotic cells, down regulation of allergic reactions and resolution of inflammation. Their basic structures include a triple-helical collagen region and a C-terminal homotrimeric lectin or carbohydrate recognition domain (CRD). The trimeric CRDs can recognise carbohydrate or charge patterns on microbes including fungi, allergens and dying cells, while the collagen region may interact with receptor molecules, present on a variety of immune cells, and thus initiate clearance mechanisms. The pentraxins are structurally unrelated to the collectins and include small pentraxins such as C-reactive proteins (CRP), serum amyloid protein (SAP) and long pentraxins such as PTX3. They belong to a superfamily of evolutionarily conserved proteins characterised by a structural motif, the pentraxin domain. The pentraxins are an essential component of humoral innate immunity,with CRP and SAP being acute phase reactants in human and mouse, respectively. CRP and SAP can function as opsonins for a range of pathogens.