Localization and functions of SP-A and SP-D at mucosal surfaces

Pulmonary surfactant protein A (SP-A) and D (SP-D), members of the collecti n family, are implicated in innate host defense of the lung. Collectins con sist of a collagen-like domain and a carbohydrate recognition domain. SP-A and SP-D recognize and interact with glycoconjugates on the surface of micr oorganisms. They protect the lung by interacting with a wide variety of pot ential pathogens, including viruses, bacteria, and fungi. This may result i n enhanced killing and/or clearance by phagocytes. Although most extensivel y studied in the lung, both SP-A and SP-D, or proteins closely resembling S P-A and SP-D, are found in a number of other sites in the body and therefor e may play a protective role at other sites than the lung. SP-A and SP-D pr otein and/or mRNA have been detected at various sites of the body: the resp iratory tract, the gastrointestinal tract, the middle ear, and in the perit oneal cavity. The presence of SP-A and SP-D at these mucosal surfaces, in c lose contact with numerous potentially harmful microorganisms, supports a r ole for these "lung"-collectins in innate mucosal defense. SP-A and SP-D ma y be important molecules in a threefold innate defense, particularly in the neonatal period between maternally acquired immunity and a fully developed adaptive immune system; the time interval between first exposure to a path ogen and generation of specific antibodies; and states of impaired immune f unction.