SP-A ENHANCES PHAGOCYTOSIS OF KLEBSIELLA BY INTERACTION WITH CAPSULARPOLYSACCHARIDES AND ALVEOLAR MACROPHAGES

We found that surfactant protein A (SP-A) enhances phagocytosis of Kle bsiella pneumoniae K21a but not of K2 serotypes by alveolar macrophage s. SP-A interacted with the capsule of K21a (containing Man alpha 1Man sequences) as shown by SP-A-induced agglutination of the bacteria, by binding of SP-A-coated particles onto the bacterial surface, and by b inding of SP-A to immobilized parent K21a strain and recombinant strai ns that switched their capsule from K2 to K21a. In contrast, only marg inal binding of SP-A to K2 parent strain (lacking this sequence) could be detected. Furthermore, binding of capsular polysaccharide of K21a to immobilized SP-A was inhibited by mannan but not by lipopolysacchar ide and K2 capsular polysaccharide. SP-A-treated macrophages bound inc reased numbers of parent K21a strain and recombinant strains of K21a c apsule type but considerably less parent K2 strain. SP-A also enhanced killing of K21a strains by macrophages. The enhanced binding of K21a by macrophages pretreated with SP-A was inhibited by mannan, suggestin g that binding is mediated by the mannose receptor on macrophages. We conclude that SP-A increases phagocytosis of the Klebsiella by two mec hanisms, one of which is by serving as an opsonin, which binds to the capsular polysaccharides of the bacteria and potentially to SP-A recep tors on the macrophages, and the other by activating the macrophages, resulting in increased activity of the mannose receptor.