NONOPSONIC PHAGOCYTOSIS OF MICROORGANISMS

Nonopsonic phagocytosis mediated by phagocyte receptors that recognize corresponding adhesins on microbial surfaces has attracted increasing interest as a potential host defense mechanism against extracellular pathogens and as a means of survival in the host for intracellular pat hogens. Three types of nonopsonic phagocytosis involving carbohydrate- protein interactions (also termed lectinophagocytosis), protein-protei n interactions, and hydrophobic interactions are discussed. A prominen t receptor on phagocytic cells involved in recognizing pathogens belon gs to the CD11/CD18 integrins. It mediates both opsonophagocytosis and nonopsonic phagocytosis and exhibits multiple specificity for differe nt microbial adhesins. In other cases, similar specificity toward a mi crobial ligand (e.g. the Klebsiella pneumoniae capsule) is shared by d ual molecules, one of which (e.g. the mannose-binding protein in serum ) mediates opsonophagocytosis and the other (e.g. the macrophage manno se receptor) mediates nonopsonic phagocytosis of the microorganisms. I n addition, we discuss how nonopsonic phagocytosis can trigger the pha gocytes to release inflammatory agents and cause tissue injury. Furthe r studies of the molecular mechanisms of nonopsonic phagocytosis, in p articular those underlying the up-regulation of the phagocytic recepto rs by various agents, should lead to the development of new approaches for the prevention of infectious diseases.