Receptors on phagocytic cells involved in microbial recognition

There are two general concepts that we hope to have stressed concerning the recognition of microbes by phagocytic cells. The first is the concept of receptor redundancy and receptor cooperatively. Multiple receptors on leukocytes often participate in a given microbial recognition event. This concept can be illustrated by leishmania promastigotes that utilize both mannose receptors and Mac-1 to bind efficiently to macrophages. Likewise, macrophages use receptors for both IgG and complement to phagocytize encapsulated bacteria. This cooperativity between multiple receptors often changes the apparent affinity of the receptors for their ligand. Consequently the physiology of these receptors is altered. Fibronectin ligation, for example, results in the internalization of C3b-coated particles by the CR1. The second concept concerns the transduction of specific cellular signals following receptor ligation. Often, the receptor to which a microbe binds orchestrates many of the subsequent intracellular events during phagocytosis by transducing specific cellular signals. Some receptors, for example the mannose and Fc gamma-receptors, are particularly well suited to direct particles to phagolysosomes and trigger a respiratory burst, whereas other receptors, for example the CR1, may not. Indeed, from this perspective, one can view the immune response as being designed to target microbes preferentially to those receptors on phagocytic cells capable of making the appropriate cellular responses. In the case of leishmania, phagocytosis mediated by the Fc gamma receptors leads to parasite killing even by resident macrophages, while complement-mediated phagocytosis leads to parasite survival.