Bacterial inhibition of phagocytosis.
The concerted study of molecular mechanisms of phagocytosis and the inhibition of phagocytosis by specific products of extracellular bacterial pathogens has borne considerable fruit. The importance of tyrosine phosphorylation and of the Rho family of GTPases has become clear to cell biologists, but pathogenic bacteria recognized the importance of these signalling pathways in phagocytic cells long ago. The discoveries described in this review are only the beginning. The simultaneous pursuit of the mechanisms and molecules involved in the initiation and regulation of phagocytosis and that pathogenic bacteria use to inhibit phagocytosis will surely identify more interesting pathways on each side of the contest. Are there any obvious possibilities? There are several bacterial factors that have the potential to inhibit known mechanisms of phagocytosis. Clostridium species, for example, make a number of exotoxins of interest. Clostridium botulinum and Clostridium tetani neurotoxins inactivate the regulated secretory machinery by proteolytic cleavage of SNARE proteins, and targets of tetanus toxin and botulinum b toxin inhibit the exocytotic delivery of membrane vesicles needed for phagocytosis of large particles (Hackam et al., 1998). Moreover, the C3 exotoxin of C. botulinum catalyses ADP ribosylation and inactivation of rho family GTPases (Wiegers et al., 1991), and toxins A and B of C. difficile UDP-glucosylate and inactivate rho GTPases and thereby disrupt the actin cytoskeleton (Just et al., 1995a,b). However, as Clostridia lack the machinery for type III secretion, these proteins are not rapidly targeted to the phagocyte cytoplasm. More searching may reveal a pathogen that has combined the type III secretory machinery with clostridia toxin-like substrates. A potentially unique strategy for remaining outside phagocytes is exhibited by Helicobacter pylori, which contain a type IV secretion system. Unopsonized virulent strains of H. pylori bind readily to macrophages but are only internalized after a delay of several minutes. Such a delay appears to be sufficient for the bacteria to remain extracellular (Allen et al., 2000). Elucidation of the mechanism used by H. pylori to delay phagocytosis may reveal one or more novel virulence factors as well as one or more novel targets in the phagocyte that will add to the understanding of a fundamental process in host defence. Another field ripe for further mechanistic investigation is complement receptor-mediated phagocytosis. Dedicated study of the molecular events and molecular mediators of phagocytosis downstream of CR3 is likely to reveal interesting differences from FcgammaR phagocytosis and is just as likely to reveal that microbes have discovered unique mechanisms for circumventing them. Study of extracellular pathogens and the mechanisms that they use to remain outside phagocytic cells has revealed a great deal about the initial encounter between pathogen and phagocyte. We can look forward to additional discoveries about the host-pathogen interactions and the mechanisms and factors that each side uses to battle against the other.