Infection of phagocytic cells

Abstract

Phagocytosis represents an important effector mechanism for the eradication of infectieus agents. It is performed prhnarily by specialized cells of two different lineages, namely polymorphonuclear neutrophilic granulocytes (PNG) and mononuclear phagocytes (MP). Because of their major function, PNG and MP are often referred to as ‘professional phagocytes’ to distinguish them from most other host cells which are called ‘non-professional phagocytes’. Phagocytosis alone, of course, cannot eradicate invading microbes; it must be followed by intracellular killing mechanisms. To fulhl these functions, PNG and MP have specihc receptors for microorganisms, and spe&ized organelles and molecules for killIng and degrading them. Furthermore, they must respond to specihc signals which Indicate the presence of infectieus agents and also collaborate with the elements of spectic im munity. PNG interact with antibodies and complement alone, whIle MP are also controlled by T lymphocytes. At Iirst glance, it may appear that host cells specialized to engulf, kill and degrade invading microbes would represent a rather unattractive target for pathogens. However, a variety of bacterial, fungal and protozoan pathogens have chosen exactly this cell type as their preferred habitat (see Table 1). Furthermore, MP allow the replication of viruses from many families and genera, which, apart from their common ability to infect MP, are quite different in their host-cell tropisms (sec Table 2). This indicates that MP provide viruses with a mode of entry which circumvent5 the constraints of virus-type speciIic interaction with host target cells. Indeed, the exquisite capacity of professional phagocytes to actively engulf microbial pathogens represents a major Achilles’ heel, as it also enables even those pathogens that lack special adhesion and invasion factors to enter the host cell. Furthermore, professional 1 phagocytes are highly mobile cells and hence provide the pathogen with a safe vehicle for dissemination. A major differente between PNG and MP is their lifespan, PNG live for less than a day and MP often last for many weeks. Perhaps because of this, MP are far more often abused than PNG. In this brief review, we w-ill discuss microbial strategies for entry into professional phagocytes and for evasion of intracellular defence mechanisms.