Mechanisms of survival of protozoan parasites in mononuclear phagocytes

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Abstract
SUMMARY: The understanding of the mechanisms whereby intracellular parasites counteract the microbicidal processes of macrophages has progressed considerably in recent years. Various factors contribute to intracellular parasite destruction; from a biochemical standpoint, particularly important is the oxidative burst triggered by phagocytosis and by macrophage ‘activation’, that leads to the generation of toxic metabolites of oxygen. At the ultrastructural level, fusion of the parasitophorous vacuole with surrounding lysosomes appears to be a pre-requisite for the final digestion and elimination of the infecting microorganisms. The counter-measures evolved by microorganisms to escape intracellular destruction are best illustrated by studiesin vitroon the interaction of parasites of theLeishmania, ToxoplasmaandTrypanosomaspp. with mononuclear phagocytes. Some microbes are able to inhibit the fusion of phagosomes with lysosomes, thus avoiding the potentially harmful action of lysosomal hydrolases. Other microorganisms are able to resist the effects of such enzymes, perhaps by secreting inhibitory substances. Others still avoid lysosomes by leaving the phagocytic vacuole, to reach the cytoplasmic matrix where their development is unhindered. Particularly critical is the capacity of certain parasites to subvert the lethal effects of the oxidative burst. This can be achieved either by failing to evoke this metabolic response, or by producing scavengers that can detoxify harmful oxygen metabolites. Intracellular death or survival will thus depend on a delicate balance between the potency of macrophage cidal mechanisms, and the efficacy of the protective measures evolved by the infecting agents.