Nitric oxide is produced by a number of different cell types in response to
cytokine stimulation and thus has been found to play a role in immunologically mediated protection against a growing list of protozoan and helminth parasites in vitro and in animal models. The biochemical basis of its effects on the parasite targets appears to involve primarily inactivation of
enzymes crucial to energy metabolism and growth, although it has other
biologic activities as well. NO is produced not only by macrophages and macrophage-like cells commonly associated with the effector arm of cell-mediated immune reactivity but also by cells commonly considered to lie outside the immunologic network, such as hepatocytes and endothelial cells, which are intimately involved in the life cycle of a number of parasites. NO production is stimulated by
gamma interferon in combination with
tumor necrosis factor alpha or other secondary activation signals and is regulated by a number of
cytokines (especially
interleukin-4,
interleukin-10, and
transforming growth factor beta) and other mediators, as well as through its own inherent inhibitory activity. The potential for design of prevention and/or intervention approaches against
parasitic infection (e.g., vaccination or combination chemo- and
immunotherapy strategies) on the basis of induction of cell-mediated immunity and NO production appears to be great, but the possible pathogenic consequences of overproduction of NO must be taken into account. Moreover, more research on the role and regulation of NO in human
parasitic infection is needed before its possible clinical relevance can be determined.