Monocytes and macrophages

By secreting antiviral cytokines such as 1FN and TNFa and by serving as antigen-presenting accessory cells which stimulate specific immune responses, monocytes and macrophages play fundamental roles in resistance to viruses. Intravenous injection of mice with radiolabeled virus results in a rapid clearance of the label from the bloodstream and concentration by the liver Kupffer cell macrophage population. The ability of viruses to replicate in monocytes, macrophages, and the more differentiated macrophage-like histiocytes such as Kupffer cells is thought to be an important virulence factor. This concept is supported by the fact that nearly all persistent virus infections involve extensive infections of macrophages.

Many viruses replicate in macrophages, but few viruses have macrophage-specific tropism or bind to macrophage-specific receptors. Lactic dehydrogenase (LDH) virus, which causes a persistent infection in mice, has a predilection for infecting macrophages expressing major histocompatibility complex (MHC) class II antigens and may use these molecules as receptors. Some macrophages express CD4, which leave them permissive to HIV. Some viruses enter macrophages by an aberration of opsonization known as 'immune enhancement'; macrophages have high-affinity receptors for complement and for the antibody Fc region, which facilitate the adsorption, penetration and productive infection by some antibody- and/or complement-coated viruses, such as dengue virus and HIV. More conventionally, antibody-coated viruses are suitably disposed of. Infection of macrophages by vaccinia virus normally results in entry of virus into the cytoplasm, followed by productive replication; antibody-coated vaccinia virus suffers a different fate, as it is engulfed in a phagolysosome and degraded.

Viral infections of macrophages may be abortive, productive, latent and/or transforming, and the outcome of infection commonly depends on the macrophage activation state. Many viruses, such as herpes simplex virus (HSV) and influenza, grow poorly in activated macrophages, but others, such as murine CMV or LDH virus may grow better in activated macrophages. The reasons for these responses vary with the virus, but abortive infection in activated macrophages may be due in part to the lack of requisite host cell factors not found in nonrep-licating, poorly differentiated cells. The ability of viruses to grow in macrophages is extensively influenced by cytokines. Genetically determined resistance of mouse macrophages to influenza virus has been shown to be due to an IFN-induced Mx gent-product, which is a protein essential for resistance to influenza virus but not to VSV. IFNy, produced by NK cells or T cells, has been shown to induce nitric oxide synthase in macrophages, and the liberated nitric oxide inhibits the replication of several viruses, including HSV-1, vaccinia virus, murine CMV, and VSV.

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