The virus

Cytomegalovirus is a herpesvirus and shares physical features with other members of this family, including a linear, double-stranded DNA genome packaged in an icosahedral nucleopsid which is surrounded by tegument enveloped in amorphous lipid bilayer derived from the host cell endoplasmic reticulum-Golgi complex. CMV is large, around 200 nm in diameter, with roughly 230 kbp of DNA that encode upwards of 150 gene products. In human fibroblast tissue culture CMV grows slowly, producing foci of enlarged, rounded, refractile cells. Although there are many cytomegaloviruses, they are species-specific; human CMV infects only humans, and grows exclusively in human cells in the laboratory. In its human host, CMV infects a variety of cell types (hematopoietic, endothelial, epithelial, stromal) in multiple organs, including salivary gland, gastrointestinal tract, kidney, liver, spleen, lung, adrenal gland, reproductive organs, vasculature, brain, inner ear, and eye. An important biologic feature of CMV is its ability to establish latency and persist indefinitely in the host. Hematopoietic progenitor cells, monocyte-derived macrophages, and dendritic cells appear to play a key role in harboring CMV genome in the latent state. Reactivation of CMV from latency is stimulated by activation and differentiation of these cells. Another key feature of human CMV is the large number of viral genes that code for proteins, which have the potential to interact with host inflammatory responses or interfere with host innate and adaptive immune responses. These include gene products which interfere with host cell apoptosis, mimic host cytokines or their receptors, interfere with innate immune mechanisms, and evade host T-cell responses by interfering with antigen processing in the context of the major histocompatibility complex. The complex biology of CMV has been recently reviewed (Mocarski et al., 2007). The large number of human CMV genes aimed at host cell functions is indicative of a virus that is remarkably well adapted to its host.

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