| Summary: | HCMV infection is usually asymptomatic in immunocompetent hosts, but serious disease can occur in immunocompromised individuals and in congenitally infected newborns. The importance of virus fitness determinations became evident in 1976, when it was proposed that the infecting strain of HCMV is important for clinical outcome, along with the intensity and duration of viral replication. HCMV strains exhibit different levels of virulence in vivo, depending on their passage history in cell culture. High and low passage HCMV strains exhibit tropism differences in vitro, suggesting that different tissue tropism may occur in vivo. In addition, approximately 13kb of novel DNA sequences located near the right edge of the unique long component of the genome has been identified in Toledo and clinical strains. This region (UL/b') encodes several open reading frames, which are missing from the high passage laboratory-adapted variants of Towne and AD 169 and are thought to play important roles in pathogenesis. One of the aims of this thesis was to determine the replication dynamics of different HCMV strains in vitro as well as compare their ability to bind to cells and mediate cell-to-cell spread of infection using pair-wise competition experiments in cell culture. AD 169 was shown to replicate better than Toledo in fibroblasts. Furthermore, assessment of the replication of Toledo in a different cell type, HUVEC, indicated that the virus replicated to higher levels in fibroblasts. Towne was found to bind to HEL cells with higher affinity compared to AD 169. The results showed phenotypic differences between high and low passaged HCMV variants and also illustrated that fitness differences between them are variable and highly dependent upon the status of the virus inoculum. To begin to understand the complex relationship between tissue tropism, virulence and HCMV genome composition, a DNA microarray approach was developed to examine host and HCMV gene expression during the productive infection of two distinct cell lines, fibroblasts and endothelial cells. The results showed that genes wifrojn tye fPSIPp were expressed in a cell type-specific fashion. Ip the context of host cell gene expression, cell type-specific host gene transcriptional changes were observed, reflecting different viral modulation of distinct cell type environments. The results provided potential insight into the function of genes encoded in the UL/b' region. Of particular note was that transcriptional changes frequently occurred in genes associated with pathways involved in the pathology of HCMV in the human host.
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