Modification of human cytomegalovirus tropism through propagation in vitrois associated with changes in the viral genome

Following extensive propagation in fibroblasts, human cytomegalovirus (HCMV ) loses tropism for a number of otherwise natural host cells, in particular , endothelial cells. In this study, the hypothesis was tested that loss of endothelial tropism is associated with the appearance of genomic variants. Initial quantitative focus expansion assays on endothelial monolayers demon strated that, while the laboratory strains AD 169 and Towne failed to form detectable foci, 29 out of 30 recent clinical HCMV isolates had the potenti al to expand in endothelial cell culture. By long-term adaptation in fibrob last cultures, nonendotheliotropic strains could be selected from clinical HCMV isolates, while long-term endothelial-adapted strains of the same isol ates retained both fibroblast tropism and endothelial tropism, Such differe ntially adapted isolate pairs always displayed genomic differences in restr iction fragment length analyses. Coinfection of endothelial cells by two no nendotheliotropic HCMV strains yielded an endotheliotropic recombinant HCMV variant combining portions of the genomes of both parental viruses. When D NA purified from various isolates was transfected into fibroblasts, progeny virus retained the specific tropism of parental virus from which the DNA w as isolated. These findings demonstrate that endothelial tropism is an inhe rent property of most clinical HCMV isolates and is determined by the viral genome, Although the specific determinants of HCMV cell tropism are still unknown, this study provides the first evidence for a genetic contribution.