PERIPHERAL-BLOOD MONONUCLEAR PHAGOCYTES MEDIATE DISSEMINATION OF MURINE CYTOMEGALOVIRUS

Cytomegalovirus is transmitted with blood and organs from seropositive individuals, although the particular leukocyte population harboring l atent or persistent virus remains poorly characterized. Murine cytomeg alovirus, tagged with the Escherichia coli lacZ gene, was used to iden tify cells in which virus replicates during acute infection of immunoc ompetent mice. Recombinant murine cytomegaloviruses, RM461, RM460, and RM427, were constructed to express beta-galactosidase under control o f the human cytomegalovirus ie1/ie2 promoter/ enhancer. The lacZ gene was inserted between the ie2 and sgg1 genes in RM461 and RM460, disrup ting a 0.85-kb late transcript that was found to be dispensable for re plication in cultured cells as well as for infection of mice. In BALB/ c mice, lacZ-tagged and wild-type viruses exhibited a similar 50% leth al dose and all had the capacity to latently infect the spleen. Periph eral blood mononuclear phagocytes were the major infected leukocyte ce ll type, as demonstrated by the ability of infected cells to adhere to glass and to phagocytize latex beads; however, these cells did not ex hibit typical monocyte markers. Plaque assay for virus and -bromo-4-ch loro-3-indolyl-beta-D-galactopyranoside (X-Gal) staining of frozen sec tions of organs from infected mice revealed that the major target orga ns included the spleen, adrenal glands, liver, and salivary glands, al though tissues as diverse as brown fat and lungs were also involved. I ndividual blue-staining cells were readily identified in all infected tissues. These studies identified a mononuclear phagocyte, possibly a macrophage or dendritic cell precursor, as the vehicle of virus dissem ination during acute infection, and demonstrate the utility of using l acZ-tagged murine cytomegalovirus for tropism, pathogenesis, and laten cy studies.