Dissemination of Rat Cytomegalovirus through Infected Granulocytes and Monocytes in Vitro and in Vivo

In clinical human cytomegalovirus (HCMV) infections, virus infected granulocytes and monocytes have been implicated in the dissemination of virus through the body. In vitro, human neutrophilic granulocytes and monocytes have been shown to acquire and transfer infectious virus to uninfected cells. The aim of this study was to determine the role of leukocytes in the dissemination of cytomegalovirus in vivo. From 3 days after intraperitoneal injection with cell-free rat CMV (RCMV), RCMV-DNA was detectable in whole blood samples. Moreover, replicating virus could be cultured from leukocytes isolated from these blood samples, indicating that during active infection rats experience a viremia that is comparable to humans with an active HCMV infection. Rat cytomegalovirus (RCMV) could be transferred to rat granulocytes and monocytes by cocultivation with RCMV-infected rat embryonic fibroblasts. In vitro, these phagocytes were able to transfer RCMV to uninfected rat fibroblasts. Electron microscopy and PCR confirmed the presence of virus particles and viral DNA in both granulocytes and monocytes. Subsequently, intravenous injection of purified infected granulocytes or monocytes into irradiated rats resulted in a systemic infection. Infection of the animals proved to be efficient, since virus replication could be demonstrated in spleen and salivary glands already after a single low dose injection of infected phagocytes. We show for the first time in vivo that cytomegalovirus is able to disseminate through infected leukocytes. Based on the in vitro data on transmission of HCMV, we hypothesise that our model is a powerful tool to gain further insight on CMV dissemination and for the development of new antiviral agents.