Abstract
Acquisition of human cytomegalovirus (CMV) usually occurs by contact between contaminated bodily fluids, such as urine and saliva, and host mucosal cells. Langerhans-type dendritic cells (LC) are the only type of immune cells found in the outermost layers of the oral mucosae, where they not only provide a first line of defense against CMV but can easily be targeted by orally administered vaccines, while their bone marrow resident progenitors are important sites of virus latency. In this work, we tracked the progress of infection in CD34(+) progenitor cells, immature LC (iLC), and mature LC (mLC) exposed to the clinical-like strain TB40-BAC4 or to the vaccine strain AD169varATCC, prior to their long-term maintenance under either immature or mature conditions. We show that the genomes of both strains are efficiently maintained in CD34(+) cells during their differentiation into iLC, although this requires the presence of larger amounts of input AD169varATCC DNA. Lipopolysaccharide- and CD40 ligand-induced maturation of iLC derived from latently infected progenitors was not associated with robust viral genome replication and progeny production, while maturation of directly infected iLC increased and prolonged expression of the viral immediate early proteins. While effective replication of viral genomes from both strains occurred only in mLC, both iLC and mLC produced viral progeny, suggesting that both types of LC may contribute to CMV horizontal transmission in vivo. Importance: Human CMV is usually acquired via the oral and nasal mucosae. Langerhans-type dendritic cells (LC) are the only type of immune cells found in the outermost layers of these tissues. Understanding how CMV interacts with LC and their hematopoietic progenitors is thus essential to develop innovative means of defense against this virus. Here we show that the genomes of a virulent and an attenuated strain of CMV are maintained in hematopoietic progenitor cells during their differentiation into immature LC and that maturation of these cells by exposure to lipopolysaccharide and CD40 ligand is not sufficient to trigger virus reactivation. While the extents of viral protein expression and genome replication were broadest in directly infected mature LC populations, similar amounts of viral progeny were detected in the supernatants of immature and mature LC, suggesting that these immune cells of the oral mucosa are likely to be important for CMV transmission within the human population.