Growth kinetics of human cytomegalovirus are altered in monocyte-derived macrophages

Stimulation of monocytes/macrophages with activated nonadherent cells allows productive nonlytic growth of human cytomegalovirus (HCMV), but the viral replication cycle is delayed relative to replication of HCMV in human fibroblasts. Analysis of infected monocyte-derived macrophage (MDM) mRNA for major immediate-early (MIE 86, 72, and 55) and late (pp65 and gB) gene expression by reverse transcription PCR indicates that transcription peaks at 3 and 7 days postinfection (dpi), respectively. In contrast, in human fibroblast controls, mRNA for MIE and late gene expression peaked at 5 and 48 h postinfection, respectively. Consistent with reverse transcription PCR experiments, double-label antibody experiments first detected MIE antigen expression at 12 h postinfection, peaking at 3 dpi, and late (pp65 or gB) antigen expression at 5 dpi, peaking at 7 dpi. MIE antigen was not detected between 3 and 7 dpi but reappeared and was coexpressed with pp65 in enlarged MDM nuclei at 7 dpi. After 7 dpi, macrophages with numerous vacuoles containing large amounts of pp65 and gB were observed in culture. These vacuoles were frequently seen at cellular contact points, suggesting that cell-to-cell transfer of virus was the major mode of viral transmission. Consistent with this observation, infectious virus was recovered from MDM cellular lysates but not culture supernatant. The delayed growth and compartmentalization of HCMV in macrophages may allow the cell to accommodate the viral replication cycle without cell lysis. In addition, the macrophage may function as a vehicle for cell-to-cell transmission of HCMV.