HCMV-induced factor alters monocyte differentiation. Purified human primary monocytes were incubated with the day 2 virus-free CMV CM (picture on the left) or the mock CM (picture on the right) (see legend to Fig 3). The percentage of oxidative activity inhibition (mean ± SD) are shown. The scale is shown by the bar which is equal to 80 μm. These results are representative of two independent experiments. After 2 days of culture, approximatly 20% of the monocytes incubated with the mock CM appeared to be slightly bigger in size and some of them had started to spread. However, monocytes incubated with the CMV CM remain small, rounded with prominent membrane ridges. At this time, no significant difference in monocyte oxidative activity is detected (see Fig 3). After 4 days, morphologic changes were obvious, with the majority of the monocytes flattened and spread, increased greatly in size, and showing a higher cytoplasm to nucleus ratio. In contrast, cells treated with the CMV CM maintained the phenotype of undifferentiated monocytes. In correlation, the oxidative activity of mock CM treated cells dramatically increased from 0.272 ± 0.036 to 0.698 ± 0.011, whereas it remained low, 0.301 ± 0.019, in CMV CM–treated monocytes (Fig 3). After 6 days, the cell size increased and the cell spreading was more pronounced. The cytoplasm to nucleus ratio was significantly augmented and membrane ruffles and numerous cytoplasmic granules and lipid inclusions were observed. In parallel, the oxidative activity of mock CM treated cells increased to 0.815 ± 0.022 but remained low, 0.199 ± 0.002, in CMV CM–treated monocytes (Fig 3). After 8 days, mock CM–treated monocytes presented similar morphologic characteristics as day 6 with increasing cell size and increasing oxidative activity (Fig 3), whereas these parameters remained unchanged in CMV CM–treated monocytes.