Percentage of BrdU-labeled monocytes at 24 hours after BrdU administration as a reliable marker of monocyte turnover. (A) Kinetics of BrdU-labeled monocytes of 4 rhesus macaques after BrdU injection. Blood monocytes were defined as CD14⁺ cells after gating through CD45 and side scatter plots. Any T lymphocytes, B cells, and NK cells were further excluded by gating on CD3⁻CD8⁻CD20⁻HLA-DR⁺CD45⁺ population. Data shown is the percentage of BrdU⁺ monocytes at indicated time points after BrdU injection. Note the inverse relation in the rank order of the percentage of BrdU labeling at 1 day versus 6 days. (B) Percentage of BrdU-labeled monocytes observed at 24 hours after BrdU administration inversely correlates with the time those cells spend in circulation. In other words, animals with the greatest percentage of CD14⁺BrdU⁺ cells at 1 day have the lowest percentage of CD14⁺ cells by 6 days after labeling. Data show the kinetics of BrdU-labeled monocytes from animal CA50 (red) and N195 (green) that exhibit a high and low percentage of labeled monocytes 24 hours after BrdU administration, respectively. The dotted line highlights the differences in the dynamic behavior of the 2 representative animals within 24 hours of BrdU injection that correlate with the decline of the BrdU⁺ monocytes in circulation.