A novel method to quantify the turnover and release of monocytes from the bone marrow using the thymidine analog 5′-bromo-2′-deoxyuridine

The present study was designed to develop methods to study the production and release of monocytes from the bone marrow using the thymidine analog 5'-bromo-2'-deoxyuridine (BrdU). Dividing monocytes in bone marrow were labeled with BrdU (MOBrdU), and their release into the blood and disappearance from the circulation were monitored using a double immunostaining method. The first MOBrdU appeared in the circulation 4 h after labeling with BrdU and peaked at 18 h when 34.3 +/- 5.8% of monocytes were labeled. The calculated transit time of monocytes through bone marrow was 38.1 +/- 3.1 h in control rabbits with a half-life (T1/2) of 12.7 h. Instillation of Streptococcus pneumoniae into the lung accelerated the release of monocytes from bone marrow (peak at 10 h) and shortened their bone marrow transit time (27.1 +/- 1.8 vs. 22.6 +/- 0.6, vehicle vs. pneumonia; P < 0.05). We conclude that this nonradioisotope method provides a novel way to monitor monocyte kinetics and confirmed previous reports that a focal pneumonia shortens monocyte marrow transit and increases their release into the circulation.