Analysis of neutrophil turnover by in vivo ²H₂O labeling. Cross-sectional up- and down-labeling of murine (A) BM PMP neutrophils and (B) blood neutrophils. (C) Representative example of longitudinal up- and down-labeling of human blood neutrophils; parameter values of 5 volunteers are given in Table 1. Dashed vertical lines indicate the time of label cessation on day 7 (A-B) or day 63 (C). Curves were fitted as described in “Methods,” taking into account the actual level of label enrichment of plasma in mice or urine in humans (supplemental Figure 1). (D) Estimated median neutrophil lifespans and transit times (in days) of mice and humans. Murine estimates for transit times in the BM could be calculated directly; from the BM labeling data we calculated that labeled cells entered the PMP with a delay of Δ = 0.7 days and that they had an expected lifespan in the PMP of 1/d = 1.6 days (see “Mathematical modeling”). The total transit time in the BM of 0.7 + 1.6 days matches the estimated Δ = 2.3 days that resulted from analysis of the murine blood-labeling data. BM transit times in humans were estimated from the delay Δ with which labeled neutrophils were observed in the blood, whereas the circulating lifespan was calculated from 1/d.