Human precursor labeling in situ reveals a conveyor belt differentiation trajectory and a 6-day neutrophil life span

The kinetics of neutrophil development in human bone marrow remain largely unclear, despite the large implications for the mechanisms underlying host defense and inflammatory responses. Here, in situ DNA labeling was used to follow human neutrophil precursor maturation in bone marrow and blood. Human bone marrow samples were obtained by aspiration from healthy volunteers after short-term (6 hrs) labeling of dividing cells via oral intake of glucose containing the stable isotope deuterium. Mature and precursor populations were isolated at different times by fluorescence-activated cell sorting, and the deuterium incorporation in DNA was measured by GC/MS. Promyelocytes quickly incorporated deuterium in their DNA, whereas in mature neutrophils deuterium was only found from 6 days after intake. The enrichment curves of myelocytes, metamyelocytes, banded, and mature neutrophils mirrored that of the promyelocytes, shifted in time. This pattern indicated that neutrophil development in the bone marrow is well described with a linear conveyor belt model, i.e. following the "first-in/first-out" principle. In contrast to the current dogma, myelocytes did not divide, placing them in the postmitotic pool. Furthermore, the deuterium enrichment curves suggested that mature neutrophils constantly exchange between the bone marrow, blood, and tissue. Our unique labeling data of all stages of the neutrophil life cycle allowed us to estimate the lifespan of the mature neutrophil, deriving a value of at least 6 days. This study provides new insights into neutrophil development kinetics, informing the design and application of therapeutic strategies targeting granulopoiesis and the recruitment of mature neutrophils.