Summary Human natural killer (NK) cells form a circulating population in a state of dynamic homeostasis. We investigated NK cell homeostasis by labelling dividing cells in vivo using deuterium‐enriched glucose in young and elderly healthy subjects and patients with viral infection. Following a 24‐hr intravenous infusion of 6,6‐D2‐glucose, CD3– CD16+ NK cells sorted from peripheral blood mononuclear cells (PBMC) by fluorescence‐activated cell sorter (FACS) were analysed for DNA deuterium content by gas chromatography mass spectrometry to yield minimum estimates for proliferation rate (p). In healthy young adults (n = 5), deuterium enrichment was maximal ∼ 10 days after labelling, consistent with postmitotic maturation preceding circulation. The mean (± standard deviation) proliferation rate was 4·3 ± 2·4%/day (equivalent to a doubling time of 16 days) and the total production rate was 15 ± 7·6 × 106 cells/l/day. Labelled cells disappeared from the circulation at a similar rate [6·9 ± 4·0%/day; half‐life (T½) < 10 days]. Healthy elderly subjects (n = 8) had lower proliferation and production rates (P = 2·5 ± 1·0%/day and 7·3 ± 3·7 × 106 cells/l/day, respectively; P = 0·04). Similar rates were seen in patients chronically infected with human T‐cell lymphotropic virus type I (HTLV‐I) (P = 3·2 ± 1·9%/day). In acute infectious mononucleosis (n = 5), NK cell numbers were increased but kinetics were unaffected (P = 2·8 ± 1·0%/day) a mean of 12 days after symptom onset. Human NK cells have a turnover time in blood of about 2 weeks. Proliferation rates appear to fall with ageing, remain unperturbed by chronic HTLV‐I infection and normalize rapidly following acute Epstein–Barr virus infection.

Memory T cells can be divided into central–memory (TCM) and effector–memory (TEM) cells, which differ in their functional properties. Although both subpopulations can persist long term, it is not known whether they are maintained by similar mechanisms. We used in vivo labeling with deuterated glucose to measure the turnover of CD4+ T cells in healthy humans. The CD45R0+CCR7− TEM subpopulation was shown to have a rapid proliferation rate of 4.7% per day compared with 1.5% per day for CD45R0+CCR7+ TCM cells; these values are equivalent to average intermitotic (doubling) times of 15 and 48 d, respectively. In contrast, the CD45RA+CCR7+ naive CD4+ T cell population was found to be much longer lived, being labeled at a rate of only 0.2% per day (corresponding to an intermitotic time of approximately 1 yr). These data indicate that human CD4+ TEM cells constitute a short-lived cell population that requires continuous replenishment in vivo.

The ability to measure, describe and interpret T cell kinetics is pivotal in understanding normal lymphocyte homeostasis and diseases that affect T cell numbers. Following in vivo labeling of dividing cells with 6,6-D 2 -glucose in eight healthy volunteers, peripheral blood T cells were sorted by CD4, CD8 and CD45 phenotype. Enrichment of deuterium in DNA was measured by gas chromatography-mass spectrometry. A novel model of T cell kinetics, allowing for heterogeneity within T cell pools, was used to analyze data on acquisition and loss of label and calculate proliferation and disappearance rates for each subpopulation. CD8+ lymphocytes and CD45RA + CD4 + lymphocytes had slower proliferation rates, 0.5% and 0.6% / day, respectively (doubling time about 4 months). Disappearance rates of labeled cells were similar for all cell types (7%-12% / day) and exceeded corresponding proliferation rates. This disparity may be understood conceptually in terms of either phenotypic heterogeneity (rapid versus slow turnover pools), or history (recently divided cells are more likely to die). The new kinetic model fits the data closely and avoids the need to postulate a large external source of lymphocytes to maintain equilibrium.

Information about the kinetic behavior and lifespan of lymphocytes is crucial to understanding the mechanisms that regulate processes such as immunologic memory. We have used in vivo labeling of dividing cells with 6,6-2 H 2 -glucose, combined with cell sorting and gas-chromatography-mass spectrometry for deuterium enrichment, in order to analyze the kinetics of human total, naive, or memory B lymphocytes, separated from peripheral blood using monoclonal antibodies. We show that total blood B cells of young adults divide at an average rate of 1.9% (؎1.0%) per day and at a similar though slightly slower rate, 1.5% (