In humans, the monocyte pool comprises three subsets (classical, intermediate, and nonclassical) that circulate in dynamic equilibrium. The kinetics underlying their generation, differentiation, and disappearance are critical to understanding both steady-state homeostasis and inflammatory responses. Here, using human in vivo deuterium labeling, we demonstrate that classical monocytes emerge first from marrow, after a postmitotic interval of 1.6 d, and circulate for a day. Subsequent labeling of intermediate and nonclassical monocytes is consistent with a model of sequential transition. Intermediate and nonclassical monocytes have longer circulating lifespans (∼4 and ∼7 d, respectively). In a human experimental endotoxemia model, a transient but profound monocytopenia was observed; restoration of circulating monocytes was achieved by the early release of classical monocytes from bone marrow. The sequence of repopulation recapitulated the order of maturation in healthy homeostasis. This developmental relationship between monocyte subsets was verified by fate mapping grafted human classical monocytes into humanized mice, which were able to differentiate sequentially into intermediate and nonclassical cells.

While memory T cells are maintained by continuous turnover, it is not clear how human regulatory CD4⁺CD45RO⁺CD25^hi Foxp3⁺ T lymphocyte populations persist throughout life. We therefore used deuterium labeling of cycling cells in vivo to determine whether these cells could be replenished by proliferation. We found that CD4⁺CD45RO⁺Foxp3⁺CD25^hi T lymphocytes were highly proliferative, with a doubling time of 8 days, compared with memory CD4⁺CD45RO⁺Foxp3^–CD25^– (24 days) or naive CD4⁺CD45RA⁺Foxp3^–CD25^– populations (199 days). However, the regulatory population was susceptible to apoptosis and had critically short telomeres and low telomerase activity. It was therefore unlikely to be self regenerating. These data are consistent with continuous production from another population source. We found extremely close TCR clonal homology between regulatory and memory CD4⁺ T cells. Furthermore, antigen-related expansions within certain TCR Vβ families were associated with parallel numerical increases of CD4⁺CD45RO⁺CD25^hiFoxp3⁺ Tregs with the same Vβ usage. It is therefore unlikely that all human CD4⁺CD25⁺Foxp3⁺ Tregs are generated as a separate functional lineage in the thymus. Instead, our data suggest that a proportion of this regulatory population is generated from rapidly dividing, highly differentiated memory CD4⁺ T cells; this has considerable implications for the therapeutic manipulation of these cells in vivo.

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 (T_½) < 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.

T cell activation upon TCR signaling can lead to development of effector and memory cells. Roychoudhuri and colleagues show that the transcription factor BACH2 promotes memory CD8+ T cell generation by blocking access to genomic regulatory sites recognized by AP-1. T cell antigen receptor (TCR) signaling drives distinct responses depending on the differentiation state and context of CD8+ T cells. We hypothesized that access of signal-dependent transcription factors (TFs) to enhancers is dynamically regulated to shape transcriptional responses to TCR signaling. We found that the TF BACH2 restrains terminal differentiation to enable generation of long-lived memory cells and protective immunity after viral infection. BACH2 was recruited to enhancers, where it limited expression of TCR-driven genes by attenuating the availability of activator protein-1 (AP-1) sites to Jun family signal-dependent TFs. In naive cells, this prevented TCR-driven induction of genes associated with terminal differentiation. Upon effector differentiation, reduced expression of BACH2 and its phosphorylation enabled unrestrained induction of TCR-driven effector programs.

Increased expenditure of energy at rest has been considered a contributing factor to the negative energy balance and weight loss that occur in patients with human immunodeficiency virus (HIV) infection. However, the true determinant of energy balance is not resting but total energy expenditure. We sought to determine the contribution of total energy expenditure to weight changes in patients with HIV-associated wasting. We performed 51 assessments of energy metabolism in 27 men with HIV infection at different stages of disease, including periods of both rapid and slow weight loss. Resting energy expenditure was measured by indirect calorimetry, total energy expenditure by the doubly-labeled-water technique, and energy intake by recording the weight of food consumed. The results were compared with the rate of weight loss or gain. The mean (±SD) total energy expended by the HIV-infected men was 2750 ±670 kcal per day, no more than that expended by normal men. There was a significant positive relation between total energy expenditure and the rate of weight change (r = 0.61, P<0.001); thus, during rapid weight loss, total energy expenditure was reduced to 2180 ±580 kcal per day (P = 0.009), primarily because of reduced physical activity. During rapid weight loss, the negative energy balance (-850 ±580 kcal per day) was primarily the result of the reduction in energy intake, to 1330 ±610 kcal per day; intake correlated strongly with the rate of weight change (r = 0.84, P<0.001). In patients with HIV infection, total energy expenditure is reduced during episodes of weight loss. Reduced energy intake, not elevated energy expenditure, is the prime determinant of weight loss in HIV-associated wasting.