IntroductionNeutrophils are indispensable for host defense. 1 In addition, these cells play a detrimental role in the pathogenesis of many acute and chronic inflammatory diseases. They can cause tissue damage through aspecific activation of their repertoire of antimicrobial mechanisms. Neutrophils also inform and shape subsequent immunity 2 and can prolong inflammation by release of cytokines 3 and chemokines. 4 There is an emerging concept that neutrophils directly influence adaptive immune responses through pathogen shuttling to draining lymph nodes, 5,6 antigen presentation, 7 and modulation of T helper 1/T helper 2 responses. 8 Along this line, neutrophils have been reported to be an important component of myeloid-derived suppressor cells mediating lymphocyte suppression in various experimental models of acute 9 and chronic inflammation. 10 Targeting neutrophils in disease has mainly been focused on limiting their damaging capacity or directing their cytotoxic machinery to tumors. 11 Their immune modulatory functions have received little attention as potential targets in inflammatory diseases. This may at least in part be due to the current paradigm that these functions are of limited importance because of the generally accepted short circulatory half-life of neutrophils. Neutrophil lifespans have mainly been assessed by determination of ex vivo lifespans in culture (Ͻ 24 hours) and by transfer studies of ex vivo-manipulated neutrophils. The latter studies showed an estimated circulating half-life of approximately 8 hours in humans. 12 Ex vivo manipulation has been shown to have dramatic effects on neutrophil redistribution in vivo. 13 In mice, half-lives of 8 to 10 hours were reported when neutrophils were labeled in vivo. 14 In contrast, ex vivo labeling in mice showed that after transfer 90% of labeled neutrophils were cleared from the circulation within 4 hours, resulting in a half-life of less than 1.5 hours. 15 These differences between in vivo and ex vivo labeling strengthen our hypothesis that neutrophil transfer experiments may lead to an underestimation of neutrophil lifespan. The activation during ex vivo manipulation has probably led to retention in the lungs, 16 liver, spleen, and bone marrow (BM), 15 which may drastically reduce their circulatory half-life. To circumvent the complications introduced by ex vivo manipulation, we labeled the neutrophil pool in vivo in healthy mice and humans by administration of 2 H 2 O in drinking water. Acquisition of label and appearance of labeled neutrophils in the circulation is characterized by (1) the rate of division in the mitotic pool (MP) in the BM, (2) the transit time of newly formed neutrophils through the postmitotic pool (PMP) in the BM, and (3) the delay in mobilization of neutrophils from the PMP to the blood. With the use of a combination of gas chromatography and mass spectrometry the fraction of 2 H-labeled adenosine in the DNA of the proliferating neutrophil pool was measured, and the kinetics of the neutrophil pool was determined. Study des...

Parallels between T cell kinetics in mice and men have fueled the idea that a young mouse is a good model system for a young human, and an old mouse, for an elderly human. By combining in vivo kinetic labeling using deuterated water, thymectomy experiments, analysis of T cell receptor excision circles and CD31 expression, and mathematical modeling, we have quantified the contribution of thymus output and peripheral naive T cell division to the maintenance of T cells in mice and men. Aging affected naive T cell maintenance fundamentally differently in mice and men. Whereas the naive T cell pool in mice was almost exclusively sustained by thymus output throughout their lifetime, the maintenance of the adult human naive T cell pool occurred almost exclusively through peripheral T cell division. These findings put constraints on the extrapolation of insights into T cell dynamics from mouse to man and vice versa.

In mice, recent thymic emigrants (RTEs) make up a large part of the naïve T cell pool and have been suggested to be a distinct short-lived pool. In humans, however, the life span and number of RTEs are unknown. Although 2 H2O labeling in young mice showed high thymic-dependent daily naïve T cell production, long term upand down-labeling with 2 H2O in human adults revealed a low daily production of naïve T cells. Using mathematical modeling, we estimated human naïve CD4 and CD8 T cell half-lives of 4.2 and 6.5 years, respectively, whereas memory CD4 and CD8 T cells had half-lives of 0.4 and 0.7 year. The estimated half-life of recently produced naïve T cells was much longer than these average half-lives. Thus, our data are incompatible with a substantial short-lived RTE population in human adults and suggest that the few naïve T cells that are newly produced are preferentially incorporated in the peripheral pool. recent thymic emigrants ͉ T cell half-lives ͉ T cell production T he role of the thymus in HIV infection is still poorly understood (1, 2). On the one hand, thymic failure has been suggested to play a crucial role in CD4 T cell loss during HIV infection (3), and rapid thymic rebound has been proposed to be responsible for T cell reconstitution during anti-viral treatment (4). However, it has been argued that thymic output in adults might be too low to have a large impact on CD4 T cell depletion (5). In general, these issues are addressed with estimates of thymic output, naïve and memory T cell production rates, and life spans that are simply extrapolated from observations in mice, monkeys, and lymphopenic or irradiated humans (6-11).Naïve T cells are generally thought to turnover relatively slowly, but it has been suggested that, in mice, a considerable part of the naïve T cell pool consists of RTEs with relatively rapid turnover (9, 10, 12). In humans, naïve T cell numbers, T cell receptor excision circles (TRECs), and expression of CD31 have been used to measure thymic output (7,13,14). Dion et al. (4) observed rapid changes in the Sj/V␤ TREC ratio within 3 months after infection with HIV, which suggested the presence of a rapidly turning over RTE pool in human adults containing most of the TRECs in the periphery, similar to young rodents and chickens (15, 16). However, because TRECs are long-lived, none of these approaches is specific for T cells that have recently emigrated from the thymus (1, 2, 5), and, therefore, they fail to quantify thymic output in humans.Peripheral T cell proliferation might also contribute to the maintenance of the naïve T cell pool in human adults; however, it is unclear which fraction of these cells remains in the naïve T cell pool (17). The contribution of RTEs and peripheral T cell proliferation to the maintenance of the naïve T cell pool can only be determined by studying the fate of newly produced T cells. In vivo labeling with stable isotopes in combination with appropriate mathematical analysis of these data provides a way to obtain T cell decay and production rates ...