Maintenance of peripheral naive T cells is sustained by thymus output in mice but not humans.

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.

Closing the gap between T-cell life span estimates from stable isotope-labeling studies in mice and humans.

Quantitative knowledge of the turnover of different leukocyte populations is a key to our understanding of immune function in health and disease. Much progress has been made thanks to the introduction of stable isotope labeling, the state-of-the-art technique for in vivo quantification of cellular life spans. Yet, even leukocyte life span estimates on the basis of stable isotope labeling can vary up to 10-fold among laboratories. We investigated whether these differences could be the result of variances in the length of the labeling period among studies. To this end, we performed deuterated water-labeling experiments in mice, in which only the length of label administration was varied. The resulting life span estimates were indeed dependent on the length of the labeling period when the data were analyzed using a commonly used single-exponential model. We show that multiexponential models provide the necessary tool to obtain life span estimates that are independent of the length of the labeling period. Use of a multiexponential model enabled us to reduce the gap between human T-cell life span estimates from 2 previously published labeling studies. This provides an important step toward unambiguous understanding of leukocyte turnover in health and disease.

Sparse production but preferential incorporation of recently produced naive T cells in the human peripheral pool.

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)H(2)O labeling in young mice showed high thymic-dependent daily naïve T cell production, long term up- and down-labeling with (2)H(2)O 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.

A critical contribution of both CD28 and ICOS in the adjuvant activity of Neisseria meningitidis H44/76 LPS and lpxL1 LPS.

The development of novel vaccines against Neisseria meningitidis recently gained momentum by the generation of penta-acylated lpxL1 LPS which has similar adjuvant activity, but reduced endotoxic activity as compared to hexa-acylated wild type (H44/76) LPS. We investigated the costimulation requirements for the adjuvant activity of both forms of LPS by immunizing CD28-, ICOS- and B7.1/2/ICOS-deficient mice. Both ICOS and CD28 appeared essential for optimal adjuvant activity of H44/76 LPS or lpxL1 LPS. Interestingly, ICOS-mediated costimulation predominates in the adjuvant activity of lpxL1 LPS, while both ICOS and CD28 are required for H44/76 LPS adjuvant activity.

ICOS contributes to T cell expansion in CTLA-4 deficient mice.

Both CD28 and ICOS are important costimulatory molecules that promote Ag-specific cellular and humoral immune reactions. Whereas CD28 is generally thought to be the most important molecule in the initiation of a T cell response, ICOS is considered to act during the effector phase. We have investigated the contribution of ICOS to T cell responses in the absence of CTLA-4-mediated inhibition. Mice lacking CTLA-4, which show spontaneous CD28-mediated CD4(+) T cell activation, expansion and differentiation, were treated with antagonistic alphaICOS antibodies. Blocking the interaction between ICOS and its ligand B7RP-1 significantly reduced this aberrant T cell activation and caused a reduction in T cell numbers. In vitro analysis of CD4(+) T cells from treated mice revealed that ICOS blockade significantly reduced Th1 differentiation, while Th2 differentiation was only moderately inhibited. Further in vitro stimulation experiments demonstrated that ICOS is able to induce proliferation of murine CD4(+) and CD8(+) T cells but only in the presence of IL-2. These results indicate that ICOS is not only important for T cell effector function but also contributes to the expansion phase of a T cell response in the presence of CD28 signaling.