Comprehensive Assessment and Mathematical Modeling of T Cell Population Dynamics and Homeostasis 1 Ve ´ronique Thomas-Vaslin, 2,3 * † Hester Korthals Altes, 3,4‡ Rob J. de Boer, 5§ and David Klatzmann 5 * † Our current view of T cell differentiation and population dynamics is assembled from pieces of data obtained from separate experimental systems and is thus patchy. We reassessed homeostasis and dynamics of T cells 1) by generating a mathematical model describing the spatiotemporal features of T cell differentiation, and 2) by fitting this model to experimental data generated by disturbing T cell differentiation through transient depletion of dividing T cells in mice. This specific depletion was obtained by administration of ganciclovir to mice expressing the conditional thymidine kinase suicide gene in T cells. With this experimental approach, we could derive quantitative parameters describing the cell fluxes, residence times, and rates of import, export, pro- liferation, and death across cell compartments for thymocytes and recent thymic emigrants (RTEs). Among other parameters, we show that 93% of thymocytes produced before single-positive stages are eliminated through the selection process. Then, a post- selection peripheral expansion of naive T cells contributes three times more to naive T cell production than the thymus, with half of the naive T cells consisting of dividing RTEs. Altogether, this work provides a quantitative population dynamical framework of thymocyte development, RTEs, and naive T cells. The Journal of Immunology, 2008, 180: 2240 –2250. T he high turnover, migration, and recirculation of lym- phoid cell populations are key features of a resilient im- mune system regulated by homeostasis (1). The immune system efficacy is guaranteed both by the diverse repertoire of the naive cells (2) and by the rapid memory response of previously selected and expanded Ag-experienced T cells (3). Because im- mune responses are associated with major expansions of specific T cells followed by T cell death, homeostatic regulation is essential to maintain the equilibrium between cell production and cell death, and between naive and effector/memory T cells, as well as to pre- serve repertoire diversity. Elements of the dynamics of peripheral T lymphocyte popula- tions and their precursors in the thymus have been studied using various experimental approaches in rodents. These studies were based on cellular DNA labeling (4 – 8), targeted expression of MHC molecules (9, 10), or the tracking of migration of labeled recent thymic emigrants (RTE) 6 (11–13). They also involved the artificial increase (14) or decrease (15) of thymic output, transfer of isolated cell populations (16, 17), or in vitro systems of T cell development (18). Collectively, these studies have provided esti- mates for proliferation rates, cell lifespans, daily export, or cell death and renewal rates in various cellular compartments. Different mechanisms for homeostasis were proposed (19 –21). However, these data gathered with various experimental methods are difficult to combine into a single model accounting for the quantitative aspects of cell dynamics. In addition, the temporal parameters of the population dynamics of T cells in response to a short lym- phopenia have only partially been established (22–26). This leaves us with a puzzling, complex, and incomplete view of thymocyte and peripheral T cell dynamics. In the present study, we used a single experimental system of tran- sient perturbation of T lymphocyte homeostasis, in combination with a mathematical model, to quantify the T cell differentiation dynamics in mice. We induced a temporary cell depletion of the immature TCR - CD4 - CD8 - thymocytes through to the mature peripheral CD4 and CD8 T cells in young adult mice with a wild-type T cell repertoire. This was achieved using mice transgenic for the conditional HSV1-TK (TK) suicide gene (27–29). Expression of the TK gene allows eukary- otic cells to metabolize ganciclovir (GCV) into a triphosphate active form, which can be incorporated in the DNA of dividing cells, block- ing DNA elongation and inducing cell death. Cell killing in our sys- tem is thus strictly conditioned by three simultaneous constraints: TK expression, GCV administration, and cell division. In fact, our exper- imental system corresponds to “soft” thymectomy (Tx), as this pro- cedure temporarily blocks thymic output without the side effects as- sociated with surgery. *Universite ´ Pierre et Marie Curie-Paris 06, Unite ´ Mixte de Recherche 7087, Biologie et The ´rapeutique des Pathologies Immunitaires, Paris, France; † Centre National de la Recherche Scientifique, Unite ´ Mixte de Recherche 7087, Biologie et The ´rapeutique des Pathologies Immunitaires, Paris, France; ‡ Population Biology, Institute for Biodi- versity and Ecosystem Dynamics, University of Amsterdam, Amsterdam, The Neth- erlands; and § Theoretical Biology and Bioinformatics, Utrecht University, Utrecht, The Netherlands Received for publication July 24, 2007. Accepted for publication December 4, 2007. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. 1 This work was supported by grants from Agence Nationale de la Recherche sur le SIDA, Centre National de la Recherche Scientifique, Paris 6 University, Action The ´- matique Concerte ´e Vieillissement Institut National de la Sante ´ et de la Recherche Me ´dicale, Netherlands Organisation for Scientific Research, and CompuVac. 2 Address correspondence and reprint requests to Dr. Ve ´ronique Thomas-Vaslin, Unite ´ Mixte de Recherche 7087, Universite ´ Pierre et Marie Curie, Centre National de la Recherche Scientifique, CERVI Groupe Hospitalo-Universitaire Pitie ´-Salpe ˆtrie `re, 83 Boulevard de l’Ho ˆpital 75013 Paris, France. E-mail address: thomas@ chups.jussieu.fr 3 V.T.-V. and H.K.A. contributed equally. 4 Current address: Centre for Infectious Disease Epidemiology, National Institute for Public Health and the Environment, Antonie van Leeuwenhoeklaan 9, 3721 MA Bilthoven, The Netherlands. 5 R.J.d.B. and D.K. contributed equally. 6 Abbreviations used in this paper: RTE, recent thymic emigrant; GCV, ganciclovir; DN, CD4 - CD8 - double negative; Ct, cycle threshold; LN, lymph node; DP, CD4 + CD8 + double positive; SP, single positive; TK, HSV-1 thymidine kinase; Tx, thymectomy; 7AAD, 7-aminoactinomycin D. Copyright © 2008 by The American Association of Immunologists, Inc. 0022-1767/08/$2.00 The Journal of Immunology www.jimmunol.org