A Dual Role of IFN- in the Balance between Proliferation and Death of Human CD4  T Lymphocytes during Primary Response 1 Elisabetta Dondi, 2 * Gae ¨l Roue ´, † Victor J. Yuste, † Santos A. Susin, 3† and Sandra Pellegrini 3,4 * Type I IFNs (IFN-) enhance immune responses, notably T cell-mediated responses, in part by promoting the functional activities of dendritic cells. In this study, we analyzed the direct impact of IFN- on proliferative and apoptotic signals upon in vitro activation of human naive CD4  T lymphocytes. We demonstrate that IFN- protects T cells from the intrinsic mitochon- drial-dependent apoptosis early upon TCR/CD28 activation. IFN- acts by delaying entry of cells into the G 1 phase of the cell cycle, as well as by increasing Bcl-2 and limiting Bax activation. Later, upon activation, T cells that were exposed to IFN- showed increased levels of surface Fas associated with partially processed caspase-8, a key component of the extrinsic apoptotic pathway. Caspase-8 processing was augmented furthermore by Fas ligation. Overall, these findings support a model whereby IFN- favors an enhanced clonal expansion, yet it sensitizes cells to the Ag-induced cell death occurring at the end of an immune response. These observations point to a complex role of type I IFN in regulating the magnitude of proliferation and survival of naive CD4  T cells during primary response and underline how crucial could be the timing of exposure to this cytokine. The Journal of Immunology, 2004, 173: 3740 –3747. T he type I IFNs (IFN-) are important mediators of the innate immune response for their potent and direct anti- viral activity and their stimulatory effects on NK cells and macrophages. Recently, these cytokines have been reconsidered as critical actors in the generation of the adaptive immune responses toward virus infection and tumor growth. Evidence obtained in different model systems has highlighted their ability to support proliferation, functional activity, and survival of certain T cell sub- sets (1, 2). These activities of IFN are, at least in part, mediated by the dendritic cells (DCs). 5 A promoting effect of type I IFN in maturation, differentiation, and functional activity of DCs has been indeed reported (reviewed in Ref. 3). Upon infection, DC devel- opment from monocytes is induced by type I IFN produced by specific cell types, such as macrophages and endothelial cells. Moreover, infected DCs, and particularly plasmacytoid-like cells (pDCs), produce large amounts of type I IFN that contributes to their maturation in an autocrine and paracrine fashion (4 – 6). The secretion of type I IFN is indeed sustained in lymph nodes, where pDCs represent the principal source (7). Hence, in lymph nodes, naive T cells may encounter cognate Ags, undergo clonal expan- sion, and acquire effector functions while being exposed to type I IFN. To investigate the impact of this cytokine on T cell responses, we set up an experimental model system that allows us to analyze the direct effects of IFN- on human naive CD4 + T cells purified from umbilical cord blood (UCB). In this model, we previously found that IFN- exerts an anti-proliferative effect that is strictly dependent on the activation state of the cells (8). Here, we sought to analyze whether IFN- may intervene at the onset of the pri- mary T CD4 + response by skewing the balance between prolifer- ation and death. For this, we have characterized the apoptotic pro- grams initiated in TCR/CD28-stimulated naive CD4 + T cells and, in parallel, we have investigated whether IFN- affects them. Apoptosis occurs via two major execution programs: the extrin- sic pathway triggered by the ligation of membrane death receptors, and the intrinsic pathway that integrates the different cell stress signals at the mitochondrial level. In the extrinsic pathway, the stimulation of death receptors (e.g., Fas or TNF-R) provokes the processing and the activation of caspase-8, which, in turn, cleaves procaspase-3 and thereby initiates a proteolytic cascade. In the intrinsic pathway, disruption of the mitochondrial transmembrane potential ( m ) controls the release of several proapoptotic pro- teins (9, 10). The Bcl-2 family of proteins are pivotal intermediates in cell death signaling through the regulation of the  m . Indeed, some Bcl-2 family members, such as Bcl-2 and Bcl-x L , promote cell survival by inhibiting the  m loss, while other members of this family, such as Bax and Bak, participate actively in the dis- ruption of the mitochondrial potential (11). The extrinsic and the intrinsic pathways can be connected through the caspase-mediated activation of the proapoptotic protein Bid (12). In this study, we provide the evidence that the TCR/CD28-me- diated stimulation of naive T CD4 + cells activates the mitochon- drial-dependent apoptotic pathway. In this context, IFN- protects cells from death through the up-regulation of the anti-apoptotic *Unite ´ de Signalisation des Cytokines, and † Apoptose et Syste `me Immunitaire, In- stitut Pasteur, Paris, France Received for publication March 29, 2004. Accepted for publication July 2, 2004. 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 the Association pour la Recherche sur le Cancer (Contract 5957 to S.P. and Contract 4812 to S.A.S.), Fondation de France, Fondation pour la Recherche Me ´dicale, Ligue Nationale contre le Cancer, and the Pasteur-Weizmann Scientific Council (to S.A.S.). E.D. was supported by a postdoc- toral fellowship from the Association pour la Recherche sur la Scle ´rose en Plaque. V.J.Y. holds a postdoctoral fellowship from the Spanish Ministry of Science. 2 Current Address: Unite ´ Institut National de la Sante ´ et de la Recherche Me ´dicale U567, De ´partement d’He ´matologie, Institut Cochin, Paris, France. 3 S.A.S. and S.P. share senior coauthorship of this manuscript. 4 Address correspondence and reprint requests to Dr. Sandra Pellegrini, Unite ´ de Signalisation des Cytokines, Institut Pasteur, 25 rue du Dr. Roux, 75724 Paris Cedex 15, France. E-mail address: pellegri@pasteur.fr 5 Abbreviations used in this paper: DC, dendritic cell; pDC, plasmacytoid-like cells: UCB, umbilical cord blood;  m , mitochondrial transmembrane potential; PS, phos- phatidylserine; 7-AAD, 7-aminoactinomycin D; ROS, reactive oxygen specie; FasL, Fas ligand. The Journal of Immunology Copyright © 2004 by The American Association of Immunologists, Inc. 0022-1767/04/$02.00