[Show abstract][Hide abstract] ABSTRACT: Continuous support from follicular CD4+ T helper (Tfh) cells drives germinal center (GC) responses, which last for several weeks to produce high affinity memory B cells and plasma cells. In autoimmune Sle1 and NZB/W F1 mice, elevated numbers of Tfh cells persist, promoting the expansion of self-reactive B cells. Expansion of circulating Tfh like cells have also been described in several autoimmune diseases. Although, the signals required for Tfh differentiation have now been well described, the mechanisms that sustain the maintenance of fully differentiated Tfh are less understood. Recent data demonstrate a role for GC B cells for Tfh maintenance after protein immunization.
PLoS ONE 01/2014; 9(8):e102791. · 3.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Immune tolerance requires regulatory T (Treg) cells to prevent autoimmune disease, with the transcription factor Foxp3 functioning as the critical regulator of Treg cell development and function. We report here that Foxp3 was lethal to developing Treg cells in the thymus because it induced a unique proapoptotic protein signature (Puma(++)p-Bim(++)p-JNK(++)DUSP6(-)) and repressed expression of prosurvival Bcl-2 molecules. However, Foxp3 lethality was prevented by common gamma chain (γc)-dependent cytokine signals that were present in the thymus in limiting amounts sufficient to support only ∼1 million Treg cells. Consequently, most newly arising Treg cells in the thymus were deprived of this signal and underwent Foxp3-induced death, with Foxp3(+)CD25(-) Treg precursor cells being the most susceptible. Thus, we identify Foxp3 as a proapoptotic protein that requires developing Treg cells to compete with one another for limiting amounts of γc-dependent survival signals in the thymus.
[Show abstract][Hide abstract] ABSTRACT: The thymus generates T cells with diverse specificities and functions. To assess the contribution of cytokine receptors to the differentiation of T cell subsets in the thymus, we constructed conditional knockout mice in which IL-7Rα or common cytokine receptor γ chain (γ(c)) genes were deleted in thymocytes just before positive selection. We found that γ(c) expression was required to signal the differentiation of MHC class I (MHC-I)-specific thymocytes into CD8(+) cytotoxic lineage T cells and into invariant natural killer T cells but did not signal the differentiation of MHC class II (MHC-II)-specific thymocytes into CD4(+) T cells, even into regulatory Foxp3(+)CD4(+) T cells which require γ(c) signals for survival. Importantly, IL-7 and IL-15 were identified as the cytokines responsible for CD8(+) cytotoxic T cell lineage specification in vivo. Additionally, we found that small numbers of aberrant CD8(+) T cells expressing Runx3d could arise without γ(c) signaling, but these cells were developmentally arrested before expressing cytotoxic lineage genes. Thus, γ(c)-transduced cytokine signals are required for cytotoxic lineage specification in the thymus and for inducing the differentiation of MHC-I-selected thymocytes into functionally mature T cells.
Journal of Experimental Medicine 10/2012; · 13.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The thymic medulla is generally held to be a specialized environment for negative selection. However, many self-reactive thymocytes first encounter ubiquitous self-antigens in the cortex. Cortical epithelial cells are vital for positive selection, but whether such cells can also promote negative selection is controversial. We used the HY(cd4) model, where T cell receptor for antigen (TCR) expression is appropriately timed and a ubiquitous self-antigen drives clonal deletion in male mice. We demonstrated unambiguously that this deletion event occurs in the thymic cortex. However, the kinetics in vivo indicated that apoptosis was activated asynchronously relative to TCR activation. We found that radioresistant antigen-presenting cells and, specifically, cortical epithelial cells do not efficiently induce apoptosis, although they do cause TCR activation. Rather, thymocytes undergoing clonal deletion were preferentially associated with rare CD11c(+) cortical dendritic cells, and elimination of such cells impaired deletion.
Journal of Experimental Medicine 11/2008; 205(11):2575-84. · 13.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Producing a healthy immune system capable of defending against pathogens, while avoiding autoimmunity, is dependent on thymic selection. Positive selection yields functional T cells that have the potential to recognize both self and foreign antigens. Therefore, negative selection exists to manage potentially self-reactive cells. Negative selection results from the induction of anergy, receptor editing, clonal diversion (agonist selection), and/or clonal deletion (apoptosis) in self-reactive clones. Clonal deletion has been inherently difficult to study because the cells of interest are undergoing apoptosis and being eliminated quickly. Furthermore, analysis of clonal deletion in humans has proved even more difficult due to availability of samples and lack of reagents. Mouse models have thus been instrumental in achieving our current understanding of central tolerance, and the evolution of elegant model systems has led to an explosion of new data to be assimilated. This review will focus on recent advances in the field of clonal deletion with respect to three aspects: the development of physiological model systems, signaling pathways that lead to apoptosis, and antigen presenting cell types involved in the induction of clonal deletion.
Seminars in Immunopathology 11/2008; 30(4):399-409. · 5.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Conventional alphabeta T cell precursors undergo positive selection in the thymic cortex. When this is successful, they migrate to the medulla and are exposed to tissue-specific antigens (TSA) for purposes of central tolerance, and they undergo maturation to become functionally responsive T cells. It is commonly understood that thymocytes spend up to 2 wk in the medulla undergoing these final maturation steps before emigrating to peripheral lymphoid tissues. In addition, emigration is thought to occur via a stochastic mechanism whereby some progenitors leave early and others leave late-a so-called "lucky dip" process. However, recent research has revealed that medullary thymocytes are a heterogeneous mix of naive alphabeta T cell precursors, memory T cells, natural killer T cells, and regulatory T cells. Given this, we revisited the question of how long it takes naive alphabeta T cell precursors to emigrate. We combined the following three approaches to study this question: BrdU labeling, intrathymic injection of a cellular tag, and RAG2p-GFP reporter mice. We established that, on average, naive alphabeta T cell precursors emigrate only 4-5 d after becoming single-positive (SP) thymocytes. Furthermore, emigration occurs via a strict "conveyor belt" mechanism, where the oldest thymocytes leave first.
Journal of Experimental Medicine 11/2007; 204(11):2513-20. · 13.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A signal initiated by the newly formed Ag receptor is integrated with microenvironmental cues during T cell development to ensure positive selection of CD4+CD8+ progenitors into functionally mature CD4+ or CD8+ T lymphocytes. During this transition, a survival program is initiated, TCR gene recombination ceases, cells migrate into a new thymic microenvironment, the responsiveness of the Ag receptor is tuned, and the cells commit to a specific T lineage. To determine potential regulators of these processes, we used mRNA microarray analysis to compare gene expression changes in CD4+CD8+ thymocytes from TCR transgenic mice that have received a TCR selection signal with those that had not received a signal. We found 129 genes with expression that changed significantly during positive selection, the majority of which were not previously appreciated. A large number of these changes were confirmed by real-time PCR or flow cytometry. We have combined our findings with gene changes reported in the literature to provide a comprehensive report of the genes regulated during positive selection, and we attempted to assign these genes to positive selection process categories.
The Journal of Immunology 12/2004; 173(9):5434-44. · 5.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Conventional αβ T cell precursors with a low affinity for self undergo positive selection in the thymic cortex, then migrate to the medulla. During their residency in the medulla, they undergo further maturation to become functionally responsive T cells, after which time they emigrate. In contrast to what was previously thought, we found that thymic emigration occurs rapidly for conventional αβ T cells (1-2 days) and is dependent on the transcription factor KLF2. On the other hand long-lived antigen-experienced cells (NKT, Treg, memory cells etc.) can be found in the thymus, but this may reflect their re-circulation from the periphery. T cell precursors with a high affinity for self-antigens commonly die within the thymus—clonal deletion. We used a highly physiologic TCR transgenic model (HY cd4 ) to show that self-reactive thymocytes die in the cortex. We also show that costimulatory molecules expressed in the medulla were dispensable for deletion, as was migration to the medulla or even an organized medullary epithelium. However, the kinetics of clonal deletion in vivo indicated that apoptosis was asynchronously activated over four days after receiving a high-affinity signal. This inefficient apoptosis mechanism may allow for the generation of self-antigen specific regulatory T cell populations.