Published: 2014
Total Pages: 0
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T cells are a heterogeneous group of lymphocytes that are derived from the bone marrow and mature in the thymus before being disseminated to secondary lymphoid organs such as the spleen and lymph nodes. They are critical for anti-microbial defense via the promotion of appropriate antigen-specific primary adaptive responses against immunologic threats, the generation of immunologic memory, and the suppression of inappropriate immune responses. Antigen-specific memory responses are the hallmark of the adaptive immune system, and they are significantly more rapid and potent than primary antigen-specific responses. As a result, appropriate memory T cell responses can provide potent and long-lived protection from disease, while a lack of T cell memory may lead to failure of immunity, and inappropriate memory may result in potentially life-threatening immune-mediated diseases. The development of appropriate primary and memory T cell responses is highly complex, and requires the careful integration of diverse cytokine and cell-cell signals, the installation of specific transcriptional programs during differentiation, and profound alterations in proliferative and functional capacity. The rational design of prophylactic interventions such as vaccines to prevent infectious diseases, and the identification of therapeutic targets for the treatment of immune-mediated diseases depend upon a detailed understanding of these molecular mechanisms. In this thesis, I will discuss the cell-signaling and transcriptional bases of T cell effector and memory differentiation and homeostasis, and emphasize newly elucidated roles for the PI3K/Akt pathways, mTOR, and the FoxO transcription factors in the differentiation and regulation of CD8 T cells, and the roles of Bach2 in the differentiation of CD8 T cell memory and the development of CD4 Foxp3+ regulatory T cells. Naïve CD8 T cells respond to infection with viruses or intracellular bacteria, or the emergence of tumor cells by differentiating into cytotoxic T cells. These cytotoxic CD8 T cells are antigen-specific and their cytotoxicity is restricted to cells bearing the pathogen or tumor antigen. Antiviral CD8 T cell responses have been the best characterized. These responses are highly dynamic, and have been classically divided in to three distinct phases: expansion, contraction and memory. Only cells that successfully pass through all three phases may differentiate into bona fide memory cells capable of producing protective secondary responses. Thus, generation of appropriate memory depends upon a variety of molecular mechanisms occurring throughout the process, with critical checkpoints occurring during the initial activation of naïve cells, the successful transition between phases, and the orderly differentiation of T cell subsets within each phase. There is considerable evidence that the PI3K/Akt signaling pathway is activated via engagement of the TCR and co-stimulatory interactions, during the initial activation of naïve T cells by professional antigen-presenting cells. (APCs). Further evidence indicates that the Akt signaling pathway is concurrently modulated by changes in cellular metabolism and signaling by a variety of cytokines including IL-2, IL-7, IL-12 and IL-15. However, how these stimuli collectively activate the PI3K/Akt signaling pathway in vivo, and how this Akt signaling dictates the differentiation process of CD8 T cells during an acute viral infection are yet to be determined. Chapter Three reports the role of Akt signaling on the differentiation of CD8 T cells during an acute viral infection. Using genetic and pharmacological approaches, I have identified Akt as a signal integrator that accepts signals from TCR and cytokines like IL-2 and IL-12, and links downstream targets like mTOR and FoxO to distinct facets of CD8 T cell differentiation. Notably, sustained Akt signaling promotes the terminal differentiation of effector CD8 T cells, which results in the exaggerated contraction and the impaired formation and maintenance of memory CD8 T cells. These changes are induced at least in part through the hyper-activation of mTOR followed by the increased expression of T-bet. Moreover, inactivation of FoxO1 induced by constitutive Akt signaling downregulates IL-7R expression. Conversely, preventing excessive mTOR activation by in vivo rapamycin administration, and the forced expression of IL-7R significantly enhance the formation of memory CD8 T cells. Finally, in vivo inhibition of Akt signaling mitigates impaired generation of memory CD8 T cells. These findings imply that therapeutic modulation of Akt might be a strategy to enhance vaccine-induced immunity. One of several target genes affected by Akt signaling is the transcription factor Bach2. It has been originally identified as a B cell-specific transcription factor that maintains B cell identity and restrains differentiation of plasma cells. Notably, other groups and I have discovered that Bach2 is also expressed in the T cell compartment. Remarkably, CD8 T cells' progression towards terminal differentiation correlates with reduced expression of Bach2. Thus, it is likely that Bach2 regulates the homeostasis of naïve CD8 T cells and the differentiation of memory CD8 T cells, in which Bach2 mRNA is highly expressed. Therefore, in Chapter Four, I have investigated the effects of Bach2 on CD8 T cell differentiation during an acute viral infection. Similar to B cells, Bach2 deficiency promotes terminal differentiation of LCMV-specific CD8 T cells, and prevents efficient effector-to-memory transition in a cell-intrinsic manner. Additionally, in the absence of Bach2, tissue distribution of virus-specific CD8 T cells is affected. Remarkably, in the absence of Bach2, LCMV-specific memory T cells exhibits defective memory maintenance, which results in the gradual attrition of memory CD8 T cells. Together, my studies suggest that Bach2 exerts important effects on the formation and homeostasis of memory CD8 T cells by preventing terminal differentiation and by contributing efficient effector-to-memory transition and maintenance of virus-specific memory CD8 T cells. In contrast to a single effector subset of CD8 T cells, CD4 T cells (also known as helper T cells) can differentiate into various subsets of effector CD4 T cells, in which TH1, TH2, TH17 and follicular helper T cell (TFH) orchestrates immune responses to clear different types of pathogens such as virus, bacteria, fungi and parasites, while regulatory T cells (Treg) tone down activated immune responses in order to prevent immune-mediated pathology. A recent study has reported that the expression of Bach2 is dynamically regulated during Treg cell development. Moreover, I have found that T cells in Bach2-deficient mice show spontaneous activation. These findings have led us to investigate the function of Bach2 on Treg cell development and homeostasis. As described in Chapter Five, without Bach2, Treg cells exhibit attenuated foxp3 expression, diminished frequencies and numbers, enhanced activation and proliferation, and profound loss of competitive fitness in vivo. Importantly, Bach2 deficiency redirects the Treg differentiation program into a TH2 effector program by the increased expression of TH2-driving transcription factor, Gata3. Additionally, perturbations in the conversion of induced Treg cells in the periphery induced by Bach2 deficiency undermines optimal establishment of immune tolerance contributed by Treg cells. Strikingly, the abnormal homeostasis of Treg cells seems to be associated with systemic inflammation, especially a life-threatening eosinophilic crystalline pneumonia in Bach2-deficient mice. In summary, Bach2 enforces T cell quiescence, promotes the optimal development and homeostasis of Treg cells, and protects against immune-mediated diseases.
