Graduate School of Biomedical Sciences; Program in Immunology and Virology
Self Tolerance; Forkhead Transcription Factors; Autoimmunity; T-Lymphocytes, Regulatory; Homeostasis; Adaptor Proteins, Signal Transducing; Academic Dissertations; Dissertations, UMMS
Life Sciences | Medicine and Health Sciences
Adaptive immunity requires T cell responses to foreign pathogens to be counterbalanced with the need to limit collateral destruction of the host’s own tissues. Further, the presence of a substantial pool of lymphocytes capable of recognizing selfantigen in the periphery poses a threat to the maintenance of peripheral tolerance and prevention of autoimmunity. Regulatory T cells (Treg) that can suppress potentially self-reactive T cells are critical regulators of peripheral tolerance as well as initiation of immune responses. Treg cells employ several context-dependent mechanisms to establish regulation. In this thesis, we describe two distinct pathways of regulation used by Treg cells involving negative costimulation by CTLA-4 and immunomodulation by the morphogen, TGFβ.
CTLA-4 is a co-inhibitory receptor on T cells essential for maintaining T cell homeostasis and tolerance to self. CTLA-4 expression is induced in conventional T cells following activation, whereas it is constitutively expressed in regulatory FOXP3+CD4+ regulatory T cells. Mice lacking CTLA-4 develop an early onset, fatal breakdown in T cell tolerance. Whether this autoimmune disease occurs because of the loss of CTLA-4 function in regulatory T cells, conventional T cells, or both, is not known. We present evidence here that in addition to a critical CTLA-4 function in regulatory T cells, CTLA-4 in conventional T cells is also necessary for controlling the consequences of abnormal T cell activation. CTLA-4 expression in activated conventional T cells only in vivo is unable to compensate for the impaired function of CTLA-4-less regulatory T cells that results in systemic lymphoproliferation, but it can prevent the aberrantly activated T cells from infiltrating and fatally damaging non-lymphoid tissues. These results demonstrate that CTLA-4 has a dual function in maintaining T cell homeostasis: CTLA-4 in regulatory T cells inhibits inappropriate naïve T cell activation and CTLA-4 in conventional T cells can prevent the harmful accumulation of inappropriately activated pathogenic T cells in vital organs.
In addition, we have identified Disabled-2 (Dab2), a TGFβ signaling intermediate, as a FOXP3 target gene that is expressed exclusively in Treg cells and is critical for in vitro and in vivo regulation by Treg cells. During T cell development, DAB2 is also expressed in a Foxp3-independent manner in thymic precursor cells, and acts as a sensor of TGFβ signals that is required for programming normal TGFβ responsiveness in T cell progenies. Naïve CD4+ T cells that differentiate from Dab2-deficient precursors favor Th17 cell generation at the expense of FOXP3+ Treg cells as a result of altered sensitivity to TGFβ. Importantly, retinoic acid can restore TGFβ signaling capacity of naïve CD4+ T cells generated from Dab2-deficient precursors, emphasizing the cooperative nature of retinoic acid and TGFβ signaling pathways in promoting Treg cell development and maintenance.
Rights and Permissions
Copyright is held by the author, with all rights reserved.
Jain, Nitya, "Multifaceted Regulation of Peripheral T Cell Tolerance and Autoimmunity by FOXP3+ T Regulatory Cells: A Dissertation" (2009). University of Massachusetts Medical School. GSBS Dissertations and Theses. Paper 416.