Early development and differentiation of nascent T cells that migrate from bone marrow to become mature, naïve T cells, which are capable of responding to antigen takes place inside the thymus. Around 1010 TCR (T cell receptor) variations are generated in developing T lymphocyte clones through a random process of somatic cell gene reorganization. During this process, often T-cells recognizing self-antigens are generated. Due to the ability of these self-reactive T-cells to elicit an autoimmune attack, they are permanently removed by the thymus through negative selection and clonal deletion. But, some of them manage to escape the thymic defenses and harbor themselves in the peripheral lymphoid organs. In periphery, T lymphocytes undergo further differentiation into effectors of various immune functions.
One of many immunotolerance mechanisms that immune system has developed to distinguish between self and non-self antigens is regulatory T cells or Tregs. These cells are recently characterized specialized T-cell subsets that actively suppress a variety of immune responses. Researchers have broadly classified Tregs into natural and adaptive Tregs. Natural Tregs are CD4+CD25+ T-cells that originate in the thymus and play a significant role in immune homeostasis and protection against autoimmunity. Adaptive Tregs are non-regulatory CD4+ T-cells that have up-regulated CD25 expression during pathological and inflammatory conditions such as cancers and infections.
Although the principal immunosuppressive mechanism of Tregs remains elusive, several in vivo experimental models have indicated that Tregs secrete large amounts of immunosuppressants including IL-9, IL-10 and TGF-β upon activation. These lymphokines are capable of inhibiting activation of Th1, Th2 cells and CTLs required for cell-mediated immunity, inflammation and antibody production. Certain recent experimental data and results even indicate that IL-2-IL-2R signaling is vital for development, maintenance, survival, expansion and suppressive activity of Tregs. Increased expression of certain other characteristic markers including CTLA-4, glucocorticoid-inducible tumor necrosis factor receptor (GITR) and OX40 has been identified on Tregs whose function inside these cells is still not clear. The TCRs displayed on Tregs are capable of recognizing and interacting with any peptide-MHC class II ligand having certain range of avidity. But, the contribution of TCR signaling and role of TCR-ligand interactions towards regulatory T-cell development needs to be determined.
Several elegant experiment using transgenic mice and retrovirus mediate over expression studies, researchers have identified FoxP3, a transcription factor, to be a specific molecular marker essential for the development and function of Tregs. The primary evidence regarding the involvement of FoxP3 in the development of Tregs was provided by the experiments of Sakaguchi et al, (ref ?) in patients suffering from IPEX, a rare and fatal human autoimmune disorder. In these patients, mutated FoxP3 gene causes improper development of Tregs resulting in hyperactivation of T-cells reactive to self-antigens. Recently, experiments have clearly shown that retroviral mediated introduction of FoxP3 into conventional CD4+ T-cells converts them into regulatory T-cells.
The emergence of regulatory T-cells and role of FoxP3 as a critical player in the negative control of a of various normal and pathological immune responses holds great promise for the development of novel therapies useful for the treatment of autoimmune diseases in humans. However, there are several questions that remain to be answered including the basic biology of the Tregs, various ligands responsible for thymic selection of these cells, the exact function of FoxP3 in relation with various markers present on Tregs and most importantly, the mechanisms by which Tregs exert their suppressive effects. A better understanding of manipulating FoxP3 and Tregs would enable us to harness the tremendous therapeutic potential in various clinical situations including Type I diabetes, Multiple sclerosis, GVHD, rheumatoid arthritis, allergy, and cancers.

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