The autoimmune regulator (AIRE) is a protein that in humans is encoded by the AIREgene. AIRE is a transcription factor expressed in the medulla (inner part) of the thymus. It is part of the mechanism which eliminates self-reactive T cells that would cause autoimmune disease. It exposes T cells to normal, healthy proteins from all parts of the body, and T cells that react to those proteins are destroyed.
Each T cell recognizes a specific antigen when it is presented in complex with a MHC molecule by an antigen presenting cell. This recognition is accomplished by the T cell receptors expressed on the cell surface. T cells receptors are generated by randomly shuffled gene segments which results in a highly diverse population of T cells - each with a unique antigen specificity. Subsequently, T cells with receptors that recognize the body's own proteins need to be eliminated while still in the thymus. Through the action of AIRE, medullary thymic epithelial cells (mTEC) express major proteins from elsewhere in the body (so called "tissue-restricted antigens" - TRA) and T cells that respond to those proteins are eliminated through cell death (apoptosis). Thus AIRE drives negative selection of self-recognizing T cells. When AIRE is defective, T cells that recognize antigens normally produced by the body can exit the thymus and enter circulation. This can result in a variety of autoimmune diseases.
In the thymus, the AIRE causes transcription of a wide selection of organ-specific genes that create proteins that are usually only expressed in peripheral tissues, creating an "immunological self-shadow" in the thymus. It is important that self-reactive T cells that bind strongly to self-antigen are eliminated in the thymus (via the process of negative selection), otherwise they may later encounter and bind to their corresponding self-antigens and initiate an autoimmune reaction. So the expression of non-local proteins by AIRE in the thymus reduces the threat of autoimmunity by promoting the elimination of auto-reactive T cells that bind antigens not normally found in the thymus. Furthermore, it has been found that AIRE is expressed in a population of stromal cells located in secondary lymphoid tissues, however these cells appear to express a distinct set of TRAs compared to mTECs.
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