During their maturation in the thymus, many immature T cells that recognize antigens with high-affinity die, and some of the surviving cells in the CD4+ lineage develop into Tregs (Fig. 1). Death of immature T cells as a result of recognition of antigens in the thymus is known as deletion, or negative selection; it was described in Chapter 8 in the discussion of T-cell maturation. This process affects major histocompatibility com plex (MHC) class I– and class II–restricted T cells and is therefore important for tolerance in both CD8+ and CD4+ lymphocyte populations. Negative selection of thymocytes is responsible for the fact that the repertoire of mature T cells that leave the thymus and populate peripheral lymphoid tissues is unresponsive to many self antigens that are present in the thymus. Negative selection occurs in immature CD4+CD8+ (double-positive) T cells in the thymic cortex and newly generated single-positive T cells in the medulla. In both locations, immature thymocytes with high-affinity receptors for self antigens that encounter these antigens die by apoptosis. The two main factors that determine if a particular self antigen will induce negative selection of self-reactive thymocytes are the presence of that antigen in the thymus, by local expression or delivery by the blood, and the affinity of the thymocyte T-cell receptors (TCRs) that recognize the antigen. Thus, the important questions that are relevant to negative selection are which self antigens are present in the thymus and how immature T cells that recognize these antigens are deleted.
Fig1. Central T-cell tolerance. Recognition of self antigens by immature T cells in the thymus leads to the death of the cells (negative selection, or deletion) or to the development of regulatory T cells (Tregs) that enter peripheral tissues.
The antigens that are present in the thymus include many circulating and cell-associated proteins that are widely distributed in most tissues. The thymus also has a special mechanism for expressing numerous protein antigens that are not ubiquitously expressed but are limited to certain peripheral tissues, so that immature T cells specific for these antigens can be deleted from the developing T-cell repertoire. Many of these peripheral tissue antigens are expressed in medullary thymic epithelial cells (mTECs), and their expression is dependent on the autoimmune regulator (AIRE) protein. AIRE is a com ponent of a multiprotein complex that is expressed mainly in mTECs and is thought to be involved in transcriptional elongation and the regulation of splicing and chromatin remodeling, but the molecular basis of AIRE function remains poorly understood. Mutations in the AIRE gene are the cause of an autoimmune disease called autoimmune polyglandular syn drome type 1 (APS1). This group of diseases is characterized by antibody- and T-lymphocyte-mediated injury to multiple endocrine organs, including the parathyroids, adrenals, and pancreatic islets, as well as the skin and other tissues. A mouse model of APS1 has been developed by knockout of the AIRE gene, and it recapitulates many of the features of the human disease. Immature CD4+ or CD8+ T cells that recognize AIRE dependent peripheral tissue antigens are mostly deleted in the thymus or some develop into a subset of Tregs (Fig. 2). In the absence of functional AIRE (as in APS1 patients and AIRE knockout mice), numerous tissue-restricted antigens are not expressed in the thymus, and CD4+ and CD8+ T cells specific for the antigens escape deletion, mature, and enter the periphery, where they attack the target tissues in which the antigens are expressed independent of AIRE (see Fig. 2). Patients also make a variety of autoantibodies, probably because of failure to delete helper T cells specific for self antigens. Interestingly, some of these autoantibodies react against and neutralize the patients’ own cytokines, including interleukin-17 (IL-17), IL-22, and type I interferons (IFNs). The deficiency of IL-17 (and perhaps IL-22) makes these individuals susceptible to mucocutaneous candidiasis, which is a common feature of APS1 and reflects the essential role of Th17 cytokines in defense against this fungal infection.
Fig2. The function of autoimmune regulator (AIRE) in the elimination of self-reactive T cells in the thymus. (A) The AIRE protein is part of a complex that regulates the expression of tissue-restricted antigens (TRAs) in medullary thymic epithelial cells (mTECs) and in cells mimicking peripheral tissue cells that develop from mTECs (see Fig. 3). (Other proteins in this transcriptional regulatory complex are not shown.) Peptides derived from these antigens are displayed on the mTEC and recognized by immature antigen-specific T cells, leading to the deletion of many self-reactive T cells or the development of regulatory T cells (Tregs). (B) In the absence of functional AIRE, these immature self-reactive T cells are not eliminated; they can mature into naive T cells that can be activated in secondary lymphoid organs, generating effector T cells (not shown) that enter tissues where the antigens continue to be produced and cause injury. Although only CD4+ T cells are shown, AIRE also controls the deletion of CD8+ T cells. Another mechanism by which AIRE contributes to central tolerance to peripheral tissue antigens is illustrated in Fig. 3.
Fig4. The role of thymic cells that mimic peripheral tissue cells in central tolerance. Cells that transcriptionally resemble peripheral tissue cells (called thymomimetic cells) develop from medullary thymic epithelial cells (mTECs). These mimics of peripheral cells express tissue-restricted antigens (TRAs) of peripheral tissues and induce tolerance to these antigens. Note that the mimetic cells resemble peripheral tissue cells transcriptionally but may not express all the morphologic features of peripheral tissue cells. Treg, Regulatory T cell.
Recent studies have revealed a novel mechanism for expression of peripheral tissue-restricted antigens in the thymus. Some mTECs differentiate into populations that express dis tinct lineage determining transcription factors (Fig. 3). In this way, mTECs become mimics of different peripheral tis sue cells. Some resemble tuft cells, which are normally found in the gut epithelium; others become myocyte-like cells; yet others become keratinocyte-like, expressing numerous genes normally restricted to skin epithelium; and so on. Each of these unique “mimetic cells” produces lineage-specific proteins rep resenting a different peripheral cell type. These proteins are tissue-specific self antigens that induce central tolerance in the thymic medulla. In humans, the keratinocyte-like cells degenerate to give rise to keratin whorls that have long been known as Hassall’s corpuscles.
High-affinity TCR signaling in immature T cells triggers the mitochondrial pathway of apoptosis. Clearly, immature and mature lymphocytes interpret antigen receptor signals differently—the immature cells die and the mature cells are activated. The biochemical basis of this difference is not known.
Some self-reactive CD4+ T cells that see self antigens in the thymus are not deleted but instead differentiate into Tregs that are specific for these antigens (see Fig. 1). The regulatory cells leave the thymus and inhibit responses against self antigens in the periphery. What determines the choice between deletion and development of Tregs is not known. Possible factors include the affinity of antigen recognition, the types of antigen-presenting cells (APCs) presenting the antigen, and the availability of certain cytokines locally in the thymus. We will describe the characteristics and functions of Tregs later in the context of peripheral tolerance because these cells suppress immune responses in the periphery.
