Downloaded from genesdev.cshlp.org on September 30, 2021 - Published by Cold Spring Harbor Laboratory Press REVIEW Molecular orchestration of differentiation and function of regulatory T cells Li-Fan Lu2 and Alexander Rudensky1 Howard Hughes Medical Institute and Immunology Program, Memorial Sloan-Kettering Cancer Center, New York, New York 10065, USA During the last decade, a unique mechanism of negative CD4 T cells recognize MHC class II molecules bound to regulation of immune responses and inflammation by peptides generated in the endosomes and lysosomes, a dedicated population of so-called regulatory T cells although some important exceptions to this rule exist (Treg) has become a focus of intensive investigation. (Trombetta and Mellman 2005). Through the discovery of transcription factor Foxp3 as Sequence analyses of peptides displayed by MHC a central molecular determinant of differentiation and molecules revealed that the vast majority of peptides function of Treg cells, the complex biology of these cells, bound to MHC class I and class II molecules is derived including maintenance of immunological tolerance to from self-proteins. Furthermore, TCR have an intrinsic ‘‘self’’ and regulation of immune responses to pathogens, bias toward recognition of MHC molecules, and ‘‘tick- commensals, and tumors, has become amenable to mech- ling’’ of TCR through certain low-affinity interactions anistic studies. In this review, we discuss the molecular with self-peptide–MHC complexes displayed by thymic aspects of Treg cell lineage commitment, maintenance, cortical epithelial cells is prerequisite for thymic T-cell and function. precursors to pass a critical differentiation checkpoint dubbed positive selection. In the periphery, analogous The cornerstone of the adaptive immune system is the interactions facilitate the long-term maintenance of random generation of antigen-specific receptors in the mature CD4 and CD8 T cells and further shape TCR process of somatic gene rearrangement. The immense repertoire. Thus, this ‘‘self-referential’’ nature of T-cell diversity of this anticipatory recognition system provides recognition presents a tremendous problem of establish- an efficient counterbalance to rapidly evolving patho- ing T-cell tolerance to genome-encoded self-antigens, and gens. Two major classes of lymphocytes, key cellular to limiting reactivity to antigens derived from commen- actors of the adaptive immune system, rely on different sal microbiota, food, and environmental factors. In this principles of antigen recognition: Immunoglobulin (Ig) review, we call ‘‘self’’ a sum of the aforementioned receptors displayed by B lymphocytes recognize intact antigens, unrestrained reactivity to which causes diverse forms of antigens, whereas T-cell receptors (TCR) recog- immune-mediated pathologies including autoimmunity, nize antigens in the form of short peptides bound to the allergy, and asthma. products of the major histocompatibility gene complex (MHC). The peptide–MHC complexes are displayed for Dominant and recessive tolerance T-cell recognition on the surface of antigen-presenting cells (APCs), among which dendritic cells (DCs) are Numerous mechanisms of tolerance described during the unique in their ability to process and present antigens last several decades can be divided into two major groups. to T cells and to initiate the immune response. APC Cell-intrinsic mechanisms, also known as recessive tol- exploit the proteolytic machinery of intracellular protein erance, leading to physical elimination or functional turnover to generate antigenic peptides for T-cell recog- inactivation of a given ‘‘self’’ reactive T-cell clone, operate nition. MHC molecules sample the peptide content of both in the thymus and in the periphery and are likely main sites of intracellular proteolysis, the cytosol and responsible for neutralization of the majority of high- the endocytic compartment, without discriminating be- affinity T cells recognizing self. Immature CD4 and CD8 tween ‘‘self’’ and ‘‘foreign’’—i.e., pathogen-derived—pep- double-positive (DP) and single-positive (SP) thymocytes tides. CD8 T cells recognize MHC class I molecules undergo apoptosis, or ‘‘negative selection,’’ upon high- bound to peptides generated in the cytosol, whereas affinity TCR engagement by self-antigen displayed by thymic DCs and thymic medullary epithelial cells. Those self-reactive thymocytes that escape clonal deletion be- [Keywords: T lymphocytes; tolerance; differentiation; transcriptional regulation] come ‘‘anergic’’ or incapable of efficient proliferation in Correspondence. response to a ‘‘self’’ antigen and differentiation into 1E-MAIL [email protected]; FAX (646) 888-3160. 2E-MAIL [email protected]; FAX (646) 888-3160. functional effector cytokine-producing or cytotoxic T Article is online at http://www.genesdev.org/cgi/doi/10.1101/gad.1791009. cells (for review, see Starr et al. 2003). 1270 GENES & DEVELOPMENT 23:1270–1282 Ó 2009 by Cold Spring Harbor Laboratory Press ISSN 0890-9369/09; www.genesdev.org Downloaded from genesdev.cshlp.org on September 30, 2021 - Published by Cold Spring Harbor Laboratory Press Molecular regulation in Treg cells Beyond thymus, chronic engagement of TCR displayed Treg cells isolated from nonimmune animals, its utility by peripheral T cells by ‘‘self’’ antigens can also lead to as a definitive Treg cell marker was limited due up- activation-induced cell death or anergy induction. Pe- regulation of CD25 in all activated T cells. The inability ripheral anergy induction is reinforced by the two-signal to discriminate between protective and inflammation- requirement for initiation of the naive T-cell responses. promoting cells during the immune response impeded This requirement is fulfilled upon simultaneous engage- further understanding of dominant tolerance, especially ment of TCR and T-cell costimulatory receptor CD28 by its mechanistic aspects. Moreover, it was proposed that cognate peptide–MHC complex and inducible CD28 Treg cells are only conventional activated CD4 T cells ligands CD80 and CD86 displayed on the surface of that down-modulate immune responses by competing for APC (Lenschow et al. 1996). High levels of CD80 and interleukin-2. Thus, a search for a specific molecular CD86 expression are induced upon activation of a diverse marker of Treg cells and a genetic mechanism underlying set of dedicated sensors directly in response to microbial the differentiation and function of these cells ensued. or viral products or indirectly by metabolic or biochem- ical changes they induce. Foxp3: a key to dominant tolerance However, the recessive tolerance operating in a cell- intrinsic manner and the two-signal requirement for The key to discovery of the genetic basis of dominant the induction of productive immune response appear tolerance was provided by identification of mutations in insufficient to counter the threat of immune-mediated X-chromosome-encoded transcription factor Foxp3 in pathology without dominant tolerance, a unique immu- mice with a spontaneous scurfy mutation and in patients nological mechanism that involves the suppression of with the IPEX (immune dysregulation, polyendocrinop- ‘‘self’’-reactivity by specialized cells acting in a dominant athy, enteropathy, X-linked) syndrome (Chatila et al. fashion. The initial experimental evidence in support of 2000; Bennett et al. 2001; Wildin et al. 2001). In both thymic generation of cells capable of preventing autoim- humans and mice, loss-of-function Foxp3 mutations re- munity came from a surprising finding of spontaneous sult in a fatal early-onset T-cell-dependent autoimmunity T-cell-dependent autoimmune lesions in a variety of manifested by diabetes, thyroiditis, hemolytic anemia, organs observed in mice subjected to thymectomy on day hyper-IgE syndrome, exfoliative dermatitis, splenomeg- 3 of life (d3Tx) (Nishizuka and Sakakura 1969; Sakaguchi aly, lymphadenopathy, and ‘‘cytokine storm’’ (for review, et al. 1982; Bonomo et al. 1995; Asano et al. 1996) and see Gambineri et al. 2003). Importantly, the disease tolerance observed in chicken–quail chimeras (Ohki et al. affected only hemizygous Foxp3 mutant males. In con- 1987). Since adoptive transfers of T cells from euthymic trast, heterozygous female carriers of Foxp3 mutations mice prevented autoimmunity in d3Tx mice, these early were spared from the disease, suggesting that T cells observations suggested paucity of T cells capable of pre- expressing a wild-type Foxp3 allele were able to restrain venting autoimmunity in neonatal mice and, thus, laid activation of T cells (Godfrey et al. 1994). An implicit a foundation for the concept of dominant tolerance. assumption in this line of reasoning—namely, that Foxp3 mutations do not affect random X-chromosome inactiva- tion in T cells—was subsequently confirmed by analysis CD25+CD4+ T cells as mediators of dominant tolerance of Foxp3 reporter mice (Fontenot et al. 2005c; Gavin et al. Long-lasting labors to define the cell type-mediating do- 2007), as discussed below. This consideration along with minant tolerance resulted in the landmark discovery of systemic nature of immune inflammatory lesions affect- a subset of CD4 T cells, constitutively expressing high ing multiple tissues in Foxp3 mutant individuals promp- amounts of the interleukin-2 receptor a-chain (CD25), as ted several groups to examine expression of Foxp3 in being highly enriched in suppressor activity (Sakaguchi CD25+CD4+ Treg cells in mice. These studies revealed et al. 1995). CD25+CD4+ Tcells,