Medullary thymic epithelial cells (mTECs) are specialized for inducing central immunological tolerance to self-antigens. To accomplish this, mTECs must adopt a mature phenotype characterized by expression of the autoimmune regulator Aire, which activates the transcription of numerous genes encoding tissue-restricted self-antigens. The mechanisms that control mature Aire(+) mTEC development in the postnatal thymus remain poorly understood. We demonstrate here that, although either CD4(+) or CD8(+) thymocytes are sufficient to sustain formation of a well-defined medulla, expansion of the mature mTEC population requires autoantigen-specific interactions between positively selected CD4(+) thymocytes bearing autoreactive T cell receptor (TCR) and mTECs displaying cognate self-peptide-MHC class II complexes. These interactions also involve the engagement of CD40 on mTECs by CD40L induced on the positively selected CD4(+) thymocytes. This antigen-specific TCR-MHC class II-mediated crosstalk between CD4(+) thymocytes and mTECs defines a unique checkpoint in thymic stromal development that is pivotal for generating a mature mTEC population competent for ensuring central T cell tolerance.

Although plasmacytoid dendritic cells (pDCs) express major histocompatibility complex class II (MHCII) molecules, and can capture, process, and present antigens (Ags), direct demonstrations that they function as professional Ag-presenting cells (APCs) in vivo during ongoing immune responses remain lacking. We demonstrate that mice exhibiting a selective abrogation of MHCII expression by pDCs develop exacerbated experimental autoimmune encephalomyelitis (EAE) as a consequence of enhanced priming of encephalitogenic CD4+ T cell responses in secondary lymphoid tissues. After EAE induction, pDCs are recruited to lymph nodes and establish MHCII-dependent myelin-Ag–specific contacts with CD4+ T cells. These interactions promote the selective expansion of myelin-Ag–specific natural regulatory T cells that dampen the autoimmune T cell response. pDCs thus function as APCs during the course of EAE and confer a natural protection against autoimmune disease development that is mediated directly by their ability to present of Ags to CD4+ T cells in vivo.

IntroductionMicroRNAs (miRNAs) are small, single-stranded, noncoding RNAs that regulate mRNAs by binding to their 3Ј untranslated (3ЈUTR) regions. 1,2 More than 9000 miRNAs have been identified in more than 100 species. Most miRNA genes are transcribed by RNA polymerase II into primary miRNA transcripts that are processed in the nucleus by a complex containing the RNase III endonuclease Drosha. 1 The resulting precursor miRNAs are transported to the cytoplasm, where the mature miRNAs are excised by a complex containing the endonuclease Dicer. 1 Mature miRNAs are incorporated into the RNA-induced silencing complex, which binds to the 3ЈUTRs of target mRNAs, inducing their degradation and/or repressing their translation. Posttranscriptional regulation of gene expression by miRNAs is critical for a wide range of physiologic and pathologic processes, including cell proliferation, apoptosis, differentiation, morphogenesis, development, and oncogenesis. [1][2][3][4] Several miRNAs play pivotal roles in the immune system. 5-7 MicroRNA-155 (miR155) has emerged as a particularly prominent player in innate and adaptive immune responses. 5,7 miR155 is derived from an exon of the B-cell integration cluster (BIC) gene, which was identified as a common integration site of avian leucosis virus in chicken B-cell lymphomas. 8,9 BIC is a non-protein-coding gene for which the only known function is the production of miR155. Subsequent studies revealed that miR155 expression is deregulated in diverse cancers. 10,11 The molecular mechanisms underlying the oncogenic role of miR155 remain unclear. miR155 expression is induced during the activation of T cells, B cells, monocytes, macrophages, and dendritic cells (DCs), suggesting that it plays multiple roles in the immune system. 5 In agreement with this, the immune system of miR155-deficient mice is compromised by defects in several cell types. 12,13 Activated T cells from miR155 Ϫ/Ϫ mice exhibit a bias toward Th2 differentiation and express elevated levels of IL4, IL5, and IL10. This was attributed to the fact that miR155 targets the mRNA coding for c-Maf, a transcription factor implicated in IL-4 expression and Th2 differentiation. 12 The B-cell compartment in miR155 Ϫ/Ϫ mice exhibits defects in germinal center development and in the generation of efficient antibody responses. miR155 is critical for affinity maturation because the generation of plasma cells produces high-affinity isotype-switched antibodies and the development of memory B cells. [12][13][14] The B-cell defects in miR155 Ϫ/Ϫ mice result at least in part from miR155 repressing the expression of the transcription factor PU.1 14 and activation-induced cytidine deaminase. 15,16 Lastly, bone marrow-derived DCs (BM-DCs) from miR155 Ϫ/Ϫ mice are impaired in their ability to activate T cells. 12 We recently reported that the induction of miR155 expression in human monocyte-derived DCs (Mo-DCs) exposed to the TLR4 ligand lipopolysaccharide (LPS) leads to modulation of the IL1 signal transduction pathway. 17 Another study foun...

αβT cell development depends upon serial migration of thymocyte precursors through cortical and medullary microenvironments, enabling specialized stromal cells to provide important signals at specific stages of their development. Although conventional αβT cells are subject to clonal deletion in the medulla, entry into the thymus medulla also fosters αβT cell differentiation. For example, during postnatal periods, the medulla is involved in the intrathymic generation of multiple αβT cell lineages, notably the induction of Foxp3+ regulatory T cell development and the completion of invariant NKT cell development. Although migration of conventional αβT cells to the medulla is mediated by the chemokine receptor CCR7, how other T cell subsets gain access to medullary areas during their normal development is not clear. In this study, we show that combining a panel of thymocyte maturation markers with cell surface analysis of CCR7 and CCR4 identifies distinct stages in the development of multiple αβT cell lineages in the thymus. Although Aire regulates expression of the CCR4 ligands CCL17 and CCL22, we show that CCR4 is dispensable for thymocyte migration and development in the adult thymus, demonstrating defective T cell development in Aire−/− mice is not because of a loss of CCR4-mediated migration. Moreover, we reveal that CCR7 controls the development of invariant NKT cells by enabling their access to IL-15 trans-presentation in the thymic medulla and influences the balance of early and late intrathymic stages of Foxp3+ regulatory T cell development. Collectively, our data identify novel roles for CCR7 during intrathymic T cell development, highlighting its importance in enabling multiple αβT cell lineages to access the thymic medulla.

The thymus ensures the generation of a functional and highly diverse T-cell repertoire. The thymic medulla, which is mainly composed of medullary thymic epithelial cells (mTECs) and dendritic cells (DCs), provides a specialized microenvironment dedicated to the establishment of T-cell tolerance. mTECs play a privileged role in this pivotal process by their unique capacity to express a broad range of peripheral self-antigens that are presented to developing T cells. Reciprocally, developing T cells control mTEC differentiation and organization. These bidirectional interactions are commonly referred to as thymic crosstalk. This review focuses on the relative contributions of mTEC and DC subsets to the deletion of autoreactive T cells and the generation of natural regulatory T cells. We also summarize current knowledge regarding how hematopoietic cells conversely control the composition and complex three-dimensional organization of the thymic medulla.

No abstract

Arginine, a semiessential amino acid implicated in diverse cellular processes, is a substrate for two arginases—Arg1 and Arg2—having different expression patterns and functions. Although appropriately regulated Arg1 expression is critical for immune responses, this has not been documented for Arg2. We show that Arg2 is the dominant enzyme in dendritic cells (DCs) and is repressed by microRNA-155 (miR155) during their maturation. miR155 is known to be strongly induced in various mouse and human DC subsets in response to diverse maturation signals, and miR155-deficient DCs exhibit an impaired ability to induce Ag-specific T cell responses. By means of expression profiling studies, we identified Arg2 mRNA as a novel miR155 target in mouse DCs. Abnormally elevated levels of Arg2 expression and activity were observed in activated miR155-deficient DCs. Conversely, overexpression of miR155 inhibited Arg2 expression. Bioinformatic and functional analyses confirmed that Arg2 mRNA is a direct target of miR155. Finally, in vitro and in vivo functional assays using DCs exhibiting deregulated Arg2 expression indicated that Arg2-mediated arginine depletion in the extracellular milieu impairs T cell proliferation. These results indicate that miR155-induced repression of Arg2 expression is critical for the ability of DCs to drive T cell activation by controlling arginine availability in the extracellular environment.

Lymphoid organs exhibit complex structures tightly related to their function. Surprisingly, although the thymic medulla constitutes a specialized microenvironment dedicated to the induction of T cell tolerance, its three-dimensional topology remains largely elusive because it has been studied mainly in two dimensions using thymic sections. To overcome this limitation, we have developed an automated method for full organ reconstruction in three dimensions, allowing visualization of intact mouse lymphoid organs from a collection of immunolabeled slices. We validated full organ reconstruction in three dimensions by reconstructing the well-characterized structure of skin-draining lymph nodes, before revisiting the complex and poorly described corticomedullary organization of the thymus. Wild-type thymi contain ∼200 small medullae that are connected to or separated from a major medullary compartment. In contrast, thymi of immunodeficient Rag2−/− mice exhibit only ∼20 small, unconnected medullary islets. Upon total body irradiation, medullary complexity was partially reduced and then recovered upon bone marrow transplantation. This intricate topology presents fractal properties, resulting in a considerable corticomedullary area. This feature ensures short distances between cortex and medulla, hence efficient thymocyte migration, as assessed by mathematical models. Remarkably, this junction is enriched, particularly in neonates, in medullary thymic epithelial cells expressing the autoimmune regulator. The emergence of a major medullary compartment is induced by CD4+ thymocytes via CD80/86 and lymphotoxin-α signals. This comprehensive three-dimensional view of the medulla emphasizes a complex topology favoring efficient interactions between developing T cells and autoimmune regulator–positive medullary thymic epithelial cells, a key process for central tolerance induction.