B Cells Participate in Thymic Negative Selection of Murine Auto-reactive CD4+ T Cells

It is well documented that thymic epithelial cells participate in the process of negative selection in the thymus. In recent years it was reported that also dendritic cells enter the thymus and contribute to this process, thus allowing for the depletion of thymocytes that are specific to peripherally expressed self-antigens. Here we report that also B cells may take part in the elimination of auto-reactive thymocytes. Using a unique mouse model we show that B cells induce negative selection of self-reactive thymocytes in a process that leads to the deletion of these cells whereas regulatory T cells are spared. These findings have direct implication in autoimmunity, as expression of a myelin antigen by B cells in the thymus renders the mice resistant to autoimmune inflammation of the CNS

Hydrophobic CDR3 residues promote the development of self-reactive T cells

Studies of individual T cell antigen receptors (TCRs) have shed some light on structural features that underlie self-reactivity. However, the general rules that can be used to predict whether TCRs are self-reactive have not been fully elucidated. Here we found that the interfacial hydrophobicity of amino acids at positions 6 and 7 of the complementarity-determining region CDR3β robustly promoted the development of self-reactive TCRs. This property was found irrespective of the member of the β-chain variable region (V[subscript β]) family present in the TCR or the length of the CDR3β. An index based on these findings distinguished V[subscript β]2[superscript +], V[subscript β]6[superscript +] and V[subscript β]8.2[superscript +] regulatory T cells from conventional T cells and also distinguished CD4[superscript +] T cells selected by the major histocompatibility complex (MHC) class II molecule I-A[superscript g7] (associated with the development of type 1 diabetes in NOD mice) from those selected by a non–autoimmunity-promoting MHC class II molecule I-Ab. Our results provide a means for distinguishing normal T cell repertoires versus autoimmunity-prone T cell repertoires

A study of 82 extended HLA haplotypes in HFE-C282Y homozygous hemochromatosis subjects: relationship to the genetic control of CD8+ T-lymphocyte numbers and severity of iron overload

BACKGROUND: It has been recently demonstrated that CD8+ T-lymphocyte numbers are genetically transmitted in association with the MHC class I region. The present study was designed with the objective of narrowing the region associated with the setting of CD8+ T-lymphocyte numbers in a population of C282Y homozygous hemochromatosis subjects, in whom a high prevalence of abnormally low CD8+ T-lymphocyte counts has been described. METHODS: The study includes 43 C282Y homozygous subjects fully characterized both phenotypically and genotypically. Clinical characterization includes measurements of iron parameters at diagnosis (transferrin saturation and serum ferritin), total body iron stores and T-cell immunophenotyping determined by flow cytometry. Genetic characterization includes HLA class I alleles (A, B and C) and four additional microsatellite markers (D6S265, D6S2222, D6S105 and D6S2239) spanning 5 Megabases in the 6p21.3 region. RESULTS: Eighty-two extended C282Y carrying haplotypes were defined. Single-locus analysis revealed that the HLA-A region was associated with CD8+ T-cell numbers. Multivariate analysis showed that the combinations of the most common HLA-A alleles (HLA-A*03, -A*02 and -A*01) were associated with significantly lower numbers of CD8+ T-lymphocytes (0.30 ± 0.14 × 10(6)/ml), in comparison with subjects carrying only one copy of those alleles (0.46 ± 0.19 × 10(6)/ml) and subjects without any copy of those alleles (0.79 ± 0.15 × 10(6)/ml;p = 0.0001). No differences were observed in CD8+ T-cell counts among control subjects carrying the same combinations of HLA-A alleles (0.47 ± 0.14; 0.45 ± 0.21 and 0.41 ± 0.17 × 10(6)/ml, respectively), therefore not supporting a direct effect of HLA specificity but rather an indirect association with a locus close to HLA-A. Multivariate analysis showed that the combination of the most common HLA-A alleles also have an impact on the clinical expression of HH in terms of iron stores, in males(p = 0.0009). CONCLUSION: The present study provides evidence supporting an inextricable link between extended HLA haplotypes, CD8+ T-lymphocyte numbers and severity of iron overload in hereditary hemochromatosis(HH). It gives additional information to better define a candidate region involved in the regulation of CD8+ T-lymphocyte numbers. A new evolutionary hypothesis concerning the inheritance of the phenotype of low CD8+ T-lymphocyte numbers associated with particular ancestral HLA haplotypes carrying the C282Y mutation and its implication on the clinical heterogeneity of HH is discussed

Central CD4+ T cell tolerance: deletion versus regulatory T cell differentiation

The diversion of MHC class II-restricted thymocytes into the regulatory T (Treg) cell lineage, similarly to clonal deletion, is driven by intrathymic encounter of agonist self-antigens. Somewhat paradoxically, it thus seems that the expression of an autoreactive T cell receptor is a shared characteristic of T cells that are subject to clonal deletion and those that are diverted into the Treg cell lineage. Here, we discuss how thymocyte-intrinsic and -extrinsic determinants may specify the choice between these two fundamentally different T cell fates

Autonomous role of medullary thymic epithelial cells in central CD4+ T cell tolerance

International audienceMedullary thymic epithelial cells (mTECs) serve an essential function in central tolerance through expressing peripheral tissue-antigens. These antigens may be transferred to and presented by dendritic cells. Therefore, it is unclear whether mTECs, besides being an 'antigen-reservoir', also serve a mandatory function as antigen presenting cells. Here, we reduced MHC class II on mTECs through transgenic expression of a C2TA-specific 'designer miRNA'. This resulted in an enlarged polyclonal CD4 single-positive compartment and, among thymocytes specific for model-antigens expressed in mTECs, enhanced selection of regulatory T cells (Treg) at the expense of deletion. Our data document an autonomous contribution of mTECs to dominant and recessive mechanisms of CD4+ T cell tolerance and support an avidity model of Treg development versus deletion

Comparison of thymocyte development in normal and invariant chain-deficient mice provides evidence that maturation-related changes in TCR and co-receptor levels play a critical role in cell fate.

Cells of invariant chain-deficient mice show a substantial decrease in cell surface MHC class II protein expression, as well as a change in the occupancy of the expressed class II molecules. Taking advantage of recent advances in phenotypic identification of transitional populations of developing thymocytes, the effects of these changes in MHC class II on positive and negative selection were reanalyzed. A marked (approximately 6-fold) reduction in CD4 single-positive mature cells was seen in H-2b mutant mice, yet there was little change in the number of CD4hiCD8intTCRint cells, a population containing the cells from which mature CD4+ cells derive. In normal mice expressing I-E and MMTV-encoded vSAG, V beta-specific negative selection occurred at a later point in the maturation pathway for cells showing greater expression of CD8 than CD4. In invariant chain-deficient mice, vSAG-mediated negative selection was diminished in general and what deletion still occurred was seen in more mature populations as compared to wild-type mice. Taken together, the decrease in MHC class II expression in invariant chain mutant mice and these alterations in the timing of thymocyte deletion provide strong support for an avidity model of negative selection. Perhaps more importantly, they emphasize the importance of the increasing TCR expression, the changing co-receptor levels and the movement from one antigen-presenting cell to another that accompany T cell maturation in determining the fate of developing thymocytes

Inactivation of Notch 1 in immature thymocytes does not perturb CD4 or CD8T cell development.

Notch proteins influence cell-fate decisions in many developing systems. Several gain-of-function studies have suggested a critical role for Notch 1 signaling in CD4-CD8 lineage commitment, maturation and survival in the thymus. However, we show here that tissue-specific inactivation of the gene encoding Notch 1 in immature (CD25+CD44-)T cell precursors does not affect subsequent thymocyte development. Neither steady-state numbers nor the rate of production of CD4+ and CD8+ mature thymocytes is perturbed in the absence of Notch 1. In addition, Notch 1-deficient thymocytes are normally sensitive to spontaneous or glucocorticoid-induced apoptosis. In contrast to earlier reports, these data formally exclude an essential role for Notch 1 in CD4-CD8 lineage commitment, maturation or survival