Browsing Research from April 2016 by Publisher "American Association of Immunologists"
Now showing items 1-4 of 4
The Gli3 transcription factor expressed in the thymus stroma controls thymocyte negative selection via Hedgehog-dependent and -independent mechanismsThe Hedgehog (Hh) responsive transcription factor Gli3 is required for efficient thymocyte development in the fetus. In this study we show that Gli3, not detected in adult thymocytes, is expressed in the murine fetal and adult thymus stroma. PCR array analysis revealed Cxcl9, Rbp1, and Nos2 as novel target genes of Gli3. We show that Gli3 positively regulates the expression of these genes, most likely by suppressing an intermediate repressor. Deletion of autoreactive thymocytes depends on their interactions with the thymus stroma. Repression of the proapoptotic gene Nos2 in Gli3 mutants coincides with reduced apoptosis of double positive thymocytes undergoing negative selection in vitro and in vivo, and the production of autoreactive thymocytes. Taken together these data indicate that Gli3 controls thymocyte apoptosis and negative selection possibly via the regulation of Nos2. Defective Gli3 expression in the thymus stroma also resulted in decreased CD5 expression on mature thymocytes and inappropriate production of MHC class I-selected CD4+ cells, both consistent with reduced TCR signal strength. Overall our data indicate that Gli3 expressed in the thymus stroma regulates negative selection and TCR signal strength via Hh-dependent and -independent mechanisms, with implications for autoimmunity.
Peptide-specific, TCR-alpha-driven, coreceptor-independent negative selection in TCR alpha-chain transgenic miceAs thymocytes differentiate, Ag sensitivity declines, with immature CD4-CD8- double-negative (DN) cells being most susceptible to TCRsignaling events. We show that expression of alphabetaTCR from the DN3 stage lowers the threshold for activation, allowing recognition of MHC peptides independently of the TCR beta-chain and without either T cell coreceptor. The MHC class I-restricted C6 TCR recognizes the Y-chromosome-derived Ag HYK(k)Smcy. Positive selection in C6 alphabetaTCR females is skewed to the CD8 compartment, whereas transgenic male mice exhibit early clonal deletion of thymocytes. We investigated the effect of the HYK(k)Smcy complex on developing thymocytes expressing the C6 TCR alpha-chain on a TCR-alpha(-/-) background. On the original selecting haplotype, the skew to the CD8 lineage is preserved. This is MHC dependent, as the normal bias to the CD4 subset is seen on an H2b background. In male H2k C6 alpha-only mice, the presence of the HYK(k)Smcy complex leads to a substantial deletion of thymocytes from the DN subset. This phenotype is replicated in H2k C6 alpha-only female mice expressing an Smcy transgene. Deletion is not dependent on the beta variable segment of the C6 TCR or on a restricted TCR-beta repertoire. In contrast, binding of HYK(k)Smcy and Ag-specific activation of mature CD8+ T cells is strictly dependent on the original C6 beta-chain. These data demonstrate that, in comparison with mature T cells, alphabetaTCR+ immature thymocytes can recognize and transduce signals in response to specific MHC-peptide complexes with relaxed binding requirements.
Public T cell receptor beta-chains are not advantaged during positive selectionStudies of human and murine T cells have shown that public TCR beta-chain rearrangements can dominate the Ag-specific and naive repertoires of distinct individuals. We show that mouse T cells responding to the minor histocompatibility Ag HYDbSmcy share an invariant Vbeta8.2-Jbeta2.3 TCR gene rearrangement. The dominance of this rearrangement shows that it successfully negotiated thymic selection and was highly favored during clonal expansion in all animals examined. We hypothesized that such beta-chains are advantaged during thymic and/or peripheral selection and, as a result, may be over-represented in the naive repertoire. A sequencing study was undertaken to examine the diversity of Vbeta8.2-Jbeta2.3 CDR3 loops from naive T cell repertoires of multiple mice. Public TCR beta-chain sequences were identified across different repertoires and MHC haplotypes. To determine whether such public beta-chains are advantaged during thymic selection, individual chains were followed through T cell development in a series of novel bone marrow competition chimeras. We demonstrate that beta-chains were positively selected with similar efficiency regardless of CDR3 loop sequence. Therefore, the establishment and maintenance of public beta-chains in the periphery is predominantly controlled by post-thymic events through modification of the primary, thymus-derived TCR repertoire.
TCR-alpha CDR3 loop audition regulates positive selectionHow positive selection molds the T cell repertoire has been difficult to examine. In this study, we use TCR-β-transgenic mice in which MHC shapes TCR-α use. Differential AV segment use is directly related to the constraints placed on the composition of the CDR3 loops. Where these constraints are low, efficient selection of αβ pairs follows. This mode of selection preferentially uses favored AV-AJ rearrangements and promotes diversity. Increased constraint on the α CDR3 loops leads to inefficient selection associated with uncommon recombination events and limited diversity. Further, the two modes of selection favor alternate sets of AJ segments. We discuss the relevance of these findings to the imprint of self-MHC restriction and peripheral T cell activation.