β5t-containing thymoproteasome: specific expression in thymic cortical epithelial cells and role in positive selection of CD8+ T cells
Highlights
► The thymoproteasome contains the vertebrate-specific catalytic subunit β5t. ► The thymoproteasome is specifically expressed by thymic cortical epithelial cells. ► The thymoproteasome is essential for positive selection of a functionally competent repertoire of CD8+ T cells.
Introduction
Fifty years have passed since the immunological role of the thymus was discovered [1, 2, 3]. It is now clear that the thymus is an organ that produces T lymphocytes and shapes their repertoire of antigen recognition specificity. Molecular signals that induce the proliferation and differentiation of T-lymphoid progenitor cells in the thymus include those mediated by the IL-7 receptor and Notch1 in T-lymphoid progenitor cells, through intrathymic ligand interactions with IL-7 and DLL4, respectively [4, 5]. The sequential engagement of the IL-7 receptor and Notch1 in the thymus seems sufficient for supporting the V(D)J recombination and expression of T-cell antigen receptor (TCR) β in T-lymphoid progenitor cells [6, 7]. The successful expression of in-frame TCRβ chains is censored through the signals provided by TCRβ-containing pre-TCR complexes [8], which drive the recombination and expression of TCRα as well as the expression of coreceptors CD4 and CD8. Only the cells that succeed in the production of in-frame TCRα chains become TCRαβ+CD4+CD8+ immature T cells [9, 10].
Newly generated TCRαβ+CD4+CD8+ cells carry a virgin repertoire of TCR recognition specificities and are positively and negatively selected in the thymus according to the TCR ligand interactions with peptide–MHC complexes. High-affinity TCR interactions with self-peptide–MHC complexes induce the negative selection (i.e. deletion) of self-reactive thymocytes [11] and occasionally the generation of regulatory T cells by a yet unclear mechanism [12]. The ectopic expression of various genes representing essentially all tissues of the body, including tissue-restricted genes, by thymic medullary epithelial cells (mTECs) [13] and the proximal interactions between mTECs and dendritic cells (DCs) [14, 15] facilitate the efficient presentation of diverse self-antigens, including tissue-restricted antigens, in the thymic medulla and contribute to the establishment of self-tolerance by trimming the repertoire of T cells reactive to self-antigens.
In addition to negative selection and the generation of regulatory T cells, the thymus contributes to the formation of a T cell repertoire by inducing positive selection. Positive selection was originally identified as the thymic process that determines the MHC restriction specificity of T-cell-mediated immune responses [16]. Low-affinity TCR interactions of TCRαβ+CD4+CD8+ cells in the thymic cortex where these cells are generated induce the survival and differentiation to CD4+CD8− or CD4−CD8+ cells [10, 11] and the migration of the differentiated cells to the thymic medulla by the expression of CCR7, the chemokine ligands of which are produced by mTECs [17, 18]. Positive selection also nurtures the formation of the thymic medulla by the production of RANKL, which induce the proliferation of mTECs [19]. Cells that primarily induce positive selection are thymic cortical epithelial cells (cTECs) [20].
Until recently, however, little was known about the mechanism by which cTECs are specialized to induce the positive selection of T cells. Recent identification of the thymoproteasome, which is specifically expressed by cTECs, has advanced our understanding of how the positive selection is induced and how the positive selection contributes to the establishment of the immune system. It is now known that cTECs possess a unique protein degradation machinery that is specialized to induce the positive selection of a functionally competent T cell repertoire.
Section snippets
Structure of thymoproteasome
The proteasome is a macromolecular proteolytic enzyme that degrades regulatory proteins and aberrant proteins into small peptides in eukaryotic cells, usually in cooperation with the ubiquitin system. Peptides generated by the proteasome serve as a major source of peptides for presentation on MHC class I molecules in vertebrates [21]. The catalytic core cylinder of the proteasome, the 20S proteasome, comprises four stacked heptameric rings. The two outer rings contain seven α-type subunits
Expression of thymoproteasome
The thymoproteasome-specific subunit β5t is exclusively expressed in the mouse thymus by cTECs and not by other thymic cells including thymocytes and DCs as well as mTECs, fibroblasts, endothelial cells, and macrophages [25•]. Mouse experiments have demonstrated that β5t is not detected anywhere outside the thymus [23••, 25•]. An analysis of human thymus sections has shown that β5t is detected in a fraction of DCs in the thymic cortex, in addition to cTECs [26•]. By contrast, β5t mRNA and
Role in positive selection of CD8+ T cells
An analysis of β5t-deficient mice has shown that β5t deficiency specifically and markedly affects the generation of CD4−CD8+ T cells in the thymus and the secondary lymphoid organs [23••]. The cellularity of cTECs and the formation of the thymic cortex are unaffected. The numbers of CD4+CD8+ thymocytes and CD4+CD8− T cells as well as other lineages of immune cells, including TCRγδ+ cells, CD8αα+ intraepithelial lymphocytes, NKT cells, NK cells, B cells, DCs, and macrophages, are not altered [44
Possible role in production of positively selecting peptides in cTECs
How the thymoproteasome regulates the positive selection of CD8+ T cells is still a matter of speculation and discussion. It is clear that the deficiency of β5t in cTECs specifically impairs the development of the major and immunocompetent repertoire, but not the entire population, of CD8+ T cells. The development of CD4+ T cells or any other cell lineages is unaffected in β5t-deficient mice, indicating that the thymoproteasome specifically regulates the positive selection of the
Perspectives
In order to directly examine the thymoproteasome-dependent production of a cTEC-specific repertoire of class I MHC-associated peptides, it is apparent that class I MHC-bound peptides displayed on the surface of thymoproteasome-expressing cTECs should be biochemically identified. These peptides may be unique to cTECs and not found in other cells, owing to the unique enzymatic activity of the thymoproteasome. Alternatively, the decreased chymotrypsin-like activity of the thymoproteasome may
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgement
This study was supported by MEXT Grant-in-Aid for Scientific Research on Priority Areas ‘Immunological Self.’
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