Prss16 is not required for T-cell development

Thymus-specific serine protease, a protease that shapes the CD4 T cell repertoire

The lifespan of T cells is determined by continuous interactions of their T cell receptors (TCR) with self-peptide-MHC (self-pMHC) complexes presented by different subsets of antigen-presenting cells (APC). In the thymus, developing thymocytes are positively selected through recognition of self-pMHC presented by cortical thymic epithelial cells (cTEC). They are subsequently negatively selected by medullary thymic epithelial cells (mTEC) or thymic dendritic cells (DC) presenting self-pMHC complexes. In the periphery, the homeostasis of mature T cells is likewise controlled by the interaction of their TCR with self-pMHC complexes presented by lymph node stromal cells while they may be tolerized by DC presenting tissue-derived self-antigens. To perform these tasks, the different subsets of APC are equipped with distinct combination of antigen processing enzymes and consequently present specific repertoire of self-peptides. Here, we discuss one such antigen processing enzyme, the thymus-specific serine protease (TSSP), which is predominantly expressed by thymic stromal cells. In thymic DC and TEC, TSSP edits the repertoire of peptide presented by class II molecules and thus shapes the CD4 T cell repertoire.

Development and function of cortical thymic epithelial cells

The thymic cortex provides a microenvironment that supports the generation and T cell antigen receptor (TCR)-mediated selection of CD4(+)CD8(+)TCRαβ(+) thymocytes. Cortical thymic epithelial cells (cTECs) are the essential component that forms the architecture of the thymic cortex and induces the generation as well as the selection of newly generated T cells. Here we summarize current knowledge on the development, function, and heterogeneity of cTECs, focusing on the expression and function of β5t, a cTEC-specific subunit of the thymoproteasome.

Mesenchymal cells regulate retinoic acid receptor-dependent cortical thymic epithelial cell homeostasis

The vitamin A metabolite and transcriptional modulator retinoic acid (RA) is recognized as an important regulator of epithelial cell homeostasis in several tissues. Despite the known importance of the epithelial compartment of the thymus in T cell development and selection, the potential role of RA in the regulation of thymic cortical and medullary epithelial cell homeostasis has yet to be addressed. In this study, using fetal thymus organ cultures, we demonstrate that endogenous RA signaling promotes thymic epithelial cell (TEC) cell-cycle exit and restricts TEC cellularity preferentially in the cortical TEC compartment. Combined gene expression, biochemical, and functional analyses identified mesenchymal cells as the major source of RA in the embryonic thymus. In reaggregate culture experiments, thymic mesenchyme was required for RA-dependent regulation of TEC expansion, highlighting the importance of mesenchyme-derived RA in modulating TEC turnover. The RA-generating potential of mesenchymal cells was selectively maintained within a discrete Ly51(int)gp38(+) subset of Ly51(+) mesenchyme in the adult thymus, suggesting a continual role for mesenchymal cell-derived RA in postnatal TEC homeostasis. These findings identify RA signaling as a novel mechanism by which thymic mesenchyme influences TEC development.

Differential processing of self-antigens by subsets of thymic stromal cells

The stromal network of the thymus provides a unique environment that supports the development of mature CD4(+) and CD8(+) T cells expressing a very diverse repertoire of T cell receptors (TCR) with limited reactivity to self-antigens. Thymic cortical epithelial cells (cTECs) are specialized antigen-presenting cells (APCs) that promote the positive selection of developing thymocytes while medullary thymic epithelial cells (mTECs) and thymic dendritic cells (tDCs) induce central tolerance to self-antigens. Recent studies showed that cTECs express a unique set of proteases involved in the generation of self-peptides presented by major-histocompatibility encoded molecules (pMHC) and consequently may express a unique set of pMHC complexes. Conversely, the stromal cells of the medulla developed several mechanisms to mirror as closely as possible the constellation of self-peptides derived from peripheral tissues. Here, we discuss how these different features allow for the development of a highly diverse but poorly self-reactive repertoire of functional T cells.

Thymus-specific serine protease controls autoreactive CD4 T cell development and autoimmune diabetes in mice

Type 1 diabetes is a chronic autoimmune disease in which genetic predispositions affect the immune system, leading to a loss of T cell tolerance to β cells and consequent T cell-mediated destruction of insulin-producing islet cells. Genetic studies have suggested that PRSS16 is linked to a diabetes susceptibility locus of the extended HLA class I region in humans. PRSS16 encodes what we believe to be a novel protease, thymus-specific serine protease (TSSP), which shows predominant expression in thymic epithelial cells and is suspected to have a restricted role in the class II presentation pathway. Consistently, Tssp is necessary for the intrathymic selection of few class II-restricted T cell receptor specificities in B6 mice. To directly assess the role of Tssp in autoimmune diabetes, we generated Tssp-deficient (Tssp°) NOD mice. While remaining immunocompetent, Tssp° NOD mice were protected from diabetes and severe insulitis. Diabetes resistance of Tssp° NOD mice was a property of the CD4 T cell compartment that is acquired during thymic selection and correlated with an impaired selection of CD4 T cells specific for islet antigens. Hence, in the NOD mouse, Tssp is a critical regulator of diabetes development through the selection of the autoreactive CD4 T cell repertoire.

Thymus-specific serine protease contributes to the diversification of the functional endogenous CD4 T cell receptor repertoire

Thymus-specific serine protease expression in stromal as well as hematopoietic cells in the thymus is needed for diversification of the endogenous repertoire of TCRs specific for a particular protein antigen.

Thymus-specific serine protease (TSSP) is a novel protease that may contribute to the generation of the peptide repertoire presented by MHC class II molecules in the thymus. Although TSSP deficiency has no quantitative impact on the development of CD4 T cells expressing a polyclonal T cell receptor (TCR) repertoire, the development of CD4 T cells expressing the OTII and Marilyn transgenic TCRs is impaired in TSSP-deficient mice. In this study, we assess the role of TSSP in shaping the functional endogenous polyclonal CD4 T cell repertoire by analyzing the response of TSSP-deficient mice to several protein antigens (Ags). Although TSSP-deficient mice responded normally to most of the Ags tested, they responded poorly to hen egg lysozyme (HEL). The impaired CD4 T cell response of TSSP-deficient mice to HEL correlated with significant alteration of the dominant TCR-β chain repertoire expressed by HEL-specific CD4 T cells, suggesting that TSSP is necessary for the intrathymic development of cells expressing these TCRs. Thus, TSSP contributes to the diversification of the functional endogenous CD4 T cell TCR repertoire in the thymus.

Thymus-specific serine protease regulates positive selection of a subset of CD4+ thymocytes

Thymus-specific serine protease (TSSP) was initially reported as a putative protease specifically expressed in the endosomal compartment of cortical thymic epithelial cells (cTEC). As such, TSSP is potentially involved in the presentation of the self-peptides that are bound to MHC class II molecules expressed at the cTEC surface and are involved in the positive selection of CD4(+) thymocytes. We tested this hypothesis by generating mutant mice deprived of Prss16, the gene encoding TSSP. TSSP-deficient mice produced normal numbers of T cells, despite a decrease in the percentage of cTEC expressing high surface levels of MHC class II. By using sensitive transgenic models expressing MHC class II-restricted TCR transgenes (Marilyn and OT-II), we showed that the absence of TSSP markedly impaired the selection of Marilyn and OT-II CD4(+) T cells. In contrast, selection of CD8(+) T cells expressing an MHC class I-restricted TCR transgene (OT-I) was unaffected. Therefore, TSSP is involved in the positive selection of some CD4(+) T lymphocytes and likely constitutes the first serine protease to play a function in the intrathymic presentation of self-peptides bound to MHC class II complexes.

Association analysis in type 1 diabetes of the PRSS16 gene encoding a thymus-specific serine protease

We have previously mapped a separate type 1 diabetes (T1D) association in the extended MHC class I region, marked by D6S2223, on the DRB1*03-DQA1*0501-DQB1*0201 haplotype. The associated region encompasses a gene encoding a thymus-specific serine protease (PRSS16), presumably involved in positive selection of T cells or in T-cell regulation. Fourteen PRSS16 polymorphisms were genotyped in two steps using a total of six T1D family data sets, as well as case-control materials for both T1D and celiac disease (CD). An association with a 15 base-pair deletion in exon 12 of PRSS16 was found on the DRB1*03-DQA1*0501-DQB1*0201 haplotype for both T1D and CD, but it could not explain the more pronounced disease associations observed at marker D6S2223. We compared the performance of the 14 tested PRSS16 polymorphisms, selected after our previous comprehensive screen, against HapMap selected tag SNPs. Use of a HapMap based SNP selection strategy would result in loss of a large proportion of the genetic variation in PRSS16. Our data suggest that it is unlikely that polymorphisms within the PRSS16 gene are involved in the predisposition to T1D. However, we cannot rule out that regulatory polymorphisms located some distance away from the gene may be involved.

Gene expression profiling of precursor T-cell lymphoblastic leukemia/lymphoma identifies oncogenic pathways that are potential therapeutic targets

We compared the gene expression pattern of thymic tumors from precursor T-cell lymphoblastic lymphoma/leukemia (pre-T LBL) that arose in transgenic mice that overexpressed SCL, LMO1 or NUP98-HOXD13 (NHD13) with that of thymocytes from normal littermates. Only two genes, Ccl8 and Mrpl38, were consistently more than fourfold overexpressed in pre-T LBL from all three genotypes analyzed, and a single gene, Prss16 was consistently underexpressed. However, we identified a number of genes, such as Cfl1, Tcra, Tcrb, Pbx3, Eif4a, Eif4b and Cox8b that were over or under-expressed in pre-T LBL that arose in specific transgenic lines. Similar to the situation seen with human pre-T LBL, the SCL/LMO1 leukemias displayed an expression profile consistent with mature, late cortical thymocytes, whereas the NHD13 leukemias displayed an expression profile more consistent with immature thymocytes. We evaluated two of the most differentially regulated genes as potential therapeutic targets. Cfl1 was specifically overexpressed in SCL-LMO1 tumors; inactivation of Cfl1 using okadaic acid resulted in suppression of leukemic cell growth. Overexpression of Ccl8 was a consistent finding in all three transgenic lines, and an antagonist for the Ccl8 receptor-induced death of leukemic cell lines, suggesting a novel therapeutic approach.