The T Cell Repertoire–Diversifying Enzyme TSSP Contributes to Thymic Selection of Diabetogenic CD4 T Cell Specificities Reactive to ChgA and IAPP Autoantigens

Abstract

Multiple studies highlighted the overtly self-reactive T cell repertoire in the diabetes-prone NOD mouse. This autoreactivity has primarily been linked to defects in apoptosis induction during central tolerance. Previous studies suggested that thymus-specific serine protease (TSSP), a putative serine protease expressed by cortical thymic epithelial cells and thymic dendritic cells, may edit the repertoire of self-peptides presented by MHC class II molecules and shapes the self-reactive CD4 T cell repertoire. To gain further insight into the role of TSSP in the selection of self-reactive CD4 T cells by endogenous self-Ags, we examined the development of thymocytes expressing distinct diabetogenic TCRs sharing common specificity in a thymic environment lacking TSSP. Using mixed bone marrow chimeras, we evaluated the effect of TSSP deficiency confined to different thymic stromal cells on the differentiation of thymocytes expressing the chromogranin A–reactive BDC-2.5 and BDC-10.1 TCRs or the islet amyloid polypeptide–reactive TCR BDC-6.9 and BDC-5.2.9. We found that TSSP deficiency resulted in deficient positive selection and induced deletion of the BDC-6.9 and BDC-10.1 TCRs, but it did not affect the differentiation of the BDC-2.5 and BDC-5.2.9 TCRs. Hence, TSSP has a subtle role in the generation of self-peptide ligands directing diabetogenic CD4 T cell development. These results provide additional evidence for TSSP activity as a novel mechanism promoting autoreactive CD4 T cell development/accumulation in the NOD mouse.

Tissue-Specific Factors Differentially Regulate the Expression of Antigen-Processing Enzymes During Dendritic Cell Ontogeny

“…Interestingly, we found that although the regulation of Tssp expression during thymic cDC2 differentiation is conserved between B6 and NOD mice, its expression is significantly higher in the cDC1 subset of B6 mice as compared with NOD mice. The biological significance of this difference is unclear given that TSSP limits the presentation of self-antigen by thymic cDCs and consequently the negative selection of the corresponding CD4 T cell (16)(17)(18)20). Second, we found that Ctsl expression is barely detectable in pre-cDCs and further extinguished in the two cDC subset regardless of their tissue origin or whether cDCs are generated in in vitro cultures of BM with the exception of thymic cDC2, which maintain the low expression level of their BM precursor.…”

“…In addition, we showed that thymic cDCs uniquely express the thymus-specific serine protease (TSSP). In thymic cDCs, TSSP limits the presentation of several self-antigens and thus limits the deletion of self-reactive CD4 T cells (16)(17)(18)(19). Consequently, TSSP-deficient non-obese diabetic (NOD) mice are completely protected from autoimmune diabetes and develop less severe experimental autoimmune encephalomyelitis (16,20).…”

Thymic-Specific Serine Protease Limits Central Tolerance and Exacerbates Experimental Autoimmune Encephalomyelitis

“…We had shown previously that, likewise, lack of TSSP expression by thymic APCs favors the deletion of several islet-reactive CD4 T cells and, therefore, leads to complete protection from autoimmune diabetes (28)(29)(30). Hence, TSSP significantly curtailed the peptide repertoire presented by I-A g7 -expressing thymic DCs and, thereby, predisposed to autoimmunity.…”

“…Tssp-deficient (Tssp˚) B6 and NOD (NOD Tssp˚), NOD wild-type (WT), NODscid, Tssp˚NODscid, NOD-Tcra-deficient (NOD Ca˚), and TsspN OD Ca˚mice have been described previously (24,28). For generation of bone marrow (BM) chimeras, 10-12-wk-old NOD Ca˚and Tssp˚NOD Cam ice were irradiated (9.5 Gy) the day before reconstitution with 8 3 10 6 T cell-depleted NOD WT BM cells mixed at a 3:1 ratio with NODscid or Tssp˚NODscid BM cells, as previously described (30). This ratio is sufficient to induce deletion of islet-reactive CD4 T cells by TSSPdeficient APCs (28)(29)(30)(31).…”

“…In NOD mice, expression of TSSP by thymic stromal cells limits the presentation of several islet Ags by unknown mechanisms and, thus, limits central tolerance to these self-antigens (28)(29)(30). Hence, in NOD mice, lack of TSSP expression by thymic DCs or thymic epithelial cells (TECs) enhances thymic negative selection, as demonstrated by the increased deletion of several islet-reactive CD4 T cells, and prevents diabetes development (28)(29)(30).…”