In healthy individuals, T cells react against incoming pathogens, but remain tolerant to self-antigens, thereby preventing autoimmune reactions. CD4 regulatory T cells are major contributors in induction and maintenance of peripheral tolerance, but a regulatory role has been also reported for several subsets of CD8 T cells. To determine the molecular basis of peripheral CD8 T-cell tolerance, we exploited a double transgenic mouse model in which CD8 T cells are neonatally tolerized following interaction with a parenchymal self-antigen. These tolerant CD8 T cells have regulatory capacity and can suppress T cells in an antigen-specific manner during adulthood. Dickkopf-3 (DKK3) was found to be expressed in the tolerant CD8 T cells and to be essential for the observed CD8 T-cell tolerance. In vitro, genetic deletion of DKK3 or blocking with antibodies restored CD8 T-cell proliferation and IL-2 production in response to the tolerizing self-antigen. Moreover, exogenous DKK3 reduced CD8 T-cell reactivity. In vivo, abrogation of DKK3 function reversed tolerance, leading to eradication of tumors expressing the target antigen and to rejection of autologous skin grafts. Thus, our findings define DKK3 as a immune modulator with a crucial role for CD8 T-cell tolerance.immune tolerance | parenchymal cells | autoimmunity

Biglycan is a proteoglycan ubiquitously present in extracellular matrix of a variety of organs including heart and has been reported to be overexpressed in myocardial infarction. Myocardial infarction may be complicated by perimyocarditis through to date unclear mechanisms. Our aim was to investigate the capacity of TLR2/TLR4 ligand biglycan to enhance the presentation of specific Ags released upon cardiomyocyte necrosis. In vitro, Ova-pulsed bone-marrow derived dendritic cells from WT (C57BL/6), TLR2-, TLR4-, MyD88- or TRIF- deficient mice were co-treated with LPS, biglycan or vehicle and incubated with Ova-recognizing MHCI- or MHCII-restricted T cells. Biglycan enhanced Ova-specific cross-priming by more than 80% to MHCI-restricted T cells in both TLR2 - and TLR4 – pathway dependent manner. Accordingly, biglycan-induced cross-priming by both MyD88- and TRIF-deficient DCs was strongly diminished. Ova-specific activation of MHCII-restricted T cells was predominantly TLR4-dependent. Our first in vivo correlate was a model of experimental autoimmune perimyocarditis triggered by injection of cardiac-Ag pulsed DCs (BALB/c). Biglycan-treated DCs triggered perimyocarditis in extent and intensity comparable to LPS-treated DCs (average scores 1.3±0.3 and 1.5±0.4, respectively). Substitution with TLR4-deficient DCs abolished this effect. In a second in vivo approach, WT and biglycan-deficient mice were followed two weeks after induction of myocardial infarction. WT mice demonstrated significantly higher myocardial T-lymphocyte infiltration in comparison to biglycan-deficient animals. We conclude that TLR2/4 ligand biglycan, a component of the myocardial matrix, may enhance Ag-specific T cell priming via MyD88 and TRIF and stimulate autoimmune perimyocarditis.

The adaptive immune system protects organisms from harmful environmental insults. In parallel, regulatory mechanisms control immune responses in order to assure preservation of organ integrity. Yet, molecules involved in the control of T-cell responses in peripheral tissues are poorly characterized. Here, we investigated the function of Dickkopf-3 in the modulation of local T-cell reactivity. Dkk3 is a secreted, mainly tissue-derived protein with highest expression in organs considered as immune-privileged such as the eye, embryo, placenta, and brain. While T-cell development and activation status in naïve Dkk3-deficient mice was comparable to littermate controls, we found that Dkk3 contributes to the immunosuppressive microenvironment that protects transplanted, class-I mismatched embryoid bodies from T-cell-mediated rejection. Moreover, genetic deletion or antibody-mediated neutralization of Dkk3 led to an exacerbated experimental autoimmune encephalomyelitis (EAE). This phenotype was accompanied by a change of T-cell polarization displayed by an increase of IFNγ-producing T cells within the central nervous system. In the wild-type situation, Dkk3 expression in the brain was up-regulated during the course of EAE in an IFNγ-dependent manner. In turn, Dkk3 decreased IFNγ activity and served as part of a negative feedback mechanism. Thus, our findings suggest that Dkk3 functions as a tissue-derived modulator of local CD4+ and CD8+ T-cell responses.