Thymic expression of a gastritogenic epitope results in positive selection of self-reactive pathogenic T cells

Involvement of thymus in amiodarone-treated autoimmune myocarditis in rats

We previously showed that amiodarone, an iodine-rich benzofuranic derivative, interferes with the progression of myocarditis using a rat model of experimental autoimmune myocarditis. Further studies have also revealed that intraperitoneal treatment with 12.5 mg/kg amiodarone, which is below the range of its therapeutic plasma concentration, also effectively interferes with the progression of myocarditis using the same model. The relationship between myocarditis and the thymus has not been studied in either patients or animal models. To investigate whether the thymus is involved in the effect of amiodarone on experimental autoimmune myocarditis, we examined its phenotypic distribution in thymocytes and peripheral blood lymphocytes using flow cytometry. We found no significant differences in the proportions of CD4(+) and CD8(+) T cells and the CD4(+)/CD8(+) T cell ratio in the control group compared with amiodarone-treated group. However, amiodarone-treated group induced a decrease in the proportion of CD4(+)TNF(+) and CD4(+)IL-4(+) T cells and an increase in CD4(+)IFN(+) T cells, resulting in a significant reduction of the CD4(+)TNF(+)/CD4(+)IFN(+) and CD4(+)IL-4(+)/CD4(+)IFN(+) T cell ratios. These results suggest for the first time that the thymus is actively involved when myocarditis is treated with amiodarone.

Spontaneous large-scale lymphoid neogenesis and balanced autoimmunity versus tolerance in the stomach of H+/K+-ATPase-reactive TCR transgenic mouse

Autoimmunity is often accompanied by the development of ectopic lymphoid tissues in the target organ, and these tissues have been believed to have close relevance to the severity of the disease. However, the true relationship between the extent of such lymphoid structures and the intensity or type of immune responses mediated by self-reactive T cells has remained unclear. In the present study, we generated transgenic mice expressing TCR from an autoimmune gastritis (AIG)-inducing Th1 cell clone specific for one of the major stomach self-Ags, H(+)/K(+)-ATPase alpha subunit. The transgenic mice spontaneously develop massive lymphoid neogenesis with a highly organized tissue structure in the gastric mucosa, demonstrating Ag-specific, T cell-mediated induction of the lymphoid tissues. Nevertheless, the damage of surrounding tissue and autoantibody production were considerably limited compared with those in typical AIG induced by neonatal thymectomy. Such a moderate pathology is likely due to the locally restricted activation and Th2 skewing of self-reactive T cells, as well as the accumulation of naturally occurring regulatory T cells in the target organ. Altogether, the findings suggest that lymphoid neogenesis in chronic autoimmunity does not simply correlate with the destructive response; rather, the overall activation status of the T cell network, i.e., the balance of self-reactivity and tolerance, in the local environment has an impact.

T-cell avidity and tuning: the flexible connection between tolerance and autoimmunity

Thymic T-cell selection mechanisms generate a cross-reactive, self-MHC restricted peripheral T-cell pool. Affinity and avidity are of profound influence on this selection and the generation of immunity. Autoreactive T cells can escape thymic deletion by lowering their avidity and retain this "tuned" state in the periphery. Upon activation, tuned T cells can cause autoimmunity, while immunotherapeutic strategies may be hampered by existing T-cell tolerance. The regulation of T-cell avidity and tuning therefore determines the balance between tolerance and autoimmunity and should be taken into account in the design of therapeutic strategies aimed at T-cell reactivity.

CD4+ CD25+ regulatory T cells modulate the T-cell and antibody responses in helicobacter-infected BALB/c mice

Gastric Helicobacter spp. induce chronic gastritis that may lead to ulceration and dysplasia. The host elicits a T helper 1 (Th1) response that is fundamental to the pathogenesis of these bacteria. We analyzed immune responses in Helicobacter-infected, normal mice depleted of CD4+ CD25+ T cells to investigate the in vivo role of regulatory T cells (Tregs) in the modulation of Helicobacter immunopathology. BALB/c and transgenic mice were depleted of CD4+ CD25+ T cells by administration of an anti-CD25 antibody either at the time of infection with Helicobacter or during chronic infection and gastritis. Depletion of CD25+ Tregs prior to and during infection of mice with Helicobacter spp. did not affect either bacterial colonization or severity of gastritis. Depletion of CD25+ Tregs was associated with increased Helicobacter-specific antibody levels and an altered isotype distribution. Paragastric lymph node cells from CD25+ Treg-depleted and control infected mice showed similar proliferation to Helicobacter antigens, but only cells from anti-CD25-treated animals secreted Th2 cytokines. CD25+ Tregs do not control the level of gastritis induced by gastric Helicobacter spp. in normal, thymus-intact BALB/c mice. However, CD25+ Tregs influence the cytokine and antibody responses induced by infection. Autoimmune gastritis is not induced in Helicobacter-infected mice depleted of CD25+ Tregs but is induced in CD25+ Treg-depleted mice, which have a higher frequency of autoreactive T cells.

Modulation of T cell function by TCR/pMHC binding kinetics

The interaction between the T cell receptor (TCR) and the peptide-MHC complex (pMHC) at the interface between the T cell and the antigen presenting cell (APC) is the main event controlling the specificity of antigen recognition by T cells. It is thought that TCR/pMHC binding kinetics are critical for the selection of the T cell repertoire in the thymus, as well as the activation of mature T cells in the periphery. One of the binding parameters that conditions T cell activation by pMHC ligands is the half-life of the TCR/pMHC interaction. This kinetic parameter is highly significant for the regulation of T cell activation and therefore determines the capacity of T cells to respond against pathogen- and tumor-derived antigens, avoiding self-reactivity. Several studies support the notion that T cells are activated only by TCR/pMHC interactions that are above a threshold of half-life. pMHC complexes that bind TCRs with half-lives below that threshold behave as null or antagonistic ligands. However, since prolonged half-lives can also impair T cell activation, there seems to be a ceiling for the TCR/pMHC half life that leads to efficient activation of T cells. According to these observations, efficient T cell activation would require an optimal half-life of TCR/pMHC interaction. These kinetic restrictions for T cell activation are important to generate a protective adaptive immune response minimizing cross-reactivity against self-constituents. The nature of the TCR/pMHC interaction defines in the thymus whether a thymocyte develops into a mature T cell or is eliminated by apoptosis. In addition, the kinetics of TCR/pMHC binding can determine the type of response shown by mature T cells in the periphery. Although several studies have focused on the modulation of T cell function by the affinity of the TCR/pMHC interaction, the binding kinetics rules governing T cell activation remain poorly understood. Here we review recent data and propose a new model for the regulation of T cell function by TCR/pMHC binding kinetics.

Shaping the T cell repertoire to a bona fide autoantigen: lessons from autoimmune gastritis

Murine autoimmune gastritis is one of the most well-defined organ-specific autoimmune diseases. CD4(+) T cells, which mediate the disease, recognize the highly abundant gastric H(+)/K(+) ATPase heterodimer. The H(+)/K(+) ATPase alpha subunit is also expressed in the thymus, in an aire-independent manner, whereas the H(+)/K(+) ATPase beta subunit is absent from the thymus. Analysis of both H(+)/K(+) ATPase-specific T cell receptor transgenic mice with different affinities for the gastric antigen and mice deficient in the H(+)/K(+) ATPase subunits has provided information on thymic and peripheral selection events. The H(+)/K(+) ATPase antigens play an important role in purging the repertoire of gastritogenic T cells, and recent data have suggested that this tolerance induction occurs primarily in the periphery. The gastritis system provides a powerful approach to determine the impact of peripheral antigen presentation in the target organ draining lymph node on tolerance and autoimmune disease.

Promiscuous Thymic Expression of an Autoantigen Gene Does Not Result in Negative Selection of Pathogenic T Cells

"Promiscuous" thymic expression of peripheral autoantigens can contribute to immunological tolerance in some cases. However, in this study we show that thymic mRNA expression alone cannot predict a contribution to thymic tolerance. Autoimmune gastritis is caused by CD4+ T cells directed to the alpha (H/Kalpha) and beta (H/Kbeta) subunits of the gastric membrane protein the H+/K+ ATPase. H/Kalpha mRNA is expressed in the thymus, but H/Kbeta expression is barely detectable. In this study, we demonstrate that thymic H/Kalpha in wild-type mice or mice that overexpressed H/Kalpha did not result in negative selection of pathogenic anti-H/Kalpha T cells. However, negative selection of anti-H/Kalpha T cells did occur if H/Kbeta was artificially overexpressed in the thymus. Given that H/Kalpha cannot be exported from the endoplasmic reticulum and is rapidly degraded in the absence of H/Kbeta, we conclude that H/Kalpha epitopes are unable to access MHC class II loading compartments in cells of the normal thymus. This work, taken together with our previous studies, highlights that thymic autoantigen expression does not necessarily result in the induction of tolerance.