T-cells in SSc
Evidence for the role of T cells in the pathogenesis of SSc initially comes from the presence of the inflammatory infiltrate in the dermis, gastric mucosa and other affected tissue of patients with SSc, which appears to drive fibrosis. Analysis of skin infiltrates shows that there is an increase in CD3 cells in SSc patients. Another study has shown that the infiltrate contains a higher proportion of CD4 T cells, implicating a role of these cells in causing fibrosis of the skin. CD4 T cells have been associated with the pathogenesis of other autoimmune diseases, including rheumatoid arthritis. However, in these diseases there is a bias toward a Th1 response, whereas in SSc patients there appears to be a bias toward a Th2 response, as shown by the increase in Th2 cytokines, such as IL-4, -6 and -13 in the serum, skin and lung of bleomycin-treated mice (a mouse model of fibrosis) and in SSc patients. Furthermore, in radiation-induced fibrosis in rats, there is a predominance of CD4 T cells at sites of fibrosis, which are mainly comprised of Th2 subset. The role of Th2 cells in inducing fibrosis is controversial since a recent study has shown that these cells block collagen production in healthy dermal fibroblasts; however, it was also shown that fibroblasts derived from SSc patients were less susceptible to Th2 inhibition of collagen production. In addition, mice that genetically lack CD28 on T cells, the molecule required for full T-cell activation, have much reduced lung fibrosis compared with wild-type mice after bleomycin challenge. Importantly, adoptive transfer of CD28 T cells from wild-type mice recovered the blocked fibrosis in the CD28-deficient mice.
There is also evidence of a change in the proportion of Th17 and Treg cells in causing fibrosis in SSc patients, although this evidence is somewhat controversial. Examination of the skin and peripheral blood of SSc patients demonstrates an increase in Th17 cells and a decrease in Tregs. Mathian et al. demonstrated that Treg cells from SSc patients can inhibit anti-CD3-induced T-cell proliferation, thus suggesting that the immunosuppressive function of Treg cells is preserved. However, when examining the total number of Treg cells in patients, it was shown that there is a decrease in these cells and FoxP3 expression compared with controls, and this correlates with diagnosis of disease. This study also examines the levels of IL-17 and -22 in the serum of patients. Although there is no change in the concentration of IL-17 in SSc patients compared with controls, an increase in IL-22 is observed. This suggests a differentiation of Treg cells to Th17 cells, resulting in fibroblast activation and, thus, fibrosis. These observations have been confirmed, with an increase in CD4 cells observed but a decrease in Treg cell numbers. However, several other studies have shown an increase or no change in Treg numbers in the peripheral blood or skin of SSc patients compared with healthy controls.
Epigenetic modification (inheritable changes in gene expression that are not due to modification of DNA sequences) of inflammatory genes in T cells has been shown to have a role in the pathogenesis of SSc. Jiang et al. have shown a role for the epigenetic modification of the CD70 promoter TNFSF7. CD70 is a costimulatory molecule expressed by activated immune cells; its expression is tightly regulated by epigenetic modifications of its promoter region. CD70 interacts with CD27 to induce plasma cell proliferation and antibody production on B cells, it enhances the survival of antigen-specific T cells, and hence the interaction has a key role in the inflammatory process. It has been shown that CD70 expression is increased in SSc patients and bisulphite sequencing revealed that the overexpression of CD70 on SSc T cells is due to the hypomethylation of TNFSF7 promotor region. Epigenetic modification of the CD40 ligand promoter region has also been observed in SSc patients and has been suggested as a reason for the predominance of SSc in females. CD40 ligand is present on the X chromosome; in order to prevent overexpression of proteins encoded on the X chromosome in females, one chromosome is silenced by epigenetic modifications, such as DNA methylation. It has also been shown that in female SSc patients, there is an increase in CD40 ligand compared with healthy female controls, owing to the demethylation of the promoter region on the X chromosome. Interestingly, there was no change observed in the levels of methylation between male SSc patients and healthy male controls.
During infection or injury, leukocytes infiltrate the affected tissue due to the presence of adhesion molecules at the inflammatory site and on the surface of lymphocytes. The importance of the adhesion molecules L-selectin (expressed on leukocytes) and ICAM-1 (expressed by endothelial cells and fibroblasts) in causing fibrosis has been demonstrated in a bleomycin mouse model. Yoshizaki et al. showed in ICAM-1 and L-selectin-knockout bleomycin mice that ICAM-1 and L-selectin are essential for: T-cell infiltration to skin and the lungs; differentiation of T-helper cells to their Th17 and Th2 subtypes; and the increase in proinflammatory cytokine expression, which is usually observed in the bleomycin mouse model. The cell-adhesion molecule DNAX-1 (CD226) is increased in SSc patients' skin and in a majority of cells and, using bleomycin DNAX mice, DNAX-1 has shown to be involved in the development of dermal thickness and T-cell infiltration into the skin.