T cells in systemic sclerosis: a reappraisal

SSc is an autoimmune disease characterized by inflammation and extracellular matrix deposition that ultimately leads to loss of organ function. T cells appear to play a prominent role in its pathogenesis. The evidence for this comes from their being at the site of fibrosis, their activated phenotype and alteration in their number and frequency in peripheral blood. This review examines the role of T cells in the pathogenesis of SSc and specifically examines the key soluble profibrotic mediators (IL-4, IL-6, IL-13) secreted by Th2 cells and their interactions with fibroblasts that deposit excess extracellular matrix leading to fibrosis. We finally examine possible therapeutic options in targeting T-cell mediators to disrupt the cellular interactions between T cells and fibroblasts that serve to drive the fibrotic response. One of the factors driving fibrosis is IL-6 and this can be neutralized in vivo not only to limit IL-6-driven tissue fibrosis but concomitantly to suppress switching of Tregs to Th17 T cells that will provide more IL-6, thus perpetuating the fibrosis. Taken together, these data implicate the role of T cells in SSc and suggest that Th2-polarized T cells and the fibrotic mediators subsequently released directly induce fibrosis. Targeting such cytokines may be therapeutic not only in SSc but more generally in diseases where fibrosis is directed by inflammatory signals.