Aberrant TLR responses driving organ fibrosis in systemic sclerosis

Unresolving tissue fibrosis due to persistent myofibroblast activation, the distinguishing hallmark of systemic sclerosis (SSc), accounts for its high mortality. Since current SSc treatments are only marginally effective in slowing or reversing fibrosis, there is a need to identify “drugable” targets. A series of recent findings, including genetic studies, suggest that Toll-like receptor 4 (TLR4) pathways may play a key pathogenic role in SSc by preventing fibrosis resolution. During the previous funding cycle, we showed that TLR4 expression, and levels of endogenous TLR4 ligands, are up-regulated, and TLR4 pathway activity is enhanced, in SSc biopsies despite only sparse inflammation within these tissues. Furthermore, we demonstrated that tenascin-C, a “damage-associated molecular pattern” (DAMP) produced by mesenchymal cells, is persistently elevated in the lesional microenvironment, and co-localizes with TLR4 expressed on myofibroblasts in lesional tissues. Remarkably, tenascin-C elicits myofibroblast activation via TLR4-MD2. The ubiquitin-editing enzyme A20, previously linked to autoimmune diseases and implicated in inflammation, is a candidate gene in SSc. We found that A20 is expressed in fibroblasts and negatively regulates fibrotic responses. Remarkably, both TLR4-mutant as well tenascin-C-null mice developed reduced fibrosis and demonstrated accelerated resolution. Our working hypothesis, based on these observations, is that fibrosis chronicity in SSc is maintained by on-going TLR4 signaling in tissue myofibroblasts responding to DAMPs within their extracellular microenvironment, impeding fibrosis resolution and tissue regeneration. Persistent cross-talk between the environment and resident fibroblasts, resulting from dysregulation of DAMPs and TLR4 pathways, represents a perturbation fundamental to the pathogenesis of non-resolving fibrosis in SSc. Aim 1 will employ genetic and pharmacological approaches to determine cell- and ligand-specific roles and mechanisms of TLR4/MD2 signaling in fibrosis. Aim 2 will evaluate fibroblast-specific regulation, function and modulation of A20 as a novel endogenous anti-fibrotic mechanism in SSc. Aim 3 will determine the site, source, and correlation with clinical phenotypes of TLR4 pathway expression in SSc skin biopsies. Employing human samples, cell-based assays, disease models and engineered mice, we will generate new knowledge to advance the understanding of SSc and fibrosis in general. In turn, this information can be leveraged to guide discovery and application of novel precision therapies that selectively target persistent myofibroblast activation in SSc.