Activation of Toll-like receptor 4 (TLR4) by its cognate damage-associated endogenous ligands (DAMPs) elicits potent profibrotic effects and myofibroblasts activation in systemic sclerosis (SSc), while genetic targeting of TLR4 or its DAMPs in mice accelerates fibrosis resolution. To prevent aberrant DAMP-TLR4 activity, a variety of negative regulators evolved to dampen the magnitude and duration of the signaling. These include radioprotective 105 KDa (RP105), a transmembrane TLR4 homolog that competitively inhibits DAMP recognition of TLR4, blocking TLR4 signaling in immune cells. The role of RP105 in TLR4-dependent fibrotic responses in SSc is unknown. Using unbiased transcriptome analysis of skin biopsies, we found that both TLR4 and its adaptor protein MD2 were elevated in SSc skin and significantly correlated with each other (r=0.54, p=0.0062). Expression of RP105 was negatively associated with myofibroblast differentiation in SSc (r=-0.53). Importantly, RP105-TLR4 association was reduced while TLR4- TLR4 showed strong association in SSc fibroblasts as shown by PLA assays. Moreover, RP105 adaptor MD1 expression was significantly reduced in SSc skin biopsies and explanted SSc skin fibroblasts. Exogenous RP105-MD1 abrogated, while loss of RP105 exaggerated, fibrotic cellular responses. Importantly, ablation of RP105 in mice was associated with augmented TLR4 signaling and aggravated skin fibrosis in complementary disease models. Thus, we identify RP105-MD1 as a novel cell-intrinsic negative regulator of TLR4-MD2-driven sustained fibroblast activation, representing a critical regulatory network governing the fibrotic process. Impaired RP105 function in SSc might contribute to persistence of progression of the disease.
Wenxia Wang, Swarna Bale, Bharath Yalavarthi, Priyanka Verma, Pei-Suen Tsou, Ken M. Calderone, Dibyendu Bhattacharyya, Gary J. Fisher, John Varga, Swati Bhattacharyya