Expression of toll‐like receptor 3 and toll‐like receptor 7 in muscle is characteristic of inflammatory myopathy and is differentially regulated by Th1 and Th17 cytokines

Abstract Objective To assess the expression of Toll‐like receptor 3 (TLR‐3) and TLR‐7 in muscle tissue from patients with polymyositis (PM) and dermatomyositis (DM) and to investigate the function and regulation of TLR‐3 in cultured muscle cells. Methods The expression of TLR‐3, TLR‐7, HLA class I, and CD56, a marker of immature myoblast precursors, was analyzed using immunohistochemistry. TLR‐3 regulation and signaling were assessed in myoblasts and in differentiated myotubes with the TLR‐3 agonist poly(I‐C), necrotic myoblasts, and Th1 and Th17 cytokines, in the presence or absence of neutralizing anti–TLR‐3 antibody. Levels of TLR‐3 messenger RNA (mRNA) were quantified by reverse transcription–polymerase chain reaction. Levels of interleukin‐6 (IL‐6), CCL20, and IL‐8 were determined by enzyme‐linked immunosorbent assay. Results TLR‐3 and TLR‐7 were expressed in PM/DM tissues, but not in noninflammatory muscle tissues, and were primarily detected in inflammatory infiltrates, although a few muscle cells were also positive. These TLR‐3– and TLR‐7–positive fibers expressed high levels of CD56 and HLA class I antigens. A synergy between poly(I‐C) and IL‐17 was observed for the production of IL‐6 and CCL20. Similarly, stimulation with necrotic myoblasts increased IL‐6 production, and stimulation with necrotic myoblasts in combination with IL‐17 further increased the induction of IL‐6. TLR‐3 blockade decreased the inducing effect of necrotic myoblasts and IL‐17 on IL‐6 production. Stimulation with interferon‐γ (IFNγ) increased TLR‐3 mRNA levels, but IL‐17 down‐regulated the inducing effect of IFNγ. Conclusion Our findings indicate that TLR‐3 and TLR‐7 are expressed in inflammatory myopathic tissues, particularly in immature myoblast precursors. Necrotic muscle cells activate cytokine production, in part, through the TLR‐3 pathway, with a differential regulatory effect of Th1 and Th17 cytokines.