TLR7/8 Activation in Immune Cells and Muscle by RNA‐Containing Immune Complexes: Role in Inflammation and the Pathogenesis of Myositis

Objective Activation of endosomal toll‐like receptors (TLRs) is one possible driver of inflammation in idiopathic inflammatory myopathies (IIM). We investigated the potential contribution of TLR7 and TLR8 to IIM pathogenesis. Methods Activation of TLR7/8 in healthy donor peripheral blood mononuclear cells (PBMCs) by immune complexes from patients with IIM and lupus was tested. Autoantibody profiling of patient IgG samples was performed using a 1581‐antigen array. TLR7 and/or TLR8 activation by RNA molecules associated with autoantibodies was assessed. Gene expression in human myoblasts and satellite cells following treatment with supernatants from TLR7/8‐activated PBMCs was evaluated by NanoString. C57BL/6 mice were dosed intramuscularly with the TLR7/8 agonist R848 and single‐cell RNA‐sequencing was performed on the muscle to ascertain the cell types responding to TLR7/8 activation and the downstream effects. Results Overall, 69 patients with IIM were included with representation of dermatomyositis (DM), polymyositis (PM), and inclusion body myositis (IBM) subsets. Immune complexes from patients with IIM, as well as autoantibody‐associated RNAs His‐tRNA, Y1, Y4 and U1, activated PBMCs to produce IFN‐α and IL‐6 via TLR7/8. Several canonical (Ro60, Ro52, HIST1H4A) and novel (IL‐36RN) autoreactivities correlated highly with TLR7/8 activation. Supernatants from TLR7/8‐activated PBMCs had a negative impact on human myoblasts and satellite cells. Endothelial cells were activated by R848 in mouse muscle in vivo , in addition to immune cells such as monocytes and macrophages. Conclusion Our results suggest that patients with IIM have autoantibodies in their blood causing TLR7/8 activation, which leads to inflammation in muscles with potential deleterious effects. image