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.