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 IIMs 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 IIMs were included with representation of dermatomyositis, polymyositis, and inclusion body myositis subsets. Immune complexes from patients with IIMs, as well as autoantibody-associated RNAs histidyl-transfer RNA, Y1, Y4, and U1, activated PBMCs to produce interferon-α and IL-6 via TLR7/8. Several canonical (Ro60, Ro52, and 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 IIMs have autoantibodies in their blood causing TLR7/8 activation, which leads to inflammation in muscles with potential deleterious effects.