A distinct subset of neutrophils called low-density granulocytes (LDGs) contribute to the pathogenesis of a number of systemic autoimmune diseases, including systemic lupus erythematosus, in part through the formation of neutrophil extracellular traps (NETs). New findings implicate aberrant NET formation by LDGs in idiopathic inflammatory myopathies (IIM).

Credit: N. Smith/Springer Nature Limited

“We found that the presence of LDGs and NETs associates with clinical features of IIM and that NETs are elevated in the circulation and various organs of patients with IIM,” reports Mariana Kaplan, corresponding author on the new study. Increased levels of circulating NETs were associated with disease activity as well as with the presence of certain myositis-associated antibodies — in particular, antibodies against melanoma differentiation-associated protein 5 (MDA5). Notably, treatment of normal-density granulocytes with anti-MDA5 antibodies isolated from patients with IIM promoted NET formation in vitro.

RNA sequencing analysis identified a neutrophil gene signature in the skeletal muscle of patients with dermatomyositis (a subtype of IIM) that was associated with the expression of type I and type II interferon-regulated genes as well as indicators of muscle damage.

To investigate the pathogenic role of NETs in IIM, the researchers looked at the effects of NETs on muscle myotubes (formed from precursor myoblasts in vitro). NETs isolated from patients with IIM impaired the viability of the skeletal myotubes in vitro, but did not reduce myoblast proliferation.

The damaging in vitro effects of NETs were mediated in part by citrullinated histones (a component of NETs) and could be partially inhibited with an anti-histone H4 antibody. Importantly, treatment with citrullinated H4, but not treatment with native H4, was toxic to myotubes, recapitulating the effects of NETs in vitro.

treatment of normal-density granulocytes with anti-MDA5 antibodies… promoted NET formation

“Our results implicate neutrophil dysregulation in the pathogenesis of adult and childhood-onset IIM and suggest that targeting pathways of NET formation could have therapeutic roles in this disease,” concludes Kaplan.