Myeloperoxidase and anti-MPO IgG synergistically enhance TLR9-mediated immune activation in ANCA-associated vasculitis.

Anti-neutrophil cytoplasmic antibodies (ANCAs) target antigens in neutrophil granules, such as myeloperoxidase (MPO) or proteinase 3, causing small-vessel inflammation. ANCA pathogenicity is typically attributed to direct neutrophil activation, but a comprehensive understanding of its mode of action is lacking. In this study, we elucidate an alternative mechanism underlying anti-MPO IgG (αMPO)-dependent immune activation. We demonstrate that MPO binds to extracellular DNA and protects it from nuclease degradation. Antibodies against MPO promote its FcγR-mediated uptake into murine GM-CSF-derived bone marrow myeloid cells (GM-BMCs). Importantly, complexing DNA with MPO and αMPO greatly enhances its ability to activate GM-BMCs in a Toll-like receptor 9 (TLR9)-dependent manner. This immunostimulatory effect occurs with both a bacterial plasmid-templated polymerase chain reaction (PCR) product and neutrophil "self" DNA and is validated in human pDCs. Our results reveal how MPO and αMPO cooperate to facilitate TLR9-dependent sensing of extracellular DNA and thus immune activation in ANCA-associated vasculitis.