What is myeloperoxidase doing in ANCA-associated glomerulonephritis?

O'Sullivan et al. describe glomerular localization of myeloperoxidase (MPO) in anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) and correlate the amount of deposition with severity of injury. MPO is the antigen against which anti-MPO ANCAs are directed, the antigen to which pathogenic T cells that can induce antibody-independent AAV are directed, and MPO can induce glomerular injury directly by interacting with H2O2 and a halide to halogenate glomerular structures. All three of these roles are likely involved in human disease.