In a mouse model of myeloperoxidase-antineutrophil cytoplasmic antibody- (MPO-ANCA-) induced vasculitis, mast cell deficiency was shown to reduce delayed hypersensitivity response to anti-MPO, regulatory T cells (Tregs), and lymph node-derived interleukin-10 (IL-10) as well as increase anti-MPO CD4+ T cells and enhance the disease severity [6]. This evidence concerns the gene MPO and vasculitis.