We evaluate whether microbial-derived peptides, including those from S. aureus, with sequence homology to the immunodominant MPO CD4+ T-cell epitope can induce the expansion of naive CD4+ T cells that recognize MPO, with the subsequent development of cross-reactive anti-MPO autoimmunity leading to glomerulonephritis and AAV. This evidence concerns the gene MPO and glomerulonephritis.