MPO and systemic lupus erythematosus: VH4-34 gene usage has been shown to be obligatory for the production of most pathogenic IgM cold-agglutinins and has been for demonstrated in IgM-RhF, IgG anti-dsDNA antibodies in systemic lupus erythematosus (SLE), and IgM anti-myeloperoxidase antibodies in systemic vasculitis [24]–[27].