In my search to figure out why p-anca pauci-immune crescentic glomerulonephritis reacts to treatment differently than c-anca pauci-immune crescentic glomerulonephritis I found the info below. It appears that some get results from rituxan and others with cytoxan. However, they don't mention IgM.

Pauci-immune renal vasculitis is associated strongly with antineutrophil cytoplasmic antibodies (ANCAs) of the immunoglobulin G (IgG) class, which are detected in 80% to 90% of affected patients. IgA ANCAs have been reported in association with various conditions, but never in the setting of pauci-immune vasculitis.

Pauci-immune crescentic glomerulonephritis associated with ANCA of ... - PubMed - NCBI

I wonder if the immunoglobulins involved have anything to do with treatment results?