A case of kidney transplantation for complete C4 deficiency with recurrent IgM-monoclonal gammopathy of renal significance (MGRS) associated nephropathy

We present a rare case of a patient with complete C4 deficiency who underwent kidney transplantation and experienced immunoglobulin M-monoclonalgammopathyofrenalsignificance (IgM-MGRS) recurrence after the procedure. A 45-year-old male patient presented with end-stage renal failure due to membranoproliferative glomerulonephritis (MPGN). The initial immunosuppressive regimen consisted of tacrolimus, steroids, mycophenolate mofetil, basiliximab, and rituximab. He underwent ABO-incompatible kidney transplantation from his mother in August 2021. The clinical course after kidney transplantation was uneventful for a month. A biopsy of the transplanted kidney was performed due to decreased renal function. The allograft biopsy result led to the suspicion of primary macroglobulinemia-associated nephropathy. Bone marrow biopsy revealed an increase in plasma cells; however, no diagnosis of primary macroglobulinemia was made. At this point, IgM-MGRS was diagnosed instead of primary macroglobulinemia. A follow-up allograft biopsy was performed, and IgM-MGRS-associated nephropathy was diagnosed. Eventually, his retrieved autologous kidney biopsy from the initial examination showed that the primary disease was not MPGN but recurrent IgM-MGRS-associated nephropathy. Dexamethasone, rituximab, and cyclophosphamide (DRC) were started to treat IgM-MGRS due to worsening renal function (serum creatinine levels were in the 4–5 mg/dL range). Additional doses of DRC with 20 cycles of plasma exchange were introduced. Severe side effects occurred but did not result in death.