In Russia, PMN is a common type of glomerulopathy, the specific features of which should include the low rates of spontaneous remissions and detection of anti-PLA2R Abs. For renal protection, the majority of patients with PMN require timely diagnosis and treatment; individualization of the choice of treatment and its enhanced efficiency call for further investigations.

BACKGROUND AND AIMS Monoclonal IgM (mIgM) secreted by B-cell lineage clone is able to damage renal tissue by the variety of mechanisms that leads to kidney dysfunction and progression of chronic kidney disease. In this study, we analyse the prevalence of mIgM-related kidney disease, morphological spectrum of kidney lesions, lymphoproliferative disorders (LPD) spectrum, haematological response (HR) and renal response (RR) as well as long-term renal prognosis in patients with this condition treated in tertiary referral nephrological centre of Saint-Petersburg, Russian Federation. METHOD In this one-centre prospective study performed from 1 January 2011 till 1 September 2021, patients with biopsy-proven kidney injury associated with monoclonal gammopathy (MG; n = 221) cases with mIgM-related kidney disease were enrolled. The prevalence, clinical and morphological parameters, type of treatment, HR and RR were analysed. HR was assessed according to the accepted criteria. The presence of RR was considered as a decrease in daily proteinuria > 30% from the initial level or less than 0.5 g in the absence of a decrease in eGFR > 25% at the time of the end of follow-up. The progression of renal dysfunction was documented with a decrease in eGFR > 25% from baseline. Other cases were assessed as stable kidney function. Kidney outcome was determined as initiation of renal replacement therapy or eGFR < 15 mL/min/1.73 m2 at the end of follow-up. Long-term renal survival was assessed by the Kaplan–Meier method. The median follow-up period was 6 (3; 25) months for studied group and 21 (6; 52) months for the kidney disease associated with non-IgM MG group. RESULTS The prevalence of mIgM-related kidney disease (n = 16) was 7.2% of all MG-associated kidney injury and 0.6% of all performed kidney biopsies (n = 2544). Morphological spectrum of mIgM-related kidney disease is shown in Figure1A. Interestingly, in three cases thrombotic microangiopathy accompanied other types of renal injury. As an LPD the majority of patients has IgM-related disease (Figure1B) and there was no cases of smoldering Waldenström macroglobulinaemia. MYD88 mutation was available only in three cases, all were positive. Clinical and demographic parameters at the time of renal biopsy are presented in Figure 2. The majority of patients (n = 14) were treated with clone-specific therapy: rituximab-based schemes were used in nine patients combined with bortezomib in five cases. Due to contraindications to rituximab in four patients bortezomib- (n = 2), melphalan- (n = 1) and cyclophosphamide-based (n = 1) regimens were applied. HR and RR assessment was available in 13 patients (Figure 1C). Four patients died, all of them were non-responders to chemotherapy. Five-year cumulative renal survival was 33% in patients with mIgM-related kidney disease (Figure 1D) and was significantly lower than in patients with kidney disease associated with non-IgM MG. CONCLUSION Monoclonal IgM-related kidney disease is a rare entity with diverse clinical and morphological manifestations and characterized by very poor renal prognosis comparing with non-IgM-related kidney injury that requires in time diagnostics and clone-directed treatment.