Untangling immunotactoid glomerulopathy in the MGRS era

Clinical Outcomes of Monoclonal Gammopathy of Renal Significance Without Detectable Clones

Introduction

Monoclonal gammopathy of renal significance (MGRS) is characterized by monoclonal immunoglobulin deposition in kidneys. However, monoclonal immunoglobulin and responsible clone(s) are not always detectable. Treatment response and kidney outcome of MGRS without detectable clones remain unclear.

Methods

In this single-center, retrospective cohort study, we identified MGRS without detectable clones from our biopsy repository between 2010 and 2022. We investigated the correlations between treatment regimens and kidney outcomes defined by proteinuria and estimated glomerular filtration rate (eGFR), and the impact of repeat kidney biopsy.

Results

Our study cohort included 29 cases (27 native kidney and 2 transplant allograft biopsies) of MGRS without detectable clones. At diagnosis, median serum creatinine was 1.8 mg/dl (interquartile range [IQR] 1.3-2.7), with proteinuria 4.6 g/gCr (IQR 2.3-7.9). Treatment regimens were variable: 6 (21%) received conservative therapy, 13 (45%) received plasma cell clone-directed therapy, 8 (28%) received lymphocytic clone-directed therapy, and 2 (7%) received nonclone-directed immunosuppressive therapy. Of 24 patients with proteinuria >0.5 g/gCr at diagnosis, 9 (38%) and 6 (25%) achieved complete response (CR) and partial response (PR), respectively. If interstitial fibrosis and tubular atrophy (IFTA) was >50% at the initial biopsy, less proportion of patients achieved CR. Six of 7 repeat biopsies showed progression of chronic changes (e.g., IFTA) but provided limited information on treatment response.

Conclusion

Treatment regimens and outcomes of MGRS without detectable clones were extremely variable. Repeat biopsy provided limited information to assess disease activity or the need for additional treatment. More sensitive tools are needed to detect clones and to assess treatment response.

Diagnosis of monoclonal immunotactoid glomerulopathy with positive λ chain by immunoelectron microscopy

We report the case of a 73-year-old-man who developed immunotactoid glomerulopathy (ITG). ITG is a rare disease characterized by proliferative glomerulonephritis and capillary wall deposits with a 10-60 nm diameter microtubular substructure. In monoclonal ITG, immunofluorescence analysis typically exhibits IgG with light chain restriction. Recent reviews recommend distinguishing monoclonal ITG from polyclonal ITG because monoclonal ITG is associated with a higher incidence of hematological disorders and better responsiveness to clone-directed therapy and renal prognosis. In our case, IgG, IgA, and IgM were negative by routine immunofluorescence; however, immunoelectron microscopy revealed positive λ chain. At 6 months after renal biopsy, the IgG λ chain was detected in the serum and urine, reflecting possible monoclonality. Therefore, it is useful to perform immunoelectron microscopy and follow-up with serum and urine protein electrophoresis and immunofixation to diagnose monoclonal ITG, even when routine immunofluorescence shows negative or nonspecific findings.

Acute glomerulonephritis

Glomerulonephritis is a heterogeneous group of disorders that present with a combination of haematuria, proteinuria, hypertension, and reduction in kidney function to a variable degree. Acute presentation with full blown nephritic syndrome or rapidly progressive glomerulonephritis is uncommon and is mainly restricted to patients with post-infectious glomerulonephritis, anti-neutrophil cytoplasmic antibodies-associated vasculitis, and anti-glomerular basement membrane disease. Most frequently, patients present with asymptomatic haematuria and proteinuria with or without reduced kidney function. All glomerulonephritis disorders can show periods of exacerbation, but disease flairs characteristically occur in patients with IgA nephropathy or C3 glomerulopathy. The gold standard for the diagnosis of a glomerulonephritis is a kidney biopsy, with a hallmark glomerular inflammation that translates into various histopathological patterns depending on the location and severity of the glomerular injury. Traditionally, glomerulonephritis was classified on the basis of the different histopathological patterns of injury. In the last few years, substantial progress has been made in unravelling the underlying causes and pathogenetic mechanisms of glomerulonephritis and a causal approach to the classification of glomerulonephritis is now favoured over a pattern-based approach. As such, glomerulonephritis can be broadly classified as immune-complex glomerulonephritis (including infection-related glomerulonephritis, IgA nephropathy, lupus nephritis, and cryoglobulinaemic glomerulonephritis), anti-neutrophil cytoplasmic antibodies-associated (pauci-immune) glomerulonephritis, anti-glomerular basement membrane glomerulonephritis, C3 glomerulopathy, and monoclonal immunoglobulin-associated glomerulonephritis. We provide an overview of the clinical presentation, pathology, and the current therapeutic approach of the main representative disorders in the spectrum of glomerulonephritis.