MGRS — Monoclonal Gammopathy of Renal Significance
MGRS is a clonal plasma-cell or B-cell disorder that secretes a nephrotoxic monoclonal immunoglobulin yet does not meet the tumor-burden criteria to treat an overt hematologic malignancy. The term was coined so these patients are no longer dismissed as having a gammopathy of undetermined significance — once a monoclonal protein is shown to be injuring the kidney, the “significance” is anything but undetermined, and the patient can be offered clone-directed therapy.
of monoclonal-gammopathy + CKD patients have MGRS on biopsy
Klomjit & Zand, Kidney Int 2025 · PMID 40403931
MGUS prevalence at age ≥50 — the pool MGRS hides within
Kyle et al., NEJM 2006 · PMID 16571879
of MGRS biopsies are AL amyloidosis — the commonest lesion
Klomjit & Zand, Kidney Int 2025 · PMID 40403931
of PGNMID cases have a detectable serum or marrow clone
Bridoux et al., NDT 2021 · PMID 33494099
The lesions of MGRS
The monoclonal immunoglobulin injures the kidney through a handful of recurring patterns, grouped by how the protein deposits — as organized fibrils and microtubules, as non-organized granular deposits or complement dysregulation, or by directly injuring the proximal tubule.
Organized deposits
Immunoglobulin self-assembles into fibrils or microtubules with a recognizable ultrastructure.
AL / AH / AHL Amyloidosis
AmyloidMisfolded monoclonal light chains (AL, most common), heavy chains (AH) or both aggregate into β-pleated-sheet fibrils that deposit in glomeruli, vessels and interstitium. Often systemic — heart, liver, nerve.
- Deposit
- Monoclonal light chain (λ > κ in AL), heavy chain, or both
- Pattern
- Nephrotic-range proteinuria; Congo-red positive with apple-green birefringence
- Ultrastructure
- Randomly arranged fibrils ~8–12 nm; typed best by laser microdissection + mass spectrometry
Cryoglobulinemic GN (Type I / II)
Cryo-GNCold-precipitating immunoglobulins deposit in glomerular capillaries. Type I is a single monoclonal Ig (the MGRS-relevant form); Type II is monoclonal IgM with rheumatoid-factor activity plus polyclonal IgG.
- Deposit
- Monoclonal IgM/IgG (Type I); monoclonal IgM + polyclonal IgG (Type II)
- Pattern
- Membranoproliferative morphology with intraluminal pseudothrombi
- Ultrastructure
- Subendothelial deposits with microtubular / 'fingerprint' substructure
Immunotactoid Glomerulopathy
ITGRare glomerular disease with proteinuria, hematuria and kidney dysfunction. Monoclonal ITG has an underlying hematologic disorder in ~two-thirds of cases; renal response tracks the hematologic response.
- Deposit
- Monoclonal Ig (usually IgG) with light-chain restriction
- Pattern
- Proliferative GN; frequent recurrence after transplant
- Ultrastructure
- Hollow-cored microtubules, typically >30 nm, in parallel arrays
Fibrillary GN
FGNGlomerular deposition of randomly arranged fibrils (12–24 nm, Congo-red negative).
- Deposit
- Usually polyclonal IgG (NOT MGRS); rare monoclonal light-chain-restricted variant
- Pattern
- Mesangial/MPGN; usually no detectable serum monoclonal protein
- Ultrastructure
- Randomly arranged fibrils ~12–24 nm; DNAJB9-positive by IHC/mass spec
The DNAJB9 discovery reclassified this entity: the vast majority of fibrillary GN is DNAJB9-positive and polyclonal — and is NOT MGRS. Only the rare monoclonal subset qualifies.
Non-organized deposits
Granular or amorphous deposition along basement membranes, or complement-driven injury.
Monoclonal Ig Deposition Disease
MIDDNon-amyloid, Congo-red-negative granular deposition of monoclonal light chains (LCDD), heavy chains (HCDD) or both (LHCDD) along basement membranes, causing Randall-type nodular glomerulosclerosis.
- Deposit
- Monoclonal κ light chain (LCDD); truncated heavy chain (HCDD); both (LHCDD)
- Pattern
- Nodular mesangial sclerosis with nephrotic proteinuria; often systemic
- Ultrastructure
- Granular, powdery electron-dense deposits along tubular & glomerular basement membranes (linear on IF)
Proliferative GN with Monoclonal Ig Deposits
PGNMIDGranular glomerular deposits of monotypic IgG (most often IgG3 κ), a single heavy-chain subclass and light chain, plus complement. A detectable serum/marrow clone is found in only ~30%.
- Deposit
- Monotypic IgG (predominantly IgG3, κ-restricted); rarer light-chain-only / IgA / IgM
- Pattern
- Membranoproliferative or endocapillary proliferative, glomerular-limited
- Ultrastructure
- Granular amorphous deposits, predominantly subendothelial / mesangial
immunoglobulin-repertoire sequencing showed PGNMID-IgG3 is most often oligo/polyclonal — not from a clonal disorder — so these cases arguably should no longer be classified as MGRS. PGNMID is now understood as heterogeneous.
C3 Glomerulopathy with Monoclonal Gammopathy
C3G-MGThe monoclonal Ig acts not as a structural deposit but by dysregulating the alternative complement pathway — as an autoantibody to complement regulators or a C3-nephritic-factor-like driver — producing immunoglobulin-poor, C3-dominant injury.
- Deposit
- C3-dominant glomerular deposits; the pathogenic monoclonal Ig is in serum
- Pattern
- Membranoproliferative; dense-deposit disease or C3GN subtypes
- Ultrastructure
- C3-dominant staining with scant/absent immunoglobulin
Tubular / crystalline
Light chains injure the proximal tubule directly — as crystals in tubular cells or histiocytes.
Light Chain Proximal Tubulopathy / Fanconi
LCPTFiltered monoclonal light chains (usually κ) are endocytosed by proximal tubular cells and either crystallize or accumulate, impairing reabsorption and producing acquired Fanconi syndrome with slowly progressive CKD. Usually low tumor-burden clones.
- Deposit
- Monoclonal light chain (predominantly κ) within proximal tubular cytoplasm
- Pattern
- Fanconi syndrome — glycosuria, phosphaturia, aminoaciduria, proximal RTA
- Ultrastructure
- Intracytoplasmic crystalline (rhomboid/needle) inclusions, or amorphous lysosomal accumulation
Crystal-Storing Histiocytosis
CSHMonoclonal light chains crystallize within histiocytes/macrophages (rather than tubular cells), which accumulate in the renal interstitium and extrarenal sites. Associated with low-grade lymphoplasmacytic disorders.
- Deposit
- Monoclonal light chain (usually κ) within histiocyte cytoplasm
- Pattern
- Tubulointerstitial sheets of crystal-laden histiocytes
- Ultrastructure
- Intracytoplasmic crystalline inclusions within histiocytes
The EM bench — narrow the differential
9/9 fitSet what the biopsy shows and the lesions narrow. Congo red first — it alone separates amyloid from everything else — then the deposit's substructure.
9 of 9 lesions fit the selected findings.
- AL / AH / AHL AmyloidosisOrganized deposits
Congo red positive · randomly-arranged fibrils · 8–12 nm
The only Congo-red-positive lesion — apple-green birefringence settles it.
- Cryoglobulinemic GN (Type I / II)Organized deposits
Congo red negative · microtubules (organized) · fingerprint / annular-tubular
Intraluminal pseudothrombi + MPGN morphology; a cryocrit and hepatitis-C serology reframe the workup.
- Immunotactoid GlomerulopathyOrganized deposits
Congo red negative · microtubules (organized) · >30 nm (parallel arrays)
Hollow microtubules in parallel arrays — wider than amyloid/fibrillary fibrils and organized, not random.
- Fibrillary GNOrganized deposits
Congo red negative · randomly-arranged fibrils · 12–24 nm
Random fibrils like amyloid but Congo-red NEGATIVE and DNAJB9-positive — and usually polyclonal, so rarely MGRS.
- Monoclonal Ig Deposition DiseaseNon-organized deposits
Congo red negative · granular / powdery
Powdery, non-organized deposits with LINEAR basement-membrane staining on IF (vs the granular staining of PGNMID).
- Proliferative GN with Monoclonal Ig DepositsNon-organized deposits
Congo red negative · granular / powdery
Granular (not linear) IF for a single IgG subclass + light chain, glomerular-limited — but only ~30% have a detectable clone.
- C3 Glomerulopathy with Monoclonal GammopathyNon-organized deposits
Congo red negative · c3-dominant, scant ig
C3-dominant with scant/absent Ig — the monoclonal protein drives complement from the serum rather than depositing; factor-B inhibition (iptacopan) is now an option.
- Light Chain Proximal Tubulopathy / FanconiTubular / crystalline
Congo red negative · intracytoplasmic crystals
Crystals inside PROXIMAL TUBULAR cells with Fanconi syndrome — a low-burden κ clone; a non-crystalline lysosomal variant also exists.
- Crystal-Storing HistiocytosisTubular / crystalline
Congo red negative · intracytoplasmic crystals
Crystals inside HISTIOCYTES/macrophages filling the interstitium — not tubular cells (that is LCPT).
Teaching aid, not a diagnosis — immunofluorescence isotype/light-chain restriction, mass-spectrometry typing and the clonal work-up are what confirm the lesion. Findings and clues are drawn from the lesion cards; nothing here is computed.
What a MGRS biopsy shows — one cohort's yield
PMID 40403931 (opens PubMed in a new tab)Which lesion, among the 92 patients who had one, in the Klomjit & Zand, Mayo cohort · Kidney Int 2025 — 92 of 280 biopsied. Every percentage below is a share of those 92, not of everyone biopsied. AL amyloidosis dominates; PGNMID and the deposition diseases follow.
The named lesions are 87% of these 92; “other” is the source's own residual of rarer lesions, not an estimate. Several atlas lesions are too rare to be quantified individually here.
How a paraprotein injures the kidney
A single clone, secreting a single abnormal antibody, injures the nephron in several distinct ways.
The intact immunoglobulin or its free light chains precipitate along basement membranes and in the mesangium — granular in MIDD and PGNMID, expansile and nodular in Randall-type sclerosis — physically distorting filtration.
Misfolded light chains self-assemble into ordered ultrastructures: β-pleated-sheet amyloid fibrils, the hollow microtubules of immunotactoid GN, or the substructured deposits of cryoglobulinemia — congophilic or not, but unmistakable on EM.
In C3 glomerulopathy the antibody never deposits as a structure. It acts catalytically — as an autoantibody to a complement regulator or a C3-nephritic-factor-like driver — driving the alternative pathway for immunoglobulin-poor, C3-dominant injury.
Filtered light chains are endocytosed by proximal tubular cells, where they crystallize or overwhelm lysosomes — driving acquired Fanconi syndrome — or precipitate inside histiocytes as crystal-storing histiocytosis.
The MGRS workup
MGRS sits between hematology and nephrology — confirming it requires assessing both the clone and the kidney. The sequence runs from serum and urine screening to the indispensable biopsy.
- 1
Serum & urine electrophoresis + immunofixation
Detect and characterize the monoclonal protein (SPEP/UPEP/IFE).
- 2
Serum free light chains + involved/uninvolved ratio
More sensitive for light-chain-only clones; an abnormal ratio is a key MGRS predictor.
- 3
Bone marrow biopsy + flow / FISH
Identify and size the clone — plasma-cell vs B-cell — to direct therapy.
- 4
Kidney biopsy with IF + EM
Indispensable: light microscopy, immunofluorescence (isotype + light/heavy chain) and electron microscopy define the lesion.
- 5
Ancillary techniques
Pronase-digested IF, IgG-subclass staining, DNAJB9 IHC, and laser microdissection + mass spectrometry for definitive typing.
- 6
Mayo MGRS Prediction Tool (2025)
Estimates the probability of an MGRS lesion on biopsy from 8 predictors (AUC 0.896); helps decide whom to biopsy.
Who to biopsy — the Mayo MGRS predictor
Klomjit/Zand · PMID 40403931 (opens PubMed in a new tab)In a patient with CKD and a monoclonal gammopathy, eight readily-available factors predict whether a kidney biopsy will show an MGRS lesion (AUC 0.896, 0.836 optimism-corrected). Enter the values and the probability is computed from the published logistic model.
Enter the five numeric values (ratio, protein, creatinine all > 0) to compute the probability.
| Threshold | Sensitivity | Specificity | Reading |
|---|---|---|---|
| ≥ 0.10 | 98.9% | 50.5% | rule-out — up to ~10 biopsies to find 1 MGRS |
| ≥ 0.25 | 88.0% | 70.2% | balanced — up to ~4 biopsies to find 1 MGRS |
Which way each factor pushes
- Abnormal / high affected:unaffected FLC ratio→ more likely MGRS
- Higher urinary protein (g/day)→ more likely MGRS
- Positive UPEP or urine immunofixation→ more likely MGRS
- Hematuria→ more likely MGRS
- Lower C3 level→ more likely MGRS
- Higher serum creatinine→ less likely MGRS
- Higher systolic blood pressure→ less likely MGRS
- Diabetes→ less likely MGRS
A shared-decision aid, not a rule — it weighs the merits of a biopsy against its risks, and the threshold you accept depends on the patient (bleeding risk, the cost of missing MGRS). The model was derived in patients whose clone did NOT already meet criteria to treat (myeloma, high-risk smoldering myeloma, Waldenström, CLL); in those, treat the clone regardless. Educational estimate — not a diagnosis.
Free light-chain ratio — read it against kidney function
iStopMM · PMID 36100605 (opens PubMed in a new tab)The κ/λ ratio drifts up as eGFR falls, so the standard range flags a monoclonal gammopathy that isn't there. Enter the values and the kidney-function band, and the ratio is read against the interval that actually applies.
Enter κ and λ (or the ratio) and pick a kidney-function band.
Educational aid — not a diagnosis. A ratio outside the applicable interval suggests a monoclonal light-chain excess and prompts SPEP/serum immunofixation and hematology input; it does not by itself diagnose a gammopathy. CKD bands are the iStopMM eGFR-stratified intervals; the older single “renal range” for eGFR<60 was 0.37–3.1.
A clone below the treatment line
MGRS is a hematologic diagnosis that presents to nephrology. The clone sits at the low-burden end of the plasma-cell / B-cell spectrum: by marrow criteria it usually looks like MGUS or a low-risk smoldering disorder, so what makes it significant is the kidney, not the tumor burden.
A clone below the treatment threshold
PMID 30510265By definition the clone does not meet the tumor-burden criteria used to treat multiple myeloma, Waldenström macroglobulinemia or CLL — the same boundary that separates MGUS and smoldering myeloma from active disease. Hematologically these patients would otherwise be watched, not treated.
The kidney re-frames the clone
PMID 41324260A monoclonal immunoglobulin proven to be injuring the kidney is itself the indication to treat — clone-directed therapy is offered regardless of tumor burden. That inversion, treating a low-burden clone because of the organ it damages, is the whole point of the MGRS concept and the 2026 treatment standard.
Smoldering myeloma & the myeloma-defining events
PMID 40403931Renal failure from cast nephropathy is a myeloma-defining event that upgrades smoldering to active myeloma and mandates myeloma therapy. MGRS lesions are NOT cast nephropathy — they are treated on their own merits — so the Mayo prediction cohort deliberately excludes patients whose clone already meets criteria to treat (myeloma, high-risk smoldering myeloma, Waldenström, CLL).
Sizing and typing the clone directs therapy
PMID 39644070The marrow biopsy with flow cytometry and FISH characterizes and sizes the clone — plasma-cell vs B-cell / lymphoplasmacytic lineage and its cytogenetics — and that lineage selects the clone-directed regimen. In MGRS the cytogenetic and tumor-burden work-up guides WHICH therapy, while the kidney lesion decides WHETHER to treat.
MGRS is the renal member of a wider family
PMID 42545758Monoclonal gammopathy of clinical significance (MGCS) is the umbrella for clones that are pathogenic without meeting criteria for hematologic malignancy, and MGRS is its kidney member. The entities outside the kidney still arrive in nephrology clinics — POEMS syndrome, TEMPI syndrome, monoclonal gammopathy-associated systemic capillary leak syndrome and hematologic autoantibody-mediated disorders — and what classifies them all is the pathogenic mechanism of the paraprotein rather than the size of the clone. The same inversion as MGRS, applied beyond the kidney.
Treatment: treat the clone
Treat the clone, not just the inflammation. Therapy is clone-directed — target the specific plasma-cell or B-cell clone producing the nephrotoxic immunoglobulin. The depth of hematologic response strongly predicts renal recovery.
For AL amyloidosis the standard of care is daratumumab + CyBorD (Dara-VCd), established by the ANDROMEDA trial. Autologous stem-cell transplant is an option in eligible patients.
Targets the CD20+ clone driving lesions such as cryoglobulinemic GN and many cases of monoclonal immunotactoid glomerulopathy.
The central unsolved challenge — when no clone is detectable (e.g. some PGNMID/ITG), empiric therapy is often used while the field searches for the driver.
Kidney transplantation is potentially transformative but the untreated clone causes high allograft recurrence; achieving a deep hematologic response before transplant reduces recurrence and improves outcomes.
Treat the clone — lesion → clone → regimen
clone-directed therapyMGRS therapy targets the clone, not the deposit. Pick a lesion to route it to the clone its therapy aims at and the regimen — each anchored to a verified reference. Where the clone is genuinely variable or often undetectable, the path says so rather than implying certainty.
ANDROMEDA raised the hematologic complete-response rate to 53.3% vs 18.1% and 6-month renal response to 53.0% vs 23.9% — depth of hematologic response predicts organ recovery.
Teaching aid, not a prescription — regimens follow current guidelines, product labeling and institutional protocol with hematology and nephrology input. Each lesion → clone mapping was verified against the linked reference.
Evidence
Every claim above traces to a verified PubMed citation. The most recent consensus and reclassification papers are listed first; the foundational literature follows.
Educational synthesis grounded in PubMed and the 2025 RPS/IKMG terminology consensus. Not medical advice.