What was known:
The disease spectrum of intratubular light chain amyloidosis (AL) remained unclear, with poorly understood clinical and prognostic differences among its subtypes, necessitating this study to define its spectrum, key features, and renal prognosis.
This study adds:
Intratubular AL presents three distinct subtypes with varying clinical and pathological characteristics; isolated intratubular AL may progress to multiple myeloma or systemic AL independent predictor of renal outcome; and serum creatinine at biopsy is an independent predictor of renal outcome.
Potential impact:
Recognizing intratubular AL subtypes facilitates risk stratification and clinical management, isolated intratubular AL requires vigilant monitoring for progression to myeloma or systemic AL, and early evaluation of renal function may help improve renal prognosis in these patients.
INTRODUCTION
Amyloidosis (AL) is characterized by the extracellular deposition of amyloid fibrils across various organs [ 1 ]. These amyloid fibrils exhibit a characteristic fibrillary ultrastructure and demonstrate specific binding to dyes such as Congo red [ 2 ]. To date, more than 40 types of amyloid fibril precursor proteins have been identified. Renal amyloid involvement predominates in the glomeruli, followed in descending order by vascular walls, interstitium, and tubules [ 3 ]. Furthermore, amyloid deposits have been identified in casts within renal tubular lumens [ 4 , 5 ], or within renal tubular epithelial cells [ 6 ].
Intratubular AL, first reported in the 1960s [ 4 , 5 ], was initially regarded as a complication of MM associated with concurrent AL and light chain cast nephropathy (LCCN). However, the exact relationship between these entities remains unclear. This ambiguity has provoked a long-standing debate regarding the clinical relevance of the condition. A competing perspective argued that the free deposition of amyloid fibrils within renal tubules was clinically trivial and did not warrant a formal diagnosis of AL [ 7 ]. This view prevailed until Gibier et al . conducted a systematic comparison of patients with LCCN, both with and without intratubular AL. Their study identified amyloid casts as a potential risk factor for the development of systemic AL [ 6 ], challenging the traditional view and underscoring the need for further investigation. Despite these findings, the literature on intratubular AL remains dominated by case reports [ 8–23 ], focusing principally on amyloid casts associated with myeloma cast nephropathy (MCN) [ 24 ].
Notably, we have also observed scattered amyloid casts in classic AL affecting the glomeruli, vascular walls, and renal interstitium. Moreover, these scattered amyloid casts may present as an isolated finding in patients without MM, classic AL or LCCN. Given the paucity of large-scale studies defining this disease spectrum, we conducted a retrospective analysis to determine the incidence of intratubular AL at our center, compare the similarities and differences among three subgroups of intratubular AL, and provide a new insight into this unique pathological entity.
MATERIALS AND METHODS
Patients and clinical data
A retrospective review of native kidney biopsy specimens was conducted at the National Clinical Research Center for Kidney Diseases, Jinling Hospital, Nanjing, from January 2008 to January 2025. The inclusion criterion was the presence of amyloid casts within the renal tubular lumens. The exclusion criteria were as follows: incomplete baseline data; nonlight chain AL. To better characterize clinicopathologic heterogeneity, eligible patients were divided into three groups based on the distribution of amyloid deposits and the presence of LCCN. Group 1 (intratubular with extra-tubular AL) comprised patients with amyloid deposits located in the tubular lumens as well as other renal compartments (glomeruli, vascular walls, interstitium); Group 2 (intratubular AL with LCCN) included patients exhibiting both intratubular amyloid cast and typical myeloma casts (Congo red-negative, light chain-restricted, surrounded by multinucleated giant cells, indicating LCCN); Group 3 (isolated intratubular AL) consisted of patients with amyloid deposits restricted to the tubular lumens, without involvement of other renal compartments or concurrent LCCN. Medical records were reviewed to extract demographic information, baseline clinical and laboratory data, treatment regimens, and follow-up outcomes.
Definitions
The following definitions were applied: anemia: hemoglobin < 120 g/l in adult males and <110 g/l in adult females; nephrotic-range proteinuria: urinary protein excretion >3 g/24 h; hypoproteinemia: serum albumin <35 g/l; nephrotic syndrome: nephrotic-range proteinuria, hypoproteinemia, with possible concurrent edema or hypertension; hematuria: erythrocytes >10 × 10 4 /ml on urinary sediment analysis; complete Fanconi syndrome: hypokalemia, hypocalcemia, hypouricemia, hypocarbonatemia, and glucosuria [ 25 ]. Monoclonal gammopathy of renal significance (MGRS) was defined according to the consensus report of the International Kidney and Monoclonal Gammopathy Research Group [ 26 ]. MM was diagnosed in accordance with the updated criteria established by the International Myeloma Working Group [ 27 ].
Criteria for evaluation of hematological outcomes were as follows [ 28 ]: complete response (CR), normal serum free light chain (FLC) ratio and negative serum/urine immunofixation; very good partial response (VGPR), difference between involved and uninvolved FLCs (dFLC) <40 mg/l or dFLC reduction >90%; partial response (PR), dFLC reduction ≥50%; and no response, failure to meet criteria for CR, VGPR or PR. Renal outcomes were defined as follows [ 29 ]: CR: proteinuria to <0.5 g/24 h with normalized Scr; PR: reduction of proteinuria by at least 50% and to <2 g/24 h with no more than a 20% increase in Scr; end-stage renal disease (ESRD): estimated glomerular filtration rate <15 ml/min/1.73 m² or initiation of chronic renal replacement therapy; persistent renal insufficiency: failure to achieve CR or PR without progression to ESRD. The primary endpoint of follow-up was progression to ESRD or being dependent on renal replacement therapy. Overall hematological or renal response was defined as achievement of PR or better after treatment.
Pathology
All renal biopsies were processed using light microscopy (LM), immunofluorescence (IF), and electron microscopy. For LM, all specimens were stained with hematoxylin-eosin, periodic acid-Schiff (PAS), periodic acid-silver methenamine-Masson (PASM-Masson), Masson trichrome, and Congo red. Interstitial fibrosis/tubular atrophy (IFTA) and acute tubular injury were scored semiquantitatively based on the percentage of affected tubulointerstitial cortex, categorized as 0%, 1%–25% (mild), 26%–50% (moderate), or >50% (severe). IF was performed on cryosections using fluorescein isothiocyanate-conjugated rabbit anti-human IgG, IgA, IgM, C3, C1q, κ, and λ light chains (DAKO, Denmark). IF for κ and λ light chains was also performed on paraffin-embedded tissue sections. Ultrastructural evaluation was performed using a FEI Tecnai G2 Spirit transmission electron microscope.
Statistical analysis
Statistical analysis was performed using SPSS software, version 27.0 (IBM Corp., Armonk, NY, USA). Continuous variables were reported as mean ± SD or median [interquartile range (IQR)], and categorical variables as number (%). Group comparisons were performed using one-way ANOVA with LSD-Bonferroni post hoc tests for normally distributed data, or the Kruskal-Wallis test with Dunn-Bonferroni post hoc analysis for nonparametric data. Categorical variables were compared using the χ² test with Bonferroni-corrected Z-tests. Correlations were assessed using Spearman’s rank correlation coefficient. Survival analyses included Kaplan-Meier curves, univariate analysis, and multivariable Cox proportional hazards models. All tests were two-sided, with statistical significance defined as P < .05.
RESULTS
Demographic and clinical characteristics
From January 2008 to January 2025, a review of renal biopsies at our center identified 747 cases with Congo red-positive staining. Intratubular AL was identified in 137 cases (18.3%). We further excluded cases from these 137 patients, including unclassified AL ( n = 6), heavy-chain AL ( n = 1), Finnish-type AL ( n = 1), serum amyloid A (SAA) AL ( n = 1), and lack of baseline light-chain data ( n = 2). A total of 126 patients were ultimately enrolled. The cohort was stratified into three groups: Group 1 ( n = 90), Group 2 ( n = 23), and Group 3 ( n = 13) (Fig. 1 ). These subgroups accounted for 12.0% (90/747), 3.1% (23/747), and 1.7% (13/747) of all Congo red-positive biopsies, respectively.
The baseline clinical characteristics at the time of biopsy are summarized in Table 1 . The study cohort comprised 82 males (65.1%) and 44 females (34.9%), with a mean age of 57.4 ± 10.9 years. Hypertension was present in 38 patients (30.2%) and diabetes in 14 patients (11.1%). The median Scr was 0.9 mg/dl, with a median proteinuria of 3.7 g/24 h and a mean serum albumin of 31.7 ± 9.0 g/l. Nephrotic syndrome was documented in 44 patients (34.9%). There were no significant differences among the three groups in terms of mean age, gender distribution, or the prevalence of hypertension or diabetes ( P > .05). Notably, patients in Group 1 exhibited significantly lower Scr and serum albumin, as well as higher 24-h proteinuria compared with Group 2 and Group 3 ( P < .05). Renal parameters did not differ significantly between Group 2 and Group 3 ( P > .05).
Table 1:
Comparison of clinical and biological characteristics among three groups.
Abbreviations: eGFR, estimated glomerular filtration rate; dFLC, difference between involved and involved free light chains; MM, multiple myeloma; MGRS, monoclonal gammopathy of renal significance; AML, acute myeloid leukemia. a Group 1 vs group 2: P < .05. b Group 1 vs group 3: P < .05. c Group 2 vs group 3: P < .05. d Abnormal serum FLC ratio means the ratio of free serum κ and λ light chain <0.5 or >3.1 renal range.
| Total ( n = 126) | Group 1 ( n = 90) | Group 2 ( n = 23) | Group 3 ( n = 13) | P -value | |
| Age (years) | 57.4 ± 10.9 | 58.1 ± 10.3 | 53.9 ± 12.8 | 59.3 ± 10.4 | .31 |
| Sex (F/M) | 44/82 | 36/54 | 6/17 | 2/11 | .14 |
| Hypertension ( n , %) | 38 (30.2%) | 25 (27.8%) | 6 (26.1%) | 7 (53.8%) | .14 |
| Diabetes ( n , %) | 14 (11.1%) | 8 (8.9%) | 3 (13.0%) | 3 (23.1%) | .30 |
| Anemia ( n , %) | 46 (36.5%) | 24 (26.7%) a | 19 (82.6%) c | 3 (23.1%) | <.001 |
| Serum creatinine (mg/dl) | 0.9 (0.7, 1.6) | 0.9 (0.7, 1.2) ab | 2.9 (1.5, 5.8) | 1.5 (1.1, 1.8) | <.001 |
| eGFR ( ml/min/1.73 m 2 ) | 80.5 (45.5, 100.5) | 90.8 (62.0, 104.0) ab | 25.6 (10.0, 50.0) | 49.0 (39.9, 69.0) | <.001 |
| Nephrotic syndrome ( n , %) | 44 (34.9%) | 37 (41.1%) | 4 (17.4%) | 3 (23.1%) | .06 |
| Albuminemia (g/l) | 31.7 ± 9.0 | 27.8 ± 6.1 ab | 42.2 ± 6.2 | 40.0 ± 9.9 | <.001 |
| Proteinuria (g/24 h) | 3.7 (1.9, 6.6) | 3.9 (2.7, 6.7) ab | 3.2 (1.0, 9.5) | 1.2 (0.9, 4.9) | .03 |
| Bence–Jones proteinuria ( n , %) | 6 (8.2%, n = 73) | 1 (1.9%, n = 51) ab | 3 (18.8%, n = 16) | 2 (33.3%, n = 6) | .01 |
| Hematuria ( n , %) | 43 (34.1%) | 36 (40.0%) | 4 (17.4%) | 3 (23.1%) | .08 |
| Light chain isotype (κ/λ) | 27/99 | 12/78 b | 7/16 | 8/5 | <.001 |
| dFLC (mg/l) | 115.2 (62.6, 197.0) | 105.2 (55.5, 160.5) a | 170.4 (118.0, 4642.4) | 195.7 (58.6, 394.8) | .02 |
| Abnormal serum FLC ratio d ( n , %) | 110 (87.3%) | 80 (88.9%) | 19 (82.6%) | 11 (84.6%) | .69 |
| Involved: uninvolved serum FLC | 8.7 (5.0,15.9) | 7.2 (4.8, 13.7) a | 27.9 (9.9, 218.0) | 10.0 (2.9, 14.7) | <.001 |
| Heavy chain isotype (no/IgA/IgG/IgM/IgG + IgA) | (70/21/25/2/1) | (44/17/22/1/0) | (18/2/2/0/0) | (8/2/1/1/1) | .07 |
| % Plasma cell bone marrow infiltration | 4.0 (1.6, 8.0) | 3.0 (1.3, 5.3) a | 14.5 (8.0, 25.0) c | 3.6 (2.5, 6.0) | <.001 |
| Hemotological disease ( n , %) | <.001 | ||||
| MM | 26 (20.6%) | 5 (5.6%) a | 20 (87.0%) c | 1 (7.7%) | |
| MGRS | 97 (77.0%) | 84 (93.3%) a | 3 (13.0%) c | 10 (76.9%) | |
| AML | 1 (0.8%) | 0 (0.0%) b | 0 (0.0%) | 1 (7.7%) | |
| Lymphoplasmacytic lymphoma | 2 (1.6%) | 1 (1.1%) | 0 (0.0%) | 1 (7.7%) |
Hematological characteristics
The hematological profiles are summarized in Table 1 . The underlying hematological disorders included MM ( n = 26), MGRS ( n = 97), acute myeloid leukemia ( n = 1), and lymphoplasmacytic lymphoma ( n = 2). MGRS was the predominant diagnosis in Groups 1 and 3, accounting for 93.3% and 76.9% of cases, respectively. Conversely, MM was the primary diagnosis in Group 2 (87.0%), associated with a significantly higher burden of bone marrow plasma cells compared with Groups 1 and 3 ( P < .05). Monoclonal immunoglobulin light chains were of the λ isotype in 99 patients (78.6%) and the κ type in 27 patients (21.4%). Notably, the κ isotype was more prevalent in Group 3, whereas the λ isotype predominated in Groups 1 and 2. The detailed information of all patients within Group 3 is showen in Table 6 .
Table 6:
Hematological characteristics, pathological features, treatment and prognosis of patients in Group 3.
Abbreviations: NA, not available; FLC, free light chains; sIFE, serum immunofixation electrophoresis; MM, multiple myeloma; MGRS, monoclonal gammopathy of renal significance; AML, acute myeloid leukemia; LCPT: light chain proximal tubulopathy; PIs, proteasome inhibitor; IMiD, immunomodulatory drug; Allo-HSCT: allogeneic hematopoietic stem cell transplantation; Auto-HSCT: autologous stem cell transplant. a The proportion of plasma cells in the patient’s bone marrow aspirate was within normal limits, though the exact value was not provided. Cytogenetic karyotyping revealed 46, XX, t(8;21)(q22; q22), del(11)(q14q23). Multiplex PCR showed positive AML1/ETO fusion transcripts with a quantitative WT1 level of 10.04%. Gene mutation testing confirmed a positive c-kit exon 17 mutation.
| Case No. | Serum κ/λ FLC (mg/l) | sIFE | Bence-Jones proteinuria | % Plasma cell bone marrow infiltration | Hemotological disease | LCPT | Treatments | Hematological response | Renal response |
| 1 | 50.7/127.0 = 0.4 | λ-IgA | Negative | 2.5 | MGRS→MM | No | PIs and IMiD based chemotherapy | No | No |
| 2 | 490.0/36.4 = 13.5 | κ-IgG | Negative | 5 | MGRS | No | Daratumumab | Yes | Yes |
| 3 | 47.7/10.2 = 4.7 | Negative | Negative | 72 | MM | Yes | PIs and IMiD based chemotherapy; Auto-HSCT | No | No |
| 4 | 29.6/295.0 = 0.1 | Negative | NA | 4.6 | MGRS | No | Supportive therapy only | NA | No |
| 5 | 21.6/19.4 = 1.1 | λ-IgA | Positive | 0.55 | MGRS | No | Supportive therapy only | NA | Yes |
| 6 | 26.5/362.5 = 0.07 | Negative | NA | NA a | AML | No | Chemotherapy (Cytarabine + Anthracycline); Allo-HSCT | No | No |
| 7 | 152.0/47.2 = 3.2 | κ-IgM | Negative | 1 | Lymphoplasmacytic lymphoma | No | Supportive therapy only | NA | No |
| 8 | 355.0/69.4 = 5.1 | κ | NA | 2.5 | MGRS | No | Supportive therapy only | NA | No |
| 9 | 13.3/209.0 = 0.06 | λ-IgG+λ-IgA | Positive | 2.5 | MGRS | No | Supportive therapy only | NA | No |
| 10 | 5640.0/22.1 = 255.2 | Negative | NA | 9.5 | MGRS | Yes | IMiD based chemotherapy | No | Yes |
| 11 | 1970/35.5 = 55.5 | κ | NA | NA | MGRS | Yes | Supportive therapy only | Lost to follow-up | Lost to follow-up |
| 12 | 207.0/18.0 = 11.5 | κ | NA | 6 | MGRS | No | Supportive therapy only | NA | No |
| 13 | 111.0/70.1 = 1.6 | κ | NA | NA | MGRS | No | Supportive therapy only | NA | No |
Abnormal sFLC ratios were observed in 110 patients (87.3%), with a median dFLC of 115.2 mg/dl and a median involved to uninvolved light chain ratio of 9.1. Patients in Group 2 demonstrated significantly higher median dFLC and involved-to-uninvolved light chain ratio compared with Group 1 ( P < .05), whereas no significant differences were observed between Group 3 and 1 ( P > .05).
Pathological characteristics
Amyloid casts were identified by Congo red staining, demonstrating characteristic apple-green birefringence under polarized light, with or without amyloid deposits in glomeruli, blood vessels, and interstitium. The casts appeared weakly eosinophilic on H&E and weakly positive on PAS staining. On PASM-Masson staining, they were either argyrophilic (exhibiting typical ‘cilium-like’ features) or non-argyrophilic. Morphologically, amyloid casts frequently exhibited concentric or lamellar patterns. Internal fingerprint-like textures and the spiculated margins were commonly observed. In some instances, amyloid deposits were confined solely to the cast periphery. A surrounding cellular reaction was present in a subset of cases. Rupture of renal tubules was observed in certain cases, with amyloid casts leaking into the interstitium. There were also some casts lacking fingerprint-like texture and surrounding peritubular cellular reactions (Figs 2 – 4 ).
The renal pathological findings are described in Table 2 . The overall median amyloid cast count was 1.5 per section (density: 0.2/mm²). The cast density in Group 2 (1.8/mm²) was significantly higher than in Group 3 (0.2/mm²) and Group 1 (0.1/mm²; P < .05). Mild, moderate, and severe IFTA were present in 66 (52.4%), 16 (12.7%), and 8 (6.3%) cases, respectively. Mild, moderate, and severe acute tubular injury were present in 34 (27.0%), 18 (14.3%), and 5 (4.0%) cases, respectively. Histologically, Group 2 exhibited significantly more severe acute tubular injury and IFTA compared with Group 1 ( P < .05). However, no significant differences were found between Group 2 and Group 3 ( P > .05).
Table 2:
Comparison of pathological characteristics among the three groups.
Abbreviations: LCPT, light chain proximal tubulopathy; IFTA, tubular atrophy and interstitial fibrosis. a Group 1 vs group 2: P < .05. b Group 1 vs group 3: P < .05. c Group 2 vs group 3: P < .05.
| Total ( n = 126) | Group 1 ( n = 90) | Group 2 ( n = 23) | Group 3 ( n = 13) | P -value | |
| Glomeruli | 24.5 (18.0, 36.0) | 24.5 (18.0, 37.5) | 24.0 (16.0, 32.0) | 27.0 (21.0, 36.5) | .77 |
| %Mean sclerotic glomeruli | 11.0 (4.0, 22.6) | 11.1 (3.8, 20.0) | 5.6 (0.0, 22.7) | 23.8 (8.2, 28.7) | .05 |
| Number of amyloid casts | 1.5 (1.0, 3.0) | 1.0 (1.0, 2.0) a | 12.0 (6.0, 21.0) c | 2.0 (1.0, 3.0) | <.001 |
| Mean amyloid casts (/mm 2 ) | 0.2 (0.1, 0.3) | 0.1 (0.1, 0.2) a | 1.8 (0.5, 2.9) c | 0.2 (0.1, 0.2) | <.001 |
| IFTA (%) (no/mild/moderate/marked) | 28.6/52.4/12.7/6.3 | 37.8/48.9/7.8/5.6 a | 4.3/52.2/30.4/13 | 7.7/76.9/15.4/0 | .001 |
| Acute tubular injury (%) (no/mild/moderate/marked) | 54.8/27.0/14.3/4.0 | 62.2/30.0/6.7/1.1 a | 26.1/17.4/39.1/17.4 | 53.8/23.1/23.1/0 | <.001 |
| LCPT ( n , %) | 4 (3.2%) | 0 (0.0%) b | 1 (4.3%) | 3 (23.1%) | <.001 |
| Amyloid in renal tubular epithelial cells ( n , %) | 3 (2.4%) | 0 (0.0%) a | 3 (13.0%) | 0 (0.0%) | .001 |
Of the 126 patients, 4 (1 in Group 2, 3 in Group 3) had concurrent light chain proximal tubulopathy (LCPT) without Congo-red positive amyloid deposits in the cytoplasm of renal tubular epithelial cells. In contrast, Congo-red positive amyloid deposits were detected within tubular epithelial cell cytoplasm in three cases (all in Group 2). Group 3 demonstrated heterogeneously pathological manifestations, including three cases with LCPT, three cases with membranous nephropathy, four cases with diabetes or hypertension related mesangial proliferative lesions, two cases with focal segmental glomerulosclerosis, and 1 case with chronic interstitial nephritis.
Extra-renal specimens–including bone marrow, skin and adipose tissue, gastrointestinal mucosa, and liver–were evaluated in 89 patients (70.6%). Among them, 63 cases (70.8%) were Congo red positive. Compared with Groups 2 and 3, Group 1 exhibited a significantly higher frequency of performed extra-renal biopsies and a higher rate of Congo red positivity ( P < .05) (Table 3 ).
Table 3:
Extrarenal pathological specimens assessed for amyloid light chain amyloidosis.
a Group 1 vs group 2: P < .05. b Group 1 vs group 3: P < .05. c Group 2 vs group 3: P < .05.
| Group 1 ( n = 90) | Group 2 ( n = 23) | Group 3 ( n = 13) | P -value | |
| Patients with ≥1 specimen ( n , %) | 72 (80.0%) a b | 11 (47.8%) | 6 (46.2%) | .001 |
| Bone marrow biopsies ( n , %) | 21 (23.3%) | 5 (21.7%) | 1 (7.7%) | .44 |
| Skin and subcutaneous adipose biopsies ( n , %) | 58 (64.4%) | 9 (39.1%) | 5 (38.5%) | .03 |
| Gastrointestinal mucosal biopsies ( n , %) | 25 (27.8%) | 0 (0.0%) | 3 (23.1%) c | .02 |
| Liver biopsies ( n , %) | 1 (1.1%) | 0 (0.0%) | 0 (0.0%) | 1.00 |
| Extrarenal organ involvement by amyloid ( n , %) | 58 (64.4%) a b | 4 (17.4%) | 1 (7.7%) | <.001 |
Treatment and prognosis
The treatment regimens and prognostic outcomes are presented in Table 4 . Proteasome inhibitors and immunomodulatory agents were the therapeutic mainstays, with a comparable distribution among the three groups ( P > .05). In Group 3, five patients received hematologic antineoplastic therapy: 1 was diagnosed with MM complicated by LCPT; 1 had acute myeloid leukemia (AML); 1 was diagnosed with MGRS and LCPT; 1 patient with isolated intratubular AL (without LCPT), 2.5% bone marrow plasma cells, an abnormal serum FLC ratio and detectable λ monoclonal band received no chemotherapy at biopsy yet later progressed to MM during follow-up with a poor response to subsequent chemotherapy; another patient with isolated intratubular AL (without LCPT), 5% bone marrow plasma cells, abnormal FLC ratio and κ monoclonal band achieved hematological and renal remission following four cycles of daratumumab initiated immediately after biopsy (Table 6 ).
Table 4:
Treatments and renal survival.
Abbreviations: PIs, proteasome inhibitor, including bortezomib and carfilzomib; IMiD, immunomodulatory drug, including lenalidomide and pomalidomide; Auto-HSCT, autologous stem cell transplant. Allo-HSCT: allogeneic hematopoietic stem cell transplantation. a Group 1 vs group 2: P < .05. b Group 1 vs group 3: P < .05. c Group 2 vs group 3: P < .05.
| Group 1 ( n = 90) | Group 2 ( n = 23) | Group 3 ( n = 13) | P -value | |
| Chemotherapy ( n , %) | 75 (83.3%) b | 17 (73.9%) | 4 (30.8%) c | <.001 |
| PIs (n,%) | 54 (72.0%) | 11 (64.7%) | 2 (50.0%) | .57 |
| IMiDs ( n , %) | 31 (41.3%) | 9 (52.9%) | 3 (75%) | .30 |
| Cytarabine ( n , %) | 0 (0%) b | 0 (0%) | 1 (25%) | .04 |
| Vincristine ( n , %) | 0 (0%) a | 3 (17.6%) | 0 (0.0%) | .01 |
| Anthracycline ( n , %) | 0 (0%) ab | 3 (17.6%) | 1 (25%) | .01 |
| Daratumumab ( n , %) | 11 (12.2%) | 1 (4.3%) | 1 (7.7%) | .51 |
| Allo-HSCT ( n , %) | 0 (0.0%) | 0 (0.0%) | 1 (7.7%) | .10 |
| Auto-HSCT ( n ,%) | 30 (33.3%) | 3 (13.0%) | 1 (7.7%) | .09 |
| Supportive therapy only ( n , %) | 13 (14.4%) b | 6 (26.1%) c | 8 (61.5%) | <.001 |
| Follow-up time (month) ( n , %) | 20.7 (6.7, 61.8) | 12.4 (1.6, 21.9) | 10.6 (4.6, 23.5) | .42 |
| Hematological response ( n , %) | 40 (54.8%, n = 73) | 9 (75.0%, n = 12) | 1 (16.7%, n = 5) | .06 |
| Renal response ( n , %) | 21 (26.6%, n = 79) | 4 (28.6%, n = 14) | 3 (25.0%, n = 12) | .98 |
| ESRD ( n , %) | 12 (13.3%) a | 9 (39.1%) c | 0 (0.0%) | .004 |
| Death ( n , %) | 1 (1.1%) | 2 (8.7%) | 0 (0.0%) | .09 |
Follow-up data were available for 109 patients, with a median follow-up duration of 16.4 months (no significant difference among groups, P > .05). Hematological remission was achieved in 55.6% (50/90) of patients. The hematological remission rate was significantly higher in Group 2 (9/12, 75.0%) than Group 1 (40/73, 54.8%) and Group 3 (1/7, 16.7%) ( P < .05). Renal remission was achieved in 26.7% (28/105) of the cohort, with no significant differences observed among groups ( P > .05). Notably, nine patients (39.1%) in Group 2 progressed to ESRD, a rate significantly higher than those in Groups 1 and 3 ( P < .05). Three deaths were recorded during follow-up.
Survival analysis revealed a mean renal survival time of 101.0 months (95% CI: 87.5–118.5) for the entire cohort. Group 2 exhibited significantly shorter mean kidney survival (17.3 months; 95% CI: 7.6–27.1) compared with Group 1 (112.8 months; 95% CI: 96.8–128.7), whereas no patient in Group 3 progressed to ESRD (Fig. 5 ). Univariate analysis identified plasma cell percentage >10%, involved serum free light chain levels, Scr, albumin levels, amyloid cast density, and the presence of at least moderate acute tubular injury or IFTA as predictive factors for progression to ESRD ( P < .05). However, in multivariable analysis, only serum creatinine (Scr) remained independently associated with kidney outcomes ( P < .05) (Table 5 ).
Table 5:
Predictive factors for progression to end-stage renal disease (ESRD).
a The variables marked with this letter remained significantly associated with renal prognosis in the multivariate analysis. Abbreviations: IFTA, tubular atrophy and interstitial fibrosis.
| Univariate analysis | Multivariate analysis | |||
| Characteristics | HR (95% CI) | P -value (uni) | HR (95% CI) | P -value (mul) |
| % Plasma cell bone marrow infiltration >10% | 4.1 (1.1, 15.5) | .04 | 0.02 (0.02, 9.3 ) | .62 |
| Involved serum FLC (mg/dl) | 1.0 (1.0, 1.0) | .01 | 1.00 (1.00, 1.01) | .51 |
| Serum creatinine (mg/dl) | 1.4 (1.2, 1.6) | <.001 | 1.5 (1.1, 2.2) | .04 a |
| Mean amyloid casts (/mm 2 ) | 1.6 (1.1, 2.3) | .01 | 0.7 (0.3, 1.7) | .43 |
| Acute tubular injury (moderate and severe) | 9.0 (3.6, 22.5) | <.001 | 3.6 (0.1, 14.6) | .50 |
| IFTA (moderate and severe) | 4.3 (1.8, 10.3) | <.001 | 0.6 (0.1, 7.1) | .75 |
Correlation analysis demonstrated no correlation between the number of amyloid casts and Scr in any Group (Group 1: R = −0.13, P = .23; Group 2: R = 0.03, P = .89; Group 3: R = 0.40, P = .22).
DISCUSSION
This study reported the largest cohort of 126 patients with intratubular light chain AL and compared the similarities and differences across three subgroups: Group 1 (intratubular with extra-tubular AL), Group 2 (intratubular AL with LCCN), and Group 3 (isolated intratubular AL). The cohort was predominantly composed of middle-aged and elderly individuals, with a significantly higher proportion of males than females. Consistent with previous studies on AL [ 30 ], the λ light chain isotype was predominant. Notably, among 61 patients diagnosed with LCCN during the study period, the prevalence of concurrent intratubular AL (Group 2) was 37.7%, higher than the previously reported rate of 28% [ 7 ].
It is worth noting that Group 1 exhibited the lowest serum albumin levels and the highest urinary protein excretion. This finding may be explained by glomerular involvement due to amyloid deposits [ 31 ], consistent with the manifestations of classic AL [ 32 ]. Compared with Group 1, Group 2 demonstrated significantly higher Scr, dFLC levels, incidence of anemia, prevalence of MM, and amyloid cast density, indicating a heavier hematological tumor burden, a closer association with MM, and more severe tubular injury. Group 3 displayed intermediate clinical manifestations between Groups 1 and 2, which may reflect relatively mild tubular damage from a limited number of amyloid casts or glomerular lesions caused by metabolic abnormalities (e.g. diabetes and hypertension). Notably, both Group 3 and 1 displayed low and statistically comparable hematological tumor burdens, consistent with the characteristic low tumor load observed in patients with AL [ 32 ].
Pathologically, the λ light chain isotype remained predominant across the entire cohort, consistent with prior reports [ 7 ]. In contrast, Group 3 showed a slight predominance of the κ light chain over the λ light chain, differing from the series reported by Wang et al ., in which all cases exhibited λ light-chain restriction [ 33 ]. We also observed three cases with amyloid deposits within renal tubular epithelial cells. The frequent coexistence of intratubular cytoplasmic AL with amyloid casts may be linked to the underlying disease pathogenesis [ 34 ]. Additionally, in four patients with concurrent LCPT, the pathogenic light chains demonstrated the dual properties of inducing amyloid folding and directly damaging lysosomes.
Despite distinct morphological features (e.g. textured appearance, argyrophilic properties), amyloid casts are often overlooked during pathological examination, particularly when the cast density is extremely low. Consequently, Congo red staining on biopsy section and light chain staining on the adjacent section should be performed on renal biopsy specimens from patients with serum M-protein, bone marrow monoclonal proliferation, or LCCN. This approach is essential for enhancing diagnostic sensitivity in patients with a low burden of amyloid casts.
We observed marked differences in the extrarenal amyloid deposition among three subgroups of intratubular AL. The proportion of extrarenal biopsy samples and the positive rate for Congo red staining in Group 1 were significantly higher than in Groups 2 and 3. Additionally, one case in Group 3 demonstrated extrarenal amyloid deposition, suggesting that even patients with isolated intratubular AL may develop systemic involvement. Consequently, patients with intratubular AL should undergo targeted biopsies of highly sensitive tissues, such as skin and adipose tissue [ 3 ], and should be closely monitored for clinical manifestations of systemic involvement in the heart, liver, and peripheral nerves to enable timely treatment. Extrarenal amyloid deposition was also detected in four patients of Group 2. Given that Gibier et al . previously established amyloid casts as a risk factor for systemic AL in patients with LCCN [ 7 ], further exploration was not performed.
Group 3 was analyzed as an independent cohort based on its hematological, renal, and pathological characteristics. Currently, there is insufficient evidence to clarify whether this subgroup represents a sampling bias, an early stage of Group 1 or Group 2, or a separate entity requiring early intervention. Ten patients in Group 3 did not fulfill the diagnostic criteria for hematological malignancies at the time of biopsy, who showed laboratory evidence of circulating monoclonal immunoglobulin or abnormal FLC ratios, and some of whom showed low grade small clonal proliferations. On this hematological background, renal pathology consistently demonstrated intratubular amyloid casts and two of these 10 patients additionally presented pathological features consistent with LCPT. Notably, one case with MGRS (without LCPT) in Group 3 achieved both hematological and renal remission after four courses of daratumumab therapy. This finding implies that hematological antineoplastic therapy may be effective in patients with isolated intratubular AL, even without concomitant LCPT or MM. Overall, we are inclined to classify these 10 patients as MGRS rather than MGUS (monoclonal gammopathy of undetermined significance) to draw appropriate clinical attention to this pattern. However, further studies with larger samples and longer follow-up in this subgroup are needed.
Antineoplastic therapy can induce a hematological remission. Notably, the rates of hematological remission in Groups 1 and 2 were significantly higher than the rates of renal remission. Indeed, we observed a dissociation in outcomes, where reduced proteinuria coincided with rising Scr in some patients during follow-up. The discordance between renal and hematological remission may be attributed to two factors: irreversible chronic renal lesions or refractory severe acute renal injury and difficulty in clearing or reducing established renal amyloid deposits [ 35 ]. Similarly, prior studies have described patients achieving hematological but not renal remission, in whom repeat biopsies identified secondary focal segmental glomerulosclerosis (FSGS) [ 36–38 ].
Multivariate analysis identified Scr as an independent predictor of renal outcomes. For patients with significantly abnormal Scr, particularly those with AKI, active measures should be considered to optimize renal function. For those with LCCN, dialysis strategies, such as PMMA or HCO dialyzers, can reduce circulating pathogenic FLC loads, thereby facilitating partial recovery of renal function [ 39 , 40 ]. While the number of light chain casts was a decisive factor for renal response in patients with LCCN [ 41 ], amyloid cast density was correlated with renal outcomes in univariate analysis but lost statistical significance in multivariate analysis in this study. Furthermore, subgroup analysis showed no significant correlation between the number of amyloid casts and Scr in any of the three groups, precluding confirmation of a direct link between amyloid cast density and renal function impairment [ 33 ].
Genetic factors are recognized contributors to the pathogenic heterogeneity of monoclonal immunoglobulin light chains. Five germline genes (IGLV10-44, IGLV2-14, IGLV3-1, IGLV6-57, and IGKV1-33) encode 55% to 60% of amyloidogenic light chains [ 42 ]. Among these, IGLV6-57 shows the strongest association with renal AL, accounting for ∼20% of pathogenic λ light chains [ 43 ]. Whether the mechanisms underlying intratubular AL are identical across the three groups or linked to genetic heterogeneity remains unclear. Emerging technologies such as RACE-Repseq [ 44 ] and other technologies, which enable comprehensive immunoglobulin gene analysis, will be pivotal for subsequent investigations.
The study has several limitations. First, as a retrospective study, it relies on medical records, which may be subject to information bias and selection bias. Second, the proportion of patients in Groups 2 and 3 who underwent extrarenal tissue biopsy was low. Finally, loss of follow-up of some patients may have compromised the assessment of renal outcomes.
In conclusion, this study reported the largest cohort of patients with intratubular AL to date. Intratubular AL is commonly observed in patients with renal AL and LCCN, exhibiting overlapping clinical and pathological features with both conditions. Isolated intratubular AL may progress to MM or systemic AL, underscoring the need for rigorous clinical follow-up.
ACKNOWLEDGEMENTS
This study was approved by the Ethics Committee of Jinling Hospital [2026DZKY-042-01] and complies with the Declaration of Helsinki. This article does not contain personal and/or medical information about an identifiable individual.
Contributor Information
Xiaoyu Wang, National Clinical Research Center for Kidney Diseases, Jinling Hospital, Affiliated Hospital of Medical School, Nanjing University, Nanjing, China.
Yuxuan Yuan, National Clinical Research Center for Kidney Diseases, Jinling Hospital, Affiliated Hospital of Medical School, Nanjing University, Nanjing, China.
Dan Zhou, National Clinical Research Center for Kidney Diseases, Jinling Hospital, Affiliated Hospital of Medical School, Nanjing University, Nanjing, China.
Zhengyun Zhu, National Clinical Research Center for Kidney Diseases, Jinling Hospital, Affiliated Hospital of Medical School, Nanjing University, Nanjing, China.
Yujie Wang, National Clinical Research Center for Kidney Diseases, Jinling Hospital, Affiliated Hospital of Medical School, Nanjing University, Nanjing, China.
Feng Xu, National Clinical Research Center for Kidney Diseases, Jinling Hospital, Affiliated Hospital of Medical School, Nanjing University, Nanjing, China.
Xiaodong Zhu, National Clinical Research Center for Kidney Diseases, Jinling Hospital, Affiliated Hospital of Medical School, Nanjing University, Nanjing, China.
Mingchao Zhang, National Clinical Research Center for Kidney Diseases, Jinling Hospital, Affiliated Hospital of Medical School, Nanjing University, Nanjing, China.
Fan Yang, National Clinical Research Center for Kidney Diseases, Jinling Hospital, Affiliated Hospital of Medical School, Nanjing University, Nanjing, China.
Shaoshan Liang, National Clinical Research Center for Kidney Diseases, Jinling Hospital, Affiliated Hospital of Medical School, Nanjing University, Nanjing, China.
Caihong Zeng, National Clinical Research Center for Kidney Diseases, Jinling Hospital, Affiliated Hospital of Medical School, Nanjing University, Nanjing, China.
AUTHORS' CONTRIBUTIONS
Each author contributed significant intellectual content during the drafting or revision of the manuscript and assumes accountability for the work as a whole, ensuring that any questions regarding the accuracy or integrity of any part of the work are properly investigated and resolved. C.Z. and S.L. designed the study and and contributed to supervision and mentorship. X.W. selected eligible patients from the renal biopsy patients in our center. X.W. drafted the initial manuscript and YY revised the manuscript. Y.Y., D.Z., Z.Z., and Y.W. were responsible for collecting medical records and the follow-up data of patients. F.X., X.Z., M.Z., and F.Y. were responsible for preparing pathological sections and performing staining.
CONFLICT OF INTEREST STATEMENT
All authors of this article declare no conflict of interest.
FUNDING
This work was supported by the Medical Scientific Research Project of Jiangsu Provincial Health Commission (No. ZD2021018).
DATA AVAILABILITY STATEMENT
All data generated or analyzed during this study are included in this article. Further enquiries can be directed to the corresponding author.
REFERENCES
1. Merlini G, Bellotti V. Molecular mechanisms of amyloidosis. N Engl J Med. 2003;349:583–96. 10.1056/NEJMra023144.
2. Iadanza MG, Jackson MP, Hewitt EW et al. A new era for understanding amyloid structures and disease. Nat Rev Mol Cell Biol. 2018;19:755–73. 10.1038/s41580-018-0060-8.
3. Nader R, Angel-Korman A, Havasi A. Amyloidosis and the kidney: an update. Semin Nephrol. 2022;42:151343. 10.1016/j.semnephrol.2023.151343.
4. Vassar PS, Culling CF. Fluorescent amyloid staining of casts in myeloma nephrosis. Arch Pathol. 1962;73:59–63.
5. Azzopardi JG, Lehner T. Systemic amyloidosis and malignant disease. J Clin Pathol. 1966;19:539–48. 10.1136/jcp.19.6.539.
6. Jea H. Unique pattern of renal κ light chain amyloid deposition with histiocytic transdifferentiation of tubular epithelial cells. Am J Surg Pathol. 2012;36:1253–7.
7. Gibier J-B, Gnemmi V, Glowacki F et al. Intratubular amyloid in light chain cast nephropathy is a risk factor for systemic light chain amyloidosis. Mod Pathol. 2018;31:452–62. 10.1038/modpathol.2017.124.
8. Limas C, Wright JR, Matsuzaki M et al. Amyloidosis and multiple myeloma. A reevaluation using a control population. Am J Med. 1973;54:166–73. 10.1016/0002-9343(73)90220-9.
9. Defronzo RA, Humphrey RL, Wright JR et al. Acute renal failure in multiple myeloma. Medicine. 1975;54:209–23. 10.1097/00005792-197505000-00003.
10. Melato M, Falconieri G, Pascali E et al. Amyloid casts within renal tubules: a singular finding in myelomatosis. Virchows Arch A Path Anat and Histol. 1980;387:133–45. 10.1007/BF00430695.
11. Hill GS, Morel-Maroger L, Wry J-P et al. Renal lesions in multiple myeloma: their relationship to associated protein abnormalities. Am J Kidney Dis. 1983;2:423–8. 10.1016/S0272-6386(83)80075-4.
12. Pirani CL, Silva F, D’Agati V et al. Renal lesions in plasma cell dyscrasias: ultrastructural observations. Am J Kidney Dis. 1987;10:208–21. 10.1016/S0272-6386(87)80176-2.
13. Rota S, Mougenot B, Baudouin B et al. Multiple myeloma and severe renal failure: a clinicopathologic study of outcome and prognosis in 34 patients. Medicine. 1987;66:126–37. 10.1097/00005792-198703000-00004.
14. El-Zoghby Z, Lager D, Gregoire J et al. Intra-tubular amyloidosis. Kidney Int. 2007;72:1282–8. 10.1038/sj.ki.5002411.
15. Nasr SH, Alobeid BB, Otrakji JA et al. Myeloma cast nephropathy, direct renal infiltration by myeloma, and renal extramedullary hematopoiesis. Kidney Int. 2008;73:517–8. 10.1038/sj.ki.5002416.
16. Sethi S, Hanna MH, Fervenza FC Unusual casts in a case of multiple myeloma. Am J Kidney Dis. 2009;54:970–4. 10.1053/j.ajkd.2009.01.273.
17. Kato H, Fujigaki Y, Asakawa S et al. Rapid deterioration of the renal function caused by the coexistence of intratubular amyloidosis and myeloma cast nephropathy. Intern Med. 2015;54:3023–8. 10.2169/internalmedicine.54.5174.
18. Iliuta IA, Garneau AP, Latulippe E et al. Amyloid cast tubulopathy: a unique form of immunoglobulin-induced renal disease. Blood Cancer J. 2016;6:e474. 10.1038/bcj.2016.74.
19. Rajagopal MD, Nachiappa Ganesh R, Parameswaran S et al. Unusual morphology of amyloid cast nephropathy in renal biopsy portending poor prognosis. BMJ Case Rep. 2018; 11:e225899doi:. 10.1136/bcr-2018-225899.
20. Kurien AA, Fernando ME. Amyloid proximal tubulopathy and amyloid casts: an unusual finding in multiple myeloma. Indian J Nephrol. 2018;28:160–3. 10.4103/ijn.IJN_297_16.
21. Ichimata S, Hata Y, Abe R et al. An autopsy case of amyloid tubulopathy exhibiting characteristic spheroid-type deposition. Virchows Arch. 2020;477:157–63. 10.1007/s00428-019-02740-4.
22. Sun LJ, Dong HR, Xu XY et al. Two kinds of rare light chain cast nephropathy caused by multiple myeloma: case reports and literature review. BMC Nephrol. 2021;22:42. 10.1186/s12882-021-02250-z.
23. Mizuno S, Kosukegawa H. Amyloid cast nephropathy with systemic AL amyloidosis. Int J Hematol. 2021;113:1–2. 10.1007/s12185-020-03036-z.
24. Nasr SH, Royal V, Best Rocha A et al. Renal Pathology Society/International Kidney and Monoclonal Gammopathy Research Group consensus on pathologic definitions and terminology of monoclonal gammopathy-associated kidney lesions. Kidney Int. 2025;108:184–93. 10.1016/j.kint.2025.04.007.
25. Lin Y, Xing G, Hu R et al. Clinicopathological characteristics of light chain proximal tubulopathy: a multicentre case series. J Clin Pathol. 2025;78:562–8. 10.1136/jcp-2024-209620.
26. Leung N, Bridoux F, Batuman V et al. Publisher correction: the evaluation of monoclonal gammopathy of renal significance: a consensus report of the International Kidney and Monoclonal Gammopathy Research Group. Nat Rev Nephrol. 2019;15:121. 10.1038/s41581-018-0102-7.
27. Rajkumar SV, Dimopoulos MA, Palumbo A et al. International Myeloma Working Group updated criteria for the diagnosis of multiple myeloma. Lancet Oncol. 2014;15:e538–e48. 10.1016/S1470-2045(14)70442-5.
28. Palladini G, Dispenzieri A, Gertz MA et al. New criteria for response to treatment in immunoglobulin light chain amyloidosis based on free light chain measurement and cardiac biomarkers: impact on survival outcomes. J Clin Oncol. 2012;30:4541–9. 10.1200/JCO.2011.37.7614.
29. Liang S, Liang D, Zhu X et al. Light chain only variant of proliferative glomerulonephritis with monoclonal immunoglobulin deposits: a clinicopathological study of three cases. J Clin Pathol. 2024;77:551–6. 10.1136/jcp-2023-208795.
30. Zhu Z, Yue C, Sun Y et al. Light-chain amyloidosis with renal involvement: renal outcomes and validation of two renal staging systems in the Chinese population. Amyloid. 2019;26:186–91. 10.1080/13506129.2019.1639149.
31. Gise H, Mikeler E, Gruber M et al. Investigations on the cause of the nephrotic syndrome in renal amyloidosis. A discussion of electron microscopic findings. Virchows Arch A Path Anat and Histol. 1978;379:131–41. 10.1007/BF00432482.
32. Huang XH, Liu ZH. The clinical presentation and management of systemic light-chain amyloidosis in China. Kidney Dis. 2016;2:1–9. 10.1159/000444287.
33. Wang L, Yue SL, Huang L et al. Clinicopathological characteristics of light chain amyloid cast nephropathy. J Clin Nephrol. 2023;23:292–99.
34. Javaugue V, Rocha AB, Said SM et al. Clinicopathologic and proteomic characteristics of intratubular cytoplasmic AL amyloidosis. Kidney Int. 2022;102:926–9. 10.1016/j.kint.2022.07.020.
35. Zeier M, Perz J, Linke RP et al. No regression of renal AL amyloid in monoclonal gammopathy after successful autologous blood stem cell transplantation and significant clinical improvement. Nephrol Dial Transplant. 2003;18:2644–7. 10.1093/ndt/gfg436.
36. Angel-Korman A, Jaberi A, Sanchorawala V et al. The utility of repeat kidney biopsy in systemic immunoglobulin light chain amyloidosis. Amyloid. 2020; 27:17–24. 10.1080/13506129.2019.1672650.
37. Okuyama H, Yamaya H, Fukusima T et al. A patient with persistent renal AL amyloid deposition after clinical remission by HDM/SCT therapy. Clinical Nephrol. 2013;79:233–6. 10.5414/CN107162.
38. Zhang P, Chen X, Zou Y et al. Value of repeat renal biopsy in the evaluation of AL amyloidosis patients lacking renal response despite of complete hematologic remission: a case report and literature review. BMC Nephrol. 2022;23:127. 10.1186/s12882-022-02752-4.
39. Finkel KW, Cohen EP, Shirali A et al. Paraprotein-related kidney disease: evaluation and treatment of myeloma cast nephropathy. Clin J Am Soc Nephrol. 2016;11:2273–9. 10.2215/CJN.01640216.
40. Sens F, Chaintreuil D, Jolivot A et al. Effectiveness of IHD with adsorptive PMMA membrane in myeloma cast nephropathy: a cohort study. Am J Nephrol. 2017;46:355–63. 10.1159/000481461.
41. Ecotière L, Thierry A, Debiais-Delpech C et al. Prognostic value of kidney biopsy in myeloma cast nephropathy: a retrospective study of 70 patients. Nephrol Dial Transplant. 2016;31:850. 10.1093/ndt/gfw056.
42. Herrera GA, Teng J, Turbat-Herrera EA et al. Understanding mesangial pathobiology in AL-amyloidosis and monoclonal Ig light chain deposition disease. Kidney Int Rep. 2020;5:1870–93. 10.1016/j.ekir.2020.07.013.
43. Kourelis TV, Dasari S, Theis JD et al. Clarifying immunoglobulin gene usage in systemic and localized immunoglobulin light-chain amyloidosis by mass spectrometry. Blood. 2017;129:299–306. 10.1182/blood-2016-10-743997.
44. Bender S, Javaugue V, Saintamand A et al. Immunoglobulin variable domain high-throughput sequencing reveals specific novel mutational patterns in POEMS syndrome. Blood. 2020;135:1750–8. 10.1182/blood.2019004197.
Associated Data
Data Availability Statement
All data generated or analyzed during this study are included in this article. Further enquiries can be directed to the corresponding author.