BACKGROUND AND HYPOTHESIS:Diabetic nephropathy (DN), a leading cause of end-stage kidney disease (ESKD), often progresses despite standard care. Evidence suggests a DN subgroup exhibit overlapping features of primary podocytopathy, potentially responsive to immunosuppression, has not yet systematically studied. We hypothesized that DN patients with biopsy-proven concurrent ultrastructural features of primary podocytopathy will demonstrate superior renal outcomes following immunosuppressive therapy, compared to those with isolated DN pathology. METHODS:A retrospective analysis was conducted on 88 patients with biopsy-confirmed DN recruited from Peking University First Hospital between 2017 and 2022. The patients were categorized into two groups: Group A (patients with DN and primary podocytopathy) and Group B (patients with DN without primary podocytopathy). Seven patients with diabetes and primary podocytopathy but without DN served as disease controls. Primary podocytopathy was defined by electron microscopy foot process width (FPW) ≥821 nm. RESULTS:Patients in Group A had a shorter duration of nephropathy (p = 0.005), lower serum albumin levels (p < 0.001), and higher proteinuria (p = 0.032) than those in Group B. Light microscopy demonstrated milder glomerular lesions in Group A (p < 0.001), with most cases classified as Class I or II (63.6%). Electron microscopy showed significantly broader FPW in Group A than in Group B (1310.5 ± 510.1 nm vs. 569.1 ± 97.5 nm, p < 0.001). While Group B showed no such benefit and differed significantly from the control group, patients in Group A benefited from immunosuppressive therapy, with a significantly reduced risk of ESKD. CONCLUSION:DN with concurrent primary podocytopathy represents a distinct clinical entity characterized by nephrotic-range proteinuria, ultrastructural podocyte effacement, and favorable response to immunosuppressive therapy. Early renal biopsy incorporating FPW assessment enables targeted therapy to mitigate ESKD progression in this high-risk phenotype.

Introduction Alternative complement activation is pivotal in antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) pathogenesis. Growing evidence highlights an important role of complement factor H (FH) in AAV. FH-related protein 5 (FHR5) functions as a competitive antagonist of FH. This study aimed to investigate the association of FHR5 with disease severity and renal outcomes in patients with AAV. Methods In this retrospective cohort study, plasma FHR5 levels were measured using enzyme-linked immunosorbent assay in 154 patients with AAV at active stage and 55 patients in remission. Renal FHR5 deposition was assessed using immunohistochemistry (IHC) in 71 of the 154 patients. The association of plasma FHR5 levels and renal FHR5 deposition with clinicopathological parameters was analyzed. Results Plasma levels of FHR5 were significantly higher in patients with active AAV than in those in remission and healthy controls, and correlated with increased inflammatory parameters and more severe renal damage. Positive FHR5 staining was observed in the glomeruli of 49 (69%) renal specimens. Patients with moderate or strong glomerular FHR5 deposition (grade > 1+) demonstrated significantly worse baseline renal function, more severe histopathological kidney injury, and poorer treatment response than those with negative or few deposition of FHR5 (grade ≤ 1+). Multivariable Cox regression analysis indicated that moderate or strong glomerular FHR5 deposition was independently associated with an increased risk of end-stage kidney disease (ESKD). Conclusion Both plasma levels and glomerular deposition of FHR5 were elevated in patients with AAV and associated with more active disease and more severe renal damage. Glomerular FHR5 deposition was an independent risk factor for ESKD in AAV.

BACKGROUND:Proliferative glomerulonephritis with monoclonal immunoglobulin deposits (PGNMID) is a rare renal disorder for which prospective controlled treatment evidence is limited. Clone-directed therapies targeting plasma cells or B cells are increasingly used, but comparative prospective data are lacking. METHODS:This single-center, open-label, pilot RCT aimed to evaluate the feasibility and preliminary efficacy of plasma cell-targeted therapy (bortezomib-cyclophosphamide-dexamethasone, BCD) versus B-cell-targeted therapy (rituximab, RTX). Twenty eligible patients were randomized (1:1) to receive either 6 cycles of BCD or 4 weekly doses of rituximab at375 mg/m². In patients with B-cell reconstitution > 5/μL, a 500 mg rituximab booster dose was administered at 6 months according to the prespecified protocol. The primary efficacy endpoint was overall renal response at 12 months. Changes in proteinuria, serum albumin and estimated glomerular filtration rate were also assessed at 6 and 12 months as secondary outcomes. Safety outcomes included infections and hematologic toxicity. RESULTS:Baseline characteristics were generally comparable between groups. The median age was 57 years, and mean proteinuria 4.1 ± 3.1 g/d. In the primary intention-to-treat analysis (n = 20), the primary outcome of overall renal response at 12 months was observed in 60% (6/10) of patients in the BCD group and 70% (7/10) in the RTX group (P = 1.0). Treatment failure occurred in 2 patients in the BCD group and none in the RTX group. At 6 months, complete response occurred in five patients in the BCD group and three patients in the RTX group. Both groups showed reductions in proteinuria and improvements in serum albumin over 12 months. A transient decline in estimated glomerular filtration rate was observed in the BCD group at 6 months and recovered by 12 months. Safety profiles were manageable, with one serious adverse event, pneumonia, in the BCD group. CONCLUSION:Both BCD and rituximab were associated with renal responses in this exploratory pilot trial of PGNMID. Given the small sample size, open-label design, and differences in treatment exposure structure, these findings are hypothesis-generating. Larger multicenter studies with longer follow-up are needed to define optimal therapy for PGNMID.Trial registration: chictr.org.cn Identifier: ChiCTR2500102928.

Introduction: Light chain cast nephropathy (LCCN) results from the coprecipitation of monoclonal light chains with Tamm-Horsfall protein (THP) within the distal nephron, which forms obstructive casts. Although previous studies have suggested that crystalline LCCN variants may develop independently of THP, the role of THP in classic LCCN remains unclear. We investigated THP involvement in diverse LCCN variants and compared the clinicopathological profiles and outcomes of THP-positive and THP-negative cohorts. Methods: We retrospectively analyzed 32 patients with newly diagnosed multiple myeloma and biopsy-proven LCCN. Cases were classified as THP-positive (n = 19) or THP-negative (n = 13) based on THP immunohistochemical staining. Clinical, laboratory, and histopathological data were compared and supplemented by a proteomic analysis of cast composition using laser microdissection coupled with liquid chromatography-tandem mass spectrometry. Results: Immunoreactive-negative THP LCCN, accounting for 40.6% (13/32) of the cohort, presented with significantly lower hemoglobin, higher serum creatinine, more frequent acute kidney injury (AKI), and greater dialysis dependence than their THP-immunoreactive-positive counterparts. Immunoreactive-negative THP patients exhibited exacerbated tubular atrophy, interstitial inflammation, and acute tubular injury. Mass spectrometry further confirmed that THP was undetectable in a portion of immunoreactive-negative THP casts. No significant between-group difference was observed in survival. Conclusion: Immunoreactive-negative THP LCCN correlates with severe anemia, AKI requiring dialysis, and marked tubular damage. Therefore, immunohistochemical staining for THP should be carefully evaluated in patients with LCCN. Further exploration of the mechanisms underlying LCCN pathogenesis is warranted.

Malakoplakia is a rare chronic granulomatous disorder predominantly occurring in immunocompromised patients, with renal parenchymal involvement being exceptionally rare, likely because of dysfunctional macrophage clearance of bacteria‌. Here, we report a case of a 69-year-old woman with elevated creatinine levels and bilateral kidney lesions, but no identifiable immunosuppressive factors. Enterococcus faecalis was isolated from urine analysis. Histopathological examination showed nearly complete replacement of renal parenchyma by 3 distinct morphological zones: a granulomatous area rich in CD68+ macrophages and giant cells with eosinophilic granular cytoplasm, an inflammatory zone dominated by mixed inflammatory cells, and a collagen-dominant fibrotic zone. Numerous Michaelis-Gutmann (MG) bodies with significant size variation (maximum dimension 187 μm × 96 μm) were observed. Electron microscopy showed abnormally enlarged lysosomes containing fibrillary components, as well as crystalloid or targetoid bodies, representing different developmental stages of MG bodies. After treatment with levofloxacin and prednisone, the patient's creatinine levels improved, and renal cortical thickness normalized. This case not only demonstrates the histological features of malakoplakia at different stages but also shows the ultrastructural progression of MG bodies, suggesting that malakoplakia should be considered in the differential diagnosis of elderly patients with unexplained kidney function decline.

BACKGROUND:The association between immunoglobulin light chain variable (IGVL) gene usage and clinicopathological features in renal AL amyloidosis requires further research. This study analyzed IGVL genes and their associations with clinicopathological characteristics, organ involvement and survival outcomes in a Chinese cohort. METHODS:We retrospectively enrolled 273 renal AL amyloidosis patients. Amyloid deposits were microdissected and subjected to mass spectrometry (MS)-based proteomics to identify IGVL genes. Clinicopathological features, organ involvement and survival data were systematically analyzed. RESULTS:IGVL genes were successfully identified in 250 patients (92%). IGLV6-57 (32%) was predominant, IGKV1 family (12%) was the most prevalent in AL-κ cases. IGLV6-57 was associated with a higher rate of full nephrotic syndrome, while was linked to milder vascular amyloid deposition and total amyloid deposition. IGKV1 was associated with heavier amyloid burden, severe interstitial inflammation and a higher rate of hepatic involvement, IGLV6-57 was correlated with reduced hepatic involvement. IGLV1-51 potentially predicted rapid renal progression and dialysis risk. CONCLUSION:The IGVL gene usage is associated with distinct clinicopathological features in renal AL amyloidosis, IGLV6-57 is linked to a higher frequency of full nephrotic syndrome, IGKV1 is associated with severe kidney structural damage and hepatic tropism, and IGLV1-51 potentially predicts poor renal survival.

Light chain proximal tubulopathy (LCPT) is a rare kidney disorder associated with monoclonal gammopathy, traditionally defined by crystalline inclusions in proximal tubular epithelial cells. A poorly recognized variant, fibrillary LCPT, is characterized by large nonamyloid fibrillar aggregates but has remained inconsistently classified, leading to diagnostic uncertainty. We aimed to clarify its clinicopathologic features and refine its position within the LCPT spectrum. We retrospectively analyzed 41 biopsy-proven LCPT cases, reclassified by ultrastructural features into crystalline LCPT (n = 22), fibrillary LCPT (n = 11), amyloid LCPT (n = 2), and LCPT with lysosomal indigestion (n = 6). Fibrillary LCPT showed irregular cytoplasmic fibrils measuring 6 to 18 nm in diameter, arranged in intersecting, fishbone-like, or compact bundle-like patterns, confined to proximal tubular epithelial cells. These fibrils were Congo red negative and not apparent on light microscopy. Conventional immunofluorescence (IF) on frozen tissue was uniformly negative in fibrillary LCPT, and pronase-digested paraffin IF detected κ restriction in only 7 of 11 cases; immunoelectron microscopy confirmed κ light chain labeling in all paraffin IF-negative cases, yielding 100% diagnostic sensitivity. Clinically, most fibrillary LCPT cases were associated with monoclonal gammopathy of renal significance (72.7%), with the remainder linked to multiple myeloma (27.3%). This subtype was commonly associated with Fanconi syndrome and reduced estimated glomerular filtration rate, with a subset of cases developing acute kidney injury. Following clone-directed therapy, a majority of patients achieved stable renal function, and 85% showed improvement in tubular dysfunction, although proteinuria reduction was less pronounced than in crystalline LCPT. Fibrillary LCPT represents a distinct, nonamyloid entity characterized by Congo red-negative fibrils, κ light chain restriction, and frequent association with Fanconi syndrome and monoclonal gammopathy of renal significance. We propose a refined 4-tier ultrastructural classification comprising 4 LCPT subtypes (crystalline LCPT, fibrillary LCPT, amyloid LCPT, and LCPT with lysosomal indigestion) to improve diagnostic precision and guide management.

Leukocyte chemotactic factor 2 is a recently identified amyloidogenic protein, whose abnormal aggregation defines a systemic amyloidosis termed ALECT2 amyloidosis. Due to the lack of reliable biomarkers, diagnosis relies primarily on histological demonstration and typing of amyloid deposits in renal biopsies. However, immunohistochemical detection of ALECT2 is often inconsistent, leading to diagnostic uncertainty. The underlying basis remains poorly understood, reflecting our limited knowledge of ALECT2 deposits. Here, using cryo-electron microscopy (cryo-EM), we determined the structures of ALECT2 filaments from renal biopsies of five living patients. Unlike filaments assembled from recombinant proteins in vitro, all 133 residues of mature LECT2 are incorporated into the filament cores, with native disulfide linkages preserved. The filaments consistently adopt the shared six-layered folds in all five patients, indicating a common mechanism of amyloidogenesis. Because all residues are incorporated into the fibril core, epitope accessibility is limited. This can explain variability in immunohistochemical detection and thus highlights the need for conformation-specific antibodies and antibody-independent detection strategies for improving diagnostic accuracy. This biopsy-based workflow not only expands the availability of patient-derived tissue for cryo-EM studies but also demonstrates the potential of cryo-EM as a tool for precise diagnosis of systemic amyloidosis.

Acute kidney injury (AKI) often leads to incomplete recovery of renal function, progressing to chronic kidney disease (CKD). Key pathological features in the AKI-CKD transition include microvascular rarefaction and fibrosis. However, the direct effects of activated fibroblasts on microvasculature and endothelial cells remain unclear. We constructed a single-cell RNA sequencing (scRNA-seq) database from unilateral ischemia reperfusion injury (uIRI) mouse model and identified five heterogeneous fibroblast subpopulations, with C0-Sema3dhi fibroblasts significantly increasing post-injury and correlating with reduced endothelial cells. Conditioned medium from Sema3dhi-NRK49F cells inhibited focal adhesion formation and induced cytoskeletal collapse in human umbilical vein endothelial cells (HUVECs), preventing migration and angiogenesis. Mechanistically, Sema3dhi fibroblasts secrete Sema3d, activating the endothelial Plexin D1 receptor, leading to Arf6 activation and integrin β1 internalization, thus suppressing endothelial function. Systemic administration of Sema3d-shRNA using adeno-associated virus serotype 9 (AAV9) effectively reduced Sema3d levels and significantly alleviated renal fibrosis in mice. The presence of SEMA3D+ fibroblasts was confirmed by analyzing human scRNA-seq data and through immunofluorescence staining of kidney sections from patients with kidney diseases. This study reveals new target for mitigating renal fibrosis and microvascular loss, suggesting that targeting the Sema3d signaling pathway may provide a novel strategy for preventing AKI fibrotic progression.

Phospholipase A2 receptor (PLA2R) is recognized as the predominant targeted antigen in patients with idiopathic membranous nephropathy (MN). The immunostaining of PLA2R in kidney biopsy tissue from MN patients plays a crucial role in elucidating pathogenesis and guiding clinical treatment. Nevertheless, significant heterogeneity in PLA2R immunohistochemistry (IHC) staining pattern has been consistently observed in paraffin-embedded specimens undergoing extended preservation, which critically compromises the reliability of the diagnostic confirmation. Our study demonstrates that the preservation protocols of paraffin-embedded slides critically determine PLA2R IHC staining outcomes, thus offering a methodological avenue to address this technical bottleneck by optimizing and standardizing specimen preservation protocols. This study involved 3 patients with PLA2R-associated MN. Consecutive 3µm sections from these patients were sectioned and paraffin-embedded, then the specimens were divided into two groups, group 1 was preserved at room temperature before subjected to IHC staining for PLA2R on days 1, 10, 20, and 30 post sectioning; and group 2 was preserved at -20°C before subjected to IHC staining on day 30 post sectioning. Group 1, the positivity rate of PLA2R in consecutive tissue sections significantly decreased with prolonged storage time at room temperature, while samples of group 2 stored at -20°C exhibited no significant changes in positivity rates between day 1 and day 30. For optimal PLA2R IHC results, freshly cut paraffin-embedded tissue sections should undergo antigen retrieval and staining; alternatively, unstained slides must be stored at -20°C for long-term storage.

Background Antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is characterized by necrotizing inflammation of small vessels, typically affecting glomerular capillaries. However, small arteries can also be affected. The clinical significance of this subgroup of patients remains unclear. This study aimed to analyze the clinicopathological features and prognostic significance of renal arteritis in Chinese AAV patients. Methods A large multicenter cohort of 511 patients with renal biopsy-proven AAV, diagnosed between 2002 and 2023, was retrospectively enrolled. Clinicopathological parameters were compared between those with and without renal arteritis (termed AAV_A+ and AAV_A-, respectively). The prognostic value of renal arteritis was further analyzed. Results Among the 511 AAV patients, 73 (14.3%) had renal arteritis. Compared with AAV_A- patients, AAV_A+ patients were significantly older at diagnosis, with more pronounced systemic inflammation and more frequent extrarenal involvement. Although baseline renal function was comparable, AAV_A+ patients exhibited a distinct histopathological phenotype, with fewer crescentic lesions but more inflammatory and necrotizing injuries. Despite AAV_A+ patients showed a better response to immunosuppressive therapy, renal survival was similar between the two groups. However, among patients classified as low or moderate risk by the ANCA kidney risk score (AKRiS), renal survival was significantly worse in AAV_A+ patients. For this subgroup, renal arteritis was independently associated with an increased risk of end-stage kidney disease (ESKD). Conclusions AAV with renal arteritis represents a distinct clinical and pathological phenotype in Chinese population. In patients classified as low or moderate risk by the AKRiS, renal arteritis was independently associated with an increased risk of ESKD.

Introduction:It has been recognized that periodic acid-Schiff (PAS) staining of amyloid deposits is typically weak or negative in the majority of AL amyloidosis. However, a subset of patients exhibits PAS-positive deposits, and their clinicopathological significance remains unclear. Methods:We retrospectively analyzed 269 patients with renal biopsy-proven AL amyloidosis from our institution; 72 were classified as PAS positive and 197 as PAS negative. Baseline demographics, clinical data, renal pathology features, treatment strategies, and prognostic information were collected. Multivariable linear regression and Cox proportional hazards models were used to determine the impact of PAS-positive amyloid deposition on renal function and patients' outcomes. Results:Compared with the PAS-negative group, PAS-positive patients had higher serum creatinine (94.40 vs. 79 μmol/L, p = 0.005) and increased renal insufficiency ratio (31.94% vs. 14.72%, p = 0.002). Histopathology showed more severe glomerular and interstitial amyloid deposition (p = 0.037, p = 0.049, respectively). In multivariable linear regression, PAS-positive deposits (β = 0.158, p = 0.005) were independently associated with higher serum creatinine. PAS-positive patients showed significantly worse renal survival (HR: 3.05, 95% CI: 1.40-6.65, p = 0.005), independent of age, treatment regimens, renal stage, and pathological cores. Meanwhile, PAS-positive staining was an adverse prognostic factor (HR: 2.34, 95% CI: 1.24-4.43, p = 0.009) for overall survival after adjusting for AL type, age, treatment, organ involvement, interstitial fibrosis and tubular atrophy, and total amyloid burden. Conclusion:PAS-positive amyloid deposits are associated with more severe renal injury and poorer prognosis in AL amyloidosis.

Rapidly progressive IgA nephropathy (IgAN) and IgA vasculitis nephritis (IgAVN) are characterized by marked glomerular inflammation, extensive extracapillary proliferation, and rapid renal function decline. Despite increasing insights into their shared pathogenic mechanisms, optimal treatment strategies remain uncertain. Here, we report two patients with rapidly progressive crescentic disease, one with primary IgAN and one with IgAVN, who received sequential complement inhibition combined with BAFF/APRIL blockade. Both patients showed substantial reductions in proteinuria and stabilization of kidney function during follow-up. These cases suggest that multi-pathway targeted therapy may represent a promising approach for selected patients with aggressive IgAN or IgAVN.

BackgroundPulmonary alveolar proteinosis (PAP) is a rare respiratory disorder characterized by the excessive accumulation of surfactant material on alveolar surfaces and the dysfunction of alveolar macrophages. Indium-tin-oxide (ITO), a substance utilized in liquid crystal displays and solar panels, could be a potential cause of secondary PAP. Here, we report a rare case of interstitial lung disease in a solar panel processing worker with ITO exposure, which was pathologically confirmed as PAP and then progressed to pulmonary fibrosis.Case presentationA 35-year-old male was admitted with a symptom of dysponea. Chest computed tomography revealed diffuse, bilateral ground-glass opacities and interlobular septal thickening in both lungs. Transbronchial lung biopsy was PAS-positive, confirming the diagnosis of PAP. The indium levels in the patient’s bronchoalveolar lavage fluid and plasma were far above the normal range. He underwent whole-lung lavage, but approximately 4 months later, he still developed progressive pulmonary fibrosis. Nintedanib showed no significant effect in slowing the decline of his lung function.ConclusionWhen a patient has a definite history of indium exposure, the diagnosis of PAP should be considered. Workers need to wear protective measures daily, as lung diseases caused by ITO exposure are associated with a poor prognosis.

Systemic immunoglobulin light-chain (AL) amyloidosis is characterized by clonal immunoglobulin-secreting cells that produce a monoclonal light chain prone to misfolding and amyloid fibril formation in tissues. Understanding its molecular basis requires accurate full-length sequencing of amyloidogenic light chains and linkage of circulating light chains to renal deposits. Achieving this is technically challenging because the low abundance and N-glycosylation of amyloidogenic light chains can complicate protein purification and peptide-level sequence analysis. To address these challenges, we implemented a robust analytical pipeline that integrates intact-mass measurement by Q-TOF MS before and after deglycosylation with multi-protease digestion and de novo peptide sequencing to systematically characterize urinary N-glycosylated light chains. Mass shifts observed before and after deglycosylation supported the presence of N-glycosylation, whereas deglycosylated intact masses were used to constrain full-length sequence assembly. The assembled urinary light-chain sequences were subsequently compared with matched renal amyloid proteomes isolated by laser microdissection and analyzed by bottom-up liquid chromatography-tandem mass spectrometry (LC-MS/MS). Enzymatic deglycosylation reduced glycan-induced spectral interference. Eight monoclonal light-chain sequences were assembled from urinary light chains, including five derived from the immunoglobulin kappa variable 1 (IGKV1), two from the immunoglobulin lambda variable 2 (IGLV2), and one from IGKV4. The close agreement between theoretical and experimental intact masses confirmed the accuracy and completeness of sequence assembly. In each renal amyloid proteome, the corresponding urine-derived sequence had the highest score among light-chain identifications of the patient's clinically determined isotype (κ or λ) and showed 87.6-100% variable-region peptide coverage. Together, these findings support urinary N-glycosylated monoclonal light chains as the precursor proteins of the corresponding renal amyloid fibrils. In conclusion, we have demonstrated a robust workflow that applies established de novo peptide sequencing to characterize urinary N-glycosylated light chains and trace the corresponding sequences in renal amyloid deposits, with potential applications across light chain-related diseases.

Background: It has been recognized that Periodic acid–Schiff (PAS) staining of amyloid deposits reveals weak or negative in the majority of AL amyloidosis. However, a subset of patients exhibits PAS-positive deposits, and their clinicopathological significance remains unclear. Methods: We retrospectively analyzed 269 patients with renal biopsy-proven AL amyloidosis from our institution; 72 were classified as PAS-positive and 197 as PAS-negative. Baseline demographics, clinical data, renal pathology features, treatment strategies, and prognostic information were collected. Multivariable linear regression and Cox proportional hazards models were used to determine the impact of PAS-positive amyloid deposition on renal function and patients’ outcomes. Results: Compared with the PAS-negative group, PAS-positive patients had higher serum creatinine (94.40 vs. 79 μmol/L, P = 0.005) and increased renal insufficiency ratio (31.94 % vs. 14.72%, P = 0.002). Histopathology showed more severe glomerular and interstitial amyloid deposition (P = 0.037, P = 0.049, respectively). In multivariable linear regression, PAS-positive deposits (β = 0.158, P = 0.005) were independently associated with higher serum creatinine. PAS-positive patients showed significantly worse renal survival (HR 3.05, 95% CI: 1.40 – 6.65, P = 0.005), independent of age, treatment regimens, renal stage, and pathological cores. Meanwhile, PAS-positive staining was an adverse prognostic factor (HR 2.34, 95% CI: 1.24 – 4.43, P = 0.009) for overall survival after adjusting for AL-type, age, treatment, organ involvement, interstitial fibrosis and tubular atrophy, and total amyloid burden. Conclusion: PAS-positive amyloid deposits are associated with more severe renal injury and poorer prognosis in AL amyloidosis.

Monoclonal free light chains drive diverse renal pathologies in plasma cell dyscrasias. However, the mechanism of light chain proximal tubulopathy remains poorly understood because sequence data are scarce. Here we characterize full-length kappa free light chains from five patients with light chain proximal tubulopathy, four of whom had intracellular crystals. All showed the highest homology to the IGKV1-33 germline, while crystal-forming light chains contained distinct single-residue substitutions. These light chains generated a stable ~12-kDa fragment resistant to digestion by cathepsin B and D, a pattern not observed in non-crystal-forming light chains. Mechanistically, a single-residue substitution contributed substantially to cathepsin D resistance and stable fragment formation. In vivo comparison of recombinant κWJJ and its single-amino acid mutant further supported an important role for this substitution in intracellular crystal formation. These findings establish a sequence-based link between impaired lysosomal processing and crystal accumulation in light chain proximal tubulopathy.

The aim of this study is to explore the quantity profiles of amyloid signature proteins (serum amyloid P component, SAP; apolipoprotein E, ApoE; apolipoprotein A-IV) in common types of renal amyloidosis by mass spectrometry and immunostaining methods. Twenty-one patients with renal amyloidosis of different types evaluated at the Renal Pathological Center of Peking University First Hospital from 2000 to 2021 were enrolled. Immunohistochemistry (IHC) and laser microdissection combining with mass spectrometry (LMD-MS) were applied to investigate the localization and quantity profiles of signature proteins in renal amyloidosis. The co-localization relationships among signature proteins and amyloid fibrils, as well as the ultrastructural localization of SAP were examined by laser scanning confocal microscopy (LSCM) and immuno-electron microscopy (IEM), respectively. By MS-based proteomic analysis, large spectra numbers of ApoE and its higher abundance were noted in four types of amyloidosis when compared with SAP, and ApoA-IV was absent in ALECT2 amyloidosis. LSCM showed ApoE and SAP co-localized with amyloid fibrils in renal AL-κ, AL-λ and ALECT2 amyloidosis. ApoA-IV co-localized with amyloid fibrils in AL-κ and AL-λ amyloidosis, but was not found in ALECT2 amyloidosis. By semi-quantitative analysis based on LSCM and IEM, the quantity levels of signature proteins in AL-κ appeared to be lower than that in AL-λ (P < 0.05) or ALECT2 (P < 0.05), while there was no significant difference between AL-λ and ALECT2 amyloidosis. Both of SAP and ApoE were the ubiquitous signature components of renal amyloidosis (AL, AA, ALECT2), as well as ApoA-IV in AL and AA, but not in ALECT2. ApoE was the key signature protein in renal amyloidosis. The quantity levels of signature proteins investigated through LCSM/IEM demonstrated variability among different types, with AL-κ amyloidosis appeared to have a lower level. Not applicable.