Introduction: Although anti-neutrophil cytoplasmic autoantibody (ANCA)-associated vasculitis (AAV) is rare in children, kidney involvement is both common and potentially severe. Data on the clinico-pathological presentation and progression of kidney involvement in pediatric AAV are limited. Methods: This multicentric, retrospective, observational study aims to characterize kidney involvement in pediatric AAV, through a centralized pathology review of kidney biopsies and comparisons with an adult AAV cohort. Results: Eighty-one pediatric patients (median age 12.7 years, 23% male) were included over 20 years. Compared with adults (median age 66 years, 52% male), children presented with more frequent nephrotic-range proteinuria (42% vs. 18%, P < 0.001), lower hemoglobin levels (8.7 vs. 9.7 g/dl, P < 0.001), and more common cutaneous (24% vs. 9%, P = 0.001) and gastrointestinal involvement (18% vs. 3%, P < 0.001); histologically, more crescentic (P < 0.001) and fewer focal forms (P < 0.001), less interstitial fibrosis and tubular atrophy (IF/TA 0: 41% vs. 19%, P = 0.001) and more interstitial inflammation (ti1-2-3 per Banff classification: 66% vs. 47%, P = 0.025). For induction treatment children had a more frequent use of rituximab (65% vs. 43%, P = 0.001) and plasma exchange (31% vs. 15%, P = 0.004). Kidney outcomes were comparable to adults. The Berden classification, ANCA Renal Risk Score (ARRS), and ANCA Kidney Risk Score (AKRiS) showed good predictive accuracy for pediatric patients. Repeat biopsies demonstrated reduced activity and progression towards fibrosis. Conclusion: Pediatric patients present a more active kidney histology than adults, a more frequent nephrotic-range proteinuria and more severe anemia at presentation. Despite this, outcomes are comparable to adults. Berden, ARRS, and AKRiS can be used for predicting kidney prognosis in the pediatric population.

The XVIIth Banff Conference on Allograft Pathology, held in Paris in 2024 in collaboration with the Paris Institute for Transplantation and Organ Regeneration of Université de Paris Cité, convened the heart transplant working group to discuss 2 main topics: (1) the microvascular inflammatory burden on endomyocardial biopsy (EMB), with a focus on new advancements in inflammation-endothelium interactions within the allograft; and (2) the impact of molecular and noninvasive diagnostic tools in rejection monitoring. The discussions addressed the implications of molecular pathology performed on formalin-fixed paraffin-embedded tissue in the context of the inflammatory burden on EMBs, and explored the emerging characteristics of the endothelium in the microvasculature and its relationship with alloantibodies. The integration of tissue and blood molecular analyses has enhanced the reliability and accuracy of EMB interpretation by pathologists, while highlighting the complexity of inflammatory mechanisms occurring in the allograft. The high negative predictive value of noninvasive biomarkers and novel imaging modalities suggests their potential applicability in clinical practice and their capacity to modify follow-up strategies in heart transplant recipients.

Peripheral Arterial Disease (PAD) is a prevalent but underdiagnosed pathology. Soluble CD146 (sCD146) was described as a marker of endothelial dysfunction and vascular congestion. We hypothesize that sCD146 may represent a novel biomarker of PAD. Our objective was to evaluate the association between plasma sCD146 levels and the occurrence and severity of PAD. In this case–control study, 184 Caucasian men with symptomatic PAD were compared to 163 age-matched healthy control patients. PAD diagnosis was confirmed using ankle-brachial index (ABI) and imaging. Plasma sCD146 was quantified using ELISA. Associations with clinical and biochemical parameters were analyzed through multivariable logistic regression models. sCD146 level was significantly reduced in PAD patients (mean [95

Kidney transplantation requires the anastomosis of the graft’s kidney vein to the recipient's iliac vein or inferior vena cava (IVC). The venous reconstruction can be challenging in case of extended ilio-caval thrombosis. We report the case of a child with Denis Drash syndrome, with right Wilms tumor and intra-caval tumoral extension, left nodules of nephroblastomatosis on a horseshoe kidney, proteinuria, and progressive kidney failure, resulting in bilateral nephrectomy and iliac veins and vena cava thrombosis extended to the retro-hepatic IVC. At the age of 7 years, 32 months after the end of the oncological treatment, she underwent kidney transplantation. Venous reconstruction was achieved by using the donor IVC as a venous conduit anastomosed to the recipient's retro-hepatic IVC at the level of the hepatic veins. Postoperative course was uneventful, and the child is alive and well, free of tumor and with normal kidney function, 5 years after the end of oncological treatment, and 32 months after transplantation. This technique appears as a safe and physiological alternative to previously described venous anastomoses to the portal system or pelvic varices, preventing chronic venous hypertension of the graft.

BACKGROUND AND AIMS:Tissular gene expression profiling applicable to formalin-fixed, paraffin-embedded (FFPE) endomyocardial biopsies (EMBs) may refine the diagnosis of cardiac rejection while being easily implemented in clinical practice. This study aimed to develop and validate the first FFPE-based molecular diagnostic system dedicated to heart transplant rejection. METHODS:An international study was conducted (NCT06436027), establishing a deeply phenotyped cohort of heart transplant recipients. EMBs were graded according to international classifications, and gene expression was analysed on FFPE-EMBs using the Banff Human Organ Transplant Panel. Molecular classifiers for antibody-mediated rejection (AMR) and acute cellular rejection (ACR) were developed, with discrimination and calibration assessed in internal and external validation cohorts. RESULTS:A total of 671 biopsies were included, with 591 in the main cohort (AMR: n = 188, ACR: n = 289, matched non-rejection: n = 114); this was split into a derivation set (n = 475) and an internal validation set (n = 116). The external validation cohort comprised 80 biopsies (AMR: n = 20, ACR: n = 32, non-rejection: n = 28). AMR was associated with significant transcripts related to the interferon-gamma pathway, endothelial activation, and monocyte-macrophage recruitment, while ACR was characterized by transcripts related to T-cell receptor signalling, CD3 receptor activation, and CD28 signalling. Molecular ACR and AMR models accurately identified rejection in the validation cohorts (internal: ROC-AUC: AMR = 0.812, ACR = 0.849; external: ROC-AUC: AMR = 0.822, ACR = 0.815) and were strongly associated with pathologic severity. Calibration was adequate. An automated report was developed to enhance the clinical applicability of these classifiers. CONCLUSIONS:A novel FFPE-based molecular diagnostic system accurately identified cardiac allograft rejection. This tool is readily applicable in clinical practice as a companion to pathology and has the potential to refine the diagnosis of rejection.

Background and hypothesis : Maladaptive repair drives progression from acute kidney injury (AKI) to chronic kidney disease. Injured proximal tubular cells overexpress complement C3 mRNA, but its functional role is not well understood. We hypothesized that complement proteins produced and active locally within proximal tubules drive maladaptive repair and remain inaccessible to plasma-targeted inhibitors. Methods : We investigated complement, encompassing extracellular, autocrine and intracellular pools, in human biopsies and mouse model of rhabdomyolysis-induced AKI (RIAKI). Hyperplex sequential immunofluorescence combined with RNAscope was used to localize complement proteins and transcripts in situ and immune infiltrate. C3 or factor B (FB) deficient mice and pharmacological FB inhibition were tested in a RIAKI model. Bulk transcriptomics and C3/CFB knockdown in tubular cells with rescue experiments dissected mechanisms in vivo and in vitro . Results : In human and experimental RIAKI, injured VCAM-1 proximal tubules showed reabsorption and local transcription of C3 and CFB in a subset of tubules, induced by inflammatory cytokines. Genetic deletion of C3 or Cfb attenuated AKI, reducing tubular necrosis, immune infiltration and signatures of epithelial-to-mesenchymal transition and maladaptive repair. Pharmacological FB inhibition reduced extracellular C3 activation but did not preserve renal function. In vitro , C3 or FB knockdown in tubular cells suppressed proliferation, inflammation and mTOR-linked metabolic pathways and was not rescued by exogenous purified protein, consistent with cell-intrinsic/intracellular mode of action. Conclusion : Locally produced complement in proximal tubules drives inflammatory and maladaptive repair programs during AKI. The dissociation between genetic deletion and pharmacological FB inhibition indicates that the relevant complement activity occurs in a compartment poorly accessible to systemic inhibitors and supports development of locally acting, tissue-penetrant or cell-permeable complement-targeted therapies for AKI.

Introduction:Immune checkpoint inhibitor (ICI)-induced acute interstitial nephritis (AIN) (ICI-AIN) is the leading cause of ICI-associated acute kidney injury (AKI). ICI-AIN is characterized by mononuclear immune infiltrates, although the mechanisms behind their toxicity remain unclear. We aimed to characterize these infiltrates in kidney biopsies and assess their correlation with clinical outcomes and therapeutic responses. Methods:We retrospectively analyzed 49 biopsy-proven ICI-AIN cases using multiplex immunofluorescence to quantify immune cells (macrophages, neutrophils, B cells, T cells, and plasmocytes). Unsupervised clustering was used to identify patient groups, which we then correlated with clinical presentation and outcomes. Finally, we explored the role of C5a/C5aR1 in neutrophil recruitment. Results:Unsupervised clustering revealed 3 immune phenotypes as follows: (i) low mononuclear (cluster 1), (ii) high mononuclear (cluster 2), and (iii) neutrophil-rich (cluster 3). Cluster 3 was associated with higher systemic inflammation (C-reactive protein: 84 vs. 15-24 mg/l, P = 0.0002; neutrophil-to-lymphocyte ratio (NLR): 7 vs. 3.2-2.3, P < 0.0001) and more severe initial AKI (peak creatinine: 360 vs. 215-208 μmol/l, P = 0.0001). Histologically, it was marked by granular casts and neutrophilic tubulitis (P < 0.0001). Despite the pyelonephritis-like appearance, urine cultures and metatranscriptomic analysis both ruled out infection. At 12 months, renal response rates to steroids were 93% (cluster 2), 67% (cluster 1), and 38% (cluster 3) (P = 0.004). Relapses occurred more frequently in cluster 3 (38% vs. 11% in cluster 1, 0% in cluster 2, P = 0.01). Urine C5a correlated with C5aR1+ neutrophil infiltration (rho = 0.78). Conclusion:Our findings identify distinct ICI-AIN subtypes, with a neutrophil-rich cluster linked to complement activation and poor prognosis, offering insights into refining diagnosis and treatment strategies.

RATIONALE & OBJECTIVE:Autoantibodies targeting complement factor B (anti-FB) are among the most recently described alternative pathway anomalies. Although they have been strongly associated with post-infectious glomerulonephritis in children, the clinical phenotypes associated with such antibodies, along with their effects on the alternative pathway in adults, remain elusive and require further characterization, which was the goal of this study. STUDY DESIGN:Retrospective case series. SETTING & PARTICIPANTS:Patients for whom anti-FB IgG was systematically detected by the French reference center for complement abnormalities between October 2017 and June 2020 and who underwent concurrent kidney biopsy confirming kidney disease were included as cases. FINDINGS:Seventy-one patients tested positive for anti-FB antibodies using ELISA and single-bead antigen assays in the setting of newly diagnosed biopsy-proven kidney disease. Detection of both anti-FB and anti-C3b autoantibodies occurred for 36/71 (51%) patients. In vitro, total purified IgG from these patients enhanced alternative pathway activity, with anti-FB titers being correlated with C3bB proconvertase formation (R2 = 0.40, p < 0.001) and anti-C3b titers being correlated with C3bBb convertase stabilization (R2 = 0.46, p < 0.001), suggesting distinct and potentially synergistic functional effects. Clinically, the most frequent diagnosis associated with anti-FB detection was infection-related glomerulonephritis (IR-GN) (46/71 - 65%), with higher anti-FB titers in patients with IR-GN compared to those with other diagnoses (median 791 [IQR 287-2000] vs. 326 [173-790] AU/mL; Hodges-Lehmann difference 354 AU/mL [95% CI: 58-1020]; p = 0.01). No difference was detected in anti-C3b titers between IR-GN and other diagnoses. Anti-FB antibodies became undetectable in 21 of 36 retested patients (58%); among the 10 patients with persistent detection beyond 3 months, 8 (80%) had uncontrolled infection. After a median follow-up of 12.5 (5-24) months, 18/67 (27%) patients experienced a major adverse kidney event (kidney failure or a sustained >50 decline in eGFR below the baseline value). LIMITATIONS:Retrospective design, the series was enriched in alternative pathway anomalies due to reference center recruitment. CONCLUSIONS:Detection of anti-FB antibodies is strongly associated with infection-related glomerulonephritis in adult patients, highlighting an important mechanism of alternative pathway deregulation in such diseases. PLAIN LANGUAGE SUMMARY:Antibodies targeting factor B, a key protein of the complement cascade, have been shown to activate the complement system and promote kidney inflammation. However, the clinical characteristics and outcomes of patients with such autoantibodies remain largely understudied, particularly in adults. This study systematically screened over 700 adults referred for complement investigation and identified 71 patients with anti-factor B antibodies and concurrent biopsy-proven kidney disease. These antibodies were predominantly associated with infection-related glomerulonephritis, activated the alternative complement pathway, and tended to disappear once the underlying infection was controlled. These findings highlight the importance of screening for anti-factor B antibodies in patients with complement-mediated kidney disease, as their detection may prompt the search for an underlying infection and guide therapeutic decisions.

The inability of graft endothelial cells to deliver HLA-I-dependent inhibitory signals to recipient natural killer (NK) cells (missing self, MS), drives donor-specific antibody-independent microvascular inflammation (MVI), leading to graft failure. This study aimed to elucidate the signaling pathways involved in MS-associated NK cell activation and explore therapeutic strategies. Analyses of kidney graft biopsies identified calcium signaling pathways and mTOR as a key regulator of MS-induced NK cell activation. Two experimental models were developed to mimic the pathological condition: in vitro cocultures of human NK cells with allogeneic microvascular endothelial cells and a murine heart transplantation model. These models showed that while calcineurin inhibitor (CNI) alone had a limited impact, combining CNI with mTOR inhibitors (mTORinh) synergistically reduced NK cell activation and endothelial damage. In a pilot clinical study involving 50 renal transplant recipients with MS-associated NK cell-mediated microvascular inflammation, patients who tolerated mTORinh introduced on top of CNI at diagnosis demonstrated reduced MVI lesions and improved graft survival compared to a historical cohort left on CNI and mycophenolate mofetil. This translational study identifies mTOR inhibition as a pivotal adjunct to CNI in mitigating MS-associated NK cell-mediated inflammation, potentially improving long-term graft outcomes.

Vascularized Composite Allotransplantation (VCA) requires immunosuppression to prevent allosensitization and subsequent allograft rejection. This study evaluated the prevalence, characteristics, and clinical implications of donor-specific antibodies (DSA) in VCA recipients. Thirty-seven VCA recipients (median age 32.5 years) from six international centers, grafted between 2000 and 2019, were studied. They included 23 upper extremity transplantations (UETs), 13 face transplantations and one patient with simultaneous UET and face transplantation. A total of 402 pre-transplant and follow-up sera were screened for DSA using a centralized assay. Among the 31 evaluable recipients, DSA appeared in a range of time between 7 and 2740 days post-transplantation with a median time of 360 days. The cumulative incidence of de novo DSA was 26% at five years and 48% at ten years. The incidence of de novo DSA did not differ between UET and face recipients. Both preformed and de novo DSA displayed comparable disappearance kinetics. A higher incidence of chronic rejection was observed in patients with DSA when an MFI cutoff of 500 was used (46.7% vs. 17.6% in patients with and without DSA, respectively), which became significant (p = 0.02) when the cutoff was increased to 1000. In conclusion, humoral immunization is a common event following VCA. DSA positivity was associated with chronic rejection and graft vasculopathy. Further mechanistic studies are needed to confirm the role of DSA in VCA rejection.

Kidney transplantation (KTx) is the main therapy for patients suffering from end-stage kidney disease.Unfortunately, the occurrence of acute rejection, mainly within the first year after transplantation, remains a major problem and is a significant risk factor for the development of future transplant failure.The current gold standard for diagnosing acute rejection is by histomorphological evaluation of a KTx biopsy using the Banff classification of renal allograft pathology, an international consensusbased classification updated every two years.Challenges with Borderline T Cell-Mediated Rejection (bTCMR): A difficult diagnosis within this classification is category 3, Suspicious (Borderline) for acute T cell-mediated rejection (bTCMR).This diagnosis is based on minimal interstitial inflammation and the presence of tubulitis.Over the different updates of the Banff classification, there have been changes in the amount of interstitial inflammation needed to diagnose bTCMR, varying from <10% (i0) to 10-25% (i1).During the discussions at the 2019 Banff meeting and research publications, the threshold for the amount of interstitial inflammation in bTCMR was increased to 10-25% of the non-scarred cortical tissue.(1)These changes have had significant clinical implications, as they may have significantly affected the treatment and management strategies for patients.

The generation of donor-specific antibodies (DSA) requires that alloreactive B cells receive help from follicular helper T (TFH) cells. Recent works have suggested that γδ T cells could contribute to T cell-dependent humoral responses, leading us to investigate their role in DSA generation. Analysis of a cohort of 331 kidney transplant recipients found no relation between the number of circulating γδ T cells and the risk to develop DSA. Coculture models demonstrated that activated γδ T cells were unable to promote the differentiation of B cells into plasma cells, ruling out that they can be “surrogate” TFH. In line with this, γδ T cells preferentially localized outside the B cell follicles, in the T cell area of lymph nodes, suggesting that they could instead act as “antigen-presenting cell” (APC) to prime αβ TFH. This hypothesis was proven wrong since γδ T cells failed to acquire APC functions in vitro. These findings were validated in vivo by the demonstration that following transplantation with an allogeneic Balb/c (H2d) heart, wild-type and TCRδKO C57BL/6 (H2b) mice developed similar DSA responses, whereas TCRαKO recipients did not develop DSA. We concluded that the generation of DSA is unfazed by the absence of γδ T cells.

C3 glomerulopathy (C3G), a prototype of complement-mediated disease, is characterized by significant heterogeneity, in terms of not only clinical, histological and biological presentation but also prognosis, and response to existing therapies. Recent advancements in understanding the factors responsible for alternative pathway dysregulation in the disease have highlighted its even more complex nature. Here, we propose a reexamination of the diversity of C3G presentations in light of the drivers of complement activation. Autoantibodies targeting complement proteins, genetic abnormalities in complement genes and monoclonal immunoglobulins are now well-known to drive disease occurrence. This review discusses how these drivers contribute to the heterogeneity in disease phenotype and outcomes, providing insights into tailored diagnostic and therapeutic approaches. In recent years, a broad spectrum of complement inhibitory therapies has emerged, soon to be available in clinical practice. The recognition of specific clinical, biological and histological patterns associated with different forms of C3G is crucial for personalized management, particularly treatment strategies.

Background. Kidney transplant recipients with preformed donor-specific antibodies (DSAs) are at higher risk for antibody-mediated rejection (AMR) and graft failure. This study assesses outcomes during a 15-y period, comparing patients with high levels of preformed DSAs to a matched cohort of recipients without DSA. Methods. This retrospective study includes 95 patients with high levels of preformed DSAs, defined by DSA levels (mean fluorescence intensity > 3000). Using propensity score matching, 88 of these patients were matched to 154 recipients without DSA based on age, sex, transplant year, cold ischemia time, and other clinical factors. Outcomes included occurrence of AMR, T cell–mediated rejection, and long-term allograft and patient survival. Results. The mean follow-up after transplantation was 11.9 ± 5.6 y. Among the matched patients, kidney allograft survival rates after transplantation at 5, 10, and 15 y were 90.5%, 79.5%, and 72.8% for the patients without preformed DSA and 86.7%, 60.2%, and 47.6% for the patients with high levels of preformed DSAs (P = 0.002). The patient survival rates after transplantation at 5, 10, and 15 y were 91.3%, 72.3%, and 57.9% for the patients without preformed DSA and 86.4%, 71.2%, and 49.0% for the patients with high levels of preformed DSAs (P = 0.206). More AMR occurred in the patients with high levels of preformed DSAs (P < 0.001) without significant difference in T cell–mediated rejection (P = 0.203). Conclusions. Although patients with high levels of preformed DSAs have a higher risk of graft loss, their long-term survival did not differ significantly from recipients without DSA, supporting the use of intensive immunosuppression in this population.

Introduction Endomyocardial biopsies (EMB) gene expression profiling is a promising companion tool for rejection diagnosis. We developped a fully automatised molecular analysis sytem for the diagnosis of cardiac allograft rejection. Objective We aimed to properly assess the clinical validity of this tool by assessing precisely its statistical performance. Method We performed a multicenter, retrospective study (NCT06436027), collecting 591 FFPE-EMBs between 2011 and 2021 representative of the landscape of rejection (antibody-mediated rejection-AMR, n=188; acute cellular rejection-ACR, n=289; non-rejection, n=114). Tissue gene expression was analyzed using the consensus Banff Human Organ Transplant gene panel. Molecular classifiers for AMR and ACR were built using a supervised model, assessing thoroughly the performance. Models’ calibration was optimized, and discrepancies were analyzed in training and validation set. Results In the validation cohort (n=116), the molecular classifiers demonstrated strong diagnostic performance: AMR detection achieved an accuracy of 81.89% (ROC-AUC=0.831, Brier score=0.143, F1 score=0.70), while ACR detection achieved 77.58% accuracy (ROC-AUC=0.812, Brier score=0.176, F1 score=0.76). Models’ stabilization was reached around 400 samples, confirming sufficient statistical power for reliable predictions. Calibration curves resulted adequate for AMR and ACR models sets, reflecting the reliability of the probabilistic predictions (Fig. 1).In the derivation cohort, the predicted probabilities closely followed the ideal trend for both rejection-specifics models, indicating minimal bias. In the validation cohort, the mean absolute error was 0.031 for the AMR model and 0.014 for the ACR model respectively, suggesting excellent calibration. Minor deviations from the ideal line were observed at sub-clinical probability levels. Overall, the calibration curves support the reliability of the models for predicting rejection events in both cohorts. Conclusion The molecular diagnostic system demonstrated a good reliability in identifying rejection events. This system can complement standard pathology, reduce diagnostic uncertainty, and serve as a practical companion tool in the clinical management of heart transplant patients.