The Banff 2022 Kidney Meeting Report: Reappraisal of microvascular inflammation and the role of biopsy-based transcript diagnostics

Role of microvascular pericyte dysfunction in antibody-mediated rejection following kidney transplantation

OBJECTIVE

To investigate the role of microvascular pericyte dysfunction in antibody-mediated rejection (ABMR) of transplanted kidneys.

METHODS

A total of 160 patients who underwent kidney transplantation in our hospital from 2004 to 2020 were enrolled, divided into 4 groups: ABMR group (n = 79), TCMR group (n = 20), mixed rejection group (n = 25) and control group (n = 36). Postoperative renal function indicators were compared, and immunohistochemical and immunofluorescence staining was performed on graft tissues and mice models using the pericyte marker PDGFR-β. An in vitro pericyte dysfunction model was co-cultured with vascular endothelial cells for functional assessment through Western blotting, PCR, and wound healing tests. KEGG pathway analysis from the GEO database identified gene expression changes in pericytes, which were further analyzed using electron microscopy and Western blot techniques.

RESULTS

There were statistically significant differences in creatinine, urea nitrogen, urine protein, and eGFR among the groups over time, with ABMR displaying the poorest outcomes. Immunohistochemistry revealed lower pericyte expression in ABMR, which was confirmed in mouse model studies showing reduced PDGFR-β expression in ABMR. KEGG analysis highlighted decreased autophagy in pericyte dysfunction, supported by electron microscopy and Western blot findings indicating reduced autophagy and pericyte damage, which could be reversed by chloroquine.

CONCLUSION

ABMR episodes worsened the long-term prognosis of transplanted kidneys. pericyte dysfunction appears to be one of the crucial causes of poor prognosis in ABMR patients. In vitro studies demonstrated that dysfunction of microvascular pericytes can result in damage to vascular endothelial cells, with autophagy impairment being a significant mechanism contributing to pericyte dysfunction.

Donor-Derived Cell-Free DNA for Diagnosing Subclinical Rejection: A Pilot Multicenter Study in Japanese Living-Donor Kidney Transplantation

BACKGROUND

Donor-derived cell-free DNA (dd-cfDNA) offers a non-invasive biomarker for detecting allograft injury. However, its performance in predominantly living-donor transplant settings in Asian populations remains underexplored.

METHODS

In a multicenter prospective pilot study, 73 living-donor kidney transplant (LKT) recipients from Japan underwent plasma dd-cfDNA testing and protocol biopsies approximately 1 year post-transplant. Diagnostic performance of dd-cfDNA for subclinical antibody-mediated rejection (AMR) was evaluated at 1.0% and 0.5% cut-off thresholds.

RESULTS

Eight patients (11.0%) were diagnosed with subclinical active AMR. Patients with dd-cfDNA ≥1.0% showed significantly higher incidence of AMR compared to those with dd-cfDNA <1.0% (80% vs. 5.9%, p < 0.001). ROC analysis revealed an AUC of 0.85 for dd-cfDNA in diagnosing AMR. Lowering the threshold to 0.5% improved sensitivity while maintaining high specificity. Patients with persistent donor-specific antibodies (DSA) at 1-year follow-up exhibited significantly higher dd-cfDNA levels. Neither HLA nor ABO incompatibility nor donor-recipient relation significantly affected dd-cfDNA levels.

CONCLUSION

dd-cfDNA demonstrates high negative predictive value and complements DSA testing for early detection of subclinical AMR in living-donor transplants. This pilot study provides foundational evidence for dd-cfDNA-based surveillance strategies in Asian cohorts.

C3 glomerulopathy post kidney transplantation: A single center experience

BACKGROUND

C3 glomerulopathies (C3G) are a rare cause of kidney failure resulting from complement dysregulation. Small studies demonstrate a high rate of recurrence and poor outcomes in kidney transplantation. Treatment efficacy in this setting with eculizumab, a terminal complement inhibitor, is largely unknown.

AIM

To determine the outcomes of kidney transplantation in patients with C3G and the potential impact of eculizumab.

METHODS

We retrospectively studied kidney transplant recipients who underwent a post-transplant biopsy confirming C3G between January 1, 1993 and December 31, 2023 at a single center. Only the first episode of kidney transplant was reviewed. The electronic medical records were reviewed for post-transplant allograft function, indication for biopsy, time to biopsy from transplant, time to allograft failure from transplantation, post-C3G treatment, complement laboratory testing, and concurrent malignancy/infection. Reports, and when available slides and immunofluorescence/electron microscopic images, were re-reviewed by a renal pathologist.

RESULTS

A total of fifteen patients were included in this study. Fourteen patients had suspected recurrent disease, with a pre-transplant native kidney report of C3G. One patient developed de novo C3G. Median post kidney transplant clinical follow up time was 91 months. Median time to recurrence was 7 months with median graft survival of 48 months post kidney transplantation. The most common index biopsy pattern of injury was endocapillary proliferative glomerulonephritis (often with exudative features) with or without mesangial hypercellularity (56%) followed by membranoproliferative glomerulonephritis (25%). Most patients developed membranoproliferative glomerulonephritis pattern of injury on follow up biopsies (63%). Seven patients with recurrent disease received treatment with eculizumab with a median graft survival of 73 months, with five functioning grafts by the end of the study period. Seven patients with recurrent disease did not receive therapy, and all lost their graft with a median graft survival of 22 months (P = 0.003).

CONCLUSION

C3G following kidney transplantation is mostly a recurrent disorder with a poor prognosis in untreated patients. Untreated recurrence has a poor prognosis with median allograft survival < 2 years. Early treatment with eculizumab may improve transplant outcomes in patients with recurrent C3G.

Glycolytic control proteins in urinary extracellular vesicles are elevated during kidney transplant T cell-mediated rejection

BACKGROUND

A priority in kidney transplant management is the ability to monitor allograft health accurately, frequently and less-invasively. Metabolic reprogramming from fatty acid oxidation to glycolysis has been associated with kidney injury. Given the histological localisation of T cell-mediated rejection (TCMR) to the tubulointerstitium, we hypothesised that expression of glycolytic control proteins contained in urinary extracellular vesicles (UEV) may increase during TCMR.

METHODS

In this prospective observational study, urine samples were collected from kidney transplant recipients prior to indication biopsy. UEV were separated by differential ultracentrifugation. Vesicle markers, glycolytic control proteins and CD3 were assayed by immunoblotting. Differences in protein detection were compared across biopsy diagnoses (TCMR versus not) and Banff lesion scores.

RESULTS

51 paired urine and biopsy samples from 43 subjects were included. The TCMR group comprised of 6 cases of TCMR and 1 borderline TCMR. The remaining 44 samples comprised a "No TCMR" group. There was significant increase in phosphofructo-2-kinase/fructose-2,6-bisphosphatase 4 (PFKFB4) (p = 0.018) in TCMR compared to No TCMR, and similarly when tubulitis (p = 0.037) and interstitial inflammation (p = 0.047) were present. Total inflammation score ≥ 1 was associated with increases in PFKFB2 (p = 0.027), PFKFB3 (p = 0.090) and PFKFB4 (p = 0.0098). Interstitial fibrosis was associated with increased PFKFB2 (p = 0.0045) and PFKFB3 (p = 0.045). CD3 + UEV did not correlate with TCMR diagnosis. When combining the four glycolytic control proteins governing the phosphofructokinase-1 step of glycolysis (PFK-L, PFKFB2, PFKFB3 and PFKFB4), presence of ≥ 3 markers discriminated TCMR with ROC AUC of 0.73 (95% CI 0.50-0.96).

CONCLUSION

Increased rate-limiting enzymes of glycolysis in UEV were detected in association with tubulointerstitial inflammation and fibrosis. This suggests altered energy metabolism in the form of increased renal glycolysis occurring in the tubular epithelium, consistent with findings in native kidney injury. Further work is required to evaluate whether this could serve as a non-invasive strategy to study pathology in kidney transplantation.

Total plasma cfDNA methylation in pediatric kidney transplant recipients provides insight into acute allograft rejection pathophysiology

Cell-free DNA (cfDNA) is a marker of organ injury and immune response. DNA methylation is an epigenetic regulator of gene expression. Here, we elucidate total plasma cfDNA methylation from kidney transplant recipients in presence versus absence of rejection. In doing so, we exploit cfDNA as a real-time biomarker to define molecular pathways of rejection. Twenty plasma cfDNA samples from pediatric kidney transplant recipients were collected at allograft biopsy. Differentially methylated cytosine residues (>20 % methylation difference, q-value <0.05) were identified in presence (N = 7) versus absence (N = 9) of acute rejection. Separate analyses were performed comparing borderline rejection (N = 4) to rejection and non-rejection. In rejection versus non-rejection, there were 1269 differentially methylated genes corresponding to 533 pathways. These numbers were 4-13× greater than comparisons against borderline samples. Enriched pathways between rejection and non-rejection samples were related to immune cell/inflammatory response, lipid metabolism, and tryptophan-kynurenine metabolism, suggesting differential methylation of these pathways contributes to rejection.

Artificial intelligence-enhanced interpretation of kidney transplant biopsy: focus on rejection

PURPOSE OF REVIEW

The objective of this review is to provide an update on the application of artificial intelligence (AI) for the histological interpretation of kidney transplant biopsies.

RECENT FINDINGS

AI, particularly convolutional neural networks (CNNs), has demonstrated great potential in accurately identifying kidney structures, detecting abnormalities, and diagnosing rejection with improved objectivity and reproducibility. Key advancements include the segmentation of kidney compartments for accurate assessment and the detection of inflammatory cells to aid in rejection classification. Development of decision support tools like the Banff Automation System and iBox for predicting long-term allograft failure have also been made possible through AI techniques. Challenges in AI implementation include the need for rigorous evaluation and validation studies, computational resource requirements and energy consumption concerns, and regulatory hurdles. Data protection regulations and Food and Drug Administration (FDA) approval represent such entry barriers. Future directions involve the integration of AI of histopathology with other modalities, such as clinical laboratory and molecular data. Development of more efficient CNN architectures could be possible through the exploration of self-supervised and graph neural network approaches.

SUMMARY

The field is progressing towards an automated Banff Classification system, with potential for significant improvements in diagnostic processes and patient care.

Deep learning-based histopathologic segmentation of peritubular capillaries in kidney transplant biopsies

BACKGROUND

Assessing the extent of inflammation in peritubular capillaries (PTCs) is important for diagnosing antibody-mediated rejection in kidney transplant biopsies. However, this assessment is time-consuming and suffers from interobserver variability, making it a promising candidate for automation. This study introduces a deep learning-based approach for PTCs detection and segmentation in PAS-stained kidney transplant biopsies, providing a first step towards automated scoring of peritubular capillaritis (ptc).

METHODS

PAS-stained whole slide images of kidney transplant biopsies (n = 69) were manually annotated, guided by CD34 immunohistochemical re-staining, resulting in approximately 28.000 PTCs for training and evaluation.

RESULTS

We trained a U-Net model using a patch-sampling approach in a two-class setting (PTCs versus other tissue), achieving a PTCs segmentation performance of 71.3 %, 56.1 % and 73.0 % for the Dice score, Jaccard index and normalized surface Dice, respectively. For the individual PTCs detection, we achieved a precision of 67.7 %, and a recall of 71.8 %.

CONCLUSIONS

Despite satisfactory performance of the DL-model developed in this study, we observed that prominent interstitial inflammation and fibrosis, where PTCs become less recognizable, and the presence of structures mimicking PTCs (e.g. atrophic tubules), posed the largest challenges for the model. Our model is novel as it was trained on a set of transplant biopsies with morphological abnormalities, requiring only PAS-stained kidney biopsies and confirmation of ground truth with the use of CD34 immunohistochemistry. Next steps include incorporation of inflammatory cell segmentation, which may enhance accuracy, reproducibility and quantitative scoring of peritubular capillaritis to guide pathologists in routine diagnostics.

A randomized controlled trial of intravenous immunoglobulin vs standard of care for the treatment of chronic active antibody-mediated rejection in kidney transplant recipients

INTRODUCTION

Chronic active antibody-mediated rejection (AMR) is the leading cause of death-censored kidney allograft loss, with no proven treatments. While intravenous immunoglobulin (IVIG) has been used in certain cases, its efficacy is unknown.

METHODS

In this open-label multicenter randomized controlled trial (VIPAR), participants with biopsy-proven chronic active AMR, were assigned to six doses (1g/kg/month) of IVIG or no-IVIG. The primary endpoint was the difference in slopes of the chronic allograft damage index (CADI) scores between groups, across four allograft biopsies (baseline, three, six and 12-months). Secondary outcomes, assessed at baseline, three, six and 12-months, included change in estimated glomerular filtration rate (eGFR), change in donor specific anti-HLA antibodies (DSA), allograft and patient survival, and change in intra-graft mRNA expression.

RESULTS

Fifteen participants were randomized to each arm. Their median age was 54.3 years, 22 were male and mean eGFR was 43.3 ml/min/1.73m2. Participants in the no-IVIg group experienced a significant increase in mean CADI (+0.28/month, 95% confidence interval 0.14 to 0.41), while the IVIG group did not (-0.004/month, - 0.13 to 0.12). Over two years, eGFR significantly declined more rapidly in the no-IVIG group (-1.1 ml/min/month, -1.5 to -0.7 ml/min/month) than the IVIG group (-0.4 ml/min/month, - 0.8 to 0.03 ml/min/month). Differences in patient and allograft survival were not evident by 12 months. Intra-graft expression of 59 genes (mostly B-cell related) reduced with IVIG relative to no-IVIG.

CONCLUSION

IVIG therapy was associated with stabilization in allograft histology and eGFR in kidney transplant recipients with chronic active AMR.

12-Month Outcomes of a Prospective Randomized Trial Investigating Effects of IVIG on Top of rATG Versus rATG Alone in Pre-Sensitized Kidney Transplant Recipients: The INHIBIT Study

Intravenous immunoglobulins (IVIG) are commonly used in peri-transplant desensitization, but evidence supporting their efficacy is limited. We conducted a prospective, randomized single-center, open-label, Phase IIIb non-inferiority clinical pilot trial to compare the efficacy of IVIG (administered at a dose of 3 × 0.5 g/kg) versus no IVIG, in conjunction with rabbit anti-thymocyte globulin (5-7 mg/kg) induction, in pre-sensitized patients with donor-specific antibodies who had negative pre-transplantation Flow- and CDC-crossmatches, between July 2020 and November 2022. The primary endpoint was the rate of efficacy failure, defined as biopsy-proven rejection within 12-month post-transplant. Secondary endpoints included the incidence of rejection at protocol biopsies, evaluated by histology and biopsy-based transcripts diagnostics. Of the screened patients, 53 (72.6%) were excluded due to crossmatch positivity. Ten patients were randomized to the IVIG+, and 7 to the IVIG-arm. The trial was prematurely terminated due to futility at interim analysis. In the IVIG-arm, 3 patients (43%) experienced the primary endpoint compared to none in the IVIG+ arm (p = 0.026). MMDx identified one molecular ABMR in the IVIG+ and 2 in the IVIG-arm in 12-month protocol biopsies. There was one graft loss in the IVIG-arm. The results of this pilot study, although not definitive, do not support the use of IVIG-sparing regimens in HLA-incompatible kidney transplantation (NCT04302805). This study is registered on ClinicalTrials.gov under the identifier NCT04302805.

Tocilizumab-Based Treatment of Microvascular Inflammation in Kidney Transplant Recipients: A Retrospective Study

Chronic-active antibody mediated rejection (caAMR) is the leading causes of long-term kidney graft failure. Tocilizumab (TCZ), an anti-IL-6 receptor antibody, has been suggested as a treatment, but data are conflicting. We retrospectively studied consecutive adult kidney transplant recipients with caAMR or microvascular inflammation (MVI) without Donor-Specific Antibodies (DSA) and without C4d deposition (MVI + DSA-C4d-), who received TCZ as first-line therapy in two European centers. Estimated glomerular filtration rate (eGFR) and DSA were assessed one-year before and after TCZ initiation. The study included 64 patients who received TCZ between July 2018 and September 2023. The eGFR trajectory significantly decreased after TCZ treatment (-1.2 ± 0.2 vs. 0.03 ± 0.2 mL/min/1.73 m2/month pre- vs. post-TCZ, respectively; p = 0.001). The percentage of patients with DSA decreased from 63.9% to 38.9% (p < 0.001), and the average MFI decreased from 9,537 to 7,250 (p = 0.001). In multivariate analysis, younger age (OR = 0.95, p = 0.02), MVI + DSA-C4d- phenotype (OR = 5.2, p = 0.01), and lower chronic glomerulopathy score (OR = 4.5, p = 0.02) were associated with TCZ response (trajectory ≥0 after TCZ). Patient survival was 98.4%, and graft survival was 93.7% at one-year. First-line TCZ therapy for caAMR or MVI + DSA-C4d- is associated with an improvement of eGFR trajectories, reduced DSA numbers and MFI and histological inflammation in glomeruli. These data suggest a potential benefit of TCZ in these settings.

Targeting CD38 in Antibody-Mediated Rejection

Antibody-mediated rejection (AMR) remains a major challenge in clinical transplantation. Current therapies have yielded inconsistent outcomes, highlighting the need for innovative approaches. CD38, a multifunctional glycoprotein, is highly expressed on plasma cells and natural killer (NK) cells, potentially offering a dual mechanism of action that could intervene in the pathophysiologic course of AMR: depleting alloantibody-producing plasma cells and NK cells. This review focuses on recent results from CD38-targeted therapies, with felzartamab emerging as a promising option. Previous case reports and series suggested that off-label daratumumab treatment could effectively reverse AMR. Felzartamab has now demonstrated safety and efficacy in a phase 2 trial for late AMR. Reductions in microvascular inflammation, downregulation of rejection-associated transcripts, and decreases in donor-derived cell-free DNA paralleled a substantial decrease in NK cell counts. However, felzartamab did not significantly affect donor-specific antibodies, which may reflect its distinct mechanism of action, primarily involving antibody-dependent cellular cytotoxicity and phagocytosis. The effects on rejection activity may have a rapid onset, but are transient. The potential benefits of prolonged therapy are currently being investigated in a recently launched phase III trial. Future studies may expand the applications of CD38 targeting to early AMR or broader indications, such as DSA-negative microvascular inflammation.

Chronic rejection models for vascularized composite tissue allotransplantation

Vascularized composite tissue allotransplantation (VCA) has transformed patients' lives by enabling limb, face, abdominal wall, and penile transplants. Despite advancements in screening and immunosuppression, chronic rejection continues to limit the success of VCA. Lack of reliable preclinical models exacerbates this challenge. Here, we report on new mouse models of chronic rejection following heterotopic hind limb VCA. We employed different levels of MHC mismatch using CD8 knockout C57BL/6 mice as recipients along with BALB/c or B6 H2-Ab1bm12 mice as donors. Transient CD4 T cell depletion was induced to allow graft maturation. Evaluation included gross findings, changes in immune status changes, production of donor-specific antibodies (DSA), C4d levels, and histopathological alterations. Two chronic rejection models displayed common features of clinical chronic graft rejection, such as skin stricture, hair loss, adnexal atrophy, extensive fibrosis and mast cell infiltration without widespread necrotic changes common in acute rejection. Similar to chronic rejection patients, large populations of activated B and plasma cells were detected in the recipient's immune system as well as increased DSA and C4d production. Collectively, our models closely replicate the immunological and histopathological aspects of chronic graft rejection post-VCA, and could provide a new platform for evaluation of novel therapeutic interventions prior to clinical evaluation.

Alloreactive adaptive natural killer cells in renal transplantation: potential contribution to allograft microvascular inflammation

Inhibitory killer cell immunoglobulin-like receptors (iKIRs) are randomly expressed by natural killer (NK) cell subsets and recognize motifs shared by HLA class-I (HLA-I) allotypes. Such interactions prevent NK cell autoreactivity while enhancing their response against cells lacking those HLA-I molecules (missing self), a situation defined in transplantation as iKIR-HLA-I mismatch (iKIR-MM), whose genotypic prediction has been associated with microvascular inflammation (MVI). Herein, we compared iKIR-MM in kidney transplant recipients with MVI ≥2 (n = 19) and controls with MVI ≤1 (n = 36). In parallel to genetic analysis of iKIR-MM, which was more frequent in MVI ≥2 patients, putative alloreactive iKIR-MM NK cells were defined by flow cytometry as NKG2A(-) cells bearing self-specific but lacking donor-specific iKIR. Although iKIR-MM NK cells were detected in both groups, their pretransplant numbers were higher in MVI ≥2 patients (median = 11.02, interquartile range = 0-58.31 vs median = 0, interquartile range = 0-9.46), especially in the presence of donor-specific antibodies or C4d, and correlated with MVI grade. Pretransplant, a subset of MVI ≥2 patients showed high proportions and numbers of oligoclonal iKIR-MM NK cells, which displayed an NKG2C(+) adaptive phenotype associated with cytomegalovirus infection. This pilot study provides a novel perspective on the contribution of iKIR-MM NK cells to MVI, with potential practical implications.

Molecular diagnosis of kidney allograft rejection based on the Banff Human Organ Transplant gene panel: a multicenter international study

Transcriptomic analysis of kidney biopsies has demonstrated the potential to improve diagnosis of allograft rejection. Here, we developed a molecular assessment of antibody-mediated rejection (AMR) and T cell-mediated rejection (TCMR) based on the Banff Human Organ Transplant consensus gene panel. Expression assays of formalin-fixed paraffin-embedded kidney biopsies from well-phenotyped cohorts were used to develop prediction models for AMR and TCMR and an automated report of gene expression-based diagnosis. The study population consisted of 950 kidney allograft biopsies from 10 transplantation centers in Europe and North America. The development cohort included 664 renal allograft biopsies split into a training (n = 537) and test set (n = 127), and 2 external validation cohorts (n = 286). We performed gene selection using regularized regression and developed several different base models based on Banff Human Organ Transplant expression data, which were combined into a single ensemble model for each rejection diagnosis. Model performance was assessed in the test set and the 2 external validation cohorts, showing good discriminative abilities (respective areas under the precision-recall curve: AMR = 0.811, 0.891, and 0.832 and TCMR = 0.736, 0.810, and 0.782). We identified challenging biopsies with histology below diagnostic thresholds for which gene expression-based probability can refine rejection diagnosis. This automated molecular diagnostic system shows potential for improving kidney allograft rejection diagnosis in routine practice and clinical trials.

Evolving thresholds for the diagnosis of acute T cell mediated rejection

PURPOSE OF REVIEW

The Banff schema uses combinations of pathological lesions at predefined thresholds to diagnose of T cell rejection (TCMR) and grade its severity. Constant definitional changes have caused confusion among clinicians and pathologists. This review describes the evolution of lesion definitions and the rationale for the minimal thresholds.

RECENT FINDINGS

The minimal diagnostic threshold for borderline TCMR has been reset to original Banff i1/t1, where isolated tubulitis is now excluded. Arteritis can be mediated by either Grade II TCMR or caused by donor specific antibody as antibody-mediated vascular rejection. The conservative threshold for chronic active TCMR diagnosis uses moderate total and scarred inflammation with tubulitis has been challenged by recent longitudinal data to suggest lower thresholds including i-IFTA=1 as clinically relevant.

SUMMARY

Minor changes in the threshold ruleset can cause substantial alterations in the final pathological diagnoses. While minimal thresholds for borderline and active TCMR have now stabilized, future changes are likely for chronic active TCMR pending confirmatory research.

Rethinking the Diagnosis and Management of Antibody-Mediated Rejection in Multidisciplinary Transplant Meetings: A Global Survey and Banff Working Group Recommendations

INTRODUCTION

The diagnosis of antibody-mediated rejection (AMR) requires input from several transplant professionals. Bringing clinical and laboratory experts together may help standardize care. Yet, little is known about current global practices of multidisciplinary meetings for AMR management.

METHODS

The Banff Antibody-Mediated Injury Working Group approached professional societies worldwide to distribute a survey on the availability, content, participants, perceived value, and barriers to the implementation of multidisciplinary meetings.

RESULTS

Four hundred two transplant professionals from six continents caring for kidney (90.55%), liver (21.14%), pancreas (20.65%), heart (15.17%), and lung (14.18%) transplant recipients participated in the survey, and 302 (75.12%) reported attending multidisciplinary meetings. Multidisciplinary meetings were more prevalent in academic centers, in high- versus low-to-middle-income regions (81.03% and 65.99%, respectively; p < 0.001), and in mid-to-large size transplant programs compared to smaller programs. Perceived value included continued professional development (97.68%) and trainee education (95.70%). AMR was reported to be discussed at these meetings by 217 respondents with case presentations reviewing patient characteristics, histology, and HLA antibody data. A third of the respondents reviewed non-HLA/pathogenic autoantibodies and/or molecular diagnostics, with the latter being more frequently applied in high- versus low-to-middle-income regions (46.71% and 12.31%, respectively; p < 0.001). AMR case presentations allowed diagnosis revision, actionable management plans and were perceived as improving care. The primary barrier to the implementation of multidisciplinary meetings (63.27%) was the unavailability of transplant professionals (e.g., transplant immunologists).

CONCLUSION

Facilitating multidisciplinary meetings through the remote participation of pertinent experts and incentivizing participation through remuneration, protected time, or continued medical education may help standardize AMR diagnosis and harmonize its management.

Repeated HLA-DRB1 and HLA-DQB1 Mismatches Without Preformed DSA Affect Graft Survival, Rejection and DSA Development: A Multicenter Analysis

Repeat transplantations represent up to 20% of all kidney transplants. Whereas repeat transplantations in the presence of circulating donor-specific HLA-antibodies are generally avoided, the risk of repeated HLA-mismatches (RMM) without detectable antibodies remains debated. This multicenter study evaluated the hazard of RMM, stratified by HLA-class, on transplant outcomes in the absence of preformed donor-specific antibodies. We included repeat kidney transplant recipients from January 2009 onward with available HLA typing for HLA-A, -B, -C, -DRB1, -DRB3/4/5 and -DQB1 from current and previous donors. RMM were defined at the split serological antigen level, excluding patients with only HLA-DP or HLA-DRB3/4/5 RMM. Patients were included if: (a) preformed donor-specific HLA-antibodies (DSA) had never been detected and (b) this was confirmed by Luminex assays within 6 months pre-transplantation. A competing risk model, adjusting for demographic factors and total HLA-mismatch load while accounting for death as a competing risk, showed that HLA-DRB1 and/or HLA-DQB1 RMM significantly increased the risk of graft loss within 1 year post-transplant (HR 3.75 [95% CI 1.51-9.34], p = 0.004). Cox proportional hazard models further linked these HLA-class II RMM to higher risks of biopsy-proven rejection (HR 1.98 [95% CI 1.04-3.76], p = 0.037) and DSA development (HR 9.89 [95% CI 1.92-50.99], p = 0.006), while no significant risks were observed for HLA-class I RMM. Sensitivity analyses in patients screened for pretransplant DSA via single antigen bead assays, those with similar immunosuppression, and those with allelic RMM further confirmed these findings. These results suggest that avoiding HLA-DRB1 and HLA-DQB1 RMM, when feasible, may improve transplant outcomes.

Archetypal Analysis of Kidney Allograft Biopsies Using Next-generation Sequencing Technology

BACKGROUND

In kidney transplantation, molecular diagnostics may be a valuable approach to improve the precision of the diagnosis. Using next-generation sequencing (NGS), we aimed to identify clinically relevant archetypes.

METHODS

We conducted an Illumina bulk RNA sequencing on 770 kidney biopsies (540 kidney recipients) collected between 2006 and 2021 from 11 European centers. Differentially expressed genes were determined for 11 Banff lesions. An ElasticNet model was used for feature selection, and 4 machine learning classifiers were trained to predict the probability of presence of the lesions. NGS-based classifiers were used in an unsupervised archetypal analysis to different archetypes. The association of the archetypes with allograft survival was assessed using the iBox risk prediction score.

RESULTS

The ElasticNet feature selection reduced the number of the genes from a range of 859-10 830 to a range of 52-867 genes. NGS-based classifiers demonstrated robust performances (precision-recall area under the curves 0.708-0.980) in predicting the Banff lesions. Archetypal analysis revealed 8 distinct phenotypes, each characterized by distinct clinical, immunological, and histological features. Although the archetypes confirmed the well-defined Banff rejection phenotypes for T cell-mediated rejection and antibody-mediated rejection, equivocal histologic antibody-mediated rejection, and borderline diagnoses were reclassified into different archetypes based on their molecular signatures. The 8 NGS-based archetypes displayed distinct allograft survival profiles with incremental graft loss rates between archetypes, ranging from 90% to 56% rates 7 y after evaluation ( P < 0.0001).

CONCLUSIONS

Using molecular phenotyping, 8 archetypes were identified. These NGS-based archetypes might improve disease characterization, reclassify ambiguous Banff diagnoses, and enable patient-specific risk stratification.

Functional Natural Killer-cell Genetics and Microvascular Inflammation After Kidney Transplantation: An Observational Cohort Study

BACKGROUND

Recent evidence highlights the pivotal role of natural killer (NK) cells in allograft rejection.

METHODS

We explored associations of missing self and gene polymorphisms determining the phenotype and/or functionality of NK cells with microvascular inflammation (MVI) in a single-center cohort of 507 consecutive kidney transplant recipients. Patients were genotyped for killer cell Ig-like receptors and polymorphisms in 4 selected genes ( FCGR3AV/F158 [rs396991], KLRC2wt/del , KLRK1HNK/LNK [rs1049174], and rs9916629-C/T).

RESULTS

MVI was detected in 69 patients (13.6%). In a proportional odds model, the KLRC2del/del variant reduced MVI risk (odds ratio [OR] 0.26; 95% confidence interval [CI], 0.05-0.93; P  = 0.037) independent of donor-specific antibodies, HLA class II eplet mismatch, and number of biopsies. Conversely, missing self (OR 1.40; 95% CI, 1.08-1.80; P  = 0.011) and the rs9916629 T/T gene variant increased the risk (OR 1.70; 95% CI, 1.08-2.68; P  = 0.021). Graft loss tended to be more frequent among patients with missing self ≥2 (hazard ratio 1.97; 95% CI, 0.89-4.37; P  = 0.097), without influence on estimated glomerular filtration trajectories. FCGR3A variants were associated with MVI only in patients with preformed and/or de novo donor-specific antibodies (OR 4.14; 95% CI, 0.99-17.47; P  = 0.052).

CONCLUSIONS

Missing self and NK-cell genetics may contribute to MVI, underscoring the important role of NK cells in transplant rejection.

Role of intrarenal complement production in kidney transplantation

Systemic complement is a major contributor to the onset and progression of kidney graft injury. However, the kidney itself is an important site of complement production. Renal-derived complement plays a key role in graft dysfunction, unlike in some other solid organ transplants. Complement factors are generated by multiple renal cell types under both physiological and pathological conditions. Renal complement production mediates ischemia/reperfusion injury and acute cellular and humoral rejection but protective effects of the complement cascade have been reported as well. More recently, intracellular complement production and activation (complosome) has also been shown to be an important regulator of key metabolic and cellular functions in renal cells and in immune kidney infiltrates, adding complexity to the field. Herein, we review current knowledge on the role of renal-derived complement in the pathophysiology of kidney graft damage and the current landscape of complement targeted therapeutics in kidney transplantation.

Plasma exchange-sensitive syncytial glomerulopathy in a kidney transplant patient

Microvascular inflammation (MVI), defined as the presence of glomerulitis and/or peritubular capillaritis, is the key histological lesion of anti-HLA donor-specific antibodies (DSA)-related antibody mediated rejection, but recently other possible mechanisms of MVI have emerged. However, except for peritubular capillary C4d deposition that is more frequently observed in the presence of anti-HLA-DSA, histological features are similar regardless of MVI origin. Therefore, accurately describing patterns of MVI may help differentiate etiologies and drive therapeutic choices. We describe the case of a kidney transplant recipient (primary nephropathy: autosomal dominant polycystic kidney disease) who underwent kidney biopsy for worsening renal function and new onset hypertension. Histologic findings showed severe microvascular inflammation with intense glomerulitis and presence of intracapillary multinucleated cells, positive on immunostaining for endothelial marker ETS-related gene (ERG). Focal intense peritubular capillaritis and early glomerular basement membrane reduplication, C4d negative, were observed, consistent with early chronic active ABMR. HLA-DSA were absent, but high level of anti-angiotensin II type-1 receptor (AT1R) antibodies (Ab) were detected (78 U/L, normal levels < 10 U/L). Two subsequent biopsies showed intense microvascular inflammation with diffuse peritubular capillaritis, and multinucleated, ERG-positive, endothelial cells were still seen in glomerular capillary loops. The patient was started on angiotensin receptor blockers (ARBs) and plasma exchange (PEX) sessions obtaining normalization of blood pressure and AT1R Ab and proteinuria reduction, but, after subsequent liver transplant, rituximab therapy failed to maintain remission and the patient remained PEX-dependent.

Association of Ultrastructural Changes in Renal Allograft Biopsies With Diagnostic Elements of Antibody-Mediated Rejection and Graft Outcomes

BACKGROUND

The role of electron microscopy (EM) in the diagnosis of kidney allograft pathologies, particularly immunologic injury has not been well studied.

METHODS

In this retrospective, single-center cohort study, we examined EM features in 796 biopsies from 623 patients at high risk for antibody-mediated rejection, with glomerular abnormalities in light microscopy, presence of donor-specific antibody (DSA), or any degree of albuminuria/proteinuria.

RESULTS

Glomerular endothelial cell enlargement (GECE) > 50% was present in 29.1%, subendothelial expansion/basement membrane duplication in 24.5%, and peritubular basement membrane multilamellation > 4 (PTCML) in 18.5%. There was an incremental odds of worsening GECE from no DSA to class I DSA (odds ratio [OR], 2.75, P < 0.001; 95% confidence interval [CI], 1.7-4.5), class II DSA (OR, 3.44, P < 0.001, 95% CI, 2.5-4.7) and both classes (OR, 6.3, P < 0.001; 95% CI, 4.1-9.8). Moreover, the increase in number of antibodies was predictive of higher likelihood of worsening GECE (OR, 2.81, P < 0.001; 95% CI, 2.1-3.8 for 1 DSA; OR, 5.29, P < 0.001; 95% CI, 3.5-7.9 for 2-3; and OR, 8.45, P < 0.001; 95% CI, 4.7-15.3 for ≥4). Similar association was observed with PTCML. In multivariate analysis including DSA, subendothelial expansion/basement membrane duplication, and GECE >50%, but not PTCML were independently predictive of graft failure over mean follow-up of 63 mo (hazard ratio [HR], 1.6, P = 0.006, 95% CI, 1.2-2.3; HR, 2.0, P < 0.001; 95% CI, 1.4-2.9, respectively). Among a cohort with g, ptc, cg, and C4d scores 0, GECE >50% was independently associated with graft failure (HR, 2.58, P < 0.001, 95% CI, 1.6-4.3).

CONCLUSIONS

These observations support the wider use of EM in kidney transplant biopsies to help with earlier diagnosis of antibody-mediated rejection and to risk stratify the graft outcome.

Complement in Antibody-Mediated Rejection of the Kidney Graft: From Pathophysiology to Clinical Practice

Antibody-mediated rejection (AMR) is a leading cause of kidney graft failure. Complement activation is involved in the AMR process. Our aim is to provide the current understanding of the pathophysiology related to complement-mediated injury in AMR, to present the current evidence regarding complement blockade in AMR management, and to point out emerging therapies and future directions in this area. The complement system plays an important role in the onset and progression of AMR. There is a balance between complement-dependent and -independent mechanisms in the development of rejection lesions. Classic and leptin pathways are involved in this process. C4d positivity is no longer a mandatory feature for AMR diagnosis but remains an independent predictor of negative outcomes. The current evidence regarding AMR treatment is limited. Terminal and proximal complement blockade has gained recognition in clinical practice. Eculizumab and C1 inhibitors are effective in the treatment of AMR as adjuvant therapies to the standard of care. The availability of novel complement inhibitors will lead to more effective and tailored treatment strategies.

Mechanism and regulation of the complement activity in kidney xenotransplantation

Xenotransplantation is emerging as one of several potential solutions for addressing organ donor shortages, with significant progress bringing it closer to clinical application. However, challenges remain, particularly concerning complement system dysregulation caused by species differences, as well as xenoantigens and coagulopathy. Complement regulatory proteins expressed on endothelial cells of donor xenografts are less compatible with complement components in recipients. These difficulties contribute to hyperacute rejection, characterized by antibody-mediated complement activation that destroys the graft within 24 h. Moreover, because molecules are incompatible across different species, ischemia-reperfusion injury or infection can easily elicit complement activity via all three pathways, resulting in xenograft loss via complement-mediated vascular injury. Complement activity also stimulate innate and adaptive immune cells. To address this issue, genetic modifications in donor pigs and the development of novel medicines have been tested in preclinical models with promising results. Pigs modified to express human complement-regulating molecules such as CD46, CD55, and CD59 have shown longer kidney xenograft survivals over years in preclinical models with nonhuman primates, paving the way for clinical trials. Anti-complement component agents such as C1 esterase and C5 inhibitors have also been shown to increase xenograft survivals. This review examines the role of the complement system in kidney xenotransplantation, emphasizing new research and clinical trial advancements.

Microvascular inflammation in kidney allografts: New directions for patient management

Microvascular inflammation (MVI) is a key histological feature of immune-mediated injury at the capillary interface of renal allografts, characterized by immune cell infiltration into glomerular and peritubular capillaries. Although traditionally associated with antibody-mediated rejection (AMR), many MVI cases lack detectable donor-specific antibodies (DSA), suggesting the involvement of antibody-independent immune mechanisms or alternative triggers, such as viral infections or ischemia-reperfusion injury. The Banff 2022 scheme introduced a subcategory, "MVI, DSA-negative, C4d-negative," within an overarching AMR or MVI category. This subcategory-similar to AMR-was shown to carry a significant risk of graft failure. Its recognition marks a major advancement, offering a robust framework for investigating the pathophysiology of MVI, which may involve a wide array of overlapping triggers. Emerging evidence from transcriptome analyses highlights natural killer cells as possible effectors, regardless of DSA status. Therapies targeting natural killer cells, particularly the anti-CD38 antibody felzartamab, have shown promising reductions in MVI and molecular injury. Notably, the US Food and Drug Administration has approved an MVI-based primary endpoint for a phase 3 trial evaluating this approach, representing a critical step toward the development of new therapeutics. Recognizing MVI as a multifaceted histological phenotype-driven by diverse triggers-may signal a paradigm shift in transplant medicine.

Implementation of the in-house technique for the determination of donor-derived cell-free DNA in daily clinical practice: Experience from the Hospital Clinic of Barcelona

BACKGROUND AND OBJECTIVE

The introduction of donor-derived free DNA (ddcfDNA) has emerged as an accurate non-invasive biomarker to diagnose rejection, compared to classical ones. Here we evaluate our experience after its implementation in our center as an in-house technique.

MATERIALS AND METHODS

Single-center cross-sectional study with extraction of cell-free DNA in blood and quantification of the ddcfDNA using the AlloSeqcfDNA assay (CareDx) at the time of performing biopsies 'per protocol' or 'per indication' between December 2020 and December 2023.

RESULTS

172 graft biopsies were included (59 for protocol and 113 for cause) in 112 kidney transplant recipients. Among the biopsies, 19 borderline rejections, 11 T-cell mediated rejections, and 30 antibody-mediated rejections were identified. The median ddcfDNA in each diagnostic group was: 0.40% (0.23%-0.82%) in borderline, 0.60% (0.23%-1.91%) in cellular, and 1.48% (0.77%-3.4%) in antibody-mediated rejection (P < .001). In the 112 biopsies with no signs of rejection, the median ddcfDNA was 0.33% (0.17%-0.54%) (P < .001). Cases with positive DSAs and rejection showed higher levels of ddcfDNA than positive DSAs without rejection (P = .010), and ddcfDNA levels were significantly associated with microvascular inflammation and C4d positivity. The area under the ROC curves of ddcfDNA to discriminate any type of rejection from the absence of rejection was 0.74 (0.65-0.82) and, excluding borderline rejection from the analysis, 0.80 (0.72-0.89), outperforming other markers of renal function.

CONCLUSIONS

Implementing ddcfDNA analysis at our center as a clinical tool has proven valuable for distinguishing biopsy-confirmed acute rejection, particularly antibody-mediated rejection, outperforming classic renal function markers. Its hospital-based implementation supports timely and accurate diagnosis, improving transplant management and prognosis.

Chronic Rejection After Kidney Transplantation

In kidney transplantation, ongoing alloimmune processes-commonly triggered by HLA incompatibilities-can trigger chronic transplant rejection, affecting the microcirculation and the tubulointerstitium. Continuous inflammation may lead to progressive, irreversible graft injury, culminating in graft dysfunction and accelerated transplant failure. Numerous experimental and translational studies have delineated a complex interplay of different immune mechanisms driving rejection, with antibody-mediated rejection (AMR) being an extensively studied rejection variant. In microvascular inflammation, a hallmark lesion of AMR, natural killer (NK) cells have emerged as pivotal effector cells. Their essential role is supported by immunohistologic evidence, bulk and spatial transcriptomics, and functional genetics. Despite significant research efforts, a substantial unmet need for approved rejection therapies persists, with many trials yielding negative outcomes. However, several promising therapies are currently under investigation, including felzartamab, a monoclonal antibody targeting the surface molecule CD38, which is highly expressed in NK cells and antibody-producing plasma cells. In an exploratory phase 2 trial in late AMR, this compound has demonstrated potential in resolving molecular and morphologic rejection activity and injury, predominantly by targeting NK cell effector function. These findings inspire hope for effective treatments and emphasize the necessity of further pivotal trials focusing on chronic transplant rejection.

The prognostic relevance of donor-specific antibodies in facial transplantation - A retrospective cohort study

BACKGROUND

Little is known about the prognostic relevance of donor-specific antibodies (DSAs) and C4d tissue deposition for acute and chronic rejection in facial vascularized composite allotransplantation.

METHODS

We conducted a retrospective analysis of nine face transplant patients. Outcome measures, such as DSA positivity, were determined by the local tissue typing laboratory, whereas C4d deposition was assessed by immunostaining. Acute rejection events, occurrence of chronic rejection, and transplant-related outcomes were collected.

RESULTS

Five (5/9) patients developed de novo DSAs. Among the 56 time points when rejection grade, DSA, and C4d levels were measured concurrently, 25 (44.6%) had only one positive parameter (DSA or C4d), 23 (41.1%) had two negative values, and 8 (14.3%) had both parameters positive. A trend was observed where acute rejection grades were higher at time points with both DSA positivity and C4d deposition compared with time points where both parameters were negative. Chronic rejection was observed in four patients, two of whom developed DSAs and two who did not. Among the five patients who developed de novo DSAs, graft loss occurred in one patient. The remaining four patients with de novo DSAs retained their allografts [mean follow-up: 118 (63-162) months].

CONCLUSION

We identified a potential association between DSAs and grade of acute cellular rejection. Chronic rejection was observed in four patients, with no clear association with DSA status. Only one out of five patients who developed DSAs underwent graft loss. Further studies need to explore the mechanistic role of DSAs in the setting of VCA rejection.

Predicting the success of antibody removal with therapeutic plasma exchange: The role of serum dilutions

Therapeutic plasma exchange (TPE) remains a primary therapy for addressing circulating donor-specific antibodies against mismatched human leukocyte antigens, which pose a significant challenge to successful transplantation of solid organs and hematopoietic cells. In this study, we aimed to evaluate the response to 5 TPE sessions in patients undergoing desensitization, as well as in patients who developed antibody-mediated rejection posttransplant. Sera collected before and after each TPE session was treated with EDTA and tested using the single-antigen bead assay as undiluted or 1:4, 1:16, 1:64, and 1:256 diluted. The reduction in human leukocyte antigen antibodies correlated with previously established total immunoglobulin G reduction levels. However, we observed that class I antibody levels were reduced by 85% to 90% while class II antibodies were reduced by only 75% to 80% after 5 TPE sessions. We also found that dilutions performed on pretreatment serum were predictive of the response to TPE. This approach was used to design TPE-based desensitization protocols for highly sensitized patients across various organ and tissue types, as well as for treating patients with antibody-mediated rejection. Our data suggest that serum dilution may be an effective method for assessing the likelihood of response to TPE, thereby aiding in the guidance of treatment strategies.

Expanding role of antibodies in kidney transplantation

The role of antibodies in kidney transplant (KT) has evolved significantly over the past few decades. This role of antibodies in KT is multifaceted, encompassing both the challenges they pose in terms of antibody-mediated rejection (AMR) and the opportunities for improving transplant outcomes through better detection, prevention, and treatment strategies. As our understanding of the immunological mechanisms continues to evolve, so too will the approaches to managing and harnessing the power of antibodies in KT, ultimately leading to improved patient and graft survival. This narrative review explores the multifaceted roles of antibodies in KT, including their involvement in rejection mechanisms, advancements in desensitization protocols, AMR treatments, and their potential role in monitoring and improving graft survival.

Chronic Antibody-Mediated Rejection and Plasma Cell ER Stress: Opportunities and Challenges with Calcineurin Inhibitors

Chronic alloantibody-mediated rejection (cAMR) remains a major challenge in transplant immunology, with no FDA-approved targeted therapies currently available. Despite advancements in cellular immunosuppression, effective strategies to mitigate alloantibody-mediated rejection are still lacking. This review provides a comprehensive overview of transplant rejection with a particular focus on the pathophysiology and therapeutic landscape of cAMR. We highlight the role of plasma cell-driven alloantibody production and its susceptibility to endoplasmic reticulum (ER) stress, a pathway with potential for therapeutic intervention. Special attention is given to calcineurin inhibitors (CNIs), which, beyond their well-established T-cell inhibitory effects, exhibit differential impacts on ER stress and plasma cell viability. By delineating the mechanistic differences between cyclosporine and tacrolimus in regulating ER stress responses, we propose potential therapeutic implications for optimizing cAMR management. This review underscores the need for innovative strategies targeting plasma cell biology to improve long-term transplant outcomes.

Additional Diagnoses Other Than Rejection in the Kidney Allograft Biopsy: Pitfalls for Biopsy-based Transcript Diagnostics

Background

Biopsy-based transcripts associated with antibody-mediated rejection (AMR) hold promise as substitutes for C4d positivity. However, their utility in cases with additional diagnoses other than rejection remains inadequately studied.

Methods

In our comprehensive analysis of 326 kidney allograft biopsies, assessed by histology and the Molecular Microscope Diagnostic System, we identified 68 cases characterized by additional pathologies, including pyelonephritis (n = 15), BK nephropathy (n = 20), acute interstitial nephritis (n = 5), and glomerular diseases (n = 28).

Results

Among cases with pyelonephritis, 7 of 15 cases (46%) showed a rejection-like signal, 4 above (16%) and 3 (20%) below diagnostic thresholds. Notably, the T cell-mediated rejection (TCMR) archetype score R2 (median, 0.13; interquartile range [IQR], 0.04-0.34) predominantly contributed to this observation. In BK nephropathy, 13 of 20 cases (65%) showed a rejection-like signal, 10 (50%) above and 3 (15%) below diagnostic thresholds. Elevated TCMR R2 (median, 0.07; IQR, 0.00-0.41) and all AMR archetype scores R4-6 (median, 0.23; IQR, 0.07-0.53) were driving factors. Among cases with acute interstitial nephritis, 3 of 5 cases (60%) showed TCMR-like signal with elevated R2 scores (median, 0.13; IQR, 0.00-0.54). Conversely, only 5 of 28 cases (18%) showed a rejection-like signal in glomerular disease cases, whereas 57% displayed all AMR archetype scores of ≥0.30.

Conclusions

Additional pathologies can affect the Molecular Microscope Diagnostic System output, giving a molecular rejection-like signal. The prevalence of rejection-like signals below diagnostic thresholds is noteworthy, warranting caution and prompting further investigation.

The future of nephrology in 2050

As medicine advances at an unprecedented pace, the field of nephrology is poised for transformative change. By 2050, breakthroughs in kidney disease prevention, dialysis, transplantation, and omics-driven precision medicine could redefine patient care and outcomes. Here, we share our perspectives on the challenges faced and how changes in health policy, emerging technologies, novel therapies, and data-driven approaches might shape the future of nephrology. From innovative dialysis solutions to xenotransplantation and AI-powered diagnostics, we explore the possibilities that could revolutionise kidney health in the decades to come.

The Microscope and Beyond: Current Trends in the Characterization of Kidney Allograft Rejection From Tissue Samples

The Banff classification is regularly updated to integrate recent advances in the characterization of kidney allograft rejection, gathering novel diagnostic, prognostic, and theragnostic data into a diagnostic and pathogenesis-based framework. Despite ongoing research on noninvasive biomarkers of kidney rejection, the Banff classification remains, to date, biopsy-centered, primarily relying on a semiquantitative histological scoring system that overall lacks reproducibility and granularity. Besides, the ability of histopathological injuries and transcriptomics analyses from bulk tissue to accurately infer the pathogenesis of rejection is questioned. This review discusses findings from past, current, and emerging innovative tools that have the potential to enhance the characterization of allograft rejection from tissue samples. First, the digitalization of pathological workflows and the rise of deep learning should yield more reproducible and quantitative results from routine slides. Additionally, novel histomorphometric features of kidney rejection could be discovered with an overall genuine clinical implementation perspective. Second, multiplex immunohistochemistry enables in-depth in situ phenotyping of cells from formalin-fixed samples, which can decipher the heterogeneity of the immune infiltrate during kidney allograft rejection. Third, transcriptomics from bulk tissue is gradually integrated into the Banff classification, and its specific context of use is currently under extensive consideration. Finally, single-cell transcriptomics and spatial transcriptomics from formalin-fixed and paraffin-embedded samples are emerging techniques capable of producing up to genome-wide data with unprecedented precision levels. Combining all these approaches gives us hope for novel advances that will address the current blind spots of the Banff system.

Early Thrombotic Microangiopathy After ABO-Incompatible Living Donor Kidney Transplantation

Introduction

Although long-term graft survival is comparable with that of ABO-compatible (ABOc) renal transplantation, the risk of antibody-mediated rejection (ABMR) following ABO-incompatible (ABOi) transplantation is higher and can occur as an early thrombotic microangiopathy (TMA).

Methods

We designed a retrospective multicenter study, including all patients who presented with a TMA (histological and/or biological) after an ABOi transplantation (< 1 month) and compared with matched controls who had a favorable initial course with a normal biopsy.

Results

Between 2013 and 2022, 375 ABOi kidney transplants were performed and 23 patients (6.1%) developed TMA (median: 1 day, interquartile range [IQR]: 0-3 days). Twenty-one patients (91.3%) had biological TMA. Among 23 early graft biopsies, histological evidence of active TMA was found in 17 cases (80.9%). All patients received treatment: 20 of 23 received at least 1 session of plasmapheresis and 19 of 23 received at least 1 injection of eculizumab. Eight early graft losses (30.4%) occurred (median: 7 days, IQR: 3-16 days). IgG and IgM anti-blood group antibody (ABGA) levels (peak and last pregraft assay) were significantly higher in the TMA group (peak: P = 0.01 for IgG and P = 0.0006 for IgM; last assay before kidney transplantation [KT]: P < 0.0001 for IgG and P = 0.0003 for IgM). A level ≥ 1/8 for IgG and ≥ 1/4 or IgM before transplantation were significantly and independently predictive of the occurrence of TMA. No other predictive factors were found.

Conclusion

TMA after ABOi transplantation is not a rare phenomenon and is associated with a poor prognosis in nonresponders-to-treatment patients. ABGA titer performed by hemagglutination is an imperfect marker of the occurrence of such a phenomenon.

Microvascular Inflammation of Kidney Allografts and Clinical Outcomes

BACKGROUND

The heterogeneous clinical presentation of graft microvascular inflammation poses a major challenge to successful kidney transplantation. The effect of microvascular inflammation on allograft outcomes is unclear.

METHODS

We conducted a cohort study that included kidney-transplant recipients from more than 30 transplantation centers in Europe and North America who had undergone allograft biopsy between 2004 and 2023. We integrated clinical and pathological data to classify biopsy specimens according to the 2022 Banff Classification of Renal Allograft Pathology, which includes two new diagnostic categories: probable antibody-mediated rejection and microvascular inflammation without evidence of an antibody-mediated response. We then assessed the association between the newly recognized microvascular inflammation phenotypes and allograft survival and disease progression.

RESULTS

A total of 16,293 kidney-transplant biopsy specimens from 6798 patients were assessed. We identified the newly recognized microvascular inflammation phenotypes in 788 specimens, of which 641 were previously categorized as specimens with no evidence of rejection. As compared with patients without rejection, the hazard ratio for graft loss was 2.1 (95% confidence interval [CI], 1.5 to 3.1) among patients with microvascular inflammation without evidence of an antibody-mediated response and 2.7 (95% CI, 2.2 to 3.3) among patients with antibody-mediated rejection. Patients with a diagnosis of probable antibody-mediated rejection had a higher risk of graft failure beyond year 5 after biopsy than those without rejection (hazard ratio, 1.7; 95% CI, 0.8 to 3.5). Patients with a diagnosis of either newly recognized microvascular inflammation phenotype had a higher risk of progression of transplant glomerulopathy during follow-up than patients without microvascular inflammation.

CONCLUSIONS

Microvascular inflammation in kidney allografts includes distinct phenotypes, with various disease progression and allograft outcomes. Our findings support the clinical use of additional rejection phenotypes to standardize diagnostics for kidney allografts. (Funded by OrganX. ClinicalTrials.gov number, NCT06496269.).

Potential MRI Biomarkers for Predicting Kidney Function and Histological Damage in Transplanted Deceased Donor Kidney Recipients

Background/Objectives: Kidney transplantation (kTx) is the preferred treatment for end-stage kidney disease. Limited evaluation of structural changes in transplanted kidneys hinders the timely prediction of disease progression and the implementation of treatment modifications. Protocol biopsies provide valuable insights but are invasive and carry risks of biopsy-related complications. This study investigates whether multiparametric magnetic resonance imaging (MRI), including T1 and T2 mapping and diffusion-weighted imaging (DWI), can predict kidney function and the progression of interstitial fibrosis and tubular atrophy (IF/TA) in the early post-transplant period. Methods: A prospective study was conducted at The Hospital of Lithuanian University of Health Sciences Kauno Klinikos from May 2022 to March 2024. Thirty-four patients receiving kidney transplants from deceased donors underwent baseline biopsies and post-transplant MRI scans. Follow-up assessments included kidney function evaluation, biopsies, and MRI scans at three months post-transplant. Results: Significant correlations were observed between MRI parameters and kidney function: T1 and apparent diffusion coefficient (ADC) corticomedullary differentiation (CMD) correlated with eGFR at discharge (r = -0.338, p = 0.05; r = 0.392, p = 0.022, respectively). Linear and logistic regression models demonstrated that post-transplant T1 and ADC CMD values significantly predicted kidney function at discharge. Furthermore, T1 CMD values measured 10-15 days post-transplant predicted IF/TA progression at three months post-kTx, with an area under the curve of 0.802 (95% CI: 0.616-0.987, p = 0.001) and an optimal cut-off value of -149.71 ms. The sensitivity and specificity were 0.818 and 0.273, respectively (Youden's index = 0.545). T2 mapping was not predictive. Conclusions: This study highlights the potential immediate clinical utility of MRI-derived biomarkers, particularly ADC and T1 CMD, in centers equipped with advanced imaging capabilities as tools for assessing kidney function in the early post-transplant period. With an AUROC of 0.802, T1 CMD demonstrates strong discriminatory power for predicting IF/TA progression early in the post-transplant period.

Limitations of biopsy-based transcript diagnostics to detect T-cell-mediated allograft rejection

BACKGROUND

Isolated tubulitis, borderline changes and isolated arteritis suspicious for histologic T-cell-mediated rejection (hTCMR) remain findings of uncertain significance. Although the Molecular Microscope Diagnostics System (MMDx) has not been trained on those lesions, it was suggested that MMDx might reclassify a subgroup to molecular TCMR (mTCMR).

METHODS

In this single-center cohort of 326 consecutive, unselected kidney allograft biopsies assessed by histology and MMDx, we analyzed 249 cases with isolated tubulitis (i0, t1-3, v0; n = 101), borderline changes (according to Banff 2022, v0; n = 9), isolated arteritis (no borderline, v1; n = 37), no inflammation (i0, t0, v0; n = 67) and a positive control cohort (hTCMR, n = 27; mixed histologic rejection, n = 8; both according to Banff 2022; total n = 35). The first three groups were summarized as TCMR-suspicion (n = 147). Subcategorization included the presence and absence of microvascular inflammation (MVI); g+ptc ptc ≥2. Molecular rejection rates and differentiation were investigated.

RESULTS

Molecular rejection rates were 37/147 cases (25.2%; 32 with MVI) in TCMR-suspicion, 6/67 (9%; 4 with MVI) in no inflammation and 30/35 (85.7%; 19 with MVI) in the positive control cohort. Molecular antibody-mediated rejection (mAMR) was present in 39/73 (53.4%) of cases. The presence of donor-specific antibodies at the time of the biopsy was high (127/249, 51%). Only 3 mAMR/TCMR and 0 pure mTCMR cases were detected in TCMR-suspicion and no inflammation, compared with 12 mAMR/TCMR and 10 mTCMR cases in the positive control cohort (P < .001). Even though the TCMR-specific molecular (Classifier) score differentiated between TCMR-suspicion and no inflammation (P = 0.005), rejection phenotype scores (R2 and R3) did not (P = .157 and .121).

CONCLUSIONS

MMDx did not identify pure mTCMR among isolated tubulitis, borderline changes or isolated arteritis, likely due to low sensitivity for TCMR lesions. However, it identified mAMR or mAMR/TCMR, especially in cases with MVI. Subthreshold findings remain to be further studied.

Banff 2022 Kidney Commentary: Reflections and Future Directions

In September 2022, in Banff, Alberta, Canada, the XVIth Banff meeting, corresponding to the 30th anniversary of the Banff classification, was held, leading to 2 recent publications. Discussions at the Banff meeting focused on proposing improvements to the Banff process as a whole. In line with this, a unique opportunity was offered to a selected group of 16 representatives from the pathology and transplant nephrology community, experts in the field of kidney transplantation, to review these 2 Banff manuscripts. The aim was to provide an insightful commentary, to gauge any prospective influence the proposed changes may have, and to identify any potential areas for future enhancement within the Banff classification. The group expressed its satisfaction with the incorporation of 2 new entities, namely "microvascular inflammation/injury donor-specific antibodies-negative and C4d negative" and "probable antibody-mediated rejection," into category 2. These changes expand the classification, facilitating the capture of more biopsies and providing an opportunity to explore the clinical implications of these lesions further. However, we found that the Banff classification remains complex, potentially hindering its widespread utilization, even if a degree of complexity may be unavoidable given the intricate pathophysiology of kidney allograft pathology. Addressing the histomorphologic diagnosis of chronic active T cell-mediated rejection (CA TCMR), potentially reconsidering a diagnostic-agnostic approach, as for category 2, to inflammation in interstitial fibrosis and tubular atrophy and chronic active T cell-mediated rejection was also an important objective. Furthermore, we felt a need for more evidence before molecular diagnostics could be routinely integrated and emphasized the need for clinical and histologic context determination and the substantiation of its clinical impact through rigorous clinical trials. Finally, our discussions stressed the ongoing necessity for multidisciplinary decision-making regarding patient care.

Early and Late Microvascular Inflammation Have Differing Etiological Causes and Clinical Expression

BACKGROUND

Microvascular inflammation (MVI) is an important pathological feature of antibody-mediated rejection (AMR). How posttransplant time affects its clinicopathological expression is little understood.

METHODS

This retrospective, single-center study screened 3398 kidney transplant biopsies and dichotomized 202 MVI ≥ 2 (Banff glomerulitis + peritubular capillaritis ≥ 2) samples by 9-mo median incidence time for comparison.

RESULTS

The prevalence of MVI ≥ 2 was 12.4% in transplant kidneys, which failed more frequently than propensity-matched normal controls (n = 202; P  < 0.001). Epidemiological risk factors for early MVI ≥ 2 were delayed graft function, prior AMR, and circulating donor-specific antibodies (DSAs+). Prior recipient sensitization occurred in 72.3%. Early MVI ≥ 2 was classified AMR in 65.3% and cellular rejection in 34.7%, and demonstrated excellent functional recovery and graft survival comparable to normal control kidneys. Late MVI ≥ 2 was predicted by younger (18 = 29 y) age, female recipient, living-donation, prior methylprednisolone, cyclosporine (versus tacrolimus, levels <5 ng/mL), absent antiproliferative therapy, and DSA+ using multivariable epidemiological modeling. Nonadherence caused 49.5%, with iatrogenic minimization responsible for 47.5%, usually for recipient infection. Late MVI ≥ 2 was because of AMR in 93.1%, and characterized by greater interstitial fibrosis, tubular atrophy, complement degradation split-product 4d (C4d) staining of peritubular capillaries+, endothelial C4d staining of glomerular capillaries+, transplant glomerulopathy and vasculopathy scores, DSA strength, and graft failure than early MVI ≥ 2 or normal transplant kidneys. Death-censored graft survival in 149 unique MVI ≥ 2 kidneys was independently determined by nonadherence, serum creatinine, proteinuria, DSA+, Banff C4d staining of peritubular capillaries+, and chronic interstitial fibrosis scores. MVI score and time lost significance using multivariable Cox regression.

CONCLUSIONS

The changing expression of MVI ≥ 2 over time is best explained by differences in underimmunosuppression and microvascular injury from AMR impacting allograft function and survival.

Blood Gene Signature as a Biomarker for Subclinical Kidney Allograft Rejection: Where Are We?

The observation decades ago that inflammatory injuries because of an alloimmune response might be present even in the absence of concomitant clinical impairment in allograft function conduced to the later definition of subclinical rejection. Many studies have investigated the different subclinical rejections defined according to the Banff classification (subclinical T cell-mediated rejection and antibody-mediated rejection), overall concluding that these episodes worsened long-term allograft function and survival. These observations led several transplant teams to perform systematic protocolar biopsies to anticipate treatment of rejection episodes and possibly prevent allograft loss. Paradoxically, the invasive characteristics and associated logistics of such procedures paved the way to investigate noninvasive biomarkers (urine and blood) of subclinical rejection. Among them, several research teams proposed a blood gene signature developed from cohort studies, most of which achieved excellent predictive values for the occurrence of subclinical rejection, mainly antibody-mediated rejection. Interestingly, although all identified genes relate to immune subsets and pathways involved in rejection pathophysiology, very few transcripts are shared among these sets of genes, highlighting the heterogenicity of such episodes and the difficult but mandatory need for external validation of such tools. Beyond this, their application and value in clinical practice remain to be definitively demonstrated in both biopsy avoidance and prevention of clinical rejection episodes. Their combination with other biomarkers, either epidemiological or biological, could contribute to a more accurate picture of a patient's risk of rejection and guide clinicians in the follow-up of kidney transplant recipients.

Impact of Immunosuppressive Drug Concentrations on Microvascular Inflammation, Negative Donor-Specific Antibodies, and C4d-Negative Status in Kidney Transplant Recipients

INTRODUCTION

This study investigated the impact of immunosuppressive drug concentrations on microvascular inflammation (MVI) in kidney transplant recipients with negative donor-specific antibodies (DSA) against human leukocyte antigen (HLA) and negative C4d deposition in peritubular capillaries.

METHODS

We analyzed data from 268 living kidney transplant recipients at the Department of Urology, University of Osaka, Japan. Patients received immunosuppressive therapy comprising extended-release tacrolimus, mycophenolate mofetil (MMF), and/or everolimus, with or without steroids. Graft biopsies were routinely performed at 3, 12, 36 and 60 months post-surgery.

RESULTS

No significant differences were observed between the MVI+DSA-C4d- and MVI-DSAC4d groups regarding graft survival rates (95.5% vs. 96.6%, p = 0.772) or patient survival rates (95.7% vs. 95.9%, p = 0.735). Lower tacrolimus and everolimus concentrations were significantly associated with an increased risk of MVI+DSA-C4d- (tacrolimus: OR, 0.169; 95% CI, 0.055-0.515; p = 0.002; everolimus: OR, 0.386; 95% CI, 0.171-0.874; p = 0.022). In contrast, MPA concentration was not significantly associated with MVI+DSA-C4d- (OR, 0.994; 95% CI, 0.554-1.780; p = 0.984). Steroid discontinuation did not significantly impact the risk of MVI+DSA-C4d- (OR, 1.980; 95% CI, 0.318-12.000; p = 0.470).

CONCLUSION

Lower trough levels of tacrolimus and everolimus correlated with a higher incidence of antibody-independent MVI, supporting the need for tailored immunosuppressive regimens in kidney transplantation.

The Promising Effect of Tocilizumab on Chronic Antibody-Mediated Rejection (cAMR) of Kidney Transplant

Background: Chronic antibody-mediated rejection (cAMR) constitutes a serious challenge in the long-term success of organ transplantation. It is associated with donor-specific antibodies (DSAs) which activate a complement pathway in response to the presence of human leukocyte antigens (HLAs) on the graft, which results in chronic inflammation and leads to graft dysfunction. One of the recent promising methods of cAMR treatment is a recombinant humanized anti-interleukin-6 receptor (IL-6R) monoclonal antibody referred to as Tocilizumab (TCZ). The aim of the presented systematic review is to explore the existing knowledge regarding the effect of tocilizumab treatment on cAMR and to perform a meta-analysis of the available data. Methods: A systematic review was performed using the PRISMA 2020 Checklist and Flow diagram. A systematic review protocol was registered in PROSPERO: CRD42024510996. The bias assessment was obtained with Methodical Index for Non-Randomized Studies (MINORS), whereas meta-analysis was performed using MedCalc. Results: Five clinical trials with a total number of 105 patients were included in our review. The mean loss of eGFR in time was -0.141 mL/min/1.73 m2 (95% CI: -0.409 to 0.126; p = 0.298) and was found to be statistically insignificant. The heterogeneity was low and was equal to I2 = 0.00%. The authors demonstrated a reduction in DSA titer by TCZ (-0.266 MFI (95% CI: -0.861 to 0.329; p = 0.377)). In the majority of studies, eGFR stabilization was associated with a reduction in DSAs. Conclusions: TCZ pharmacotherapy insignificantly reduced DSA titer and eGFR. Despite promising outcomes of potential eGFR stabilization, there is a need for large randomized controlled trials comparing standard management of cAMR and tocilizumab treatment.

Expanding the Scope of Microvascular Inflammation: Unveiling Its Presence Beyond Antibody-Mediated Rejection Into T-Cell Mediated Contexts

Microvascular inflammation (MVI) in kidney transplant biopsies is mainly associated with antibody-mediated rejection (AMR), sparking debate within the Banff Classification of Renal Allograft Pathology regarding its exclusivity. This study reviewed the literature on MVI in T cell-mediated rejection (TCMR) and analyzed MVI in our transplant population. We searched English publications in MEDLINE, Embase, Web of Science, Cochrane, and Google Scholar until June 2024, focusing on glomerulitis (g), peritubular capillaritis (ptc), or MVI in kidney transplant biopsies classified as TCMR. Additionally, we examined g, ptc, and MVI in 69 patients with AMR, TCMR, and no rejection. Our search yielded 541 citations, with 10 studies included, covering 810 TCMR and 156 AMR biopsies. The studies showed g, ptc, and MVI were present in TCMR but were less prevalent and severe than in AMR. In our cohort, AMR had significantly higher g, ptc, and MVI scores compared to aTCMR and ATN, however, aTCMR also displayed MVI. These findings confirm that MVI occurs in aTCMR and should not be exclusively linked to AMR. These findings highlight the need to further explore MVI's significance in TCMR and investigate the inflammatory composition. This could refine the Banff Classification, improving Classification accuracy of kidney transplant pathology assessments.

Desensitization With Imlifidase for HLA-Incompatible Deceased Donor Kidney Transplantation: A Delphi International Expert Consensus

Highly sensitized (HS) patients in need of kidney transplantation (KTx) typically spend a longer time waiting for compatible kidneys, are unlikely to receive an organ offer, and are at increased risk of antibody-mediated rejection (AMR). Desensitization using imlifidase, which is more rapid and removes total body immunoglobulin G (IgG) to a greater extent than other methods, enables transplantation to occur between HLA-incompatible (HLAi) donor-recipient pairs and allows patients to have greater access to KTx. However, when the project was launched there was limited data and clinical experience with desensitization in general and with imlifidase specifically. Hence, this Delphi methodology was used to reach a consensus from a multi-disciplinary team (MDT) of experts from 15 countries on the management of HS patients undergoing imlifidase HLAi from a deceased donor (DD) KTx. This Delphi consensus provides clinical practice guidance on the use of imlifidase in the end-to-end management of HS patients undergoing an HLAi DD KTx and supports centers in the development of guidelines for the utilization and integration of imlifidase into clinical practice.

Early and late antibody mediated rejection: Which game is the complement playing?

The role of the complement system in antibody mediated rejection (AMR) emerged in the last decades, and the demonstration of the presence of complement fragments in renal allograft biopsies is a consolidated diagnostic sign of AMR. However, antibodies against donor antigens may lead to microvascular inflammation and endothelial injury even in the absence of complement activation, and growing evidence suggests that complement-independent mechanisms may be prominent in late (i.e., occurring >6 months after transplantation) vs early AMR. Different donor specific antibodies (DSA) with different biological features and complement activation ability may be involved in late or early AMR. Downregulation of tissue complement inhibitors may happen early after transplantation, partially due to ischemia reperfusion injury, and could facilitate complement activation in early vs late AMR. Clinical and histological features of late AMR and C4d negative AMR seem to converge, and this narrative review analyzes the evidence that supports lower complement activation in late vs early AMR, including differential C4d staining prevalence based on the time after transplantation, differential response to anti-complement therapy and other direct and indirect signs of the complement system activation. The therapeutic approach in early vs late AMR should take into account possible differences in the pathophysiological mechanisms of microvascular inflammation and endothelial injury in early vs late AMR.