Cytotoxic T Cells Expand in Antibody-Mediated Kidney Transplant Rejection

Redefining the Immunological Divide in Kidney Graft Rejection

Long-term kidney allograft survival remains limited by chronic active antibody-mediated rejection, a process driven by donor-specific antibodies for which no established effective treatment exists [1, 2]. While traditional guidelines distinguish between antibody-mediated and T cell-mediated phenotypes, this binary classification is increasingly viewed as an oversimplification of the underlying immune pathology [3, 4]. Standard interventions such as plasmapheresis and intravenous immunoglobulin show limited efficacy. A meta-analysis of 21 studies found no significant difference in graft survival compared to controls, yielding a hazard ratio of 0.76 (95% CI 0.35 to 1.63; p = 0.475) [2, 5]. To improve early detection, clinicians have investigated biomarkers such as donor-derived cell-free DNA (fragments of donor genetic material released into the recipient's circulation) and soluble B cell-activating factor (a cytokine that promotes B cell maturation), the latter of which is associated with a 2.04-fold increased risk of antibody-mediated rejection (95% CI 1.52 to 2.74) [6, 7, 8]. Now, a multi-cohort study analyzing 192 bulk transcriptomes and 16 single-cell RNA sequencing cases demonstrates that the intragraft environment in antibody-mediated rejection involves significant clonal expansion of cytotoxic CD8+ and CD4+ T cells, suggesting cellular mechanisms that extend beyond current histological definitions [9].

Quantifying T Cell Infiltration Beyond Traditional Histology

To characterize the immune landscape of the allograft, researchers analyzed kidney transplant biopsy datasets from four independent cohorts. The investigation utilized 192 cases of bulk transcriptomics (a technique that measures the total sum of all RNA transcripts in a tissue sample) and 16 cases of single-cell RNA and T-cell receptor sequencing to identify specific cellular populations. To validate these findings and assess the physical arrangement of immune cells within the graft, the study also incorporated 18 and 75 cases of multiplex immunostaining for spatial analysis. This multi-modal approach allowed the authors to map CD4+ and CD8+ T cell enrichment, clonal expansion, and spatial organization with high precision across different rejection phenotypes. The researchers then employed deconvolution of bulk transcriptomes, a computational method used to estimate the proportions of different cell types in a mixed tissue sample, to quantify specific immune cells. This analysis revealed a significant enrichment of CD8+ T cells in 33 antibody-mediated rejection or microvascular inflammation samples when compared to 139 samples with no rejection. Notably, the data demonstrated that CD8+ T cell levels in antibody-mediated rejection and microvascular inflammation were comparable to levels observed in 20 cases of T cell-mediated rejection or borderline T cell-mediated rejection. For practicing nephrologists and transplant surgeons, these findings suggest that the cellular infiltration typically associated with T cell-mediated rejection is equally prevalent in cases histologically classified as antibody-mediated. Consequently, relying strictly on the current diagnostic binary during biopsy interpretation may cause clinicians to overlook a substantial cytotoxic T cell component driving graft injury.

Characterizing the Cytotoxic T Cell Landscape

To define the specific cellular players involved in graft injury, the researchers analyzed single-cell RNA sequencing data using unsupervised clustering, a statistical technique that groups cells based on shared gene expression patterns rather than predefined labels. This computational approach identified five major T cell clusters within the kidney allografts, providing a high-resolution map of the immune infiltrate. The study specifically focused on clonal expansion, the proliferation of identical T cells from a single ancestor in response to a specific antigen. The data revealed that clonal expansion was primarily present in CD8+ effector memory (Tem) and effector memory T cells re-expressing CD45RA (Temra). These findings suggest that a targeted, antigen-driven T cell response is a core component of the pathology in these samples, even when the clinical diagnosis points solely to antibody-mediated rejection. The analysis further localized clonal expansion within CD4+ cytotoxic T cells and CD8+ HLA-DR+ CD27+ T cells, which are subsets typically associated with active, cell-mediated tissue destruction. In contrast, the CD4+ Naïve/central memory (Tcm) cluster was mostly not expanded, indicating that these precursor cells are not actively participating in the local immune response. Additionally, the researchers identified a specific CD8+ cluster characterized by early activation markers (CD69+, FOS+, and JUN+) that demonstrated intermediate clonality. This molecular signature points to a population of T cells in the early stages of activation and recruitment, highlighting a dynamic cellular landscape that standard light microscopy fails to capture. By identifying these specific cytotoxic populations, the study provides evidence that the immune response in antibody-mediated rejection is far more complex than a simple humoral process, potentially opening the door for T cell-directed therapies in patients failing standard antibody-depleting regimens.

Clinical Correlations and Spatial Localization in the Graft

The clinical implications of T cell involvement in antibody-mediated rejection become most evident when analyzing specific patient subgroups. The researchers found that four antibody-mediated rejection biopsies demonstrated the highest proportion of clonally expanded T cells among all the samples studied. This high level of clonal expansion was most notable in cases of early antibody-mediated rejection occurring in patients with pre-existing donor-specific antibodies. For the practicing clinician, the most striking finding is that in three cases of early antibody-mediated rejection, significant clonal expansion occurred without a histological diagnosis of T cell-mediated rejection. This indicates that a robust, antigen-specific T cell response can be actively damaging the graft even when standard microscopic examination of the tissue does not meet the formal criteria for cellular rejection. The intensity of this cellular response appears to be driven by the degree of molecular incompatibility between the donor and the recipient. The study found that both the fraction of expanded T cells and the number of distinct expanded clones correlated with the PIRCHE-II score (a metric that estimates the number of donor HLA-derived epitopes presented by the recipient's HLA molecules). This correlation provides a mechanistic link between the presentation of donor antigens and the subsequent proliferation of specific T cell clones. To further localize these cells within the graft, the researchers utilized spatial proteomics, a technique used to map the precise location of proteins within intact tissue architecture. This analysis confirmed T-cell enrichment in the intravascular and glomerular compartments of biopsies characterized by antibody-mediated rejection or microvascular inflammation. By placing these cytotoxic cells directly at the sites of active vascular injury, the findings suggest that the traditional dichotomy between antibody-mediated and T cell-mediated rejection needs reconsidering, which could eventually shift how clinicians select immunosuppressive therapies for complex rejection episodes.

References

1. Hotta K. Clinical Trials for Developing Treatment and Early Diagnostic Methods for Chronic Active Antibody-Mediated Rejection in Kidney Transplantation: A Review.. Nephron. 2025. doi:10.1159/000547781

2. Wan SS, Ying TD, Wyburn K, Roberts DM, Wyld M, Chadban SJ. The Treatment of Antibody-Mediated Rejection in Kidney Transplantation: An Updated Systematic Review and Meta-Analysis.. Transplantation. 2018. doi:10.1097/TP.0000000000002049

3. Becker JU, Seron D, Rabant M, Roufosse C, Naesens M. Evolution of the Definition of Rejection in Kidney Transplantation and Its Use as an Endpoint in Clinical Trials.. Transplant international : official journal of the European Society for Organ Transplantation. 2022. doi:10.3389/ti.2022.10141

4. Roufosse C, Becker JU, Rabant M, et al. Proposed Definitions of Antibody-Mediated Rejection for Use as a Clinical Trial Endpoint in Kidney Transplantation.. Transplant international : official journal of the European Society for Organ Transplantation. 2022. doi:10.3389/ti.2022.10140

5. Mancebo E, Diekmann F, Palou E, et al. Spanish guidelines for kidney transplantation in highly sensitized patients with donor-specific anti-HLA antibodies.. Transplantation reviews (Orlando, Fla.). 2025. doi:10.1016/j.trre.2025.100919

6. Akifova A, Budde K, Amann K, et al. Donor-derived cell-free DNA monitoring for early diagnosis of antibody-mediated rejection after kidney transplantation: a randomized trial. Nephrology, Dialysis and Transplantation. 2024. doi:10.1093/ndt/gfae282

7. Wijtvliet V, Plaeke P, Abrams S, et al. Donor‐derived cell‐free DNA as a biomarker for rejection after kidney transplantation: a systematic review and meta‐analysis. Transplant International. 2020. doi:10.1111/tri.13753

8. Zhang H, Wang S, Su X, et al. The role of soluble B cell-activating factor in further stratifying the risk of antibody-mediated rejection post-renal transplant: A meta-analysis.. International immunopharmacology. 2020. doi:10.1016/j.intimp.2019.106059

9. Vaulet T, Lamarthée B, Callemeyn J, et al. Intragraft clonal expansion of cytotoxic CD8⁺ and CD4⁺ T cells in antibody-mediated kidney transplant rejection.. Kidney international. 2026. doi:10.1016/j.kint.2026.03.012