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Editorial: Methods in alloimmunity and transplantation: 2025

Year: 2026

Type: editorial

Abstract: This Research Topic is the second volume of the "Methods in Alloimmunity and Transplantation" series and part of the wider "Methods in Immunology" series, in which recent developments and methodological topics are discussed (Figure 1) [1][2][3] . Alloimmunity describes the immune response to alloantigens: immunogenic molecules from other members of the same species, typically blood group antigens and the highly polymorphic major histocompatibility complex (MHC) antigens [1][2][3][4][5][6] . Alloantigens are traditionally divided into major and minor mismatch antigens, and they are distinguished from the xenoantigens of other species and from the autoantigens or self-antigens [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] . They engage both arms of the adaptive response: alloantibody formation through primed B cells and plasma cells, measured as panel-reactive and donorspecific antibodies and autoantibodies (PRA and DSA and AABs), but also in the activation / polarization of effector and regulatory T cells (Teffs and Tregs) [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] . The clinical modalities that depend on controlling this response include hematopoietic stem cell transplantation (HSCT), solid organ transplantation (SOT; e.g. kidney, liver, lung and heart), and vascularized composite allotransplantation (VCA) [1][2][3][17][18][19][20][21][22][23][24][25][26][27] . In VCA, face allografts carry a large and highly immunogenic mucosal and skin component which makes strict rejection monitoring even more important and adds another layer of complexity [20][21][22][23][24][25][26][27] .Indeed, across all the transplant (Tx) procedures, outcomes still depend on two major variables: how precisely donor and recipient can be matched, and how narrowly immunosuppression (IS) can be targeted, without trading rejection for infection, malignancy, and drug toxicity [20][21][22][28][29][30] . The first volume of this research topic has described several established and emerging methods (Figure 1), which have now moved much closer to routine, including allo/auto-antibody-profiling with Luminex and other technologies (e.g. whole autoantibodyome assessment with HuProt-Assays), next-generation-sequencing (NGS)-based methods (e.g. RNAseq and single-cell RNAseq (scRNAseq), T-and B-cell receptor repertoire sequencing (TCRseq and BCRseq) [31][32][33] , and NGS analysis of donor-derived cell-free DNA (dd-cfDNA) [34][35][36][37][38][39][40] .Importantly, dd-cfDNA is now increasingly used for surveillance rather than as a research readout in a growing number of transplant centers 41 . However, the timing and ideal detection thresholds and fractional versus absolute quantification across different organ types are all still under investigation (e.g. KTx vs LuTx) 35 . Molecular diagnostics on biopsy tissue increasingly run alongside Banff histopathology 39,42 .Single-cell sequencing, spatial transcriptomics, and artificial intelligence and machine learning (AI/ML)assisted systems biology (SysBio)-analysis now generate comprehensive readouts, where the data interpretation and not the acquisition is the hardest part 14,[42][43][44][45][46][47] . In the fields of transplantation, allo-and auto-immune diagnostics, technologies are now moving forward from monitoring individual molecular markers and determinants, to assessing whole transcriptome and larger allo/auto-antibody-panels in combination with scRNAseq profiling of cellular composition, to complement the traditional Banff criteria and to better understand the whole picture of acute and chronic rejection processes and autoimmunepathology in individual patients and larger cohorts 10,11,[42][43][44][48][49][50][51][52][53] .In clinical practice, novel concepts of transplantation (Tx), immunosuppression (IS), cell therapy and patient care are moving rapidly forward 28,29 . This includes therapy with advanced therapy medicinal products and cell and gene therapies (ATMPs and CGTs), such as immunomodulatory and regenerative mesenchymal stromal/stem cells (MSCs) and Tregs to improve graft tolerization and ameliorate endogenous rejection processes, but also CD19-directed chimeric antigen receptor (CAR)-modified immune cell therapy for depletion of pathogenic allo/auto-antibody-producing B-and plasma-cell memory 2, 15, 17, 29, 45, 46, 54-63 . Two promising recent case studies from independent centers have shown that depletion of anti-HLA antibodies with CD19-directed CAR-T cell therapy can facilitate KTx in patients previously at risk of rejection due to high allo-antibody sensitization 15,16,64 . Similar applies in the treatment of autoimmunepathology and oncology, where CAR-T therapy has first emerged approximately one decade ago as promising new treatment modality for cancer, but now also realizing its enormous potential for the treatment of autoimmune and transplant pathology 65 .The eighteen contributions gathered in this research topic fall into four groups: non-invasive and tissue-based diagnostics of rejection; experimental and clinical models of alloimmune and complementmediated injury; the humoral and cellular mechanisms these methods aim to capture; and cell-and genebased therapeutics together with their clinical outcomes.The largest cluster in this volume addresses a problem the field has not yet solved: how to detect allograft injury early and non-invasive -without the need for repeated biopsies. 22 . The authors describe a discordance with direct consequences for how rejection is graded and treated. These findings matter for VCA and build on earlier reports: skin and mucosa reject with distinct spatial and temporal patterns and through different mechanisms. B cells, for example, are found in mucosa but not skin, and the discordance between skin and mucosal grades widens as the mucosal rejection grade rises [23][24][25][26][27] . This is in line with other studies listed below that clearly outline immunological differences in tissue compartments and how they are affected by the host's immune system. Understanding the signals these diagnostics detect requires controlled experimental systems and detailed mechanistic work. In VCA, Thomé et al. establish a rodent model that distinguishes acute from chronic skin rejection. Schweizer et al. use fully mismatched rat hind-limb transplants to test donor adipose-derived mesenchymal stromal/stem cells (MSCs) against FK-506, finding that the MSCs partially delayed, but did not reverse progression from grade II to grade III rejection. The authors demonstrate that the agent was able to reduce graft vasculitis and pro-inflammatory cytokines. That negative result on rejection reversal is as informative as the vasoprotective one: it argues that stromal cell therapy in VCA might be an adjunct to conventional immunosuppression, and not a replacement. The methodological direction across these eighteen contributions is consistent: away from single-analyte, single-time-point diagnostics and toward composite monitoring that combines a molecular signal, an antibody / autoantibody / autoantibodyome profile, and a cellular readout in the same patient over time.Two obstacles become clear. Assays originally validated in kidney transplantation are being transferred to liver, lung, heart, and VCA without re-devised thresholds. Multi-center standardization and comparable data-integration still lags well behind analytical innovations and single-center advancements, thus calling for continued cooperation towards future advancements. Novel treatment concepts and cell therapy approaches now offer safe and effective new treatment modalities for pathology previously refractory to treatment. We thank the authors, reviewers, and the Frontiers editorial office, and we hope the next volume can report on how far that standardization has come.

Source: Frontiers in Immunology

DOI: 10.3389/fimmu.2026.1970388

Authors: Martin Kauke-Navarro, Guido Moll