---
title: "Non‑engrafting donor leukocytes and improved allogeneic transplant outcomes: mechanisms and clinic"
id: "frontiers-in-immunology-5-three-in-a-bed-and-changing-allogeneic-transplant-outcomes-with-a-second-non"
canonical_url: "https://medichelpline.com/clinical-feed/frontiers-in-immunology-5-three-in-a-bed-and-changing-allogeneic-transplant-outcomes-with-a-second-non"
content_type: "clinical_feed_article"
specialty: "Infectious Disease"
source_name: "Frontiers in Immunology"
source_url: "https://www.frontiersin.org/articles/10.3389/fimmu.2026.1904787"
published_at: "2026-09-21T00:00:00.000Z"
evidence_level: "Journal Feed"
license: "CC-BY-NC-4.0 / Informational Use"
---
# Non‑engrafting donor leukocytes and improved allogeneic transplant outcomes: mechanisms and clinic
## Provenance & Clinical Metadata
- **Canonical URL:** https://medichelpline.com/clinical-feed/frontiers-in-immunology-5-three-in-a-bed-and-changing-allogeneic-transplant-outcomes-with-a-second-non
- **Specialty:** [Infectious Disease](https://medichelpline.com/clinical-feed/infectious-disease.md)
- **Primary Source:** Frontiers in Immunology
- **Source URL:** [Original Journal Publication](https://www.frontiersin.org/articles/10.3389/fimmu.2026.1904787)
- **Published At:** 2026-09-21T00:00:00.000Z
- **Evidence Rating:** Journal Feed
## Executive GIST (TL;DR)
- Relapse is the primary cause of mortality after **allogeneic hematopoietic stem cell transplant (HSCT)** for leukemia despite advances in biology and targeted therapies. Cure after allogeneic transplant depends largely on the **graft‑versus‑leukemia (GVL)** effect mediated by donor immune cells. - Donor T cells recognize recipient leukemic cells via **HLA** and minor histocompatibility antigen mismatches; GVL is closely linked to graft‑versus‑host disease (GVHD). - There is historical and emerging evidence that transfer of **non‑engrafting allogeneic leukocytes** (from blood transfusions, granulocyte infusions, or third‑party sources) can provoke anti‑leukemic immune responses and occasional remissions. - In solid‑organ transplantation, transfusion‑induced HLA sensitization is well documented and is associated with graft rejection, demonstrating that transferred leukocytes are highly immunogenic and can prime antigen‑specific responses. - Cord blood (CB) transplantation, particularly **T‑cell replete cord blood (TRCB)**, shows lower relapse in high‑risk leukemia cohorts despite more frequent HLA mismatch; proposed reasons include distributed mismatches across both haplotypes and reduced genomic loss of mismatched HLA. - In double cord blood transplantation (DCBT), one unit commonly engrafts while the other is rejected; if the rejected (“losing”) unit shares HLA mismatches with the host, the immune response primed by rejection may be transferred and reduce relapse risk without increasing GVHD or transplant‑related mortality in some series. - Published case reports and retrospective series describe spontaneous remissions after leukocyte‑replete transfusions and granulocyte infusions; leukocyte reduction (e.g., irradiation/filtration) appears to diminish this effect. - The authors synthesize these observations and propose that a better understanding of **non‑engrafting cellular therapies** could enable transplant approaches that enhance leukemia‑directed immunity while limiting GVHD. - The review summarizes immunotherapy modalities and highlights gaps in mechanistic understanding and the need for prospective studies to define how third‑party, non‑engrafting cells can be harnessed to reduce relapse risk.
## Clinical Analysis & Structured Key Points
Frontiers | Three in a bed and changing allogeneic transplant outcomes with a second, non-engrafting cell source PERSPECTIVE article Front. Immunol. , 21 September 2026 Sec. Alloimmunity and Transplantation Volume 17 - 2026 | https://doi.org/10.3389/fimmu.2026.1904787 Published in Frontiers in Immunology Alloimmunity and Transplantation 7 impact factor 11.3 citescore Editor & Reviewers Edited by S I Sawa Ito Reviewed by A H Amir Hossein Kheirkhah Y H Yuta Hasegawa Outline Figures and Tables Figure 1 View in article Figure 2 View in article Table 1 Summary of key immunotherapy modalities. View in article Table 2 Summary of published studies using different modalities of (Non-Engrafting) leukocyte transfusion for treatment of AML. View in article Table 3 Possible mechanisms of anti-leukemic effects in non-engrafting cellular therapies. View in article PERSPECTIVE article Front. Immunol. , 21 September 2026 Sec. Alloimmunity and Transplantation Volume 17 - 2026 | https://doi.org/10.3389/fimmu.2026.1904787 Three in a bed and changing allogeneic transplant outcomes with a second, non-engrafting cell source N J Nathan Jeffreys 1 * K P Kay Poulton 2 R W Robert Wynn 1 1. Pediatric Blood and Marrow Transplant Programme, Royal Manchester Children’s Hospital, Manchester, United Kingdom 2. Transplantation Laboratory, Manchester University NHS Foundation Trust, Manchester, United Kingdom Article metrics View details Abstract Relapse remains the most significant cause of mortality after allogeneic transplant for leukemia despite advances in understanding of disease biology and newer, targeted treatment modalities. Cure after allogeneic transplant is mediated through the graft-vs-leukemia effect, as donor immune cells recognize and reject leukemic cells, largely through recognition of mismatched HLA and/or minor histocompatibility antigens. There is growing evidence that adjuvant but non-engrafting cellular therapies, either alone or in conjunction with conventional allogeneic transplant, improves leukemia outcomes and immune responses to such therapies have also been described during solid-organ transplants and following double cord unit transplantation, and are there associated with clinical consequences. We review these immune responses induced by third party and non-engrafting leukocytes and suggest possible mechanisms for the action of such cells. We further suggest that improved understanding of the nature of such responses might allow development of transplant technologies to make allogeneic transplants both better and safer, by targeting the immune response of the allogeneic immune system more specifically to leukemia. Introduction Relapse remains a significant barrier to improved outcomes in leukemia Outcomes of patients with leukemia have improved substantially over recent decades as a direct result of advances in our understanding of the underlying tumor biology and its molecular and genetic features. The use of new targeted treatments, alongside existing treatment options and advances in supportive care, has led to improvements in overall survival and significant reductions in treatment-related toxicity. Hematopoietic stem cell transplantation is part of the therapeutic landscape and is required often for cure of disease but the commonest cause of treatment failure, even with such transplant, remains disease relapse. There remains therefore significant unmet need to improve outcomes for children and for adults with difficult-to-cure leukemia. How transplant cures leukemia Hematopoietic stem cell transplantation (HSCT) has become a mainstay of treatment for hematologic malignancies and cures many patients who are refractory to other treatment modalities, or who have relapsed after previously achieving remission. In general, HSCT might result in cure through dose escalation of chemotherapy or radiotherapy given during conditioning since such escalation might overcome resistance, and this is the principle of autologous transplantation. However, the principal benefit in allogeneic transplant is immunologic, and through a graft-versus-leukemia (GVL) effect during which donor-derived immune cells recognize and destroy host leukemic stem cells and their progeny. Transplant remains the oldest and numerically most successful immune therapy of malignancy. In recent years other immune therapies of cancer have reached the clinic, including monoclonal antibodies, CAR-T approaches, co-stimulation blockade that blocks an inhibitory signal to tumor-reactive T-cells, tumor infiltrating lymphocytes (TIL), TCR-T (engineered T-cells with a transgenic T cell receptor that recognizes tumor antigen expressed with HLA) and Bispecific T-cell engager (BiTE) antibody therapies that brings T cells to the tumor with an antibody that binds both tumor and T cells (all approaches summarized in Table 1 ). The plethora and efficacy of such treatments further indicate that cancer can be controlled by immunologic means and that this is true for both hematologic and solid tumors. Table 1 Immunotherapy modality Mechanism of action Licensed examples Cell therapy based Allogeneic HCT ( 1 ) Primarily mediated by graft-vs-leukaemia effect N/A Chimeric antigen receptor (CAR-T) ( 2 ) Genetically engineered autologous T cells that possess an artificial transmembrane protein that binds to tumour surface antigens and then initiates intracellular signalling, to promote antitumour T cell mediated cytotoxicity Tisangenlecleucel T-cell receptor engineered T-cell therapy (TCT-T) ( 3 ) Genetically engineered autologous T cells that possess a modified T cell receptor that binds to tumour intracellular proteins presented on HLA molecules, to promote antitumour T cell mediated cytotoxicity Afamitresgene autoleucel Tumour infiltrating lymphocytes (TILs) ( 4 ) Autologous T cells from within host tumour are isolated and expanded ex vivo before reinfusion, promoting antitumour T cell mediated cytotoxicity Lifileucel Drug- or antibody- based Monoclonal antibodies and antibody-drug conjugates ( 5 ) Range of mechanisms of action, including: • Antibody dependent cellular cytotoxicity • Antibody dependent phagocytosis • Compliment mediated cytotoxicity • Direct cytotoxicity and apoptosis • For ADCs, delivery of cytotoxic payload Rituximab (anti-CD20) Brentuximab (anti-CD30) Gemtuzumab ozogamicin (anti-CD33) Bispecific T cell engagers (BiTEs) ( 6 ) Simultaneously binds to both T cell receptor and tumour cells, promoting direct T cell mediated cytotoxicity Blinatumomab (CD3 x CD19) Immune checkpoint inhibitors ( 7 ) Blocks inhibitory immune checkpoints, e.g. PD-1, CTLA-4, restoring anti-tumour T cell mediated cytotoxicity Pembrolizumab (PD-1) Nivolumab (PD-1) Ipilimumab (CTLA-4) Tumour vaccines ( 8 ) Induce tumour specific immunity by presenting tumour antigens to prime immune mediated recognition and responses Clinical trials only Summary of key immunotherapy modalities. Table 1 summarizes the main immunotherapeutic treatment modalities currently available or in development. T-cells are the principal mediator of GVL. There is increased relapse when the graft is depleted of donor T-cells ( 9 ), or when there is a high level of immunosuppressive drug ( 10 ). Furthermore, remission can be achieved where there is post-transplant disease relapse with donor T-cell infusion ( 11 ). Donor T-cells recognize recipient leukemic cells through recognition of HLA-mismatch, or minor Histocompatibility Antigen-mismatch (MiHA), namely mismatched peptides presented by matched HLA. GVL is closely associated with Graft versus Host Disease (GVHD) since the target antigens are shared, and not leukemia-specific, and GVHD remains an important and much feared cause of morbidity and mortality after allogeneic transplant. HLA matching between transplant and donor at the major HLA loci contributes to successful engraftment and long-term graft function in recipients of stem cell transplant. Recipients with a HLA-matched donor do better than those without such a donor ( 12 ). Relapse rates may be reduced when there is HLA-mismatch as the leukemia is more foreign to the donor, but this comes at the cost of increased incidence of GVHD. When there is major HLA mismatch between donor and recipient, as during haplo-identical or other mismatched donor transplant, loss of HLA expression on the leukemic cells contributes to relapse after transplant and occurs through several mechanisms. This was explored by Vago et al. who has described both genomic loss of the mismatched HLA genes and epigenetic modification to downregulate mismatched HLA gene expression in the leukemic clone at relapse ( 13 – 15 ). At relapse after haploidentical transplant, there is copy-neutral loss of heterozygosity (CN-LOH), i.e. duplication of the compatible haplotype and loss of the mismatched haplotype. This implies that major HLA-mismatch is a GVL target and by down regulation of this mismatched antigen, there is reduced immunologic recognition of the disease by the donor which also limits the utility of therapeutic options such as donor leukocyte infusion (DLI). They have recently presented evidence that this mechanism of relapse relates to transplant type, with reduced rates of genomic loss of the mismatched HLA at relapse in CB transplant compared to MUD, likely as the mismatches are on both haplotypes and the mismatched antigen expression cannot be achieved replacement of the mismatched haplotype by the matched haplotype. Improved therapeutic options are still required Despite improved outcomes with hematopoietic stem cell transplantation, relapse after transplant remains an issue and significant cause of treatment failure ( 16 ). This occurs in 20-70% cases and depends on several factors including disease subtype ( 17 ), donor factors ( 18 ), the presence of residual disease before transplant ( 19 ) and the intensity of conditioning therapy ( 20 ). Its efficacy is derived from donor T cell action, and relapse might be expected to be reduced by better focusing the recognition of host malignancy by donor T cells. Existing therapies such as DLI are used in select cases only to augment the GVL effect. More widespread use is limited by excess GVHD, leading to high treatment related morbidity and mortality ( 21 , 22 ). Substantial and long-standing evidence suggests that direct leukocyte-induced responses may have a beneficial effect in the treatment of hematological malignancies. We describe the history of such therapies that describe leukocyte-induced immune responses in allogeneic transplantation and how this may influence clinical decisions around donor source and adjuvant therapies to reduce relapse risk and improve outcomes. Studies were identified through PubMed indexing and analysis of references for selected landmark studies; this approach is non-exhaustive but permits a broad oversight of the current landscape and provides perspectives for future work in the area. Leukocytes and their effects Historical perspectives on leukocyte transfusion The ability of blood transfusion to induce remission of acute leukemia was first reported in the 1960s, with several reports of prolongation of remission and even some examples of spontaneous remission following leukocyte-replete blood transfusion. In 1967, Schwarzenberg ( 23 ) reported on 24 patients with ‘acute leukemia in the evolving phase’ who were treated with leukocyte transfusion to treat concurrent infection, and noted six complete remissions and three incomplete responses in the absence of concomitant chemotherapy, therefore suggesting an antileukemic effect of the transfusions themselves. Subsequent studies suggest that the anti-leukemic effect of blood transfusion was reduced following leukocyte reduction, such as irradiation or white cell filtration ( 24 ), suggesting the importance of allogeneic leukocytes as the key agent in the anti-leukemic effect seen. A more recent case report by Creasey et al. details a 45-year-old woman with poor-risk AML who received sixteen irradiated granulocyte pools on days 82–93 for supportive treatment during a period of severe neutropenia and suspected aspergillus lung infection. A sustained improvement in neutrophil, lymphocyte and platelet count was seen following the granulocyte infusions, and bone marrow evaluation on day 105 showed spontaneous morphological and cytogenetic remission with <5% blasts. The team went on to consolidate with a 10/10 HLA-matched MUD on day 130 and report sustained complete remission at 1 year follow-up. Leukocytes in solid-organ transplant medicine The above examples suggest that allogeneic leukocyte transfer may have a role in leukemia treatment but do not specify the underlying mechanisms for the responses seen. In the solid organ transplantation setting, specific HLA-mediated immune responses following (unintended) leukocyte transfer are well described and such immune responses are an important mediator of graft rejection ( 25 ). Frequent blood transfusions can provoke HLA sensitization, i.e. the development of HLA-specific immune responses. Evidence suggests that in solid organ transplantation such an immune response is strongly associated with graft rejection ( Figure 1A ). This remains evident in the post-transplant period where blood transfusion is adversely associated with graft survival, specifically if the transfusion bears the same HLA-mismatch with the patient as the transplanted kidney. Hassan et al. ( 26 ) reported on 1104 renal and simultaneous kidney-pancreas transplant recipients in whom post-transplant blood transfusion was shown to be independently associated with the development of de novo donor-specific HLA antibodies, and this was associated with graft loss even when the antibodies resulting from such immunological priming were not donor-directed. The mechanism underlying the increased likelihood of graft loss in transplant recipients who develop transfusion-induced HLA-specific antibodies is unclear. It is evident though, that transferred leukocytes are highly immunogenic and prime an antigen specific immune response by the recipient directed against any HLA-mismatch, which then has the potential to become directed against the transplanted organ leading to rejection. It is possible that similar immune responses to third party leukocytes are transferred to leukemic cells and explain the beneficial effects of leukocytes in patients with leukemia. Figure 1 (A) Diagram showing that presence of mismatched HLA expression on the surface of third-party leucocytes (e.g. from blood transfusion) can be recognised and potentiates an immune response. If the transplanted solid organ, e.g. kidney, shares the same HLA mismatch with the third-party leucocyte, this can lead to both elimination of the third-party leucocytes and accelerated graft loss, as in Hassan et al. (B) Diagram showing the mechanism behind great anti-leukemic effect in mismatched double CBT, wherein shared mismatch between the LU and host promotes GvL effect by the WU which has been “primed” by rejection of the LU during early engraftment ( 26 ). T-replete cord blood as an attractive transplant source in high-risk malignancy Cord blood (CB) transplantation has been shown in several studies to improve outcomes in high-risk malignancy compared to other cell sources. CB transplants are more often performed with HLA-mismatch and T-cell replete donors than other cell sources, which may contribute to the improved survival rates seen in several papers. This is mechanistically plausible as GVL is optimized through identification of mismatched HLA. In CB transplant, the mismatched HLA loci are spread throughout both inherited copies of the major histocompatibility complex which encodes the HLA genes and not positioned on the same strand of genes as in a haploidentical transplant, therefore genomic loss as a mechanism of HLA loss is reduced, leading to lower relapse rates. We have previously published, using UK retrospective data from all children transplanted with AML, the incidence of relapse is reduced following transplant in all children if T-cell replete cord blood (TRCB) is used as the transplant cell source. The differences are particularly marked in children with the highest risk disease, those with measurable residual disease (MRD) at the time of transplant. In those with MRD, there was a very significant difference in relapse-free survival (RFS), of 50% in the TRCB group and 21% in those transplanted with other donors. In those without MRD, RFS was improved in the TRCB group, but not significantly so. Despite that TRCB transplants were more often mismatched and were more often performed T-cell replete, the rates of chronic GVHD were lower in this group (5% vs 19%) and the composite endpoint of chronic GVHD-free, relapse-free survival (GFRS) was much better in this TRCB cohort than in those transplanted from other cell sources (48% vs 11%). Others have reported similar findings that in those AML patients at highest risk of relapse after transplant, the use of TRCB has reduced relapse rates and better RFS and GRFS than other cell sources. Most notably, Milano et al. published institutional data from Seattle that was remarkably like our UK retrospective data. Mismatch at multiple HLA loci between donor and recipient has been associated with reduced post-transplant relapse risk, however overall survival was comparable between CB and HLA-matched unrelated donor transplantation. Some guidelines for CB selection specifically advocating for the selection of mismatched CB units to utilize HLA-mediated GVL immunogenicity and reduce relapse risk in the setting of high-risk malignancy. However, there is the potential that such benefits from lower relapse risk after CB transplantation may be offset by higher rates of non-relapse mortality in some cohorts. Double CB transplant setting In double CB unit transplantation (DCBT) for hematological malignancy, where two cords are used to increase cell dose for heavier subjects, it has been shown that HLA alloimmune responses have a beneficial impact on overall survival ( 27 ). When DCBT is performed, one cord tends to predominantly engraft - termed the “winning” unit - while the other cord is actively rejected by the first cord in a HLA-mediated process - termed the “losing” unit ( 28 ). We proposed that when there is shared HLA mismatch between the “losing” unit and the host (and therefore host-derived leukemic cells), the T-cell response, primed by the “winning” unit against the “losing” unit, is transferred and rejects the leukemic clone, clinically resulting in reduced relapse rates ( Figure 1B ). Evidence for this comes from a retrospective registry study wherein there was a significant difference in relapse risk with increasing HLA disparity only when these mismatches were shared with the recipient (RR 7% vs 29%, P<0.005), and especially if the shared mismatch was at HLA-A. Indeed, if there were 2 such shared mismatches including one at HLA-A, then the relapse rate in this large series was zero. Such shared mismatches did not increase the risk of Graft versus Host Disease (GVHD) or Transplant-related mortality (TRM), unless the shared mismatch was at class II, including HLA-DR. The results are dramatic, and there is no increased GVHD or TRM where there is such shared mismatch. Further prospective studies are needed to elucidate the mechanisms around beneficial immune responses in DCBT further. Leukocytes for treatment of high-risk leukemia Allogeneic leukocyte transfer can therefore induce antigen-specific T-cell responses in the solid organ setting, and during DCBT. There is emerging evidence that the direct transfer of allogeneic leukocytes may prime an immune response against leukemia cells to promote disease control. A range of potential leukocyte sources have been reported in the literature, and there is varying nomenclature with the terms “non-engrafting alloreactive cellular therapy (NEACT)”
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