Home > Proceedings in Hematology > Proceedings of the 4th European Congress Controversies in Leukemia > Immunotherapeutic strategies after allogeneic stem cell transplantation

Immunotherapeutic strategies after allogeneic stem cell transplantation

Alexandros Spyridonidis (email)× Alexandros Spyridonidis (orcid) (email)
* First author

Hematology Div, BMT Unit and Institute of Cell Therapy, University of Patras, 26504, Greece.
Dionysia Kefala (email)× Dionysia Kefala (orcid) (email)

Hematology Div, BMT Unit and Institute of Cell Therapy, University of Patras, 26504, Greece.
Maria Liga (email)× Maria Liga (email)

Hematology Div, BMT Unit and Institute of Cell Therapy, University of Patras, 26504, Greece.

Tregs, DLI, VSTs, HLA-G

In this article, we describe current immunotherapeutic strategies to reduce transplant-related morbidity and mortality and to enhance disease control strategies after allogeneic hematopoietic cell transplantation.


Both the success (cure) as well as the disaster (transplant-related death) that may occur after allogeneic hematopoietic cell transplantation (allo-HCT) are ascribed to the non-optimal immune-reconstitution in the immune-ablated recipient. Thus, harnessing the immune system after allo-HCT by applying immunotherapeutic (IT) strategies to reduce transplant-related morbidity and mortality (TRM) and to enhance disease control remains a challenge for the transplant physician.1


IT strategies to enhance the anti-viral immunity and reduce TRM include the application of mono- or multi-virus-specific T-cells manufactured from T-cells of the original donor 2. As such advanced IT strategies can be produced only in experienced and well-equipped centres with good manufacturing practice (GMP) facilities legislated by national authorities to produce point of care (POC) advanced therapeutic medicinal therapies (ATMPs), still only a minority of patients have access to them. “Off the shelf" ready-to-use T-cell therapies generated from third-party donor-derived T-cells are currently tested in prospective clinical trials, with one of such products against EBV+ PTLD (tabelecleucel) having gained FDA and EMA approval.


IT strategies to reduce Graft versus Host Disease (GvHD) are more challenging as these should also not harm the Graft versus Leukemia (GvL) effect. Emerging evidence suggests a central role of T-regulatory reconstitution in establishing the desired optimal GvHD and GvL balance after allo-HCT. T-cell depletion (TCD) strategies with ATG or alemtuzumab are effective in preventing GvHD not only by depleting naïve T-cells but also by establishing a higher regulatory to naïve T-cell ratio after allo-HCT.3 Post-transplant cyclophosphamide (PTCY) prevents efficiently GvHD, especially chronic GvHD, mainly via a preferential recovery of Tregs and myeloid suppressors which conquer surviving alloreactive T-cells.4 As the current paradigm indicates that immune tolerance is determined by a balance of Tregs over T-effector cells, Tregs hold a great promise to combat GvHD and several approaches have been attempted to translate an adoptive Treg cell immunotherapy to the clinic. As far as manufacturing of Tregs is concerned, the most documented and common approach is to isolate natural Tregs (nTregs) through CD4+CD25+ selection, however, this approach was accompanied by co-infusion of a high proportion of alloreactive CD25+ T-cells along with the intended nTreg population. To circumvent such purity and safety issues, a novel approach implementing a GMP-compliant closed-system flow cytometry (FACS) sorter to isolate the CD127low fraction of the CD4+CD25+ nTregs has been successfully employed in the first and only phase III randomized clinical trial of nTregs, with exceptional preliminary data.5 Since nTregs constitute a rare population in the periphery challenging their clinical application, many groups, including ours, have sought to convert conventional T-cells to induced Treg (iTregs).

Our approach aspired to mimic the mechanism of successful physiological immunotolerance during pregnancy, the best example of semi-allogeneic tolerance in nature, in which a highly potent immunomodulatory molecule called human-leucocyte-antigen-G (HLA-G) is expressed in the placenta to protect the foetus from the maternal immune attack. HLA-G is epigenetically silenced after prenatal life and in normal healthy tissues but may putatively re-expressed in pathological conditions aiming to mitigate immune aggression by suppressing various immune effector cells. We were the first to show that in allo-HSCT recipients, HLA-G is de novo expressed in GvHD sites and in peripheral blood T-cells which upon their isolation by flow cytometry (FACS)-sorting proved to have strong in vitro immune-suppressive properties (HLAG+ T-suppressors).6 Subsequently, we managed to robustly ex vivo generate on small and large-clinical scale a highly in vitro and in vivo immunosuppressive induced T-regulatory (iTreg) population enriched in HLA-G expressing cells, termed iG-Tregs.7,8 We successfully translated our iG-Treg production methods into GMP-compatible manufacturing processes, and we initiated the first-in-human phase 1/2 clinical trial of ex vivo generated iG-Tregs in adult patients undergoing allo-HCT from an HLA-matched sibling donor to prevent GvHD or treat refractory chronic GvHD.9

Our preliminary exploratory results hint towards long-term persistence of infused iG-Treg clonotypes and the emergence of increased diversity of the Treg repertoire. Besides our ex-vivo generated iG-Treg products, other iTregs have proceeded to phase 1/2 clinical studies showing the feasibility and safety of this approach with encouraging results.10,11 All these approaches have in common the exposure of T-cells to regulatory-inducing mediators e.g. decitabine, TGF-beta, or tolerogenic dendritic cells. Novel manufacturing approaches which are now tested in phase I/II clinical trials include the use of genetically engineered Tregs, like viral-based systems to induce chimeric antigen receptor CAR-Tregs or CRISPRCas9 genome editing to deplete or activate endogenous antigens.12 The major challenge of the use of Tregs in the clinic remain in identifying the most effective Treg subsets with stable regulatory function and long-term persistence in vivo. Other challenges include production issues as an ATMP, as these are produced on an individual patient basis, in time-consuming, complex and still expensive manufacturing processes. A third-party and “off-the-shelf” Treg bank could overcome such limitations.


Minimal residual disease (MRD) after allo-HCT may be used as a predictor of impending relapse and should be part of routine follow-up for transplanted patients, however, a clear recommendation on how to best implement MRD testing and MRD-directed therapy after allo-HCT is still lacking.13 The MRD techniques continue to advance (eg, error-corrected NGS, MRD from circulating DNA) and are expected to improve the accuracy of assessment of clonal and/or immunological changes (e.g. HLA loss) in low-volume residual disease, thus enabling a more rational therapeutic intervention than is currently possible. Less progress has been made in monitoring the speed and quality of GvL reconstitution. Unlike chemotherapy, which induces an antileukemic effect of short duration, the GvL effect is prolonged, with unique and non-quantifiable dynamics in different individuals, and may require several months to eradicate any persisting tumour cells. Recent reports suggest that the increased frequency of regulatory T-cells and exhausted leukaemia-specific T-cells in bone marrow or the co-expression of inhibitory molecules on circulating T-cells represents a dysfunctional GvL pattern that permits eventual relapse.14,15 Understanding the interplay between GvL and MRD post-allo-HCT remains a major challenge.

Prophylactic or preemptive donor lymphocyte infusion (DLI) may improve outcomes, yet convincing evidence from randomized trials is lacking.16-18 Open questions remain about the dose intensity and the total number of infusions that are necessary to achieve long-term remissions. The landscape of cellular and targeted immunotherapy is evolving rapidly and is increasingly used also as an IT strategy after allo-HCT. Hypomethylating agents (azacitidine and decitabine) may beneficially influence the balance between GVL and GVHD by enhancing the immunological visibility of leukaemia cells (eg, through the expression of silenced cancer/testis antigens and activation of interferon responses) while mitigating GvHD through expansion of regulatory T-cells.19 Better results were found when azacitidine was given together with DLIs.20 Extended azacitidine dosing using the oral formulation of the drug and panobinostat (deacetylase inhibitor) have shown promising results in prophylactic phase 1/2 studies.21 Case series reported the efficacy of immune checkpoint inhibitors (anti-CTLA- 4 and anti-PD-1/PD-L1) in relapsed disease after allo-HCT, but their use is associated with high rates of severe, often life-threatening GvHD and thus the administration of these drugs in the preemptive MRD setting is not justified outside a clinical trial.22,23 FLT3 inhibitors enhance the GvL effect and have been shown efficacious in preventing relapse or in treating FLT3-ITD–mutant acute myeloid leukaemia (AML) relapse, especially when combined with DLI. While the phase 3 BMT-CTN 1506/MORPHO trial of gilteritinib did not demonstrate statistically significant improvement of relapse-free survival (RFS), there was a clinical improvement of RFS among patients with detectable MRD before and after allo-HCT.24, 25 The isocitrate dehydrogenase (IDH) inhibitors (ivosidenib and enasidenib) are currently tested in IDH–mutated AML as maintenance and salvage therapy after allo-HCT. Interferon-a and interleukin-2 alone or together with DLIs have also been tested as immunomodulators in the MRD preemptive setting, but with doubtful effects and safety concerns.


Much remains unknown regarding the dynamic evolution of the immune system in the allo-HCT recipient and how we can dictate it. How much immunosuppression after allo-HCT do we need? How can we establish immune tolerance long-term? How can we detect and correct a dysfunctional GvL pattern? IT strategies have been developed and have shown promising results in small patient series. A significant challenge will be to perform well-designed prospective clinical trials of IT in these relatively small patient populations. 26


None related to this work.


This work was supported by Hellenic Foundation for Research and Innovation (HFRI-FM17-2971) and European Union and Greek national funds (GSRI-T2EDK–02437).


We thank Maria Tsima and Georgia Tzirou for administrative help.


  1. Spyridonidis A, Liga M. A long road of T-cells to cure cancer: from adoptive immunotherapy with unspecific cellular products to donor lymphocyte infusions and transfer of engineered tumor-specific T-cells. Am J Blood Res. 2012;2(2):98-104.
  2. Koukoulias K, Papayanni PG, Georgakopoulou A, Alvanou M, Laidou S, Kouimtzidis A, et al. "Cerberus" T Cells: A Glucocorticoid-Resistant, Multi-Pathogen Specific T Cell Product to Fight Infections in Severely Immunocompromised Patients. Front Immunol 2021:11:608701. doi: 10.3389/fimmu.2020.608701
  3. Spyridonidis A, Liga M, Triantafyllou E, Themeli M, Marangos M, et al. Pharmacokinetics and clinical activity of very low-dose alemtuzumab in transplantation for acute leukemia. Bone Marrow Transplant. 2011;46(10):1363-8. doi: 10.1038/bmt.2010.308
  4. Fletcher RE, Nunes N, Patterson MT, Vinod N, Khan SM, Mendu SK, et al. Post-transplantation cyclophosphamide expands functional myeloid-derived suppressor cells and indirectly influences Tregs. Blood Adv. 2023;11;7(7):1117-1129. doi: 10.1182/bloodadvances.2022007026
  5. Oliai C, Hoeg RT, Pavlova A, Gandhi A, Muffly L, Mehtaet RS, al. Precision-Engineered Cell Therapy Orca-T Demonstrates High Relapse-Free Survival at 1 Year While Reducing Graft-Versus-Host Disease and Toxicity. Blood. 2022;140:654–6.
  6. Lazana I, Zoudiari A, Kokkinou D, Themeli M, Liga M, Papadaki H, et al. Identification of a novel HLA-G+ regulatory population in blood: expansion after allogeneic transplantation and de novo HLA-G expression at graft-versus-host disease sites. Haematologica. 2012;97(9):1338-47. doi: 10.3324/haematol.2011.055871
  7. Stamou P, Marioli D, Patmanidi AL, Sgourou A, Vittoraki A, Theofani E, et al. Simple in vitro generation of Human Leukocyte Antigen-G–expressing T-regulatory cells through pharmacological hypomethylation for adoptive cellular immunotherapy against graft-versus-host disease. Cytotherapy. 2017;19:521–30. doi: 10.1016/j.jcyt.2017.01.004
  8. Lysandrou M, Stamou P, Kefala D, Pierides C, Kyriakou M, Savvopoulos N, et al. Hypomethylation-induced regulatory programs in T cells unveiled by transcriptomic analyses. Front Immunol. 2023;14:1235661. doi: 10.3389/fimmu.2023.1235661
  9. Lysandrou M, Kefala D, Christofi P, Savvopoulos N, Papayanni PG, Theodorellou ., et al. Study protocol: Phase I/II trial of induced HLA-G+ regulatory T cells in patients undergoing allogeneic hematopoietic cell transplantation from an HLA-matched sibling donor. Front Med (Lausanne). 2023;10:1166871. doi: 10.3389/fmed.2023.1166871
  10. MacMillan ML, Hippen KL, McKenna DH, Kadidlo D, Sumstad D, DeFor TE, et al. First-in-human phase 1 trial of induced regulatory T cells for graft-versus-host disease prophylaxis in HLA-matched siblings. Blood Adv. 2021;5(5):1425–36. doi: 10.1182/bloodadvances.2020003219
  11. Meyer EH, Laport G, Xie BJ, MacDonald K, Heydari K, Sahaf B, et al. Transplantation of donor grafts with defined ratio of conventional and regulatory T cells in HLA-matched recipients. JCI Insight. 2019;4(10):e127244. doi: 10.1172/jci.insight.127244
  12. Stucchi A, Maspes F, Montee-Rodrigues E, Fousteri G. Engineered Treg cells: The heir to the throne of immunotherapy. J Autoimmun. 2023;11:102986. doi: 10.1016/j.jaut.2022.102986
  13. Spyridonidis A. How I treat measurable (minimal) residual disease in acute leukemia after allogeneic hematopoietic cell transplantation. Blood. 2020;135(19):1639-1649. doi: 10.1182/blood.2019003566
  14. Noviello M, Manfredi F, Ruggiero E, Perini T, Oliveira G, Cortesi F, et al. Bone marrow central memory and memory stem T-cell exhaustion in AML patients relapsing after HSCT. Nat Commun. 2019;10(1):1065. doi: 10.1038/s41467-019-08871-1
  15. Hutten TJA, Norde WJ, Woestenenk R, Wang RC, Maas F, Kester M, et al. Increased coexpression of PD-1, TIGIT, and KLRG-1 on tumor-reactive CD81 T cells during relapse after allogeneic stem cell transplantation. Biol Blood Marrow Transplant. 2018;24(4):666-677. doi: 10.1016/j.bbmt.2017.11.027
  16. Liga M, Triantafyllou E, Tiniakou M, Lambropoulou P, Karakantza M, Zoumbos NC, et al. High alloreactivity of low-dose prophylactic donor lymphocyte infusion in patients with acute leukemia undergoing allogeneic hematopoietic cell transplantation with an alemtuzumab-containing conditioning regimen. Biol Blood Marrow Transplant. 2013;19(1):75-81. doi: 10.1016/j.bbmt.2012.07.021.
  17. Schmid C, Labopin M, Schaap N, Veelken H, Schleuning M, Stadler M, et al; EBMT Acute Leukaemia Working Party. Prophylactic donor lymphocyte infusion after allogeneic stem cell transplantation in acute leukaemia - a matched pair analysis by the Acute Leukaemia Working Party of EBMT. Br J Haematol. 2019;184(5):782-787. doi: 10.1111/bjh.15691
  18. Tsirigotis P, Liga M, Gkirkas K, Stamouli M, Triantafyllou E, Marangos M, et al. Low dose alemtuzumab for GvHD prevention followed by prophylactic donor lymphocyte infusions in high-risk leukemia. Bone Marrow Transplant. 2017;52(3):445-451. doi: 10.1038/bmt.2016.272
  19. Platzbecker U, Middeke JM, Sockel K, Herbst R, Wolf D, Baldus CD, et al. Measurable residual disease-guided treatment with azacitidine to prevent haematological relapse in patients with myelodysplastic syndrome and acute myeloid leukaemia (RELAZA2): an open-label, multicentre, phase 2 trial. Lancet Oncol. 2018;19(12):1668-1679. doi: 10.1016/S1470-2045(18)30580-1
  20. Schroeder T, Rachlis E, Bug G, Stelljes M, Klein S, Steckel NK, et al. Treatment of acute myeloid leukemia or myelodysplastic syndrome relapse after allogeneic stem cell transplantation with azacitidine and donor lymphocyte infusions–a retrospective multicenter analysis from the German Cooperative Transplant Study Group. Biol Blood Marrow Transplant. 2015;21(4):653-660. doi: 10.1016/j.bbmt.2014.12.016
  21. de Lima M, Oran B, Champlin RE, Papadopoulos EB, Giralt SA, Scott BL, et al. CC-486 maintenance after stem cell transplantation in patients with acute myeloid leukemia or myelodysplastic syndromes. Biol Blood Marrow Transplant. 2018;24(10): 2017-2024. doi: 10.1016/j.bbmt.2018.06.016
  22. Davids MS, Kim HT, Bachireddy P, Costello C, Liguori R, Savell A, et al; Leukemia and Lymphoma Society Blood Cancer Research Partnership. Ipilimumab for patients with relapse after allogeneic transplantation. N Engl J Med. 2016;375(2): 143-153. doi: 10.1056/NEJMoa1601202
  23. Haverkos BM, Abbott D, Hamadani M, Armand P, Flowers ME, Merryman R, et al. PD-1 blockade for relapsed lymphoma post allogeneic hematopoietic cell transplant: high response rate but frequent GVHD. Blood. 2017;130(2):221-228. doi: 10.1182/blood-2017-01-761346
  24. Mathew NR, Baumgartner F, Braun L, O'Sullivan D, Thomas S, Waterhouse M, et al. Sorafenib promotes graft-versus-leukemia activity in mice and humans through IL-15 production in FLT3-ITD-mutant leukemia cells. Nat Med. 2018;24(3): 282-291. doi: 10.1038/nm.4484
  25. Bazarbachi A, Bug G, Baron F, Brissot E, Ciceri F, Dalle IA, et al. Clinical practice recommendation on hematopoietic stem cell transplantation for acute myeloid leukemia patients with FLT3-internal tandem duplication: a position statement from the Acute Leukemia Working Party of the European Society for Blood and Marrow Transplantation. Haematologica. 2020;105(6):1507-1516. doi: 10.3324/haematol.2019.243410
  26. Spyridonidis A. How much immunosuppression do we need? Blood. 2017;129(10):1241-1243. doi: 10.1182/blood-2017-01-761627

Table of Contents

©2024 the author(s). Published with license by Medicom Medical Publishers.
This an Open Access article distributed under the terms of the Creative Commons attribution-non Commercial license (http://creativecommons.org/licenses/by-nc/4.0/), which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Posted on