Immunotherapeutic strategies after allogeneic stem cell transplantation

INTRODUCTION

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 ANTI-VIRAL IMMUNITY

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 COMBAT GRAFT VERSUS HOST DISEASE

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 CD127^low 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.

IT STRATEGIES TO REDUCE RELAPSE

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.

CONCLUSION

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

CONFLICT OF INTEREST

None related to this work.

FUNDING

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

ACKNOWLEDGEMENTS

We thank Maria Tsima and Georgia Tzirou for administrative help.

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Posted on 1 May 202412 September 2024