The Elephant in The Room: AML Relapse Post Allogeneic Hematopoietic Cell Transplantation

Relapsed acute myeloid leukemia (AML) following allogeneic hematopoietic cell transplantation (allo-HCT) is an unfavorable event associated with a poor prognosis, particularly for patients with early relapses. It usually arises from resistant leukemic blasts that escaped both preparative chemotherapy regimen and the graft-versus-leukemia (GVL) effect. Independent from the choice of salvage treatment, only minority of patients can achieve durable remissions. In recent years, better understanding of the disease relapse biology post allo-HCT allowed the application of newer strategies that could induce higher rates of remission, and potential longer survival. Those strategies aim at optimizing drugs that have a direct anti-leukemia activity by targeting different oncogenic mutations, metabolism pathways or surface antigens, and concurrently enhancing the immune microenvironment to promote GVL effect. This review discusses the current treatment landscape of AML relapse post allo-HCT.

Clonal expansion of CD8+ T cells reflects graft-versus-leukemia activity and precedes durable remission following DLI

Donor lymphocyte infusion (DLI) is a standard of care for relapse of AML after allogeneic hematopoietic stem cell transplantation (aHSCT). Currently it is poorly understood how and when CD8+ αβ T cells exert graft-versus-leukemia (GvL) activity after DLI. Also, there is no reliable biomarker to monitor GvL activity of the infused CD8+ T cells. Therefore, we analyzed the dynamics of CD8+ αβ T cell clones in DLI-patients. In this prospective clinical study of 29 patients, we performed deep T cell receptor β (TRB) sequencing of sorted CD8+ αβ T cells to track patients' repertoire changes in response to DLI. Upon first occurrence of GvL, longitudinal analyses revealed a preferential expansion of distinct CD8+ TRB clones (n=14). This did not occur in samples of patients without signs of GvL (n=11). Importantly, early repertoire changes 15 days after DLI predicted durable remission for the 36 months study follow-up. Furthermore, absence of clonal outgrowth of the CD8+ TRB repertoire after DLI was an early biomarker that predicted relapse at a median time of 11.2 months ahead of actual diagnosis. Additionally, unbiased sample analysis regardless of the clinical outcome revealed that patients with decreasing CD8+ TRB diversity at day 15 after DLI (n=13) had a lower relapse incidence (P=0.0040) compared to patients without clonal expansion (n=6). In conclusion, CD8+ TRB analysis may provide a reliable tool for predicting the efficacy of DLI and holds the potential to identify patients at risk for progression and relapse after DLI.

Reduced dose of PTCy followed by adjuvant α-galactosylceramide enhances GVL effect without sacrificing GVHD suppression

Posttransplantation cyclophosphamide (PTCy) has become a popular option for haploidentical hematopoietic stem cell transplantation (HSCT). However, personalized methods to adjust immune intensity after PTCy for each patient’s condition have not been well studied. Here, we investigated the effects of reducing the dose of PTCy followed by α-galactosylceramide (α-GC), a ligand of iNKT cells, on the reciprocal balance between graft-versus-host disease (GVHD) and the graft-versus-leukemia (GVL) effect. In a murine haploidentical HSCT model, insufficient GVHD prevention after reduced-dose PTCy was efficiently compensated for by multiple administrations of α-GC. The ligand treatment maintained the enhanced GVL effect after reduced-dose PTCy. Phenotypic analyses revealed that donor-derived B cells presented the ligand and induced preferential skewing to the NKT2 phenotype rather than the NKT1 phenotype, which was followed by the early recovery of all T cell subsets, especially CD4+Foxp3+ regulatory T cells. These studies indicate that α-GC administration soon after reduced-dose PTCy restores GVHD-preventing activity and maintains the GVL effect, which is enhanced by reducing the dose of PTCy. Our results provide important information for the development of a novel strategy to optimize PTCy-based transplantation, particularly in patients with a potential relapse risk.

Molecular and cellular features of CTLA-4 blockade for relapsed myeloid malignancies after transplantation

Relapsed myeloid disease after allogeneic stem cell transplantation (HSCT) remains largely incurable. We previously demonstrated the potent activity of immune checkpoint blockade (ICB) in this clinical setting with ipilimumab or nivolumab. To define the molecular and cellular pathways by which CTLA-4 blockade with ipilimumab can reinvigorate an effective graft-versus-leukemia (GvL) response, we integrated transcriptomic analysis of leukemic biopsies with immunophenotypic profiling of matched peripheral blood samples collected from patients treated with ipilimumab following HSCT on the ETCTN 9204 trial. Response to ipilimumab was associated with transcriptomic evidence of increased local CD8+ T cell infiltration and activation. Systemically, ipilimumab decreased naïve and increased memory T cell populations and increased expression of markers of T cell activation and co-stimulation such as PD-1, HLA-DR and ICOS, irrespective of response. However, responding patients were characterized by higher turnover of T cell receptor sequences in peripheral blood and showed increased expression of proinflammatory chemokines in plasma that was further amplified by ipilimumab. Altogether, these data highlight the compositional T cell shifts and inflammatory pathways induced by ipilimumab both locally and systemically that associate with successful GvL outcomes.

Granulocyte Colony-Stimulating Factor Effectively Mobilizes TCR γδ and NK Cells Providing an Allograft Potentially Enhanced for the Graft-Versus-Leukemia Effect for Allogeneic Stem Cell Transplantation

Allogeneic hematopoietic stem cell transplantation (HSCT) is a potential cure for patients with hematological malignancies but substantial risks of recurrence of the malignant disease remain. TCR γδ and NK cells are perceived as potent innate effector cells in HSCT and have been associated with post-transplant protection from relapse in clinical studies. Immunocompetent cells from the donor are crucial for patient outcomes and peripheral blood stem cells (PBSC) are being increasingly applied as graft source. G-CSF is the preferential mobilizing agent in healthy donors for PBSC grafts, yet effects of G-CSF on TCR γδ and NK cells are scarcely uncovered and could influence the graft composition and potency of these cells. Therefore, we analyzed T and NK cell subsets and activation markers in peripheral blood samples of 49 donors before and after G-CSF mobilization and—for a subset of donors—also in the corresponding graft samples using multicolor flowcytometry with staining for CD3, CD4, CD8, TCRαβ, TCRγδ, Vδ1, Vδ2, HLA-DR, CD45RA, CD197, CD45RO, HLA-DR, CD16, CD56, and CD314. We found that TCR γδ cells were mobilized and harvested with an efficiency corresponding that of TCR αβ cells. For TCR γδ as well as for TCR αβ cells, G-CSF preferentially mobilized naïve and terminally differentiated effector (TEMRA) cells over memory cells. In the TCR γδ cell compartment, G-CSF preferentially mobilized cells of the nonVδ2 types and increased the fraction of HLA-DR positive TCR γδ cells. For NK cells, mobilization by G-CSF was increased compared to that of T cells, yet NK cells appeared to be less efficiently harvested than T cells. In the NK cell compartment, G-CSF-stimulation preserved the proportion of CD56dim NK effector cells which have been associated with relapse protection. The expression of the activating receptor NKG2D implied in anti-leukemic responses, was significantly increased in both CD56dim and CD56bright NK cells after G-CSF stimulation. These results indicate differentiated mobilization and altering properties of G-CSF which could improve the effects of donor TCR γδ and NK cells in the processes of graft-versus-leukemia for relapse prevention after HSCT.