A new tool for rapid and reliable diagnosis of HLA loss relapses after HSCT

Tricking the trickster: precision medicine approaches to counteract leukemia immune escape after transplant

ABSTRACT

Over the last decades, significant improvements in reducing the toxicities of allogeneic hematopoietic cell transplantation (allo-HCT) have widened its use as consolidation or salvage therapy for high-risk hematological malignancies. Nevertheless, relapse of the original malignant disease remains an open issue with unsatisfactory salvage options and limited rationales to select among them. In the last years, several studies have highlighted that relapse is often associated with specific genomic and nongenomic mechanisms of immune escape. In this review we summarize the current knowledge about these modalities of immune evasion, focusing on the mechanisms that leverage antigen presentation and pathologic rewiring of the bone marrow microenvironment. We present examples of how this biologic information can be translated into specific approaches to treat relapse, discuss the status of the clinical trials for patients who relapsed after a transplant, and show how dissecting the complex immunobiology of allo-HCT represents a crucial step toward developing new personalized approaches to improve clinical outcomes.

[Relapse with HLA loss after hematopoietic stem cell transplantation with non-HLA identical donor: Guidelines from the Francophone society of bone marrow transplantation and cellular therapy (SFGM-TC)]

Loss of heterozygosity or HLA loss is a genomic-type escape mechanism highlighted in certain types of relapses after allogeneic hematopoietic stem cell transplantation with a non-HLA identical donor, and especially after haplo-identical transplantation. The diagnosis must be made with certainty because the result conditions the therapy. In this article, the different mechanisms and techniques that can be used for the diagnosis of loss of heterozygosity, as well as the therapeutic options are reviewed, making it possible to establish clinico-biological recommendations for the diagnosis confirmation and management of the patients in relapse.

Relapse after allogeneic transplantation with post-transplant cyclophosphamide: Shattering myths and evolving insight

Early studies in allogeneic blood or marrow transplantation (alloBMT) demonstrated that HLA-mismatching was protective again relapse. However, benefits in relapse reduction were outweighed by a high risk of graft-versus-host disease (GVHD) when using conventional pharmacological immunosuppression. Post-transplant cyclophosphamide(PTCy)-based platforms abated the risk of GVHD thereby overcoming the negative effects of HLA-mismatching on survival. However, since its inception, PTCy has been shadowed by a reputation for a greater risk of relapse when compared with traditional GVHD prophylaxis. Specifically, whether PTCy reduces the anti-tumor efficacy of HLA-mismatched alloBMT by killing alloreactive T cells has been the subject of debate since the early 2000's. Here we review the many studies demonstrating the potent graft-versus-malignancy (GVM) properties of alloBMT with PTCy. We discuss the laboratory data from PTCy platforms supporting that T regulatory cells may be a major mechanism of prevention of GVHD and that natural killer (NK) cells may be early effectors of GVM. Finally, we propose potential paths to optimize GVM through selecting for class II mismatching and augmenting NK cell activity.

New methods for the quantification of mixed chimerism in transplantation

Background

Quantification of chimerism showing the proportion of the donor in a recipient is essential for the follow-up of hematopoietic stem cell transplantation but can also be useful to document an immune tolerance situation after solid organ transplantation. Historically, chimerism has been quantified from genomic DNA, but with technological advances, chimerism from donor-derived cell-free DNA seems particularly relevant in solid organ transplantation.

Methods

The reference method was until recently the short tandem repeat technique, but new innovative techniques as digital PCR (dPCR) and NGS, have revolutionized the quantification of chimerism, such as the so-called microchimerism analysis. After a short review of chimerism methods, a comparison of chimerism quantification data for two new digital PCR systems (QIAcuity™ dPCR (Qiagen^®) and QuantStudio Absolute Q (ThermoFisher^®) and two NGS-based chimerism quantification methods (AlloSeq HCT™ (CareDx^®) and NGStrack™ (GenDX^®)) was performed.

Results

These new methods were correlated and concordant to routinely methods (r²=0.9978 and r²=0.9974 for dPCR methods, r²=0.9978 and r²=0.9988 for NGS methods), and had similar high performance (sensitivity, reproductibility, linearity).

Conclusion

Finally, the choice of the innovative method of chimerism within the laboratory does not depend on the analytical performances because they are similar but mainly on the amount of activity and the access to instruments and computer services.

How I treat high-risk acute myeloid leukemia using preemptive adoptive cellular immunotherapy

Allogeneic hematopoietic stem cell transplantation (alloHSCT) is a potentially curative treatment for patients with high-risk acute leukemias, but unfortunately disease recurrence remains the major cause of death in these patients. Infusion of donor lymphocytes (DLI) has the potential to restore graft-versus-leukemia immunologic surveillance; however, efficacy varies across different hematologic entities. Although relapsed chronic myeloid leukemia, transplanted in chronic phase, has proven remarkably susceptible to DLI, response rates are more modest for relapsed acute myeloid leukemia and acute lymphoblastic leukemia. To prevent impending relapse, a number of groups have explored administering DLI preemptively on detection of measurable residual disease (MRD) or mixed chimerism. Evidence for the effectiveness of this strategy, although encouraging, comes from only a few, mostly single-center retrospective, nonrandomized studies. This article seeks to (1) discuss the available evidence supporting this approach while highlighting some of the inherent challenges of MRD-triggered treatment decisions post-transplant, (2) portray other forms of postremission cellular therapies, including the role of next-generation target-specific immunotherapies, and (3) provide a practical framework to support clinicians in their decision-making process when considering preemptive cellular therapy for this difficult-to-treat patient population.

Chimärismusanalyse nach Stammzelltransplantation mit hochsensitiven Methoden

Mit den hochsensitiven Methoden zur Chimärismusanalyse kann ein Rezidiv bedeutend früher prognostiziert werden. Für die Prognose ist hierbei vor allem die Dynamik des Anstiegs im Eigenanteil entscheidend, dies gilt auch schon für sehr niedrige Werte (< 0,1%). KM-Proben können über ihren variablen Eigenanteil durch Stromazellen die Erkennung einer Dynamik im niedrigen Prozentbereich (< 1%) verschleiern und somit das Erkennen eines Rezidives erschweren. Die frühe Erkennung einer Dynamik in den sehr niedrigen Prozentbereichen erlaubt bei der Anwendung der sensitiven Methoden eine Reduktion der Anzahl der KM-Proben, was den Patienten weniger belastet. Durch zellfreie DNA und andere Faktoren kann ein „Grundrauschen“ bei sehr niedrigen Eigenanteilen auftreten, der diagnostische Wert von Eigenanteilen von 0,02% und kleiner ist daher fraglich.

Assessment of Patient-Specific Human Leukocyte Antigen Genomic Loss at Relapse After Antithymocyte Globulin-Based T-Cell-Replete Haploidentical Hematopoietic Stem Cell Transplant

IMPORTANCE

Patient-specific human leukocyte antigen (HLA) genomic loss (HLA loss) is one of the reputed mechanisms of leukemia immune escape and relapse after haploidentical hematopoietic stem cell transplant (HSCT). However, clinical characteristics and prognosis of this distinct relapse type in the setting of haploidentical HSCT based on antithymocyte globulin (ATG) T-cell-replete conditioning are still unknown, especially for patients with lymphoid leukemia.

OBJECTIVE

To identify the incidence of and patient characteristics associated with HLA loss at hematologic cancer relapse after ATG-based haploidentical HSCT and to assess overall survival after HLA loss at relapse.

DESIGN, SETTING, AND PARTICIPANTS

This retrospective and multicenter case series study used data from medical records to identify patients who experienced relapse of hematologic cancer after receipt of ATG-based haploidentical HSCT. The study included 788 consecutive patients aged 8 to 70 years with lymphoid or myeloid leukemia who received ATG T-cell-replete haploidentical HSCT at the Zhejiang Cooperative Group for Blood and Marrow Transplantation between May 1, 2012, and May 31, 2021.

EXPOSURES

Relapse after ATG-based haploidentical HSCT.

MAIN OUTCOMES AND MEASURES

Incidence, risk factors, and postrelapse overall survival among patients with HLA loss at hematologic cancer relapse after receipt of haploidentical HSCT. Logistic regression analysis was used to identify characteristics associated with the likelihood of HLA loss, and Kaplan-Meier and Cox regression analyses were performed to evaluate postrelapse survival.

RESULTS

A total of 788 patients who received haploidentical HSCT for hematologic cancer were identified, 180 of whom experienced relapse after HSCT. Of those, 106 evaluable patients (median age, 30.9 years [range, 8.3-64.6 years]; 54 female [50.9%] and 52 male [49.1%]) were screened for HLA loss, which was detected in 54 patients (50.9%). Patients with HLA loss experienced relapse later than those without HLA loss (lymphoid group: median, 323 days [range, 98-2056 days] vs 151 days [range, 57-2544 days]; P = .01; myeloid group: median, 321 days [range, 55-1574 days] vs 223 days [range, 68-546 days]; P = .03). Among patients with lymphoid leukemia, those with minimal residual disease positivity before hematologic relapse (odds ratio [OR], 28.47; 95% CI, 1.99-407.98; P = .01), those with chronic graft-vs-host disease (OR, 27.68; 95% CI, 1.40-546.88; P = .03), and those with more than 180 days between HSCT and relapse (OR, 6.91; 95% CI, 1.32-36.22; P = .02) were more likely to lose unshared HLA at relapse, whereas male patients (OR, 0.03; 95% CI, 0.003-0.32; P = .04) were more likely to preserve their HLA genome at relapse. Patients with myeloid leukemia had different factors associated with HLA loss, including underweight status (OR, 0.10; 95% CI, 0.02-0.60; P = .01) and acute graft-vs-host disease (OR, 4.84; 95% CI, 1.14-20.53; P = .03). The receipt of preemptive donor lymphocyte infusion among patients with minimal residual disease recurrence did not postpone hematologic cancer relapse in those with HLA loss (median, 322 days [range, 204-1030 days]) compared with no receipt of donor lymphocyte infusion (median, 340 days [range, 215 days to not available]; P > .99).

CONCLUSIONS AND RELEVANCE

In this study, HLA loss at leukemia relapse occurred frequently after receipt of ATG-based haploidentical HSCT. The identification of risk factors associated with HLA loss would help to prompt screening, avoid potentially harmful infusions of donor T cells, and develop alternative therapeutic strategies.

Chimerism analysis for clinicians: a review of the literature and worldwide practices

This review highlights literature pertinent to chimerism analysis in the context of hematopoietic cell transplantation (HCT). We also conducted a survey of testing practices of program members of CIBMTR worldwide. Questions included testing methods, time points, specimen type, cell lineage tested and testing indications. Recent literature suggests that detection of low level mixed chimerism has a clinical utility in predicting relapse. There is also increasing recognition of HLA loss relapse to potentially guide rescue decisions in cases of relapse. These developments coincide with wider access to high sensitivity next generation sequencing (NGS) in clinical laboratories. Our survey revealed a heterogeneity in practices as well as in findings and conclusions of published studies. Although the most commonly used method is STR, studies support more sensitive methods such as NGS, especially for predicting relapse. There is no conclusive evidence to support testing chimerism in BM over PB, particularly when using a high sensitivity testing method. Periodic monitoring of chimerism especially in diagnoses with a high risk of relapse is advantageous. Lineage specific chimerism is more sensitive than whole blood in predicting impending relapse. Further studies that critically assess how to utilize chimerism testing results will inform evidence based clinical management decisions.

Clinical Characteristics and Outcome Analysis for HLA Loss Patients Following Partially Mismatched Related Donor Transplantation Using HLA Chimerism for Loss of Heterozygosity Analysis by Next-Generation Sequencing

Genomic loss of mismatched human leukocyte antigen (HLA loss) is one of the most vital immune escape mechanisms of leukemic cells after allogeneic hematopoietic stem cell transplantation (allo-HSCT). However, the methods currently used for HLA loss analysis have some shortcomings. Limited literature has been published, especially in lymphoid malignancies. This study aims to evaluate the incidences, risk factors of HLA loss, and clinical outcomes of HLA loss patients. In all, 160 patients undergoing partially mismatched related donor (MMRD) transplantation from 18 centers in China were selected for HLA loss analysis with the next-generation sequencing (NGS)-based method, which was validated by HLA-KMR. Variables of the prognostic risk factors for HLA loss or HLA loss–related relapse were identified with the logistic regression or the Fine and Gray regression model. An HLA loss detection system, HLA-CLN [HLA chimerism for loss of heterozygosity (LOH) analysis by NGS], was successfully developed. Forty (25.0%) patients with HLA loss were reported, including 27 with myeloid and 13 with lymphoid malignancies. Surprisingly, 6 of those 40 patients did not relapse. The 2-year cumulative incidences of HLA loss (22.7% vs 22.0%, P = 0.731) and HLA loss–related relapse (18.4% vs 20.0%, P = 0.616) were similar between patients with myeloid and lymphoid malignancies. The number of HLA mismatches (5/10 vs <5/10) was significantly associated with HLA loss in the whole cohort [odds ratio (OR): 3.15, P = 0.021] and patients with myeloid malignancies (OR: 3.94, P = 0.021). A higher refined-disease risk index (OR: 6.91, P = 0.033) and donor–recipient ABO incompatibility (OR: 4.58, P = 0.057) contributed to HLA loss in lymphoid malignancies. To sum up, HLA-CLN could overcome the limitations of HLA-KMR and achieve a better HLA coverage for more patients. The clinical characteristics and outcomes were similar in patients with HLA loss between myeloid and lymphoid malignancies. In addition, the results suggested that a patient with HLA loss might not always relapse.

A practical guide to chimerism analysis: Review of the literature and testing practices worldwide

BACKGROUND AND PURPOSE

Currently there are no widely accepted guidelines for chimerism analysis testing in hematopoietic cell transplantation (HCT) patients. The objective of this review is to provide a practical guide to address key aspects of performing and utilizing chimerism testing results. In developing this guide, we conducted a survey of testing practices among laboratories that are accredited for performing engraftment monitoring/chimerism analysis by either the American Society for Histocompatibility & Immunogenetics (ASHI) and/or the European Federation of Immunogenetics (EFI). We interpreted the survey results in the light of pertinent literature as well as the experience in the laboratories of the authors.

RECENT DEVELOPMENTS

In recent years there has been significant advances in high throughput molecular methods such as next generation sequencing (NGS) as well as growing access to these technologies in histocompatibility and immunogenetics laboratories. These methods have the potential to improve the performance of chimerism testing in terms of sensitivity, availability of informative genetic markers that distinguish donors from recipients as well as cost.

SUMMARY

The results of the survey revealed a great deal of heterogeneity in chimerism testing practices among participating laboratories. The most consistent response indicated monitoring of engraftment within the first 30 days. These responses are reflective of published literature. Additional clinical indications included early detection of impending relapse as well as identification of cases of HLA-loss relapse.

Relevance of Polymorphic KIR and HLA Class I Genes in NK-Cell-Based Immunotherapies for Adult Leukemic Patients

Simple Summary

Immunotherapies are promising approaches to curing different acute leukemias. Natural killer (NK) cells are lymphocytes that are efficient in the elimination of leukemic cells. NK-cell-based immunotherapies are particularly attractive, but the landscape of the heterogeneity of NK cells must be deciphered. This review provides an overview of the polymorphic KIR and HLA class I genes that modulate the NK cell repertoire and how these markers can improve the outcomes of patients with acute leukemia. A better knowledge of these genetic markers that are linked to NK cell subsets that are efficient against hematological diseases will optimize hematopoietic stem-cell donor selection and NK immunotherapy design.

Abstract

Since the mid-1990s, the biology and functions of natural killer (NK) cells have been deeply investigated in healthy individuals and in people with diseases. These effector cells play a particularly crucial role after allogeneic hematopoietic stem-cell transplantation (HSCT) through their graft-versus-leukemia (GvL) effect, which is mainly mediated through polymorphic killer-cell immunoglobulin-like receptors (KIRs) and their cognates, HLA class I ligands. In this review, we present how KIRs and HLA class I ligands modulate the structural formation and the functional education of NK cells. In particular, we decipher the current knowledge about the extent of KIR and HLA class I gene polymorphisms, as well as their expression, interaction, and functional impact on the KIR⁺ NK cell repertoire in a physiological context and in a leukemic context. In addition, we present the impact of NK cell alloreactivity on the outcomes of HSCT in adult patients with acute leukemia, as well as a description of genetic models of KIRs and NK cell reconstitution, with a focus on emergent T-cell-repleted haplo-identical HSCT using cyclosphosphamide post-grafting (haplo-PTCy). Then, we document how the immunogenetics of KIR/HLA and the immunobiology of NK cells could improve the relapse incidence after haplo-PTCy. Ultimately, we review the emerging NK-cell-based immunotherapies for leukemic patients in addition to HSCT.

Allogeneic Stem Cell Transplantation for Acute Myeloid Leukemia: Who, When, and How?

Although the majority of patients with acute myeloid leukemia (AML) treated with intensive chemotherapy achieve a complete remission (CR), many are destined to relapse if treated with intensive chemotherapy alone. Allogeneic stem cell transplant (allo-SCT) represents a pivotally important treatment strategy in fit adults with AML because of its augmented anti-leukemic activity consequent upon dose intensification and the genesis of a potent graft-versus-leukemia effect. Increased donor availability coupled with the advent of reduced intensity conditioning (RIC) regimens has dramatically increased transplant access and consequently allo-SCT is now a key component of the treatment algorithm in both patients with AML in first CR (CR1) and advanced disease. Although transplant related mortality has fallen steadily over recent decades there has been no real progress in reducing the risk of disease relapse which remains the major cause of transplant failure and represents a major area of unmet need. A number of therapeutic approaches with the potential to reduce disease relapse, including advances in induction chemotherapy, the development of novel conditioning regimens and the emergence of the concept of post-transplant maintenance, are currently under development. Furthermore, the use of genetics and measurable residual disease technology in disease assessment has improved the identification of patients who are likely to benefit from an allo-SCT which now represents an increasingly personalized therapy. Future progress in optimizing transplant outcome will be dependent on the successful delivery by the international transplant community of randomized prospective clinical trials which permit examination of current and future transplant therapies with the same degree of rigor as is routinely adopted for non-transplant therapies.

Clinical Utility of the Detection of the Loss of the Mismatched HLA in Relapsed Hematological Patients After Haploidentical Stem Cell Transplantation With High-Dose Cyclophosphamide

Haploidentical hematopoietic stem cell transplantation (Haplo-HSCT) with high-dose cyclophosphamide (PTCy) has resulted in a low incidence of graft-vs.-host disease (GVHD), graft failure, and non-relapse mortality. However, post-transplantation relapse remains a common cause of treatment failure in high-risk patients. Unraveling the mechanisms of relapse is therefore crucial for designing effective relapse treatment strategies. One of these mechanisms is the loss of the mismatched HLA on the recipient's leukemic cells. To study the incidence and clinical relevance of this phenomenon, we analyzed 181 patients treated with Haplo-HSCT with PTCy (2007–2019), of which 37 relapsed patients after transplantation. According to the kit employed for HLA-loss analysis, among 22 relapsed patients, we identified HLA loss at relapse in 6 of the 22 patients (27%) studied. Based on the results obtained, the genomic loss of HLA was more common in females than males (66 vs. 33%) and HLA-loss relapses occurred later than classical relapses (345 vs. 166 days). Moreover, the patients with HLA-loss had a greater presence of active disease at the time of transplantation and had undergone a larger number of treatment lines than the group with classical relapses (66 vs. 43% and 66 vs. 18%, respectively). Four of these relapses were studied retrospectively, while two were studied prospectively, the results of which could be considered for patient management. Additionally, two relapsed patients analyzed retrospectively had myeloid neoplasms. One patient had not undergone any treatment, and three had undergone donor lymphocyte infusions (DLIs) and chemotherapy. All presented severe GVHD and disease progression. In contrast, the two patients studied prospectively had a lymphoid neoplasm and were not treated with DLIs. One of them was treated with chemotherapy but died from disease progression, and the other patient underwent a second Haplo-HSCT from a different donor and is still alive. We can conclude that the detection of HLA-loss at the onset of relapse after Haplo-HSCT with PTCy could help in clinical practice to select appropriate rescue treatment, thereby avoiding the use of DLIs or a second transplantation from the same donor.

How I treat measurable (minimal) residual disease in acute leukemia after allogeneic hematopoietic cell transplantation

Although allogeneic hematopoietic cell transplantation (allo-HCT) is currently the standard curative treatment of acute leukemia, relapse remains unacceptably high. Measurable (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. Patients with MRD may respond to therapies aiming to unleash or enhance the graft-versus-leukemia effect. However, evidence-based recommendations on how to best implement MRD testing and MRD-directed therapy after allo-HCT are lacking. Here, I describe our institutional approach to MRD monitoring for preemptive MRD-triggered intervention, using patient scenarios to illustrate the discussion.

Novel agents targeting leukemia cells and immune microenvironment for prevention and treatment of relapse of acute myeloid leukemia after allogeneic hematopoietic stem cell transplantation

Relapse remains the worst life-threatening complications after allogeneic hematopoietic stem cell transplantation (allo-HSCT) in patients with acute myeloid leukemia (AML), whose prognosis has been historically dismal. Given the rapid development of genomics and immunotherapies, the interference strategies for AML recurrence have been changing these years. More and more novel targeting agents that have received the U.S. Food and Drug Administration (FDA) approval for de novo AML treatment have been administrated in the salvage or maintenance therapy of post-HSCT relapse. Targeted strategies that regulate the immune microenvironment of and optimize the graft versus leukemia (GVL) effect of immune cells are gradually improved. Such agents not only have been proven to achieve clinical benefits from a single drug, but if combined with classic therapies, can significantly improve the poor prognosis of AML patients who relapse after allo-HSCT. This review will focus on currently available and promising upcoming agents and also discuss the challenges and limitations of targeted therapies in the allogeneic hematopoietic stem cell transplantation community.

Mechanisms of Leukemia Immune Evasion and Their Role in Relapse After Haploidentical Hematopoietic Cell Transplantation

Over the last decade, the development of multiple strategies to allow the safe transfer from the donor to the patient of high numbers of partially HLA-incompatible T cells has dramatically reduced the toxicities of haploidentical hematopoietic cell transplantation (haplo-HCT), but this was not accompanied by a similar positive impact on the incidence of post-transplantation relapse. In the present review, we will elaborate on how the unique interplay between HLA-mismatched immune system and malignancy that characterizes haplo-HCT may impact relapse biology, shaping the selection of disease variants that are resistant to the “graft-vs.-leukemia” effect. In particular, we will present current knowledge on genomic loss of HLA, a relapse modality first described in haplo-HCT and accounting for a significant proportion of relapses in this setting, and discuss other more recently identified mechanisms of post-transplantation immune evasion and relapse, including the transcriptional downregulation of HLA class II molecules and the enforcement of inhibitory checkpoints between T cells and leukemia. Ultimately, we will review the available treatment options for patients who relapse after haplo-HCT and discuss on how a deeper insight into relapse immunobiology might inform the rational and personalized selection of therapies to improve the largely unsatisfactory clinical outcome of relapsing patients.

Detection of 6pLOH in an aplastic anemia patient by in phase HLA genotyping

Recent studies have reported loss of heterozygosity in the chromosome 6p arms (6pLOH) of acquired aplastic anemia (AA) patients, and in tumor cells trying to escape the autoimmune system. We thus sought to establish detection methods for LOH to investigate the mechanisms underlying AA and tumor immunity. Herein, we report our evaluation of 6pLOH in a patient with severe AA patient using super-high resolution, single-molecule, sequence-based typing (SS-SBT). The highest ratios of 6pLOH detection were observed during the patient's treatment with granulocyte colony stimulating factor (G-CSF). This result suggested that most of the neutrophil precursor cells stimulated by G-CSF already had LOH in the HLA lesion. The SS-SBT method is a simple NGS method that provides complete HLA allele coverage.

Clonal evolution and immune evasion in posttransplantation relapses

Despite the considerable improvements witnessed over the last few decades in the feasibility and safety of allogeneic hematopoietic cell transplantation (allo-HCT) for hematological malignancies, disease relapse continues to represent a frequent occurrence, with largely unsatisfactory salvage options. Recent studies have shed new light on the biology of posttransplantation relapses, demonstrating that they can frequently be explained using an evolutionary perspective: The changes in disease clonal structure and immunogenicity that are often documented at relapse may in fact represent the end results of a process of selection, allowing the outgrowth of variants that are more capable of resisting the therapeutic control of allo-HCT. This review provides an overview of the mechanisms forming the basis of relapse, including clonal evolution, gain of tropism for privileged sites, genomic and nongenomic changes in the HLA asset, and enforcement of immune checkpoints. Finally, this review discusses how these mechanisms may combine in complex patterns and how understanding and untangling these interactions may provide key knowledge for the selection of personalized therapeutic approaches.

Mechanisms of immune escape after allogeneic hematopoietic cell transplantation

Relapse of the original disease is a major cause of death after allogeneic hematopoietic cell transplantation for acute leukemias. There is growing evidence that relapses may be explained not only by resistance to chemotherapy but also by the escape of tumor cells from the control of the allogeneic immune response. Mechanisms of immune evasion can involve abrogation of leukemia cell recognition due to loss of HLA genes, immunosuppression by immune-checkpoint ligand expression, production of anti-inflammatory factors, release of metabolically active enzymes, loss of proinflammatory cytokine production, and acquisition of novel driver mutations that promote leukemia outgrowth. These mechanisms, and therapeutic targeting of immune escape, will be discussed. We divide the evidence in support of immune-escape mechanisms into animal studies, human laboratory studies, and human clinical experience. A better understanding of the molecular pathways connected to immune escape and relapse may help to improve our therapeutic armamentarium against acute myeloid leukemia relapse.

Targeted RNA-sequencing for the quantification of measurable residual disease in acute myeloid leukemia

Great effort is spent on developing therapies to improve the dire outcomes of those diagnosed with acute myeloid leukemia. The methods for quantifying response to therapeutic intervention have however lacked sensitivity. Patients achieving a complete remission as defined by conventional cytomorphological methods therefore remain at risk of subsequent relapse due to disease persistence. Improved risk stratification is possible based on tests designed to detect this residual leukemic burden (measurable residual disease). However, acute myeloid leukemia is a genetically diverse set of diseases, which has made it difficult to develop a single, highly reproducible, and sensitive assay for measurable residual disease. Here we present the development of a digital targeted RNA-sequencing-based approach designed to overcome these limitations by detecting all newly approved European LeukemiaNet molecular targets for measurable residual disease in acute myeloid leukemia in a single standardized assay. Iterative modifications and novel bioinformatics approaches resulted in a greater than 100-fold increase in performance compared with commercially available targeted RNA-sequencing approaches and a limit of detection as low as one leukemic cell in 100,000 cells measured, which is comparable to quantitative polymerase chain reaction analysis, the current gold standard for the detection of measurable residual disease. This assay, which can be customized and expanded, is the first demonstrated use of high-sensitivity RNA-sequencing for measurable residual disease detection in acute myeloid leukemia and could serve as a broadly applicable standardized tool.

Immune checkpoint molecules in acute myeloid leukaemia: managing the double-edged sword

New immunotherapeutic interventions have revolutionized cancer treatment. The immune responsiveness of acute myeloid leukaemia (AML) was first demonstrated by allogeneic stem cell transplantation. In addition, milder immunotherapeutic approaches are exploited. However, the long-term efficacy of these therapies is hampered by various immune resistance and editing mechanisms. In this regard, co-inhibitory signalling pathways have been shown to play a crucial role. Via up-regulation of inhibitory checkpoints, tumour-reactive T cell and Natural Killer cell responses can be strongly impeded. Accordingly, the introduction of checkpoint inhibitors targeting CTLA-4 (CTLA4) and PD-1 (PDCD1, CD279)/PD-L1 (CD274, PDCD1LG1) accomplished a breakthrough in cancer treatment, with impressive clinical responses. Numerous new co-inhibitory players and novel combination therapies are currently investigated for their potential to boost anti-tumour immunity and improve survival of cancer patients. Although the challenge here remains to avoid severe systemic toxicity. This review addresses the involvement of co-inhibitory signalling in AML immune evasion and discusses the opportunities for checkpoint blockers in AML treatment.