Interferon-alpha, but not the ABL-kinase inhibitor imatinib (STI571), induces expression of myeloblastin and a specific T-cell response in chronic myeloid leukemia

Novel treatment strategies for chronic myeloid leukemia

ABSTRACT

Starting with imatinib, tyrosine kinase inhibitors (TKIs) have turned chronic myeloid leukemia (CML) from a lethal blood cancer into a chronic condition. As patients with access to advanced CML care have an almost normal life expectancy, there is a perception that CML is a problem of the past, and one should direct research resources elsewhere. However, a closer look at the current CML landscape reveals a more nuanced picture. Most patients still require life-long TKI therapy to avoid recurrence of active CML. Chronic TKI toxicity and the high costs of the well-tolerated agents remain challenging. Progression to blast phase still occurs, particularly in socioeconomically disadvantaged parts of the world, where high-risk CML at diagnosis is common. Here, we review the prospects of further improving TKIs to achieve optimal suppression of BCR::ABL1 kinase activity, the potential of combining different classes of TKIs, and the current state of BCR::ABL1 degraders. We cover combination therapy approaches to address TKI resistance in the setting of residual leukemia and in advanced CML. Despite the unprecedented success of TKIs in CML, more work is needed to truly finish the job, and we hope to stimulate innovative research aiming to achieve this goal.

Single-cell analysis of immune recognition in chronic myeloid leukemia patients following tyrosine kinase inhibitor discontinuation

Immunological control of residual leukemia cells is thought to occur in patients with chronic myeloid leukemia (CML) that maintain treatment-free remission (TFR) following tyrosine kinase inhibitor (TKI) discontinuation. To study this, we analyzed 55 single-cell RNA and T cell receptor (TCR) sequenced samples (scRNA+TCRαβ-seq) from patients with CML (n = 13, N = 25), other cancers (n = 28), and healthy (n = 7). The high number and active phenotype of natural killer (NK) cells in CML separated them from healthy and other cancers. Most NK cells in CML belonged to the active CD56dim cluster with high expression of GZMA/B, PRF1, CCL3/4, and IFNG, with interactions with leukemic cells via inhibitory LGALS9-TIM3 and PVR-TIGIT interactions. Accordingly, upregulation of LGALS9 was observed in CML target cells and TIM3 in NK cells when co-cultured together. Additionally, we created a classifier to identify TCRs targeting leukemia-associated antigen PR1 and quantified anti-PR1 T cells in 90 CML and 786 healthy TCRβ-sequenced samples. Anti-PR1 T cells were more prevalent in CML, enriched in bone marrow samples, and enriched in the mature, cytotoxic CD8 + TEMRA cluster, especially in a patient maintaining TFR. Our results highlight the role of NK cells and anti-PR1 T cells in anti-leukemic immune responses in CML.

Immunomodulatory Effects of IFNα on T and NK Cells in Chronic Myeloid Leukemia Patients in Deep Molecular Response Preparing for Treatment Discontinuation

A deep and stable molecular response (DMR) is a prerequisite for a successful treatment-free remission (TFR) in chronic myeloid leukemia (CML). In order to better identify and analyze potential candidates of successful TFR, we examined the phenotypic and functional host immune compartment in DMR patients who had received TKI treatment only (TKI-only) or had been previously treated with interferon-alpha (IFNα + TKI) or had received IFNα treatment only (IFNα-only). The T/NK-cell subset distribution, NK- and T-cell cytokine production, activation and maturation markers were measured in 44 patients in DMR treated with IFNα only (9), with IFNα + TKI (11) and with TKI-only (24). IFNα + TKI and TKI-only groups were eligible to TKI discontinuation according to the NCCN and ESMO guidelines (stable MR4 for more than two years). In IFNα-treated patients, we documented an increased number of lymphocytes capable of producing IFNγ and TNFα compared to the TKI-only group. In INFα + TKI patients, the percentage of NKG2C expression and its mean fluorescence intensity were significantly higher compared to the TKI-only group and to the INFα-only group in the CD56dim/CD16+ NK cell subsets (INFα + TKI vs. TKI-only p = 0.041, p = 0.037; INFα + TKI vs. INFα-only p = 0.03, p = 0.033, respectively). Furthermore, in INFα-only treated patients, we observed an increase of NKp46 MFI in the CD56bright/CD16- NK cell subset that becomes significant compared to the INFα + TKI group (p = 0.008). Our data indicate that a previous exposure to IFNα substantially and persistently modified the immune system of CML patients in memory T lymphocytes, differentiated NKG2C+ “long-lived” NK cells responses, even years after the last IFNα contact.

Chronic myeloid leukemia-from the Philadelphia chromosome to specific target drugs: A literature review

Chronic myeloid leukemia (CML) is a myeloproliferative neoplasm and was the first neoplastic disease associated with a well-defined genotypic anomaly - the presence of the Philadelphia chromosome. The advances in cytogenetic and molecular assays are of great importance to the diagnosis, prognosis, treatment, and monitoring of CML. The discovery of the breakpoint cluster region (BCR)-Abelson murine leukemia (ABL) 1 fusion oncogene has revolutionized the treatment of CML patients by allowing the development of targeted drugs that inhibit the tyrosine kinase activity of the BCR-ABL oncoprotein. Tyrosine kinase inhibitors (known as TKIs) are the standard therapy for CML and greatly increase the survival rates, despite adverse effects and the odds of residual disease after discontinuation of treatment. As therapeutic alternatives, the subsequent TKIs lead to faster and deeper molecular remissions; however, with the emergence of resistance to these drugs, immunotherapy appears as an alternative, which may have a cure potential in these patients. Against this background, this article aims at providing an overview on CML clinical management and a summary on the main targeted drugs available in that context.

Interferons: role in cancer therapy

Interferons (IFNs) are a group of signaling cytokines, secreted by host cells to induce protection against various disorders. IFNs can directly impact on tumor cells or indirectly induce the immune system to protect host cells. The expression levels of IFNs and its functions of are excellently modulated in a way to protect host cells from probable toxicities caused by extreme responses. The efficacy of anticancer therapies is correlated to IFNs signaling. Although IFN signaling is involved in induction of antitumor responses, chronic stimulation of the IFN signaling pathway can induce resistance to various antineoplasm therapies. Hence, IFNs are expressed by both cancer and immune cells, and modulate their biological function. Understanding this mechanism of action might be a key target of combination therapies.

C/EBPβ is a critical mediator of IFN-α-induced exhaustion of chronic myeloid leukemia stem cells

Even in the era of ABL tyrosine kinase inhibitors, eradication of chronic myeloid leukemia (CML) stem cells is necessary for complete cure of the disease. Interferon-α (IFN-α) has long been used for the treatment of chronic-phase CML, but its mechanisms of action against CML stem cells remain unclear. We found that IFN-α upregulated CCAAT/enhancer binding protein β (C/EBPβ) in BCR-ABL-expressing mouse cells by activating STAT1 and STAT5, which were recruited to a newly identified 3' distal enhancer of Cebpb that contains tandemly aligned IFN-γ-activated site elements. Suppression or deletion of the IFN-γ-activated site elements abrogated IFN-α-dependent upregulation of C/EBPβ. IFN-α induced differentiation and exhaustion of CML stem cells, both in vitro and in vivo, in a C/EBPβ-dependent manner. In addition, IFN-α upregulated C/EBPβ and induced exhaustion of lineage^- CD34⁺ cells from CML patients. Collectively, these results clearly indicate that C/EBPβ is a critical mediator of IFN-α-induced differentiation and exhaustion of CML stem cells.

Differential roles of STAT1 and STAT2 in the sensitivity of JAK2V617F- vs. BCR-ABL-positive cells to interferon alpha

BACKGROUND

Interferon alpha (IFNa) monotherapy is recommended as the standard therapy in polycythemia vera (PV) but not in chronic myeloid leukemia (CML). Here, we investigated the mechanisms of IFNa efficacy in JAK2V617F- vs. BCR-ABL-positive cells.

METHODS

Gene expression microarrays and RT-qPCR of PV vs. CML patient PBMCs and CD34+ cells and of the murine cell line 32D expressing JAK2V617F or BCR-ABL were used to analyze and compare interferon-stimulated gene (ISG) expression. Furthermore, using CRISPR/Cas9n technology, targeted disruption of STAT1 or STAT2, respectively, was performed in 32D-BCR-ABL and 32D-JAK2V617F cells to evaluate the role of these transcription factors for IFNa efficacy. The knockout cell lines were reconstituted with STAT1, STAT2, STAT1Y701F, or STAT2Y689F to analyze the importance of wild-type and phosphomutant STATs for the IFNa response. ChIP-seq and ChIP were performed to correlate histone marks with ISG expression.

RESULTS

Microarray analysis and RT-qPCR revealed significant upregulation of ISGs in 32D-JAK2V617F but downregulation in 32D-BCR-ABL cells, and these effects were reversed by tyrosine kinase inhibitor (TKI) treatment. Similar expression patterns were confirmed in human cell lines, primary PV and CML patient PBMCs and CD34+ cells, demonstrating that these effects are operational in patients. IFNa treatment increased Stat1, Stat2, and Irf9 mRNA as well as pY-STAT1 in all cell lines; however, viability was specifically decreased in 32D-JAK2V617F. STAT1 or STAT2 knockout and reconstitution with wild-type or phospho-deficient STAT mutants demonstrated the necessity of STAT2 for IFNa-induced STAT1 phosphorylation in BCR-ABL- but not in JAK2V617F-expressing cells. STAT1 was essential for IFNa activity in both BCR-ABL- and JAK2V617F-positive cells. Furthermore, ChIP experiments demonstrate higher repressive and lower active chromatin marks at the promoters of ISGs in BCR-ABL-expressing cells.

CONCLUSIONS

JAK2V617F but not BCR-ABL sensitizes MPN cells to interferon, and this effect was dependent on STAT1. Moreover, STAT2 is a survival factor in BCR-ABL- and JAK2V617F-positive cells but an IFNa-sensitizing factor solely in 32D-JAK2V617F cells by upregulation of STAT1 expression.

Can increased immunogenicity in chronic myeloid leukemia improve outcomes?

INTRODUCTION

Chronic myeloid leukemia (CML) has long been thought to be the model disease for immunotherapy with its characteristic BCR-ABL fusion protein. Although targeted therapy using tyrosine kinase inhibitors (TKIs) is highly effective at inducing remission, most patients require life-long TKI to decrease the risk of relapse. In recent years, much effort has been devoted to finding ways to eliminate CML stem cells (LSCs); the source of disease persistence. Areas covered: In this review, the authors present recent immunologic findings pertinent to CML, vaccinations targeting leukemia antigens, interferon combination therapies, and other emerging strategies aimed at increasing immunogenicity and improving outcomes in patients with CML. Recent publications and abstracts found in Pubmed and hematology/oncology meetings related to these topics were identified and incorporated into this review. Expert commentary: Further understanding of the immune system and antigenic composition of LSCs has allowed for novel therapeutic development. Immunotherapies are effective at the malignant stem cell level and combining these approaches with TKI is a promising option. Despite ongoing challenges, it is increasingly recognized that a cure may be achievable through immunotherapies.

Development, Function, and Clinical Significance of Plasmacytoid Dendritic Cells in Chronic Myeloid Leukemia

Plasmacytoid dendritic cells (pDC) are the main producers of a key T-cell-stimulatory cytokine, IFNα, and critical regulators of antiviral immunity. Chronic myeloid leukemia (CML) is caused by BCR-ABL, which is an oncogenic tyrosine kinase that can be effectively inhibited with ABL-selective tyrosine kinase inhibitors (TKI). BCR-ABL-induced suppression of the transcription factor interferon regulatory factor 8 was previously proposed to block pDC development and compromise immune surveillance in CML. Here, we demonstrate that pDCs in newly diagnosed CML (CML-pDC) develop quantitatively normal and are frequently positive for the costimulatory antigen CD86. They originate from low-level BCR-ABL-expressing precursors. CML-pDCs also retain their competence to maturate and to secrete IFN. RNA sequencing reveals a strong inflammatory gene expression signature in CML-pDCs. Patients with high CML-pDC counts at diagnosis achieve inferior rates of deep molecular remission (MR) under nilotinib, unless nilotinib therapy is combined with IFN, which strongly suppresses circulating pDC counts. Although most pDCs are BCR-ABL-negative in MR, a substantial proportion of BCR-ABL + CML-pDCs persists under TKI treatment. This could be of relevance, because CML-pDCs elicit CD8+ T cells, which protect wild-type mice from CML. Together, pDCs are identified as novel functional DC population in CML, regulating antileukemic immunity and treatment outcome in CML.Significance: CML-pDC originates from low-level BCR-ABL expressing stem cells into a functional immunogenic DC-population regulating antileukemic immunity and treatment outcome in CML. Cancer Res; 78(21); 6223-34. ©2018 AACR.

BCR-ABL-specific CD4+ T-helper cells promote the priming of antigen-specific cytotoxic T cells via dendritic cells

The advent of tyrosine kinase inhibitor (TKI) therapy markedly improved the outcome of patients with chronic-phase chronic myeloid leukemia (CML). However, the poor prognosis of patients with advanced-phase CML and the lifelong dependency on TKIs are remaining challenges; therefore, an effective therapeutic has been sought. The BCR-ABL p210 fusion protein's junction region represents a leukemia-specific neoantigen and is thus an attractive target for antigen-specific T-cell immunotherapy. BCR-ABL p210 fusion-region-specific CD4+ T-helper (Th) cells possess antileukemic potential, but their function remains unclear. In this study, we established a BCR-ABL p210 b3a2 fusion-region-specific CD4+ Th-cell clone (b3a2-specific Th clone) and examined its dendritic cell (DC)-mediated antileukemic potential. The b3a2-specific Th clone recognized the b3a2 peptide in the context of HLA-DRB1*09:01 and exhibited a Th1 profile. Activation of this clone through T-cell antigen receptor stimulation triggered DC maturation, as indicated by upregulated production of CD86 and IL-12p70 by DCs, which depended on CD40 ligation by CD40L expressed on b3a2-specific Th cells. Moreover, in the presence of HLA-A*24:02-restricted Wilms tumor 1 (WT1)235-243 peptide, DCs conditioned by b3a2-specific Th cells efficiently stimulated the primary expansion of WTI-specific cytotoxic T lymphocytes (CTLs). The expanded CTLs were cytotoxic toward WT1235-243-peptide-loaded HLA-A*24:02-positive cell lines and exerted a potent antileukemic effect in vivo. However, the b3a2-specific Th-clone-mediated antileukemic CTL responses were strongly inhibited by both TKIs and interferon-α. Our findings indicate a crucial role of b3a2-specific Th cells in leukemia antigen-specific CTL-mediated immunity and provide an experimental basis for establishing novel CML immunotherapies.

CML patients with deep molecular responses to TKI have restored immune effectors and decreased PD-1 and immune suppressors

Increased immune suppressors and PD-1 abrogates effector responses in CML patients at diagnosis. Enhanced net effector immune responses and decreased PD-1 and immune suppressors may promote sustained deep molecular response in CML.

Imatinib withdrawal syndrome and longer duration of imatinib have a close association with a lower molecular relapse after treatment discontinuation: the KID study

The aim of the Korean Imatinib Discontinuation Study was to identify predictors for safe and successful imatinib discontinuation. A total of 90 patients with a follow-up of ≥12 months were analyzed. After a median follow-up of 26.6 months after imatinib discontinuation, 37 patients lost the major molecular response. The probability of sustained major molecular response at 12 months and 24 months was 62.2% and 58.5%, respectively. All 37 patients who lost major molecular response were retreated with imatinib therapy for a median of 16.9 months, and all achieved major molecular response again at a median of 3.9 months after resuming imatinib therapy. We observed newly developed or worsened musculoskeletal pain and pruritus in 27 (30%) patients after imatinib discontinuation. Imatinib withdrawal syndrome was associated with a higher probability of sustained major molecular response (P=0.003) and showed a trend for a longer time to major molecular response loss (P=0.098). Positivity (defined as ≥ 17 positive chambers) of digital polymerase chain reaction at screening and longer imatinib duration before imatinib discontinuation were associated with a higher probability of sustained major molecular response. Our data demonstrated that the occurrence of imatinib withdrawal syndrome after imatinib discontinuation and longer duration of imatinib were associated with a lower rate of molecular relapse. In addition, minimal residual leukemia measured by digital polymerase chain reaction had a trend for a higher molecular relapse. (Trial registered at ClinicalTrials.gov: NCT01564836).

The implication of cancer progenitor cells and the role of epigenetics in the development of novel therapeutic strategies for chronic myeloid leukemia

SIGNIFICANCE

Chronic myeloid leukemia (CML) involves the malignant transformation of hematopoietic stem cells, defined largely by the Philadelphia chromosome and expression of the breakpoint cluster region-Abelson (BCR-ABL) oncoprotein. Pharmacological tyrosine kinase inhibitors (TKIs), including imatinib mesylate, have overcome limitations in conventional treatment for the improved clinical management of CML.

RECENT ADVANCES

Accumulated evidence has led to the identification of a subpopulation of quiescent leukemia progenitor cells with stem-like self renewal properties that may initiate leukemogenesis, which are also shown to be present in residual disease due to their insensitivity to tyrosine kinase inhibition.

CRITICAL ISSUES

The characterization of quiescent leukemia progenitor cells as a unique cell population in CML pathogenesis has become critical with the complete elucidation of mechanisms involved in their survival independent of BCR-ABL that is important in the development of novel anticancer strategies. Understanding of these functional pathways in CML progenitor cells will allow for their selective therapeutic targeting. In addition, disease pathogenesis and drug responsiveness is also thought to be modulated by epigenetic regulatory mechanisms such as DNA methylation, histone acetylation, and microRNA expression, with a capacity to control CML-associated gene transcription.

FUTURE DIRECTIONS

A number of compounds in combination with TKIs are under preclinical and clinical investigation to assess their synergistic potential in targeting leukemic progenitor cells and/or the epigenome in CML. Despite the collective promise, further research is required in order to refine understanding, and, ultimately, advance antileukemic therapeutic strategies.

Interferon alpha 2 maintenance therapy may enable high rates of treatment discontinuation in chronic myeloid leukemia

A minority of chronic myeloid leukemia (CML) patients is capable of successfully discontinuing imatinib. Treatment modalities to increase this proportion are currently unknown. Here, we assessed the role of interferon alpha 2a (IFN) on therapy discontinuation in a previously reported cohort of 20 chronic phase CML patients who were treated upfront with IFN alpha plus imatinib followed by IFN monotherapy to maintain cytogenetic or molecular remission (MR) after imatinib discontinuation. After a median follow-up of 7.9 years (range, 5.2-12.2), relapse-free survival was 73% (8/11 patients) and 84% (5/6 patients) for patients who discontinued imatinib in major MR (MMR) and MR4/MR4.5, respectively. Ten patients discontinued IFN after a median of 4.5 years (range, 0.24-9.3). After a median of 2.8 years (range, 0.7-5.1), nine of them remain in ongoing treatment-free remission with MR5 (n=6) and MR4.5 (n=3). The four patients who still administer IFN are in stable MR5, MR4.5, MR4, and MMR, respectively. In conclusion, an IFN/imatinib induction treatment followed by a temporary IFN maintenance therapy may enable a high rate of treatment discontinuation in CML patients in at least MMR when stopping imatinib.

Is there a role for allogeneic transplantation in chronic myeloid leukemia?

Allogeneic hemopoietic cell transplantation has moved away from being the standard of care for patients with chronic myeloid leukemia (CML). Its role is currently limited to an unsatisfactory response to therapy with tyrosine kinase inhibitors as well as advanced stages of the disease. The advent of tyrosine kinase inhibitors has been one of the most remarkable advances in any form of cancer. Never-the-less, as a definitive procedure, allogeneic transplantation remains the only curative modality and its use in carefully selected patients, who have an inadequate response, has been increasingly recognized. It remains a standard of care for patients who present with blast crisis CML and is often used also in accelerated phase. The future for patients with CML has become so much brighter over the past decade but new issues and considerations continually emerge.

Enlarged memory T-cell pool and enhanced Th1-type responses in chronic myeloid leukemia patients who have successfully discontinued IFN-α monotherapy

A small proportion of chronic myeloid leukemia patients treated with interferon-α (IFN-α) monotherapy are able to discontinue the treatment without disease relapse although residual leukemia cells are present. Recently, we showed that these patients have increased amount of NK-cells and a distinct blood cytokine profile. We now aimed to study the function of NK- and T-cells in order to understand the role of the immune system in maintaining the treatment response after IFN-α discontinuation. The study included 13 patients: 5 patients were still treated with IFN-α monotherapy (IFN-ON, median treatment time 163 months) and 8 had stopped the treatment successfully (IFN-OFF, median time without therapy 42 months). Detailed immunophenotype and cytokine production of NK- and T-cells was analyzed with flow cytometry. In addition, the cytotoxicity of NK-cells was studied using K562 as target cells and both the degranulation and direct killing was measured. Compared to healthy controls, IFN-OFF patients had increased proportion of CD4⁺ effector memory (CCR7⁻CD45RA⁻; median 23% vs. healthy 16%, p = 0.009) and CD8⁺ central memory T-cells (CCR7⁺CD45RA⁻; median 26% vs. healthy 14%, p = 0.004). Further, upon stimulation the IFN-γ/TNF-α cytokine secretion by CD4⁺ T-cells was significantly enhanced in IFN-OFF patients (median 13.7% vs. healthy 7.8%, p = 0.01), and CD4+ effector and central memory cells were the main cytokine producers. No similar increase was observed in IFN-ON group (6.5%). In addition, the proportion of NK-cells was significantly increased in IFN-OFF patients (median IFN-OFF 24%, healthy 13%, p = 0.04), but their direct killing of K562 cells was impaired. The cytotoxicity of NK-cells was also diminished in IFN-ON patients. To conclude, in addition to elevated NK-cell count, IFN-OFF patients have increased amount of memory T-cells, which are able to induce strong cytokine response upon stimulation. This activity may contribute to the maintenance of prolonged remission after successful IFN-α discontinuation.

Safety and efficacy of imatinib cessation for CML patients with stable undetectable minimal residual disease: results from the TWISTER study

Most patients with chronic myeloid leukemia (CML) treated with imatinib will relapse if treatment is withdrawn. We conducted a prospective clinical trial of imatinib withdrawal in 40 chronic-phase CML patients who had sustained undetectable minimal residual disease (UMRD) by conventional quantitative polymerase chain reaction (PCR) on imatinib for at least 2 years. Patients stopped imatinib and were monitored frequently for molecular relapse. At 24 months, the actuarial estimate of stable treatment-free remission was 47.1%. Most relapses occurred within 4 months of stopping imatinib, and no relapses beyond 27 months were seen. In the 21 patients treated with interferon before imatinib, a shorter duration of interferon treatment before imatinib was significantly associated with relapse risk, as was slower achievement of UMRD after switching to imatinib. Highly sensitive patient-specific BCR-ABL DNA PCR showed persistence of the original CML clone in all patients with stable UMRD, even several years after imatinib withdrawal. No patients with molecular relapse after discontinuation have progressed or developed BCR-ABL mutations (median follow-up, 42 months). All patients who relapsed remained sensitive to imatinib re-treatment. These results confirm the safety and efficacy of a trial of imatinib withdrawal in stable UMRD with frequent, sensitive molecular monitoring and early rescue of molecular relapse.

Imatinib mesylate and nilotinib affect MHC-class I presentation by modulating the proteasomal processing of antigenic peptides

Imatinib (IM) has been described to modulate the function of dendritic cells and T lymphocytes and to affect the expression of antigen in CML cells. In our study, we investigated the effect of the tyrosine kinase inhibitors IM and nilotinib (NI) on antigen presentation and processing by analyzing the proteasomal activity in CML cell lines and patient samples. We used a biotinylated active site-directed probe, which covalently binds to the proteasomally active beta-subunits in an activity-dependent fashion. Additionally, we analyzed the cleavage and processing of HLA-A3/11- and HLA-B8-binding peptides derived from BCR-ABL by IM- or NI-treated isolated 20S immunoproteasomes using mass spectrometry. We found that IM treatment leads to a reduction in MHC-class I expression which is in line with the inhibition of proteasomal activity. This process is independent of BCR-ABL or apoptosis induction. In vitro digestion experiments using purified proteasomes showed that generation of epitope-precursor peptides was significantly altered in the presence of NI and IM. Treatment of the immunoproteasome with these compounds resulted in an almost complete reduction in the generation of long precursor peptides for the HLA-A3/A11 and -B8 epitopes while processing of the short peptide sequences increased. Treatment of isolated 20S proteasomes with serine-/threonine- and tyrosine-specific phosphatases induced a significant downregulation of the proteasomal activity further indicating that phosphorylation of the proteasome regulates its function and antigen processing. Our results demonstrate that IM and NI can affect the immunogenicity of malignant cells by modulating proteasomal degradation and the repertoire of processed T cell epitopes.

Is going for cure in chronic myeloid leukemia possible and justifiable?

After more than a decade of treatment of chronic myeloid leukemia (CML) patients with the BCR-ABL tyrosine kinase inhibitor imatinib, and despite the impressive clinical results of this targeted therapeutic, many questions remain unresolved. One major question is how to cure CML, and the next step for the future will be to address this key issue. CML is a good model of cancer. The fact that the majority of CML patients who respond very well but discontinue tyrosine kinase inhibitors later show evidence of molecular recurrence focuses attention on the need for further research on leukemic stem cells. The challenge now is to understand why, after stopping treatment, the leukemia recurs in some patients but not in others. If we win this battle, this progress will certainly benefit the treatment and management of other leukemias and solid tumors and will validate this new topic.

Re-emergence of interferon-α in the treatment of chronic myeloid leukemia

Treatment for chronic myeloid leukemia (CML) has evolved from chemotherapy (busulfan, hydroxyurea) to interferon-α (IFNα), and finally to tyrosine kinase inhibitors such as imatinib. Although imatinib has profoundly improved outcomes for patients with CML, it has limitations. Most significantly, imatinib cannot eradicate CML primitive progenitors, which likely accounts for the high relapse rate when imatinib is discontinued. IFNα, unlike imatinib, preferentially targets CML stem cells. Early studies with IFNα in CML demonstrated its ability to induce cytogenetic remission. Moreover, a small percentage of patients treated with IFNα were able to sustain durable remissions after discontinuing therapy and were probably cured. The mechanisms by which IFNα exerts its antitumor activity in CML are not well understood; however, activation of leukemia-specific immunity may have a role. Some clinical studies have demonstrated that the combination of imatinib and IFNα is superior to either therapy alone, perhaps because of their different mechanisms of action. Nonetheless, the side effects of IFNα often impede its administration, especially in combination therapy. Here, we review the role of IFNα in CML treatment and the recent developments that have renewed interest in this once standard therapy for patients with CML.

Broad cross-presentation of the hematopoietically derived PR1 antigen on solid tumors leads to susceptibility to PR1-targeted immunotherapy

PR1 is a HLA-A2-restricted peptide that has been targeted successfully in myeloid leukemia with immunotherapy. PR1 is derived from the neutrophil granule proteases proteinase 3 (P3) and neutrophil elastase (NE), which are both found in the tumor microenvironment. We recently showed that P3 and NE are taken up and cross-presented by normal and leukemia-derived APCs, and that NE is taken up by breast cancer cells. We now extend our findings to show that P3 and NE are taken up and cross-presented by human solid tumors. We further show that PR1 cross-presentation renders human breast cancer and melanoma cells susceptible to killing by PR1-specific CTLs (PR1-CTL) and the anti-PR1/HLA-A2 Ab 8F4. We also show PR1-CTL in peripheral blood from patients with breast cancer and melanoma. Together, our data identify cross-presentation as a novel mechanism through which cells that lack endogenous expression of an Ag become susceptible to therapies that target cross-presented Ags and suggest PR1 as a broadly expressed tumor Ag.

Pushing the limits of targeted therapy in chronic myeloid leukaemia

Tyrosine kinase inhibitor (TKI) therapy targeting the BCR-ABL1 kinase is effective against chronic myeloid leukaemia (CML), but is not curative for most patients. Minimal residual disease (MRD) is thought to reside in TKI-insensitive leukaemia stem cells (LSCs) that are not fully addicted to BCR-ABL1. Recent conceptual advances in both CML biology and therapeutic intervention have increased the potential for the elimination of CML cells, including LSCs, through simultaneous inhibition of BCR-ABL1 and other newly identified, crucial targets.

Development of an effective therapy for chronic myelogenous leukemia

Targeted small-molecule drugs have revolutionized treatment of chronic myeloid leukemia (CML) during the last decade. These agents interrupt a constitutively active BCR-ABL, the causative agent for CML, by interfering with adenosine 5' triphosphate-dependent ABL tyrosine kinase. Although the efficacy of tyrosine kinase inhibitors (TKIs) has resulted in overall survival of greater than 90%, TKIs are not curative. Moreover, no currently approved TKIs are effective against the T315I BCR-ABL variant. However, a new generation of TKIs with activity against T315I is on the horizon. We will highlight the clinical utility of historical CML therapeutics, those used today (first- and second-generation TKIs), and discuss treatment modalities that are under development. Recent advances have illuminated the complexity of CML, especially within the marrow microenvironment. We contend that the key to curing CML will involve strategies beyond targeting BCR-ABL because primitive human CML stem cells are not dependent on BCR-ABL. Ultimately, drug combinations or exploiting synthetic lethality may transform responses into definitive cures for CML.

Biological therapy and the immune system in patients with chronic myeloid leukemia

Chronic myeloid leukemia (CML) is a clonal myeloproliferative disorder of hematopoietic stem cells that has been recognized as a disease responsive to immunotherapy. Despite the huge success of the tyrosine kinase inhibitors (TKIs), CML remains for the most part incurable, probably due to treatment resistance of leukemic stem cells, which are responsible for rapid disease relapse after discontinuation of therapy. Only allogeneic stem cell transplantation enables disease eradication. In addition to the Bcr-Abl1 oncoprotein, TKIs also inhibit off-target kinases (e.g. c-kit, Src, Tec), some of them having physiological functions in immune responses. In vitro studies have implied immunomodulatory effects of TKIs and interferon-alpha (IFN-α), but comprehensive information from in vivo analyses is missing. This review summarizes the recent advances in the field of immunology of CML, including basic information about leukemia-associated antigens and peptide vaccines, that could lead to the incorporation of TKIs and IFN-α in future therapeutic, potentially curative, interventions for CML.

Chronic myeloid leukemia patients in prolonged remission following interferon-α monotherapy have distinct cytokine and oligoclonal lymphocyte profile

Before the era of tyrosine kinase inhibitors (TKIs), interferon-alpha (IFN-α) was the treatment of choice in chronic myeloid leukemia (CML). Curiously, some IFN-α treated patients were able to discontinue therapy without disease progression. The aim of this project was to study the immunomodulatory effects of IFN-α in CML patients in prolonged remission and isolate biological markers predicting response. Due to rarity of patients on IFN-α monotherapy, a relatively small cohort of patients still on treatment (IFN-ON, n = 10, median therapy duration 11.8 years) or had discontinued IFN-α therapy but remained in remission for >2 years (IFN-OFF, n = 9) were studied. The lymphocyte immunophenotype was analyzed with a comprehensive flow cytometry panel and plasma cytokine levels were measured with multiplex bead-based assay. In addition, the clonality status of different lymphocyte subpopulations was analyzed by TCR γ/δ rearrangement assay. Median NK-cell absolute number and proportion from lymphocytes in blood was higher in IFN-OFF patients as compared to IFN-ON patients or controls (0.42, 0.19, 0.21×10⁹/L; 26%, 12%, 11%, respectively, p<0.001). The proportion of CD8+ T-cells was significantly increased in both patient groups and a larger proportion of T-cells expressed CD45RO. Most (95%) patients had significant numbers of oligoclonal lymphocytes characterized by T-cell receptor γ/δ rearrangements. Strikingly, in the majority of patients (79%) a distinct clonal Vγ9 gene rearrangement was observed residing in γδ⁺ T-cell population. Similar unique clonality pattern was not observed in TKI treated CML patients. Plasma eotaxin and MCP-1 cytokines were significantly increased in IFN-OFF patients. Despite the limited number of patients, our data indicates that IFN-α treated CML patients in remission have increased numbers of NK-cells and clonal γδ⁺ T-cells and a unique plasma cytokine profile. These factors may relate to anti-leukemic effects of IFN-α in this specific group of patients and account for prolonged therapy responses even after drug discontinuation.

Combination of pegylated IFN-α2b with imatinib increases molecular response rates in patients with low- or intermediate-risk chronic myeloid leukemia

Biologic and clinical observations suggest that combining imatinib with IFN-α may improve treatment outcome in chronic myeloid leukemia (CML). We randomized newly diagnosed chronic-phase CML patients with a low or intermediate Sokal risk score and in imatinib-induced complete hematologic remission either to receive a combination of pegylated IFN-α2b (Peg-IFN-α2b) 50 μg weekly and imatinib 400 mg daily (n = 56) or to receive imatinib 400 mg daily monotherapy (n = 56). The primary endpoint was the major molecular response (MMR) rate at 12 months after randomization. In both arms, 4 patients (7%) discontinued imatinib treatment (1 because of blastic transformation in imatinib arm). In addition, in the combination arm, 34 patients (61%) discontinued Peg-IFN-α2b, most because of toxicity. The MMR rate at 12 months was significantly higher in the imatinib plus Peg-IFN-α2b arm (82%) compared with the imatinib monotherapy arm (54%; intention-to-treat, P = .002). The MMR rate increased with the duration of Peg-IFN-α2b treatment (< 12-week MMR rate 67%, > 12-week MMR rate 91%). Thus, the addition of even relatively short periods of Peg-IFN-α2b to imatinib markedly increased the MMR rate at 12 months of therapy. Lower doses of Peg-IFN-α2b may enhance tolerability while retaining efficacy and could be considered in future protocols with curative intent.

Interferon: current status and future prospects in cancer therapy

Type I interferons (IFNs) exhibit antiproliferative activity and apoptotic effects, and regulate an immune response by activating multiple cells types, including dendritic cells, cytotoxic T cells, and natural killer cells. Most recently, a report in the literature identified dysfunctional induction of a type I IFN response in cancer stem cells--specifically, breast cancer-initiating cells, implicating this defect in progression to breast cancer. Indeed, accumulating evidence suggests that cancer stem cells/cancer-initiating cells are prevalent in leukemias and solid tumors, are resistant to chemotherapy and radiation therapy, and therefore likely contribute to tumor recurrence. IFN-β treatment of human glioma xenografts leads to disruption of the vascular niche of glioma stem cells, in further support of a potential therapeutic effect of IFN treatment in limiting cancer stem cells. The implications are that restoring an IFN response, or enhancing an IFN response, may invoke a reduction, or elimination of both cancer stem cells and tumor cells. In this review, the clinical application of type I IFNs, mainly IFN-αs, will be reviewed.

Interferon-α in acute myeloid leukemia: an old drug revisited

Interferon-α (IFN-α), a type I IFN, is a well-known antitumoral agent. The investigation of its clinical properties in acute myeloid leukemia (AML) has been prompted by its pleiotropic antiproliferative and immune effects. So far, integration of IFN-α in the therapeutic arsenal against AML has been modest in view of the divergent results of clinical trials. Recent insights into the key pharmacokinetic determinants of the clinical efficacy of IFN along with advances in its pharmaceutical formulation, have sparked renewed interest in its use. This paper reviews the possible applicability of IFN-α in the treatment of AML and provides a rational basis to re-explore its efficacy in clinical trials.

[Chronic myeloid leukemia. Diagnostics, therapy and future strategy]

Survival of patients with chronic myeloid leukemia (CML) has dramatically improved with the introduction of the BCR-ABL-specific tyrosine kinase inhibitor imatinib. As a rule patients on therapy with imatinib achieve permanent complete cytogenetic and molecular remission. Patients who are primarily refractive to imatinib or lose remission achieved using imatinib are in the minority. This group has a poor prognosis. This article gives a transparent review of the diagnostics necessary when CML is primarily diagnosed and for assessment of the response during the course of the therapy. The guidelines developed for this procedure by the European leukemia network on the type and frequency of surveillance controls as well as the diagnostic criteria for imatinib resistance or suboptimal response will be presented. The indications for allogenic stem cell transplantation and the administration of second generation BCR-ABL inhibitors will be discussed as therapeutic alternatives in cases of imatinib failure in a stage-specific manner. Finally a view on therapy targets and forms of future first-line therapy of CML will be given.

Minimal residual disease and discontinuation of therapy in chronic myeloid leukemia: can we aim at a cure?

Patients with chronic myeloid leukemia (CML) who have achieved a complete molecular response (CMR) defined by no detectable BCR-ABL mRNA on imatinib (IM) treatment often ask whether it is necessary for treatment to continue. We now know that approximately 40% of patients with a stable CMR for at least 2 years are able to stop IM treatment and remain in molecular remission for at least 2 years. This exciting observation has raised hopes that many patients can be cured of CML without the need for transplantation and its attendant risks. One might argue that for many patients maintenance therapy with IM or an alternative kinase inhibitor is so well tolerated that there is no imperative to stop treatment; however, chronic medical therapy may be associated with impaired quality of life and reduced compliance. Inferences about the biology of CML in patients responding to kinase inhibitors can be drawn from clinical experience, molecular monitoring data, and experimental observations. We summarize this information herein, and propose 3 possible pathways to "cure" of CML by kinase inhibitors: stem-cell depletion, stem-cell exhaustion, and immunological control.

Changes of immunological profiles in patients with chronic myeloid leukemia in the course of treatment

In the previous paper of ours we compared, prior to start any treatment, a number of immunological parameters in 24 chronic myeloid leukemia patients with the same number of healthy subjects matched by age and sex. We found significant differences in the levels of immunoglobulins, the C4 component of complement, the C-reactive protein, interleukin 6, the composition of lymphocyte population and the production of some cytokines by stimulated CD3+ cells. Eleven of these patients were followed longitudinally. After treatment with hydroxyurea, interferon alpha, imatinib mesylate and dasatinib, or various combinations thereof, hematological remission was achieved in all patients and complete cytogenetic remission in nine of them. There was a nearly general tendency towards normalization of the abnormalities observed in the patients at their enrollment.

Immunotherapy of chronic myeloid leukemia: present state and future prospects

In spite of the considerable successes that have been achieved in the treatment of chronic myeloid leukemia (CML), cure for the disease can only be obtained by the present means in a rather small minority of patients. During the past decade, considerable progress has been made in the understanding of the immunology of CML, which has raised hopes that this disease may be curable by supplementing the current targeted chemotherapy with immunotherapeutic approaches. More than ten small-scale clinical trials have been carried out with experimental vaccines predominantly based on the p210bcr-abl fusion protein. Their results suggested beneficial effects in some patients. Recent data obtained in human patients as well as in animal models indicate that the p210bcr-abl protein does not carry the immunodominant epitope(s). These observations, combined with the recognition of an ever increasing number of other immunogenic proteins in CML cells, strongly support the concept that gene-modified, cell-based vaccines containing the full spectrum of tumor antigens might be the most effective immunotherapeutic approach. Recently created mathematical models have provided important leads for the timing of the combination of targeted drug therapy with vaccine administration. A strategy of how targeted drug therapy might be combined with vaccination is outlined.

Do we have to kill the last CML cell?

Previous experience in the treatment of chronic myeloid leukaemia (CML) has shown that the achievement of clinical, morphological and cytogenetic remission does not indicate eradication of the disease. A complete molecular response (CMR; no detectable BCR-ABL mRNA) represents a deeper level of response, but even CMR is not a guarantee of elimination of the leukaemia, because the significance of CMR is determined by the detection limit of the assay that is used. Two studies of imatinib cessation in CMR are underway, cumulatively involving over 100 patients. The current estimated rate of stable CMR after stopping imatinib is approximately 40%, but the duration of follow-up is relatively short. The factors that determine relapse risk are yet to be identified. The intrinsic capacity of any residual leukaemia [corrected] cells to proliferate following the withdrawal of treatment may be important, but there may also be a role for immunological suppression of the leukaemia [corrected] clone. No currently available test can formally prove that the leukaemic clone is eradicated. Here we discuss the sensitive measurement of minimal residual disease, and speculate on the biology of BCR-ABL-positive cells that may persist after effective therapy of CML.

Patients with chronic myeloid leukemia who maintain a complete molecular response after stopping imatinib treatment have evidence of persistent leukemia by DNA PCR

Around 40-50% of patients with chronic myeloid leukemia (CML) who achieve a stable complete molecular response (CMR; undetectable breakpoint cluster region-Abelson leukemia gene human homolog 1 (BCR-ABL1) mRNA) on imatinib can stop therapy and remain in CMR, at least for several years. This raises the possibility that imatinib therapy may not need to be continued indefinitely in some CML patients. Two possible explanations for this observation are (1) CML has been eradicated or (2) residual leukemic cells fail to proliferate despite the absence of ongoing kinase inhibition. We used a highly sensitive patient-specific nested quantitative PCR to look for evidence of genomic BCR-ABL1 DNA in patients who sustained CMR after stopping imatinib therapy. Seven of eight patients who sustained CMR off therapy had BCR-ABL1 DNA detected at least once after stopping imatinib, but none has relapsed (follow-up 12-41 months). BCR-ABL1 DNA levels increased in all of the 10 patients who lost CMR soon after imatinib cessation, whereas serial testing of patients in sustained CMR showed a stable level of BCR-ABL1 DNA. This more sensitive assay for BCR-ABL1 provides evidence that even patients who maintain a CMR after stopping imatinib may harbor residual leukemia. A search for intrinsic or extrinsic (for example, immunological) causes for this drug-free leukemic suppression is now indicated.

The response to imatinib and interferon-alpha is more rapid than the response to imatinib alone: a retrospective analysis of 495 Philadelphia-positive chronic myeloid leukemia patients in early chronic phase

Before the introduction of imatinib, interferon alpha-based regimens were the gold standard for treatment of early chronic phase chronic myeloid leukemia patients. The combination of IFN-alpha with imatinib is currently being investigated in at least two large clinical trials, the German CML Study IV and the French SPIRIT trial. We reviewed the cytogenetic and molecular responses of 76 early chronic phase chronic myeloid leukemia patients who were treated with imatinib and interferon-alpha and of 419 early chronic phase chronic myeloid leukemia patients treated with imatinib alone front-line. The complete cytogenetic response rate was higher in the IM+IFN-alpha group than in the imatinib group at six months (60% vs. 42%; P=0.003), but not at 48 months (88% vs. 88%). The durability of the complete cytogenetic response was similar in the two groups with 94% and 91% of complete cytogenetic responders in continuous complete cytogenetic response at 48 months (P=0.56). The major molecular response rate was higher in the IM+IFN-alpha group at six months (58% vs. 34%; P=0.0001) and 12 months (67% vs. 47%; P=0.001) but not later on (65% vs. 57% at 48 months; P=0.25). Overall and progression free survival were comparable in the two groups; a significant trend to a better event free survival was observed in patients treated with PegIFNalpha (91% vs. 78%; P=0.02). These data suggest that the response to the combination treatment is more rapid. It is not yet known how much a rapid reduction will influence the longer-term overall and progression free survival, and the cure rate.

Sustained molecular response with interferon alfa maintenance after induction therapy with imatinib plus interferon alfa in patients with chronic myeloid leukemia

PURPOSE

Imatinib induces sustained remissions in patients with chronic myelogenous leukemia (CML), but fails to eradicate CML stem cells. This is of major concern regarding the issues of cure, long-term imatinib tolerability, and imatinib resistance. We therefore asked whether interferon alfa-2a (IFN) alone could maintain molecular remissions achieved by a prior combination therapy with imatinib and IFN.

PATIENTS AND METHODS

Imatinib therapy was stopped in 20 patients who had concomitantly been pretreated with imatinib and IFN for a median of 2.4 years (range, 0.2 to 4.8 years) and 2.5 years (range, 0.2 to 4.9 years), respectively. After imatinib discontinuation, remission status was monitored monthly by quantitative analysis of the peripheral-blood BCR-ABL mRNA levels using real-time polymerase chain reaction. Proteinase-3 expression and proteinase-3-specific cytotoxic T cells (CTLs) were longitudinally measured to assess putative markers of IFN response.

RESULTS

With a median time of 2.4 years after imatinib withdrawal (range, 0.5 to 4.0 years), 15 (75%) of 20 patients remained in remission. The number of patients in complete molecular remission increased under IFN from two patients at baseline to five patients after 2 years. Relapses occurred in five patients within 0.4 years (range, 0.2 to 0.8 years), but patients underwent rescue treatment with imatinib, re-establishing molecular remission. IFN therapy was associated with an increase in the expression of leukemia-associated antigen proteinase 3 and induction of proteinase-3-specific CTLs.

CONCLUSION

Treatment with IFN enables discontinuation of imatinib in most patients after prior imatinib/IFN combination therapy and may result in improved molecular response. Induction of a proteinase-3-specific CTL response by IFN may contribute to this effect.

ICSBP-mediated immune protection against BCR-ABL-induced leukemia requires the CCL6 and CCL9 chemokines

Interferon (IFN) is effective at inducing complete remissions in patients with chronic myelogenous leukemia (CML), and evidence supports an immune mechanism. Here we show that the type I IFNs (alpha and beta) regulate expression of the IFN consensus sequence-binding protein (ICSBP) in BCR-ABL-transformed cells and as shown previously for ICSBP, induce a vaccine-like immunoprotective effect in a murine model of BCR-ABL-induced leukemia. We identify the chemokines CCL6 and CCL9 as genes prominently induced by the type I IFNs and ICSBP, and demonstrate that these immunomodulators are required for the immunoprotective effect of ICSBP expression. Insights into the role of these chemokines in the antileukemic response of IFNs suggest new strategies for immunotherapy of CML.

Graft-versus-leukemia effects of transplantation and donor lymphocytes

Allogeneic transplantation of hematopoietic cells is an effective treatment of leukemia, even in advanced stages. Allogeneic lymphocytes produce a strong graft-versus-leukemia (GVL) effect, but the beneficial effect is limited by graft-versus-host disease (GVHD). Depletion of T cells abrogates GVHD and GVL effects. Delayed transfusion of donor lymphocytes into chimeras after T cell-depleted stem cell transplantation produces a GVL effect without necessarily producing GVHD. Chimerism and tolerance provide a platform for immunotherapy using donor lymphocytes. The allogeneic GVL effects vary from one disease to another, the stage of the disease, donor histocompatibility, the degree of chimerism, and additional treatment. Immunosuppressive therapy before donor lymphocyte transfusions may augment the effect as well as concomitant cytokine treatment. Possible target antigens are histocompatibility antigens and tumor-associated antigens. Immune escape of tumor cells and changes in the reactivity of T cells are to be considered. Durable responses may be the result of the elimination of leukemia stem cells or the establishment of a durable immune control on their progeny. Recently, we have learned from adoptive immunotherapy of viral diseases and HLA-haploidentical stem cell transplantation that T-cell memory may be essential for the effective treatment of leukemia and other malignancies.