Immunotherapy in Chronic Myelogenous Leukemia

Immunotherapy in Chronic Myelogenous Leukemia François Guilhot, Lydia Roy, Geraldine Martineau, Joëlle Guilhot, Frederic Millot

Abstract Chronic myelogenous leukemia is one of the more responsive leukemic disorders to immunotherapy. Interferon-based regimens were the first treatment to produce complete cytogenetic responses, and this agent has been classified as an immunotherapeutic agent. Although most patients are now treated with imatinib as first-line therapy, a combination of interferon and imatinib could increase the rate of molecular responses and prevent patients from relapse. Thus, large phase III trials are currently exploring this strategy. Allogeneic stem cell transplantation also involves the immune system, with fewer patients in relapse in case they experience graft-versus-host disease. Vaccine strategies are also promising with phase II ongoing trials. These vaccine strategies include the use of oligopeptides derived from the Bcr-Abl junction. Initial results indicate the safety of these therapies with patients exhibiting complete cytogenetic response and molecular responses. These 3 different approaches of immunotherapies are extensively described herein. Although these results obtained with imatinib are promising, this tyrosine kinase inhibitor does not eradicate the leukemic stem cells. Thus, immunotherapeutic strategies are still being investigated in chronic myelogenous leukemia.

Clinical Lymphoma & Myeloma, Vol. 7, Suppl. 2, S64-S70, 2007 Key words: Antileukemic effects, Antitumor efficacy, Imatinib, Interferon-α

Introduction Chronic myelogenous leukemia (CML) has long been recognized as one of the more responsive leukemic disorders to immunotherapy. The chimeric fusion protein is a tumor-specific antigen. This unique neoantigen is immunogenic in animal models because the functional region of p210 contains a sequence of amino acids that is not expressed in normal hematopoietic stem cells. The role of the immune system has been suspected in the context of allogeneic stem cell transplantation (SCT; alloSCT). However, the demonstration that vaccine strategies using the junction peptide could reduce the leukemic burden in humans is more recent. The third therapeutic approach based on immune reactivity is interferon (IFN) therapy. Interferon is an immunomodulatory agent that has activity in CML and has resulted in sustained cytogenetic remission in selected groups of patients. The recent success of imatinib has dramatically changed the way of treating CML. However, despite the impressive rate of cytogenetic and molecular responses and the survival improvement obtained with imatinib, a subgroup of patients might experience resistance or relapse. This review Department of Oncology-Hematology and Cell Therapy, Clinical Research Centre, Centre Hospitalier Universitaire de Poitiers, France Submitted: Nov 14, 2006; Revised: Jan 22, 2007; Accepted: Jan 30, 2007 Address for correspondence: François Guilhot, MD, Department of OncologyHematology and Cell Therapy, Clinical Research Centre, CHU La Milétrie, 2 Rue de la Milétrie, 86021 Poitiers Cedex, France Fax: 33-5-49-44-38-63; e-mail: [email protected]

summarizes the 3 different immunotherapeutic approaches that can be used in CML. The use of these therapies in the imatinib era will also be discussed.

Immunotherapeutic Effect of Interferon in Chronic Myelogenous Leukemia Interferons are cellular glycoproteins with antiproliferative, antiviral, and immunoregulatory properties. They are produced by various cell types in response to viral infection. Interferon-α and IFN-β are acid stable, bind to the same receptors, and are produced primarily by leukocytes and fibroblasts, respectively. Interferon-α, the molecule used in CML, initially prepared from human leukocytes, is now essentially produced by recombinant techniques. Interferon-α exerts various effects in the immune system, such as modulation of immunoglobulin production, inhibition of T-cell cytotoxicity, monocytes/macrophage function, and natural killer cell activity.1 The mechanism of action in CML has been extensively studied. These mechanisms suggest that IFN could be described as an immunotherapeutic agent. Interestingly, the IFN consensus sequence binding protein (ICSBP) in mice is of interest in order to understand the fundamental role of IFN-α in CML pathogenesis. Interferon consensus sequence binding protein (also known as IFN regulatory factor 8) is a member of the transcriptional factors family called IFN regulatory factors, implicated in the regulation of transcription of IFN-activated genes.2 Interferon consensus sequence binding protein is expressed mainly in hematopoietic cells.3,4 Its expression is strongly induced by IFN-γ5,6 and in vivo

Dr François Guilhot has no relevant relationships to disclose. Dr Roy has no relevant relationships to disclose. Dr Martineau has no relevant relationships to disclose. Dr Joëlle Guilhot has no relevant relationships to disclose. Dr Millot has no relevant relationships to disclose. This article includes discussion of investigational and/or unlabeled uses of drugs, including the use of modified interferon, CMLVAX100 vaccine, and a heat-shock protein–70–based vaccine in the treatment of imatinib-resistant CML.

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by IFN-α treatment in patients with CML, among whom ICSBP expression is impaired.7,8 The overexpression of bcr-abl in mice leads to a CML-like disease associated with a downregulation of ICSBP, whereas the forced coexpression of ICSBP in such mice inhibits the bcr-abl–induced cell proliferation.9,10 Furthermore, ICSBP-negative mice develop a granulocytic leukemia with enlarged lymph nodes, liver, and spleen, resembling CML in humans.11 Finally, ICSBP is essential for the generation of plasmacytoid dendritic cells (pDCs, also named IFN-producing cells [IPCs]), the major IFN-α producing cells in vivo,12,13 because ICSBP-negative mice have a completely depleted pDC compartment, in addition to various phenotypic and functional defects of the other dendritic cell subsets.14,15 Rescuing such mice with retroviral ICSBP transduction in ICSBP-negative progenitors restores a functional pDC population.16 Lessons Learned from Large Trials Interferon-α has the potential to control progression in the chronic phase and has been the first nonmyelotoxic drug shown to cause a marked reduction in Philadelphia (Ph) positivity in some patients. Using recombinant IFN-α2a, IFN-α2b, or IFN-α2c, several studies from single institution or cooperative groups have subsequently confirmed the efficacy of IFN-α in CML. Patients treated in early chronic phase CML with IFN-α had major cytogenetic response rates (MCyR, < 35% Ph-positive residual cells) of 20%-40% and complete cytogenetic response (CCyR) rates of 15%-30%. Combining IFN-α with cytarabine might increase the rate of CCyR to 25%-35%.17 Several trials have demonstrated that having MCyRs or CCyRs significantly improved the survival.18 Some studies have not found an association between having a major or complete responses and survival, probably because the number of responders was too small. However, using a multivariate analysis including MCyR as a time-dependent variable, achievement of such a response was independently associated with prolonged survival. Therefore, achievement of a cytogenetic response might be the most important prognostic feature for patients with CML treated with IFN-α alone or in combination with cytarabine.19,20 Patient compliance to prolonged treatment with IFN-α in CML was an important factor for achieving clinical benefit. In some studies, antitumor efficacy appears related to higher dose schedules of IFN-α daily therapy. Similarly, trials in CML suggested that an increased area under the curve and associated prolonged tumor exposure to IFN-α could be important in mediating the antileukemic effects. Therapy with IFN-α was associated with significant side effects, requiring dose reductions and temporary or permanent treatment interruptions in 10%-50% of patients. Before the availability of imatinib, the combination of IFN-α plus cytarabine was considered standard therapy for patients with CML not undergoing alloSCT.21,22 A randomized trial (IRIS) comparing imatinib 400 mg per day with IFN-α plus cytarabine in newly diagnosed patients with chronic phase CML enrolled 553 patients on each treatment and demonstrated significant superiority of imatinib in all parameters measured with a median follow-up of 19 months.23 Because of the superior response rates, protection from disease

progression, and tolerance of imatinib, a large percentage of patients crossed over from the IFN-α arm to imatinib. Indeed, 64% of patients assigned to the IFN-α plus cytarabine arm switched to imatinib after a median duration of treatment of 9 months. Thus, overall survival analysis based on intent to treat was assessable but could not show a difference. A retrospective analysis comparing the outcome of patients first treated with imatinib in the IRIS trial and patients assigned to IFN-α and cytarabine in the French CML91 trial was recently performed.24 Patients selected for this analysis were those who actually received their assigned experimental treatment: the IFN-α plus cytarabine combination in the CML91 trial (n = 325) or imatinib as first-line therapy in the IRIS trial (n = 551). A cutoff date of 42 months for the common follow-up was selected, corresponding to the last update available for the IRIS study at the time of this analysis. This comparison provides confirmatory evidence that imatinib is superior to the combination of IFN-α/cytarabine in terms of cytogenetic responses, survival free of transformation, and overall survival. Current Use of Interferon-α in the Imatinib Area The success of imatinib therapy in CML has changed the therapeutic algorithm of CML. Thus, the use of IFN-α as firstline therapy cannot be considered for patients in first chronic phase. However, it could be a valuable option for patients who become resistant to imatinib and who have never been previously exposed to IFN-α. Imatinib is effective in treating CML; however, some patients ultimately experience relapse with resistant disease. Resistance might develop through several mechanisms, such as point mutations within the tyrosine kinase binding site, gene amplification, clonal evolution, or decreased imatinib bioavailability.25 In addition, despite the high rate of CCyR, a minority of patients had a complete and durable molecular negativity, and most of the patients who stopped imatinib might experience relapse. This suggests that although imatinib is highly active against a mature CML cell population, it does not, at least at standard dose, eradicate all residual leukemic cells. There is evidence that CML stem cells are not eliminated by imatinib in vivo with patients in CCyR having detectable Ph-positive CD34+ cells and long-term culture-initiating cells.26,27 Thus, research is currently in progress to further elucidate the mechanisms of imatinib resistance and to develop strategies that will expand the usefulness of imatinib. In order to obtain a higher rate of cytogenetic response and to overcome resistance, new strategies using the combination of imatinib and IFN, pegylated or nonpegylated forms, are actively tested. The conjugation of a 40 kDa branched polyethylene glycol (PEG) molecule to IFN-α2a (PEG IFN-α2a) results in the formation of a novel IFN with properties, including sustained absorption and a prolonged half-life, allowing for a once-weekly dosing regimen. Thus, this new compound could be better tolerated in patients with CML. An open-label trial has included 144 patients comparing subcutaneous PEG IFN-α2a 450 μg once weekly with regular IFN-α2a, 9 MU per day. After 12 months, MCyR, CCyR, and hematologic response

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Immunotherapy in Chronic Myelogenous Leukemia Table 1 Cytogenetic Responses with the Combination of Imatinib (400 mg Daily) plus Polyethylene Glycol Interferon Total n = 76

Months

50 μg per Week n = 27

100 μg per Week n = 18

150 μg per Week n = 31

CCyR

MCyR

CCyR

MCyR

CCyR

MCyR

CCyR

MCyR

3

29

61

22

59

33

67

32

58

6

46

67

44

78

50

61

45

61

9

54

63

48

63

56

61

58

65

12

68

79

74

93

67

78

65

68

Overall

70

83

78

93

67

78

65

77

Values are percentages.

were significantly better with PEG IFN-α2a compared with regular IFN-α2a: 35% and 18% (P = 0.0016), 15% and 7%, and 66% and 41% (P = 0.0008), respectively.28 An exploratory study was conducted in order to investigate the effects of a standard 400 mg daily imatinib dose and a variable PEG IFN dose (50 μg weekly, 100 μg weekly, and 150 μg per week).29 The criteria for dose adjustment were designed to ensure the delivery of the imatinib dose and to protect quality of life. There were 76 patients with previously untreated Ph-positive CML enrolled in the study. There were 3 patients who discontinued imatinib and 45 patients who discontinued PEG IFN. The severity of adverse events increased with increasing PEG IFN dose. The imatinib dose could be administered to the patients who were assigned to receive 50 μg weekly or 100 μg weekly PEG IFN but not to those who were assigned to receive 150 μg weekly. The median administered dose of PEG IFN ranged between 32 μg weekly and 36 μg weekly. In this group of patients, 70% had a CCyR and 83% MCyR. The Bcr-Abl transcript was reduced by ≥ 3 logs in 68% of CCyR patients (Table 1). These phase II trials were essential for the design of the current large phase III trials. Two groups are conducting in parallel large phase III trials exploring dosage of imatinib as well as combination therapies. In July 2002, the German CML study group activated a 4-armed randomized controlled trial comparing imatinib 400 mg with imatinib plus IFN-α, imatinib/cytarabine, and imatinib after IFN-α failure. In this trial, high-risk patients are randomly assigned to receive primary imatinib-based therapies including a treatment arm with 800 mg daily.30 A recent evaluation was based on 416 patients with 12 months of follow-up. Of the 335 patients with cytogenetic evaluation, 63% had an MCyR and 53% a CCyR. The number of patients who experienced progression each year was very low, and 27% of patients had a major molecular response. In September 2003, the French CML study group started a similar phase III trial.31 The experimental arms are imatinib 400 mg daily in combination with PEG IFN-α2a 90 μg weekly, imatinib 400 mg daily in combination with cytarabine (20 mg/m2 per day on days 15-28 of 28-day cycles), or imatinib 600 mg daily. The reference arm is imatinib 400 mg daily. A first evaluation based on 315 patients with a median time of observation of 12 months demonstrated the feasibility of combination therapies with a complete hematologic response

rate of 82% at 3 months. Cytogenetic data were available from 154 patients. At 6 months, 135 patients (87%) had an MCyR, being complete in 105 patients (68%). A substantial number of patients experienced grade 3/4 hematologic as well as nonhematologic toxicities. A final analysis of these ongoing trials will determine the role of combination therapies using imatinib and IFN.

Allogeneic Stem Cell Transplantation as a Model of Immune Therapy For many years, alloSCT has been considered standard curative therapy and was proposed as first-line therapy, especially in young patients. Because of the recent success of imatinib, alloSCT is now offered to patients in case of failure or insufficient response with imatinib therapy.32 However, in the context of hematopoietic SCT, CML has long been recognized as one of the more responsive malignancies to immunotherapy. The role of the immune system during alloSCT has been extensively studied and is usually referred to as graftversus-leukemia. Numerous observations underlined this effect, such as increased risk of leukemia relapse in recipients of transplants from genetically identical twins compared with allotransplant recipients and decreased risk of leukemia relapse in allotransplant recipients who experienced graft-versus-host disease (GVHD). In addition, depletion of T cells from donor marrow to reduce the risk of GVHD resulted in a significantly increased risk of disease relapse compared with T cell–replete alloSCTs (even when adjusted for GVHD). Finally, an antileukemia effect of stopping posttransplantation immune suppression was described together with the infusion of donor lymphocytes without additional therapy.33 These clinical observations provide evidence that donor T cells play an important role in mediating graft-versus-leukemia as well as GVHD. The early detection of minimal residual disease is essential in order to initiate treatment when the leukemia burden is less than a cytogenetic or hematologic relapse. The treatment by donor lymphocytes has been described first by Van Rhee et al, who treated 14 relapse patients with CML.34 Of the 7 patients in hematologic relapse, 3 had a CCyR after donor lymphocytes; however, 2 of these patients developed aplasia. Three patients with hematologic relapse and 2 with molecular relapse were also treated. All had a complete response

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François Guilhot et al without side effect. There was no clear difference of severe GVHD among the 3 groups of patients. Other studies provided evidence that donor lymphocytes were a powerful treatment if administered in patients with minimal leukemia burden.35,36 The main complication of donor lymphocytes is GVHD. Although imatinib is a powerful drug for patients with CML, donor lymphocytes have been considered standard therapy for patients with CML experiencing relapse after alloSCT. Donor lymphocytes have proven to restore full donor chimerism and produce long-term complete molecular remissions.37,38 Decreasing the dose of donor lymphocytes is associated with less GVHD but also with a longer interval between treatment and CCyR. It has been postulated that combining IFN-α with donor lymphocytes would enable a decrease in the dose of donor lymphocytes, thereby limiting GVHD, and at the same time, a decrease in the interval between donor lymphocytes and CCyR for patients with a hematologic or cytogenetic relapse. For molecular relapses, it was hypothesized that because of a lower tumor load, very low doses of donor lymphocytes without IFN-α could be an effective treatment. Two groups of patients with chronic phase CML treated with donor lymphocytes at a very low dose of 0.5-1 × 107 mononuclear cells per kg, containing 2-6 × 106 CD3+ T cells per kg, were recently analyzed: 13 patients with a cytogenetic or a hematologic relapse after alloSCT (group A) were treated with additional IFN-α therapy at a dose of 3 × 106 U for 5 days a week, and 8 patients with a molecular relapse were treated without IFN-α (group B).39 Based on the results, it was concluded that very low-dose donor lymphocytes in combination with IFN-α as treatment for cytogenetic or hematologic relapses of chronic phase CML after alloSCT reduced the interval to obtain a CCyR with acceptable GVHD when compared with the literature. Patients with a CCyR also reached complete donor chimerism and complete molecular remissions. For patients with a molecular relapse, very low-dose donor lymphocytes alone are sufficient to induce molecular remissions in most patients and are associated with limited GVHD. There is no definitive recommendation concerning the treatment of patients who relapse after SCT. Two therapeutic options can be offered: imatinib or donor lymphocytes.40 A retrospective analysis was recently performed on patients relapsing after SCT who were treated with imatinib alone (n = 10) or with donor lymphocytes (n = 21). In this retrospective comparison, imatinib resulted in a higher incidence of relapse and inferior leukemia-free survival (P = 0.006 and P = 0.016, respectively). Although this observation is of interest, it should be confirmed by large prospective trials.

Vaccine Strategies in Chronic Myelogenous Leukemia Chronic myelogenous leukemia has been recognized as a potent model for immune therapy in humans because there is a specific gene rearrangement, bcr-abl, whose product, p210Bcr-Abl, can be the target antigen for immune therapy. Peptides spanning the junction between Bcr and Abl in p210Bcr-Abl are specific to CML cells; they are not present in other normal cells, in patients with CML, or in cells in normal individuals without CML, although

polymerase chain reaction positivity for Bcr-Abl has been described in some healthy individuals. There are also other potential targets for vaccines in CML including PR1, Wilms’ tumor protein (WT1), minor histocompatibility antigens, CML-66, CML-28, the ribonucleoprotein telomerase (hTERT), and survivin. Cytotoxic T Cells as Adoptive Immunotherapy A number of studies have focused on different antigens derived from normal tissue proteins than can play a role as tumor antigens in CML. Of these, proteinase 3, a differentiation antigen associated with granule formation in myeloid cells, is aberrantly expressed in tumor cells. PR1, a human leukocyte antigen (HLA)–A2–restricted peptide derived from proteinase 3, elicits cytotoxic T lymphocytes that kill myeloid leukemia cells but not normal marrow cells.41,42 They are present at significant frequencies in patients with CML in remission.43,44 Thus, PR1-specific cytotoxic T cells might help to eradicate leukemic cells after IFN-α or SCT.44 Two different populations of PR1-specific cytotoxic T cells have been identified based on their avidity. High-avidity PR1-specific T cells kill CML cells more effectively than low-avidity T cells. Low-avidity PR1-specific T cells have been identified and selectively expanded in vitro from the blood of newly diagnosed patients with CML. Circulating high-avidity PR1-specific T cells were identified in patients who had cytogenetic responses after IFN-α therapy. Thus, high-avidity PR1-specific T cells expanded in vitro could be used for therapeutic purposes. The product of the Wilms’ tumor gene WT1, a transcription factor expressed at low levels in immature CD34+ progenitors cells, is overexpressed in the leukemia cells of a large number of patients with CML. The HLA-A2 restricted WT1-126 peptide elicits cytotoxic T cells that specifically kill HLA-A2 leukemia CD34+ leukemia cells. Ex vivo responses of CD8 T cells from patients with CML to extrajunction Bcr-Abl peptides and telomerase 540-548 hTert, PR1, and WT1 peptides were recently characterized.45 Chronic myelogenous leukemia–specific CD8 T cells were present in most treated patients and were usually multiepitopic: WT1, hTert, PR1, and bcr74 tetramer-positive cells were detected in 85%, 82%, 67%, and 61% of patients, respectively. Chronic myelogenous leukemia–specific tetramer-positive CD8 T cells had a predominantly memory phenotype, an intermediate perforin content, and low intracellular IFN- accumulation in the presence of the relevant peptide. However, in shortterm culture with HLA-matched leukemia cells, the patients’ memory T cells were specifically reactivated to become IFN- –producing effector cells, suggesting that CD8 T-cell precursors with lytic potential are present in vivo and can be activated by appropriate stimulation. These cells could be used in patients exhibiting minimum residual disease after imatinib therapy. Immunotherapy Using Peptide Vaccines to Bcr-Abl Philadelphia-positive patients with CML express b2a2 or b3a2 fusion transcripts, depending on whether exon b3 has been included. When translated, b2a2 and b3a2 messenger RNAs each

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Immunotherapy in Chronic Myelogenous Leukemia Table 2 Immune Strategies in Chronic Myelogenous Leukemia: Risks and Benefits Treatment

Advantages

Risks

Disadvantages

AlloSCT

• Long-term survival in patients who had sustained CCyR • Donor lymphocytes for relapse

Early transplantation-related mortality still approximately 20%

• Treatment restricted to young patients • Need a suitable donor

IFN

Long-term outcome for patients who had sustained CCyR

• Serious side effects • Treatment compliance is poor

• Percentage of response is low • Nonspecific stimulation

• Subcutaneous injection with good compliance • Specific targeting

• Single antigen targeting might lead to tumor escape mechanism • Complete eradication of leukemic stem cell is not demonstrated

Specific HLA type could have better responses compared with other subtypes

Vaccines with Peptides

generate a 210-kDa Bcr-Abl protein (p210), which is necessary and sufficient for leukemic transformation. The Bcr-Abl junction in b3a2 messenger RNA disrupts a triplet codon, producing a novel lysine (K) at the junction in the b3a2 Bcr-Abl protein product. A codon disruption also occurs at the b2a2 fusion junction (Asp altered to Glu), but in this case, the novel amino acid might not be recognized because it is also present at the normal a1a2 junction. Oligopeptides derived from the Bcr-Abl junction are potential novel CML-specific antigens and might, therefore, elicit an immune response. In addition, a number of class I HLA molecules have been reported to bind strongly to peptides spanning the bcr-abl fusion junction.46-50 Bocchia et al46 identified Bcr-Abl b3a2 junctional 9-mers that bound strongly to HLA-A3 and A11 (sequence KQSSKALQR) and B8 (sequence GFKQSSKAL). These peptides also elicited specific class I-restricted cytotoxic T-lymphocyte activity.47 Human leukocyte antigen–A2 is capable of binding a different b3a2 fusion peptide that is able to elicit cytotoxic T-lymphocyte responses in healthy donors and patients with CML.48 These data suggest that certain Bcr-Abl junctional peptides might preferentially bind to certain HLA alleles. However, no sequences from the b2a2 junction bound to any of the HLA class I molecules.46 It has also been demonstrated that the HLA-A3 binding peptide from the bcr3abl2 region is endogenously processed and presented on leukemic cells.51 Finally, the expression of HLA-B8 and HLA-A3 coexpressed with HLA-B8 reduced the risk of acquiring CML.52 Together, these data suggest that T-cell immunity against Bcr-Abl might possibly be of clinical relevance. Thus, based on these previous observations, phase I/II vaccine trials, using a mixture of 5 b3a2-derived peptides plus the immunologic adjuvant QS-2117, have shown peptidespecific T-cell responses without tumor responses.53,54 More recently, Bocchia et al performed a phase II vaccine multicenter trial in Italy, enrolling patients with b3a2-related CML (b3a2-CML) from 4 centers.55 Patients with persistent stable disease during conventional treatment (minimum duration of previous treatment, 12 months [imatinib] or 24 months [IFN-α]) and with ≥ 1 of the following HLA molecules were eligible for the study: HLA-A3, HLA-A11, HLA-B8, HLA-DR11, HLA-DR1, or HLA-DR4. They used CMLVAX100, a vaccine that consisted of 5 b3a2 breakpoint-derived peptides. They enrolled 16 patients with chronic phase CML previously treated with imatinib or IFN-α. They were given 6 vaccinations with a peptide vaccine derived from the sequence p210-b3a2 plus molgramostim and

QS-21 as adjuvants (CMLVAX100); imatinib (10 patients) and IFN-α (6 patients) therapy was continuously administered during vaccine therapy. Patients had cytogenetic improvement, including 2 patients in complete molecular response. Of interest, 70% of patients had a positive delayed-type hypersensitivity reaction, and in some cases, in vitro CD4+ proliferative response to b3a2 peptides. Although this phase II trial is of interest, the vaccine strategy dose needs specific requirement, and a longer follow-up of the patients is needed to assess the benefit of antileukemic T-cell–mediated immunotherapy. Multivalent Heat-Shock Protein Vaccines A third approach is to use heat-shock protein (Hsp)–based vaccines. Heat-shock protein encompasses a family of chaperone proteins that participates in the degradation process of intracellular proteins. These could include multiple potentially antigenic peptides. Peptides chaperoned from the tumor cell might provide the specificity of the Hsp. This vaccine strategy does not need structurally defined potential CML antigens or immune adjuvants. The vaccine made from autologous Hsp is multivalent, and different peptides might be immunogenic in different people. Similar to other vaccine approaches, CD4+ and CD8+ T cells are activated by Hsps. This approach was used in patients with CML who did not exhibit response after imatinib therapy, and favorable responses were noted.56 A phase I trial of Hsp-70 showed feasibility and safety of an Hspbased vaccine in 14 patients with CML. Several had nonspecific and specific immune responses (reduced T regulatory cells and IFN-γ release assessed by enzyme-linked immunospot assay, after culture with autologous prevaccination CML cells). A substantial number of patients had a reduced number of CML cells after vaccination (assessed by cytogenetics, polymerase chain reaction, and fluorescence in situ hybridization). Because the patients were also receiving concurrent therapies, it is not possible to know if there was a real clinical benefit. However, an ongoing phase II trial of Hsp-70 in patients with chronic phase CML in whom imatinib thearapy failed will provide more information concerning this new immunologic approach of the treatment of CML.

Conclusion The clinical results, with respective risks and benefits (Table 2) obtained with the 3 main immune strategies (IFN, alloSCT, and vaccines) clearly demonstrate the important role of the immune system. The current question is to define

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François Guilhot et al the role of these different strategies in the context of imatinib therapy. Imatinib has been proposed as first-line therapy for all newly diagnosed patients. The goal of imatinib therapy is the achievement of a certain degree of responses at various time points after the initiation of the treatment. It has been proposed to define failure and suboptimal responses for patients without a sufficient response achieved in a precise period of time. Failure of treatment implies that the patient should be moved to other treatment; suboptimal response suggests that the long-term outcome is not likely to be optimal, so that the patient becomes eligible for other treatment. In this context, young patients with a fully matched donor should be offered SCT. The vast majority of patients will have sustained CCyR. However, imatinib does not eradicate leukemic stem cells. Thus, most patients who discontinue imatinib will experience relapse. This is why several groups are investigating treatment that would complete the initial antileukemic effects of imatinib. Vaccine strategies are currently being investigated for these patients. The initial results indicate that a molecular response can be achieved in patients who have already obtained a sufficient response with imatinib. Interferon could also play an important role based on immunologic effects. Thus, IFN is currently being used in combination with imatinib as initial treatment. However, IFN could also be used in patients with minimal residual disease after imatinib therapy in order to prevent relapse after discontinuation of imatinib.

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