Augmentation of immune response by altered peptide ligands of the antigenic peptide in a human CD4+ T-cell clone reacting to TEL/AML1 fusion protein

Pediatric precursor B acute lymphoblastic leukemia: are T helper cells the missing link in the infectious etiology theory?

Precursor B acute lymphoblastic leukemia (BCP-ALL), the most common childhood malignancy, arises from an expansion of malignant B cell precursors in the bone marrow. Epidemiological studies suggest that infections or immune responses to infections may promote such an expansion and thus BCP-ALL development. Nevertheless, a specific pathogen responsible for this process has not been identified. BCP-ALL cells critically depend on interactions with the bone marrow microenvironment. The bone marrow is also home to memory T helper (Th) cells that have previously expanded during an immune response in the periphery. In secondary lymphoid organs, Th cells can interact with malignant cells of mature B cell origin, while such interactions between Th cells and malignant immature B cell in the bone marrow have not been described yet. Nevertheless, literature supports a model where Th cells—expanded during an infection in early childhood—migrate to the bone marrow and support BCP-ALL cells as they support normal B cells. Further research is required to mechanistically confirm this model and to elucidate the interaction pathways between leukemia cells and cells of the tumor microenvironment. As benefit, targeting these interactions could be included in current treatment regimens to increase therapeutic efficiency and to reduce relapses.

Tumour antigens recognized by T lymphocytes: at the core of cancer immunotherapy

In this Timeline, we describe the characteristics of tumour antigens that are recognized by spontaneous T cell responses in cancer patients and the paths that led to their identification. We explain on what genetic basis most, but not all, of these antigens are tumour specific: that is, present on tumour cells but not on normal cells. We also discuss how strategies that target these tumour-specific antigens can lead either to tumour-specific or to crossreactive T cell responses, which is an issue that has important safety implications in immunotherapy. These safety issues are even more of a concern for strategies targeting antigens that are not known to induce spontaneous T cell responses in patients.

Vaccines against advanced melanoma

Research shows that cancers are recognized by the immune system but that the immune recognition of tumors does not uniformly result in tumor rejection or regression. Quantitating the success or failure of the immune system in tumor elimination is difficult because we do not really know the total numbers of encounters of the immune system with the tumors. Regardless of that important issue, recognition of the tumor by the immune system implicitly contains the idea of the tumor antigen, which is what is actually recognized. We review the molecular identity of all forms of tumor antigens (antigens with specific mutations, cancer-testis antigens, differentiation antigens, over-expressed antigens) and discuss the use of these multiple forms of antigens in experimental immunotherapy of mouse and human melanoma. These efforts have been uniformly unsuccessful; however, the approaches that have not worked or have somewhat worked have been the source of many new insights into melanoma immunology. From a critical review of the various approaches to vaccine therapy we conclude that individual cancer-specific mutations are truly the only sources of cancer-specific antigens, and therefore, the most attractive targets for immunotherapy.

HLA-DPβ1 Asp84-Lys69 antigen-binding signature predicts event-free survival in childhood B-cell precursor acute lymphoblastic leukaemia: results from the MRC UKALL XI childhood ALL trial

We previously reported that children in the UKALL XI ALL trial with HLA-DP 1 and -DP 3 supertypes had significantly worse event-free survival (EFS) than children with other DP supertypes. As DP 1 and DP 3 share two of four key antigen-binding amino-acid polymorphisms (aspartic acid84–lysine69), we asked whether Asp84-Lys69 or Asp84 alone were independent prognostic indicators in childhood acute lymphoblastic leukemia (ALL). We analysed EFS in 798 UKALL XI patients, stratified by Asp84-Lys69 vs non-Asp84-Lys69, for a median follow-up of 12.5 years. Asp84-Lys69 was associated with a significantly worse EFS than non-Asp84-Lys69 (5-year EFS: Asp84-Lys69: 58.8% (95% CI (confidence of interval): 52.7–64.9%); non-Asp84-Lys69: 67.3% (63.4–71.2%); 2P=0.007). Post-relapse EFS was 10% less in Asp84-Lys69 than non-Asp84-Lys69 patients. EFS was significantly worse (P=0.03) and post-relapse EFS marginally worse (P=0.06) in patients with Asp84 compared with Gly84. These results suggest that Asp84-Lys69 predicted adverse EFS in the context of UKALL XI because of Asp84, and may have influenced post-relapse EFS. We speculate that this may be due to the recruitment of Asp84-Lys69-restricted regulatory T cells in the context of this regimen, leading to the re-emergence of residual disease. However, functional and molecular studies of the prognostic value of this and other HLA molecular signatures in other childhood ALL trials are needed.

Transmission of HLA-DP variants from parents to children with B-cell precursor acute lymphoblastic leukemia: log-linear analysis using the case-parent design

Childhood B-cell precursor acute lymphoblastic leukemia (BCP ALL) is usually initiated in utero and is thought to progress to overt leukemia under the influence of delayed exposure to a common infection. Based on the hypothesis that polymorphic HLA-DP variants can restrict T-cell responses to infection, we previously compared DP supertype frequencies in BCP ALL patients with that of unrelated newborn controls. We reported that the DP2 supertype was associated with susceptibility, whereas DP1 was associated with protection. However, the association of genetic variants in children with early-onset diseases such as ALL may be a proxy for parental effects. Here we examine whether maternal DP1 and DP2 are associated with BCP ALL by fitting log-linear models in a combined series of family triads (both parents and case child) and dyads (1 parent and case child; n = 571) in comparison with similar models in non-BCP leukemia (n = 198). We report no evidence of maternal DP1 or DP2 associations with BCP ALL, but we did identify suggestive evidence of maternal undertransmission of the infrequent supertypes DP11 and DP15. Although these results require confirmation, they suggest that DP11 and DP15 may be protective or that there is transmission ratio distortion of these supertypes in BCP ALL.

Five HLA-DP molecules frequently expressed in the worldwide human population share a common HLA supertypic binding specificity

Compared with DR and DQ, knowledge of the binding repertoires and specificities of HLA-DP alleles is somewhat limited. However, a growing body of literature has indicated the importance of DP-restricted responses in the context of cancer, allergy, and infectious disease. In the current study, we developed high-throughput binding assays for the five most common HLA-DPB1 alleles in the general worldwide population. Using these assays on a comprehensive panel of single-substitution analogs and large peptide libraries, we derived novel detailed binding motifs for DPB1*0101 and DPB1*0501. We also derived more detailed quantitative motifs for DPB1*0201, DPB1*0401, and DPB1*0402, which were previously characterized on the basis of sets of eluted ligands and/or limited sets of substituted peptides. Unexpectedly, all five DP molecules, originally selected only on the basis of their frequency in human populations, were found to share largely overlapping peptide motifs. Testing panels of known DP epitopes and a panel of peptides spanning a set of Phleum pratense Ags revealed that these molecules also share largely overlapping peptide-binding repertoires. This demonstrates that a previously hypothesized DP supertype extends far beyond what was originally envisioned and includes at least three additional very common DP specificities. Taken together, these DP supertype molecules are found in >90% of the human population. Thus, these findings have important implications for epitope-identification studies and monitoring of human class II-restricted immune responses.

ALK a Novel Lymphoma-associated Tumor Antigen for Vaccination Strategies

The discovery of Tumor Associated Antigens (TAAs) demonstrated that tumor cells can be specifically recognized by the immune system raising the hypothesis that tumors express antigens that Cytotoxic T Lymphocytes (CTLs) can potentially attack. The identification of immunogenic epitopes led to their use as targets to mediate the specific clearance of neoplastic cells by TAA targeting strategies such as vaccination strategies. One of the critical issues in the development of efficient vaccination protocols is the identification of the appropriate TAAs. The TAA should be effective as a "tumor rejection antigen" able to induce an immune response that will affect tumor growth. A distinct pathologic entity characterized by the expression of the Anaplastic Lymphoma Kinase (ALK) protein and named "ALKoma" has recently emerged within the heterogeneous group of CD30+ Anaplastic Large Cell Lymphoma (ALCL). ALK is a receptor tyrosine kinase whose expression is normally restricted to a few scattered cells in the nervous system. Its pathological expression in lymphoma cells is due to a chromosomal translocation that leads to the formation of an ALK-derived oncogenic fusion proteins. ALK fusion proteins ectopically over-expressed and constitutively activated in lymphoid cells play a key role in the neoplastic transformation by the aberrant phosphorylation of intracellular substrates that likely contributes to the molecular pathogenesis of ALCL. The high level of ALK expression in lymphoma cells and its direct role in lymphomagenesis, combined with the fact that normal ALK is expressed at low levels in the immune privileged nervous system, makes ALK an ideal lymphoma-specific target for immunotherapy of ALK+ALCL.

Targets for active immunotherapy against pediatric solid tumors

The potential role of antibodies and T lymphocytes in the eradication of cancer has been demonstrated in numerous animal models and clinical trials. In the last decennia new strategies have been developed for the use of tumor-specific T cells and antibodies in cancer therapy. Effective anti-tumor immunotherapy requires the identification of suitable target antigens. The expression of tumor-specific antigens has been extensively studied for most types of adult tumors. Pediatric patients should be excellent candidates for immunotherapy since their immune system is more potent and flexible as compared to that of adults. So far, these patients do not benefit enough from the progresses in cancer immunotherapy, and one of the reasons is the paucity of tumor-specific antigens identified on pediatric tumors. In this review we discuss the current status of cancer immunotherapy in children, focusing on the identification of tumor-specific antigens on pediatric solid tumors.

The good, the bad and the ugly: how altered peptide ligands modulate immunity

BACKGROUND

The basis of T cell immune responses is the specific recognition of an immunogenic peptide epitope by a T cell receptor. Peptide alterations of such T cell epitopes with single or few amino acid variations can have drastic effects on the outcome of this recognition. These altered peptide ligands can act as modulators of immune responses as they are capable of downregulating or upregulating responses.

OBJECTIVE/METHODS

We review how altered peptide ligands can have 'good' 'bad' and 'ugly' outcomes in treating diseases.

RESULTS/CONCLUSION

Altered peptide ligands have been used as immunotherapeutics in autoimmune (and allergic) diseases, infectious diseases and cancer. In the next five years we anticipate seeing a number of altered peptide ligands in clinical trials, progressing from contradictory classifications of good, bad or ugly, to the exciting outcome of 'useful'.

HLA-associated susceptibility to childhood B-cell precursor ALL: definition and role of HLA-DPB1 supertypes

Childhood B-cell precursor (BCP) ALL is thought to be caused by a delayed immune response to an unidentified postnatal infection. An association between BCP ALL and HLA class II (DR, DQ, DP) alleles could provide further clues to the identity of the infection, since HLA molecules exhibit allotype-restricted binding of infection-derived antigenic peptides. We clustered >30 HLA-DPB1 alleles into six predicted peptide-binding supertypes (DP1, 2, 3, 4, 6, and 8), based on amino acid di-morphisms at positions 11 (G/L), 69 (E/K), and 84 (G/D) of the DPβ₁ domain. We found that the DPβ11-69-84 supertype GEG (DP2), was 70% more frequent in BCP ALL (n=687; P<10⁻⁴), and 98% more frequent in cases diagnosed between 3 and 6 years (P<10⁻⁴), but not <3 or >6 years, than in controls. Only one of 21 possible DPB1 supergenotypes, GEG/GKG (DP2/DP4) was significantly more frequent in BCP ALL (P=0.00004) than controls. These results suggest that susceptibility to BCP ALL is associated with the DP2 supertype, which is predicted to bind peptides with positively charged, nonpolar aromatic residues at the P4 position, and hydrophobic residues at the P1 and P6 positions. Studies of peptide binding by DP2 alleles could help to identify infection(s) carrying these peptides.

HLA-DPB1 supertype-associated protection from childhood leukaemia: relationship to leukaemia karyotype and implications for prevention

Most childhood B cell precursor (BCP) acute lymphoblastic leukaemia (ALL) cases carry the reciprocal translocation t(12;21)(p13;q22) ( approximately 25%), or a high hyperdiploid (HeH) karyotype (30%). The t(12;21) translocation leads to the expression of a novel fusion gene, TEL-AML1 (ETV6-RUNX1), and HeH often involves tri- and tetrasomy for chromosome 21. The presence of TEL-AML1+ and HeH cells in utero prior to the development of leukaemia suggests that these lesions play a critical role in ALL initiation. Based on our previous analysis of HLA-DP in childhood ALL, and evidence from in vitro studies that TEL-AML1 can activate HLA-DP-restricted T cell responses, we hypothesised that the development of TEL-AML1+ ALL might be influenced by the child's DPB1 genotype. To test this, we analysed the frequency of six HLA-DPB1 supertypes in a population-based series of childhood leukaemias (n = 776) classified by their karyotype (TEL-AML1+, HeH and others), in comparison with newborn controls (n = 864). One DPB1 supertype (GKD) conferred significant protection against TEL-AML1+ ALL (odds ratio (OR), 95% confidence interval (95% CI): 0.42, 0.22-0.81; p < 0.005) and HeH ALL (OR; 95% CI: 0.44, 0.30-0.65; p < 0.0001). These negative associations were almost entirely due to a single allele, DPB1*0101. Our results suggest that DPB1*0101 may afford protection from the development of TEL-AML1+ and HeH BCP ALL, possibly as the result of a DP-restricted immune response to BCP ALL-associated antigen(s), the identification of which could have important implications for the design of prophylactic vaccines.

A neoepitope generated by an FLT3 internal tandem duplication (FLT3-ITD) is recognized by leukemia-reactive autologous CD8+ T cells

The FLT3 receptor tyrosine kinase is expressed in more than 90% of acute myelogeneous leukemias (AMLs), up to 30% of which carry an internal tandem duplication (ITD) within the FLT3 gene. Although varying duplication sites exist, most FLT3-ITDs affect a single protein domain. We analyzed the FLT3-ITD of an AML patient for encoding HLA class I–restricted immunogenic peptides. One of the tested peptides (YVDFREYEYY) induced in vitro autologous T-cell responses restricted by HLA-A*0101 that were also detectable ex vivo. These peptide-reactive T cells recognized targets transfected with the patient's FLT3-ITD, but not wild-type FLT3, and recognized the patient's AML cells. Our results demonstrate that AML leukemic blasts can in principle process and present immunogenic FLT3-ITD neoepitopes. Therefore, FLT3-ITD represents a potential candidate target antigen for the immunotherapy of AML.

A listing of human tumor antigens recognized by T cells: March 2004 update

The technological advances occurred in the last few years have led to a great increase in the number of tumor associated antigens (TAA) that are currently available for clinical applications. In this review we provide a comprehensive list of human tumor antigens as reported in the literature updated at February 2004. The list includes all T cell-defined epitopes, while excluding analogs or artificially modified epitopes, as well as virus-encoded and antibodies-recognized antigens. TAAs are listed in alphabetical order along with the epitope sequence and the HLA allele which restricts recognition by T cells. Data on the tissue distribution of each antigen are also provided together with an extensive bibliography that allows a rapid search for any additional information may be needed on each single antigen or epitope. Overall, the updated list is a database tool for clinicians, scientists and students who have an interest in the field of tumor immunology and immunotherapy.

Degenerate recognition and response of human CD4+ Th cell clones: implications for basic and applied immunology

It was once considered that the T cell response is an all or nothing type event, but recent studies have clearly indicated that T cells show many different types of activation in recognition of altered ligands for T cell receptors (TCR). In this review, we summarize our recent findings on the response of human CD4+ helper T (Th) cell clones to altered peptide ligands (APL); peptides carrying single or multiple residue substitutions in antigenic peptides. The extensive analyses revealed that TCR-antagonism and partial agonism are frequently observed by the stimulation with APLs substituted at particular amino acid residues of antigenic peptides. We observed unique partially agonistic APLs inducing prolongation of T cell survival without cell proliferation. Superagonistic APLs stimulated enhanced proliferation and production of cytokines in Th cell clones reactive to tumor-associated antigens. The other APL induced enhanced production of interleukin-12 by antigen presenting cells and subsequent enhancement of IFN-gamma production by T cells reactive to allergens. By utilizing an HLA-DR-restricted T cell epitope library generated by mutated invariant chain genes, it was revealed that human Th cell clones recognize a more diverse array of peptides with multiple and simultaneous amino acid substitutions in an antigenic peptide. APLs also induced altered intracellular signaling events including intracellular calcium increase and phosphorylation of signaling molecules. This information provides basic knowledge regarding the characteristics of antigen recognition by human Th cells and the subsequent activation, and a novel method for manipulation of human Th cell responses by APLs, as a possible candidate for antigen-specific immuno-potentiating or immunosuppressive therapy.

Retroviral transduction of acute myeloid leukaemia-derived dendritic cells with OX40 ligand augments their antigen presenting activity

Recent studies have shown that human myeloid leukaemia cells can differentiate into dendritic cell (DC)-like cells (leukaemia-DCs) when cultured with a combination of cytokines. In the present study, we examined whether the transduction of leukaemia-DCs with OX40 ligand (OX40L), a member of the tumour necrosis factor (TNF) family, resulted in augmentation of their antigen presenting activity. Bicistronic retroviral vectors expressing both human OX40L and enhanced green fluorescent protein (EGFP) or EGFP alone were generated and used for transduction. Fresh leukaemic cells from five patients with acute myeloid leukaemia (AML) were isolated and retrovirally transduced with OX40L during the culture with a combination of cytokines from stem cell factor, fms-like tyrosine kinase (Flt)-3 ligand, granulocyte-macrophage colony stimulating factor (GM-CSF), interleukin-4 (IL-4) and TNF-alpha. After 7 d, the majority of cells showed DC-like morphology, and expressed higher levels of CD80, CD86 and HLA-DR than fresh leukaemic cells. The transduction efficiency was 8.5-27.2%. Leukaemia-DCs transduced with OX40L elicited higher proliferative response of allogeneic CD4+ T cells than fresh leukaemic cells, non-transduced, or mock-transduced leukaemia-DCs. Co-culture of allogeneic CD4+ T cells with OX40L-transduced leukaemia-DCs was superior in the generation of interferon (IFN)-gamma producing CD4+ T cells and in production of IFN-gamma. Furthermore, OX40L-transduced leukaemia-DCs could elicit significant proliferative response of human leucocyte antigen-matched T cells from the donor in allogeneic stem cell transplantation. These results indicate that retroviral transduction of leukaemia-DCs with OX40L augments their antigen presenting cell activity and thus renders them more suitable for tumour vaccines or ex vivo stimulation of leukaemia-specific T cells.

Identification of human tumor antigens recognized by T-cells and their use for immunotherapy

Identification of human tumor antigens led to development of new immunotherapies for patients with cancer. Identification of T-cell epitopes allowed us to monitor antitumor T-cell responses quantitatively and qualitatively during immunotherapy as well as to develop more efficient immunotherapies with sufficient amounts of antigens in more immunogenic forms. Various immunotherapies, passive immunotherapies, including adoptive transfer of tumor reactive T-cells or allogeneic antigen-specific donor T-cells, and active immunization with identified tumor antigens or dendritic cells pulsed with tumor antigens are being evaluated in clinical trials. Although tumor regression has been observed in some patients, further improvement is required.

Absence of leukaemic fusion gene transcripts in preterm infants exposed to diagnostic x rays

BACKGROUND

Childhood leukaemias express novel, clonotypic fusion genes that may already be present at birth before the clinical manifestation of leukaemia. Exposure of the fetus to diagnostic x rays is reported to increase the risk of childhood leukaemia, and may do so by generating leukaemic fusion genes. Advances in neonatal medicine in the past decade that have extended the limits of viability of preterm babies down to 23 weeks of gestation have resulted in the increased use of diagnostic x rays to monitor neonatal progress.

AIM

To investigate whether exposure of very preterm infants to diagnostic x rays in the neonatal period leads to the development of leukaemic fusion genes.

METHODS

Peripheral blood samples were collected at birth from very preterm infants (23-30 weeks gestation) and following exposure to diagnostic x rays at intervals of two weeks, until discharge. Cord blood samples from normal full term infants served as controls. Total RNA was extracted from the blood and the expression of the fusion genes TEL-AML1, MLL-AF4, and BCR-ABL, characteristic of three subtypes of childhood leukaemia, was investigated in the preterm and full term infant samples using a nested reverse transcriptase polymerase chain reaction method. Serial pre- and post-x ray samples from 42 preterm babies, pre-x ray samples from an additional 46 preterm infants, and cord blood samples from 100 normal full term infants were screened for fusion gene transcripts.

RESULTS

No leukaemic fusion gene transcripts were detected in preterm infants following exposure to diagnostic x rays. A BCR-ABL transcript was identified in a single preterm infant prior to x ray exposure. TEL-AML1 transcripts were detected in cord blood samples from two full term infants. MLL-AF4 transcripts were not detected in any of the pre- or full term infants tested.

CONCLUSIONS

Exposure of the preterm infants to x rays in this small series and at the doses used for diagnostic purposes did not induce leukaemic fusion gene expression, but we cannot exclude the possibility that a small proportion of preterm infants may be unusually sensitive to x rays.

Cytogenetics and molecular genetics of acute lymphoblastic leukemia

An important factor in the diagnosis of acute lymphoblastic leukemia (ALL) is that karyotype is an independent prognostic indicator, with an impact on the choice of treatment. Outcome is related to the number of chromosomes. For example, high hyperdiploidy (51-65 chromosomes) is associated with a good prognosis, whereas patients with near haploidy (23-29 chromosomes) have a poor outcome. The discovery of recurring chromosomal abnormalities in the leukemic blasts of patients with ALL has identified a large number of genes involved in leukemogenesis. Certain specific genetic changes are related to prognosis. The ETV6/AML1 fusion arising from the translocation (t12;21) (p13;q22) has been associated with a good outcome; the BCR/ABL fusion of (t9;22)(q34;q11), rearrangements of the MLL gene, and abnormalities of the short arm of chromosomes 9 involving the tumor suppressor genes p16INK4A have a poor prognosis. Unfortunately, the classification of patients into prognostic groups based on cytogenetics is not always as predicted. Even when other clinically based risk factors are taken into account, some patients with good-risk cytogenetic features will relapse. In the search for new measures of prognosis, it has recently emerged that the level of minimal residual disease following induction therapy can be a reliable predictor of outcome in ALL.

Clonal expansion of freshly isolated CD4T cells by randomized peptides and identification of peptide ligands using combinatorial peptide libraries

We synthesized Xn (n = 9 -- 19) peptides that consist of 9 to 19 residues with random sequences. X19 is considered to deliver antigenic stimuli to CD4 T cells, because: (a) X19 induces proliferation of peripheral blood mononuclear cells (PBMC), in the presence of IL-2, which is abrogated by monoclonal antibodies to class II HLA; (b) X19 + IL-2 induces proliferation of CD4 T cell clones of distinct specificities; and (c) T cell clones recognizing the same TCR ligands with distinct V beta usage are equally stimulated by X19 + IL-2. We next co-cultured single peripheral CD4 T cells with X19 and mitomycin-treated autologous PBMC. Indeed, single T cells of CD45RA(-) memory phenotype exhibited clonal expansion, with variable rates of proliferation, when IL-4, IL-7, IL-9, IL-15 and agonistic antibody to CD29 were included in the culture. These T cell clones showed heterogeneous proliferation patterns against KGXXXXXXXXXGK-based and KGXXXXXXXXXGKGKK-based combinatorial peptides libraries, in the presence of IL-2. Pattern-match search on a T cell clone resulted in peptide ligand candidates, one of which induced proliferation, as did protein molecules carrying the corresponding sequence. These results indicate that X19 can induce proliferation of peripheral memory T cells, the peptide ligands of which can be determined using combinatorial peptide libraries.

Prospects for immunotherapy of acute lymphoblastic leukemia

Acute lymphoblastic leukemia (ALL) is diagnosed in approximately 100000 people worldwide per year and 70% of the patients are children. Most children have a good prognosis, as almost 80% will be cured, however only 30% of adults are cured. Additionally, the current chemotherapies have long-lasting and severe side-effects. These findings indicate that the search for better and safer treatment modalities for ALL is still important. As leukemia directly affects the human immune cells, immunotherapeutic approaches have long been ignored as treatment options for this disease. However, increased knowledge of the immune system has opened new opportunities for immune modulation that could be of benefit to leukemia patients. Several recent advances towards immunotherapy of ALL will be discussed.

CML vaccines as a paradigm of the specific immunotherapy of cancer

T cells are implicated in the effective control of chronic myeloid leukemia (CML). Recently, several clinical observations supported by laboratory data, indicate the presence of CML-specific T cells. Many proteins potentially act as leukemia-specific antigens for MHC-restricted cytotoxicity in CML. These include the bcr-abl fusion protein, myeloid-specific differentiation antigens and minor histocompatibility antigens. There is recent evidence to suggest that bcr-abl junctional peptides are capable of eliciting both CD4 and CD8 responses in normal healthy donors and in patients with CML. Moreover, T cell lines can be generated that react with autologous or HLA-matched fresh CML cells, suggesting that the bcr-abl fusion protein can be processed and expressed in the MHC cell surface molecules. Clinical trials exploiting the new understanding of the immunology of CML are underway.