A CD4+ T CELL CLONE SELECTED FROM A CML PATIENT AFTER DONOR LYMPHOCYTE INFUSION RECOGNIZES BCR-ABL BREAKPOINT PEPTIDES BUT NOT TUMOR CELLS1

Experimental and clinical approaches for optimization of the graft-versus-leukemia effect

The goal of allogeneic (allo)-hematopoietic stem-cell transplantation (HSCT) in the treatment of hematologic malignancies is to harness the graft-versus-leukemia (GVL) effect, while minimizing the risk of graft-versus-host disease (GVHD). Allo-HSCT research has focused on the GVL target antigens and effector mechanisms, and on potential approaches to exploit GVL independently of GVHD. Donor lymphocyte infusion (DLI) achieves the most powerful anti-leukemic responses, and this approach is often used in combination with nonmyeloablative transplant regimens to optimize GVL and reduce GVHD. Serial, dose-escalating, and CD8(+) T-cell-depleted DLI have been introduced into clinical practice, while other variants of DLI have so far been explored only in animal models. The role of naturally occurring regulatory T cells in transplantation tolerance is being increasingly acknowledged, and murine studies indicate the potential ability of T cells to regulate GVHD while maintaining GVL. Experimental and clinical studies have demonstrated the importance of host-type chimerism, particularly for antigen-presenting cells, in determining the occurrence of DLI-induced GVL. Murine studies could assist in the development of clinical strategies targeted at antigen-presenting cells. Clinical studies exploiting natural killer-cell-mediated antitumor reactivity in the context of killer inhibitory receptor-ligand-mismatched allo-HSCT have provided promising results.

Circulating myeloid dendritic cell directly isolated from patients with chronic myelogenous leukemia are functional and carry the bcr-abl translocation

Leukemic bcr-abl positive dendritic cells (DCs) are likely to be present in vivo in chronic myelogenous leukemia (CML) patients, but no data are available on their functional qualities. We analyzed the circulating BDCA-1+ myeloid DC compartment in 15 chronic phase CML patients. Phenotypic features of CML DCs were comparable with that of normal DCs, except for the CD80 and CD40 antigens, significantly under-represented in CML patients. Nonetheless, no differences were found between normal samples and leukemic DCs in the allostimulatory ability, as well as in the production of cytokines and polarization of T cell responses. CML DCs were analyzed by fluorescence in situ hybridization (FISH) and found positive for the bcr-abl translocation. However, when bcr-abl+ DCs were tested for their ability to stimulate an autologous T-cell response in vitro, we could not detect a specific recognition. We conclude that an apparently normal circulating DC compartment carrying the Ph+ chromosome can be identified in CML patients; however, these cells appear unable to trigger a protective anti-leukemic immune response in autologous T cells.

Priming with Dendritic Cells Can Generate Strong Cytotoxic T Cell Responses to Chronic Myelogenous Leukemia Cells In Vitro

Dendritic cells (DC) are antigen-presenting cells that can elicit potent antigen-specific responses. Since the development of techniques to cultivate these cells from peripheral blood, there has been a great deal of interest in their use in immunotherapeutic strategies. Here we show that morphologically, phenotypically, and functionally characteristic DC can be generated in vitro from peripheral blood mononuclear cells (PBMC) isolated from frozen apheresis product (AP) of cancer patients. These DC, when pulsed with whole-tumor lysate, protein, or RNA from a chronic myelogenous leukemia (CML) cell line, can induce anti-CML specific cytotoxicity in vitro by autologous cytotoxic T lymphocytes (CTL). RNA and protein-pulsed DC were more effective than lysate-pulsed DC at inducing cytotoxicity at low effector:target (E:T) ratios. These results were comparable to those obtained when fresh healthy peripheral blood was used as the source of PBMC, indicating that neither the malignant state of the patient nor the storage period detrimentally affected the generation or functionality of DC. CML cells were found to increase their level of MHC class I expression after exposure to CTL and pulsed DC thereby becoming better targets. These investigations lend support for the utilization of DC to generate anti-tumor responses in CML.

Identification of new MHC-restriction elements for presentation of the p210(BCR-ABL) fusion region to human cytotoxic T lymphocytes

Chronic myelogenous leukemia (CML) is characterized by a t(9;22) translocation resulting in expression of BCR-ABL fusion oncoproteins which are unique to the leukemic cells, necessary for oncogenesis, and potentially immunogenic. We have previously shown that human dendritic cells transduced with an adeno-associated virus vector encoding the fusion region of the b3a2 splice variant (p210(b3a2)) of the BCR-ABL oncoprotein elicit specific T-cell responses in vitro. Two cytotoxic T lymphocyte (CTL) clones generated in this fashion displayed restriction with previously unreported HLA alleles. The first, T1/B9, was CD4(+) and restricted by DRB5*0101 (autologous) or DRB1*1101 (allogeneic). The minimum cytotoxic epitope (MCE) binding to DRB5*0101 for this clone was identified as FKQSSKALQ, overlapping the p210(b3a2) fusion point (boldface). The MCE of DRB1*1101 for this clone differed from DRB5*0101, but also included the fusion point. The clonality of CTL T1/B9 was verified by analyses of TCRalpha/beta chain usage and DNA sequence analyses. To our knowledge, this is the first description of a single clone recognizing both DRB5*0101 and DRB1*1101. The other CTL clone, T1/33, was CD8+ and recognized HLA-B*3501 or B*3503 complexed with an MCE, RPVASDFEP, derived from the c-abl sequence in proximity to the p210(b3a2) fusion point. K562 cells transfected with plasmids encoding HLA-DRA + B5*0101, B*3501, or B*3503 but not controls expressing DRA + DRB1*1501 were lysed by cognate CTL clones, confirming that DRB5*0101 and B*3501/3 could present p210(b3a2) joining region epitopes via endogenous processing. The identification of three additional HLA alleles (DRB5*0101, B*3501, and B*3503) presenting the p210(b3a2) fusion-region antigen will broaden the application of vaccine strategies for targeting CML cells. The findings of single CTL clones cross-recognizing autologous (DRB5*0101 or B*3501) and allogeneic (DRB1*1101 or B*3503) HLA alleles presenting BCR-ABL fusion-region epitopes implies the potential separation of graft-versus-leukemia from graft-versus-host effects.

A multivalent bcr-abl fusion peptide vaccination trial in patients with chronic myeloid leukemia

A tumor-specific, bcr-abl-derived fusion peptide vaccine can be safely administered to patients with chronic myelogenous leukemia (CML) and can elicit a bcr-abl peptide-specific T-cell immune response. In the present phase 2 trial, 14 patients with CML in chronic phase were vaccinated with 6 fusion peptides mixed with Quillaja saponaria (QS-21). No significant toxic effects were observed. In 14 of 14 patients, delayed-type hypersensitivity (DTH) and/or CD4 proliferative responses developed after beginning vaccinations, and 11 of 14 patients showed interferon-gamma (IFN-gamma) release by CD4 enzyme-linked immunospot (ELISPOT) at one or more time points. These responses were CD4(+)CD45RO(+). A peptide-specific CD8(+) interferon-gamma ELISPOT was found in 4 patients. Four patients in hematologic remission had a decrease in Philadelphia chromosome (Ph) percentage (3 concurrently receiving interferon-alpha and 1 on imatinib mesylate), and 3 patients in molecular relapse after allogenic transplantation became transiently polymerase chain reaction (PCR) negative after vaccination; 2 of these patients received concurrent donor lymphocyte infusion (DLI). All 5 patients on IFN-alpha ultimately reached a complete cytogenetic remission. In conclusion, a tumor-specific bcr-abl breakpoint peptide-derived vaccine can be safely administered and can reliably elicit measurable peptide-specific CD4 immune responses, including in patients after bone marrow transplantation, on interferon, or on imatinib mesylate. A relationship between the clinical responses and vaccination cannot be determined from this trial.

Adoptive cellular therapy: a therapeutic reality?

The past decade has seen numerous lines of evidence emerging that suggest human malignancies may be sensitive to the effects of cellular immunotherapy. An increasing understanding of the nature of the effector cells and their target antigens is now leading to more focused efforts to harness these responses for therapeutic benefit. However, clinical application has proven more challenging than initially envisaged. Advances in the setting of allogeneic stem cell transplantation now allow attempts to augment both immunological recovery and anti-tumour activity. Some of the most attractive targets here are allospecific rather than truly tumour-specific. Application outside of this setting is based on attempts to delineate further tumour-specific or, increasingly, tumour-selective targets. This review summarizes these developments and highlights some of the issues that remain to be resolved.

BCR-ABL as a target for novel therapeutic interventions

The BCR-ABL oncogene is the result of a reciprocal translocation between the long arms of chromosome 9 and 22 t(9; 22). There is good experimental evidence demonstrating that BCR-ABL is the single causative abnormality in chronic myeloid leukaemia (CML), making it a unique model for the development of molecular targets. In addition to CML, BCR-ABL transcripts can be found in a minority of acute lymphoblastic leukaemias and very rarely in acute myeloid leukaemia (AML). Elucidating the molecular mechanisms and downstream pathways of BCR-ABL has led to the design of several novel therapeutic approaches. In this review, molecular targeting of BCR-ABL will be discussed based on the inhibition of protein tyrosine kinase activity, antisense strategies and immunomodulation.

Cellular therapy: donor lymphocyte infusion

The dose escalation of chemoradiotherapy that is achievable with stem cell transplantation is often insufficient to eradicate malignancy, and an associated immune-mediated graft-versus-malignancy effect may be equally important for many diseases. The most directly compelling evidence for its presence is the efficacy of donor lymphocyte infusions in generating anti-tumor responses, particularly for relapsed chronic-phase chronic myeloid leukemia. Response rates and durability appear lower with myeloma and acute myeloid leukemia and myelodysplasia syndrome, and minimal with acute lymphoblastic leukemia. There is relatively little data on indolent lymphoid malignancies. Issues that remain to be resolved include the precise nature of the effector cells and their target antigens, the best strategies for separating graft-versus-malignancy from graft-versus-host disease (GVHD) and their effect on the durability of responses, and the role of adjuvant chemotherapy/cytokines. Similar issues surround routine combination with nonmyeloablative transplantation protocols and preliminary data suggests that GVHD may continue to provide a major obstacle.