Hematopoietic cell transplantation in older patients with hematologic malignancies: replacing high-dose cytotoxic therapy with graft-versus-tumor effects

Intensive chemotherapy for progressive chronic lymphocytic leukemia administered early after a nonmyeloablative allograft

A 51-year-old patient with refractory CLL elected to participate in a trial of nonmyeloablative trans- plantation from an HLA-matched unrelated donor. He received low-dose fludarabine/TBI, with infusion of donor PBPC and cyclosporin (CsA)/MMF. Early post transplant he experienced explosive tumor growth with respiratory insufficiency. After immunosuppression discontinuation and rituximab administration, no response was observed. This prompted treatment with cyclophosphamide (2 g/m(2)/day x 2), paclitaxel (250 mg/m(2) over 24 h), doxorubicin (50 mg/m(2)), solumedrol (500 mg/day), and a second dose of rituximab, from days +11 to +14. A rapid response was achieved. Chemotherapy did not cause an obvious compromise of donor stem cell engraftment or establishment of stable donor chimerism.

Cellular therapy: exploiting NK cell alloreactivity in transplantation

Allogeneic hematopoietic transplantation relies on T-cell alloreactions for engraftment and the graft-versus-leukemia (GVL) effect. In human leukocyte antigen (HLA) haplotype-mismatched transplants, extensive T-cell depletion of the graft is essential to prevent GVHD. This raises the question of whether mismatched transplants exert any GVL effect and whether it will ever be possible to reduce the intensity of preparative regimens. Because natural killer (NK) cells are negatively regulated by MHC class I-specific inhibitory receptors, mismatched transplants may trigger NK-cell alloreactivity. HLA class I disparities driving NK-cell alloreactions in the GVH direction mediate strong GVL effects, produce higher engraftment rates, and do not cause GVHD. In murine MHC-mismatched transplant models with no donor T-cell reactivity against the recipient, the pre-transplant infusion of donor-vs-recipient alloreactive NK cells conditioned the recipients to bone marrow transplantation without GVHD. NK-cell alloreactivity may be a unique therapeutic tool for tolerance induction and clearance of leukemia in hematopoietic transplantation.

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.

Nonmyeloablative transplantation in children. Current status and future prospects

Over the last few years a variety of conditioning regimens have been developed that allow allogeneic hematopoietic stem cell engraftment with significantly decreased transplant related-toxicity. While these reduced-intensity regimens have offered hope for patients with malignancies formerly not eligible for myeloablative transplantation due to excessive morbidity (older patients and patients with significant organ toxicity), the role of nonmyeloablative hematopoietic cell transplantation (NM-HCT) in children is unclear. A review of the available literature for pediatric and adult studies shows several malignancies in which approaches designed to limit long-term complications in children may be appropriate. In addition, NM-HCT may offer a safer approach for children with inherited disorders curable by marrow transplantation, such as immunodeficiencies, hemoglobinopathies, or storage diseases. Finally, use of this approach to establish partial donor chimerism may provide an immunologic platform that will allow specific cellular therapies, targeted gene therapy, or immunologic tolerance in solid organ transplantation.

Controversy in multiple myeloma transplants: tandem autotransplants and mini-allografts

Autologous stem cell transplantation appears to enhance outcome in multiple myeloma patients. To improve upon these results, various groups have utilized tandem autografts, as well as used reduced-conditioning allogeneic stem cell transplantation. These two approaches, discussed herein, have been promising. Inherent patient selection, however, appears to play a role and much of the data have not yet been subjected to peer-review scrutiny. At present, these strategies remain investigational and cannot be considered the standard-of-care for multiple myeloma patients.

Allogeneic hematopoietic cell transplantation for multiple myeloma

Some patients with multiple myeloma (MM) who have undergone allogeneic hematopoietic stem cell transplants remain free of disease 5 to 13 years later-a major accomplishment for a malignancy that had been resistant to all investigational therapies. Although it will require longer follow-up to determine how many are truly cured, results for patients with MM transplanted from identical twins suggest that long-term progression-free survival is possible. While 3- to 5-year survival is similar after allogeneic or autologous stem cell transplant for MM, only allograft recipients appear to enjoy long-term disease-free survival, most likely due to an allogeneic graft-versus-myeloma (GVM) effect. The very high transplant-related mortality associated with standard allogeneic stem cell transplantation is currently the major limitation to wider use of this potentially curative treatment. The challenge for clinical investigators will be to reduce the incidence of posttransplant complications. Strategies include the use of nonablative conditioning for allografts, the administration of peripheral blood stem cells (PBSC) rather than bone marrow, and the application of more focused, targeted conditioning therapies such as bone-seeking radioisotopes.

Non-myeloablative transplants for malignant disease

This article discusses changes in the way hematopoietic stem cell allotransplants may be carried out in the future to treat patients with malignant hematological diseases. Specifically, the focus has shifted away from attempts at eradicating underlying diseases through toxic high-dose chemoradiation therapy towards using the stem cell donor's immune cells for that purpose (allogeneic graft-versus-tumor effect). The non-myeloablative transplant approaches hold promise in reducing the morbidity and mortality associated with conventional high-dose chemoradiation therapy, and they allow allogeneic transplants in elderly or medically infirm patients who are at present not candidates for transplantation. In the future, specific graft-versus-tumor responses may become possible by eliciting donor T cell responses to tumor-associated minor histocompatibility antigens. In Section I, Dr. Rainer Storb describes experimental studies in random-bred dogs that rely on non-cytotoxic immunosuppressive agents to establish stable allografts. Powerful postgrafting immunosuppression, traditionally directed at preventing graft-versus-host disease (GVHD), is also used to overcome host-versus-graft (HVG) reactions, thereby dramatically reducing the need for intensive immunosuppressive conditioning programs. Preclinical canine studies have been translated into the clinical setting for treatment of elderly or medically infirm patients with malignant hematological diseases. The pretransplant conditioning has been reduced to a single dose of 2 Gy total body irradiation (TBI) with or without fludarabine. The lack of toxicity makes it possible for transplants to be conducted in the outpatient setting. Multicenter trials have been initiated, and more than 300 patients have been successfully treated with hematopoietic stem cell grafts both from related and unrelated HLA-matched donors. In Section II, Dr. Richard Champlin describes clinical studies with therapeutic strategies that utilize relatively non-toxic, nonmyeloablative disease-specific preparative regimens incorporating fludarabine, together with other chemotherapeutic agents, to achieve disease suppression and engraftment of allogeneic hematopoietic cells and to allow subsequent infusions of donor lymphocytes. Remissions have been seen in patients with acute myelocytic, chronic myelocytic, chronic lymphocytic, leukemias, lymphomas, and myelomas. In Section III, Dr. Stanley Riddell and colleagues describe studies on isolation of T cells reactive with minor histocompatibility (H) antigens and involved both in GVHD and graft-versus-leukemia (GVL) responses. For example, the gene encoding a novel H-Y antigen in humans has been identified and shown to exhibit restricted tissue expression. Acute myelocytic leukemia stem cells were demonstrated to express the H-Y antigen and additional minor H antigens, and engraftment of such cells in NOD/SCID mice could be selectively prevented by minor antigen-specific T-cell clones. An autosomal encoded human minor H antigen associated with chronic GVHD has been demonstrated. A trial evaluating therapy of relapsed acute myelocytic leukemia or acute lymphoblastic leukemia after allogeneic stem cell transplantation with T-cell clones specific for recipient minor H antigens has been initiated.