Allogeneic immunotherapy to optimize the graft-versus-tumor effect: concepts and controversies

Serial Evaluation of T cell Subsets in Paediatric Acute Myeloid Leukaemia- a Prospective Study

PURPOSE

There is paucity of data regarding T-cells in paediatric AML patients. The aim of this prospective study was to evaluate trend of T-cell subset during disease course of paediatric AML patients and to see its correlation with patient characteristics and survival outcome.

METHODS

T-cell subsets (CD3, CD4 and CD8) were evaluated by flow-cytometry at diagnosis, post-induction, post-treatment completion, at 3 months and 6 months post-treatment completion, and relapse in 29 pediatric AML patients. Trend of T-cells was plotted between group A (those in continuous remission) and group B (those who relapsed) patients.

RESULTS

Patients with high WBC count had significantly higher number of CD3, CD4 and CD8 cell. Baseline Tcell subsets did not affect CR, EFS and OS; however, higher than median CD4 count predicted improved DFS [58% vs 25%; HR = 0.306 (0.10-0.93); P = 0.037]. On serial follow-up from post-induction till 3 months after completion of therapy, there was no difference in the absolute values of T cell subsets between group A and B patients.

CONCLUSION

Our study demonstrated T cell subsets are increased in AML subjects with high WBC count. CD4 cells have a positive impact on DFS. Serial follow-up has no impact on T cell subsets. Further studies in larger patient cohorts are needed to evaluate if CD4 population may serve as an immune biomarker for AML.

Beta-Adrenergic Signaling in Tumor Immunology and Immunotherapy

Communication between the nervous and immune systems is required for the body to regulate physiological homeostasis. Beta-adrenergic receptors expressed on immune cells mediate the modulation of immune response by neural activity. Activation of beta-adrenergic signaling results in suppression of antitumor immune response and limits the efficacy of cancer immunotherapy. Beta-adrenergic signaling is also involved in regulation of hematopoietic reconstitution, which is critical to the graft-versus-tumor (GVT) effect and to graft-versus-host disease (GVHD) following allogeneic hematopoietic cell transplantation (HCT). In this review, the function of beta-adrenergic signaling in mediating tumor immunosuppression will be highlighted. We will also discuss the implication of targeting beta-adrenergic signaling to improve the efficacy of cancer immunotherapy including the GVT effect, and to diminish the adverse effects including GVHD.

Transplantation

Mature T-cell non-Hodgkin lymphomas (T-cell NHL) are a heterogeneous group of lymphoid malignancies including NK/T-cell lymphomas. Hematopoietic cell transplantation (HCT) is an important component of the management of T-cell NHL; however, the optimal timing and type of transplant for each different subtype is an ongoing debate. For the purpose of this chapter, PTCL will be classified as (1) systemic PTCL that includes nodal as well as non-nodal histologies in PTCL (2) CTCL-or cutaneous T-cell lymphomas that arise primarily in the skin and (3) NK/T-cell lymphomas both nasal and extranasal types. It is difficult to do any large trials in PTCL as they are rare diseases with variable clinical and biological characteristics and most patients are not transplant eligible due to various reasons including poor disease control. There are no randomized trials in transplant for PTCL but there is an experience based on retrospective as well as some well-designed prospective trials that have helped outline the role of HSCT in the treatment paradigm of PTCL. High-dose therapy and autologous HCT is recommended in first complete remission for most systemic (non-cutaneous) nodal subtypes, or peripheral T-cell lymphomas (PTCL). Autologous HCT can provide long-term remission for relapsed PTCL but is ineffective for refractory/chemoresistant disease. Allogeneic stem cell transplantation harnesses the graft-versus-lymphoma effect, providing long-term remission for relapsed PTCL. AlloHCT is also being used successfully to provide long-term disease control for advanced cutaneous T-cell lymphoma (CTCL). The use of transplant in NK/T-cell lymphoma is increasingly being recommended in the relapsed setting only as there are more effective treatments available for the upfront setting in limited stage disease.

Survivin-peptide vaccination elicits immune response after allogeneic nonmyeloablative transplantation: a safe strategy to enhance the graft versus tumor effect

Allogeneic hematopoietic cell transplantation (allo-HCT) is an adoptive immunotherapy strategy whose effectiveness relies on graft-versus-tumor (GVT) effect. We explored the feasibility of enhancing GVT after allo-HCT by peptide vaccination. Two myeloma patients were transplanted with a fludarabine-total body irradiation conditioning regimen and vaccinated with an HLA-A*0201-restricted modified survivin nonapeptide, plus montanide as adjuvant. At time of first vaccination, one patient had just attained serological remission despite documented relapse after transplant, while the other patient was in stable disease. Both patients had an immune response to vaccination: the frequency of survivin-specific CD8⁺ T cells increased between second and sixth vaccination and accounted for 0.5-0.8% of CD8⁺ cells; CD8⁺ cells were functional in ELISPOT assay. The first patient persists in complete remission with a follow-up of >5 years, while the second patient did not have a clinical response and vaccination was halted. We analyzed the T-cell receptor (TCR) repertoire of the first patient by spectratyping and found that vaccination did not affect the diversity of TCR profile, indicating that survivin clonotypes were probably spread in multiple TCR families. We generated a limited number (n = 4) of survivin-specific T cell clones: three were reactive only against the modified peptide, whereas one clone recognized also the naive peptide. Peptide vaccination is safe and applicable after allo-HCT and elicits an efficient antigen-specific T cell response without causing graft-versus-host disease.

Combination of anti-CD4 antibody treatment and donor lymphocyte infusion ameliorates graft-versus-host disease while preserving graft-versus-tumor effects in murine allogeneic hematopoietic stem cell transplantation

Allogeneic hematopoietic stem cell transplantation (allo‐HSCT) is not only a well‐established immunotherapy for hematologic malignancies, but is potentially useful for treating solid tumors refractory to available therapies. However, application of allo‐HSCT to solid tumors is limited, despite the beneficial antitumor effects, by the risk of graft‐versus‐host disease (GVHD). CD4⁺ T cells have been implicated in several aspects of GVHD, and suppress antitumor CD8⁺ T‐cell responses. In the present study, we investigated clinically applicable allo‐HSCT protocols designed to maximize antitumor effects while reducing the risk of GVHD. We used a mouse model of allo‐HSCT with s.c. tumors. We found that myeloablative conditioning was associated with better inhibition of tumor growth but with severe acute GVHD. Early treatment with anti‐CD4 mAb substantially ameliorated GVHD while preserving antitumor effects, leading to improved survival in myeloablative allo‐HSCT. Late treatment with anti‐CD4 mAb also ameliorated GVHD to some extent. Donor lymphocyte infusion in GVHD mice treated with anti‐CD4 mAb further suppressed tumor growth without exacerbating GVHD. Collectively, our results suggest that myeloablative allo‐HSCT followed by anti‐CD4 mAb treatment and donor lymphocyte infusion could be a potent and safe immunotherapy for patients with cancers refractory to available therapies.

Time to unrelated donor leukocyte infusion is longer, but incidence of GVHD and overall survival are similar for recipients of unrelated DLI compared to matched sibling DLI

Donor leukocyte infusion (DLI) is used to treat relapsed leukemia after allogeneic hematopoietic stem cell transplant (HCT). Data comparing outcomes after unrelated DLI (uDLI) to matched sibling DLI (msDLI) are scant. We performed a retrospective analysis to assess differences in time to administer uDLI versus msDLI, and impact on outcomes. Fifty three patients with relapsed acute myeloid leukemia (AML), acute lymphoblastic leukemia (ALL), or myelodysplastic syndrome (MDS) after allogeneic HCT received uDLI (n = 18) or msDLI (n = 35) from 2000 to 2011. Median time from relapse to uDLI request was 15 days (range 0-66). Median time from relapse to uDLI was 56 days versus 40 days for msDLI patients (p = 0.034). 35% of msDLI and 44% of uDLI patients developed acute GVHD (p = 0.50). There was no significant difference in Grade C/D GVHD among uDLI and msDLI (28% and 21%, p = 0.58) or median OS after DLI between uDLI and msDLI (95 versus 75 days, p = 0.76). For patients with relapsed acute leukemia and MDS after allogeneic HCT, time from relapse to uDLI was longer than to msDLI, but incidence of GVHD and overall survival were similar. Access to uDLI does not appear to be a barrier to DLI administration. Outcomes unfortunately remain poor regardless of donor source.

New strategies of DLI in the management of relapse of hematological malignancies after allogeneic hematopoietic SCT

DLI is an effective strategy for patients with recurrent hematological malignancies after allogeneic hematopoietic SCT (allo-HSCT). DLI has been widely applied to boost the graft vs tumor (GVT) or GVL effects. However, given the potentially severe complications associated with conventional DLI and transient GVL effect, new strategies for DLI are emerging. In this review, we have discussed the recent important studies on DLI as a prophylactic or therapeutic modality for relapsed hematological disorders after allo-HSCT. The strategies to separate GVL from GVHD have also been discussed. Leukemia-targeting therapy and lymphodepletion combined with DLI, and prophylactic DLI after allo-HSCT are often employed for patients with high risk of relapse, which has been reviewed as well. In addition, we have also discussed the issues on DLI to be further addressed, such as the doses, timing and frequency of DLI in different clinical settings, leukemic antigen-specific DLI as well as how to augment GVL effect while attenuating GVHD.

Pushing the envelope-nonmyeloablative and reduced intensity preparative regimens for allogeneic hematopoietic transplantation

Allogeneic hematopoietic cell transplantation (HCT) was originally developed to allow delivery of myeloablative doses of chemotherapy and radiotherapy. With better understanding of disease pathophysiology, the graft vs malignancy (GVM) effect of allogeneic hematopoietic transplantation and toxicities associated with myeloablative conditioning (MAC) regimens, the focus shifted to developing less toxic conditioning regimens to reduce treatment-related morbidity without compromising survival. Although HCT with MAC is preferred to reduced intensity conditioning (RIC) for most patients ⩽60 years with AML/myelodysplastic syndrome and ALL, RIC and nonmyeloablative (NMA) regimens allow HCT for many otherwise ineligible patients. Reduced intensity preparative regimens have produced high rates of PFS for diagnoses, which are highly sensitive to GVM. Relapse of the malignancy is the major cause of treatment failure with RIC/NMA HCT. Incorporation of novel agents like bortezomib or lenalidomide, addition of cellular immunotherapy and use of targeted radiation therapies could further improve outcome. In this review, we discuss commonly used RIC/NMA regimens and promising novel regimens.

Impact of Chronic Graft-versus-Host Disease on Late Relapse and Survival on 7,489 Patients after Myeloablative Allogeneic Hematopoietic Cell Transplantation for Leukemia

PURPOSE

Malignancy relapse remains a major obstacle for successful allogeneic hematopoietic cell transplantation (HCT). Chronic graft-versus-host disease (cGVHD) is associated with fewer relapses. However, when studying effects of cGVHD on relapse, it is difficult to separate from acute GVHD effects as most cases of cGVHD occur within the first year after transplant at the time when acute GVHD is still active.

EXPERIMENTAL DESIGN

This study based on CIBMTR registry data investigated cGVHD and its association with the incidence of late relapse and survival in 7,489 patients with acute myelogenous leukemia (AML), acute lymphoblastic leukemia (ALL), chronic myelogenous leukemia (CML), and myelodysplastic syndromes (MDS), who were leukemia free at 12 months after myeloablative allogeneic HCT.

RESULTS

Forty-seven percent of the study population was diagnosed with cGVHD at 12 months after transplant. The protective effect of cGVHD on late relapse was present only in patients with CML [RR, 0.47; 95% confidence interval (CI), 0.37-0.59; P < 0.0001). cGVHD was significantly associated with higher risk of treatment-related mortality (TRM; RR, 2.43; 95% CI, 2.09-2.82; P < 0.0001) and inferior overall survival (RR, 1.56; 95% CI, 1.41-1.73; P < 0.0001) for all diseases. In patients with CML, all organ sites and presentation types of cGVHD were equally associated with lower risk of late relapse.

CONCLUSIONS

These results indicate that clinically relevant antileukemia effects of cGVHD on late relapses are present only in CML but not in AML, ALL, or MDS. Chronic GVHD in patients who are 1-year survivors after myeloablative allogeneic HCT is primarily associated with higher TRM and inferior survival.

Gamma delta T cell reconstitution is associated with fewer infections and improved event-free survival after hematopoietic stem cell transplantation for pediatric leukemia

After hematopoietic stem cell transplantation (HSCT), successful engraftment and immune recovery is necessary to protect the patient from relapse and infection. Many studies highlight the importance of conventional αβ T cell recovery after HSCT, but the impact of γδ T cell recovery has not been well described. Here, we investigate the recovery of γδ T cells in 102 pediatric patients with acute leukemia in first clinical remission who underwent allogeneic HSCT at St. Jude Children's Research Hospital from 1996 to 2011. Mean patient age was 10.5 ± 5.9 years (range, .6 to 25.2), and mean survivor follow-up was 2.7 ± 1.8 years (range, .12 to 6.0). Diagnoses included 59% patients with acute lymphoblastic leukemia and 41% patients with acute myelogenous leukemia. Multivariate analysis demonstrated significant impact of the maximum number of CD3(+), CD4(+), and CD8(+) T cells and donor source on the γδ T cell recovery (P < .0001, P < .0001, P < .0001, and P < .004, respectively). Univariate and multivariate models found the number of γδ T cells after HSCT to be associated with infections (P = .026 and P = .02, respectively). We found the probability of infections for patients with an elevated number of γδ T cells was significantly lower compared with patients with low or normal γδ T cells after HSCT (18% versus 54%; P = .025). Bacterial infections were not observed in patients with elevated γδ T cells. Finally, event-free survival was significantly higher in patients with enhanced γδ T cell reconstitution compared with patients with low/normal γδ T cell reconstitution after HSCT (91% versus 55%; P = .04). Thus, γδ T cells may play an important role in immune reconstitution after HSCT.

Hematopoietic stem cell transplantation-50 years of evolution and future perspectives

Hematopoietic stem cell transplantation is a highly specialized and unique medical procedure. Autologous transplantation allows the administration of high-dose chemotherapy without prolonged bone marrow aplasia. In allogeneic transplantation, donor-derived stem cells provide alloimmunity that enables a graft-versus-tumor effect to eradicate residual disease and prevent relapse. The first allogeneic transplantation was performed by E. Donnall Thomas in 1957. Since then the field has evolved and expanded worldwide. New indications beside acute leukemia and aplastic anemia have been constantly explored and now include congenital disorders of the hematopoietic system, metabolic disorders, and autoimmune disease. The use of matched unrelated donors, umbilical cord blood units, and partially matched related donors has dramatically extended the availability of allogeneic transplantation. Transplant-related mortality has decreased due to improved supportive care, including better strategies to prevent severe infections and with the incorporation of reduced-intensity conditioning protocols that lowered the toxicity and allowed for transplantation in older patients. However, disease relapse and graft-versus-host disease remain the two major causes of mortality with unsatisfactory progress. Intense research aiming to improve adoptive immunotherapy and increase graft-versus-leukemia response while decreasing graft-versus-host response might bring the next breakthrough in allogeneic transplantation. Strategies of graft manipulation, tumor-associated antigen vaccinations, monoclonal antibodies, and adoptive cellular immunotherapy have already proved clinically efficient. In the following years, allogeneic transplantation is likely to become more complex, more individualized, and more efficient.

Changes in T lymphocyte subsets in mice with CT26 colon tumors after treatment with donor lymphocyte infusion

The objective of this study was to detect changes in T lymphocyte subpopulations in mice with CT26 subcutaneous colon cancer after treatment with donor lymphocyte infusion (DLI) and cyclophosphamide (CP) chemotherapy. A colon cancer model was established by subcutaneous injection of CT26 carcinoma cells into BALB/C mice. The mice were randomized into different treatment groups. We recorded survival times, tumor growth inhibition rates, histopathological changes, and T lymphocyte subsets in peripheral blood of the mice. Mice treated with DLI and CP survived 33.5 ± 5.02 days, which was significantly longer than the survival time of untreated control mice (16.7 ± 2.98 days, P < 0.01). In addition, the tumor inhibitory rate was higher in mice treated with DLI and CP (89.3 %) than that in mice treated with CP or DLI alone (67.1 and 34.5 %, respectively). There were higher levels of T lymphocytes that were CD3(+) and CD4(+) in mice treated with DLI alone or the combination of CP and DLI (P < 0.05), and the ratio of CD4(+)/CD8(+) cells was significantly improved in these mice (P < 0.05). DLI combined with chemotherapy significantly prolonged survival and inhibited tumor growth in mice with CT26 colon cancer. This treatment might also improve immune function in these mice. Donor spleen cells that include high numbers of allogeneic lymphocytes and a few stem cells could induce a graft-versus-tumor effect, leading to elimination of residual cancer cells. This indicates that it is potentially a feasible adoptive cellular immunotherapy strategy for the management of solid tumors.

Stem cell Transplantation for Eradication of Minimal PAncreatic Cancer persisting after surgical Excision (STEM PACE Trial, ISRCTN47877138): study protocol for a phase II study

Background

Pancreatic cancer is the third most common cancer related cause of death. Even in the 15% of patients who are eligible for surgical resection the outlook is dismal with less than 10% of patients surviving after 5 years. Allogeneic hematopoietic (allo-HSCT) stem cell transplantation is an established treatment capable of to providing cure in a variety of hematopoietic malignancies. Best results are achieved when the underlying neoplasm has been turned into a stage of minimal disease by chemotherapy. Allo-HSCT in advanced solid tumors including pancreatic cancer have been of limited success, however studies of allo-HSCT in solid tumors in minimal disease situations have never been performed. The aim of this trial is to provide evidence for the clinical value of allo-HSCT in pancreatic cancer put into a minimal disease status by effective surgical resection and standard adjuvant chemotherapy.

Methods/Design

The STEM PACE trial is a single center, phase II study to evaluate adjuvant allogeneic hematopoietic stem cell transplantation in pancreatic cancer after surgical resection. The study will evaluate as primary endpoint 2 year progression free survival and will generate first time state-of-the-art scientific clinical evidence if allo-HSCT is feasible and if it can provide long term disease control in patients with effectively resected pancreatic cancer. Screened eligible patients after surgical resection and standard adjuvant chemotherapy with HLA matched related stem cell donor can participate. Patients without a matched donor will be used as a historical control. Study patients will undergo standard conditioning for allo-HSCT followed by transplantation of allogeneic unmanipulated peripheral blood stem cells. The follow up of the patients will continue for 2 years. Secondary endpoints will be evaluated on 7 postintervention visits.

Discussion

The principal question addressed in this trial is whether allo-HSCT can change the unfavourable natural course of this disease. The underlying hypothesis is that allo-HSCT has the capacity to provide long-term disease control to an extent otherwise not possible in pancreatic cancer, thereby substantially improving survival of affected patients.

Trial registration

This trial has been registered: ISRCTN47877138

Lentivirus-induced dendritic cells for immunization against high-risk WT1(+) acute myeloid leukemia

Wilms' tumor 1 antigen (WT1) is overexpressed in acute myeloid leukemia (AML), a high-risk neoplasm warranting development of novel immunotherapeutic approaches. Unfortunately, clinical immunotherapeutic use of WT1 peptides against AML has been inconclusive. With the rationale of stimulating multiantigenic responses against WT1, we genetically programmed long-lasting dendritic cells capable of producing and processing endogenous WT1 epitopes. A tricistronic lentiviral vector co-expressing a truncated form of WT1 (lacking the DNA-binding domain), granulocyte-macrophage colony-stimulating factor (GM-CSF), and interleukin-4 (IL-4) was used to transduce human monocytes ex vivo. Overnight transduction induced self-differentiation of monocytes into immunophenotypically stable "SmartDC/tWT1" (GM-CSF(+), IL-4(+), tWT1(+), IL-6(+), IL-8(+), TNF-α(+), MCP-1(+), HLA-DR(+), CD86(+), CCR2(+), CCR5(+)) that were viable for 3 weeks in vitro. SmartDC/tWT1 were produced with peripheral blood mononuclear cells (PBMC) obtained from an FLT3-ITD(+) AML patient and surplus material from a donor lymphocyte infusion (DLI) and used to expand CD8(+) T cells in vitro. Expanded cytotoxic T lymphocytes (CTLs) showed antigen-specific reactivity against WT1 and against WT1(+) leukemia cells. SmartDC/tWT1 injected s.c. into Nod.Rag1(-/-).IL2rγc(-/-) mice were viable in vivo for more than three weeks. Migration of human T cells (huCTLs) to the immunization site was demonstrated following adoptive transfer of huCTLs into mice immunized with SmartDC/tWT1. Furthermore, SmartDC/tWT1 immunization plus adoptive transfer of T cells reactive against WT1 into mice resulted in growth arrest of a WT1(+) tumor. Gene array analyses of SmartDC/tWT1 demonstrated upregulation of several genes related to innate immunity. Thus, SmartDC/tWT1 can be produced in a single day of ex vivo gene transfer, are highly viable in vivo, and have great potential for use as immunotherapy against malignant transformation overexpressing WT1.

Delta-like ligand 4 identifies a previously uncharacterized population of inflammatory dendritic cells that plays important roles in eliciting allogeneic T cell responses in mice

Graft-versus-host disease (GVHD) reflects an exaggerated inflammatory allogeneic T cell response in hosts receiving allogeneic hematopoietic stem cell transplantation (HSCT). Inhibition of pan-Notch receptor signaling in donor T cells causes reduction of GVHD. However, which Notch ligand(s) in what APCs is important for priming graft-versus-host reaction remains unknown. We demonstrate that δ-like ligand-4 (Dll4) and Dll4-positive (Dll4(high)) inflammatory dendritic cells (i-DCs) play important roles in eliciting allogeneic T cell responses. Host-type Dll4(high) i-DCs occurred in the spleen and intestine of HSCT mice during GVHD induction phase. These Dll4(high) i-DCs were CD11c(+)B220(+)PDCA-1(+), resembling plasmacytoid dentritic cells (pDCs) of naive mice. However, as compared with unstimulated pDCs, Dll4(high) i-DCs expressed higher levels of costimulatory molecules, Notch ligands Jagged1 and Jagged2, and CD11b, and produced more Ifnb and Il23 but less Il12. In contrast, Dll4-negative (Dll4(low)) i-DCs were CD11c(+)B220(-)PDCA-1(-), and had low levels of Jagged1. In vitro assays showed that Dll4(high) i-DCs induced significantly more IFN-γ- and IL-17-producing effector T cells (3- and 10-fold, respectively) than Dll4(low) i-DCs. This effect could be blocked by anti-Dll4 Ab. In vivo administration of Dll4 Ab reduced donor-alloreactive effector T cells producing IFN-γ and IL-17 in GVHD target organs, leading to reduction of GVHD and improved survival of mice after allogeneic HSCT. Our findings indicate that Dll4(high) i-DCs represent a previously uncharacterized i-DC population distinctive from steady state DCs and Dll4(low) i-DCs. Furthermore, Dll4 and Dll4(high) i-DCs may be beneficial targets for modulating allogeneic T cell responses, and could facilitate the discovery of human counterparts of mouse Dll4(high) i-DCs.

Isolation of human MHC class II-restricted T cell receptors from the autologous T-cell repertoire with potent anti-leukaemic reactivity

Adoptive transfer of T cells genetically modified with tumour-specific T-cell receptors (TCR) is a promising novel approach in the treatment of cancer. We have previously isolated an allorestricted MHC class I-restricted TCR with specificity for Formin-like protein 1 (FMNL1) with potent activity against chronic lymphocytic leukaemia cells. CD4(+) T cells have been described to be highly important for tumour elimination although TCR derived from CD4(+) T cells with anti-tumour reactivity have been only rarely described. In this study we aimed to isolate MHC class-II-restricted CD4(+) T cells and TCR with specificity for leukaemia antigens. We used professional antigen-presenting cells pulsed with the leukaemia-associated and tumour-associated antigen FMNL1 for stimulation of autologous T cells in vitro. We isolated two CD4(+) HLA-DR-restricted T-cell clones and T-cell-derived TCR with so far unknown specificity but high reactivity against lymphoma cells and native malignant cells derived from HLA-matched patients with diverse leukaemias. Moreover, characterization of the TCR after TCR gene transfer revealed that specific characteristics of isolated TCR as reactivity in response to Toll-like receptors were transferable on effector cells. Our results have a major impact on the development of novel immunotherapies. They demonstrate that TCR with potent HLA-DR-restricted anti-leukaemic reactivity against so far undefined self-restricted antigens can be isolated from the healthy autorestricted CD4(+) T-cell repertoire and these TCR are highly interesting candidate tools for novel immunotherapies.

Successful allogeneic hematopoietic cell engraftment after a minimal conditioning regimen in children with relapsed or refractory solid tumors

Children with relapsed or refractory solid tumors face dismal prognoses, and novel therapies are desperately needed. Allogeneic hematopoietic cell transplantation (HCT) offers potential for cell-based therapy, but the toxicity of myeloablation limits this approach in heavily pretreated patients. We sought to determine the feasibility of HCT in a cohort of 24 children with incurable solid tumors using human leukocyte antigen-matched sibling or unrelated donors and a minimal conditioning regimen. Before stem cell infusion, all patients received 3 daily doses of 30 mg/m(2) fludarabine followed by 2 Gy of total body irradiation. Hematopoietic cell recovery was rapid and reliable. Median time to neutrophil engraftment was 13.5 days for sibling donors and 12 days for unrelated donors. Donor lymphocyte infusions were used safely in 4 patients, all of whom had either improved chimerism or apparent tumor response. Graft-versus-host disease was comparable across donor sources and did not affect survival. Relapse remains a substantial barrier, although objective graft-versus-tumor effect was observed in several patients. Four patients with detectable disease before HCT achieved a complete response for at least 30 days after HCT, and two remain long-term survivors. Three patients were in complete response before HCT and remained in remission for 3, 6, and 74 months after HCT. Early disease response was associated with improved survival. Allogeneic HCT using this conditioning regimen offers a potential platform for novel immunotherapies.