Clinical Phase II Study of WT1 Peptide Vaccination in Pediatric Leukemia Patients after Stem Cell Transplantation and Analysis of Immunological Monitoring

Introduction

Allogeneic stem cell transplantation (allo-SCT) is a radical treatment for pediatric patients with relapsed and/or chemotherapy-resistant leukemia. Because these leukemia cells seem refractory to chemotherapy and irradiation, immunotherapy are needed. WT1 is highly expressed in pediatric leukemia and essential for leukemia cell growth, leading to the hypothesis that the WT1 protein-based immunization after allo-SCT may improve the clinical outcome of the SCT therapy. We investigated the clinical efficacy and immunological effect of immunotherapy targeting WT1 protein for pediatric patients after allo-SCT. Method

Major inclusion criteria were as follows: Patients with HLA-A*2402 and/or HLA-*A0201 aged 20 years or younger and WT1 mRNA expression in leukemic cells; donors with HLA-A*2402 and/or HLA-*A0201. The HLA A*2402 restricted 9mer modified WT1 peptide or HLA A*0201 restricted natural WT1 peptide emulsified in Montanide ISA 51 adjuvant was injected intradermally. The dose of WT1 peptide depended on patient weight. The vaccinations were scheduled to be given weekly for 12 consecutive weeks, and if no recurrence and severe adverse effect were observed, vaccination was continued. Absolute number and frequency of WT1 specific cytotoxic T lymphocytes were analyzed by WT1 tetramer assay.

Results

A total of 18 patients with 9 AML/MDS, 8 ALL and 1 NHL were enrolled. Seven patients were those with high risk (HR) of relapse who had received SCT at 3^rd CR or on disease. The other 11 patients were at standard risk (SR) but had either relapsed after SCT or shown induction failure. 16 had HLA-A*2402 and the other 2 had HLA-A*0201. 12 of the 18 patients received WT1 peptide vaccination monthly after the first 12-time consecutive vaccinations with one-week interval and are still in complete remission for 16-72 months (median 31 mo.). One patient discontinued the vaccination because of pancreatitis due to treatment with steroid for GVHD. One year probabilities of PFS is 66.7% in all, and it is notable that 4 of 7 HR patients are maintained in long-term CR. Although, of these 12 patients with persistence of CR, seven patients had high WT1 mRNA level before vaccination in peripheral blood (PB) or bone marrow (BM), WT1 mRNA levels decreased to within normal level after vaccination. WT1-specific CTLs in PB were detected and increased after vaccination in all cases and absolute numbers of these cells were significantly increased after vaccination in all cases (p< 0.05). The fold increase of WT1-specific CTL numbers after the vaccination was significantly higher in cases which maintained CR (p< 0.05). The WT1-specific CTLs with high tetramer staining were detected during the first 12-time vaccinations in 14 patients, and of these patients, 12 patients are still in CR. In the other 4 patients, only very few WT1 specific CTLs were detected, and three of the four patients showed recurrence. Five patients (1AMKL, 4 ALL) had recurrence. There was no significant difference in the fold increase of WT1-specific CTL numbers after the vaccination between cases with immunosuppressive agents for the prevention of GVHD and those without the agents.

Discussion

We reported the interim result of the phase II clinical study of WT1 peptide immunotherapy after SCT for pediatric patients, in which promising clinical efficacy of the therapy was suggested. In addition, we found that an increase in WT1-specific CTLs was associated with a decrease in WT1 mRNA, suggesting occurrence of WT1-driven graft versus leukemia (GVL) effect, which is consistent with strategies to enhance leukemia antigen-specific GVL response without deterioration of GVHD. The following three mechanisms are suggested. 1) After SCT, many kinds of cytokines are produced and proliferation of T lymphocytes occur; 2) lymphodepletion after SCT allows development of T cells specific to WT1 peptide; and resultantly, 3) efficient homeostatic expansion of WT1-specific CTLs is induced. Post-SCT vaccination with WT1 peptide can be a safe and effective strategy to prevent the recurrence of the disease without deterioration of GVHD in pediatric hematological malignancy. For avoiding late side effect due to preconditioning regimen and GVHD, effective post SCT immunotherapy is a promising strategy for pediatric patient. Larger studies are warranted on application of WT1 targeted immunotherapy to prevent recurrence after pediatric SCT