Immunogenicity and Antileukemic Activity of Dendritic Cells Electroporated with Wilms' Tumor WT1 mRNA: A Phase I/II Trial in Acute Myeloid Leukemia

Abstract

The Wilms' tumor protein WT1 is a target for immunotherapy in malignancies, such as acute myeloid leukemia (AML). Following our demonstration that dendritic cells (DC) can be efficiently transfected by messenger (m)RNA electroporation (Van Tendeloo VF et al. Blood 2001; 98:49–56) and that WT1 mRNA-electroporated DC stimulate WT1-specific T cells in vitro (Van Driessche A et al. Leukemia 2005; 19:1863–1871), we performed a phase I/II dose-escalation trial, in which patients with AML in remission but at high risk of relapse and without a direct sib allo-transplant option (9 patients) or with slowly progressive AML (1 patient) received intradermal injections of WT1 RNA-loaded DC. Following apheresis and CD14 immunomagnetic monocyte separation, DC were generated in 6-day cultures in clinical-grade medium supplemented with serum, granulocytemacrophage colony-stimulating factor (GM-CSF) and interleukin (IL)-4, matured with prostaglandin (PG)E2 and tumor necrosis factor (TNF)-alpha, harvested, electroporated with WT1 mRNA and used as vaccines. The patients received four biweekly DC vaccines and a delayed-type hypersensitivity (DTH) test was performed 2 weeks following the last vaccination. Patients were monitored for minimal residual disease (MRD) by analyzing WT1 RNA expression in peripheral blood by quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) (Cilloni D et al. Leukemia 2002; 16:2115–2121 & Cilloni D et al. Haematologica 2008; 93:921–924). Before and after vaccination, peripheral blood was collected for immunomonitoring purposes. Feasibility, safety, immunogenicity and effect on MRD were investigated. There was successful DC generation and vaccine production in all 10 patients. No serious adverse events or toxicity were observed and vaccinations were well tolerated. A decrease in WT1 RNA expression was observed during the course of the vaccination in 4/7 patients who had an increased WT1 mRNA level in peripheral blood. Three of those patients are still in complete hematological remission. An in vivo vaccine-specific immune response was demonstrated in 10/10 patients by DTH. Ex vivo immunomonitoring analysis showed a significant increase in circulating activated HLA-DR+ CD4+ T cells and in IL-2 plasma levels following vaccination. Importantly, in vitro restimulation assays of peripheral blood mononuclear cells revealed a significant postvaccination increase in interferon (IFN)-gamma-producing WT1-specific CD8+ T cells (n= 8 evaluable patients), but not in cytokine-producing WT1-specific CD4+ T cells. There was no significant change in WT1-specific antibodies following vaccination. We conclude that vaccination of AML patients with WT1 RNA-loaded DC is feasible and safe. Furthermore, the DC elicit vaccine-specific and WT1-specific CD8+ T-cell responses. The correlation between reduction of circulating WT1 mRNA and the administration of the DC vaccines strongly suggests that this DC vaccine elicits an antileukemic activity.