Vaccination of Acute Leukemia Patients with WT1 Peptide-Pulsed Dendritic Cells Induces Immunological and Clinical Responses: A Pilot Study

Background and Purpose: Long term survival rate for acute leukemia has improved significantly with the development of monoclonal antibodies and molecular targeting agents. However, new therapeutic approaches are needed for patients with refractory or relapsed disease. Active immunizations using dendritic cells (DCs) loaded with tumor lysates or peptides have shown safety and efficacy. Wilms’ tumor 1 (WT1) gene is frequently overexpressed in leukemia and is considered to be one of the ideal tumor antigens for cancer immunotherapy. In the present pilot study, we treated acute leukemia patients with WT1 peptide-pulsed DC and examined safety, clinical and immunological responses to the vaccination.

Patients and Methods: 5 AML and 2 ALL patients (4 males, 3 female; aged 16-77 years) were enrolled in the present study. 2 patients relapsed following allogeneic stem cell transplantation and 3 relapsed following standard and salvage chemotherapies. 2 patients were in CR with residual disease. 6 patients were HLA-A*24:02(A24) positive and 1 was positive for HLA-A*:02:01(A2). Autologous DCs were generated from patients’ peripheral blood monocytes which were separated by an adherence technique from mononuclear cells collected by apheresis. Monocytes were cultured with GM-CSF and IL-4 followed by maturation with OK432 and PGE₂. HLA-A2 or HLA-A24-restricted 9-mer WT1 peptide-loaded mature DCs were administered intradermally every 2 weeks 5-7 times in combination with OK432. Induction of vaccine-induced T cell responses was monitored by a HLA-tetramer assay and a flow cytometry analysis. Cytotoxic activity was evaluated by an LDH release assay

Results: The treatment was well tolerated and none of the patients experienced more than grade 2 adverse events during the treatment period. The most common adverse events were mild, transient erythema at injection sites and low grade fever. Of 7 patients, 3 had SD and 4 had PD following one course of the treatment. Decrease in WT1 mRNA was observed in 2 patients with SD. Survival of patients achieving SD (responder) after one course of DC vaccination was longer than those who did not respond to the treatment (median overall survival 37 vs 5.5 months). Duration of survival in responders was 46, 37 and 10 months after initiation of therapy. Two responders are still alive in remission. Increase in positivity of WT1-specific CD8⁺ T cells was observed in all the responders after one course of treatment. The percentage of WT1 tetramer positive cells was 0.10±0.09% before vaccination; it increased to 2.49±1.58% after the first course (approximately 25 times increase). Cytotoxic T cells (CTLs) generated by in vitro stimulation with WT1 peptide showed cytotoxic activity in an LDH release assay. CTL did not lyse irrelevant HIV peptide-pulsed HLA-A24⁺ target and WT1 peptide-pulsed HLA-A24^- target, demonstrating antigen specificity and HLA restriction. Although absolute number of CD3⁺ T cells increased by 1.03±0.51 and 1.27±0.36 fold, respectively both in responders and non-responders, CD4/CD8 ratio increased by 1.45±0.97 fold in responders but decreased by 0.77±0.47 in non-responders, following the first course of DC vaccination. On the other hand, while the absolute number of CD4⁺CD25⁺Foxp3⁺ regulatory T cells decreased by 29.0% in responders, it increased by 43.0% in non-responders.

Conclusions: The present study demonstrated that a vaccination with WT1 peptide-pulsed DC was well tolerated and no serious adverse event was observed. DC vaccination elicits both innate and acquired cellular immune responses correlated with clinical effects. These results suggest WT1 peptide-pulsed DC vaccination might be a promising novel strategy for the treatment of acute leukemia patients with relapsed or refractory disease.