A Therapeutic Vaccine In Adult T-Cell Leukemia Patients With Autologous Dendritic Cells Pulsed With Peptides Corresponding To Tax-Specific Cytotoxic T Lymphocyte Epitopes

Adult T-cell leukemia (ATL) is an aggressive peripheral T-cell neoplasm with extremely poor prognosis. It typically develops due to long-term infection with human T-cell leukemia virus type 1 (HTLV-1). Among HTLV-1 antigens, the viral regulatory protein Tax is widely recognized as a target of cytotoxic T lymphocytes (CTLs), and Tax-specific CTLs are often activated in ATL patients following hematopoietic stem cell transplantation (HSCT). This strongly suggests the presence of Tax expression in vivo and potential contributions of CTL to GVL effects. Based on these findings, we developed an anti-ATL therapeutic vaccine consisting of autologous dendritic cells (DC) pulsed with Tax peptides corresponding to the major epitopes of Tax-specific CTL that were previously identified from human leukocyte antigen (HLA)-A2, A24 or A11-possessing ATL patients post-HSCT (Cancer Res. 2004, J Virol. 2005). In preliminary experiments, the DC induced with a conventional method showed matured phenotype and produced interleukin (IL)-12 in two of three ATL patients tested.

After approval by the institutional ethics committees, we conducted a phase I clinical trial of anti-ATL immunotherapy for pretreated ATL patients in stable condition in order to evaluate whether HTLV-1 Tax targeting immunotherapy is safe and feasible. The vaccine protocol consists of three subcutaneous injections of peptide-pulsed DC at 2-week intervals in a total 8-week observation period to assess adverse events. Three acute-type patients enrolled and completed the course of the study. Modest clinical symptoms, such as injection site reaction, fever, and skin damage, were observed in all cases. DC vaccine-related toxicities were grade 1 or 2 and were considered relatively mild, and non-hematological toxicity was acceptable. In addition, significant reduction in sIL-2R levels were observed at least in one patient, and the HTLV-1 proviral loads were maintained below 60 copies /1000 PBMCs during the monitored period after administration of DC in the all patients. Enhanced proliferative responses of Tax-specific CTLs were also observed in vitro after therapy. Clinical effects, as evaluated by RECIST, were partial remission in two patients and stable disease in one patient. Durable remissions and improvements in performance status were observed in all three patients without the requirement for new treatment for 4-10 months after treatment with Tax-targeted DC vaccination. These results indicate that Tax-specific DC vaccination is safe and well tolerated, and may be promising as a new therapeutic strategy for ATL patients.