The Diversity of Long-Term Persistent WT1-Specific Cytotoxic T Lymphocytes after Wilms' Tumor 1 Peptide Vaccination

[Introduction] Remarkable advances have been made in cancer immunotherapy, including the development of peptide-based cancer vaccines. Wilms' tumor 1 (WT1) is one of the cancer-testis antigens, and the WT1 gene is overexpressed in hematologic malignancies. Several clinical trials of WT1 peptide vaccines showed promising efficacy and high safety of the vaccines for hematologic malignancies. In these trials, immunological responses were assessed within 2 years after vaccination, and the transient WT1-specific immune response observed in many cases early after vaccination was confirmed. However, the long-term durability of the response of WT1-specific CD8+ cytotoxic T lymphocytes (CTLs) after peptide vaccine therapy and the T-cell receptor (TCR) diversity in those CTLs has not been clarified. More than 10 years ago, a patient with chronic myeloid leukemia (CML) received WT1 peptide vaccination after the failure of tyrosine kinase inhibitor therapy. After vaccination, WT1-specific CD8+ CTLs were observed. We continued the immunological assessment of the patient for more than 10 years after the WT1 peptide vaccination. Herein, we report our findings from the long-term monitoring of WT1-specific CTLs in the patient with CML and describe the results of our detailed analysis, including the functionality and clonality of the CTLs.

[Methods] After obtaining written consent from a patient whose CML was difficult to control by imatinib, HLA-A*24:02-restricted modified-type WT1 peptide (WT1 peptide; 9 mer peptide of CYTWNGMNL) was administered to the patient. Post-vaccination, we followed up with the patient. Immune monitoring was performed using a WT1 tetramer assay after mixed lymphocyte peptide culture (MLPC assay). The limiting-dilution (mononuclear cells divided into 20 wells or more, equally containing 3 × 10 ⁵ cells), 2-week cultures with WT1 peptide stimulation and counting of "positive wells" containing expanded WT1 tetramer+ CD8+ T cells were performed for the MLPC assay. The MLPC assay was used to detect functional WT1-specific CD8+ T cells that can expand in response to the WT1 peptide and estimate the frequency of these WT1-specific CD8+ T cells among all CD8+ T cells. For the functionality of WT1-specific CD8+ T cells, we evaluated WT1-specific cytotoxicity and cytokine production in the presence and absence of WT1 peptide pulse to T2A24 cells transfected with the GFP gene (T2A24-GFP). The phenotype and TCR of the WT1-specific CD8+ T cells expanded by MLPC were analyzed using flow cytometry and next-generation sequencing, respectively.

[Results] After the WT1 peptide vaccination, the copy numbers of major bcr-abl transcripts gradually decreased, and a therapy free remission was achieved in the patient. No severe adverse effects were observed. The estimated frequency of WT1-specific CD8+ T cells peaked in the third year after vaccination (27 cells per 10 ⁶ CD8+ T cells, 0.00027%) and then declined to 1 - 5 per 10 ⁶ CD8+ T cells at 13 years after vaccination. The WT1-specific CD8+ T cells showed that WT1 peptide-specific cytotoxicity and WT1 peptide-specific IFN-γ release in vitro. These WT1-specific CTLs had different TCRs in each MLPC well. This result was confirmed by three independent analyses, and no common TCRs were detected. Twelve different TCRs were detected in the three analyses.

[Conclusion] The WT1 peptide vaccine successfully generated long-lasting and diverse WT1-specific immune responses in a patient with CML. The WT1 peptide vaccine may be a efficient immune therapy for CML patients.