Spontaneous high clonal expansion of Wilms' tumor gene 1-specific cytotoxic T-lymphocytes in patients with Wilms' tumor gene 1-expressing solid tumor.

Wilms' tumor protein 1 (WT1)-targeted immunotherapy has been used in patients with leukemia and solid tumors. However, the spontaneous WT1-specific immune response before WT1 peptide vaccination in patients with WT1-expressing tumors (PTs) remains unclear. Therefore, we investigated whether WT1-specific cytotoxic CD8⁺ T-lymphocytes (CTLs) are clonally expanded in the peripheral blood outside of tumor sites. Clonal expansion of WT1₁₂₆ peptide (a.a.126-134)-specific CTLs (WT1₁₂₆-CTLs) was compared between seven PTs and five healthy volunteers (HVs), and their T-cell receptors (TCRs) were analyzed at the single-cell level. Overall, 433 and 351 TCR β-chains of WT1₁₂₆-CTLs were detected from PTs and HVs, respectively, and complementarity-determining region 3 was sequenced for clonality analysis. The frequencies of WT1₁₂₆-CTLs were higher in human leukocyte antigen (HLA)-A*02:01⁺ PTs than in HLA-A*02:01⁺ HVs, although the difference was not statistically significant. WT1₁₂₆-CTLs of differentiated types, including memory and effector, were higher in PTs than in HVs; whereas, those of the naïve type were higher in HVs than in PTs. WT1₁₂₆-CTL clonality was significantly higher in PTs than in HVs. Furthermore, the frequency of effector WT1₁₂₆-CTLs positively correlated with WT1₁₂₆-CTL clonality in PTs; whereas, the frequency of naïve phenotype WT1₁₂₆-CTLs tended to be negatively correlated with clonality. In conclusion, these results suggest that the WT1 protein in tumor cells is highly immunogenic, thereby stimulating endogenous naïve-type WT1₁₂₆-CTLs and enabling them to clonally expand and differentiate into effector-type WT1₁₂₆-CTLs.