Identification of two distinct populations of WT1-specific cytotoxic T lymphocytes in co-vaccination of WT1 killer and helper peptides

Simultaneous induction of tumor antigen-specific cytotoxic T lymphocytes (CTLs) and helper T lymphocytes (HTLs) is required for an optimal anti-tumor immune response. WT1₃₃₂, a 16-mer WT1-derived helper peptide, induce HTLs in an HLA class II-restricted manner and enhance the induction of WT1-specific CTLs in vitro. However, in vivo immune reaction to WT1₃₃₂ vaccination in tumor-bearing patients remained unclear. Here, a striking difference in WT1-specific T cell responses was shown between WT1 CTL + WT1 helper peptide and WT1 CTL peptide vaccines in patients with recurrent glioma. WT1-specific CTLs were more strongly induced in the patients who were immunized with WT1 CTL + WT1 helper peptide vaccine, compared to those who were immunized with WT1 CTL vaccine alone. Importantly, a clear correlation was demonstrated between WT1-specific CTL and WT1₃₃₂-specific HTL responses. Interestingly, two novel distinct populations of WT1-tetramer^low WT1-TCR^low CD5^low and WT1-tetramer^high WT1-TCR^high CD5^high CTLs were dominantly detected in WT1 CTL + WT1 helper peptide vaccine. Although natural WT1 peptide-reactive CTLs in the latter population were evidently less than those in the former population, the latter population showed natural WT1 peptide-specific proliferation capacity comparable to the former population, suggesting that the latter population highly expressing CD5, a marker of resistance to activation-induced cell death, should strongly expand and persist for a long time in patients. These results demonstrated the advantage of WT1 helper peptide vaccine for the enhancement of WT1-specific CTL induction by WT1 CTL peptide vaccine.