CD4+ TH1 cells generated by Ii-PADRE DNA at prime phase are important to induce effectors and memory CD8+ T cells.

The requirement for CD4 T cells in priming and maintaining cytotoxic T-lymphocyte responses presents a long-standing paradox in cellular immunology. In this study, we used sequential coadministration of a DNA vaccine encoding an invariant (Ii) chain in which the class II-associated Ii-peptide region is replaced with CD4 T-helper epitope, PADRE [Pan human leukocyte antigen-DR reactive epitope (Ii-PADRE)] or Bcl-xL with a DNA vaccine encoding Sig/E7/LAMP-1 to verify the role of CD4 T cells for the generation of effectors and memory E7-specific CD8 T-cell immune responses. Sequential vaccination, with Ii-PADRE+Sig/E7/LAMP-1 priming followed by Bcl-xL+Sig/E7/LAMP-1 boosting led to generation of E7-specific CD8 T cells, and was nearly equivalent in effect to coadministration with Ii-PADRE+Sig/E7/LAMP-1 or Bcl-xL+Sig/E7/LAMP-1 at both prime and boost. The mice vaccinated with the Ii-PADRE+Sig/E7/LAMP-1 prime-Bcl-xL+Sig/E7/LAMP-1 boost regimen exhibited better long-term E7-specific immune responses and tumor prevention effects in vivo than the mice vaccinated with the reverse sequential coadministration. After CD4 T-cell depletion, mice primed with Ii-PADRE+Sig/E7/LAMP-1 generated low numbers of E7-specific CD8 T cells and suppressed long-term memory CD8 T-cell response regardless of the sequence or combination of DNA vaccines administered. Mice primed with Bcl-xL+Sig/E7/LAMP-1 only suppressed long-term memory CD8 T-cell response after depletion of CD4 T cells before priming. Our findings suggest that activated CD4 T cells at prime phase are important to generate the antigen-specific CD8 T-cell immune responses and CD4 T cells, which are naive or activated, play a role to maintain the long-term memory responses.