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 BclxL 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 BclxL + 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.