Role of IL-2 secreted by PADRE-specific CD4⁺ T cells in enhancing E7-specific CD8⁺ T-cell immune responses

CD4⁺ T helper cells are known to play an integral role in the generation of CD8⁺ T-cell immune responses. We have previously shown that co-administration of DNA vaccines containing E6 or E7 protein of human papillomavirus 16 (HPV-16) combined with DNA encoding invariant (Ii) chain in which class II-associated Ii peptide (CLIP) region is replaced with the CD4⁺ T helper epitope, PADRE (Pan-DR-epitope) (Ii-PADRE DNA) enhanced HPV antigen-specific CD8⁺ T-cell immune responses in vaccinated mice. In the current study, we investigated the enhancement of HPV E7-specific CD8⁺ T-cell immune responses by PADRE-specific CD4⁺ T cells. We showed that intradermal administration of Ii-PADRE DNA at the same location as E7-expressing DNA is necessary to generate strong E7-specific CD8⁺ T-cell immune responses. We also showed that PADRE-specific CD4⁺ T cells generated by Ii-PADRE DNA vaccination expressed Th1 cytokine profile. Furthermore, our in vitro study demonstrated that PADRE-specific CD4⁺ T cells stimulated with PADRE-loaded dendritic cells secrete IL-2 that leads to the proliferation of E7-specific CD8⁺ T cells. Thus, our data suggest that activated PADRE-specific CD4⁺ T helper cells may be required at the vicinity of E7-specific CD8⁺ T cells where they secrete IL-2, which enhances the E7-specific CD8⁺ T-cell immune responses generated by DNA vaccination.