Cytotoxic T cell responses to DNA vaccination: dependence on antigen presentation via class II MHC

This study was designed to test whether cytotoxic T cell (CTL) responses to DNA vaccination are dependent upon MHC class II-restricted priming of CD4+ T cells. Because DNA vaccination may directly transfect dendritic cells, and dendritic cells may be capable of directly stimulating CD8+ T cell responses, such priming might be unnecessary. To test this hypothesis, C57BL/6 mice were immunized intramuscularly or intradermally with DNA encoding either whole OVA, a class I (Kb)-restricted peptide epitope of OVA (amino acids 257-264, SIINFEKL), or this class I-restricted epitope plus the adjacent class II (I-Ab)-restricted epitope of OVA (amino acids 265-280, TEWTSSNVMEERKIKV). Very low to negligible CTL responses were observed in mice vaccinated with the SIINFEKL construct, whereas mice vaccinated with the SIINFEKLTEWTSSNVMEERKIKV or with the complete OVA construct made equally robust CTL responses. These responses were sensitive to blocking by anti-CD8 mAb and were shown to be SIINFEKL-specific by using SIINFEKL peptide-pulsed EL-4 cells as targets. To ensure that the generation of these CTL responses was indeed dependent upon CD4+ T cell help, mice were depleted of either CD4+ or CD8+ cells before immunization. Depletion of CD4+ cells completely abrogated the CTL response to OVA DNA, as did depletion of CD8+ cells. Thus, we conclude that the CTL response to both intramuscular and intradermal DNA vaccination is highly dependent upon the generation of CD4+ T cell help via a class II MHC-dependent pathway. These results will be relevant for the construction of minimal-epitope vaccines for DNA immunization.