In the current study, we observed that mice vaccinated with the E7 peptide-based vaccine combined with PADRE peptide and poly(I:C) generate the strongest E7-specific CD8+ T cell immune responses and therapeutic anti-tumor effects against TC-1 tumors among the different vaccination groups. Furthermore, we found that intratumoral vaccination with the E7 peptide-based vaccine in combination with PADRE peptide and poly(I:C) generates significantly higher frequency of E7-specific CD8+ T cells as well as better survival compared to subcutaneous vaccination with the same regimen in treated mice.
We observed that the inclusion of PADRE peptide could significantly improve the E7-specific immune responses generated by the E7 peptide-based vaccine in vaccinated mice. Our data is consistent with our previous studies using DNA-based vaccines. We have previously employed a DNA vaccine encoding an invariant (Ii) chain in which the CLIP region is replaced with the PADRE epitope (Ii-PADRE) . We demonstrated that mice vaccinated with DNA encoding Ii-PADRE showed significantly greater PADRE-specific CD4+ T cell immune responses compared to mice vaccinated with DNA encoding Ii chain alone . More importantly, co-administration of DNA encoding HPV E7 antigen with Ii-PADRE DNA led to significantly higher frequency of E7-specific CD8+ T cell immune responses and more potent protective and therapeutic antitumor effects against TC-1 tumors in treated mice . Thus, the induction of CD4+ T help by employment of the PADRE strategy may be used in combination with DNA or peptide-based vaccination in order to enhance the antigen-specific immune responses and antitumor effects.
In our study, we found that intratumoral administration of the E7 peptide-based vaccines in conjunction with poly(I:C) generated significantly greater E7-specific immune responses and antitumor effects compared to subcutaneous vaccination (See Figures 4, 5 and 6). A potential mechanism for the observed effect may be related to the fact that the E7 CTL peptide may directly bind to MHC class I molecule of tumor cells, thus rendering them more susceptible to direct killing by E7-specific CD8+ T cells. This may also result in release of E7 antigen from the apoptotic tumor cells which may be taken up by antigen-presenting cells, resulting in further presentation of E7 antigen to CD8+ T cells (so called cross-priming mechanism). These mechanisms may potentially contribute to the observed enhancement in the E7-specific CD8+ T cell immune responses and antitumor effects against E7-expressing tumors.
Another important mechanism for the observed enhancement in immune responses and antitumor effects by intratumoral administration of the peptide-based vaccine with poly(I:C) may be related to the alteration of the tumor microenvironment. Poly(I:C) has previously been shown to trigger the maturation of DCs and promote the production of inflammatory Th1 cytokines such as IL-12, while suppressing Th2 cytokines, such as IL-10 in vitro [23, 24]. Thus, we speculate that intratumoral administration of peptide-based vaccines with poly(I:C) may potentially generate an Th1 anti-tumor inflammatory response in the tumor microenvironment, thus contributing to the destruction of the tumor. Furthermore, the released tumor antigen, such as E7 may potentially be taken up by antigen-presenting cells, leading to further activation of tumor-specific CD8+ T cells (cross-priming mechanism). Thus, intratumoral administration of poly(I:C) with the peptide-based vaccine may alter the tumor microenvironment to enhance the E7-specific immune responses as well as antitumor effects generated by the E7 peptide-based vaccine.
In summary, our study demonstrates that intratumoral administration of an E7-peptide-based vaccine in combination with PADRE peptide and poly(I:C) leads to enhanced antitumor effects in treated mice. The employment of intratumoral administration of the peptide-based vaccines in conjunction with PADRE peptide and poly(I:C) can potentially be applied for advanced cervical tumors which are not surgically resectable to improve the clinical outcome. However, this approach is restricted to a particular E7 peptide. For future clinical translation, we would require the employment of long overlapping peptides to overcome the limitation of MHC restriction and include more E7 CTL epitopes. Recent studies employing peptide vaccination using an overlapping set of long peptides comprising the sequences of the HPV16 E6 and E7 oncoproteins have been shown to demonstrate significant tumor-specific immune responses [25–27]. Thus, the employment of PADRE peptide and poly(I:C) may potentially be used in combination with overlapping peptide-based vaccines to enhance the antigen-specific immune responses and antitumor effects for the control of HPV-associated malignancies.