Boosting with recombinant vaccinia increases HPV-16 E7-specific T cell precursor frequencies and antitumor effects of HPV-16 E7-expressing sindbis virus replicon particles

Immunotherapy using the heterologous prime-boost regimen has emerged as an attractive approach for generating antigen-specific T-cell-mediated immune responses against tumors and infectious diseases. We have previously linked the Mycobacterium tuberculosis heat-shock protein 70 (HSP70) to the HPV-16 E7 antigen creating a chimera, E7/HSP70. We found that nucleic acid vaccines encoding E7/HSP70 can generate strong antitumor immunity. Recently, replication-defective Sindbis virus replicon particle vaccines have been considered as an important vector system for vaccine development. In this study, we assessed whether the combination of E7/HSP70 Sindbis virus replicon particles (SINrep5-E7/HSP70) and E7/HSP70 vaccinia (Vac-E7/HSP70) can further enhance E7-specific immune responses using sequential vaccination. We found that priming with SINrep5-E7/HSP70 and boosting with Vac-E7/HSP70 generated the highest number of E7-specific CD8⁺ T cells and best antitumor effect compared to other combinations. Moreover, our data showed that at the dosage and route of immunization used in this study, mice treated with the Sindbis virus replicon particle prime-vaccinia boost regimen generated stronger antitumor responses compared to mice treated with the DNA prime-vaccinia boost vaccine regimen. Our results encourage the use of the Sindbis virus replicon particle prime-vaccinia boost regimen in future clinical trials.