T-cell vaccines that elicit effective immune responses against HCV in chimpanzees may create greater immune pressure for viral mutation

A prime/boost vaccine strategy that transfects antigen-presenting cells using ligand-modified immunoliposomes to efficiently deliver plasmid DNA, followed by boosting with non-replicating recombinant adenovirus was used in chimpanzees to generate HCV-specific memory T-cells. Three chimpanzees (two vaccines, one control) were immunized with immunoliposomes complexed with DNA expressing NS3-NS5B or complexed with empty vector. Animals were boosted with adenovirus expressing NS3-NS5B, or non-recombinant adenovirus (control). Using liposome delivery we were able to obtain specific HCV responses following DNA priming in the chimpanzees. This data and mouse immunization studies confirm this as a more efficient delivery system than direct intramuscular inoculations with naked DNA. Subsequent to the adenovirus boost significant increases in peripheral HCV-specific T-cell responses and intrahepatic IFN-γ and CD3ɛ mRNA were also observed in the two vaccinated animals. Following challenge (100 CID50) both vaccinated animals showed immediate and significant control of viral replication (peak titers 3.7 × 104 and 9 × 103 IU/mL at weeks 1 and 2), which coincided with increases in HCV-specific T-cell responses. Viral kinetics in the control animal were comparable to historical controls with exponential increases in titer during the first several weeks. One vaccinated animal developed a low-level persistent infection (2 × 103 IU/mL) which correlated with a decrease in HCV-specific T-cell responses. Circulating virus isolated from both vaccinated animals showed ∼2-fold greater nonsynonymous mutation rates compared to controls and the nonsynonymous/synonymous mutation rate ratio was indicative of positive selection. These data suggest that although T-cell vaccines can induce immune responses capable of controlling HCV, they also induce high levels of immune pressure for the potential selection of escape mutants.