Improving Protective Immunity Induced by DNA-Based Immunization: Priming with Antigen and GM-CSF-Encoding Plasmid DNA and Boosting with Antigen-Expressing Recombinant Poxvirus1 2

Intramuscular immunization with a naked DNA plasmid expressing the Plasmodium yoelii circumsporozoite protein (pPyCSP) protects mice against challenge with P. yoelii sporozoites. This protection can be improved either by coadministration of a plasmid expressing murine GM-CSF (pGMCSF) or by boosting with recombinant poxvirus expressing the PyCSP. We now report that combining these two strategies, by first mixing the priming dose of pPyCSP with pGMCSF and then boosting with recombinant virus, can substantially increase vaccine effectiveness. Not only were immune responses and protection improved but the pPyCSP dose could be lowered from 100 μg to 1 μg with little loss of immunogenicity after boost with recombinant poxvirus. Comparing mice primed by the 1-μg doses of pPyCSP plus 1 μg pGMCSF with mice primed by 1-μg doses of pPyCSP alone, the former were better protected (60% vs 0) and had higher concentrations of Abs (titers of 163, 840 vs 5, 120 by indirect fluorescent Ab test against sporozoites), more ex vivo CTL activity (25% vs 7% specific lysis), and more IFN-γ-secreting cells by enzyme-linked immunospot assay (1460 vs 280 IFN-γ spot-forming cells/106 cells). Priming with plasmid vaccine plus pGMCSF and boosting with recombinant poxviruses strongly improves the immunogenicity and protective efficacy of DNA vaccination and allows for significant reduction of dose.