Complete, long-lasting protection against malaria of mice primed and boosted with two distinct viral vectors expressing the same plasmodial antigen

Abstract

We report that complete protection against malaria and total inhibition of liver stage development and parasitemia was obtained in 100% of BALB/c mice primed with a replication-defective recombinant adenovirus expressing the circumsporozoite (CS) protein of Plasmodium yoelii (AdPyCS), followed by a booster with an attenuated recombinant vaccinia virus, expressing the same malaria antigen, VacPyCS. We found increased levels of activated CS-specific CD8(+) and CD4(+) T cells, higher anti-sporozoite antibody titers, and greater protection in these mice, when the time between priming and boosting with these two viral vectors was extended from 2 to 8 or more weeks. Most importantly, by using this immunization regimen, the protection of the immunized mice was found to be long-lasting, namely complete resistance to infection of all animals 3 1/2 months after priming. These results indicate that immunization with AdPyCS generates highly effective memory T and B cells that can be recalled long after priming by boosting with VacPyCS.

Figures

Figure 1

Time course (in weeks) of immune responses and inhibition of liver stages in mice (groups of three) immunized with 108 pfu of reAd (AdPyCS). (a) Number of CS-specific CD8+ and CD4+, IFN-γ-producing T cells and corresponding antibody titers (IFA). (b) Inhibition of liver stage development expressed as number of copies of Plasmodium 18S rRNA. Mice immunized and challenged at the same time points as in a. Data in a and b represent the mean of two identical experiments.

Figure 2

(#) Effects of a booster given at varying time points (weeks after reAd priming) with 107 pfu VacPyCS (+) and controls (−). (a) CS-specific T cell responses and anti-sporozoite antibody titers and (b) inhibition of liver stage development after sporozoite inoculation. Data in a and b were obtained 2 weeks after booster and are each representative of four experiments.

Figure 3

Activation status and apoptosis rates of CD8+ T cell populations obtained from the liver of mice immunized with reAd alone (control) or boosted with VacPyCS, 2 or 8 weeks after priming with AdPyCS. All data obtained 5 days after booster. Flow cytometry of lymphocytes showing (a) the percentage of CS-specific CD8+ T cells, (b) the percentage of CS-specific (Tet+) CD8+ activated (CD62L−) liver lymphocytes, and (c) the percentage of apoptosis of CS-specific (Tet+) cells.

Figure 4

(*) Persistence of immunity and inhibition of liver-stage development, at various time points (2, 6, 10, and 16 weeks) after recombinant vaccinia virus booster, in mice primed with reAd 8 weeks earlier. (a) Persistence of CS-specific CD8+ and CD4+ T cells and antibody titers. (b) Degree of inhibition of liver stage development.