Combination of protein and viral vaccines induces potent cellular and humoral immune responses and enhanced protection from murine malaria challenge

Abstract

The search for an efficacious vaccine against malaria is ongoing, and it is now widely believed that to confer protection a vaccine must induce very strong cellular and humoral immunity concurrently. We studied the immune response in mice immunized with the recombinant viral vaccines fowlpox strain FP9 and modified virus Ankara (MVA), a protein vaccine (CV-1866), or a combination of the two; all vaccines express parts of the same preerythrocytic malaria antigen, the Plasmodium berghei circumsporozoite protein (CSP). Mice were then challenged with P. berghei sporozoites to determine the protective efficacies of different vaccine regimens. Two immunizations with the protein vaccine CV-1866, based on the hepatitis B core antigen particle, induced strong humoral immunity to the repeat region of CSP that was weakly protective against sporozoite challenge. Prime-boost with the viral vector vaccines, FP9 followed by MVA, induced strong T-cell immunity to the CD8+ epitope Pb9 and partially protected animals from challenge. Physically mixing CV-1866 with FP9 or MVA and then immunizing with the resultant combinations in a prime-boost regimen induced both cellular and humoral immunity and afforded substantially higher levels of protection (combination, 90%) than either vaccine alone (CV-1866, 12%; FP9/MVA, 37%). For diseases such as malaria in which different potent immune responses are required to protect against different stages, using combinations of partially effective vaccines may offer a more rapid route to achieving deployable levels of efficacy than individual vaccine strategies.

FIG. 1.

Combination of prime-boost and protein immunizations induces potent cell-mediated and humoral immunity. BALB/c mice were immunized with the F/M, C/C, F+C/M+C, F,C/M,C, or Fnr+C/Mnr+C regimen with a 2-week interval between prime and boost. Two weeks after boosting (day 28), blood was collected from the tail vein. PBMC were assayed for CD8⁺ T-cell responses to Pb9 by ELISPOT assay (A), and sera were assayed for anti-DP4 IgG responses (B) and anti-DP4 isotype profiles (C) by ELISA. Results are from several pooled experiments, except for the F,C/M,C immunized group, where n = 12. ELISPOT assay results are expressed as spot-forming cells (SFC) per million PBMC ± 95% confidence interval. ELISA results are presented as log-transformed end point titers ± 95% confidence interval (n = 12 to 22 per group). ***, P ≤ 0.001.

FIG. 2.

Antibody avidity does not differ significantly between F+C/M+C- and C/C-immunized mice. BALB/c mice were immunized with the C/C or F+C/M+C regimen. The interval between prime and boost was 2 weeks. Two weeks after boosting (day 28), blood was collected from the tail vein, sera were diluted in PBS to similar concentrations, and an avidity ELISA was performed. The rate at which DP4-specific antibodies were dissociated from the DP4 peptide by sodium thiocyanate was used as a measure of antibody avidity. Results are from one experiment and are expressed as the percentages of antibody bound at various NaSCN concentrations. Error bars represent ±95% confidence interval; n = 10.

FIG. 3.

Antibodies induced by C/C and F+C/M+C immunization bind specifically to the linear B-cell epitope DP4 and P. berghei sporozoites. BALB/c mice were immunized with the C/C or F+C/M+C regimen. The interval between prime and boost was 2 weeks. Two weeks after boosting (day 28), blood was collected from the tail vein and sera assayed for anti-DP4 specific IgG responses by ELISA and IFA. Results are representative of two experiments and are expressed as log-transformed end point titers ± 95% confidence interval (n = 4). **, P ≤ 0.01.