DNA prime/Adenovirus Boost Malaria Vaccine Encoding P. Falciparum CSP and AMA1 Induces Sterile Protection Associated With Cell-Mediated Immunity

PLoS One. 2013;8(2):e55571. doi: 10.1371/journal.pone.0055571. Epub 2013 Feb 14.

Abstract

Background: Gene-based vaccination using prime/boost regimens protects animals and humans against malaria, inducing cell-mediated responses that in animal models target liver stage malaria parasites. We tested a DNA prime/adenovirus boost malaria vaccine in a Phase 1 clinical trial with controlled human malaria infection.

Methodology/principal findings: The vaccine regimen was three monthly doses of two DNA plasmids (DNA) followed four months later by a single boost with two non-replicating human serotype 5 adenovirus vectors (Ad). The constructs encoded genes expressing P. falciparum circumsporozoite protein (CSP) and apical membrane antigen-1 (AMA1). The regimen was safe and well-tolerated, with mostly mild adverse events that occurred at the site of injection. Only one AE (diarrhea), possibly related to immunization, was severe (Grade 3), preventing daily activities. Four weeks after the Ad boost, 15 study subjects were challenged with P. falciparum sporozoites by mosquito bite, and four (27%) were sterilely protected. Antibody responses by ELISA rose after Ad boost but were low (CSP geometric mean titer 210, range 44-817; AMA1 geometric mean micrograms/milliliter 11.9, range 1.5-102) and were not associated with protection. Ex vivo IFN-γ ELISpot responses after Ad boost were modest (CSP geometric mean spot forming cells/million peripheral blood mononuclear cells 86, range 13-408; AMA1 348, range 88-1270) and were highest in three protected subjects. ELISpot responses to AMA1 were significantly associated with protection (p = 0.019). Flow cytometry identified predominant IFN-γ mono-secreting CD8+ T cell responses in three protected subjects. No subjects with high pre-existing anti-Ad5 neutralizing antibodies were protected but the association was not statistically significant.

Significance: The DNA/Ad regimen provided the highest sterile immunity achieved against malaria following immunization with a gene-based subunit vaccine (27%). Protection was associated with cell-mediated immunity to AMA1, with CSP probably contributing. Substituting a low seroprevalence vector for Ad5 and supplementing CSP/AMA1 with additional antigens may improve protection.

Trial registration: ClinicalTrials.govNCT00870987.

Publication types

  • Clinical Trial, Phase I
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adenoviruses, Human / genetics*
  • Adenoviruses, Human / immunology
  • Adolescent
  • Adult
  • Antigens, Protozoan / genetics*
  • Antigens, Protozoan / immunology
  • CD8-Positive T-Lymphocytes / immunology
  • Female
  • Humans
  • Immunity, Cellular
  • Interferon-gamma / immunology
  • Malaria Vaccines / adverse effects
  • Malaria Vaccines / genetics
  • Malaria Vaccines / immunology
  • Malaria Vaccines / therapeutic use*
  • Malaria, Falciparum / immunology
  • Malaria, Falciparum / parasitology
  • Malaria, Falciparum / prevention & control*
  • Male
  • Membrane Proteins / genetics*
  • Membrane Proteins / immunology
  • Middle Aged
  • Plasmodium falciparum / genetics*
  • Plasmodium falciparum / immunology
  • Protozoan Proteins / genetics*
  • Protozoan Proteins / immunology
  • Vaccines, DNA / adverse effects
  • Vaccines, DNA / genetics
  • Vaccines, DNA / immunology
  • Vaccines, DNA / therapeutic use*
  • Young Adult

Substances

  • Antigens, Protozoan
  • Malaria Vaccines
  • Membrane Proteins
  • Protozoan Proteins
  • Vaccines, DNA
  • apical membrane antigen I, Plasmodium
  • circumsporozoite protein, Protozoan
  • Interferon-gamma

Associated data

  • ClinicalTrials.gov/NCT00870987

Grant support

This work was supported by USAID “Development of Adenovirus-Vectored Malaria Vaccines” Grant #: GHA-P-00-03-00006-01, Project Number 936-3118; and the Congressionally Directed Medical Research Program “Development of Recombinant Adenoviral-based Vaccines against Malaria” Grant #: W81XWH-05-2-0041. Website: https://cdmrp.org. Military Infectious Research Program “Phase 1/2a clinical trials assessing the safety, tolerability, immunogenicity &protective efficacy of Ad5-CA, a two-antigen, adenovirus-vectored Plasmodium falciparum malaria vaccine, in healthy, malaria-nave adults”, work unit number 62787A 870 F 1432. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.