Sterile Immunity to Malaria After DNA prime/adenovirus Boost Immunization Is Associated With Effector Memory CD8+T Cells Targeting AMA1 Class I Epitopes

PLoS One. 2014 Sep 11;9(9):e106241. doi: 10.1371/journal.pone.0106241. eCollection 2014.


Background: Fifteen volunteers were immunized with three doses of plasmid DNA encoding P. falciparum circumsporozoite protein (CSP) and apical membrane antigen-1 (AMA1) and boosted with human adenovirus-5 (Ad) expressing the same antigens (DNA/Ad). Four volunteers (27%) demonstrated sterile immunity to controlled human malaria infection and, overall, protection was statistically significantly associated with ELISpot and CD8+ T cell IFN-γ activities to AMA1 but not CSP. DNA priming was required for protection, as 18 additional subjects immunized with Ad alone (AdCA) did not develop sterile protection.

Methodology/principal findings: We sought to identify correlates of protection, recognizing that DNA-priming may induce different responses than AdCA alone. Among protected volunteers, two and three had higher ELISpot and CD8+ T cell IFN-γ responses to CSP and AMA1, respectively, than non-protected volunteers. Unexpectedly, non-protected volunteers in the AdCA trial showed ELISpot and CD8+ T cell IFN-γ responses to AMA1 equal to or higher than the protected volunteers. T cell functionality assessed by intracellular cytokine staining for IFN-γ, TNF-α and IL-2 likewise did not distinguish protected from non-protected volunteers across both trials. However, three of the four protected volunteers showed higher effector to central memory CD8+ T cell ratios to AMA1, and one of these to CSP, than non-protected volunteers for both antigens. These responses were focused on discrete regions of CSP and AMA1. Class I epitopes restricted by A*03 or B*58 supertypes within these regions of AMA1 strongly recalled responses in three of four protected volunteers. We hypothesize that vaccine-induced effector memory CD8+ T cells recognizing a single class I epitope can confer sterile immunity to P. falciparum in humans.

Conclusions/significance: We suggest that better understanding of which epitopes within malaria antigens can confer sterile immunity and design of vaccine approaches that elicit responses to these epitopes will increase the potency of next generation gene-based vaccines.

Trial registration: NCT00392015 NCT00870987.

Publication types

  • Clinical Trial
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Adenoviridae / immunology
  • Adult
  • Antigens, Protozoan / administration & dosage
  • Antigens, Protozoan / immunology*
  • CD8-Positive T-Lymphocytes / immunology
  • DNA / administration & dosage
  • DNA / immunology
  • Epitopes / immunology
  • Humans
  • Immunologic Memory
  • Interferon-gamma / immunology
  • Interleukin-2 / immunology
  • Malaria Vaccines / administration & dosage
  • Malaria Vaccines / immunology
  • Malaria, Falciparum / immunology*
  • Malaria, Falciparum / parasitology
  • Malaria, Falciparum / pathology
  • Membrane Proteins / administration & dosage
  • Membrane Proteins / immunology*
  • Plasmodium falciparum / immunology*
  • Plasmodium falciparum / pathogenicity
  • Protozoan Proteins / administration & dosage
  • Protozoan Proteins / immunology*
  • Tumor Necrosis Factor-alpha / immunology


  • Antigens, Protozoan
  • Epitopes
  • Interleukin-2
  • Malaria Vaccines
  • Membrane Proteins
  • Protozoan Proteins
  • Tumor Necrosis Factor-alpha
  • apical membrane antigen I, Plasmodium
  • circumsporozoite protein, Protozoan
  • Interferon-gamma
  • DNA

Associated data


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; 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-naive 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.