Vaccine Strain-Specificity of Protective HLA-Restricted Class 1 P. falciparum Epitopes

Abstract

A DNA prime/adenovirus boost malaria vaccine encoding Plasmodium falciparum strain 3D7 CSP and AMA1 elicited sterile clinical protection associated with CD8+ T cell interferon-gamma (IFN-γ) cells responses directed to HLA class 1-restricted AMA1 epitopes of the vaccine strain 3D7. Since a highly effective malaria vaccine must be broadly protective against multiple P. falciparum strains, we compared these AMA1 epitopes of two P. falciparum strains (7G8 and 3D7), which differ by single amino acid substitutions, in their ability to recall CD8+ T cell activities using ELISpot and flow cytometry/intracellular staining assays. The 7G8 variant peptides did not recall 3D7 vaccine-induced CD8+ T IFN-γ cell responses in these assays, suggesting that protection may be limited to the vaccine strain. The predicted MHC binding affinities of the 7G8 variant epitopes were similar to the 3D7 epitopes, suggesting that the amino acid substitutions of the 7G8 variants may have interfered with TCR recognition of the MHC:peptide complex or that the 7G8 variant may have acted as an altered peptide ligand. These results stress the importance of functional assays in defining protective epitopes. Clinical Trials Registrations: NCT00870987, NCT00392015.

Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1. ELISpot and CD8+ T cell IFN-γ responses of DNA/HuAd5 and HuAd5 immunized subjects to P. falciparum strains 3D7 and 7G8 AMA1 A*03 protective epitopes.

ELISpot and CD8+ T cell IFN-γ activities are shown in Panels A–D. Panel A: ELISpot IFN-γ response of the A*03 protected subject (v11) are positive with Ap8 and the 3D7 A*03 epitope but not the 7G8 epitope (arrow). Panel B: ELISpot activity of v11 is not affected by CD4+-depletion but is abolished after CD8+ depletion (arrow). Panel C: CD8+ T cell IFN-γ responses of v11 are much higher (p = 0.001) to the 3D7 epitope than to the 7G8 epitope (arrow). Panel D: ELISpot IFN-γ responses of two of four non-protected subjects from the HuAd5 trial were weakly positive with the 3D7 epitope but all four subjects were negative with the 7G8 epitope (arrows).

Fig 2. ELISpot and CD8+ T cell IFN-γ responses of DNA/HuAd5 and HuAd5 immunized subjects to P. falciparum strains 3D7 and 7G8 AMA1 B*58 protective epitopes.

ELISpot and CD8+ T cell IFN-γ activities are shown in Panels A–D. Panel A: ELISpot IFN-γ responses of the B*58 protected subjects (v10, v18) are positive with Ap10 and the 3D7 B*58 epitope but not 7G8 epitopes (arrows). Panel B: ELISpot activity of v10 is reduced but still remains positive after CD4+-depletion, but is abolished after CD8+-depletion (arrow); activity of v18 is unaffected by CD4+-depletion but is abolished after CD8+-depletion (arrow). Panel C: CD8+ T cell responses of v10 and v18 are much higher (p = 0.001) against the 3D7 B*58 epitope rather than the 7G8 B*58 epitope (arrows). Panel D: ELISpot IFN-γ response of DNA/HuAd5 non-protected B*58 subject (v15) was negative and non-protected v194 was weakly positive with the 3D7 B*58 epitope; two non-protected subjects from the HuAd5 trial that express A*01 (v135, v179) were weakly positive with the 3D7 B*58 epitope; all these subjects were negative with the 7G8 B*58 epitope (arrows).