Sterile Immunity to Malaria after DNA Prime/Adenovirus Boost Immunization Is Associated with Effector Memory CD8+T Cells Targeting AMA1 Class I Epitopes

Abstract

BackgroundFifteen 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 FindingsWe 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/SignificanceWe 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.

Polyfunctional and IFN-γ monofunctional human CD4+ T cell populations are molecularly distinct

“…Some T cells produce only one cytokine (monofunctional T cells), while others can simultaneously produce multiple cytokines (polyfunctional T cells) (1). Polyfunctional T cells that simultaneously produce the key Th1 cytokines IFN-γ, IL-2, and TNF-α have been reported following infection or immunization in a number of infectious diseases states including Leishmania major (2), HIV (3,4), Mycobacterium tuberculosis (5), and Plasmodium (6)(7)(8)(9)(10).…”

Discriminating Protective from Nonprotective Plasmodium-Specific CD8+ T Cell Responses

“…In parallel, efforts are underway to evaluate viral vectored subunit vaccines, expressing one of a few potential target antigens, that would ideally elicit CD8+ T cell responses to liver-stage antigens (13, 14). However, controlled human challenge trials have not revealed robust sterilizing immunity after viral vectored subunit immunizations (15-17). One possible path forward for subunit vaccines would be immunizations with a combination of target antigens identified from RAS immunized hosts and there are ongoing efforts in such malaria antigen-discovery.…”

Human CD8+ T cells mediate protective immunity induced by a human malaria vaccine in human immune system mice

“…In a phase I clinical trial, 15 volunteers were primed with plasmid DNA encoding P. falciparum CSP (PfCSP) and apical membrane antigen-1 and then boosted with human adenovirus serotype 5 (Ad5) expressing the same antigens. This DNA priming/adenovirus boost immunization regimen induced sterile protection in four (27%) vaccinated subjects [18]. In a phase IIa clinical trial, vaccination using a priming-boost regimen based on chimpanzee adenovirus and modified Ankara vaccinia virus, both expressing P. falciparum thrombospondin adhesive protein fused to multiple epitopes derived from several malaria antigens, induced sterile protection in 21% (3 out of 14) of subjects and delayed patency in 36% (5 out of 14) of subjects [19].…”