Alternating vector immunizations encoding pre‐erythrocytic malaria antigens enhance memory responses in a malaria endemic area

Abstract

A heterologous prime-boost strategy has been developed to potently induce T cell responses to pre-erythrocytic malaria antigens. Efficacy in the field is likely to depend on both peak immunogenicity and the durability of responses. To improve both immunogenicity and durability of responses, 54 adult males from a malaria endemic area were immunized with different vaccination regimens, systematically varying antigenic insert and the number and sequence of component vaccinations. The component vaccinations were recombinant attenuated viruses, either fowlpox (FP) 9 or modified vaccinia virus Ankara (MVA). These were recombinant for either of two preerythrocytic malaria antigens (multiple epitope-thrombospondin-related adhesion protein, ME-TRAP, or circumsporozoite antigen (CS). ELISPOT assays were used to measure the effector and resting memory T cell responses. Sequence, antigen insert and number of vaccinations influenced immunogenicity, but the novel alternating vector immunizations generated the largest resting memory T cell populations. Effector responses were maintained at 84% of the peak response after 270 days. This durability of response is unprecedented. Classical prime-boost vaccination responses were at 5% of the peak after 270 days. Vaccines administered by heterologous prime-boost regimes are being developed for diverse pathogens and cancer. These data suggest these vaccines should also be administered by alternating vector regimens in clinical development. IntroductionOne million deaths per year are attributed to malaria [1]. A vaccine is urgently needed. Experimental studies on mice [2], field studies in humans [3] and irradiated sporozoite immunization [4] suggest that T cell responses are protective. A heterologous prime-boost strategy that induces T cells has been developed [5], using sequential immunization with different vectors delivering a common pre-erythrocytic malaria antigen. Two antigens have been used, multiple epitope-thrombospondin-related adhesion protein (ME-TRAP; multiple pre-erythrocytic stage epitopes joined with the whole pre-erythrocytic stage antigen TRAP) and CS (the circumsporozoite antigen with one additional Plasmodium falciparum epitope). Each antigen can be delivered by two different viral vectors, either the attenuated fowlpox strain FP9, or modified virus Ankara (MVA). In previous studies of ME-TRAP encoding vaccinations in non-immune volunteers, some individuals were fully protected and many immunized groups showed mean delays in time to parasitemia after controlled bites from P. falciparum-infected mosquitoes [6]. The direct, or ex vivo ELISPOT identifies T cells that produce IFN-c after overnight incubation with antigen. The cultured ELISPOT assay uses a 10-day pre-culture with first antigen then IL-2, to identify resting memory cells [7]. The cell populations identified by these two assays are different. A study of naturally acquired immunity to CS protein demonstrated no correlation between cultured ELISPOT and ex vivo ELISPOT results [8]. The cells identified by ...

Early Gamma Interferon and Interleukin-2 Responses to Vaccination Predict the Late Resting Memory in Malaria-Naïve and Malaria-Exposed Individuals

“…In malaria-exposed volunteers, the alternating-vector vaccinations with the ME-TRAP insert provoked a rise in ex vivo IFN-␥ production and resting memory cells (2 ; P ϭ 0.07). However, although FP9/MVA regimens encoding CS generated a rise in IFN-␥ (2), there was no evidence of a rise in IL-2 production (P ϭ 0.6).…”

“…However, experiments that selectively depleted CCR7-positive T cells before culture suggest that the cultured ELISPOT assay does detect central memory cells (14). In previous studies, we have shown that resting memory cells are expanded following vaccination in malaria-na ïve subjects (19) and malaria-exposed subjects (2) and are not simply due to prior exposure.…”

“…The medical supervision of vaccination, follow-up, and adverse events is described elsewhere (2). DNA was given as a 4-mg dose (intramuscular).…”

“…Phase 1 studies of candidate malaria vaccines were conducted in an area where malaria is not endemic and then in an area where it is endemic. Immunogenicity is described in detail elsewhere (2,9). Resting memory cell responses were significantly greater after vaccination in malarianaïve (19) and malaria-exposed (2) subjects.…”

Memory CD8 T cell responses exceeding a large but definable threshold provide long-term immunity to malaria

“…Although the mouse in general and our approach specifically may have limitations as a preclinical model, the results still may have relevance for understanding why subunit vaccines that evoke sterilizing immunity against human malaria have been difficult to obtain. For example, accumulating data from human clinical trials show that current prime-boost immunizations generate Plasmodium-specific T cell responses in the range of 0.1% of PBL at the peak after boosting and Ͻ0.01% at memory stages (20,(33)(34)(35)(36). These frequencies are 10-fold and 100-fold lower than required to protect mice from Plasmodium infection and consist mainly of CD4 T cells, which may explain why these vaccines delay the onset but rarely prevent blood-stage parasitemia (20,36).…”