Safety, infectivity and immunogenicity of a genetically attenuated blood-stage malaria vaccine

Abstract

Background There is a clear need for novel approaches to malaria vaccine development. We aimed to develop a genetically attenuated blood-stage vaccine and test its safety, infectivity, and immunogenicity in healthy volunteers. Our approach was to target the gene encoding the knob-associated histidine-rich protein (KAHRP), which is responsible for the assembly of knob structures at the infected erythrocyte surface. Knobs are required for correct display of the polymorphic adhesion ligand P. falciparum erythrocyte membrane protein 1 (PfEMP1), a key virulence determinant encoded by a repertoire of var genes. Methods The gene encoding KAHRP was deleted from P. falciparum 3D7 and a master cell bank was produced in accordance with Good Manufacturing Practice. Eight malaria naïve males were intravenously inoculated (day 0) with 1800 (2 subjects), 1.8 × 105 (2 subjects), or 3 × 106 viable parasites (4 subjects). Parasitemia was measured using qPCR; immunogenicity was determined using standard assays. Parasites were rescued into culture for in vitro analyses (genome sequencing, cytoadhesion assays, scanning electron microscopy, var gene expression). Results None of the subjects who were administered with 1800 or 1.8 × 105 parasites developed parasitemia; 3/4 subjects administered 3× 106 parasites developed significant parasitemia, first detected on days 13, 18, and 22. One of these three subjects developed symptoms of malaria simultaneously with influenza B (day 17; 14,022 parasites/mL); one subject developed mild symptoms on day 28 (19,956 parasites/mL); and one subject remained asymptomatic up to day 35 (5046 parasites/mL). Parasitemia rapidly cleared with artemether/lumefantrine. Parasitemia induced a parasite-specific antibody and cell-mediated immune response. Parasites cultured ex vivo exhibited genotypic and phenotypic properties similar to inoculated parasites, although the var gene expression profile changed during growth in vivo. Conclusions This study represents the first clinical investigation of a genetically attenuated blood-stage human malaria vaccine. A P. falciparum 3D7 kahrp– strain was tested in vivo and found to be immunogenic but can lead to patent parasitemia at high doses. Trial registration Australian New Zealand Clinical Trials Registry (number: ACTRN12617000824369; date: 06 June 2017).

Pathogenicity and virulence of malaria: Sticky problems and tricky solutions

“…In addition to PfEMP1-mediated adhesion, the altered structure of the IE compared to non-IE is also important for sequestration. Knobless parasites that express PfEMP1 have impaired infectivity in monkeys [ 66 ] and in humans [ 67 , 68 ] and this is thought to be due to reduced sequestration, allowing for efficient splenic clearance. Using a 3D in vitro model of the arteriole-capillary venules, parasites lacking knobs or PfEMP1 had reduced sequestration in narrow capillary constriction zones [ 69 ].…”

A genetically engineered Plasmodium falciparum parasite vaccine provides protection from controlled human malaria infection

“…Thus, an aseptic, purified, cryopreserved, vialed, and DVI-administered formulation PfGAP3KO will allow for safe and precise dose finding trials to achieve complete sterilizing protection with this candidate vaccine, as has been done with PfSPZ RAS vaccine (44)(45)(46). At the same time, ongoing research using the GAP platform might lead to creation of genetically attenuated Pf vaccine strains with optimized immunogenicity and potentially better efficacy such as later liver stage-arresting GAPs (47) and blood stage GAPs (27). These efforts will further improve the prospects of a highly efficacious, liveattenuated malaria vaccine.…”

“…Furthermore, with the advent of CRISPR-Cas9 technology, GAP can be further genetically engineered and optimized to iteratively improve vaccine safety and efficacy. In addition, efforts are underway to create and evaluate blood stage GAP vaccines ( 27 ).…”

Live Vaccination with Blood-Stage Plasmodium yoelii 17XNL Prevents the Development of Experimental Cerebral Malaria

“…KAHRP may therefore be a good target for malaria vaccines. A group in Australia has been conducting the first clinical investigation using a “genetically attenuated blood-stage human malaria vaccine”, and they generated a parasite that lost KAHRP in Pf 3D7 by gene engineering [ 65 ]. We await the results of this clinical investigation eagerly, as it may represent a new paradigm for malaria vaccines.…”