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

Abstract

Genetically engineered live Plasmodium falciparum sporozoites constitute a potential platform for creating consistently attenuated, genetically defined, whole-parasite vaccines against malaria through targeted gene deletions. Such genetically attenuated parasites (GAPs) do not require attenuation by irradiation or concomitant drug treatment. We previously developed a P. falciparum (Pf) GAP with deletions in P52, P36, and SAP1 genes (PfGAP3KO) and demonstrated its safety and immunogenicity in humans. Here, we further assessed safety, tolerability, and immunogenicity of the PfGAP3KO vaccine and tested its efficacy against controlled human malaria infection (CHMI) in malaria-naïve subjects. The vaccine was delivered by three (n = 6) or five (n = 8) immunizations with ~200 PfGAP3KO-infected mosquito bites per immunization. PfGAP3KO was safe and well tolerated with no breakthrough P. falciparum blood stage infections. Vaccine-related adverse events were predominately localized urticaria related to the numerous mosquito bites administered per vaccination. CHMI via bites with mosquitoes carrying fully infectious Pf NF54 parasites was carried out 1 month after the last immunization. Half of the study participants who received either three or five PfGAP3KO immunizations remained P. falciparum blood stage negative, as shown by a lack of detection of Plasmodium 18S rRNA in the blood for 28 days after CHMI. Six protected study participants received a second CHMI 6 months later, and one remained completely protected. Thus, the PfGAP3KO vaccine was safe and immunogenic and was capable of inducing protection against sporozoite infection. These results warrant further evaluation of PfGAP3KO vaccine efficacy in dose-range finding trials with an injectable formulation.

Figure 1.. Study design for vaccination and both CHMI phases.

PfGAP3KO sporozoites were delivered to eligible study participants five (Aim 1) or three times (Arm 2) via ~200 PfGAP3KO infected mosquito bites per administration. The interval between doses was one month, except for the interval between the last two doses, which was two months. One month after the last immunization, vaccinated study participants and a group of malaria-naïve study participants as infectivity controls underwent CHMI induced by five wild-type Pf NF54-infected mosquito bites. Vaccinated study participants who were protected against the first CHMI were eligible for a second CHMI six months later, and these study participants were accompanied at CHMI by another group of malaria-naïve study participants as infectivity controls.

Figure 2.. Consort diagram for enrollment through study completion.

The number of study participants who were eligible, enrolled, and completed aspects of the trial are listed by study arm.

Figure 3.. Frequency and maximum severity of local and systemic AEs during vaccination phase.

Local (top) and systemic (bottom) AEs are shown as a histogram for study participants in Arm 1 (left) and Arm 2 (right). Grading is shown as in the key and the number of study participants with each listed AE are listed in the bar labels.

Figure 4.. Infection rate following the first and second CHMI.

Time-to-event analysis for the protocol-defined primary efficacy threshold of documented Plasmodium infection after the first (A) and second CHMI (B). The y-axis represents the percentage of study participants that had not reached the definition of infection post-CHMI (one positive qRT-PCR result ≥20 estimated parasites/mL). Days are numbered post-CHMI.

Figure 5.. Antibody immunogenicity to pre-erythrocytic antigens.

Vaccine-induced antibodies recognizing full-length Pf CSP (A), Pf TRAP (C), and Pf MSP3 (E) were quantified by ELISA at each time point as shown. Arm 1 is shown in the solid red line, and Arm 2 is shown in the dashed blue line. Antibody levels are expressed in median fluorescence intensity (MdFI). Vertical dashed lines correspond to immunization procedures as noted at the top. Each data point represents the geometric mean for each Study Arm at each time point; error bars represent 95% confidence intervals. When pre-CHMI antibody levels were compared for protected vs. unprotected vs. naïve controls for Pf CSP (B), Pf TRAP (D), and Pf MSP3 (F), there was no significant difference between protected and unprotected groups for responses to any antigen.

Figure 6.. Inhibition of sporozoite invasion.

Sera from immunized study participants were also evaluated for functional inhibition of sporozoite invasion and are displayed relative to vaccination (A) or CHMI (B) using an in vitro ISTI assay. Each data point represents the median inhibition for each Study Arm at each time point; error bars represent the interquartile range. Vertical dashed lines in A correspond to immunization procedures as noted at the top; vertical solid lines in B represent CHMI days.