A highly protective malaria vaccine would greatly facilitate the prevention and elimination of malaria and containment of drug-resistant parasites1. A high level (more than 90%) of protection against malaria in humans has previously been achieved only by immunization with radiation-attenuated Plasmodium falciparum (Pf) sporozoites (PfSPZ) inoculated by mosquitoes2, 3, 4; by intravenous injection of aseptic, purified, radiation-attenuated, cryopreserved PfSPZ (‘PfSPZ Vaccine’)5, 6; or by infectious PfSPZ inoculated by mosquitoes to volunteers taking chloroquine7, 8, 9, 10 or mefloquine11 (chemoprophylaxis with sporozoites). We assessed immunization by direct venous inoculation of aseptic, purified, cryopreserved, non-irradiated PfSPZ (‘PfSPZ Challenge’12, 13) to malaria-naive, healthy adult volunteers taking chloroquine for antimalarial chemoprophylaxis (vaccine approach denoted as PfSPZ-CVac)14. Three doses of 5.12 × 104 PfSPZ of PfSPZ Challenge12, 13 at 28-day intervals were well tolerated and safe, and prevented infection in 9 out of 9 (100%) volunteers who underwent controlled human malaria infection ten weeks after the last dose (group III). Protective efficacy was dependent on dose and regimen. Immunization with 3.2 × 103 (group I) or 1.28 × 104 (group II) PfSPZ protected 3 out of 9 (33%) or 6 out of 9 (67%) volunteers, respectively. Three doses of 5.12 × 104 PfSPZ at five-day intervals protected 5 out of 8 (63%) volunteers. The frequency of Pf-specific polyfunctional CD4 memory T cells was associated with protection. On a 7,455 peptide Pf proteome array, immune sera from at least 5 out of 9 group III vaccinees recognized each of 22 proteins. PfSPZ-CVac is a highly efficacious vaccine candidate; when we are able to optimize the immunization regimen (dose, interval between doses, and drug partner), this vaccine could be used for combination mass drug administration and a mass vaccination program approach to eliminate malaria from geographically defined areas.
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Extended data figures and tables
Extended Data Figures
- Extended Data Figure 1: Distribution of adverse events. (134 KB)
a, b, The number of adverse events (AEs) regardless of attribution to investigational product. Each bar represents one volunteer sorted on the number of adverse events from the time of first injection with normal saline (controls) or PfSPZ-CVac until the time of CHMI, approximately 17 weeks later (a) and adverse events in the same volunteers from initiation of CHMI until the end of follow-up (b). Mild (grade 1) adverse events are depicted in grey, moderate (grade 2) in yellow and severe (grade 3) in blue. Non-protected volunteers are marked with an ‘M’ on the x axis.
- Extended Data Figure 3: CD4 T-cell cytokine polyfunctionality. (328 KB)
PBMCs from subjects were drawn 14 days after third immunization (post-imm) or 1 day before CHMI (pre-CHMI), stimulated with PfSPZ, PfRBC, or stimulation controls, and stained for intracellular cytokine expression. a, b, The pie charts show the proportion of memory CD4 T cells expressing any combination of IFN-γ, IL-2, or TNF-α for each dose group after stimulation with PfSPZ (a) or PfRBC (b). Responses are background subtracted from control antigen stimulations 1% HSA or uninfected erythrocytes. c, d, The magnitude of the memory CD4 T-cell response for each combination of cytokines is shown in c and d. There is a trend towards higher polyfunctionality as dose increases. e, f, The median fluorescence intensity (MFI) for IFN-γ is shown for the different combination of IFN-γ+ cells following PfSPZ or PfRBC stimulation. Cells that simultaneously produce IFN-γ, IL-2, and TNF-α have the highest IFN-γ MFI.
- Extended Data Figure 4: T-cell immunogenicity. (229 KB)
a, b, Memory CD4 T cells producing IL-4 (a) or IL-10 (b) after PfRBC stimulation. Memory γδ T cells producing IFN-γ, IL-2, or TNF-α following PfSPZ (c) or PfRBC stimulation (d). For a–d, results are the percentage of cytokine-producing cells after incubation with PfSPZ minus the percentage of cells after incubation with vaccine diluent (medium with 1% HSA) as control or percentage of cytokine-producing cells after incubation with asexual Pf-infected red blood cells (PfRBC) minus uninfected RBCs as control. e, f, Total memory γδ T cells assessed before immunization (pre-imm) and 14 days after third immunization (post-imm) for the percentage of cells expressing CD38. The absolute frequencies are shown in e and the change from pre-vaccination to post-vaccination is shown in f. For a–d, within a dose group, the difference from pre-vaccine was assessed by two-way ANOVA with Bonferroni correction. Data are median ± interquartile range. For e, f, difference from pre-vaccine was assessed by Wilcoxon signed rank test. P values were corrected for multiple comparisons by the Bonferroni method. *P < 0.05, *P < 0.01. Data are median ± interquartile range. Pre-imm, 3 days before first immunization; post-imm, 14 days after third immunization; pre-CHMI, 1 day before CHMI.
- Extended Data Figure 5: Sub-family analysis of γδ T cells. (258 KB)
a–f, The frequency of the circulating γδ T-cell subsets as a percentage of total lymphocytes was assessed in unstimulated PBMCs before the first immunization (pre-imm), 2 weeks after final immunization (post-imm), and the day before CHMI (pre-CHMI). Fold change compared to pre-imm is shown for total memory γδ T cells (a), Vγ9+Vδ2+ (b), Vγ9+Vδ1+ (c), Vγ9−Vδ1+ (d), Vγ9+Vδ1−Vδ2− (e), and Vγ9−Vδ1−Vδ2− (f) subfamilies. The frequency of Vγ9−Vδ2+ subset is low to undetectable. Within a dose group, the difference from pre-imm was assessed by two-way ANOVA with Bonferroni correction. *P < 0.05, **P < 0.01. Data are geometric mean ± 95% CI.
- Extended Data Figure 6: Anti-plasmodial antibody responses in vaccinated volunteers who were immunized with three doses of 5.12 × 104 PfSPZ at 28-day, 14-day, or 5-day intervals. (160 KB)
Antibodies to PfCSP by ELISA were assessed in sera taken before any immunizations (pre-immunization), two weeks following last immunization (post-immunization) and 10 weeks after last immunization, which was one day before CHMI (pre-CHMI). PfCSP ELISA results are reported as net OD 1.0; the reciprocal serum dilution at which the optical density was 1.0 in post-immunization or pre-CHMI sera minus the OD 1.0 in pre-immunization sera. All values met criteria for positivity. Protected volunteers are represented by yellow circles and unprotected volunteers by grey circles.
- Extended Data Figure 7: Development of PfSPZ Vaccine and PfSPZ-CVac in hepatocytes. (396 KB)
Radiation-attenuated PfSPZ in PfSPZ Vaccine invade hepatocytes and partially develop, but do not replicate. They are metabolically active and non-replicating. Infectious PfSPZ in PfSPZ-CVac invade hepatocytes and fully develop. A single PfSPZ replicates exponentially producing more than 104 merozoites. These merozoites are released into the circulation, and each merozoite can invade a different erythrocyte. Chloroquine prevents complete parasite development within erythrocytes, thereby preventing the development of merozoites that can invade new erythrocytes.
- Extended Data Figure 8: Transient parasitaemia following vaccination at 5-day intervals. (186 KB)
Parasitaemia measured by qPCR over 22 days. The subjects who were protected and not protected against CHMI are in yellow and grey, respectively. The times of PfSPZ inoculations are shown as vertical red lines and the time of last CQ administration as a vertical blue line. CQ was given as 10 mg kg−1 (maximum 620 mg) loading dose on day 0 followed by 5 mg kg−1 (maximum 310 mg) chloroquine base on days 5, 10, and 15.
Extended Data Tables
- Supplementary Information (804 KB)
This file contains Supplementary Figure 1, Supplementary Tables 1-5 and 7-9.
- Supplementary Table 6 (1.7 MB)
Supplementary Table 6 shows logistic regression of peak antibody levels (2 weeks after final immunization) and baseline antibody levels on probability of sterile protection against CHMI, adjusted by dose of PfSPZ-CVac.