Protection against Plasmodium falciparum malaria by PfSPZ Vaccine

Abstract

BACKGROUND: A radiation-attenuated Plasmodium falciparum (Pf) sporozoite (SPZ) malaria vaccine, PfSPZ Vaccine, protected 6 of 6 subjects (100%) against homologous Pf (same strain as in the vaccine) controlled human malaria infection (CHMI) 3 weeks after 5 doses administered intravenously. The next step was to assess protective efficacy against heterologous Pf (different from Pf in the vaccine), after fewer doses, and at 24 weeks. METHODS: The trial assessed tolerability, safety, immunogenicity, and protective efficacy of direct venous inoculation (DVI) of 3 or 5 doses of PfSPZ Vaccine in non-immune subjects. RESULTS: Three weeks after final immunization, 5 doses of 2.7 × 10⁵ PfSPZ protected 12 of 13 recipients (92.3% [95% CI: 48.0, 99.8]) against homologous CHMI and 4 of 5 (80.0% [10.4, 99.5]) against heterologous CHMI; 3 doses of 4.5 × 10⁵ PfSPZ protected 13 of 15 (86.7% [35.9, 98.3]) against homologous CHMI. Twenty-four weeks after final immunization, the 5-dose regimen protected 7 of 10 (70.0% [17.3, 93.3]) against homologous and 1 of 10 (10.0% [-35.8, 45.6]) against heterologous CHMI; the 3-dose regimen protected 8 of 14 (57.1% [21.5, 76.6]) against homologous CHMI. All 22 controls developed Pf parasitemia. PfSPZ Vaccine was well tolerated, safe, and easy to administer. No antibody or T cell responses correlated with protection. CONCLUSIONS: We have demonstrated for the first time to our knowledge that PfSPZ Vaccine can protect against a 3-week heterologous CHMI in a limited group of malaria-naive adult subjects. A 3-dose regimen protected against both 3-week and 24-week homologous CHMI (87% and 57%, respectively) in this population. These results provide a foundation for developing an optimized immunization regimen for preventing malaria. TRIAL REGISTRATION: ClinicalTrials.gov NCT02215707. FUNDING: Support was provided through the US Army Medical Research and Development Command, Military Infectious Diseases Research Program, and the Naval Medical Research Center's Advanced Medical Development Program.

Figure 1. Consort 2010 flow diagram.

Thirty subjects were enrolled to receive 5 doses of 2.7 × 10⁵ PfSPZ/dose at weeks 0, 4, 8, 12, and 20; subjects were then randomized equally to group 1 (homologous CHMIs) or group 2 (heterologous CHMIs). Fifteen subjects were enrolled to receive 3 doses of 4.5 × 10⁵ PfSPZ/dose at weeks 0, 8, and 16 with homologous CHMIs (group 3, which began immunizations 1 month after groups 1 and 2). Twenty-four subjects were enrolled as infectivity controls for the total of 4 CHMIs (2 homologous CHMIs and 2 heterologous CHMIs); each CHMI included 6 infectivity controls, except heterologous CHMI #1, for which only 4 infectivity controls were challenged. CHMI, controlled human malaria infection.

Figure 2. Results of CHMI done 3 weeks after the last dose of PfSPZ Vaccine.

(A and B) Survival curves in volunteers undergoing CHMI 3 weeks after the last of 5 doses of 2.7 × 10⁵ PfSPZ (A and B) or 3 doses of 4.5 × 10⁵ PfSPZ (A). Homologous CHMI with Pf 3D7 parasites was analyzed in two immunized groups (group 1 [2.7 × 10⁵ PfSPZ]: n = 13; group 3 [4.5 × 10⁵ PfSPZ]: n = 15) and one control group (n = 6) (A). Heterologous CHMI with Pf 7G8 parasites was analyzed in one immunized group (group 2 [2.7 × 10⁵ PfSPZ]: n = 5) and one control group (n = 4) (B). CHMI, controlled human malaria infection; Pf, Plasmodium falciparum.

Figure 3. Results of CHMI done 24 weeks after the last dose of PfSPZ Vaccine.

Survival curves in volunteers undergoing CHMI 24 weeks after last of 5 doses of 2.7 × 10⁵ PfSPZ (A and B) or 3 doses of 4.5 × 10⁵ PfSPZ (A). Homologous CHMI with Pf 3D7 parasites was analyzed in two immunized groups (group 1 [2.7 × 10⁵ PfSPZ]: n = 10; group 3 [4.5 × 10⁵ PfSPZ]: n = 14) and one control group (n = 6) (A). Heterologous CHMI with Pf 7G8 parasites was analyzed in one immunized group (group 2 [2.7 × 10⁵ PfSPZ]: n = 10) and one control group (n = 6) (B). CHMI, controlled human malaria infection; Pf = Plasmodium falciparum.

Figure 4. Antibody and T cell responses in all immunized participants.

For all assays, protected subjects are shown as filled (black) circles and unprotected subjects as open circles. For each of the 3 immunization groups, the interquartile ranges and the median values of responses of subjects in each group are shown. Assessment of antibodies (A–C) was performed in sera from subjects before immunization and 2 and 23 weeks after the last dose of PfSPZ Vaccine, time points that were ~1 week before CHMI #1 and CHMI #2, respectively. Number of samples assessed in A–C for CHMI #1: group 1 (n = 13), group 2 (n = 5), group 3 (n = 15); CHMI #2: group 1 (n = 10), group 2 (n = 10), group 3 (n = 14). (A) Antibodies to PfCSP by ELISA are reported as net OD 1.0 (the difference in OD 1.0 between post- and pre-immunization sera). (B) Antibodies to PfSPZ by aIFA are reported as the reciprocal serum dilution at which the fluorescence units were 2 × 10⁵ (AFU 2 × 10⁵). (C) Percent inhibition of PfSPZ invasion is reported as the percent reduction of the numbers of PfSPZ that invaded a human hepatocyte line (HC-04) in the presence of post-immunization as compared with pre-immunization serum from the same volunteer, both at a dilution of 1:5. At both CHMI #1 and CHMI #2, all 3 assays correlated with each other (CHMI #1: PfCSP ELISA vs. aIFA, P < 0.001, r² = 0.61; PfCSP ELISA vs. ISI, P = 0.0002, r² = 0.30; aIFA vs. ISI, P = 0.004, r² = 0.19. CHMI #2: PfCSP ELISA vs. aIFA, P < 0.0001, r² = 0.54; PfCSP ELISA vs. ISI, P < 0.0001, r² = 0.56; aIFA vs. ISI, P = 0.0002, r² = 0.34). Assessment of T cell responses by FlouroSpot assay was performed using PBMCs from subjects prior to immunization, 2 weeks after the first dose of PfSPZ Vaccine, and just prior to CHMI #1 and CHMI #2 (D–F). Number of samples assessed in D–F — 2 weeks after dose 1: group 1 (n = 8), group 2 (n = 5), group 3 (n = 15); pre-CHMI #1: group 1 (n = 13), group 2 (n = 5), group 3 (n = 15); pre-CHMI #2: group 1 (n = 10), group 2 (n = 10), group 3 (n = 13). Results are reported as spot-forming cells (SFCs) per 10⁶ PBMCs secreting (D–F) IFN-γ only, (E) IL-2 only, or (F) IFN-γ and IL-2. Individual data points have the pre-immunization SFCs/10⁶ PBMCs subtracted from the post-immunization SFCs/10⁶ PBMCs. aIFA, automated immunofluorescence assay; ISI, inhibition of sporozoite invasion.

Figure 5. Threshold hypothesis regarding achieving >80% protective

efficacy with PfSPZ Vaccine. To move from minimal to no protection (6),past Threshold 1 to short-term homologous protection (4), past Threshold 2 to short-term heterologous protection (this trial), past Threshold 3 to long-term homologous protection, past Threshold 4 to long-term, heterologous protection, all at >80% sterile protective efficacy, one must progressively increase the dosage of PfSPZ and optimize the number of doses and intervals between doses.