Whole sporozoite immunization with Plasmodium falciparum strain NF135 in a randomized trial

Abstract

Background: Whole sporozoite immunization under chemoprophylaxis (CPS regime) induces long-lasting sterile homologous protection in the controlled human malaria infection model using Plasmodium falciparum strain NF54. The relative proficiency of liver-stage parasite development may be an important factor determining immunization efficacy. Previous studies show that Plasmodium falciparum strain NF135 produces relatively high numbers of large liver-stage schizonts in vitro. Here, we evaluate this strain for use in CPS immunization regimes.

Methods: In a partially randomized, open-label study conducted at the Radboudumc, Nijmegen, the Netherlands, healthy, malaria-naïve adults were immunized by three rounds of fifteen or five NF135-infected mosquito bites under mefloquine prophylaxis (cohort A) or fifteen NF135-infected mosquito bites and presumptive treatment with artemether/lumefantrine (cohort B). Cohort A participants were exposed to a homologous challenge 19 weeks after immunization. The primary objective of the study was to evaluate the safety and tolerability of CPS immunizations with NF135.

Results: Relatively high liver-to-blood inocula were observed during immunization with NF135 in both cohorts. Eighteen of 30 (60%) high-dose participants and 3/10 (30%) low-dose participants experienced grade 3 adverse events 7 to 21 days following their first immunization. All cohort A participants and two participants in cohort B developed breakthrough blood-stage malaria infections during immunizations requiring rescue treatment. The resulting compromised immunizations induced modest sterile protection against homologous challenge in cohort A (5/17; 29%).

Conclusions: These CPS regimes using NF135 were relatively poorly tolerated and frequently required rescue treatment, thereby compromising immunization efficiency and protective efficacy. Consequently, the full potential of NF135 sporozoites for induction of immune protection remains inconclusive. Nonetheless, the high liver-stage burden achieved by this strain highlights it as an interesting potential candidate for novel whole sporozoite immunization approaches.

Trial registration: The trial was registered at ClinicalTrials.gov under identifier NCT03813108.

Keywords: Malaria; NF135; Plasmodium falciparum; Vaccine-induced protection; Whole sporozoite immunization.

Conflict of interest statement

The authors declare they have no competing interests.

© 2023. The Author(s).

Figures

Fig. 1

Flow chart. Eighty-nine candidates were tested for eligibility and fourty-six were included in the study after screening: twenty-three in cohort A (three additional participants started mefloquine prophylaxis in case of dropouts prior to the first immunization), twenty in cohort B, and three in the control groups. Participants in cohort A were randomized to receive immunizations with either three times fifteen NF135-infected mosquitoes (high-dose) or three times five NF135-infected mosquitoes (low-dose). One participant dropped out after the first immunization and two participants after the third immunization, all three because of personal or logistical reasons. Participants in cohort B received one immunization with fifteen NF135-infected mosquitoes (high-dose). The control group was not immunized. Immunized participants from cohort A, and three malaria-naïve control participants were challenged by bites of five NF135-infected mosquitoes. In cohort B, the trial was prematurely ended after the first immunization and participants were not subsequently challenged

Fig. 2

Parasitemia on day seven after immunization 1. Each dot represents one participant, the line represents the median value. Participants in the high-dose group were immunized with fifteen NF135-infected mosquitoes. Participants in the low-dose group were immunized with five NF135-infected mosquitoes. The statistical analysis performed was Welch’s t-test. ****p < 0001

Fig. 3

Parasitemia after immunization in cohort A. Individual parasite curves are presented in gray. The median parasite density for each group is presented in green. Dashed lines indicate an immunization. A Participants immunized with the low-dose of three times five NF135-infected mosquitoes. B Participants immunized with the high dose of three times fifteen NF135-infected mosquitoes. C Number of participants with a positive qPCR and/or thick smear after each immunization

Fig. 4

Sterile protection against PfNF135 mosquito bite challenge. A Kaplan–Meier curve depicting percentage of participants that remained sterilely protected after homologous challenge infection with PfNF135. Participants in the high-dose arm received three immunizations by bites of fifteen PfNF135-infected mosquitoes. Participants in the low-dose arm received three immunizations by bites of five infected mosquitoes with the same parasite strain. The control group received no immunizations. B Percentage of sterilely protected partially protected and unprotected participants after controlled human malaria infection with the homologous NF135 strain in each study arm. Participants in the high-dose arm received three immunizations by bites of fifteen P. falciparum NF135-infected mosquitoes and participants in the low-dose arm received three immunizations by bites of five infected mosquitoes with the same parasite clone. Mosq, mosquitoes

Fig. 5

Anti-sporozoite IgG titers. IgG antibody titers against sporozoite extract in sera of cohort A participants immunized with three times fifteen PfNF135-infected mosquitoes (purple, n = 10) and of participants in a historical comparator trial immunized with three times fifteen PfNF54-infected mosquitoes (blue, n = 12), at baseline, after the first immunization, and after completion of the course of three immunizations. Thick and thin horizontal lines represent median and interquartile range, respectively. The Mann–Whitney U test was used for differences between studies. Friedman test with post hoc paired comparison using Bonferroni-corrected Wilcoxon signed rank test was used for comparisons of time points within studies

Fig. 6

Adverse events after immunization 1. A Number of grade 1, 2, 3, and 4 adverse events per participant per cohort. B Percentage of participants who experienced a grade 1, 2, 3, or 4 adverse event per cohort. AE, Adverse event