Sporozoite immunization: innovative translational science to support the fight against malaria

Abstract

Introduction: Malaria, a devastating febrile illness caused by protozoan parasites, sickened 247,000,000 people in 2021 and killed 619,000, mostly children and pregnant women in sub-Saharan Africa. A highly effective vaccine is urgently needed, especially for Plasmodium falciparum (Pf), the deadliest human malaria parasite.

Areas covered: Sporozoites (SPZ), the parasite stage transmitted by Anopheles mosquitoes to humans, are the only vaccine immunogen achieving >90% efficacy against Pf infection. This review describes >30 clinical trials of PfSPZ vaccines in the U.S.A., Europe, Africa, and Asia, based on first-hand knowledge of the trials and PubMed searches of 'sporozoites,' 'malaria,' and 'vaccines.'

Expert opinion: First generation (radiation-attenuated) PfSPZ vaccines are safe, well tolerated, 80-100% efficacious against homologous controlled human malaria infection (CHMI) and provide 18-19 months protection without boosting in Africa. Second generation chemo-attenuated PfSPZ are more potent, 100% efficacious against stringent heterologous (variant strain) CHMI, but require a co-administered drug, raising safety concerns. Third generation, late liver stage-arresting, replication competent (LARC), genetically-attenuated PfSPZ are expected to be both safe and highly efficacious. Overall, PfSPZ vaccines meet safety, tolerability, and efficacy requirements for protecting pregnant women and travelers exposed to Pf in Africa, with licensure for these populations possible within 5 years. Protecting children and mass vaccination programs to block transmission and eliminate malaria are long-term objectives.

Introduction Malaria, a devastating febrile illness caused by protozoan parasites, sickened 247,000,000 people in 2021 and killed 619,000, mostly children and pregnant women in sub-Saharan Africa. A highly effective vaccine is urgently needed, especially for Plasmodium falciparum (Pf), the deadliest human malaria parasite. Areas covered Sporozoites (SPZ), the parasite stage transmitted by Anopheles mosquitoes to humans, are the only vaccine immunogen achieving >90% efficacy against Pf infection. This review describes >30 clinical trials of PfSPZ vaccines in the U.S.A., Europe, Africa, and Asia, based on first-hand knowledge of the trials and PubMed searches of ‘sporozoites,’ ‘malaria,’ and ‘vaccines.’ Expert opinion First generation (radiation-attenuated) PfSPZ vaccines are safe, well tolerated, 80–100% efficacious against homologous controlled human malaria infection (CHMI) and provide 18–19 months protection without boosting in Africa. Second generation chemo-attenuated PfSPZ are more potent, 100% efficacious against stringent heterologous (variant strain) CHMI, but require a co-administered drug, raising safety concerns. Third generation, late liver stage-arresting, replication competent (LARC), genetically-attenuated PfSPZ are expected to be both safe and highly efficacious. Overall, PfSPZ vaccines meet safety, tolerability, and efficacy requirements for protecting pregnant women and travelers exposed to Pf in Africa, with licensure for these populations possible within 5 years. Protecting children and mass vaccination programs to block transmission and eliminate malaria are long-term objectives.

Keywords: PfSPZ Vaccine; PfSPZ-CVac; PfSPZ-LARC2 Vaccine; Plasmodium falciparum; Sporozoites; malaria vaccines; review; vaccines.

Figure 1.

Malaria sporozoite. Scanning electron micrograph of a Plasmodium cynomolgi sporozoite superimposed on a scanning electron micrograph of a rat liver sinusoid. P. falciparum sporozoites are similar in appearance and are typically 8–10 microns in length. They are large eukaryotic cells able to glide along the endothelium, traverse interstitial spaces, and penetrate Kupffer cells and hepatocytes. Pc = parenchymal cell (hepatocyte); fsc = fat-storing cells (stellate cell); SD = space of Disse. Arrows point to endothelial cell fenestrations. This composite image was made by Ute Frevert, who modified the shape of the sporozoite to better reflect the ability of the parasite to flex when gliding along natural tissue structures as shown by intravital cinematography. Reproduced with permission from Comparative Hepatology (no changes were made) [86].

Figure 2.

PfSPZ platform technologies. (a) Illustration of a liver sinusoid showing the invading sporozoites (green progressing to red as they replicate and differentiate). H = hepatocyte, KC = Kupffer cell, EC = endothelial cell. Reproduced from Nature [8] with permission. Design credit: Sumana Chakravarty. (b) radiation-attenuated and first-generation genetically attenuated (GA) PfSPZ invade hepatocytes and partially develop, but do not replicate. Infectious PfSPZ in PfSPZ-CVac invade hepatocytes and partially develop if a liver-active antimalarial is administered (e.g. pyrimethamine, azithromycin) or fully develop including normal replication (releasing merozoites into the blood stream) if a blood-stage active antimalarial is administered (e.g. chloroquine). The extent of development therefore depends on which drug is used and when it is administered. Second-generation GA parasites have a late-arresting phenotype and halt development before the release of merozoites. Potency appears to increase with further development, whereas safety concerns arise should the parasite release merozoites into the blood.

Figure 4.

Protective efficacy of PfSPZ Vaccine against naturally transmitted Pf infection in Malian adults (MLSPZV1 clinical trial). Vaccine efficacy was analyzed by time to first positive blood smear, with day 0 at 28 days after the fifth vaccination. The inverse survival curves include participants who received all five vaccinations and were evaluable for the primary exploratory efficacy end-point. Five participants (one in the PfSPZ Vaccine group and four in the placebo group) were censored from the primary efficacy analysis because they had a positive blood smear before 28 days after the fifth vaccination. PfSPZ=Plasmodium falciparum sporozoite. Reproduced with permission from the Lancet ID (no changes were made) [3].

Figure 5.

Differential antibody and T-cell responses to PfSPZ Vaccine by age in Tanzanian adults, adolescents, children, and infants. Panel a. Antibody levels to Pf circumsporozoite protein as measured by enzyme-linked immunosorbent assay (difference between two-weeks-post-immunization and pre-immunization) after immunizing 18–45-year-olds, 11–17-year-olds, 6–10-year-olds, 1–5-year-olds, and 6–11-month-olds with three doses of 9.0×10⁵ PfSPZ of PfSPZ Vaccine at eight-week intervals. VRC314, an earlier trial in which malaria-naive US adults were immunized with the same dosage regimen and antibody levels were assessed using the same PfCSP ELISA conducted in the same laboratory, is shown as a comparison (for VRC314, filled in circles indicate volunteers protected against CHMI, empty circles the unprotected volunteers). Medians with interquartile ranges are shown. Panel b. PfSPZ-specific memory CD4 T-cell responses pre- and post-vaccination after incubation with PfSPZ, expressed as the percent of cells in the blood expressing interferon gamma (IFN-γ), interleukin 2 (IL-2), or tumor necrosis factor alpha (TNF-α) at pre-immunization or 2 weeks after the first and third doses of PfSPZ Vaccine (9.0×10⁵). VRC314 data (malaria-naive adults) are included as a comparison. Reproduced with permission from the American journal of Tropical Medicine and Hygiene (no changes were made) [136].

Figure 6.

Parasitemia detected by qPCR after the first, second and third dose of 2×10⁵ PfSPZ for PfSPZ-CVac (CQ) and PfSPZ-CVac (PYR). Median parasitaemia values and interquartile ranges are shown for positive PfSPZ-CVac (CQ) participants (no PfSPZ-CVac (PYR) participants were positive, since pyrimethamine kills the parasites during liver stage development). Dose 1, PfSPZ Challenge inoculation under CQ or PYR treatment cover with follow-up for 14 days; doses 2 and 3, PfSPZ Challenge inoculation under CQ or PYR treatment cover with follow-up for 10 days. The table shows (from left to right in each cell): the number of participants who were positive by qPCR/the number of injected participants; the median peak parasite density of positive participants (parasites per ml); and the mean day of peak parasite density (positive participants). Six PfSPZ-CVac (CQ) recipients went on to heterologous CHMI and 6/6 (100%) were protected; nine PfSPZ-CVac (PYR) recipients went on to heterologous CHMI and 7/9 (78%) were protected. ND, not detected; n/a, not applicable. Reproduced with permission from Nature (no changes were made) [68].

Figure 7.

Parasite density estimated by qRT-PCR in participants in a PfSPZ-CVac (CQ) trial conducted at the Kaiser Permanente Washington Health Research Institute, Seattle. (A) Triangle symbols indicate the first, second, and third days of vaccine administration. The numbers above each peak of parasitemia display the number of persons positive with transient parasitemia divided by the total number of vaccine recipients for that dose of vaccine. Days are listed relative to the first dose of vaccine. Group 1 received a PfSPZ Challenge dose of 5.12×10⁴ PfSPZ for each of three doses administered by DVI seven days apart. Group 3 received a PfSPZ Challenge dose of 1.024×10⁵ PfSPZ for each of three doses administered by DVI five days apart. All participants underwent CHMI 10 weeks after the last vaccine dose. 7/7 participants became infected in group 1 (vaccine efficacy 0%) while only 2/8 participants became infected in group 3 (vaccine efficacy 75%). It is notable that administration of PfSPZ-CVac on a schedule where vaccine administrations #2 and #3 coincided with sub-microscopic blood-stage parasitemia (group 1) was associated with an absence of sterile protective immunity, whereas dodging parasitemia (group 3) appeared to restore the expected protective efficacy. Reproduced with permission from PLoS pathogens (no changes were made) [127].

Figure 8.

Target populations for PfSPZ vaccines. The objective for PfSPZ vaccine development is high-level prevention of Pf infection. Major risk groups potentially benefiting from such a vaccine are listed. It is anticipated that licensure will occur first in the EU with an indication to prevent malaria in travelers (including travelers who travel within their own country from areas without malaria to areas with malaria), with licensure in the U.S.A. and malaria-endemic areas thereafter. Major risk groups are identified. The ultimate objective is use in MVPs to achieve focal and regional elimination.

Figure 9.. A hypothetical mass vaccination program (MVP) using PfSPZ-LARC2 Vaccine.

As envisioned by the authors, a malaria elimination campaign would be designed for a target geographical region including appropriate community engagement. Mass drug administration (MDA) such as artesunate + pyronaridine [290] would be conducted prior to immunization to reduce the suppression of vaccine responses by existing parasitemia and to eliminate the human parasite reservoir. Intensified vector control would help to eliminate the mosquito reservoir. Immunization with PfSPZ-LARC2 Vaccine would then be performed to protect the population against the acquisition of new infections. An additional round of MDA might be needed post immunization to achieve elimination. Primaquine would be used to eliminate infectious gametocytes. AS = artesunate; PYRON = pyronaridine; PQ = primaquine.

Figure 10.

Efficacy of PfSPZ Vaccine against naturally transmitted Pf infection in Burkina Faso adults. Inverse survival curves for time after the last vaccination to first positive thick blood smear (asymptomatic + symptomatic infection) are shown. Efficacy was analyzed as 1-hazard ratio for the primary follow-up period (0–24 weeks post third vaccination) and the extended follow-up period (0–76 weeks post third vaccination). The survival curves include 79 participants who received all three vaccinations and were evaluable for the vaccine efficacy endpoint. Reproduced with permission from Science Translational Medicine (no significant changes were made) [7].

Figure 3.

The International PfSPZ Consortium. Seattle Children’s Research Institute hosted 289 researchers from 86 institutions from 28 countries in person and virtually at the International Plasmodium falciparum Consortium (i-PfSPZ-C) meeting held November 3–4, 2022, to present data and set research and clinical development strategies for PfSPZ-based vaccines and products.