Robust immune response observed in malaria vaccine trial, demonstrating liver-stage attenuation and multipronged immunity

In a recent study published in Nature Medicine, a group of researchers assessed the safety, tolerability, and efficacy of a single immunization with genetically attenuated Pf∆mei2 parasites via mosquito bite (GA2-MB) against controlled human malaria infection.

Malaria causes over 600,000 deaths annually, predominantly in children under five, making it the fifth leading cause of child mortality worldwide. While the RTS, S and R21 subunit vaccines have significantly reduced malaria-related illness and death, their limited protective efficacy and the need for booster doses underscore the urgency of developing more potent and durable vaccines capable of breaking transmission.

Immunization strategies using whole Plasmodium falciparum (Pf) sporozoites, particularly those with genetically attenuated late liver stage-arresting parasites (Pf∆mei2, GA2), show great promise in achieving high-level protection.

Studies have demonstrated that GA2 parasites offer superior efficacy compared to early-arresting counterparts. Further research is essential to validate the safety and efficacy of GA2 in broader and more diverse populations and to refine single-dose immunization protocols for widespread application.

A randomized, double-blind, placebo-controlled trial involving a controlled human malaria infection (CHMI) was conducted between February and November 2023 at Leiden University Medical Center in the Netherlands. Fifteen malaria-naive participants aged 15-30 years underwent health screenings, including medical evaluations, drug testing, and pregnancy testing for female participants.

Ten participants were immunized with GA2 parasites via 45-55 mosquito bites, while five received placebo mock-immunizations with uninfected mosquitoes. Six weeks later, all participants underwent CHMI using five wild-type Pf3D7-infected mosquitoes.

Daily follow-ups were conducted from days 6 to 21 post-CHMI for adverse event monitoring, safety assessments, blood sampling, and sensitive parasite detection using Pf quantitative polymerase chain reaction (PfqPCR). Participants with parasitemia exceeding 100 parasites/ml or those reaching day 28 without symptoms were treated with atovaquone-proguanil. Adverse events were recorded in prespecified categories, and both participants and investigators remained blinded to group allocation.

Immunological assessments included antibody measurements using enzyme-linked immunosorbent assays (ELISA) and T-cell response analysis via flow cytometry. Statistical analyses were performed to evaluate safety, parasitemia, and immune responses. The trial adhered to ethical guidelines and was approved by the Dutch Central Committee for Research involving Human Subjects. Further studies are required to confirm findings in broader populations.

In the present study, participants aged 15-30 years were recruited between February and March 2023, screened, and randomized. On April 12, 2023, participants were exposed to 50 (±5) GA2-infected or uninfected Anopheles stephensi mosquitoes. Six weeks later, all participants underwent a CHMI via bites from five wild-type Pf3D7-infected mosquitoes. Study visits included extensive monitoring for adverse events, safety assessments, and parasite detection using PfqPCR.

The trial demonstrated that single immunization with GA2-MB was safe and well tolerated. No serious adverse events or breakthrough malaria cases occurred. Itching, moderate swelling, and mild blistering were reported in a few participants and managed effectively.

Importantly, no parasite concentrations exceeding 50 parasites per ml were detected after immunization. At CHMI, nine out of ten GA2-MB participants (90%) remained PfqPCR-negative through day 28, compared to all placebo participants who became parasitemic. Median time-to-parasitemia in the placebo group was nine days, while parasitemia in the unprotected GA2-MB participant was delayed to day 13.

Secondary outcomes highlighted precise immune responses in GA2-MB participants. Antibody levels targeting Pf circumsporozoite protein were significantly higher compared to placebo, while antibodies against other antigens, such as Pf apical membrane antigen-1 and merozoite surface protein-1 showed no significant differences.

Cellular immunity assessments revealed a strong proinflammatory type-1 cytokine response, including tumor necrosis factor (TNF), interferon-gamma (IFNγ), and interleukin-2 (IL-2), in cluster of differentiation 4-positive (CD4+) T cells and Vδ2+ γδ T cells, along with moderate type-2 responses, such as IL-4, IL-5, and IL-13, and regulatory responses, such as IL-10. GA2-MB participants demonstrated higher frequencies of polyfunctional CD4+ T cells and Vδ2+ γδ T cells co-expressing multiple type-1 cytokines.

These polyfunctional CD4+ T cells, enriched among memory T cells, predominantly acquired an effector memory phenotype within two weeks of immunization and maintained elevated levels throughout the post-CHMI monitoring period.

To summarize, this study demonstrated that a single immunization with 50 GA2-infected mosquitoes protected 90% of malaria-naive individuals against homologous CHMI. Unlike traditional multi-dose malaria vaccine regimens, single-dose immunization offers practical advantages for endemic settings and travelers.

High-level protection observed after one GA2 immunization was comparable to three-dose regimens, suggesting limited boosting effects from additional doses. No breakthrough malaria occurred among 50 participants exposed to GA2 in current and previous studies, aligning with preclinical findings.