Late-liver-stage malaria vaccine delivers 89% protection and boosts cellular immunity

A new malaria vaccine achieves an unparalleled 89% efficacy by targeting late-liver-stage antigens, unlocking new horizons in the fight against global disease.

In a recent study published in The New England Journal of Medicine, researchers in the Netherlands evaluated the safety, immune response, and protective efficacy of a second-generation genetically attenuated (GA) Plasmodium falciparum parasite in healthy adults.

Background

Malaria eradication efforts have slowed, highlighting the need for more effective tools. Current malaria vaccines, such as the recombinant protein-based RTS,S/AS01 (Mosquirix) and the modified recombinant R21, target the circumsporozoite protein (CSP) but provide only modest, short-lived protection, particularly in infants. Whole-parasite vaccination strategies using GA sporozoites offer a promising alternative. These sporozoites invade liver cells but fail to progress to blood-stage infection, allowing the immune system to safely encounter a wide array of parasite antigens and generate humoral and cellular immune responses. Late-arresting GA parasites show potential for improved efficacy over early-arresting models. Further research is needed to optimize these strategies and evaluate their effectiveness in malaria-endemic regions.

About the Study

The trial utilized a dose-escalation safety phase (Stage A) and a double-blind efficacy phase (Stage B), with participants receiving up to 50 bites from GA2-infected mosquitoes across three immunization sessions​.

A clinical trial was conducted at two centers in the Netherlands, Leiden University Medical Center and Radboud University Medical Center, to assess the safety, immunogenicity, and efficacy of genetically attenuated Plasmodium falciparum parasites. The trial included two phases: stage A, an open-label dose-escalation phase where participants received 15 or 50 mosquito bites delivering the GA2 parasite, and stage B, a double-blind, placebo-controlled phase comparing the efficacy of GA2 to GA1 and placebo. Participants underwent three immunization sessions at 28-day intervals, each involving 50 mosquito bites. Due to COVID-19 restrictions, the trial was conducted with fewer participants than initially planned.

Three weeks after the final immunization, all participants underwent controlled human malaria infection (CHMI) through five mosquito bites infected with the unattenuated P. falciparum strain 3D7 (Pf3D7). Primary outcomes included adverse event frequency and blood-stage parasitemia, assessed via quantitative polymerase chain reaction (qPCR). Secondary outcomes measured humoral and cellular immune responses using enzyme-linked immunosorbent assay (ELISA) and spectral-flow cytometry. Eligible participants, aged 18-35, provided informed consent and were randomized by an independent statistician, ensuring trial blinding. Statistical analyses included paired and unpaired t-tests, chi-square tests, and Mann-Whitney tests, with a significance level of 5%.

Study Results

No evidence of breakthrough blood-stage infections was observed, even at high doses of mosquito-borne GA2 exposure, reinforcing the parasite's safety profile.

From September 13, 2021, to January 28, 2022, 75 malaria-naïve adults were screened for trial participation, with 43 participants enrolled. No participants withdrew during stage A, while three withdrew before controlled human malaria infection in stage B. Among the participants, 51% were women, with a median age of 23 years and a median body mass index (BMI) of 24.1.

The trial reported no serious adverse events or breakthrough infections following exposure to GA2-infected mosquitoes, as confirmed by qPCR. Adverse events, primarily erythema (redness of the skin) and pruritus (itching sensation) at bite sites, occurred similarly across GA2, GA1, and placebo groups. Most events were mild and treated with antihistamines or topical corticosteroids. Systemic adverse events, such as myalgia and headache, were infrequent. Two cases of elevated troponin T were assessed as unrelated to the intervention, while elevated liver function test results were attributed to antihistamine use.

In stage B, GA2 immunization provided 89% protection against CHMI, with 8 of 9 participants showing no parasitemia. In contrast, protection was observed in 1 of 8 GA1 recipients (12%) and none in the placebo group. Time to parasitemia differed significantly between groups, highlighting GA2’s superior efficacy.

Immunogenicity analysis revealed increased antibodies against Plasmodium falciparum CSP (PfCSP) in GA2 and GA1 groups compared to baseline and placebo. However, antibody titers were similar between GA2 and GA1 groups and did not correlate with protection. Cellular immunity analysis showed higher frequencies of P. falciparum-specific Cluster of Differentiation 4 Positive T cells (CD4+) and V-Delta-2 Positive Gamma Delta (Vδ2+ γδ) T cells in GA2-immunized participants. These cells exhibited a proinflammatory signature, expressing interferon-γ, tumor necrosis factor alpha (TNF-α), and interleukin-2, with GA2 inducing more polyfunctional T cells compared to GA1.

GA2-immunized participants showed higher levels of polyfunctional CD4+ and Vδ2+ γδ T cells with an effector memory phenotype, indicating a robust immune response. This response was absent in the placebo group and less pronounced in the GA1 group.

Conclusions

To summarize, this trial demonstrated that the late-arresting parasites (GA2) induced stronger protective immunity (89%) against malaria than early-arresting parasites (GA1, 12%). GA2-induced immunity correlated with polyfunctional CD4+ T cells and Vδ2+ γδ T cells, suggesting a pivotal role for late-liver-stage antigens. Unlike antibodies, these cellular responses were critical for protection. GA2 showed a favorable safety profile, with no breakthrough blood-stage infections or serious adverse events.

Further studies are needed to confirm these findings in larger populations, evaluate the durability of immunity, and assess efficacy in malaria-endemic regions.

Journal reference:

• O.A.C. Lamers, B.M.D. Franke‑Fayard, J.P.R. Koopman, et al., "Safety and Efficacy of Immunization with a Late-Liver-Stage Attenuated Malaria Parasite," N Engl J Med (2024), DOI: 10.1056/NEJMoa2313892, https://www.nejm.org/doi/full/10.1056/NEJMoa2313892