Advancements and Challenges in Developing Malaria Vaccines: Targeting Multiple Stages of the Parasite Life Cycle

Malaria Vaccines: Current Achievements and Path Forward

Malaria remains a significant global health challenge. Although the recent approval of the liver-stage vaccines RTS, S and R21 marks significant progress in malaria control, challenges remain in achieving long-lasting and broad protection. In this review, we provide an overview of the current landscape of malaria control, especially anti-malaria vaccine development. We first review the development of the RTS, S and R21 vaccines, highlighting their efficacy and limitations. We then examine other vaccines in development, including attenuated whole-sporozoite vaccines, as well as blood-stage-targeting vaccines and transmission-blocking vaccines targeting a variety of different immunogens. Additionally, we discuss emerging technologies, such as mRNA-based platforms, nanoparticle delivery systems, and novel adjuvants, assessing their potential to enhance the efficacy and mitigate the waning immunity concerns of most malaria vaccines. We believe that the identification of novel immunogen candidates, together with continued innovation in vaccine design and delivery, will enable us to win the fight against malaria in the future.

Smartphone conjunctiva photography for malaria risk stratification in asymptomatic school age children

Malaria remains a major global health challenge. Although effective control relies on testing all suspected cases, asymptomatic infections in school-age children are frequently overlooked. Advances in retinal imaging and computer vision have enhanced malaria detection. However, noninvasive, point-of-care malaria detection remains unrealized, partly because of the need for specialized equipment. Here we report radiomic analyses of 4302 photographs of the palpebral conjunctiva captured using unmodified smartphone cameras from asymptomatic 405 participants aged 5 to 15 years to predict malaria risk. Our neural network classification model of radiomic features achieves an area under the receiver operating characteristic curve of 0.76 with 95% confidence intervals from 0.68 to 0.84 in distinguishing between malaria-infected and non-infected cases in endemic regions. Photographing the inner eyelid provides the advantages of easy accessibility and direct exposure to the microvasculature. This mobile health approach has the potential for malaria prescreening and managing febrile illness in resource-limited settings.

Plasmodium yoelii as a model for malaria: insights into pathogenesis, drug resistance, and vaccine development

Malaria continues to pose a serious global health threat, with rising drug resistance making treatment more difficult. Consequently, Plasmodium yoelii, a rodent malaria parasite, has become an invaluable model for studying the malaria parasite's complex biology, particularly host-parasite interactions and drug resistance mechanisms. Originating from African rodents, P. yoelii presents unique genetic diversity, mirroring critical aspects of human malaria and allowing for detailed research on the parasite's pre-erythrocytic and erythrocytic stages. This review introduces a novel perspective by comprehensively examining P. yoelii from multiple angles, including its origin, genetic composition, and contributions to vaccine development and drug efficacy studies. It highlights the significance of P. yoelii, revealing new insights into immune evasion, pathogenesis, and disease severity, which are critical for developing future malaria interventions. Furthermore, this review addresses the challenges associated with P. yoelii as a model, such as differences in host immune responses between rodents and humans. Despite these hurdles, the research using P. yoelii offers promising directions for combating malaria. This novel synthesis of P. yoelii studies underscores its essential role in advancing malaria research, particularly in understanding drug resistance and uncovering potential therapeutic and vaccine targets.

Host directed immunotherapy for chronic infections and cancer

Host-directed immunotherapy (HDI) is emerging as a transformative strategy in managing chronic diseases by leveraging the host's immune system to combat disease. This innovative approach has shown promise in a range of conditions, including cancer and parasitic infections. In oncology, HDI aims to enhance the body's natural immune response against cancer cells through mechanisms such as immune checkpoint inhibition, monoclonal antibodies, and cytokine therapies. These strategies are designed to boost the immune system's ability to recognize and destroy tumors, improving patient outcomes and offering alternatives to traditional cancer treatments. Similarly, in parasitic infections, HDI focuses on strengthening the host's immune defenses to control and eradicate those infections. For diseases like malaria, leishmaniasis, and Chagas disease, HDI strategies may involve adjuvants or immune modulators that amplify the body's ability to target and eliminate parasites. By optimizing immune responses and reducing reliance on conventional treatments, HDI holds the potential to revolutionize therapeutic approaches across various chronic diseases. This chapter highlights the flexibility and potential of HDI in advancing treatments, offering novel ways for improving patient care and disease management.

Recent Advances in the Treatment of Malaria

Malaria is still one of the major global health challenges affecting millions annually, particularly in non-Mediterranean Africa and Southeast Asia. Over the past two decades, substantial progress has been made in reducing malaria-related morbidity and mortality, primarily due to advancements in antimalarial therapeutics. This review provides a comprehensive overview of recent developments in malaria treatment, focusing on the evolution of drug therapies, mechanisms of action, and emerging resistance patterns. The cornerstone of current treatment strategies is artemisinin-based combination therapies (ACTs), which have proven highly effective against P. falciparum and P. vivax, the most prevalent malaria-causing parasites. However, the onset of artemisinin resistance, particularly in Southeast Asian countries, poses a significant threat to these gains. Additionally, other antimalarial classes, including quinine derivatives, 8-aminoquinolines, and antifolate drugs, are examined for their efficacy, resistance mechanisms, and future potential. This review also discusses the challenges associated with drug resistance, the genetic underpinnings of resistance in malaria parasites, and the implications for future treatment protocols. Furthermore, the review examines combinational therapies, such as triple artemisinin combination therapies (TACTs), and vaccines that are approved or in development to circumvent resistance issues. The need for continuous surveillance, innovative therapeutic strategies, and advances in novel antimalarial therapeutic agents is emphasized to sustain and further progress in the control of malaria and its eventual eradication.

Genetic structure of apical membrane antigen-1 in Plasmodium falciparum isolates from Pakistan

Plasmodium falciparum apical membrane antigen-1 (PfAMA-1) is a major candidate for the blood-stage malaria vaccine. Genetic polymorphisms of global pfama-1suggest that the genetic diversity of the gene can disturb effective vaccine development targeting this antigen. This study was conducted to explore the genetic diversity and gene structure of pfama-1 among P. falciparum isolates collected in the Khyber Pakhtunkhwa (KP) province of Pakistan. A total of 19 full-length pfama-1 sequences were obtained from KP-Pakistan P. falciparum isolates, and genetic polymorphism and natural selection were investigated. KP-Pakistan pfama-1 exhibited genetic diversity, wherein 58 amino acid changes were identified, most of which were located in ectodomains, and domains I, II, and III. The amino acid changes commonly found in the ectodomain of global pfama-1 were also detected in KP-Pakistan pfama-1. Interestingly, 13 novel amino acid changes not reported in the global population were identified in KP-Pakistan pfama-1. KP-Pakistan pfama-1 shared similar levels of genetic diversity with global pfama-1. Evidence of natural selection and recombination events were also detected in KP-Pakistan pfama-1.

Immunogenicity of PvCyRPA, PvCelTOS and Pvs25 chimeric recombinant protein of Plasmodium vivax in murine model

In the Americas, P. vivax is the predominant causative species of malaria, a debilitating and economically significant disease. Due to the complexity of the malaria parasite life cycle, a vaccine formulation with multiple antigens expressed in various parasite stages may represent an effective approach. Based on this, we previously designed and constructed a chimeric recombinant protein, PvRMC-1, composed by PvCyRPA, PvCelTOS, and Pvs25 epitopes. This chimeric protein was strongly recognized by naturally acquired antibodies from exposed population in the Brazilian Amazon. However, there was no investigation about the induced immune response of PvRMC-1. Therefore, in this work, we evaluated the immunogenicity of this chimeric antigen formulated in three distinct adjuvants: Stimune, AddaVax or Aluminum hydroxide (Al(OH)3) in BALB/c mice. Our results suggested that the chimeric protein PvRMC-1 were capable to generate humoral and cellular responses across all three formulations. Antibodies recognized full-length PvRMC-1 and linear B-cell epitopes from PvCyRPA, PvCelTOS, and Pvs25 individually. Moreover, mice's splenocytes were activated, producing IFN-γ in response to PvCelTOS and PvCyRPA peptide epitopes, affirming T-cell epitopes in the antigen. While aluminum hydroxide showed notable cellular response, Stimune and Addavax induced a more comprehensive immune response, encompassing both cellular and humoral components. Thus, our findings indicate that PvRMC-1 would be a promising multistage vaccine candidate that could advance to further preclinical studies.

Persistence of Anti-SE36 Antibodies Induced by the Malaria Vaccine Candidate BK-SE36/CpG in 5-10-Year-Old Burkinabe Children Naturally Exposed to Malaria

Information on the dynamics and decline/persistence of antibody titres is important in vaccine development. A recent vaccine trial in malaria-exposed, healthy African adults and children living in a malaria hyperendemic and seasonal area (Ouagadougou, Burkina Faso) was the first study in which BK-SE36/CpG was administered to different age groups. In 5- to 10-year-old children, the risk of malaria infection was markedly lower in the BK-SE36/CpG arm compared to the control arm. We report here data on antibody titres measured in this age-group after the high malaria transmission season of 2021 (three years after the first vaccine dose was administered). At Year 3, 83% of children had detectable anti-SE36 total IgG antibodies. Geometric mean antibody titres and the proportion of children with detectable anti-SE36 antibodies were markedly higher in the BK-SE36/CpG arm than the control (rabies) arm. The information obtained in this study will guide investigators on future vaccine/booster schedules for this promising blood-stage malaria vaccine candidate.

Immune mechanisms targeting malaria transmission: opportunities for vaccine development

INTRODUCTION

Malaria continues to remain a major global health problem with nearly a quarter of a billion clinical cases and more than 600,000 deaths in 2022. There has been significant progress toward vaccine development, however, poor efficacy of approved vaccines requiring multiple immunizing doses emphasizes the need for continued efforts toward improved vaccines. Progress to date, nonetheless, has provided impetus for malaria elimination.

AREAS COVERED

In this review we will focus on diverse immune mechanisms targeting gametocytes in the human host and gametocyte-mediated malaria transmission via the mosquito vector.

EXPERT OPINION

To march toward the goal of malaria elimination it will be critical to target the process of malaria transmission by mosquitoes, mediated exclusively by the sexual stages, i.e. male, and female gametocytes, ingested from infected vertebrate host. Studies over several decades have established antigens in the parasite sexual stages developing in the mosquito midgut as attractive targets for the development of transmission blocking vaccines (TBVs). Immune clearance of gametocytes in the vertebrate host can synergize with TBVs and directly aid in maintaining effective transmission reducing immune potential.