Low dose recombinant full-length circumsporozoite protein-based Plasmodium falciparum vaccine is well-tolerated and highly immunogenic in phase 1 first-in-human clinical testing

Diversity and selection analyses identify transmission-blocking antigens as the optimal vaccine candidates in Plasmodium falciparum

BACKGROUND

A highly effective vaccine for malaria remains an elusive target, at least in part due to the under-appreciated natural parasite variation. This study aimed to investigate genetic and structural variation, and immune selection of leading malaria vaccine candidates across the Plasmodium falciparum's life cycle.

METHODS

We analysed 325 P. falciparum whole genome sequences from Zambia, in addition to 791 genomes from five other African countries available in the MalariaGEN Pf3k Database. Ten vaccine antigens spanning three life-history stages were examined for genetic and structural variations, using population genetics measures, haplotype network analysis, and 3D structure selection analysis.

FINDINGS

Among the ten antigens analysed, only three in the transmission-blocking vaccine category display P. falciparum 3D7 as the dominant haplotype. The antigens AMA1, CSP, MSP119 and CelTOS, are much more diverse than the other antigens, and their epitope regions are under moderate to strong balancing selection. In contrast, Rh5, a blood stage antigen, displays low diversity yet slightly stronger immune selection in the merozoite-blocking epitope region. Except for CelTOS, the transmission-blocking antigens Pfs25, Pfs48/45, Pfs230, Pfs47, and Pfs28 exhibit minimal diversity and no immune selection in epitopes that induce strain-transcending antibodies, suggesting potential effectiveness of 3D7-based vaccines in blocking transmission.

INTERPRETATION

These findings offer valuable insights into the selection of optimal vaccine candidates against P. falciparum. Based on our results, we recommend prioritising conserved merozoite antigens and transmission-blocking antigens. Combining these antigens in multi-stage approaches may be particularly promising for malaria vaccine development initiatives.

FUNDING

Purdue Department of Biological Sciences; Puskas Memorial Fellowship; National Institute of Allergy and Infectious Diseases (U19AI089680).

Recombinant Full-length Plasmodium falciparum Circumsporozoite Protein-Based Vaccine Adjuvanted With Glucopyranosyl Lipid A-Liposome Quillaja saponaria 21: Results of Phase 1 Testing With Malaria Challenge

BACKGROUND

Malaria is preventable yet causes >600 000 deaths annually. RTS,S, the first marketed malaria vaccine, has modest efficacy, but improvements are needed for eradication.

METHODS

We conducted an open-label, dose escalation phase 1 study of a full-length recombinant circumsporozoite protein vaccine (rCSP) administered with adjuvant glucopyranosyl lipid A-liposome Quillaja saponaria 21 formulation (GLA-LSQ) on days 1, 29, and 85 or 1 and 490 to healthy, malaria-naive adults. The primary end points were safety and reactogenicity. The secondary end points were antibody responses and Plasmodium falciparum parasitemia after homologous controlled human malaria infection.

RESULTS

Participants were enrolled into 4 groups receiving rCSP/GLA-LSQ: 10 µg × 3 (n = 20), 30 µg × 3 (n = 10), 60 µg × 3 (n = 10), or 60 µg × 2 (n = 9); 10 participants received 30 µg rCSP alone × 3, and there were 6 infectivity controls. Participants experienced no serious adverse events. Rates of solicited and unsolicited adverse events were similar among groups. All 26 participants who underwent controlled human malaria infection 28 days after final vaccinations developed malaria. Increasing vaccine doses induced higher immunoglobulin G titers but did not achieve previously established RTS,S benchmarks.

CONCLUSIONS

rCSP/GLA-LSQ had favorable safety results. However, tested regimens did not induce protective immunity. Further investigation could assess whether adjuvant or schedule adjustments improve efficacy.

CLINICAL TRIALS REGISTRATION

NCT03589794.

Diversity and selection analyses identify transmission-blocking antigens as the optimal vaccine candidates in Plasmodium falciparum

Background

A highly effective vaccine for malaria remains an elusive target, at least in part due to the under-appreciated natural parasite variation. This study aimed to investigate genetic and structural variation, and immune selection of leading malaria vaccine candidates across the Plasmodium falciparum's life cycle.

Methods

We analyzed 325 P. falciparum whole genome sequences from Zambia, in addition to 791 genomes from five other African countries available in the MalariaGEN Pf3k Rdatabase. Ten vaccine antigens spanning three life-history stages were examined for genetic and structural variations, using population genetics measures, haplotype network analysis, and 3D structure selection analysis.

Findings

Among the ten antigens analyzed, only three in the transmission-blocking vaccine category display P. falciparum 3D7 as the dominant haplotype. The antigens AMA1, CSP, MSP119 and CelTOS, are much more diverse than the other antigens, and their epitope regions are under moderate to strong balancing selection. In contrast, Rh5, a blood stage antigen, displays low diversity yet slightly stronger immune selection in the merozoite-blocking epitope region. Except for CelTOS, the transmission-blocking antigens Pfs25, Pfs48/45, Pfs230, Pfs47, and Pfs28 exhibit minimal diversity and no immune selection in epitopes that induce strain-transcending antibodies, suggesting potential effectiveness of 3D7-based vaccines in blocking transmission.

Interpretations

These findings offer valuable insights into the selection of optimal vaccine candidates against P. falciparum. Based on our results, we recommend prioritizing conserved merozoite antigens and transmission-blocking antigens. Combining these antigens in multi-stage approaches may be particularly promising for malaria vaccine development initiatives.

Funding

Purdue Department of Biological Sciences; Puskas Memorial Fellowship; National Institute of Allergy and Infectious Diseases (U19AI089680).

Exploring genetic polymorphisms among Plasmodium vivax isolates from the Thai-Myanmar borders using circumsporozoite protein (pvcsp) and ookinete surface protein (pvs25) encoding genes

Plasmodium vivax malaria cases remain high along the Thai-Myanmar and Thai-Cambodia borders. Plasmodium vivax circumsporozoite protein (pvcsp) and Plasmodium vivax ookinete surface protein (pvs25) genes are promising molecular markers of the genetic diversity of P. vivax. This study investigated the genetic diversity of pvcsp and pvs25 in P. vivax isolates collected from the Thai-Myanmar border. The DNA samples were amplified, and the genotypes were analyzed by PCR-RFLP and DNA sequencing. Pvcsp genotypes, VK210, VK247, and mixed types, were found in 203 (91.9%), 15 (6.8%), and 3 (1.3%) of the isolates, respectively. Twenty-four allelic variants were observed, of which a high prevalence of VK210E and VK247E were reported. Two pvcsp variants, VK210C and VK210M showed significantly higher parasite density (46,234 (1154-144,000) vs. 25,606 (1373-68,878), respectively). The genetic diversity of pvcsp along the Thai-Myanmar border during 2002-2015 showed dynamic changes with both positive and negative selection. The frequency and distribution of pvcsp pattern might be changed over time and might be other factors contributing to gene selection. Three amino acid substitutions of pvs25, i.e., E97Q, I130T, and Q131K, were investigated with frequencies of 10 (4.5%), 221 (100%), and 204 (92.3%) isolates, respectively. There was no association between parasite density and pvs25 polymorphisms. The frequency of pvs25 polymorphism was similar to that previously reported, with the absence of random mutation. In conclusion, the genetic variation of pvcsp was changed over times whereas the genetic diversity of pvs25 was limited; these variations would be helpful for further vaccine development against P. vivax malaria.

Malaria Vaccines: Progress to Date

Malaria is a mosquito-borne disease caused by protozoan parasites of the genus Plasmodium. Despite significant declines in malaria-attributable morbidity and mortality over the last two decades, it remains a major public health burden in many countries. This underscores the critical need for improved strategies to prevent, treat and control malaria if we are to ultimately progress towards the eradication of this disease. Ideally, this will include the development and deployment of a highly effective malaria vaccine that is able to induce long-lasting protective immunity. There are many malaria vaccine candidates in development, with more than a dozen of these in clinical development. RTS,S/AS01 (also known as Mosquirix) is the most advanced malaria vaccine and was shown to have modest efficacy against clinical malaria in phase III trials in 5- to 17-month-old infants. Following pilot implementation trials, the World Health Organisation has recommended it for use in Africa in young children who are most at risk of infection with P. falciparum, the deadliest of the human malaria parasites. It is well recognised that more effective malaria vaccines are needed. In this review, we discuss malaria vaccine candidates that have progressed into clinical evaluation and highlight the most advanced candidates: Sanaria's irradiated sporozoite vaccine (PfSPZ Vaccine), the chemoattenuated sporozoite vaccine (PfSPZ-CVac), RTS,S/AS01 and the novel malaria vaccine candidate, R21, which displayed promising, high-level efficacy in a recent small phase IIb trial in Africa.

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

Malaria, caused by Plasmodium species, remains a major global health concern, causing millions of deaths annually. While the introduction of the RTS,S vaccine has shown promise, there is a pressing need for more effective vaccines due to the emergence of drug-resistant parasites and insecticide-resistant vectors. However, the complex life cycle and genetic diversity of the parasite, technical obstacles, limited funding, and the impact of the 2019 pandemic have hindered progress in malaria vaccine development. This review focuses on advancements in malaria vaccine development, particularly the ongoing clinical trials targeting antigens from different stages of the Plasmodium life cycle. Additionally, we discuss the rationale, strategies, and challenges associated with vaccine design, aiming to enhance the immune response and protective efficacy of vaccine candidates. A cost-effective and multistage vaccine could hold the key to controlling and eradicating malaria.

Protective antibody threshold of RTS,S/AS01 malaria vaccine correlates antigen and adjuvant dose in mouse model

Mouse models are useful for the early down-selection of malaria vaccine candidates. The Walter Reed Army Institute of Research has optimized a transgenic Plasmodium berghei sporozoite challenge model to compare the efficacy of Plasmodium falciparum circumsporozoite protein (CSP) vaccines. GSK's RTS,S vaccine formulated in the adjuvant AS01 can protect malaria-naïve individuals against malaria. We report that the RTS,S/AS01 vaccine induces high level sterile protection in our mouse model. Down titration of the antigen at a constant AS01 dose revealed a potent antigen dose-sparing effect and the superiority of RTS,S/AS01 over a soluble CSP antigen. RTS,S-mediated protective immunity was associated with a threshold of major repeat antibody titer. Combined titration of the antigen and adjuvant showed that reducing the adjuvant could improve antibody boosting post-3rd vaccination and reduce the threshold antibody concentration required for protection. Mouse models can provide a pathway for preclinical assessment of strategies to improve CSP vaccines against malaria.

In-depth review of delivery carriers associated with vaccine adjuvants: current status and future perspectives

INTRODUCTION

Vaccines are powerful tools for controlling microbial infections and preventing epidemics. To enhance the immune response to antigens, effective subunit vaccines or mRNA vaccines often require the combination of adjuvants or delivery carriers. In recent years, with the rapid development of immune mechanism research and nanotechnology, various studies based on the optimization of traditional adjuvants or various novel carriers have been intensified, and the construction of vaccine adjuvant delivery systems (VADS) with both adjuvant activity and antigen delivery has become more and more important in vaccine research.

AREAS COVERED

This paper reviews the common types of vaccine adjuvant delivery carriers, classifies the VADS according to their basic carrier types, introduces the current research status and future development trend, and emphasizes the important role of VADS in novel vaccine research.

EXPERT OPINION

As the number of vaccine types increases, conventional aluminum adjuvants show limitations in effectively stimulating cellular immune responses, limiting their use in therapeutic vaccines for intracellular infections or tumors. In contrast, the use of conventional adjuvants as VADS to carry immunostimulatory molecules or deliver antigens can greatly enhance the immune boosting effect of classical adjuvants. A comprehensive understanding of the various delivery vehicles will further facilitate the development of vaccine adjuvant research.

Current approaches to malaria vaccines

The complex Plasmodium life cycle offers different vaccine approaches with distinct parasitological and clinical effects. The approaches and their rationales were established decades ago: vaccines targeting pre-erythrocytic (sporozoite and liver-stage) parasites prevent infection, those to blood-stage parasites reduce disease, and those to sexual-stage parasites or mosquito vector reduce transmission and eliminate malaria through herd immunity. The pre-erythrocytic RTS,S vaccine (Mosquirix, GlaskoSmithKline (GSK)), recommended by WHO in 2021, reduces clinical malaria in children. Knowledge of parasite biology, host-parasite interactions, and immune mechanisms is informing new concepts to improve on RTS,S and to target other parasite stages. This review emphasizes vaccine approaches and candidates currently in the clinic or likely to enter clinical testing soon.

Engineering nano-drug biointerface to overcome biological barriers toward precision drug delivery

The rapid advancement of nanomedicine and nanoparticle (NP) materials presents novel solutions potentially capable of revolutionizing health care by improving efficacy, bioavailability, drug targeting, and safety. NPs are intriguing when considering medical applications because of their essential and unique qualities, including a significantly higher surface to mass ratio, quantum properties, and the potential to adsorb and transport drugs and other compounds. However, NPs must overcome or navigate several biological barriers of the human body to successfully deliver drugs at precise locations. Engineering the drug carrier biointerface can help overcome the main biological barriers and optimize the drug delivery in a more personalized manner. This review discusses the significant heterogeneous biological delivery barriers and how biointerface engineering can promote drug carriers to prevail over hurdles and navigate in a more personalized manner, thus ushering in the era of Precision Medicine. We also summarize the nanomedicines' current advantages and disadvantages in drug administration, from natural/synthetic sources to clinical applications. Additionally, we explore the innovative NP designs used in both non-personalized and customized applications as well as how they can attain a precise therapeutic strategy.

First-in-human assessment of safety and immunogenicity of low and high doses of Plasmodium falciparum malaria protein 013 (FMP013) administered intramuscularly with ALFQ adjuvant in healthy malaria-naïve adults

The global burden of malaria remains substantial. Circumsporozoite protein (CSP) has been demonstrated to be an effective target antigen, however, improvements that offer more efficacious and more durable protection are still needed. In support of research and development of next-generation malaria vaccines, Walter Reed Army Institute of Research (WRAIR) has developed a CSP-based antigen (FMP013) and a novel adjuvant ALFQ (Army Liposome Formulation containing QS-21). We present a single center, open-label, dose-escalation Phase 1 clinical trial to evaluate the safety and immunogenicity of the FMP013/ALFQ malaria vaccine candidate. In this first-in-human evaluation of both the antigen and adjuvant, we enrolled ten subjects; five received 20 μg FMP013 / 0.5 mL ALFQ (Low dose group), and five received 40 μg FMP013 / 1.0 mL ALFQ (High dose group) on study days 1, 29, and 57. Adverse events and immune responses were assessed during the study period. The clinical safety profile was acceptable and there were no serious adverse events. Both groups exhibited robust humoral and cellular immunological responses, and compared favorably with historical responses reported for RTS,S/AS01. Based on a lower reactogenicity profile, the 20 μg FMP013 / 0.5 mL ALFQ (Low dose) was selected for follow-on efficacy testing by controlled human malaria infection (CHMI) with a separate cohort. Trial Registration:Clinicaltrials.gov Identifier NCT04268420 (Registered February 13, 2020).

Vaccine co-display of CSP and Pfs230 on liposomes targeting two Plasmodium falciparum differentiation stages

A vaccine targeting multiple stages of the Plasmodium falciparum parasite life cycle is desirable. The sporozoite surface Circumsporozoite Protein (CSP) is the target of leading anti-infective P. falciparum pre-erythrocytic vaccines. Pfs230, a sexual-stage P. falciparum surface protein, is currently in trials as the basis for a transmission-blocking vaccine, which inhibits parasite development in the mosquito vector. Here, recombinant full-length CSP and a Pfs230 fragment (Pfs230D1+) are co-displayed on immunogenic liposomes to induce immunity against both infection and transmission. Liposomes contain cobalt-porphyrin phospholipid (CoPoP), monophosphoryl lipid A and QS-21, and rapidly bind His-tagged CSP and Pfs230D1+ upon admixture to form bivalent particles that maintain reactivity with conformational monoclonal antibodies. Use of multicolor fluorophore-labeled antigens reveals liposome binding upon admixture, stability in serum and enhanced uptake in murine macrophages in vitro. Bivalent liposomes induce humoral and cellular responses against both CSP and Pfs230D1+. Vaccine-induced antibodies reduce parasite numbers in mosquito midguts in a standard membrane feeding assay. Mice immunized with liposome-displayed antigens or that passively receive antibodies from immunized rabbits have reduced parasite liver burden following challenge with transgenic sporozoites expressing P. falciparum CSP.

How to induce protective humoral immunity against Plasmodium falciparum circumsporozoite protein

The induction of protective humoral immune responses against sporozoite surface proteins of the human parasite Plasmodium falciparum (Pf) is a prime goal in the development of a preerythrocytic malaria vaccine. The most promising antibody target is circumsporozoite protein (CSP). Although PfCSP induces strong humoral immune responses upon vaccination, vaccine efficacy is overall limited and not durable. Here, we review recent efforts to gain a better molecular and cellular understanding of anti-PfCSP B cell responses in humans and discuss ways to overcome limitations in the induction of stable titers of high-affinity antibodies that might help to increase vaccine efficacy and promote long-lived protection.

Liposome-polymer complex for drug delivery system and vaccine stabilization

Liposomes have been used extensively as micro- and nanocarriers for hydrophobic or hydrophilic molecules. However, conventional liposomes are biodegradable and quickly eliminated, making it difficult to be used for delivery in specific routes, such as the oral and systemic routes. One way to overcome this problem is through complexation with polymers, which is referred to as a liposome complex. The use of polymers can increase the stability of liposome with regard to pH, chemicals, enzymes, and the immune system. In some cases, specific polymers can condition the properties of liposomes to be explicitly used in drug delivery, such as targeted delivery and controlled release. These properties are influenced by the type of polymer, crosslinker, interaction, and bond in the complexation process. Therefore, it is crucial to study and review these parameters for the development of more optimal forms and properties of the liposome complex. This article discusses the use of natural and synthetic polymers, ways of interaction between polymers and liposomes (on the surface, incorporation in lamellar chains, and within liposomes), types of bonds, evaluation standards, and their effects on the stability and pharmacokinetic profile of the liposome complex, drugs, and vaccines. This article concludes that both natural and synthetic polymers can be used in modifying the structure and physicochemical properties of liposomes to specify their use in targeted delivery, controlled release, and stabilizing drugs and vaccines.

Restricted valency (NPNA)n repeats and junctional epitope-based circumsporozoite protein vaccines against Plasmodium falciparum

The Circumsporozoite Protein (CSP) of Plasmodium falciparum contains an N-terminal region, a conserved Region I (RI), a junctional region, 25-42 copies of major (NPNA) and minor repeats followed by a C-terminal domain. The recently approved malaria vaccine, RTS,S/AS01 contains NPNAx19 and the C-terminal region of CSP. The efficacy of RTS,S against natural infection is low and short-lived, and mapping epitopes of inhibitory monoclonal antibodies may allow for rational improvement of CSP vaccines. Tobacco Mosaic Virus (TMV) was used here to display the junctional epitope (mAb CIS43), Region I (mAb 5D5), NPNAx5, and NPNAx20 epitope of CSP (mAbs 317 and 580). Protection studies in mice revealed that Region I did not elicit protective antibodies, and polyclonal antibodies against the junctional epitope showed equivalent protection to NPNAx5. Combining the junctional and NPNAx5 epitopes reduced immunogenicity and efficacy, and increasing the repeat valency to NPNAx20 did not improve upon NPNAx5. TMV was confirmed as a versatile vaccine platform for displaying small epitopes defined by neutralizing mAbs. We show that polyclonal antibodies against engineered VLPs can recapitulate the binding specificity of the mAbs and immune-focusing by reducing the structural complexity of an epitope may be superior to immune-broadening as a vaccine design approach. Most importantly the junctional and restricted valency NPNA epitopes can be the basis for developing highly effective second-generation malaria vaccine candidates.

Fast and long-lasting immune response to S-trimer COVID-19 vaccine adjuvanted by PIKA

In the face of the emerging variants of SARS-CoV-2, there is an urgent need to develop a vaccine that can induce fast, effective, long-lasting and broad protective immunity against SARS-CoV-2. Here, we developed a trimeric SARS-CoV-2 S protein vaccine candidate adjuvanted by PIKA, which can induce robust cellular and humoral immune responses. The results showed a high level of neutralizing antibodies induced by the vaccine was maintained for at least 400 days. In the study of non-human primates, PIKA adjuvanted S-trimer induced high SARS-CoV-2 neutralization titers and protected from virus replication in the lung following SARS-CoV-2 challenge. In addition, the long-term neutralizing antibody response induced by S-trimer vaccine adjuvanted by PIKA could neutralize multiple SARS-CoV-2 variants and there is no obvious different among the SARS- CoV-2 variants of interest or concern, including B.1.351, B.1.1.7, P.1, B.1.617.1 and B.1.617.2 variants. These data support the utility of S-trimer protein adjuvanted by PIKA as a potential vaccine candidate against SARS-CoV-2 infection.

Novel malaria vaccines

Malaria vaccines hold significant promise for life-saving benefit, especially to children who bear the major burden of malaria mortality. The RTS,S/AS01 malaria vaccine provides moderate efficacy and is being tested in implementation studies. In parallel, multiple strategies are being advanced to test next-generation malaria vaccines, including novel approaches that build on principles learned from RTS,S development, vaccination with radiation-attenuated sporozoites, and development of monoclonal antibodies targeting immunogenic peptides. Novel vaccine delivery approaches are also being advanced, including self-amplifying RNA vaccine delivery, self-assembling protein nanoparticle methods, circumsporozoite protein-based approaches, and whole organism vaccination. Techniques employed for COVID-19 vaccine development should also be considered for malaria vaccination, including sustained release polymer nanoparticle hydrogel vaccination and charge-altering releasable transporters. As vaccine science advances and new approaches optimize knowledge gained, highly effective malaria vaccines that provide sustained protection are within reach.