Design of a stabilized non-glycosylated Pfs48/45 antigen enables a potent malaria transmission-blocking nanoparticle vaccine

Multistage protective anti-CelTOS monoclonal antibodies with cross-species sterile protection against malaria

CelTOS is a malaria vaccine antigen that is conserved in Plasmodium and other apicomplexan parasites and plays a role in cell-traversal. The structural basis and mechanisms of CelTOS-induced protective immunity to parasites are unknown. Here, CelTOS-specific monoclonal antibodies (mAbs) 7g7 and 4h12 demonstrated multistage activity, protecting against liver infection and preventing parasite transmission to mosquitoes. Both mAbs demonstrated cross-species activity with sterile protection against in vivo challenge with transgenic parasites containing either P. falciparum or P. vivax CelTOS, and with transmission reducing activity against P. falciparum. The mAbs prevented CelTOS-mediated pore formation providing insight into the protective mechanisms. X-ray crystallography and mutant-library epitope mapping revealed two distinct broadly conserved neutralizing epitopes. 7g7 bound to a parallel dimer of CelTOS, while 4h12 bound to a novel antiparallel dimer architecture. These findings inform the design of antibody therapies and vaccines and raise the prospect of a single intervention to simultaneously combat P. falciparum and P. vivax malaria.

Structure-based design of a Plasmodium vivax Duffy-binding protein immunogen focuses the antibody response to functional epitopes

The Duffy-binding protein (DBP) is a promising antigen for a malaria vaccine that would protect against clinical symptoms caused by Plasmodium vivax infection. Region II of DBP (DBP-II) contains the receptor-binding domain that engages host red blood cells, but DBP-II vaccines elicit many non-neutralizing antibodies that bind distal to the receptor-binding surface. Here, we engineered a truncated DBP-II immunogen that focuses the immune response to the receptor-binding surface. This immunogen contains the receptor-binding subdomain S1S2 and lacks the immunodominant subdomain S3. Structure-based computational design of S1S2 identified combinatorial amino acid changes that stabilized the isolated S1S2 without perturbing neutralizing epitopes. This immunogen elicited DBP-II-specific antibodies in immunized mice that were significantly enriched for blocking activity compared to the native DBP-II antigen. This generalizable design process successfully stabilized an integral core fragment of a protein and focused the immune response to desired epitopes to create a promising new antigen for malaria vaccine development.

RBD design increases the functional antibody titers elicited by SARS-CoV-2 spike vaccination

Most COVID-19 vaccines contain the SARS-CoV-2 spike protein as an antigen, but they lose efficacy as neutralizing antibody titers wane and escape variants emerge. Modifying the spike antigen to increase neutralizing antibody titers would help counteract this decrease in titer. We previously used a structure-based computational design method to identify nine amino acid changes in the receptor-binding domain (RBD) of spike that stabilize the RBD and increase the neutralizing antibody titers elicited by vaccination. Here, we introduce those enhancing amino acid changes into a full-length spike (FL-S-2P) ectodomain representative of most approved vaccine antigens. These amino acid changes can be incorporated into the FL-S-2P protein without negatively effecting expression or stability. Furthermore, the amino acid changes improved functional antibody titers in both mice and monkeys following vaccination. These amino acid changes could increase the duration of protection conferred by most COVID-19 vaccines.

Vaccines and monoclonal antibodies: new tools for malaria control

SUMMARYMalaria remains one of the biggest health problems in the world. While significant reductions in malaria morbidity and mortality had been achieved from 2000 to 2015, the favorable trend has stalled, rather significant increases in malaria cases are seen in multiple areas. In 2022, there were 249 million estimated cases, and 608,000 malaria-related deaths, mostly in infants and children aged under 5 years, globally. Therefore, in addition to the expansion of existing anti-malarial control measures, it is critical to develop new tools, such as vaccines and monoclonal antibodies (mAbs), to fight malaria. In the last 2 years, the first and second malaria vaccines, both targeting Plasmodium falciparum circumsporozoite proteins (PfCSP), have been recommended by the World Health Organization to prevent P. falciparum malaria in children living in moderate to high transmission areas. While the approval of the two malaria vaccines is a considerable milestone in vaccine development, they have much room for improvement in efficacy and durability. In addition to the two approved vaccines, recent clinical trials with mAbs against PfCSP, blood-stage vaccines against P. falciparum or P. vivax, and transmission-blocking vaccine or mAb against P. falciparum have shown promising results. This review summarizes the development of the anti-PfCSP vaccines and mAbs, and recent topics in the blood- and transmission-blocking-stage vaccine candidates and mAbs. We further discuss issues of the current vaccines and the directions for the development of next-generation vaccines.

A Self-Assembling Pfs230D1-Ferritin Nanoparticle Vaccine Has Potent and Durable Malaria Transmission-Reducing Activity

Malaria is caused by eukaryotic protozoan parasites of the genus Plasmodium. There are 249 million new cases and 608,000 deaths annually, and new interventions are desperately needed. Malaria vaccines can be divided into three categories: liver stage, blood stage, or transmission-blocking vaccines. Transmission-blocking vaccines prevent the transmission of disease by the mosquito vector from one human to another. Pfs230 is one of the leading transmission-blocking vaccine antigens for malaria. Here, we describe the development of a 24-copy self-assembling nanoparticle vaccine comprising domain 1 of Pfs230 genetically fused to H. pylori ferritin. The single-component Pfs230D1-ferritin construct forms a stable and homogenous 24-copy nanoparticle with good production yields. The nanoparticle is highly immunogenic, as two low-dose vaccinations of New Zealand White rabbits elicited a potent and durable antibody response with high transmission-reducing activity when formulated in two distinct adjuvants suitable for translation to human use. This single-component 24-copy Pfs230D1-ferritin nanoparticle vaccine has the potential to improve production pipelines and the cost of manufacturing a potent and durable transmission-blocking vaccine for malaria control.

Extraordinary Titer and Broad Anti-SARS-CoV-2 Neutralization Induced by Stabilized RBD Nanoparticles from Strain BA.5

The receptor-binding domain (RBD) of the SARS-CoV-2 spike is a primary target of neutralizing antibodies and a key component of licensed vaccines. Substantial mutations in RBD, however, enable current variants to escape immunogenicity generated by vaccination with the ancestral (WA1) strain. Here, we produce and assess self-assembling nanoparticles displaying RBDs from WA1 and BA.5 strains by using the SpyTag:SpyCatcher system for coupling. We observed both WA1- and BA.5-RBD nanoparticles to degrade substantially after a few days at 37 °C. Incorporation of nine RBD-stabilizing mutations, however, increased yield ~five-fold and stability such that more than 50% of either the WA1- or BA.5-RBD nanoparticle was retained after one week at 37 °C. Murine immunizations revealed that the stabilized RBD-nanoparticles induced ~100-fold higher autologous neutralization titers than the prefusion-stabilized (S2P) spike at a 2 μg dose. Even at a 25-fold lower dose where S2P-induced neutralization titers were below the detection limit, the stabilized BA.5-RBD nanoparticle induced homologous titers of 12,795 ID50 and heterologous titers against WA1 of 1767 ID50. Assessment against a panel of β-coronavirus variants revealed both the stabilized BA.5-RBD nanoparticle and the stabilized WA1-BA.5-(mosaic)-RBD nanoparticle to elicit much higher neutralization breadth than the stabilized WA1-RBD nanoparticle. The extraordinary titer and high neutralization breadth elicited by stabilized RBD nanoparticles from strain BA.5 make them strong candidates for next-generation COVID-19 vaccines.

Designing an effective malaria vaccine targeting Plasmodium vivax Duffy-binding protein

Malaria caused by the Plasmodium vivax parasite is a major global health burden. Immunity against blood-stage infection reduces parasitemia and disease severity. Duffy-binding protein (DBP) is the primary parasite protein responsible for the invasion of red blood cells and it is a leading subunit vaccine candidate. An effective vaccine, however, is still lacking despite decades of interest in DBP as a vaccine candidate. This review discusses the reasons for targeting DBP, the challenges associated with developing a vaccine, and modern structural vaccinology methods that could be used to create an effective DBP vaccine. Next-generation DBP vaccines have the potential to elicit a broadly protective immune response and provide durable and potent protection from P. vivax malaria.

Structure-based design of a strain transcending AMA1-RON2L malaria vaccine

Apical membrane antigen 1 (AMA1) is a key malaria vaccine candidate and target of neutralizing antibodies. AMA1 binds to a loop in rhoptry neck protein 2 (RON2L) to form the moving junction during parasite invasion of host cells, and this complex is conserved among apicomplexan parasites. AMA1-RON2L complex immunization achieves higher growth inhibitory activity than AMA1 alone and protects mice against Plasmodium yoelii challenge. Here, three single-component AMA1-RON2L immunogens were designed that retain the structure of the two-component AMA1-RON2L complex: one structure-based design (SBD1) and two insertion fusions. All immunogens elicited high antibody titers with potent growth inhibitory activity, yet these antibodies did not block RON2L binding to AMA1. The SBD1 immunogen induced significantly more potent strain-transcending neutralizing antibody responses against diverse strains of Plasmodium falciparum than AMA1 or AMA1-RON2L complex vaccination. This indicates that SBD1 directs neutralizing antibody responses to strain-transcending epitopes in AMA1 that are independent of RON2L binding. This work underscores the importance of neutralization mechanisms that are distinct from RON2 blockade. The stable single-component SBD1 immunogen elicits potent strain-transcending protection that may drive the development of next-generation vaccines for improved malaria and apicomplexan parasite control.

A potent and durable malaria transmission-blocking vaccine designed from a single-component 60-copy Pfs230D1 nanoparticle

Malaria transmission-blocking vaccines (TBVs) reduce disease transmission by breaking the continuous cycle of infection between the human host and the mosquito vector. Domain 1 (D1) of Pfs230 is a leading TBV candidate and comprises the majority of transmission-reducing activity (TRA) elicited by Pfs230. Here we show that the fusion of Pfs230D1 to a 60-copy multimer of the catalytic domain of dihydrolipoyl acetyltransferase protein (E2p) results in a single-component nanoparticle composed of 60 copies of the fusion protein with high stability, homogeneity, and production yields. The nanoparticle presents a potent human transmission-blocking epitope within Pfs230D1, indicating the antigen is correctly oriented on the surface of the nanoparticle. Two vaccinations of New Zealand White rabbits with the Pfs230D1 nanoparticle elicited a potent and durable antibody response with high TRA when formulated in two distinct adjuvants suitable for translation to human use. This single-component nanoparticle vaccine may play a key role in malaria control and has the potential to improve production pipelines and the cost of manufacturing of a potent and durable TBV.

A human antibody epitope map of the malaria vaccine antigen Pfs25

Pfs25 is a leading antigen for a malaria transmission-blocking vaccine and shows moderate transmission-blocking activity and induction of rapidly decreasing antibody titers in clinical trials. A comprehensive definition of all transmission-reducing epitopes of Pfs25 will inform structure-guided design to enhance Pfs25-based vaccines, leading to potent transmission-blocking activity. Here, we compiled a detailed human antibody epitope map comprising epitope binning data and structures of multiple human monoclonal antibodies, including three new crystal structures of Pfs25 in complex with transmission-reducing antibodies from Malian volunteers immunized with Pfs25 conjugated to EPA and adjuvanted with AS01. These structures revealed additional epitopes in Pfs25 capable of reducing transmission and expanded this characterization to malaria-exposed humans. This work informs immunogen design to focus the antibody response to transmission-reducing epitopes of Pfs25, enabling development of more potent transmission-blocking vaccines for malaria.

Design of a stabilized RBD enables potently neutralizing SARS-CoV-2 single-component nanoparticle vaccines

Waning immunity and emerging variants necessitate continued vaccination against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Improvements in vaccine safety, tolerability, and ease of manufacturing would benefit these efforts. Here, we develop a potent and easily manufactured nanoparticle vaccine displaying the spike receptor-binding domain (RBD). Computational design to stabilize the RBD, eliminate glycosylation, and focus the immune response to neutralizing epitopes results in an RBD immunogen that resolves issues hindering the efficient nanoparticle display of the native RBD. This non-glycosylated RBD can be genetically fused to diverse single-component nanoparticle platforms, maximizing manufacturing ease and flexibility. All engineered RBD nanoparticles elicit potently neutralizing antibodies in mice that far exceed monomeric RBDs. A 60-copy particle (noNAG-RBD-E2p) also elicits potently neutralizing antibodies in non-human primates. The neutralizing antibody titers elicited by noNAG-RBD-E2p are comparable to a benchmark stabilized spike antigen and reach levels against Omicron BA.5 that suggest that it would provide protection against emerging variants.