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.

Genetic diversity, natural selection and haplotype grouping of Plasmodium vivax Duffy-binding protein genes from eastern and western Myanmar borders

Background

Merozoite proteins of the malaria parasites involved in the invasion of red blood cells are selected by host immunity and their diversity is greatly influenced by changes in malaria epidemiology. In the Greater Mekong Subregion (GMS), malaria transmission is concentrated along the international borders and there have been major changes in malaria epidemiology with Plasmodium vivax becoming the dominant species in many regions. Here, we aimed to evaluate the genetic diversity of P. vivax Duffy-binding protein gene domain II (pvdbp-II) in isolates from the eastern and western borders of Myanmar, and compared it with that from global P. vivax populations.

Methods

pvdbp-II sequences were obtained from 85 and 82 clinical P. vivax isolates from the eastern and western Myanmar borders, respectively. In addition, 504 pvdbp-II sequences from nine P. vivax populations of the world were retrieved from GenBank and used for comparative analysis of genetic diversity, recombination and population structure of the parasite population.

Results

The nucleotide diversity of the pvdbp-II sequences from the Myanmar border parasite isolates was not uniform, with the highest diversity located between nucleotides 1078 and 1332. Western Myanmar isolates had a unique R391C mutation. Evidence of positive natural selection was detected in pvdbp-II gene in P. vivax isolates from the eastern Myanmar area. P. vivax parasite populations in the GMS, including those from the eastern, western, and central Myanmar as well as Thailand showed low-level genetic differentiation (F_ST, 0.000–0.099). Population genetic structure analysis of the pvdbp-II sequences showed a division of the GMS populations into four genetic clusters. A total of 60 PvDBP-II haplotypes were identified in 210 sequences from the GMS populations. Among the epitopes in PvDBP-II, high genetic diversity was found in epitopes 45 (379-SIFGT(D/G)(E/K)(K/N)AQQ(R/H)(R/C)KQ-393, π = 0.029) and Ia (416-G(N/K)F(I/M)WICK(L/I)-424], Ib [482-KSYD(Q/E)WITR-490, π = 0.028) in P. vivax populations from the eastern and western borders of Myanmar.

Conclusions

The pvdbp-II gene is genetically diverse in the eastern and western Myanmar border P. vivax populations. Positive natural selection and recombination occurred in pvdbp-II gene. Low-level genetic differentiation was identified, suggesting extensive gene flow of the P. vivax populations in the GMS. These results can help understand the evolution of the P. vivax populations in the course of regional malaria elimination and guide the design of PvDBP-II-based vaccine.

Role of Plasmodium vivax Duffy-binding protein 1 in invasion of Duffy-null Africans

The ability of the malaria parasite Plasmodium vivax to invade erythrocytes is dependent on the expression of the Duffy blood group antigen on erythrocytes. Consequently, Africans who are null for the Duffy antigen are not susceptible to P. vivax infections. Recently, P. vivax infections in Duffy-null Africans have been documented, raising the possibility that P. vivax, a virulent pathogen in other parts of the world, may expand malarial disease in Africa. P. vivax binds the Duffy blood group antigen through its Duffy-binding protein 1 (DBP1). To determine if mutations in DBP1 resulted in the ability of P. vivax to bind Duffy-null erythrocytes, we analyzed P. vivax parasites obtained from two Duffy-null individuals living in Ethiopia where Duffy-null and -positive Africans live side-by-side. We determined that, although the DBP1s from these parasites contained unique sequences, they failed to bind Duffy-null erythrocytes, indicating that mutations in DBP1 did not account for the ability of P. vivax to infect Duffy-null Africans. However, an unusual DNA expansion of DBP1 (three and eight copies) in the two Duffy-null P. vivax infections suggests that an expansion of DBP1 may have been selected to allow low-affinity binding to another receptor on Duffy-null erythrocytes. Indeed, we show that Salvador (Sal) I P. vivax infects Squirrel monkeys independently of DBP1 binding to Squirrel monkey erythrocytes. We conclude that P. vivax Sal I and perhaps P. vivax in Duffy-null patients may have adapted to use new ligand-receptor pairs for invasion.

Preclinical Assessment of Viral Vectored and Protein Vaccines Targeting the Duffy-Binding Protein Region II of Plasmodium Vivax

Malaria vaccine development has largely focused on Plasmodium falciparum; however, a reawakening to the importance of Plasmodium vivax has spurred efforts to develop vaccines against this difficult to treat and at times severe form of relapsing malaria, which constitutes a significant proportion of human malaria cases worldwide. The almost complete dependence of P. vivax red blood cell invasion on the interaction of the P. vivax Duffy-binding protein region II (PvDBP_RII) with the human Duffy antigen receptor for chemokines (DARC) makes this antigen an attractive vaccine candidate against blood-stage P. vivax. Here, we generated both preclinical and clinically compatible adenoviral and poxviral vectored vaccine candidates expressing the Salvador I allele of PvDBP_RII – including human adenovirus serotype 5 (HAdV5), chimpanzee adenovirus serotype 63 (ChAd63), and modified vaccinia virus Ankara (MVA) vectors. We report on the antibody and T cell immunogenicity of these vaccines in mice or rabbits, either used alone in a viral vectored prime-boost regime or in “mixed-modality” adenovirus prime – protein-in-­adjuvant boost regimes (using a recombinant PvDBP_RII protein antigen formulated in Montanide^®ISA720 or Abisco^®100 adjuvants). Antibodies induced by these regimes were found to bind to native parasite antigen from P. vivax infected Thai patients and were capable of inhibiting the binding of PvDBP_RII to its receptor DARC using an in vitro binding inhibition assay. In recent years, recombinant ChAd63 and MVA vectors have been quickly translated into human clinical trials for numerous antigens from P. falciparum as well as a growing number of other pathogens. The vectors reported here are immunogenic in small animals, elicit antibodies against PvDBP_RII, and have recently entered clinical trials, which will provide the first assessment of the safety and immunogenicity of the PvDBP_RII antigen in humans.