Transmission-Blocking Vaccines against Schistosomiasis Japonica.

Control of schistosomiasis japonica, endemic in Asia, including the Philippines, China, and Indonesia, is extremely challenging. Schistosoma japonicum is a highly pathogenic helminth parasite, with disease arising predominantly from an immune reaction to entrapped parasite eggs in tissues. Females of this species can generate 1000-2200 eggs per day, which is about 3- to 15-fold greater than the egg output of other schistosome species. Bovines (water buffalo and cattle) are the predominant definitive hosts and are estimated to generate up to 90% of parasite eggs released into the environment in rural endemic areas where these hosts and humans are present. Here, we highlight the necessity of developing veterinary transmission-blocking vaccines for bovines to better control the disease and review potential vaccine candidates. We also point out that the approach to producing efficacious transmission-blocking animal-based vaccines before moving on to human vaccines is crucial. This will result in effective and feasible public health outcomes in agreement with the One Health concept to achieve optimum health for people, animals, and the environment. Indeed, incorporating a veterinary-based transmission vaccine, coupled with interventions such as human mass drug administration, improved sanitation and hygiene, health education, and snail control, would be invaluable to eliminating zoonotic schistosomiasis.

PIMMS43 is required for malaria parasite immune evasion and sporogonic development in the mosquito vector.

After being ingested by a female Anopheles mosquito during a bloodmeal on an infected host, and before they can reach the mosquito salivary glands to be transmitted to a new host, Plasmodium parasites must establish an infection of the mosquito midgut in the form of oocysts. To achieve this, they must first survive a series of robust innate immune responses that take place prior to, during, and immediately after ookinete traversal of the midgut epithelium. Understanding how parasites may evade these responses could highlight new ways to block malaria transmission. We show that an ookinete and sporozoite surface protein designated as PIMMS43 (Plasmodium Infection of the Mosquito Midgut Screen 43) is required for parasite evasion of the Anopheles coluzzii complement-like response. Disruption of PIMMS43 in the rodent malaria parasite Plasmodium berghei triggers robust complement activation and ookinete elimination upon mosquito midgut traversal. Silencing components of the complement-like system through RNAi largely restores ookinete-to-oocyst transition but oocysts remain small in size and produce a very small number of sporozoites that additionally are not infectious, indicating that PIMMS43 is also essential for sporogonic development in the oocyst. Antibodies that bind PIMMS43 interfere with parasite immune evasion when ingested with the infectious blood meal and significantly reduce the prevalence and intensity of infection. PIMMS43 genetic structure across African Plasmodium falciparum populations indicates allelic adaptation to sympatric vector populations. These data add to our understanding of mosquito-parasite interactions and identify PIMMS43 as a target of malaria transmission blocking.

Evaluation of the Pfs25-IMX313/Matrix-M malaria transmission-blocking candidate vaccine in endemic settings.

Malaria control relies heavily on the use of anti-malarial drugs and insecticides against malaria parasites and mosquito vectors. Drug and insecticide resistance threatens the effectiveness of conventional malarial interventions; alternative control approaches are, therefore, needed. The development of malaria transmission-blocking vaccines that target the sexual stages in humans or mosquito vectors is among new approaches being pursued. Here, the immunological mechanisms underlying malaria transmission blocking, status of Pfs25-based vaccines are viewed, as well as approaches and capacity for first in-human evaluation of a transmission-blocking candidate vaccine Pfs25-IMX313/Matrix-M administered to semi-immune healthy individuals in endemic settings. It is concluded that institutions in low and middle income settings should be supported to conduct first-in human vaccine trials in order to stimulate innovative research and reduce the overdependence on developed countries for research and local interventions against many diseases of public health importance.

An analytical review of vector- and pathogen-based transmission-blocking vaccine for malaria control.

Malaria is a vector borne disease, considered to be one of the most serious public health problems. The present review focused on the blocking of parasite development in mosquito vectors; one broad strategy for achieving this is Transmission Blocking Vaccines (TBV). The TBVs usually rely on immunization of vertebrate hosts with molecules derived from the vector or pathogen to reduce pathogen transmission from infected to uninfected hosts. Most of the studies on the TBVs are based on the antibodies targeted against the surface antigens of sexual stages of malaria parasite, but it is meagre to develop mosquito-based vaccine in this regard. Vector-based TBVs include surface proteins that are expressed by the mosquito midgut digestive enzymes which are induced upon blood-feeding, and receptors expressed on the epithelial line of the tissue. Many proteins are reported that can act as candidates for transmission-blocking vaccines. This review aims to summarize the vector midgut-based proteins identified till date, that can block the development and maturity of sexual stages of the parasite within mosquitoes as targets for transmission-blocking vaccine development. The TBVs intervention can block transmission of different malaria parasite species in various species of mosquitoes with future application perspective worldwide.

A novel model fitted to multiple life stages of malaria for assessing efficacy of transmission-blocking interventions.

BACKGROUND: Transmission-blocking interventions (TBIs) aim to eliminate malaria by reducing transmission of the parasite between the host and the invertebrate vector. TBIs include transmission-blocking drugs and vaccines that, when given to humans, are taken up by mosquitoes and inhibit parasitic development within the vector. Accurate methodologies are key to assess TBI efficacy to ensure that only the most potent candidates progress to expensive and time-consuming clinical trials. Measuring intervention efficacy can be problematic because there is substantial variation in the number of parasites in both the host and vector populations, which can impact transmission even in laboratory settings. METHODS: A statistically robust empirical method is introduced for estimating intervention efficacy from standardised population assay experiments. This method will be more reliable than simple summary statistics as it captures changes in parasite density in different life-stages. It also allows efficacy estimates at a finer resolution than previous methods enabling the impact of the intervention over successive generations to be tracked. A major advantage of the new methodology is that it makes no assumptions on the population dynamics of infection. This enables both host-to-vector and vector-to-host transmission to be density-dependent (or other) processes and generates easy-to-understand estimates of intervention efficacy. RESULTS: This method increases the precision of intervention efficacy estimates and demonstrates that relying on changes in infection prevalence (the proportion of infected hosts) alone may be insufficient to capture the impact of TBIs, which also suppress parasite density in secondarily infected hosts. CONCLUSIONS: The method indicates that potentially useful, partially effective TBIs may require multiple infection cycles before substantial reductions in prevalence are observed, despite more rapidly suppressing parasite density. Accurate models to quantify efficacy will have important implications for understanding how TBI candidates might perform in field situations and how they should be evaluated in clinical trials.

Insights into Theileria transmission-blocking vaccines for East Coast fever control: A disease with an "outdated vaccination approach".

Instead of using the Infection and Treatment Method (ITM)-based vaccine, is it possible to control East Coast Fever (ECF) through blocking Theileria parva transmission in ticks and cattle? This review pursues this question. It's over 100 years since Arnold Theiler (1912) first illustrated the natural ITM as a vaccination approach against ECF-cattle disease. The approach entails infecting cattle with live Theileria sporozoites and co-treatment with long-acting tetracycline. Building on the ITM principle, the "Muguga"-cocktail ECF vaccine was developed in the 1970s and it remains the only commercially available-one. Although the vaccine induces cattle-protection, the vaccination approach still raises several drawbacks. Of those, the most outstanding is the vaccine-safety. This is implied because after ITM vaccination, cattle revert to T. parva pathogen reservoirs, therefore, during blood meal-acquisition, the ticks co-ingest T. parva pathogens. Ultimately, the pathogens are further transmitted transstadial; from larvae to nymph and nymph-adults and later re-transmitted to cattle during blood-meal acquisition. Consequently, the vaccine-constituting T. parva strains are introduced and (re) spread in non-endemic/ endemic areas. Precisely, rather than eradicating the disease, the ITM vaccination-approach promotes ECF endemicity. With advent of novel vaccination approaches toward vector and vector-borne disease control, ECF-control based on ITM of vaccination is considered outdated. The review highlights the need for embracing a holistic integrative vaccination approach entailing blocking Theileria pathogen-development and transmission both in the ticks and cattle, and/or the tick-population.

Transmission-Blocking Vaccines for Canine Visceral Leishmaniasis: New Progress and Yet New Challenges.

Dogs with visceral leishmaniasis play a key role in the transmission cycle of Leishmania infantum to humans in the urban environment. There is a consensus regarding the importance of developing a vaccine to control this disease. Despite many efforts to develop a protective vaccine against CVL, the ones currently available, Leish-tec® and LetiFend®, have limited effectiveness. This is due, in part, to the complexity of the immune response of the naturally infected dogs against the parasite and the complexity of the parasite transmission cycle. Thus, strategies, such as the development of a transmission-blocking vaccines (TBVs) already being applied to other vector-borne diseases like malaria and dengue, would be an attractive alternative to control leishmaniasis. TBVs induce the production of antibodies in the vertebrate host, which can inhibit parasite development in the vector and/or interfere with aspects of vector biology, leading to an interruption of parasite transmission. To date, there are few TBV studies for CVL and other leishmaniasis forms. However, the few studies that exist show promising results, thus justifying the further development of this approach.

Malaria vaccines: the 60-year journey of hope and final success-lessons learned and future prospects.

BACKGROUND: The world has made great strides towards beating malaria, although about half of the world population is still exposed to the risk of contracting malaria. Developing an effective malaria vaccine was a huge challenge for medical science. In 2021 the World Health Organization (WHO) approved the first malaria vaccine, RTS,S/AS01 vaccine (Mosquirix™), for widespread use. This review highlights the history of development, and the different approaches and types of malaria vaccines, and the literature to date. It covers the developmental stages of RTS,S/AS01 and recommends steps for its deployment. The review explores other potential vaccine candidates and their status, and suggests options for their further development. It also recommends future roles for vaccines in eradicating malaria. Questions remain on how RTS,S vaccine will work in widespread use and how it can best be utilized to benefit vulnerable communities. CONCLUSION: Malaria vaccines have been in development for almost 60 years. The RTS,S/AS01 vaccine has now been approved, but cannot be a stand-alone solution. Development should continue on promising candidates such as R21, PfSPZ and P. vivax vaccines. Multi-component vaccines may be a useful addition to other malaria control techniques in achieving eradication of malaria.

B cell clonal expansion and mutation in the immunoglobulin heavy chain variable domain in response to Pfs230 and Pfs25 malaria vaccines.

Malaria transmission-blocking vaccines induce antibodies that target Plasmodium in the mosquito vector. We recently reported that Pfs230 vaccine achieves activity superior to Pfs25 in humans. Here, we describe clonal expansion in the variable region of immunoglobulin heavy chains (VH) of antigen-specific single B cells collected from humans immunised with Pfs230D1-EPA or Pfs25-EPA conjugate vaccines formulated in Alhydrogel®. Based on studies of CD27+ memory B cells following Pfs230 vaccination, clonal expansion and somatic hypermutation was seen in four of five subjects. Pfs25 did not induce sufficient CD27+ cells for sorting; based instead on CD19+ Pfs25-reactive B cells, clonal expansion was only seen in two of five subjects. Clonal expansions and mutations in Pfs230-specific single B cells combined with the enhanced activity of Pfs230 antibodies by complement, might justify the outstanding activity of Pfs230D1 as a TBV candidate.

Heterologous Expression and Evaluation of Novel Plasmodium falciparum Transmission Blocking Vaccine Candidates.

Malaria transmission blocking vaccines (TBV) aim to induce antibodies that can interrupt Plasmodium falciparum development in the mosquito midgut and thereby prevent onward malaria transmission. A limited number of TBV candidates have been identified and only three (Pfs25, Pfs230 and Pfs48/45) have entered clinical testing. While one of these candidates may emerge as a highly potent TBV candidate, it is premature to determine if they will generate sufficiently potent and sustained responses. It is therefore important to explore novel candidate antigens. We recently analyzed sera from naturally exposed individuals and found that the presence and/or intensity of antibodies against 12 novel putative surface expressed gametocyte antigens was associated with transmission reducing activity. In this study, protein fragments of these novel TBV candidates were designed and heterologously expressed in Drosophila melanogaster S2 cells and Lactococcus lactis. Eleven protein fragments, covering seven TBV candidates, were successfully produced. All tested antigens were recognized by antibodies from individuals living in malaria-endemic areas, indicating that native epitopes are present. All antigens induced antigen-specific antibody responses in mice. Two antigens induced antibodies that recognized a native protein in gametocyte extract, and antibodies elicited by four antigens recognized whole gametocytes. In particular, we found that antigen Pf3D7_0305300, a putative transporter, is abundantly expressed on the surface of gametocytes. However, none of the seven novel TBV candidates expressed here induced an antibody response that reduced parasite development in the mosquito midgut as assessed in the standard membrane feeding assay. Altogether, the antigen fragments used in this study did not prove to be promising transmission blocking vaccine constructs, but led to the identification of two gametocyte surface proteins that may provide new leads for studying gametocyte biology.

Rare Alleles and Signatures of Selection on the Immunodominant Domains of Pfs230 and Pfs48/45 in Malaria Parasites From Western Kenya.

Background: Malaria elimination and eradication efforts can be advanced by including transmission-blocking or reducing vaccines (TBVs) alongside existing interventions. Key transmission-blocking vaccine candidates, such as Pfs230 domain one and Pfs48/45 domain 3, should be genetically stable to avoid developing ineffective vaccines due to antigenic polymorphisms. We evaluated genetic polymorphism and temporal stability of Pfs230 domain one and Pfs48/45 domain three in Plasmodium falciparum parasites from western Kenya. Methods: Dry blood spots on filter paper were collected from febrile malaria patients reporting to community health facilities in endemic areas of Homa Bay and Kisumu Counties and an epidemic-prone area of Kisii County in 2018 and 2019. Plasmodium speciation was performed using eluted DNA and real-time PCR. Amplification of the target domains of the two Pfs genes was performed on P. falciparum positive samples. We sequenced Pfs230 domain one on 156 clinical isolates and Pfs48/45 domain three on 118 clinical isolates to infer the levels of genetic variability, signatures of selection, genetic diversity indices and perform other evolutionary analyses. Results: Pfs230 domain one had low nucleotide diversity (π = 0.15 × 10-2) with slight variation per study site. Six polymorphic sites with nonsynonymous mutations and eight haplotypes were discovered. I539T was a novel variant, whereas G605S was nearing fixation. Pfs48/45 domain three had a low π (0.063 × 10-2), high conservation index, and three segregating sites, resulting in nonsynonymous mutation and four haplotypes. Some loci of Pfs230 D1 were in positive or negative linkage disequilibrium, had negative or positive selection signatures, and others (1813, 1955) and (1813, 1983) had a history of recombination. Mutated loci pairs in Pfs48/45 domain three had negative linkage disequilibrium, and some had negative and positive Tajima's D values with no history of recombination events. Conclusion: The two transmission blocking vaccine candidates have low nucleotide diversity, a small number of zone-specific variants, high nucleotide conservation index, and high frequency of rare alleles. With the near fixation a polymorphic site and the proximity of mutated codons to antibody binding epitopes, it will be necessary to continue monitoring sequence modifications of these domains when designing TBVs that include Pfs230 and Pfs48/45 antigens.

Plasmodium's journey through the Anopheles mosquito: A comprehensive review.

The malaria parasite has an extraordinary ability to evade the immune system due to which the development of a malaria vaccine is a challenging task. Extensive research on malarial infection in the human host particularly during the liver stage has resulted in the discovery of potential candidate vaccines including RTS,S/AS01 and R21. However, complete elimination of malaria would require a holistic multi-component approach. In line with this, under the World Health Organization's PATH Malaria Vaccine Initiative (MVI), the research focus has shifted towards the sexual stages of malaria in the mosquito host. Last two decades of scientific research obtained seminal information regarding the sexual/mosquito stages of the malaria. This updated and comprehensive review would provide the basis for consolidated understanding of cellular, biochemical, molecular and immunological aspects of parasite transmission right from the sexual stage commitment in the human host to the sporozoite delivery back into subsequent vertebrate host by the female Anopheles mosquito.

In vivo Characterization of Plasmodium berghei P47 (Pbs47) as a Malaria Transmission-Blocking Vaccine Target.

An effective vaccine to reduce malaria transmission is central to control and ultimately achieve disease eradication. Recently, we demonstrated that antibodies targeting the Plasmodium falciparum surface protein P47 (Pfs47) reduce parasite transmission to Anopheles gambiae mosquitoes. Here, Plasmodium berghei (Pb) was used as a model to assess the in vivo efficacy of a P47-targeted transmission blocking vaccine (Pbs47). Mice were immunized following a prime/boost regimen and infected with P. berghei. The effect of immunization on infectivity to mosquitoes was evaluated by direct feeding on P. berghei-infected mice. The key region in Pbs47 where antibody binding confers protection was mapped, and the immunogenicity of this protective antigen was enhanced by conjugation to a virus-like particle. Passive immunization with 100 and 50 µg/mL of anti-Pbs47 IgG reduced oocyst density by 77 and 67%, respectively. Furthermore, affinity purified Pbs47-specific IgG significantly reduced oocyst density by 88 and 77%, respectively at doses as low as 10 and 1 µg/mL. These studies suggest that P47 is a promising transmission blocking target and show that antibodies to the same specific region in Pfs47 and Pbs47 confer protection.

High Plasmodium infection intensity in naturally infected malaria vectors in Africa.

The population dynamics of human to mosquito malaria transmission in the field has important implications for the genetics, epidemiology and control of malaria. The number of oocysts in oocyst-positive mosquitoes developing from a single, naturally acquired infectious blood meal (herein referred to as a single-feed infection load) greatly influences the efficacy of transmission blocking interventions but still remains poorly documented. During a year-long analysis of malaria parasite transmission in Burkina Faso we caught and dissected wild malaria vectors to assess Plasmodium oocyst prevalence and load (the number of oocysts counted in mosquitoes with detectable oocysts) and the prevalence of salivary gland sporozoites. This was compared with malaria endemicity in the human population, assessed in cross-sectional surveys. Data were analysed using a novel transmission mathematical model to estimate the per bite transmission probability and the average single-feed infection load for each location. The observed oocyst load and the estimated single-feed infection load in naturally infected mosquitoes were substantially higher than previous estimates (means ranging from 3.2 to 24.5 according to seasons and locations) and indicate a strong positive association between the single-feed infection load and parasite prevalence in humans. This work suggests that highly infected mosquitoes are not rare in the field and might have a greater influence on the epidemiology and genetics of the parasite, and on the efficacy of novel transmission blocking interventions.

Transmission-Blocking Vaccines for Malaria: Time to Talk about Vaccine Introduction.

Malaria kills more than 600 000 people yearly, mainly children, and eradication is a global priority. Malaria transmission-blocking vaccines are advancing in clinical trials, and strategies for their introduction must be prioritized among stakeholders and the vulnerable populations exposed to the disease.

Arbovirosis and potential transmission blocking vaccines.

Infectious diseases caused by arboviruses (viruses transmitted by arthropods) are undergoing unprecedented epidemic activity and geographic expansion. With the recent introduction of West Nile virus (1999), chikungunya virus (2013) and Zika virus (2015) to the Americas, stopping or even preventing the expansion of viruses into susceptible populations is an increasing concern. With a few exceptions, available vaccines protecting against arboviral infections are nonexistent and current disease prevention relies on vector control interventions. However, due to the emergence of and rapidly spreading insecticide resistance, different disease control methods are needed. A feasible method of reducing emerging tropical diseases is the implementation of vaccines that prevent or decrease viral infection in the vector. These vaccines are designated 'transmission blocking vaccines', or TBVs. Here, we summarize previous TBV work, discuss current research on arboviral TBVs and present several promising TBV candidates.

Membrane Feeding Assay to Determine the Infectiousness of Plasmodium vivax Gametocytes.

The evaluation of Plasmodium vivax gametocyte infectiousness by the membrane feeding assay is herein described. While P. vivax cannot be cultured and different parasite isolates may infect mosquitoes at different rates, the protocol described in this chapter identifies critical parameters to be considered when performing the assay such as methods for the preparation of the mosquitoes, the size of the blood cup, and the blood volume used. In previous studies the data have shown that the membrane feeding assay is useful for studies of parasite biology, and the effects of transmission blocking drugs and vaccines.

Transmission blocking vaccines to control insect-borne diseases: a review

Insect-borne diseases are responsible for severe mortality and morbidity worldwide. As control of insect vector populations relies primarily on the use of insecticides, the emergence of insecticide resistance as well to unintended consequences of insecticide use pose significant challenges to their continued application. Novel approaches to reduce pathogen transmission by disease vectors are been attempted, including transmission-blocking vaccines (TBVs) thought to be a feasible strategy to reduce pathogen burden in endemic areas. TBVs aim at preventing the transmission of pathogens from infected to uninfected vertebrate host by targeting molecule(s) expressed on the surface of pathogens during their developmental phase within the insect vector or by targeting molecules expressed by the vectors. For pathogen-based molecules, the majority of the TBV candidates selected as well as most of the data available regarding the effectiveness of this approach come from studies using malaria parasites. However, TBV candidates also have been identified from midgut tissues of mosquitoes and sand flies. In spite of the successes achieved in the potential application of TBVs against insect-borne diseases, many significant barriers remain. In this review, many of the TBV strategies against insect-borne pathogens and their respective ramification with regards to the immune response of the vertebrate host are discussed.