The Problematic Parasite

Home , Jenner Institute

Vaccines outlook DENNIS KUNKEL MICROSCOPY/SPL KUNKEL DENNIS Transmission electron micrograph of merozoite-stage malarial parasites, which have caused a red blood cell to rupture. The problematic parasite

This year, the first vaccine for malaria was given to children. Scientists are working on improvements, but there is little agreement on how to do this. By Anthony King

n 1991, when immunologist Patrick Duffy After 30 years in development, RTS,S is the researchers are not popping the champagne was joining the US National Institute first malaria vaccine shown to offer protection corks and relaxing just yet. “RTS,S is leading of Allergy and Infectious Diseases, the to young children in a phase III trial — malaria the pack, but it’s a suboptimal vaccine,” says research community was sceptical that takes 1,200 lives each day, mostly those of chil- Michael Good, a vaccine researcher at Griffith it would be possible to vaccinate against dren under 5. “The biggest news is that we have University in Brisbane, Australia. Iparasites — especially the Plasmodium parasites malaria vaccines that work,” says Duffy. “Some feel it doesn’t give you enough that cause malaria. These organisms, each just RTS,S, developed by pharmaceutical firm protection to be worthwhile,” says Stefan a few micrometres across, can shape-shift and GlaxoSmithKline, based in London, works by Kappe, an infectious-disease scientist at hide from the human immune system, moving introducing the immune system to a fragment Seattle Children’s Hospital in Washington. from the blood into liver cells, before burst- of a protein that is present on the surface of Over the course of the vaccine’s four-year ing out in a new form to take over red blood Plasmodium when the parasite enters the phase III trial, it prevented around 30% of seri- cells. They deploy similar cellular machinery to bloodstream through an infected mosquito. ous cases of malaria1. The trials also showed humans, and are much more complex than viral The protein stimulates the production of a rise in mortality among girls who were or bacterial foes — for instance, whereas the antibodies, and allows the body to mount a vaccinated. “If that is real, that is the end of that Ebola virus can encode 7 proteins, Plasmodium swift response to the parasite the next time story,” says Adrian Hill, a vaccine researcher at boasts genes for 5,000. it is encountered. The vaccine’s pilot pro- the Jenner Institute in Oxford, UK. But in 2019, children in Ghana, Kenya and gramme, coordinated by the World Health Malaria-vaccine researchers are working on Malawi began receiving the RTS,S vaccine Organization, is expected to immunize at least several approaches to achieve more robust against malaria as part of a pilot programme. 360,000 children a year until 2022. But malaria protection than that offered by RTS,S. Some

Vaccines outlook are looking to empower the immune system One sporozoite reaching the liver can result in doses one-fifth the size, uses more-modern to go after Plasmodium in the liver, including full-blown malaria, so the vaccine must prime production methods and has a cheaper adju- using live attenuated parasites as immuniz- the body with enough antibodies to destroy vant. A phase II trial in Burkina Faso to test ing agents. Others have their eyes on targeting every single sporozoite before they make it efficacy in adults and children is now under the parasite inside red blood cells — if not to to the liver. It takes a lot, says Hill — around way, with results expected in 2020. neutralize it entirely, then at least to prevent 500 times more antibodies than are produced it spreading to other people. There are numer- by a meningitis vaccine. This requires the help Clear the hideouts ous strategies all jostling for the limelight, of an adjuvant, which chemically stimulates The way in which sporozoite-targeting and considerable disagreement about which the immune system. RTS,S consists of a region vaccines prepare the body to mount a defence should be prioritized. of repeating amino acids from the surface is only good for a tight window of time after circumsporozoite protein and a hepatitis B infection. The immune response, therefore, Following the leader surface antigen — a set-up that is the legacy of needs to be rapid and effective. An alternative When a person is bitten by a mosquito carrying the vaccine’s long development history. Back strategy is to target the Plasmodium parasite Plasmodium falciparum, the deadliest of in 1987, researchers “couldn’t make what they once it is inside liver cells. With this approach, the five species of malaria parasite that can wanted, which is a virus-like protein with just any immune response will have at least five infect humans, a handful of protozoa in their malaria on the surface”, says Hill. And although days to eliminate infected liver cells. T cells needle-like ‘sporozoite’ form enter the body technology has improved, once a vaccine has — immune cells that react to pathogens lurk- (see ‘Breaking the cycle’). The RTS,S vaccine entered clinical trials, you cannot simply ing in cells — can be trained to recognize pro- stimulates the body to produce antibodies overhaul the whole programme. teins on the surface of infected liver cells, and against proteins present on the surface of a Hill and his colleagues are working on kill the cells. The symptoms associated with sporozoite, but the parasite does not linger an alternative vaccine, R21, that has many malaria occur only after the parasite leaves the in this form for long — in half an hour, it can similarities to RTS,S. R21 fuses together liver and starts destroying red blood cells, so ensconce itself inside liver cells. There, it a hepatitis B antigen and half the circum- eliminating infected liver cells would prevent multiplies, and 7–10 days later emerges as sporozoite protein — a larger portion than in illness and transmission. 30,000 merozoite-stage parasites, each RTS,S2. Hill thinks that this combination will Denise Doolan, a vaccine scientist at the Aus- invisible to the RTS,S-induced antibodies. be at least as effective as RTS,S, but it is less tralian Institute of Tropical Health and Medicine This presents RTS,S with a daunting task. expensive because it can be administered in in Cairns, has systematically evaluated all of the parasite’s proteins. She looked for proteins that BREAKING THE CYCLE would be spotted by antibodies or T cells found Plasmodium falciparum, the parasite responsible for the most severe form of malaria in people, spends a in the blood serum of people with a tolerance considerable part of its life cycle inside the human body. As the parasite takes on dierent forms and infects to malaria. Now, she has a promising list of pro- dierent parts of the body, it presents researchers with several distinct targets for vaccines (white boxes). teins associated with protection against malaria that could be useful for vaccine development. Parasite life cycle The RTS,S and R21 “We have three antigens that we have selected vaccines target from many years of screening that I would love 1. A mosquito carrying sporozoites before to get to the clinic,” says Doolan. She is espe- P. falciparum injects a they reach the liver. form of the parasite cially keen on antigens that elicit a response called a sporozoite against both the liver and blood stages of the into the bloodstream. parasite’s life cycle, and those that show poten- 2. Sporozoites infiltrate Sporozoite tial to stimulate an immune response in multi- liver cells and multiply. ple Plasmodium species. Liver 3. Tens of thousands Wiping out all Plasmodium-infected liver of merozoite-stage cells requires very large numbers of T cells. parasites burst from One way to ramp up a T-cell response is with a liver cells and enter the bloodstream. one–two punch: prime the immune system by Red blood cell injecting a circular piece of Plasmodium DNA or 4. Merozoites hijack red blood cells and a viral vector expressing a gene found in Plas- multiply. modium, and then boost it with adenoviruses engineered to express the same parasite gene. 5a. Merozoites burst out and infect more The combination triggers the immune system red blood cells, to muster a robust T-cell response that it will releasing toxic remember in future. But delivering it is logisti- substances that cause many of the clinical cally challenging because it requires two differ- symptoms of malaria. ent vaccines to be administered sequentially. Transmission- Candidate 5b. Some merozoites blocking vaccines such Hill points to encouraging results (67% effi- mature into a sexual vaccines as PfSPZ train cacy)3 from a small trial in Kenya, in which two form. These are taken target the immune cells viral-vector injections carry an antigen from up by mosquitoes, and sexual stage Vaccines aimed at the called killer sexual reproduction of the merozoite-stage can target T cells to find the liver stage of the parasite. But this might occurs and new parasite. them in the blood or train and destroy not be good enough — if a single infected liver sporozoites are T cells to find and destroy infected liver generated. infected red blood cells. cells. cell goes undetected, a battalion of merozoites can burst out to target red blood cells.

Stephen Hoffman, a malaria researcher and blood cells. He and Hoffman might collaborate in mosquitoes,” he says, “but we have made chief executive of biotechnology company to insert P. vivax genes into attenuated P. falci- vaccines in pus and poo.” Sanaria in Rockville, Maryland, has an idea parum sporozoites, with the aim of providing During the life cycle of malaria parasites, the about how to elicit a T-cell response without protection against both. Together, this pair of microorganisms exit the liver and hijack red relying on segments of protein. In the early parasites accounts for 95% of malaria. blood cells. Once inside, merozoites multiply 1970s, research suggested that people gained Despite their promise, live attenuated inside the cells and burst out. That can lead to immunity to P. falciparum if they were bitten by vaccines have engendered some scepticism in seizures, severe anaemia and coma. Researchers irradiated mosquitoes carrying sporozoites. the malaria-vaccine community. Some suggest have long targeted this part of the malarial life Hoffman founded Sanaria in the 1990s to con- that their efficacy might not be as impressive cycle with vaccines, but have had little success. tinue this work and develop a vaccine contain- in the real world as it seems in studies that In the 1960s, hopes were raised when it was ing live, but radiation-weakened, parasites. deliberately expose volunteers to malaria. In shown6 that parasite load could be dampened Sanaria injected volunteers with weakened these, a defined parasite strain is introduced, by giving someone blood serum from an adult P. falciparum parasites that travelled to and says Doolan, “whereas in the field you typi- living in a malaria-endemic region. But trials developed inside liver cells for a few days, but cally have hundreds of thousands of differ- focusing on the blood phase of the life cycle then stopped replicating and did not leave ent strains of malaria and multiple species”. fell flat — researchers targeted vaccines at a the cells to cause disease. Because the para- Doolan also warns that gene-edited vaccines small number of surface antigens within which site remained in the liver stage, immune cells could “revert to virulence”, shaking off the merozoites have evolved extreme variability learnt how to identify it in future infections. shackles of attenuation and causing malaria to outwit their host’s immune system. “The Sanaria says that its vaccine, PfSPZ, has rather than preventing it. targets of some of the earlier vaccines had a lot shown 100% protection in studies4,5 in which Whether Sanaria can make its vaccine of genetic diversity, meaning they looked dif- adults who have not been previously exposed affordable is also a subject of debate. The man- ferent in every parasite,” says Simon Draper, an to malaria, mostly in the United States, are ufacturing process involves infecting sterile immunologist at the University of Oxford, UK. infected with P. falciparum. However, in mosquitoes with the parasite, incubating them To avoid this problem, Draper is looking at a endemic regions, people can be infected by for several weeks, and then removing their sali- protein that P. falciparum uses to harpoon red other species, such as Plasmodium vivax. In vary glands by hand. The parasites in these tiny blood cells, called RH5. “We know RH5 is highly February, Hoffman says, a trial will begin on glands are then purified and packaged. “It’s conserved. It looks nearly identical in every the island of Bioko, 32 kilometres off the coast obviously a tedious process, and will make the parasite,” says Draper. His group currently tar- of West Africa, where malaria is endemic. The vaccine more expensive,” says Good. gets the entire protein, but it is redesigning the trial will involve 2,100 people between the molecule to elicit a stronger immune response ages of 2 and 50, and gather safety and efficacy “Many people say it is and putting RH5 on a virus-like particle to ramp data for regulatory approval. “I’m pretty con- crazy to make a malaria up the immune response. A trial to study safety fident that the vaccine also works,” says Duffy. and efficacy in UK volunteers showed highly Duffy is involved in another PfSPZ study, to vaccine in mosquitoes, but promising RH5 antibody responses. A trial in be based in Mali, which is now recruiting we have made vaccines in Tanzania, where malaria is endemic, finished 900 women to assess the protection the vac- pus and poo.” in July, and a second is on the cards for 2020. cine offers during pregnancy. Hoffman is also In Australia, Good led the first clinical evalu- planning a trial in Indonesia to see whether ation7 of a whole-parasite blood-stage vaccine PfSPZ protects against P. vivax, and is trying But improvements in production method- in 2018. Merozoites were grown in red blood to produce P. vivax sporozoites for testing in ology are possible. “If the vaccine works, there cells, disabled with an antimalarial drug, and controlled human studies. will be research to speed up the process,” Good then injected into eight volunteers. “The vol- “Whole-parasite vaccines have shown says. Hill takes the opposing view, saying that unteers all developed a good T-cell response,” significant promise, and we can make them it is “non-manufacturable to even a few thou- says Good. His group is now infecting vacci- even better,” says Kappe. Rather than training sand people without huge expense”. Storage nated volunteers with malaria to see whether the immune system to recognize and respond and administration are problematic: in its cur- this approach limits the number of parasites to one parasite protein, the live parasite rent form, the vaccine must be transported in in their blood. Again, Hill sees manufacturing teaches it about lots of antigens at once, and dry ice and delivered intravenously. problems ahead — growing parasites in blood should, therefore, generate a considerable Hoffman brushes off cost criticisms, saying cells will be just as difficult as carving them out immune response. Kappe is also working on that vaccines such as Prevnar, used to protect of mosquitoes’ salivary glands, he says. a live attenuated malaria vaccine. Instead of against pneumococcal bacteria, take longer to hobbling the parasite with radiation, Kappe make and require more arduous quality con- Stopping the spread uses gene editing. A crucial DNA-packaging trol. He also says that it might be possible to Vaccinating against the malaria parasite is protein is sabotaged using the gene-editing reduce the number of parasites required for a “an incredibly complex problem, like send- tool CRISPR–Cas9, creating a parasite that single vaccination. Each dose of standard PfSPZ ing someone to the Moon”, says Kappe. He can enter liver cells but never leave them. “Our harbours 900,000 sporozoites. But when vol- thinks that greater investment is required, new parasite can infect the liver, replicate and unteers also took antimalarial medication, a but this is not likely to be forthcoming. become an antigen-making machine,” says vaccine containing only 50,000 sporozoites Parasite-targeting vaccines commonly fail to Kappe. gave superior results — all of the nine partici- excite pharmaceutical companies — few have Kappe has shown that this strategy prevents pants exposed to the parasite were protected5. invested substantially in malaria-vaccine parasites from coming out of the liver and that Kappe is also confident about the pros- research. And there is a feeling among the it triggers a potent immune response, but, so pects of live attenuated vaccines. “Many peo- researchers Nature spoke to that philanthropic far, only for mice hosting human liver and ple say it is crazy to make a malaria vaccine funding organizations have begun to place

Vaccines outlook more attention on tackling the mosquitoes Q&A that carry the disease and preventing transmission, rather than priming people to fight off the parasite. Hill thinks eradication of malaria is not possible with the tools currently available, and that new vaccines are ultimately Jeffrey Bethony: necessary. Duffy agrees: “An effort like elimi- nation means you have to bring lots of tools Taking on worms together,” he says. “A vaccine can be an important addition.” About two billion of the world’s poorest Haem is potentially toxic, and antibodies Vaccines could be useful in the battle to people are infected with parasitic worms. against the degradation protein reduce the prevent transmission, too. One way to stop Treatments are available, but Jeffrey worm’s ability to eliminate haem from its the parasite jumping between human and Bethony, a microbiologist at George blood meals. The other protein prevents hook- mosquito is to sabotage the parasite’s sexual Washington University in Washington DC, worms from breaking down haemoglobin for

stage. Duffy’s team has developed a transmis- explains why only vaccines can eradicate consumption. We have done separate phase I SCIENCES HEALTH SCHOOL OF MEDICINE AND OLIVIA ANDERSON/GW sion-blocking vaccine that targets the sexual infection. trials of our two vaccines in the United States, forms that are taken up by mosquitoes when Brazil and Gabon, and have just received fund- they ingest human blood cells. In an initial Why is it more difficult to develop a vaccine ing to test both proteins using a controlled field trial of this type of vaccine, conducted for parasites than for many viruses? human infection model in endemic areas of in Mali in 2018, it proved safe8. Next, Duffy Parasites go through a series of life stages and Brazil. We then plan to test the simultaneous is planning to report the results of a trial of occupy several different niches in the body. delivery of both proteins in a single vaccine. a vaccine that combines Pfs230, a protein They’ve also developed clever mechanisms on the outside of the parasite gamete, with to evade the immune system. So parasitic So a CHIM study involves injecting healthy GlaxoSmithKline’s adjuvant ASO1, which is infections are the ultimate challenge. people with parasites? included in the RTS,S vaccine to boost T-cell Yes. We borrowed the idea from malaria response. A major challenge with these trans- Which diseases caused by parasitic worms researchers. I immunize people against mission-blocking vaccines, however, will be do we most need a vaccine for? hookworms by administering the proteins in developing the methods to measure any Schistosomiasis results in the greatest disease the subunit vaccines, and then challenge the reduction in malaria transmission in the field, burden, especially in sub-Saharan Africa volunteers with hookworms. If the vaccines and to prove that the vaccines work. and Brazil. There is an effective treatment, don’t work, we can get rid of the infection Hill doubts that a stand-alone transmis- praziquantel, but without a vaccine we can’t with drugs. If we had to wait for people to sion-blocking vaccine will ever emerge, but prevent reinfection, so it has proved impos- get infected, studies would take longer and can see it being part of a multi-component sible to eliminate the parasite in low-income cost more; CHIM studies accelerate vaccine vaccine. A combination vaccine that includes countries. And more than 500 million peo- development. a transmission blocker could be more effective ple are infected with hookworms, which can and practical. impair physical and mental development. Isn’t it tricky to get people to volunteer to The need for a malaria vaccine is not dimin- be infected with a parasitic worm? ishing, and researchers are optimistic. “We can What progress are you making on a We have no problem getting volunteers. see the top of the mountain now,” says Kappe schistosomiasis vaccine? There are lots of people who think that of his team’s work. Although scientists might We are developing a candidate vaccine based hookworms, because they can modulate not agree on the best approach to take, they on proteins found on the outer surface of the the immune system, can be therapeutic for can see progress being made — not least the worms. We’re currently running a phase II coeliac disease, Crohn’s disease or irritable wide distribution of the RTS,S vaccine. “Our trial in Uganda, funded by the US Depart- bowel syndrome. That strategy is being best products will be combined in different ment of Defense, that targets a fragment trialled by other researchers now. We suspect ways,” says Duffy. “RTS,S has found a very of such a protein on the worm Schistosoma that’s why some people volunteer. specific role to reduce clinical malaria in chil- mansoni. A group at Leiden University Medi- dren.” It might not be perfect, Hill says, but cal Centre in the Netherlands is working on a Do you have any problems getting funding? malaria is such a horrendous problem that controlled human infection model, or CHIM, People who need a vaccine against parasitic even a partially effective vaccine could make for schistosomiasis. This approach allows worms can’t afford to pay hundreds of dollars a big difference. researchers to give people an experimental for it. So there’s not lots of money spilling vaccine and then challenge them with a dose around. Malaria is better funded than disease Anthony King is a freelance science writer of pathogen. This would allow us to use fewer caused by parasitic worms — it’s considered based in Dublin. volunteers and reduce the costs of trials. more important. Malaria researchers are

1. RTS,S Clinical Trials Partnership Lancet 386, 31–45 (2015). usually one or two steps ahead of us. But 2. Venkatraman, N. et al. Preprint at medRxiv https://doi. What vaccine strategies are you developing success with a malaria vaccine would help org/10.1101/19009282 (2019). against hookworms? all of us. If they can do it, so can we. 3. Ogwang, C. et al. Sci. Transl. Med. 7, 286re5 (2015). 4. Seder, R. A. Science 341, 1359–1365 (2013). We have developed two subunit vaccines, each 5. Mordmüller, B. Nature 542, 445–449 (2017). containing a protein given with an immune By Anthony King 6. Cohen, S, McGregor, I. A. & Carrington, S. Nature 192, stimulant. One protein degrades haem, a com- This interview has been edited for length and 733–737 (1961). 7. Stanisic, D. I. et al. BMC Med. 16, 184 (2018). ponent of the blood protein haemoglobin. clarity. 8. Sagara, I. et al. Lancet Infect. Dis. 18, 969–982 (2018).