By Luciana Constantino | Agência FAPESP – Considered one of the world’s 17 neglected tropical diseases (NTDs), schistosomiasis is still an important public health problem in Brazil, but the only medication available to treat it was discovered more than 40 years ago. A scientific study reported in the journal Nature Communications points to a strategy for the development of novel therapies and even a vaccine against this parasitosis.
A research group including Brazilian scientists affiliated with Butantan Institute and the University of São Paulo (USP), as well as colleagues at institutions in several other countries, discovered the mechanism whereby the Rhesus macaque Macaca mulatta naturally develops a lasting immune response against schistosomiasis leading to self-cure after first contact with the parasite Schistosoma mansoni, and enabling its to organism to react faster to a second infection.
The research, which was supported by FAPESP, found nine genes belonging to S. mansoni’s autophagy pathway to be inhibited by the primate’s immune system, preventing the parasite from multiplying and contaminating the host. Autophagy is the destruction of damaged or redundant cellular components by organelles known as lysosomes, playing a role in cell growth, differentiation and homeostasis.
“The autophagy pathway, executed by lysosomes that remove dead cells or clear damaged cells of detritus, is affected in the parasite by the macaque’s antibodies. The pathway is important to the basal physiology of S. mansoni, and its involvement in self-cure had never been demonstrated before. Actually, it hadn’t even been studied very much until now,” said Murilo Sena Amaral, first author of the article, told Agência FAPESP. Amaral is a researcher at Butantan Institute’s Parasitology Laboratory.
The principal investigator for the study was Sergio Verjovski-Almeida, a professor at USP and a researcher at Butantan Institute.
Autophagy is important to the parasite as a mechanism for tissue remodeling during its life cycle, especially in the transition from the larval stage (cercaria) to the adult stage. “We located more than 100 genes that are affected but in different pathways,” he said. “That doesn’t mean they aren’t important, but when you find that nine out of ten genes in the same pathway are affected, you can say there’s substantial evidence that the pathway could be the key to a vaccine.”
Another part of the research stressed by Verjovski-Almeida was a follow-up study of the macaques between self-cure and a second challenge executed 42 weeks after the first infection, showing their resistance to reinfection and rapid elimination of the parasite.
In humans, schistosomiasis can be cured provided it is diagnosed in the initial stage so that the parasite is eliminated from the organism by a drug and there is no time for complications such as enlargement of the liver and spleen, anemia, or gastrointestinal bleeding. However, humans do not acquire immunity like macaques and can be reinfected.
The number of people with the disease worldwide is estimated to be about 200 million, almost half of whom are children, and it causes some 200,000 deaths annually. In Brazil, it is present in 18 states and the Federal District. Eight states have endemic transmission (Alagoas, Bahia, Pernambuco, Rio Grande do Norte, Paraíba, Sergipe, Espírito Santo, and Minas Gerais).
According to federal government data, 1.5 million people live in areas where there is a risk of contracting the disease. In the Northeast and Southeast, its occurrence is directly linked to the presence of the vector, a freshwater snail (Lymnaea luteola).
Brazil recorded 423,117 cases of the disease between 2009 and 2019, according to the latest bulletin from the Health Ministry, issued in March 2021.
Transmission is endemic in places without proper basic sanitation, given that snails are contaminated by the parasite via human feces or urine containing its eggs. The cycle begins when a person who has the disease excretes schistosome eggs, which find their way into untreated water. The eggs hatch and release larvae, which contaminate L. luteola, the intermediate host.
The larvae mature and multiply in the snails, and after about four weeks are released as cercariae back into the water, where they can survive for 48 hours. Anyone who bathes, washes laundry, swims, fishes or plays in the contaminated water can be infected by cercariae, which penetrate the intact skin via an enzyme released from a head gland.
Inside the human body, they progress to the schistosomule stage, during which they enter the bloodstream and may reach the lungs and heart. As microscopic adult worms, they lodge in the veins of the intestines, where the females lay some 300 eggs per day. The eggs are carried by the blood to the liver, where they cause damage, and to the feces and urine.
The disease is diagnosed by laboratory analysis of feces or urine. Simple cases can be treated by a single dose of praziquantel, a drug discovered in the 1970s and distributed in Brazil by the national health system (Sistema Único de Saúde, SUS).
In the study, 12 macaques were infected with 700 S. mansoni cercariae and followed for 42 weeks during the establishment, maturation and self-cure stages. They were then exposed to reinfection by 700 cercariae as a secondary challenge and monitored for 20 more weeks, until the 62nd week after the initial infection. Blood samples were taken to estimate worm burden using the level of circulating anodic antigen (CAA) via the measurement of inflammatory and hematological markers.
The researchers also analyzed samples of the animals’ feces to determine the number of eggs per gram, and the actual worm burden in each animal. Separately, they conducted in vitro assays to analyze plasma containing antibodies and incubated with young parasites, detecting their deaths. “We performed the assays to evaluate gene expression and chromatin marks regulating genes expressed from DNA, arriving at the nine autophagy pathway genes,” Amaral explained.
By monitoring blood levels of the parasite-derived antigen, the researchers found that the infection was eliminated and resistance to reinfection created as of the tenth week. Antibody profiles suggested that antigens mediating protection were the released products of developing schistosomules. In culture, they were killed by addition of plasma collected from the animals from the eighth week after infection, and even more efficiently by post-challenge plasma.
In addition, cultured schistosomules lost chromatin-activating marks at the transcription start site of genes associated with worm development, and showed decreased expression of genes relating to lysosomes involved in autophagy.
According to Verjovski-Almeida, the researchers are now working on ways to identify the antibodies’ targets. “We’re looking at the phenotype, which is a consequence of the antibodies that alter pathway gene expression,” he said. “The next step will be to find out whether these and other genes are specific targets.”
Once the targets have been identified, they will be tested for a candidate vaccine.
The research was supported by FAPESP via six projects (15/06366-2, 18/15049-9, 18/23693-5, 19/09404-3, 16/10046-6, and 18/18117-5).
The article “Rhesus macaques self-curing from a schistosome infection can display complete immunity to challenge” is at: www.nature.com/articles/s41467-021-26497-0.