One of the biggest reasons why it’s hard to eradicate malaria is that a dormant form of the parasite lurks in the livers of some patients. This dormant form is resistant to most antimalarial drugs and can reawaken months or even years later, causing a relapse. Malaria researchers know little about the biology of these dormant parasites, so it has been difficult to develop drugs that target them — until now.

Researchers at the Massachusetts Institute of Technology have shown they can grow the dormant parasite in engineered human liver tissue for several weeks, allowing them to closely study how the parasite becomes dormant, what vulnerabilities it may have, and how it springs back to life.

“I’m really excited because I believe it will open the door to both the basic biology of dormancy as well as the possibility of better medicines,” says Dr Sangeeta Bhatia, a biological engineer at MIT and senior author of the study.

In India, most human cases of malaria are caused by one of two parasite species, Plasmodium falciparum and Plasmodium vivax. P. vivax, while less deadly, produces dormant forms known as hypnozoites (so called because they are “hypnotized”), and can lead to recurring infections.

“This dormant form has been seen as the critical barrier to eradication,” Bhatia says. “You can treat the symptoms of vivax malaria by killing all the parasites in the blood, but if hypnozoites linger in someone’s liver, these forms can reactivate and reinfect the blood of a patient. If a mosquito comes along and takes a blood meal, the cycle starts all over again. So, if we want to eradicate malaria, we have to eradicate the hypnozoite.”

Scientists have known this since 1991, when a small island in the Southwest Pacific was chosen as a site to test possible measures to eradicate malaria. Researchers sprayed against mosquito larvae and supplied bed nets and malaria medicine across the entire island. These efforts led to the complete eradication of Plasmodium falciparum within a year. In contrast, it took five years to eliminate P. vivax.

The only existing drug that can kill hypnozoites is primaquine, but this drug cannot be used in large-scale eradication campaigns because it causes blood cells to rupture in people with a certain enzyme deficiency.

Bhatia’s team has developed special micropatterned surfaces on which human liver cells can be grown, surrounded by supportive cells. This architecture creates a microenvironment in which human liver cells function much the same way as they do in humans, making it easier to establish, maintain, and study infections of the liver. Bhatia, who initially used this technology to model hepatitis infections, realized it was also well-suited to studying the liver stage of malaria. She and team lead, Sandra March, began with Plasmodium falciparum, the strain that can be cultured in lab settings, and found that parasites grown in these liver tissue followed the same life cycle observed in natural infections. They also found that the system could be used to test responses to experimental malaria vaccines.

Following that success, Bhatia’s lab began working with Plasmodium vivax. Efforts to bring the parasite-infected mosquitoes into the United States were unsuccessful, so Gural, the paper’s lead author, traveled to collaborator Jetsumon Prachumsri’s lab in Thailand repeatedly to obtain samples from infected patients and perform the experiments there.

Using their new technology, the researchers showed that they could grow small forms of the parasite that had all of the known features of hypnozoites: persistence, sensitivity to primaquine, and the ability to “wake up” after a few weeks. They then tested six candidate antimalarial medications against the hypnozoites and found none effective.

The researchers were also able to sequence the dormant parasites’ RNA, finding it expressed a different subset of genes than those found in active counterparts.

In future studies, Bhatia plans to use single cell RNA-sequencing to identify gene signatures to uncover the signaling pathways that control hypnozoite dormancy and reactivation. The researchers will also study corresponding changes in gene expression of the infected liver cells. This approach could yield potential new drug candidates that would specifically target the dormant forms of the parasite, bringing the field closer to its goal of eradicating malaria. The researchers also hope to identify biomarkers that could be used to diagnose patients who have an otherwise undetectable dormant infection.