Once thought to be caused by ‘mala aria’—Italian for ‘bad air’—from swampy areas, malaria is now attributed to infection by the Plasmodium parasite. Spread by the Anopheles mosquito, the parasite claims more than 400,000 lives around the world each year.
After entering a person’s bloodstream via a mosquito bite, the Plasmodium parasite takes up residence in liver cells, or hepatocytes, where they multiply before being released into the bloodstream once more. The released parasites multiply in the bloodstream and are eventually taken up by mosquitoes, for the next phase of their life cycle.
Some parasites will become dormant in the liver, where they can persist for weeks, months or even years. These dormant parasites, responsible for malaria relapse, are known as hypnozoites, and little is known about their biology, in part due to the lack of appropriate models to study them outside a living host.
“A majority of the existing two-dimensional liver models are physiologically-limited, short-term assays of less than two weeks, which are unable to sustain the liver-specific functions required for the proper development of the malaria parasite,” said Pablo Bifani, a Principal Investigator at A*STAR’s Singapore Immunology Network (SIgN).
Together with SIgN colleague Adeline Chua and international collaborators, Bifani created liver organoids by seeding hepatocytes onto a biologically compatible biomaterial, called 3D Cellusponge. The liver spheroids mimic the liver microenvironment, allowing for a longer-term investigation of the life cycle of the Plasmodium parasite outside a living host.
By infecting their liver organoids with human-associated Plasmodium strains, the researchers were able to recapitulate the life cycle of relapsing malaria in a petri dish. Importantly, the spheroids could be maintained for more than three weeks, and the parasites that multiplied within the liver spheroids during that time remained capable of re-infecting red blood cells.
To validate their liver organoid model, the researchers used a drug called KDU691, which was previously shown to prevent malaria relapse in monkeys when administered at the time of infection, but which was ineffective if administered after an infection had been established. Bifani’s team reported similar results with their liver organoid model, suggesting that the liver organoid model is a useful tool for screening antimalarial drugs and predicting drug efficacy in vivo.
Going forward, Bifani and colleagues plan to use their liver organoid model to identify genes involved in hypnozoite formation and reactivation from dormancy. “This [knowledge] will shed light on the liver-stage biology of the parasite, allowing for the development of predictable assays that can be used to screen compounds in the search for new antimalarial drugs,” he said.
The A*STAR-affiliated researchers contributing to this research are from the Singapore Immunology Network (SIgN) and the Institute of Bioengineering and Nanotechnology (IBN).