According to the World Health Organization, there were an estimated 219 million cases of malaria in 2017, resulting in the deaths of approximately 435,000 people. Infected mosquitoes transmitPlasmodiumparasites to humans in the form of rod-shaped sporozoites that travel to the liver and invade liver cells (hepatocytes). Once inside these cells, thePlasmodiumsporozoites develop into spherical exoerythrocytic forms (EEFs) that eventually give rise to thousands of merozoites capable of spreading into red blood cells and causing malaria.

"It seems likely that the transformation ofPlasmodiumsporozoites into EEFs is tightly controlled so that it only occurs in hepatocytes and not at earlier stages of the parasite's life cycle," says Masahiro Yamamoto, a professor at the Research Institute for Microbial Diseases of Osaka University. "However, we know very little about the host factors that regulate the differentiation of sporozoites in infected hepatocytes."

In the new study, Yamamoto and colleagues discovered that a hepatocyte protein called CXCR4 helpsPlasmodiumsporozoites transform into EEFs. Depleting this protein from human liver cells reduced the ability of sporozoites to develop into EEFs. Moreover, mice pretreated with a drug that inhibits CXCR4 were resistant to malaria, showing reduced levels of parasites in the blood and significantly higher survival rates followingPlasmodiuminfection.

Yamamoto and colleagues also identified a cell signaling pathway that causes hepatocytes to produce more CXCR4 in response toPlasmodiuminfection and determined that the protein aids the parasite's development by raising the levels of calcium inside the cells.

"Our study reveals that CXCR4 blockade inhibitsPlasmodiumsporozoite transformation in hepatocytes," Yamamoto says. "Most anti-malaria drugs targetingPlasmodium-derived molecules eventually lead to drug resistance in these parasites. However, inhibitors targeting human proteins such as CXCR4 might avoid this problem and could be used prophylactically to prevent the development of malaria. Moreover, the CXCR4 inhibitor used in this study is already widely used in humans undergoing treatment for blood cancers, which could accelerate its repurposing as a new way of combating malaria."