Plasmodium, the causative agent of malaria, must first infect hepatocytes to initiate a mammalian infection. Sporozoites migrate through several hepatocytes, by breaching their plasma membranes, before infection is finally established in one of them. Here we show that wounding of hepatocytes by sporozoite migration induces the secretion of hepatocyte growth factor (HGF), which renders hepatocytes susceptible to infection. Infection depends on activation of the HGF receptor, MET, by secreted HGF. The malaria parasite exploits MET not as a primary binding site, but as a mediator of signals that make the host cell susceptible to infection. HGF/MET signaling induces rearrangements of the host-cell actin cytoskeleton that are required for the early development of the parasites within hepatocytes. Our findings identify HGF and MET as potential targets for new approaches to malaria prevention.

Malaria starts with the infection of the liver of the host by Plasmodium sporozoites, the parasite form transmitted by infected mosquitoes. Sporozoites migrate through several hepatocytes by breaching their plasma membranes before finally infecting one with the formation of an internalization vacuole. Migration through host cells induces apical regulated exocytosis in sporozoites. Here we show that apical regulated exocytosis is induced by increases in cAMP in sporozoites of rodent (P. yoelii and P. berghei) and human (P. falciparum) Plasmodium species. We have generated P. berghei parasites deficient in adenylyl cyclase α (ACα), a gene containing regions with high homology to adenylyl cyclases. PbACα-deficient sporozoites do not exocytose in response to migration through host cells and present more than 50% impaired hepatocyte infectivity in vivo. These effects are specific to ACα, as re-introduction of ACα in deficient parasites resulted in complete recovery of exocytosis and infection. Our findings indicate that ACα and increases in cAMP levels are required for sporozoite apical regulated exocytosis, which is involved in sporozoite infection of hepatocytes.

Autophagy plays an important role in the defence against intracellular pathogens. However, some microorganisms can manipulate this host cell pathway to their advantage. In this study, we addressed the role of host cell autophagy during Plasmodium berghei liver infection. We show that vesicles containing the autophagic marker LC3 surround parasites from early time-points after invasion and throughout infection and colocalize with the parasitophorous vacuole membrane. Moreover, we show that the LC3-positive vesicles that surround Plasmodium parasites are amphisomes that converge from the endocytic and autophagic pathways, because they contain markers of both pathways. When the host autophagic pathway was inhibited by silencing several of its key regulators such as LC3, Beclin1, Vps34 or Atg5, we observed a reduction in parasite size. We also found that LC3 surrounds parasites in vivo and that parasite load is diminished in a mouse model deficient for autophagy. Together, these results show the importance of the host autophagic pathway for parasite development during the liver stage of Plasmodium infection.