Organelle segregation into Plasmodium liver stage merozoites

Hostile takeover by Plasmodium: reorganization of parasite and host cell membranes during liver stage egress

The protozoan parasite Plasmodium is transmitted by female Anopheles mosquitoes and undergoes obligatory development within a parasitophorous vacuole in hepatocytes before it is released into the bloodstream. The transition to the blood stage was previously shown to involve the packaging of exoerythrocytic merozoites into membrane-surrounded vesicles, called merosomes, which are delivered directly into liver sinusoids. However, it was unclear whether the membrane of these merosomes was derived from the parasite membrane, the parasitophorous vacuole membrane or the host cell membrane. This knowledge is required to determine how phagocytes will be directed against merosomes. Here, we fluorescently label the candidate membranes and use live cell imaging to show that the merosome membrane derives from the host cell membrane. We also demonstrate that proteins in the host cell membrane are lost during merozoite liberation from the parasitophorous vacuole. Immediately after the breakdown of the parasitophorous vacuole membrane, the host cell mitochondria begin to degenerate and protein biosynthesis arrests. The intact host cell plasma membrane surrounding merosomes allows Plasmodium to mask itself from the host immune system and bypass the numerous Kupffer cells on its way into the bloodstream. This represents an effective strategy for evading host defenses before establishing a blood stage infection.

Malaria is one of the most important infectious diseases in the developing world. It is caused by Plasmodium parasites, which are transmitted by female Anopheles mosquitoes during blood feeding. In the mammalian host, Plasmodium first develops within liver cells, growing from one parasite into many thousands. After this extensive replication, the parasites are released into the blood stream in vesicles termed merosomes that are surrounded by membrane. However, the origin of this membrane was unclear due to the absence of typical host cell membrane markers. Here, we analyzed several parasite- and host cell-derived membranes and show that the merosome membrane is of host cell origin. We also demonstrate that characteristic markers are lost from the host cell membrane once the parasite is liberated from its enclosure within the cell and moves freely in the host cell. The disappearance of membrane markers seems to be a consequence of the host cell death that is triggered toward the end of parasite development in the liver cell. The simultaneous induction of host cell death and retention of an intact host cell membrane enables the Plasmodium parasite to hide from the host immune system and thus to escape elimination before establishing a blood stage infection.