Parasite-filled merosomes (yellow) move from an infected liver cell (pink) into a blood vessel full of red blood cells but avoid being 'eaten' by white blood cells of the immune system (blue).

Credit: Science

Related articles

• Malaria vaccine announced
• Genetically engineered mosquito resistant to malaria
• Girl switches blood type after liver transplant
• Scientists grow human liver
• Sydney harbours missing malaria link

BEIJING, 5 August 2006 - Just how malaria parasites avoid the human immune system when they move from the liver and into red blood cells - a journey that culminates in the blood cells bursting, causing chills and fever - has finally been unmasked by Germans researchers.

The finding could help researchers develop ways of stopping the parasite in its tracks. The study, led by Volker Heussler of the Bernhard Nocht Institute for Tropical Medicine in Hamburg, Germany, was published online section of the U.S. journal, Science, on August 4.

After arriving in a human body when an infected mosquito bites, malaria parasites head to the liver. Here, they change into a new form that can infect red blood cells, and begin to reproduce.

But how the parasites evade the immune system while travelling from the liver to the blood has eluded scientists until now.

Heussler's team tracked the parasite's migration from the liver in infected mice using a method called intra-vital imaging. This involved using a laser scanning microscope to detect parasites that had been genetically modified to produce a fluorescent protein.

The researchers found that the parasites kill the liver cell they occupy and make it detach from its neighbours. The infected cells then squeeze through tiny gaps in the walls of blood vessels in the liver.

As they do this, the cells break up into smaller cell-like structures called merosomes, each full of malaria parasites.

The parasites avoid being engulfed by the white blood cells of the immune system by accumulating calcium ions released from within the host cell. This blocks, on the merosomes' outer surface, the appearance of a protein that would normally act as an 'eat me' signal for the immune cells.

Heussler said his team had observed the same process in human liver cells in the laboratory.

He added that interfering with the enzymes the malaria parasites use to control the death and movement of the host cell could be a way of stopping the migration from the liver to blood.