Sure shot!

How the malaria parasite takes out an 'insurance policy' to deal with the immune system

Published: Monday 15 November 1999

when the malaria parasite multiplies, each of the replica may contain a different protein signature to prevent the immune system from wiping out all the them. This is tantamount to an insurance policy taken out by the parasite. The consequence would be an increased capacity for invasiveness and a continuing ability to maintain an infection for a long period, while the immune system keeps battling against one or the other variant separately. These are the findings of scientists at the National Institute of Medical Research in London, uk ( Nature , Vol 398, p618-622).

Malaria is caused by parasites of the group Plasmodium , which multiply in the blood of an infected animal. In the first step of infection, the parasite, known as merozoite, gets into the red blood cell of the host. The merozoite then changes shape, acquiring a ring-like appearance, and is known as tropozoite. Each tropozoite undergoes maturation within the red blood cell to give rise to what is called a schizont, which divides repeatedly to create 6 to 32 merozoites through a phenomenon called schizogony. The merozoites look for a new cell, and the cycle goes on.

The schizont reproduces asexually, meaning that all 32 merozoites are expected to be genetically identical. To facilitate a successful invasion, merozoites have complex organs at their tips called rhoptries -- sensors that help a merozoite recognise a red blood cell prior to entering it. It can be assumed that interfering with rhoptry will interfere with the invasion. Thus, a successful immune response against the rhoptry is all that the host needs to do to prevent the invasion. Clearly, it is in the interest of the parasite to do what it can to evade the immune system of the host.

The researchers at London have found out how the malarial parasite tricks the immune system from recognising the proteins in its rhoptry. The parasite is Plasmodium yoelii , which afflicts a small African rodent. The study focused on a group of proteins collectively known as p235 , which are localised in the rhoptry, and are encoded by a large number of genes.

The scientists found that even in a single schizont, each resulting merozoite contains the product of a different p235 gene in its rhoptry. In other words, each rhoptry has a different 'protein signature' from the other; an immune response directed against one rhoptry will remain restricted to that one alone and will be relatively ineffective against another.

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