Structure function studies of the Plasmodium berghei circumsporozoite protein repeat region

Malaria is caused by protozoan parasites of the genus, Plasmodium, and is responsible for the death of 400,000 children every year. Infection results from an infected mosquito bite, which transmits the sporozoite stage of Plasmodium into the skin, where the sporozoite migrates until finding a blood vessel which it actively enters to use the blood circulation to the liver and establish infection. Circumsporozoite protein (CSP) is a multi-functional protein that covers the sporozoite surface and is composed of three domains: an N-terminal domain, central repeat region, and a thrombospondin repeat type-1 (TSR) domain. Structure and/or function studies have elucidated some functionalities of the N-terminus and TSR domains, but no function has been described for the repeat region linking these two terminal domains. This is particularly surprising as the repeat region of CSP is the antibody target of the only malaria vaccine that has shown efficacy in Phase III clinical trials, RTS,S. We sought to elucidate the function of the CSP repeat region through mutagenesis in the rodent model system, Plasmodium berghei. We replaced the endogenous csp locus with csp repeat mutations or deletions including: complete deletion of the repeat region, trunctations of the repeats, charge modifications, and a scrambled repeat sequence. Through these mutants we found that the repeats play a critical role early in sporozoite development and motility, which is critical for infection of the liver. These studies are the first to show that the CSP repeat region has a functional role during sporozoite infection, thus expanding our knowledge of this promising vaccine target