Abstract
BackgroundThe evasion of host immune response by the human malaria parasite Plasmodium falciparum has been linked to expression of a range of variable antigens on the infected erythrocyte surface. Several genes are potentially involved in this process with the var, rif and stevor multigene families being the most likely candidates and coding for rapidly evolving proteins. The high sequence diversity of proteins encoded by these gene families may have evolved as an immune evasion strategy that enables the parasite to establish long lasting chronic infections. Previous findings have shown that the hypervariable region (HVR) of STEVOR has significant sequence diversity both within as well as across different P. falciparum lines. However, these studies did not address whether or not there are ancestral stevor that can be found in different parasites.MethodsDNA and RNA sequences analysis as well as phylogenetic approaches were used to analyse the stevor sequence repertoire and diversity in laboratory lines and Kilifi (Kenya) fresh isolates.ResultsConserved stevor genes were identified in different P. falciparum isolates from different global locations. Consistent with previous studies, the HVR of the stevor gene family was found to be highly divergent both within and between isolates. Importantly phylogenetic analysis shows some clustering of stevor sequences both within a single parasite clone as well as across different parasite isolates.ConclusionThis indicates that the ancestral P. falciparum parasite genome already contained multiple stevor genes that have subsequently diversified further within the different P. falciparum populations. It also confirms that STEVOR is under strong selection pressure.
Highlights
The evasion of host immune response by the human malaria parasite Plasmodium falciparum has been linked to expression of a range of variable antigens on the infected erythrocyte surface
Identification of common stevor genes in laboratory lines and Kilifi field isolates The P. falciparum genome sequencing project [2] as well as subsequent sequencing efforts has given some insight into the diversity of stevor in different P. falciparum lines
This is indicative of the amplification of multiple stevor sequences from both laboratory lines and field isolates
Summary
The evasion of host immune response by the human malaria parasite Plasmodium falciparum has been linked to expression of a range of variable antigens on the infected erythrocyte surface. Previous findings have shown that the hypervariable region (HVR) of STEVOR has significant sequence diversity both within as well as across different P. falciparum lines. These studies did not address whether or not there are ancestral stevor that can be found in different parasites. Rif as well as stevor have been described [2,3,4] as the major variant surface antigens (VSAs) These multigene families are predominantly situated at the sub-telomeric ends of chromosomes [2,5,6], where gene rearrangements are frequent [7,8] leading to high rates of recombination, facilitating their rapid evolution and diversity. The high sequence diversity of proteins encoded by these gene families may have evolved as an immune evasion strategy that enables the parasite to establish long lasting chronic infections [9]
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