Parasite-derived variant surface antigens (VSA) expressed on the infected erythrocyte surface are important targets of naturally acquired protective immune responses against malaria. The three major VSA-encoding multigene families of Plasmodium falciparum, var, rif and stevor, exhibit high inter- and intra-strain genomic variability. The VSA component encoded by the var gene family, PfEMP1, comprises the major cytoadhesive ligand and the major antigenic target of protective antibodies. However, other VSA families, including those encoded by the rif and stevor multigene families, may also play important roles in malaria pathogenesis and immunity. However, the biological relevance of non-PfEMP1 VSA remains largely unknown.
This thesis represents the first extensive analysis of sequence diversity and expression patterns of rif and stevor variant gene families in African field isolates of P. falciparum. The work details the characterization of rif and stevor gene repertoires of P. falciparum parasites from diverse geographical locations and identification of conserved genes in both P. falciparum and the related chimpanzee malaria parasite P. reichenowi.
Both capillary sequencing and clone-free, 454 deep sequencing methods have been used to study changes in variant gene expression during asexual development of wild and culture-adapted isolates. Isolate- and stage-specific transcription patterns of rif and stevor genes were observed, and the major sets of transcripts in multiple isolates, including parasites that have been selected for different cytoadhesive phenotypes (resetting and adhesion to human brain endothelial cell lines) identified.
The role of RIFIN antigens in the natural acquisition of antibodies to malaria was tested using a strain transcendent variant PF3D7_1253700 (PFL2585c). The hypervariable region of this RIFIN, expressed as a recombinant protein was used to purify naturally acquired human antibodies from sera from malaria-immune African adults. These antibodies recognized native RIFIN antigen on the surface of intact trophozoites showing that RIFINs for additional targets of naturally acquired antibodies that recognize the surface of parasite-infected red blood cells
