Medical Biology - Theses
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ItemPlasmodium chabaudi adami: vaccine antigens and antigenic variation( 2003)There is an abundance of information available on the molecular mechanisms of antigenic variation in Plasmodium falciparum. The variant antigen PfEMP1, which mediates antigenic variation as well as cytoadherence and rosetting, has been extensively characterised. Genes coding for the antigen belong to the gene family var, and several var genes have been cloned and characterised. The rodent malaria parasite P. chabaudi is a widely studied in vivo model for P. falciparum. The P. c. chabaudi AS parasite strain has been shown to exhibit antigenic variation and the variant antigen has been detected by surface fluorescence. As with P. falciparum, there is a link between antigenic variation and cytoadherence, however genes coding for the variant antigen in P. chabaudi have not been cloned to date. Therefore, potentially useful in vivo experiments on antigenic variation are restricted. In this thesis it is shown for the first time that the P. c. adami DS parasite strain also exhibits antigenic variation. Chapter 3 describes efforts to locate genes coding for variant antigens in P. c. adami DS. The main strategy involved a genome survey, by sequencing and analysing randomly selected clones from a P. c. adami DS genomic library. DNA sequences were compared to Plasmodium spp. sequence databases to look for similarity to var genes or other genes encoding variant antigens. Of the 297 clones analysed none had significant sequence similarity to genes coding for variant antigens. However, in a small proportion of sequences some similarity to var genes was noted. Several genes of potential interest were identified, most importantly the gene coding for the vaccine candidate rhoptry associated protein 1 (RAP1), which was subsequently cloned and characterised. Further attempts to locate var gene homologues in P. c. adami involved amplification of P. c. adami genomic DNA using degenerate oligonucleotide primers corresponding to conserved regions of var genes. This strategy proved to be unsuccessful, most likely due to lack of sequence similarity between P. falciparum and P. c. adami genes. In several vaccination studies with the apical membrane antigen 1 (AMA1) of P. c. adami DS, mice were significantly protected against homologous parasite challenge. However, some mice developed late, low-level breakthrough parasitaemias. In Chapter 4, the characterisation of two such breakthrough parasitaemias is described. The ama1 genes of the breakthrough parasites were found to be identical to the ama1 gene of the parental parasites. Similarly, no alteration in AMA1 expression was observed. However, the breakthrough parasites were found to be more resistant than the parental parasites to the effects of passive immunisation with rabbit antisera to AMA1, RAP1 and possibly also MSP119. P. chabaudi infections in mice have been previously shown to consist of a primary parasitaemia followed by a short period of subpatency, and a recrudescent parasitaemia. In surface immunofluorescence studi Chapter 4 describes similar surface immunofluorescence assays carried out with P. c. adami infected erythrocytes, and quantitation of fluorescence by flow cytometry. As with P. c. chabaudi, the recrudescent parasites were found to be antigenically distinct from the primary parasitaemia, indicating that antigenic variation had taken place. Because breakthrough parasites from the AMA1 vaccination trial were similar to recrudescences in peak and duration, we hypothesised that breakthrough parasitaemias, like recrudescent parasitaemias, occur as a result of antigenic variation. In Chapter 4 it was shown by surface immunofluorescence and flow cytometry using hyperimmune sera raised against different parasite populations, that breakthrough parasites express antigens on the surface of late trophozoite- and schizont infected erythrocytes that differ from those expressed by the parental and recrudescent parasites. These results support the hypothesis that switching of the variant antigen on the infected erythrocyte surface enables parasites to evade protective antibody responses directed against merozoite antigens. Chapter 5 describes the cloning and characterisation of P. c. adami RAP1 which was identified in the process of the genomic survey described in Chapter 3, as well as P. berghei RAP1. Both rodent parasite orthologues of RAP1 were found to have 30% sequence similarity to P. falciparum RAP1, and 6 of 8 cysteines were conserved in the rodent parasite orthologues. However the three polypeptides vary significantly in size. P. c. adami RAP1 and P. berghei RAP1 consist of 691 aa and 604 aa respectively, whereas P. falciparum RAP1 consists of 783 aa residues. These size differences reflect very different N-terminal sequences prior to the first cysteine, whereas the cysteine-rich C-terminal regions are more conserved. Both P. falciparum RAP1 and P. c. adami RAP1 contain N-terminal repeats, however they bear no sequence similarity to each other. P. berghei RAP1 lacks N-terminal sequence repeats that are characteristic of P. falciparum and P. c. adami RAP1. The large cysteine-rich C-terminal region P. c. adami RAP1 (PcRAP1 C3) was expressed in E. coli as a hexa-his fusion protein. Rabbit antiserum to recombinant PcRAP1 C3 was used to characterise the expression and sub-cellular localisation of the RAP1 antigen. P. c. adami RAP1 was found to have a Mr of approximately 80,000 and was shown by immunofluorescence to localise to the merozoite rhoptries. Passive immunisation of mice with rabbit anti-RAP1 serum was shown to protect against fulminant parasitaemia and mortality. In a mouse vaccination trial using the recombinant PcRAP1 C3 polypeptide partial protection was conferred against homologous parasite challenge.