Sequence diversity of the merozoite surface protein 1 of Plasmodium falciparum in clinical isolates from the Kilombero District, Tanzania

Merozoite surface protein 1 of Plasmodium falciparum (PfMSP-1) is regarded as a key candidate antigen for malaria vaccine development. It exhibits sig nificant antigenic polymorphism and has been divided into 17 building block s based on the analysis of sequence diversity. Differences in the antigenic composition of PfMSP-1 in local P. falciparum populations may result in di fferences in the efficacy of vaccines, which contain sequences of particula r allelic variant(s) of PfMSP-1. To contribute to the required knowledge of genetic diversity of malaria parasites in geographically diverse regions, we have used the polymerase chain reaction (PCR) to analyze the sequence di versity of blocks 1-4 of PfMSP-1 in disease isolates from the Kilombero Dis trict in Tanzania. In the semi-conserved block 1, in which dimorphic amino acid variances have been described at three positions, we found three of th e five previously described combinations of these three pairs of amino acid s. In addition one combination was found, which has not been reported befor e in parasite isolates from different locations worldwide. Of the two seque nce variants, which were dominating, one (S-44-Q(47)-V-52) corresponded to the 83.1 sequence incorporated into the SPf66 malaria peptide vaccine, whil e the other one (G(44)-H-47-I-52) differed from the previous in all three d imorphic amino acids. The partial protection observed in a phase III SPf66 trial conducted in the Kilombero District in children aged 1-5, thus does n ot seem to be associated with a clear dominance of favourable variants of b lock 1 of PfMSP-1 in this area. All three different principle types of bloc k 2, the major polymorphic region of PfMSP-1, were found in the Tanzanian i solates. Most of the sequences contained K1-type tripeptide repeats, but cl ones with MAD20-type repeats or no repetitive sequence (RO33-type block 2) were also present. K1- and MAD20-type tripeptide repeat motifs were never m ixed within one parasite clone. In one sequence a hexapeptide repeat was fo und at the end of block 2, which has not been reported before. Dimorphism i n 13 of the 17 previously described variable positions of the semi-conserve d block 3 and three of four recombination types of block 4 (K/K, M/K and M/ M) were found among the Tanzanian isolates. Apart from previously described dimorphic amino acid positions, polymorphism was rare in the non-repeated building blocks. Selection and spreading of parasite variants, which contai n amino acid exchanges at other than the dimorphic positions thus, is not a common event. Parasite isolates frequently harboured more than one PfMSP-1 allele. Three of the four heterogeneous isolates analysed contained two di fferent general types of sequences. One isolate contained at least four dis tinct clones,demonstrating the high endemicity of malaria in the Kilombero District, which isa well-established site for malaria vaccine field trials. (C) 2000 Elsevier Science B.V. All rights reserved.