Noranate Nitchakarn, Prugnolle Franck, Jouin Hélène, Tall Adama, Marrama Laurence, Sokhna Cheikh, Ekala Marie-Thérèse, Guillotte Micheline, Bischoff Emmanuel, Bouchier Christiane, Patarapotikul Jintana, Ohashi Jun, Trape Jean-François, Rogier Christophe, Mercereau-Puijalon Odile
Institut Pasteur, Unité d'Immunologie Moléculaire des Parasites, CNRS URA 2581, 28 rue du Dr ROUX, 75724 Paris Cedex 15, France.
BMC Microbiol. 2009 Oct 15;9:219. doi: 10.1186/1471-2180-9-219.
Genetic evidence for diversifying selection identified the Merozoite Surface Protein1 block2 (PfMSP1 block2) as a putative target of protective immunity against Plasmodium falciparum. The locus displays three family types and one recombinant type, each with multiple allelic forms differing by single nucleotide polymorphism as well as sequence, copy number and arrangement variation of three amino acid repeats. The family-specific antibody responses observed in endemic settings support immune selection operating at the family level. However, the factors contributing to the large intra-family allelic diversity remain unclear. To address this question, population allelic polymorphism and sequence variant-specific antibody responses were studied in a single Senegalese rural community where malaria transmission is intense and perennial.
Family distribution showed no significant temporal fluctuation over the 10 y period surveyed. Sequencing of 358 PCR fragments identified 126 distinct alleles, including numerous novel alleles in each family and multiple novel alleles of recombinant types. The parasite population consisted in a large number of low frequency alleles, alongside one high-frequency and three intermediate frequency alleles. Population diversity tests supported positive selection at the family level, but showed no significant departure from neutrality when considering intra-family allelic sequence diversity and all families combined. Seroprevalence, analysed using biotinylated peptides displaying numerous sequence variants, was moderate and increased with age. Reactivity profiles were individual-specific, mapped to the family-specific flanking regions and to repeat sequences shared by numerous allelic forms within a family type. Seroreactivity to K1-, Mad20- and R033 families correlated with the relative family genotype distribution within the village. Antibody specificity remained unchanged with cumulated exposure to an increasingly large number of alleles.
The Pfmsp1 block2 locus presents a very large population sequence diversity. The lack of stable acquisition of novel antibody specificities despite exposure to novel allelic forms is reminiscent of clonal imprinting. The locus appears under antibody-mediated diversifying selection in a variable environment that maintains a balance between the various family types without selecting for sequence variant allelic forms. There is no evidence of positive selection for intra-family sequence diversity, consistent with the observed characteristics of the antibody response.
多样化选择的遗传证据表明，裂殖子表面蛋白1第二结构域（PfMSP1第二结构域）是恶性疟原虫保护性免疫的一个假定靶点。该基因座表现出三种家族类型和一种重组类型，每种类型都有多种等位基因形式，这些等位基因形式在单核苷酸多态性以及三个氨基酸重复序列的序列、拷贝数和排列变异方面存在差异。在流行地区观察到的家族特异性抗体反应支持在家族水平上起作用的免疫选择。然而，导致家族内等位基因多样性大的因素仍不清楚。为了解决这个问题，在疟疾传播强烈且常年存在的一个塞内加尔农村社区，研究了群体等位基因多态性和序列变异特异性抗体反应。
在调查的10年期间，家族分布没有明显的时间波动。对358个PCR片段进行测序，鉴定出126个不同的等位基因，包括每个家族中的许多新等位基因以及重组类型的多个新等位基因。寄生虫群体由大量低频等位基因以及一个高频和三个中频等位基因组成。群体多样性测试支持在家族水平上的正选择，但在考虑家族内等位基因序列多样性以及所有家族合并时，没有显示出与中性有显著偏离。使用展示多种序列变异的生物素化肽分析血清阳性率，结果为中等水平且随年龄增加。反应性谱是个体特异性的，映射到家族特异性侧翼区域以及家族类型内许多等位基因形式共享的重复序列。对K1、Mad20和R033家族的血清反应性与村内相对家族基因型分布相关。随着累积接触越来越多的等位基因，抗体特异性保持不变。
Pfmsp1第二结构域基因座呈现出非常大的群体序列多样性。尽管接触了新的等位基因形式，但缺乏新抗体特异性的稳定获得，这让人联想到克隆印记。在可变环境中，该基因座似乎处于抗体介导的多样化选择之下，这种环境在维持各种家族类型之间的平衡，而不选择序列变异等位基因形式。没有证据表明对家族内序列多样性进行正选择，这与观察到的抗体反应特征一致。
