BACKGROUND Vaccines are the most reliable alternative to elicit sterile immunity against malaria but their development has been hindered by polymorphisms and strain-specificity in previously studied antigens. New vaccine candidates are therefore urgently needed. Highly conserved Plasmodium falciparum reticulocyte-binding protein homologue-5 (PfRH5) has been identified as a potential candidate for anti-disease vaccine development. PfRH5 is essential for erythrocyte invasion by merozoites and crucial for parasite survival. However, there is paucity of data on the extent of genetic variations on PfRH5 in field isolates of Plasmodium falciparum. This study described genetic polymorphisms at the high affinity binding polypeptides (HABPs) 36718, 36727, 36728 of PfRH5 in Nigerian isolates of P. falciparum. This study tested the hypothesis that only specific conserved B and T cell epitopes on PfRH5 HABPs are crucial for vaccine development. METHODS One hundred and ninety-five microscopically confirmed P. falciparum samples collected in a prospective cross-sectional study of three different populations in Lagos, Nigeria. Genetic diversity and haplotype construct of Pfrh5 gene were determined using bi-directional sequencing approach. Tajima's D and the ratio of nonsynonymous vs synonymous mutations were utilized to estimate the extent of balancing and directional selection in the pfrh5 gene. RESULTS Sequence analysis revealed three haplotypes of PfRH5 with negative Tajima's D and dN/dS value of - 1.717 and 0.011 ± 0.020, respectively. A single nucleotide polymorphism, SNP (G → A) at position 608 was observed, which resulted in a change of the amino acid cysteine at position 203 to tyrosine. Haplotype and nucleotide diversities were 0.318 ± 0.016 and 0.0046 ± 0.0001 while inter-population genetic differentiation ranged from 0.007 to 0.037. Five polypeptide variants were identified, the most frequent being KTKYH with a frequency of 51.3%. One B-cell epitope, 151 major histocompatibility complex (MHC) class II T-cell epitopes, four intrinsically unstructured regions (IURs) and six MHC class I T-cell epitopes were observed in the study. Phylogenetic analysis of the sequences showed clustering and evidence of evolutionary relationship with 3D7, PAS-2 and FCB-2 RH5 sequences. CONCLUSIONS This study has revealed low level of genetic polymorphisms in PfRH5 antigen with B- and T-cell epitopes in intrinsically unstructured regions along the PfRH5 gene in Lagos, Nigeria. A broader investigation is however required in other parts of the country to support the possible inclusion of PfRH5 in a cross-protective multi-component vaccine.

中文翻译：

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尼日利亚拉各斯人群中疟疾疫苗候选恶性疟原虫网状细胞结合蛋白同系物5的遗传多态性。

背景技术疫苗是引发针对疟疾的无菌免疫的最可靠的替代方法，但是它们的发展已被先前研究的抗原的多态性和菌株特异性所阻碍。因此，迫切需要新的候选疫苗。高度保守的恶性疟原虫网织细胞结合蛋白同源物5（PfRH5）已被确定为抗疾病疫苗开发的潜在候选者。PfRH5对于裂殖子入侵红细胞至关重要，对寄生虫的生存至关重要。但是，关于恶性疟原虫田间分离株中PfRH5的遗传变异程度的数据很少。这项研究描述了尼日利亚恶性疟原虫分离株中PfRH5的高亲和力结合多肽（HABP）36718、36727、36728的遗传多态性。这项研究检验了以下假设：PfRH5 HABP上只有特定的保守B和T细胞表位对疫苗开发至关重要。方法在尼日利亚拉各斯的三个不同人群的前瞻性横断面研究中，收集了一百九十五个显微镜确认的恶性疟原虫样品。使用双向测序方法确定了Pfrh5基因的遗传多样性和单倍型构建。Tajima的D和非同义与同义突变的比率被用来估计pfrh5基因的平衡和方向选择的程度。结果序列分析显示三种PfRH5单倍型，田岛的D和dN / dS值为负，分别为-1.717和0.011±0.020。在608位观察到单核苷酸多态性SNP（G→A），导致203位的氨基酸半胱氨酸变为酪氨酸。单倍型和核苷酸多样性分别为0.318±0.016和0.0046±0.0001，而群体间遗传分化范围为0.007至0.037。鉴定出五个多肽变体，最常见的是KTKYH，频率为51.3％。在研究中观察到一个B细胞表位，151个主要组织相容性复合体（MHC）II类T细胞表位，四个内在非结构化区域（IUR）和六个MHC I类T细胞表位。序列的系统发生分析显示与3D7，PAS-2和FCB-2 RH5序列发生聚类并证明进化关系。结论这项研究揭示了PfRH5抗原的遗传多态性水平较低，在拉各斯沿PfRH5基因的内在非结构化区域具有B细胞和T细胞表位，尼日利亚。但是，该国其他地区需要进行更广泛的调查，以支持将PfRH5包含在交叉保护性多组分疫苗中。