Malaria has exerted selective pressure on the human genome that has led to changes, such as haemoglobin S which confers some resistance against severe malaria . Understanding the mechanisms leading to increased susceptibility or resistance against the disease would assist in the elaboration of better tools to fight the disease. In this search, immune responses that are on the front of the defence are particularly investigated. Tregs are suggested to be central in the control of P. falciparum infection. Therefore, investigation of human polymorphisms in genes associated with Treg activity may underlie both susceptibility to infection and level of Treg expression.
The present study evaluated the contribution of promoter SNPs of FOXP3, IL10RA, IL10RB, STAT6 and TNFRSF18 genes in P. falciparum infection outcome among Congolese children. Earlier studies have identified some regulatory SNPs on the promoter loci of these five genes from 40 unrelated Gabonese individuals who had earlier infection episodes with various parasites, including P. falciparum[14, 19–21]. These SNPs were then validated for their allelic gene expression using transient transfection assays. Some of the identified were already pre-described, while others were newly discovered. All the SNPs selected for the current investigation were found to alter expression of their respective gene [14, 19–21]. Therefore any possible correlations of these SNP variants with resistance or susceptibility to mild malaria attacks and/or the control of P. falciparum infection levels were evaluated.
The results obtained in this study demonstrated that the STAT6 rs3024944C/C genotype was significantly more prevalent in asymptomatic children. This suggests the contribution of this genotype to protection of Congolese children against mild clinical malaria. The SNP rs3024944 is located in a transcription factor-binding site TCF-1 which is a member of family of genes with homology to high mobility group I (HMG) proteins . TCF-1, in part with β-catenin, plays a pivotal role in T cell activation and is specifically expressed in T lymphocytes [25, 26]. And the fact that in the uncomplicated malaria group, children with the minor allele rs3024944C have low parasite densities compared to those having the major allele rs3024944G, that speculates that the rs3024944C/C variant directs activation of T cells in response to P. falciparum infection, thus contributing to protection against the disease through parasite elimination. In haplotype level, the major haplotype CGG was found to be correlated with the marginal risk of uncomplicated malaria. This emphasizes the role of the SNP rs3024944 on susceptibly/resistance to uncomplicated malaria among Congolese children. This is the first study that shows the correlation between a promoter SNP STAT6 rs3024944 and protection against uncomplicated malaria. However, in previous investigations, the promoter SNP rs324013 was found to be associated with the control of S. haematobium infection levels in a Dogon population in Mali . Similarly, the variant rs324015 was associated to the control of A. lumbricoides infection levels in Chinese children .
Two novel promoter SNPs were identified including, ss491228441 and ss491228440 of IL10RA and TNFRSF18 respectively. In both cases the frequency of the minor allele was low. Meanwhile the minor allele ss491228441G of IL10RA appeared to contribute to protection against uncomplicated malaria. Interestingly, the haplotype CTG involving this minor allele ss491228441G was significantly more present in asymptomatic children. This provides an additional evidence of the involvement of this allele in protection against uncomplicated clinical malaria. However, larger association studies in different malaria endemic areas including children and adults as well as functional studies of this ss491228441 SNP variant would be of great interest to verify the current observations.
The data also showed the association between the carriage of the FOXP3 rs11091253 SNP variant and P. falciparum parasite density levels. In particular, the rs11091253CT genotype was found to be associated with high infection levels, whereas the homozygous for the minor allele, the rs11091253TT genotype, was associated with lower infection levels. This suggests that FOXP3 contributes to the modulation of the immune response against P. falciparum infection. The SNP rs11091253 is located in a putative binding site for the transcription factors nuclear factor kappa B (NF-κB) and c-Rel, which is also a member of NF-κB family. These transcription factors have been demonstrated to be critical for regulation and transcription of FOXP3[27, 28]. Therefore the differential binding properties of the promoter carrying the rs11091253CT genotype and that of the promoter carrying the rs11091253TT genotype may explain the associations observed in this study. One possible explanation may be that the binding of NF-κB and/or c-Rel with the promoter carrying the rs11091253C/T genotype results in upregulation of FOXP3 which in turn leads to increased number of fully functional Tregs. Consequently, parasite multiplication is facilitated by suppression of either P. falciparum-responsive effector cells or pro-inflammatory cytokine response. This is in line with what was reported by previous study showing that upregulation of FOXP3, and CD4+CD25+ regulatory cells correlates with more rapid parasite growth in human malaria infection .