Human host immune response to malaria infection is triggered by the innate immune system that shapes adaptive immunity. Toll-like receptors (TLRs), one of the pattern recognition receptors (PRRs) of the innate immune system, recognize several components of Plasmodium sp., described as pathogen associated molecular patterns (PAMPs)1 .
Glycosylphosphatidylinositol-anchored (GPI), merozoite surface proteins (MSP1 and MSP2), microparticles, hemozoin, and parasite deoxyribonucleic acid (DNA) are TLR ligands. TLR-2, TLR-4, and TLR-9 are responsible for the recognition of these components to trigger a cascade of reactions leading to the liberation of the transcription factor NF-κB in the cytoplasm to enter the nucleus and transcribe inflammatory mediators2 .
The immune system has been shaped by the process of evolution into an efficient system that allows the coexistence of pathogenic agents in hosts while controlling infections3. Knockout murine models of genes involved in the innate immune response have shown the key role played by TLRs in Plasmodium spp. infection4 .
In humans, Plasmodium infection results in a wide spectrum of clinical manifestations that depend mostly on the interaction between the vector-parasite-host, associated environmental factors, parasite load, immunological status, and host genetic background (Figure 1)5 .
FIGURE 1: Venn diagram of the characteristics involved in pathogenesis and severe clinical manifestations of the Plasmodium sp.infection.
Several single nucleotide polymorphisms (SNPs) in the TLR genes are associated with infectious diseases. Some of these single base pair changes, called point mutations, are non-synonymous, causing an amino acid exchange in the gene and others in the promoter regions influence the expression of the receptors6 .
The study by Dhangadamajhi et al.7 published in this issue of the Journal of the Brazilian Society of Tropical Medicine entitled “The meta-analysis of TLR4 and TLR9 SNPs implicated in severe malaria” highlights the importance of the SNPs in TLR4 (Asp299Gly) and TLR9 (-1237C/T and -1486C/T) in malaria. This meta-analysis study suggests the implications of these TLR polymorphisms on the pathogenesis of malaria and the association with severe clinical manifestations of the disease.
Several studies have reported controversial results about the association of these genetic variants with Plasmodium falciparum infection. Here, the authors point out that the differences in the design of the studies, ethnic structure of the population, and methodologies applied make it difficult to compare the studies. This holds truth with respect to the study design and population ethnicities. However, allelic discrimination by any genotyping technology should not influence the allele frequencies observed in any study, especially in research laboratories. As suggested by the authors, selecting healthy controls for case-control studies in genetic infectious diseases is always problematic. However, asymptomatic individuals infected with the parasite will always be the best control, especially when they have never experienced any clinical manifestation.
The authors noted an association of the variants -1486C/T and -1237C/T of TLR9 with severe malaria by using conventional and stratified analysis models for a total of 665 patients with malaria and 1,187 individuals with uncomplicated malaria. Only five studies were included, two from India and three from Africa. This meta-analysis study has its merits; however, the sample size is still quite small. Unfortunately, in the Indian studies, both variants of TLR9 are not in Hardy-Weinberg equilibrium and this may skew any association.
In Brazil, polymorphisms in TLR4 and TLR9 are described to be associated with malaria susceptibility and with increased parasitemia in P. falciparum infection or mixed infections8,9,10. These studies are not included in the meta-analysis presented in this issue. However, these studies from Latin America are important as they provide the allelic frequencies of each SNP in addition to their associations with parasitemia.
Altogether, this meta-analysis study indicates the implications of these TLR SNPs, which may be in part responsible for inter-individual variation in severe clinical manifestation of the disease. Studies of these SNPs using a large sample size of individuals with uncomplicated, asymptomatic, or severe malaria may contribute to the development and practical application of an effective vaccine against the parasite and in the design of tools for disease control and elimination.
