Dissecting the complexity of human susceptibility to malaria: genetic approaches

There are many basic aspects of the immunology of Plasmodium falciparum infection that are not fully understood, thus hampering our understanding of how people become immune to malaria and of immune‐based pathogenesis. The understanding of immunity and susceptibility to malaria has been hindered by the complexity of parasite‐host interaction and by the inherent difficulty of distinguishing epiphenomena from events truly on the causative pathway, as well as protective from pathological responses. We discuss genetic approaches that are of great value for dissecting the complexity of immune responses to malaria in natura by providing new insights into molecular interactions between the parasite and the host. Genetics of susceptibility to malaria therefore can represent a complementary research tool to experimental immunology in vitro and in vivo. The work presented in this thesis had two major aims: I) to investigate the role of Interferon (IFN)‐γ signalling in susceptibility to malaria and II) to understand the biological basis of the low susceptibility to malaria shown by the Fula people of West Africa. In order to investigate the molecular mechanisms of protective immunity to malaria and pathogenesis regulated by IFN‐γ, we conducted genetic epidemiology association studies of complementary design to investigate the role of four candidate loci: IFNG, IFNGR1, IFNGR2 and IRF1. The most interesting findings concerned the IRF1 gene: we observed significant associations between common genetic variation at the IRF1 locus and the ability to control P. falciparum infection, both in healthy adult individuals and in children affected by uncomplicated and severe malaria. On the other hand, our studies did not provide evidence for a major role of this gene in determining susceptibility to severe disease. Furthermore, using the methodology of allele‐specific transcript quantification mapping, we obtained preliminary results suggesting the existence of a regulatory element(s) in the 5’ upstream region of the IRF1 locus. Thus, our current hypothesis is that IRF1 polymorphisms entail different abilities to control P. falciparum infection by affecting IRF1 gene expression and ultimately the production of inflammatory cytokines, but that they are not involved in immune‐based pathogenesis of severe disease. As a first step to understand the biological basis of the resistance to malaria shown by the Fula people of West Africa, we analysed HLA class II polymorphism to confirm previous data showing that the Fula from Burkina Faso are genetically differentiated from sympatric Mossi and Rimaibé. We then compared the expression profiles of healthy adults of Fula and Mossi ethnicity. Quantitative (QT)‐PCR analysis of Peripheral Blood Mononuclear Cells (PBMCs) isolated from Fula showed higher expression of several genes related to Th1 and Th2 function and reduced expression of two important genes related to immune tolerance: FOXP3 and CTLA4. Microarray analysis of CD4+CD25+ cells also revealed a lower expression of several genes affecting T regulatory activity such as FOXP3, CTLA4, TGFB and TGFBRs in the Fula. These results suggest a functional deficit of T regulatory cells (Tregs) in the Fula and identify key genes as good candidates for future genetic association studies.