Investigating the evolutionary link between malaria and autoimmunity: a large scale immunogenetic study in two West African populations.

AIM. Despite equivalent exposure to infection and comparable use of protective measures, the Fulani of West Africa have been shown to mount stronger immune responses to Plasmodium falciparum antigens and to be less susceptible to infection and mild disease than sympatric populations (Modiano et al. 1996, PNAS). The Fulani also show a higher response to other pathogens, and both their Th1 and Th2 responses are enhanced, suggesting that their resistance to malaria could result from a generally stronger immune activation. Key genes related to T regulatory cell function are indeed down-regulated in the Fulani (Torcia et al. 2008 PNAS). This disorder of immune homeostasis could be driven by genetic factors positively selected by P. falciparum and may underlie the higher susceptibility of the Fulani to diseases with autoimmune pathogenesis reported in the literature (Fish et al. 1987, Diabetologia; Mahe et al. 1996, Br J Dermatol; Brieger et al. 1997, Trop Med Int Health). The general aim of the proposed investigation is to explore the genetic basis of the lower susceptibility to malaria observed in the Fulani, and in particular to evaluate the role of autoimmunity loci. MATERIALS AND METHODS. To investigate this hypothesis, we conducted a large-scale epidemiological study in rural villages of Burkina Faso inhabited by Fulani, Mossi and Rimaibe communities. The field study lasted 2 years (2007-8) and consisted in a combination of cross sectional and longitudinal surveys. At each survey we collected parasitological (P. falciparum index and parasite density), clinical (fever, anemia, spleen size) and serological data (IgG levels against P. falciparum and self antigens). We genotyped 363 Single Nucleotide Polymorphisms (SNPs) on 2186 samples using the Sequenom System, based on allele-specific primer extension and MALDI-TOF Mass Spectrometry. SNPs included polymorphisms previously shown to be involved in resistance to severe malaria, in resistance to infection and/or in antibody production, as well as polymorphisms at autoimmunity loci. We conducted population genetic analyses and genetic association analysis with parasitological, clinical and serological phenotypes using the free software package R. RESULTS. Principal component analysis revealed that Mossi and Rimaibe (Non-Fulani) are not genetically distinct among themselves, whereas the Fulani are a clearly distinct group, in agreement with data obtained on HLA class I-II alleles (Modiano et al. 2001, Tissue Antigens; Lulli et al. 2009, Hum Immunol). We therefore compared allele frequencies and calculated Fst, a measure of population genetic differentiation, between Fulani and Non-Fulani. We observed that the proportion of autoimmunity SNPs with Fst>0.05 (indicating moderate/high differentiation and corresponding to at least a two-fold difference in allele frequency) is 20%, versus 10% shown by other loci (p=0.03). Genetic association analysis of susceptibility to infection and infection levels showed association signals among genes involved in resistance to severe malaria (TNF, DDC, ABO, IFNG-IL22, GNAS, MECP2, G6PD). Furthermore we observed strong signals of association, both in Fulani and Non-Fulani, in the 5q31 region of the genome, which has been previously linked to P. falciparum infection levels (Rihet et al. 1998, Am J Hum Genet; Mangano et al. 2008, Genes Immun). Finally, association signals were also observed among genes related to T regulatory cell function and/or involved in autoimmunity (TGFBR3, CD25, FCGR2A, CR1, IL1R1L-IL18RAP, IL1A-IL1B, IL21, BLK, ORMDL3, TGFB1). CONCLUSIONS. The results of our investigation support the hypothesis that malaria has exerted a selective pressure on the immune system and has affected is evolution, and provide evidence that common gene regulatory networks could underlie susceptibility to malaria and to immunological disorders such as autoimmune diseases.