Modulation of host immunity to malaria by Schistosoma haematobium: observational and experimental investigations

Malaria represents a major public health problem, with 247 million clinical cases and 619000 deaths reported in 2021, the great majority of which occurring in children and pregnant women in Sub-Saharan African countries and caused by Plasmodium falciparum (World Malaria Report 2022). In these countries, diseases caused by helminths such as intestinal nematodes, lymphatic filariae and schistosomes are also endemic but often neglected. Although few studies suggest that co-infection with helminths might modulate the immune response towards intracellular pathogens including Plasmodium (Salgame et al. 2013), conclusive evidence is not available. In particular, little is known on the immunomodulatory role of Schistosoma haematobium. The major aim of this thesis was to therefore investigate the impact of S. haematobium infection on immunity to malaria combining in natura, in vitro and in vivo approaches. An observational study in natura was conducted in human populations living in rural villages from Burkina Faso, who had been previously shown to have different susceptibility to P. falciparum malaria, and where S. haematobium is also endemic. Indirect (anti-SEA and anti-SWAP IgM and IgG antibodies) and direct (Circulating Anodic Antigen and circulating DNA) markers of S. haematobium infection were measured in plasma samples collected at baseline of a repeated cross-sectional malariological study. As expected, the seroprevalence of specific antibodies (45-72%) was much higher than that of either S. haematobium circulating antigen (28%) or DNA (26%). In the multivariate logistic regression analysis of demographic factors that might affect the prevalence of S. haematobium infection, all markers showed a clear increase in seroprevalence from children 1-4 years old to 5-9 years old, with a maximum in the age group 10-19 years old and a decrease at older ages. No clear differences were observed between sexes, villages, or ethnicities. In the comparison between cDNA and CAA testing, overall agreement was 85%, with cDNA showing a 73% sensitivity and a 94% specificity compared to CAA; however, the use of a composite reference standard where a sample is considered positive when either one or both direct markers are detected can be envisaged. In the comparison between S. haematobium specific antibodies and CAA testing, anti-SEA IgG showed the best diagnostic performance. The association between CAA, as a validated marker of S. haematobium infection, and the prospective risk of P. falciparum malaria was assessed in the study populations by multivariate regression analysis adjusted for the effect of age, sex, village, ethnicity, and hemoglobin genotype. Results suggest that infection with S. haematobium, or higher infection intensity with this trematode, increases susceptibility to P. falciparum malaria, in line with those of a systematic review and meta-analysis (Degarege et al., 2016). An experimental study has been conducted in vitro to evaluate the immunomodulatory properties of S. haematobium candidate antigens tetraspanins (TSP) 2, 6 and 23 on cultured Dendritic and T CD4+ Cells from human donors. Results of flow cytometry show that the three tetraspanins were able to induce the expression of DCs maturation markers HLA-DR, CD80 and CD86. Results of Luminex assay show that the three tetraspanins were able to induce the production of inflammatory cytokines IL6, TNFalpha, IL12p70 and IL33, as well as of the Th2 cytokine IL13 and the regulatory cytokine IL10. Finally, results of RT quantitative PCR show that the supernatants of tetraspanin-stimulated DCs were able to induce changes in cytokine gene expression in T CD4+ cells, with different pattern depending on the tetraspanin. Such interesting results deserve confirmation in a larger number of donors. A second experimental study has been conducted in vivo to evaluate the impact of exposure to the known S. haematobium immunomodulatory antigen HIPSE on the course of P. berghei infection in CD1 mice. Results of two experiments using a parasite clone expressing green fluorescent protein and flow cytometry show a slower growth of parasite density in HIPSE treated versus control mice, although with inconclusive statistical evidence. A second round of experiments including a dose-response component are needed to demonstrate an impact of HIPSE treatment in reducing P. berghei parasite density in mice, and therefore a role of this antigen in modulating anti-malarial immune responses. Novel insights have been provided into the diagnostic value of different plasma biomarkers, into the ability of S. haematobium antigens to modulate the host immune response and thereby to affect immunity to malaria, and into the impact of S. haematobium infection to affect the prospective risk of P. falciparum malaria. Further investigations are needed to confirm the results of this thesis and to characterise immunomodulatory antigens and host responses. Such kind of knowledge has potential for application in products development (drugs and vaccines) and in malaria control. For instance, anti-helminthic mass treatment campaigns in the period preceding the malaria transmission season could be deployed to decrease the incidence of cases of infection/disease, or before malaria vaccination to increase the immune response to the vaccine itself and therefore its effectiveness.