Understanding the Differences in Plasmodium Falciparum Genetic Diversity and Host Immune Profiles in Asymptomatic and Symptomatic Malaria Infections

Abstract

Malaria remains a serious infectious disease in the tropics whose pathogenesis is attributed to a complex interaction between parasites with high genetic diversity and robust host immune responses. In endemic regions, frequent exposure to malaria often leads to acquisition of antidisease immunity and asymptomatic infections that serve as silent natural reservoirs of infectious parasites thereby sustaining malaria transmission. Although modulation of host immunity has been implicated in the maintenance of chronic symptomless infections, it is still unclear how the host and malaria parasites interactions are involved. To understand this phenomenon, blood samples from 425 children (<15 years) enrolled in a cohort study in Kilifi County, between 2007 and 2019, were studied. Annual cross-sectional surveys conducted in the cohort provided samples from healthy/uninfected and asymptomatic children while weekly follow-ups identified children with symptomatic malaria. Parasite genetic diversity, using merozoite surface protein 2 as a genetic marker, and gene expression profiles as well as host immune responses were then studied by comparing samples from uninfected and/or asymptomatic children with those from subsequent symptomatic malaria infections. Asymptomatic infections had significantly lower parasite density, 800 parasites/μl compared to febrile malaria 28,800 parasites/μl (p < 0.0001) that mainly comprised of new parasite clones. They were also more polyclonal (>2 parasite clones per infection) and had a significantly higher complexity of infection (COI) of 2.3 compared to symptomatic malaria infections, 2.0 (p = 0.016). Interestingly, children harboring asymptomatic infections had a parasite transcriptional signature featuring a bias towards the trophozoite stage (~12 hours-post invasion), while febrile infections featured increased ring stage parasites (~ 9 hours-post invasion). The host response during febrile/symptomatic children featured increased upregulation of genes associated with inflammatory responses compared to asymptomatic children. Similarly, uninfected children showed upregulation of genes involved in inflammatory responses compared to asymptomatic children. Furthermore, the host-responses during symptomatic malaria infections that followed an asymptomatic infection featured a significant upregulation of genes related to inflammatory responses (TNFRSFIB, TLR4, CCR7, IL1B, IFNGR1), whereas the symptomatic host responses from previously uninfected children were characterized by increased upregulation of genes related to humoral/antibody related immune responses (IGHM, IKC, IGHG4, IGHG2, C5). The results suggest that asymptomatic children in Kilifi harbor highly diverse and polyclonal parasites whereas their subsequent symptomatic..................................