Controlled human malaria infection with Plasmodium falciparum demonstrates impact of naturally acquired immunity on virulence gene expression

Abstract

The pathogenesis of Plasmodium falciparum malaria is linked to the variant surface antigen Pf EMP1, which mediates tethering of infected erythrocytes to the host endothelium and is encoded by approximately 60 var genes per parasite genome. Repeated episodes of malaria infection result in the gradual acquisition of protective antibodies against Pf EMP1 variants. The antibody repertoire is believed to provide a selective pressure driving the clonal expansion of parasites expressing unrecognized Pf EMP1 variants, however, due to the lack of experimental in vivo models there is only limited experimental evidence in support of this concept. To get insight into the impact of naturally acquired immunity on the expressed var gene repertoire early during infection we performed controlled human malaria infections of 20 adult African volunteers with life-long malaria exposure using aseptic, purified, cryopreserved P . falciparum sporozoites (Sanaria PfSPZ Challenge) and correlated serological data with var gene expression patterns from ex vivo parasites. Among the 10 African volunteers who developed patent infections, individuals with low antibody levels showed a steep rise in parasitemia accompanied by broad activation of multiple, predominantly subtelomeric var genes, similar to what we previously observed in naïve volunteers. In contrast, individuals with intermediate antibody levels developed asymptomatic infections and the ex vivo parasite populations expressed only few var gene variants, indicative of clonal selection. Importantly, in contrast to parasites from naïve volunteers, expression of var genes coding for endothelial protein C receptor (EPCR)-binding Pf EMP1 that are associated with severe childhood malaria was rarely detected in semi-immune adult African volunteers. Moreover, we followed var gene expression for up to six parasite replication cycles and demonstrated for the first time in vivo a shift in the dominant var gene variant. In conclusion, our data suggest that P . falciparum activates multiple subtelomeric var genes at the onset of blood stage infection facilitating rapid expansion of parasite clones which express Pf EMP1 variants unrecognized by the host’s immune system, thus promoting overall parasite survival in the face of host immunity.

Identification of a conserved var gene in different Plasmodium falciparum strains

“…This suggests a difference in PfEMP1 display between the two cell lines. Several investigations have reported that PfEMP1 expression is influenced by semi-immunity [20,[34][35][36] and Hoo et al [37] have shown recently that the duration of replication in the human host has a strong impact on the P. falciparum transcriptome. It is thus tempting to hypothesize that the prolonged intra-host replication of the MOA parasites selected for parasites with reduced PfEMP1 display to allow the submicroscopic parasitaemia of chronic infections.…”

<p>Immunoinformatics and Vaccine Development: An Overview</p>

“…Luckily, if the PfEMP1 protein is expressed for a long while; it comes under attack by the naturally acquired immunity. 98,99 In defence to this, the parasite has expanded the var genes coding for PfEMP1 such that the genes can exist as a polymorphic family of as much as over 50 members in every genomic haploid. Antigenic switches work well here in that members of the polymorphic family (also called antigenic-variant-protein family) can be interchanged and cannot express their proteins at the same time.…”

Host-Malaria Parasite Interactions and Impacts on Mutual Evolution

“…The P. falciparum parasite evades host immunity via mutually exclusive expression (e.g., only one PfEMP1 is expressed on the surface of an iRBC at a time) of the highly diverse var family representing an almost unlimited gene pool at the parasite population level (Su et al, 1995;Chen et al, 1998;Hviid and Jensen, 2015;Deitsch and Dzikowski, 2017). Switches of var gene expression have been demonstrated in infections of human volunteers (Peters et al, 2002;Bachmann et al, 2019). Several mechanisms to explain the mutually exclusive var expression have been reported, including: (1) cis-acting DNA elements (promoters, silencers, activators) and anti-sense RNA transcripts acting to silence or activate the var genes;…”

“…Interestingly, a vaccine may also alter parasite expression of variant antigen genes. Vaccination of 10 African volunteers with the PfSPZ vaccine showed parasites from individuals with intermediate antibody levels expressed only few var gene variants, whereas those with low antibody levels expressed a broad spectrum of multiple, predominantly subtelomeric var genes coding PfEMP1 binding to endothelial protein C receptor (EPCR) that is associated with severe childhood malaria (Bachmann et al, 2019). Therefore, vaccination certainly can impact parasite population dynamics, genetic diversity, and possibly virulence.…”