The exception that proves the rule: Virulence gene expression at the onset of Plasmodium falciparum blood stage infections

Controlled human malaria infections (CHMI) are a valuable tool to study parasite gene expression in vivo under defined conditions. In previous studies, virulence gene expression was analyzed in samples from volunteers infected with the Plasmodium falciparum (Pf) NF54 isolate, which is of African origin. Here, we provide an in-depth investigation of parasite virulence gene expression in malaria-naive European volunteers undergoing CHMI with the genetically distinct Pf 7G8 clone, originating in Brazil. Differential expression of var genes, encoding major virulence factors of Pf, PfEMP1s, was assessed in ex vivo parasite samples as well as in parasites from the in vitro cell bank culture that was used to generate the sporozoites (SPZ) for CHMI (Sanaria PfSPZ Challenge (7G8)). We report broad activation of mainly B-type subtelomeric located var genes at the onset of a 7G8 blood stage infection in naive volunteers, mirroring the NF54 expression study and suggesting that the expression of virulence-associated genes is generally reset during transmission from the mosquito to the human host. However, in 7G8 parasites, we additionally detected a continuously expressed single C-type variant, Pf7G8_040025600, that was most highly expressed in both pre-mosquito cell bank and volunteer samples, suggesting that 7G8, unlike NF54, maintains expression of some previously expressed var variants during transmission. This suggests that in a new host, the parasite may preferentially express the variants that previously allowed successful infection and transmission.Attachment of Plasmodium falciparum-infected erythrocytes to vascular endothelium via PfEMP1 is an important determinant of virulence and malaria severity, but these proteins also represent targets of the immune system and are therefore subject to antigenic variation by switching the expressed var gene. How parasites establish infection in a new host of unknown immunological history is poorly understood. Analyses of PfEMP1 expression during the course of controlled human malaria infections (CHMI) have previously focused on the parasite strain NF54 of African origin. In this study, we explored samples from volunteers infected with the South American parasite line 7G8 and confirm broad expression of subtelomeric var genes at the onset of blood stage infections, as initially discovered with NF54 infections. In addition, we expose the dominant expression of a single centromeric var gene before and after transmission as an alternative strategy of 7G8 parasites. Consistent with epigenetic retention of histone modifications during passage through mosquitoes, we provide evidence that the chromatin profile of var genes is maintained during sexual differentiation. This suggest that the PfEMP1 variant expressed in the previous malaria patient might also contribute to establishing an infection in the next human host.