Functional antibodies in pregnant women with malaria

Abstract

© 2021 Timon DamelangMalaria in pregnancy is responsible for 10,000 maternal deaths and 100,000 infant deaths every year. Pregnant women, despite lifetime malaria exposure, are more susceptible to Plasmodium falciparum malaria than their non-pregnant counterparts. This is due to the ability of parasites to sequester in the placenta by expressing VAR2CSA, a pregnancy-associated P. falciparum erythrocyte membrane protein 1 (PfEMP1) variant surface antigen that is upregulated during pregnancy. Naturally acquired antibodies (Abs) to VAR2CSA are associated with reduced adhesion of infected erythrocytes to the placenta. The two leading placental malaria vaccine candidates PAMVAC and PRIMVAC both include the VAR2CSA antigen subdomain, Duffy binding-like domain 2 (DBL2). Although Abs to DBL2 may be important in preventing placental malaria, whether these Abs need to trigger downstream effector functions, such as Ab-dependent cellular cytotoxicity and complement activation, or if the genetic characteristics of the Abs modify these functions, is poorly understood. This PhD project aimed to investigate the Ab response to VAR2CSA, exploring the functional and structural properties of these Abs to identify the mechanisms behind protection. Customised multiplex assays were developed, and systems serology approaches were used to identify key VAR2CSA domain targets of Ab immunity in women with placental malaria compared to non-placental malaria-infected women (infected with malaria, but no sequestering in placenta) from Papua New Guinea (PNG). Machine learning techniques demonstrated that IgG Abs (mainly IgG3) and C1q were the main features discriminating between placental and non-placental malaria (Chapter 2). These could be used to predict Ab-driven functional mechanisms behind protection against placental malaria. Therefore, we hypothesised that stronger Ab responses to the DBL domains will mediate more effective downstream effector functions, such as Natural Killer (NK) cell activation via Fc gamma receptor (FcgR) functions (Chapter 3). However, even though Ab-mediated NK cell activation was observed in pregnant women with malaria, no differences were associated with susceptibility to placental malaria. We also explored the importance of allotypes of immunoglobulin subclasses IgG1 and IgG3 in placental malaria infections. IgG1 and IgG3 allotypes are polymorphisms in the constant regions of immunoglobulin heavy and light chains. IgG3 allotypic variations can have structural and functional consequences, such as shorter hinge regions and extended half-lives, which may play a role in malaria susceptibility and protection in placental malaria. IgG allotypes may also have a significant impact on associated Fc-mediated effector functions. We identified a novel IgG3 allotype, that was more prevalent in an isolated population in PNG, which was associated with reduced IgG3 Ab levels in plasma. We explored the impact of this novel variant on FcgR binding, its engagement with FcgR dimers and its capacity to induce phagocytosis in the context of placental malaria infections (Chapter 4). Glycosylation patterns associated with IgG responses to malaria infections during pregnancy are relatively poorly understood but identifying such patterns could help understand the importance of IgG during placental malaria infections and could aid to develop efficient Ab-based vaccines. Here, we showed that pregnancy-associated anti-inflammatory IgG Fc glycans may dampen the Ab-mediated activation of NK cells in pregnant women with malaria infection (Chapter 3). To date, malaria-specific IgG glycosylation has not been systematically investigated, so this PhD project aimed to establish a method to purify antigen-specific Abs (Chapter 5). A detailed understanding of the glycosylation patterns of antigen-specific IgG Abs and their interactions with FcgRs and/or complement complexes may be key to uncovering novel pathways of Ab-mediated responses in placental malaria infections. In summary, this thesis analysed the functional and structural properties of Abs that could help guide the rational development of future Ab-based vaccines and therapeutic Abs to prevent clinical complications, not just placental malaria