Protective Antibodies against Placental Malaria and Poor Outcomes during Pregnancy, Benin

Immunity requires a vaccine that inhibits binding of infected erythrocytes to chondroitin sulfate.

Placental malaria is caused by Plasmodium falciparum–infected erythrocytes that bind to placental tissue. Binding is mediated by VAR2CSA, a parasite antigen coded by the var gene, which interacts with chondroitin sulfate A (CSA). Consequences include maternal anemia and fetal growth retardation. Antibody-mediated immunity to placental malaria is acquired during successive pregnancies, but the target of VAR2CSA-specific protective antibodies is unclear. We assessed VAR2CSA-specific antibodies in pregnant women and analyzed their relationships with protection against placental infection, preterm birth, and low birthweight. Antibody responses to the N-terminal region of VAR2CSA during early pregnancy were associated with reduced risks for infections and low birthweight. Among women infected during pregnancy, an increase in CSA binding inhibition was associated with reduced risks for placental infection, preterm birth, and low birthweight. These data suggest that antibodies against VAR2CSA N-terminal region mediate immunity to placental malaria and associated outcomes. Our results validate current vaccine development efforts with VAR2CSA N-terminal constructs.

The NTS-DBL2X region of VAR2CSA induces cross-reactive antibodies that inhibit adhesion of several Plasmodium falciparum isolates to chondroitin sulfate A

BACKGROUND

Binding to chondroitin sulfate A by VAR2CSA, a parasite protein expressed on infected erythrocytes, allows placental sequestration of Plasmodium falciparum-infected erythrocytes. This leads to severe consequences such as maternal anemia, stillbirths, and intrauterine growth retardation. The latter has been clearly associated to increased morbidity and mortality of the infants. Acquired anti-VAR2CSA antibodies have been associated with improved pregnancy outcomes, suggesting a vaccine could prevent the syndrome. However, identifying functionally important regions in the large VAR2CSA protein is difficult.

METHODS

Using genetic immunization, we raised polyclonal antisera against overlapping segments of VAR2CSA in mice and rabbits. The adhesion-inhibition capacities of induced antisera and of specific antibodies purified from plasma of malaria-exposed pregnant women were assessed on laboratory-adapted parasite lines and field isolates expressing VAR2CSA. Competition enzyme-linked immunosorbent assay (ELISA) was employed to analyze functional resemblance between antibodies induced in animals and those naturally acquired by immune multigravidae.

RESULTS

Antibodies targeting the N-terminal sequence (NTS) up to DBL2X (NTS-DBL2X) efficiently blocked parasite adhesion to chondroitin sulfate A in a manner similar to that of antibodies raised against the entire VAR2CSA extracellular domain. Interestingly, naturally acquired antibodies and those induced by vaccination against NTS-DBL2X target overlapping strain-transcendent anti-adhesion epitopes.

CONCLUSIONS

This study highlights an important step achieved toward development of a protective vaccine against placental malaria.

Chondroitin sulfate A-adhering Plasmodium falciparum-infected erythrocytes express functionally important antibody epitopes shared by multiple variants

Acquired protection from Plasmodium falciparum placental malaria, a major cause of maternal, fetal, and infant morbidity, is mediated by IgG specific for the P. falciparum erythrocyte membrane protein 1 variant VAR2CSA. This protein enables adhesion of P. falciparum-infected erythrocytes to chondroitin sulfate A in the intervillous space. Although interclonal variation of the var2csa gene is lower than that among var genes in general, VAR2CSA-specific Abs appear to target mainly polymorphic epitopes. This has raised doubts about the feasibility of VAR2CSA-based vaccines. We used eight human monoclonal IgG Abs from affinity-matured memory B cells of P. falciparum-exposed women to study interclonal variation and functional importance of Ab epitopes among placental and peripheral parasites from East and West Africa. Most placental P. falciparum isolates were labeled by several mAbs, whereas peripheral isolates from children were essentially nonreactive. The mAb reactivity of peripheral isolates from pregnant women indicated that some were placental, whereas others had alternative sequestration foci. Most of the mAbs were comparable in their reactivity with bound infected erythrocytes (IEs) and recombinant VAR2CSA and interfered with IE and/or VAR2CSA binding to chondroitin sulfate A. Pair-wise mAb combinations were more inhibitory than single mAbs, and all of the mAbs together was the most efficient combination. Each mAb could opsonize IEs for phagocytosis, and a combination of the eight mAbs caused phagocytosis similar to that of plasma IgG-opsonized IEs. We conclude that functionally important Ab epitopes are shared by the majority of polymorphic VAR2CSA variants, which supports the feasibility of VAR2CSA-based vaccines against placental malaria.

The impact of delayed treatment of uncomplicated P. falciparum malaria on progression to severe malaria: A systematic review and a pooled multicentre individual-patient meta-analysis

BACKGROUND

Delay in receiving treatment for uncomplicated malaria (UM) is often reported to increase the risk of developing severe malaria (SM), but access to treatment remains low in most high-burden areas. Understanding the contribution of treatment delay on progression to severe disease is critical to determine how quickly patients need to receive treatment and to quantify the impact of widely implemented treatment interventions, such as 'test-and-treat' policies administered by community health workers (CHWs). We conducted a pooled individual-participant meta-analysis to estimate the association between treatment delay and presenting with SM.

METHODS AND FINDINGS

A search using Ovid MEDLINE and Embase was initially conducted to identify studies on severe Plasmodium falciparum malaria that included information on treatment delay, such as fever duration (inception to 22nd September 2017). Studies identified included 5 case-control and 8 other observational clinical studies of SM and UM cases. Risk of bias was assessed using the Newcastle-Ottawa scale, and all studies were ranked as 'Good', scoring ≥7/10. Individual-patient data (IPD) were pooled from 13 studies of 3,989 (94.1% aged <15 years) SM patients and 5,780 (79.6% aged <15 years) UM cases in Benin, Malaysia, Mozambique, Tanzania, The Gambia, Uganda, Yemen, and Zambia. Definitions of SM were standardised across studies to compare treatment delay in patients with UM and different SM phenotypes using age-adjusted mixed-effects regression. The odds of any SM phenotype were significantly higher in children with longer delays between initial symptoms and arrival at the health facility (odds ratio [OR] = 1.33, 95% CI: 1.07-1.64 for a delay of >24 hours versus ≤24 hours; p = 0.009). Reported illness duration was a strong predictor of presenting with severe malarial anaemia (SMA) in children, with an OR of 2.79 (95% CI:1.92-4.06; p < 0.001) for a delay of 2-3 days and 5.46 (95% CI: 3.49-8.53; p < 0.001) for a delay of >7 days, compared with receiving treatment within 24 hours from symptom onset. We estimate that 42.8% of childhood SMA cases and 48.5% of adult SMA cases in the study areas would have been averted if all individuals were able to access treatment within the first day of symptom onset, if the association is fully causal. In studies specifically recording onset of nonsevere symptoms, long treatment delay was moderately associated with other SM phenotypes (OR [95% CI] >3 to ≤4 days versus ≤24 hours: cerebral malaria [CM] = 2.42 [1.24-4.72], p = 0.01; respiratory distress syndrome [RDS] = 4.09 [1.70-9.82], p = 0.002). In addition to unmeasured confounding, which is commonly present in observational studies, a key limitation is that many severe cases and deaths occur outside healthcare facilities in endemic countries, where the effect of delayed or no treatment is difficult to quantify.

CONCLUSIONS

Our results quantify the relationship between rapid access to treatment and reduced risk of severe disease, which was particularly strong for SMA. There was some evidence to suggest that progression to other severe phenotypes may also be prevented by prompt treatment, though the association was not as strong, which may be explained by potential selection bias, sample size issues, or a difference in underlying pathology. These findings may help assess the impact of interventions that improve access to treatment.

First-trimester Plasmodium falciparum infections display a typical "placental" phenotype

BACKGROUND

Plasmodium falciparum-infected erythrocytes (IEs) adhere to host cell receptors, allowing parasites to sequester into deep vascular beds of various organs. This defining phenomenon of malaria pathogenesis is key to the severe clinical complications associated with cerebral and placental malaria. The principal ligand associated with the binding to chondroitin sulfate A (CSA) that allows placental sequestration of IEs is a P. falciparum erythrocyte membrane protein 1 (PfEMP1) family member encoded by the var2csa gene.

METHODS

Here, we investigated the transcription pattern of var genes by real-time polymerase chain reaction, the expression of VAR2CSA, protein by flow cytometry, and the CSA-binding ability of IEs collected at different stages of pregnancy using a static-based Petri dish assay.

RESULTS

Through comparison with the profiles of isolates from nonpregnant hosts, we report several lines of evidence showing that parasites infecting women during pregnancy preferentially express VAR2CSA protein, and that selection for the capacity to adhere to CSA via VAR2CSA expression occurs early in pregnancy.

CONCLUSIONS

Our data suggest that the placental tropism of P. falciparum is already established in the first trimester of pregnancy, with consequent implications for the development of the pathology associated with placental malaria.

Clinical development of a VAR2CSA-based placental malaria vaccine PAMVAC: Quantifying vaccine antigen-specific memory B & T cell activity in Beninese primigravidae

BACKGROUND

The antigen VAR2CSA plays a pivotal role in the pathophysiology of pregnancy-associated malaria (PAM) caused by Plasmodium falciparum. A VAR2CSA-based vaccine candidate, PAMVAC, is under development by an EU-funded multi-country consortium (PlacMalVac project). As part of PAMVAC's clinical development, we quantified naturally acquired vaccine antigen-specific memory B and T cell responses in Beninese primigravidae recruited at the beginning of pregnancy and followed up to delivery and beyond.

METHODS

Clinical and parasitological histories were compiled from monthly clinic visits. On 4 occasions (first and fifth month of pregnancy, delivery, 6months post-delivery) peripheral blood mononuclear cells were isolated for in vitro assays. PAMVAC-specific memory B cells as well as those specific for a PAM unrelated P. falciparum antigen (PfEMP1-CIDR1a) and for tetanus toxoid were quantified by ELISpot. Memory T cell responses were assessed by quantifying cytokines (IL-5, IL-6, IL-10, IL-13, IFN-γ, TNF-α) in supernatants of cells stimulated in vitro either with PAMVAC, or mitogen (PHA).

RESULTS

Both tetanus toxoid- and PAMVAC-specific memory B cell frequencies increased to reach peak levels in the 5th month and at delivery, respectively and persisted post-delivery. The frequency of CIDR1a-specific memory B cells was stable during pregnancy, but declined post-delivery. The cumulated prevalence of infection with P. falciparum during pregnancy was 61% by microscopy. In women with a history of such infections, a significantly higher frequency of PAMVAC-specific memory B cells was observed at delivery. PAMVAC-specific pro-inflammatory (IFN-γ, TNF) responses tended to be higher at delivery in those with a history of infection. Mitogen-induced IL-5/IL-13 responses were significantly enhanced in the same women.

CONCLUSIONS

PAMVAC-specific memory B cells are induced during first pregnancies and are maintained post-delivery. Women with a T helper cell profile biased towards production of Th2-type cytokines have a greater risk of infection with P. falciparum.

Functional and immunological characterization of the var2CSA-DBL5epsilon domain of a placental Plasmodium falciparum isolate

Pregnancy-associated malaria (PAM) arises from sequestration of Plasmodium falciparum-parasitized erythrocytes (PE) in the placenta, leading to chronic symptoms in the expectant mother and serious consequences for fetal development. Placental sequestration has been linked to binding of chondroitin sulphate A (CSA) by the var2CSA variant of PfEMP1 expressed on the PE surface, and a substantial body of evidence shows that the immune response to var2CSA gives an effective protection against PAM. We have expressed the var2CSA-DBL5epsilon domain, derived from a placental isolate from Senegal, as soluble product in Escherichia coli and have shown using different criteria that the recombinant protein is obtained with the native conformation. Using surface plasmon resonance techniques, we have examined binding of DBL5epsilon to placental chondroitin sulphate proteoglycan and CSA; however, the recombinant protein also binds to other sulphated oligosaccharides, with higher affinity in some cases, indicating that the single domain lacks the specificity for CSA shown by the complete extra-cellular region of var2CSA and placental parasites. Recombinant DBL5epsilon was specifically recognized by sera from malaria-exposed Senegalese women in a parity-dependent manner but by sera not from children or males from the same endemic region. Conversely, DBL5epsilon induced antibodies in mice that recognized placental isolates from Benin but not isolates from children. The presence of universal epitopes thus supports DBL5epsilon as an interesting component of var2CSA to be considered for vaccine development.

Plasmodium falciparum VAR2CSA-Specific IgG Subclass Responses Reflect Protection Against Low Birth Weight and Pregnancy-Associated Malaria

Sequestration of Plasmodium falciparum-infected erythrocytes expressing the VAR2CSA antigen in the placenta results in poor pregnancy outcomes, including low birth weight and maternal anemia. Antigen-specific antibody-mediated immunity is acquired during successive pregnancies. Thus, evaluating VAR2CSA-specific IgG profiles among pregnant women will increase knowledge on the immunological mechanisms associated with protection, and help in the development of VAR2CSA-based placental malaria vaccines. Using the PAMVAC candidate vaccine antigen, we assessed anti-VAR2CSA IgG subclass responses of a cohort of pregnant Beninese, and analyzed their relationships with pregnancy outcomes. Cytophilic IgG1 and IgG3 responses were the most frequent, with prevalences ranging from 28% (IgG3) up to 50% (IgG1). Elevated levels of VAR2CSA-specific total IgG and cytophilic IgG3 during pregnancy were consistently associated with higher birth weights, whilst high levels of IgG4 were associated with a reduced risk of placental infections. This suggests that protective anti-VAR2CSA IgG responses are coordinated between both cytophilic and non-cytophilic antibodies.

PfEMP1-Specific Immunoglobulin G Reactivity Among Beninese Pregnant Women With Sickle Cell Trait

BACKGROUND

Sickle cell trait (HbAS) protects against severe Plasmodium falciparum malaria but not against placental malaria (PM). In this study, P falciparum erythrocyte membrane protein (PfEMP1)-specific antibodies were measured in HbAA and HbAS Beninese pregnant women as a proxy of exposure to specific PfEMP1 variants.

METHODS

Plasma samples collected at delivery from 338 HbAA and 63 HbAS women were used to measure immunoglobulin (Ig)G levels to 6 recombinant PfEMP1 proteins and 3 corresponding native proteins expressed on the infected erythrocyte (IE) surface. Immunoglobulin G-mediated inhibition of VAR2CSA+ IEs adhesion to chondroitin sulfate A (CSA) was also tested.

RESULTS

Levels of PfEMP1-specific IgG were similar in the 2 groups, except for native IT4VAR09 on IEs, where IgG levels were significantly higher in HbAS women. Adjusted odds ratios for women with positive IgG to HB3VAR06 and PFD1235w suggest a lower risk of infection with these virulent variants among HbAS individuals. The percentage of IEs binding to CSA did not differ between HbAA and HbAS women, but it correlated positively with levels of anti-VAR2CSA and parity. Women with PM had lower levels of anti-VAR2CSA-specific IgG and lower IgG-mediated inhibition of IE adhesion to CSA.

CONCLUSIONS

The findings support similar malaria exposure in HbAA and HbAS women and a lack of HbAS-dependent protection against placental infection among pregnant women.

Immunoglobulin response to Plasmodium falciparum RESA proteins in uncomplicated and severe malaria

BACKGROUND

The three members of the ring-infected erythrocyte surface antigen (RESA) proteins family share high sequence homologies, which impair the detection and assignment to one or another protein of some pathogenic processes inherent to Plasmodium falciparum malaria. The present study was intended to determine if the antibody and inflammatory responses of children living in a malaria-endemic area varied depending on the RESA-1, RESA-2 or RESA-3 proteins and the severity of the disease, two groups of severe and uncomplicated malaria cases being considered.

METHODS

Two synthetic peptides representing predicted B cell epitopes were designed per RESA protein, all located outside of the 3' and 5' repetition blocks, in order to allow an antibody detection specific of each member of the family. Recombinant rRESA-1B and rRESA-3B proteins were also engineered. Two groups of Beninese children admitted to hospital in 2009 for either uncomplicated or severe malaria were compared for their plasma levels of IgG specifically recognizing each recombinant RESA protein or synthetic peptide, and for their plasma inflammatory cytokine levels (IFN-γ, TNF-α and IL-10), taking into account host and parasite genetic factors.

RESULTS

The absence of IgG cross-reactivity between rRESA proteins and their protein carrier as well as between each RESA peptide and a non-epitopic RESA control peptide validated the use of the engineered recombinant proteins and peptides for the measurement of plasma IgG. Taking into account age, fever duration and parasitaemia, a multiple logistic regression performed on children clustered according to their antibody responses' profiles concluded to an increased risk of severe malaria for P2 (representative of RESA-1) responders (P = 0.007). Increased IL-10 plasma levels were found in children harbouring multiclonal P. falciparum infections on the basis of the T1526G resa2 gene polymorphism (P = 0.004).

CONCLUSIONS

This study provided novel tools to dissect the seroreactivity against the three members of the RESA protein family and to describe its relation to protection against malaria. It suggested the measurement of plasma antibodies raised against specific peptides to serve as predictive immunologic markers for disease severity. Lastly, it reinforced previous observations linking the T1526G resa2 gene mutation to severe malaria.

Plasmodium falciparum parasites causing cerebral malaria share variant surface antigens, but are they specific?

Background

Variant surface antigens (VSA) expressed on the surface of Plasmodium falciparum-infected red blood cells constitute a key for parasite sequestration and immune evasion. In distinct malaria pathologies, such as placental malaria, specific antibody response against VSA provides protection. This study investigated the antibody response specifically directed against VSA expressed by parasites isolated from individuals presenting a given type of clinical presentation.

Methods

Plasma and isolates were obtained from four groups of Beninese subjects: healthy adults, patients presenting uncomplicated malaria (UM), cerebral malaria (CM), or pregnancy-associated malaria (PAM). The reactivity of plasma samples from each clinical group was measured by flow cytometry against parasites isolated from individuals from each clinical group.

Results

Antibody responses against VSA_UM were predominant in CM, UM and HA plasmas. When analysed according to age in all plasma groups, anti-VSA_CM and -VSA_UM antibody levels were similar until six years of age. In older groups (6-18 and >19 years of age), VSA_UM antibody levels were higher than VSA_CM antibody levels (P = .01, P = .0008, respectively). Mean MFI values, measured in all plasmas groups except the PAM plasmas, remained low for anti-VSA_PAM antibodies and did not vary with age. One month after infection the level of anti-VSA antibodies able to recognize heterologous VSA_CM variants was increased in CM patients. In UM patients, antibody levels directed against heterologous VSA_UM were similar, both during the infection and one month later.

Conclusions

In conclusion, this study suggests the existence of serologically distinct VSA_CM and VSA_UM. CM isolates were shown to share common epitopes. Specific antibody response to VSA_UM was predominant, suggesting a relative low diversity of VSA_UM in the study area.

Functionality of Toxoplasma gondii antibodies in a population of Beninese pregnant women exposed to malaria

Plasmodium falciparum and Toxoplasma gondii are two apicomplexan parasites that can lead to severe complications for the newborn when contracted during pregnancy. This study explores the cross-reactivity of antibodies specific to both pathogens in pregnant women, exposed or not to malaria. The antibody response against full-length recombinant antigens from P. falciparum (PfAMA1, Pfs48/45) and T. gondii (TgAMA1, TgSAG1, TgGRA7), selected for their strong immunogenicity, was analysed on 150 plasma samples from women residing in Benin or France. The antibody functionality was assessed using P. falciparum in vitro Growth Inhibition Assay (GIA). As the main results, toxoplasmosis seropositive women with an ongoing P. falciparum infection better inhibited P. falciparum invasion compared to toxoplasmosis seronegative women (34.6% vs. 17.2%, p ≤ 0.01). Women with positive serologies for both parasites presented a significantly higher inhibition of P. falciparum invasion compared to those only seropositive for malaria (coef = 6.27, p = 0.076) in reference with double-negative women (coef = 11.35, p = 0.001). These data suggest that plasma samples containing anti-T. gondii IgG may contribute reducing the development of P. falciparum parasites. This study provides insight into the immune dynamics of the co-infection by these two apicomplexans with potential implications for developing cross-protective vaccines and therapies.

Natural Acquired Immunity to Malaria Antigens among Pregnant Women with Hemoglobin C Trait

Hemoglobin C is the second most common structural hemoglobinopathy in Africa, and carriers have a reduced risk of severe malaria. However, the effect of HbAC on the antibody response to malaria antigens in pregnancy has not been studied. Here, we measured PfEMP1-specific antibodies in plasma samples from 74 Beninese pregnant women with either HbAA or HbAC. IgG-mediated inhibition of VAR2CSA+ infected erythrocytes adhesion to chondroitin sulfate A (CSA) was also tested. PfEMP1-specific IgG levels to VAR2CSA were significantly lower in HbAC women, suggesting less exposure to VAR2CSA. In contrast, the percentage of VAR2CSA+-infected erythrocytes adhesion to CSA was not different between HbAA and HbAC women. Moreover, IgG levels to PfEMP1 variants associated with severe malaria were not significantly different between groups. The findings indicate similar exposure to Plasmodium falciparum parasites expressing PfEMP1 variants causing severe malaria, and justify more comprehensive studies of hemoglobinopathy-related qualitative and quantitative differences in PfEMP1-specific antibody responses.

Fc-Afucosylation of VAR2CSA-Specific Immunoglobulin G and Clinical Immunity to Placental Plasmodium falciparum Malaria

BACKGROUND

Acquired immunity to Plasmodium falciparum malaria is mainly mediated by immunoglobulin G (IgG) targeting erythrocyte membrane protein 1 (PfEMP1). These adhesins mediate infected erythrocyte (IE) sequestration, protecting IEs from splenic destruction. PfEMP1-specific IgG is therefore thought to protect mainly by inhibiting IE sequestration. VAR2CSA-type PfEMP1 mediates placental IE sequestration, putting pregnant women exposed to P falciparum parasites at risk of placental malaria (PM).

METHODS

Levels and Fc-afucosylation of VAR2CSA-specific plasma IgG were measured by a modified enzyme-linked immunosorbent assay (FEASI). We also measured the ability of the IgG to inhibit IE adhesion and to induce natural killer (NK) cell degranulation. The results were related to parity and clinical pregnancy outcomes.

RESULTS

Parity was positively correlated with levels and Fc-afucosylation of VAR2CSA-specific IgG, and with birth weight and plasma IgG inhibition of IE adhesion in vitro. Fc-afucosylation of VAR2CSA-specific IgG increased NK-cell degranulation. Women with Fc-afucosylated VAR2CSA-specific IgG had a reduced risk of delivering a low birth weight (LBW) baby, but not of PM or anemia.

CONCLUSIONS

Fc-afucosylated VAR2CSA-specific IgG effectively induced NK-cell degranulation and was associated with protection against LBW, independent of IgG levels. Our study has implications for the development of VAR2CSA-based subunit vaccines, which exclusively induce Fc-fucosylated IgG.