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Lars Hviid

Publications and source records attributed to Lars Hviid.

At least 19 recordsLinked to original sources
PubMed2006

Human pregnancy-associated malaria-specific B cells target polymorphic, conformational epitopes in VAR2CSA.

Pregnancy-associated malaria (PAM) is caused by Plasmodium falciparum-infected erythrocytes (IEs) that bind to chondroitin sulphate A (CSA) in the placenta by PAM-associated clonally variant surface antigens (VSA). Pregnancy-specific VSA (VSA(PAM)), which include the PfEMP1 variant VAR2CSA, are targets of IgG-mediated protective immunity to PAM. Here, we report an investigation of the specificity of naturally acquired immunity to PAM, using eight human monoclonal IgG1 antibodies that react exclusively with intact CSA-adhering IEs expressing VSA(PAM). Four reacted in Western blotting with high-molecular-weight (> 200 kDa) proteins, while seven reacted with either the DBL3-X or the DBL5-epsilon domains of VAR2CSA expressed either as Baculovirus constructs or on the surface of transfected Jurkat cells. We used a panel of recombinant antigens representing DBL3-X domains from P. falciparum field isolates to evaluate B-cell epitope diversity among parasite isolates, and identified the binding site of one monoclonal antibody using a chimeric DBL3-X construct. Our findings show that there is a high-frequency memory response to VSA(PAM), indicating that VAR2CSA is a primary target of naturally acquired PAM-specific protective immunity, and demonstrate the value of human monoclonal antibodies and conformationally intact recombinant antigens in VSA characterization.

Amino Acid Sequence↗
PubMed2006

Baculovirus-expressed constructs induce immunoglobulin G that recognizes VAR2CSA on Plasmodium falciparum-infected erythrocytes.

We raised specific antisera against recombinant VAR2CSA domains produced in Escherichia coli and in insect cells. All were reactive in enzyme-linked immunosorbent assay, but only insect cell-derived constructs induced immunoglobulin G (IgG) that was reactive with native VAR2CSA on the surface of infected erythrocytes. Our data show that five of the six VAR2CSA Duffy-binding-like domains are surface exposed and that induction of surface-reactive VAR2CSA-specific IgG depends critically upon antigen conformation. These findings have implications for the development of vaccines against pregnancy-associated Plasmodium falciparum malaria.

Animals↗
PubMed2006

Levels of plasma immunoglobulin G with specificity against the cysteine-rich interdomain regions of a semiconserved Plasmodium falciparum erythrocyte membrane protein 1, VAR4, predict protection against malarial anemia and febrile episodes.

Antibodies to variant surface antigen have been implicated as mediators of malaria immunity in studies measuring immunoglobulin G (IgG) binding to infected erythrocytes. Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) is an important target for these antibodies, but no study has directly linked the presence of PfEMP1 antibodies in children to protection. We measured plasma IgG levels to the cysteine-rich interdomain region 1alpha (CIDR1alpha) of VAR4 (VAR4-CIDR1alpha), a member of a semiconserved PfEMP1 subfamily, by enzyme-linked immunosorbent assay in 561 Tanzanian individuals, who were monitored clinically for 7 months. The participants resided in Mkokola (a high-transmission village where malaria is holoendemic) or Kwamasimba (a moderate-transmission village). For comparison, plasma IgG levels to two merozoite surface protein 1 (MSP1) constructs, MSP1-19 and MSP1 block 2, and a control CIDR1 domain were measured. VAR4-CIDR1alpha antibodies were acquired at an earlier age in Mkokola than in Kwamasimba, but after the age of 10 years the levels were comparable in the two villages. After controlling for age and other covariates, the risk of having anemia at enrollment was reduced in VAR4-CIDR1alpha responders for Mkokola (adjusted odds ratio [AOR], 0.49; 95% confidence interval [CI], 0.29 to 0.88; P = 0.016) and Kwamasimba (AOR, 0.33; 95% CI, 0.16 to 0.68; P = 0.003) villages. The risk of developing malaria fever was reduced among individuals with a measurable VAR4-CIDR1alpha response from Mkokola village (AOR, 0.51; 95% CI, 0.29 to 0.89; P = 0.018) but not in Kwamasimba. Antibody levels to the MSP1 constructs and the control CIDR1alpha domain were not associated with morbidity protection. These data strengthen the concept of developing vaccines based on PfEMP1.

Adolescent↗
PubMed2006

Plasmodium falciparum parasites expressing pregnancy-specific variant surface antigens adhere strongly to the choriocarcinoma cell line BeWo.

Placenta-sequestering Plasmodium falciparum parasites causing pregnancy-associated malaria express pregnancy-specific variant surface antigens (VSA(PAM)). We report here that VSA(PAM)-expressing patient isolates adhere strongly to the choriocarcinoma cell line BeWo and that the BeWo line can be used to efficiently select for VSA(PAM) expression in vitro.

Animals↗
PubMed2006

Epitope mapping and topographic analysis of VAR2CSA DBL3X involved in P. falciparum placental sequestration.

Pregnancy-associated malaria is a major health problem, which mainly affects primigravidae living in malaria endemic areas. The syndrome is precipitated by accumulation of infected erythrocytes in placental tissue through an interaction between chondroitin sulphate A on syncytiotrophoblasts and a parasite-encoded protein on the surface of infected erythrocytes, believed to be VAR2CSA. VAR2CSA is a polymorphic protein of approximately 3,000 amino acids forming six Duffy-binding-like (DBL) domains. For vaccine development it is important to define the antigenic targets for protective antibodies and to characterize the consequences of sequence variation. In this study, we used a combination of in silico tools, peptide arrays, and structural modeling to show that sequence variation mainly occurs in regions under strong diversifying selection, predicted to form flexible loops. These regions are the main targets of naturally acquired immunoglobulin gamma and accessible for antibodies reacting with native VAR2CSA on infected erythrocytes. Interestingly, surface reactive anti-VAR2CSA antibodies also target a conserved DBL3X region predicted to form an alpha-helix. Finally, we could identify DBL3X sequence motifs that were more likely to occur in parasites isolated from primi- and multigravidae, respectively. These findings strengthen the vaccine candidacy of VAR2CSA and will be important for choosing epitopes and variants of DBL3X to be included in a vaccine protecting women against pregnancy-associated malaria.

Amino Acid Motifs↗
PubMed2005

Differential induction of immunoglobulin G to Plasmodium falciparum variant surface antigens during the transmission season in Daraweesh, Sudan.

BACKGROUND: The acquisition of immunoglobulin (Ig) G to variant surface antigens (VSAs) seems important for the development of protective immunity against malaria. Unlike VSAs expressed by parasite isolates associated with uncomplicated malaria, VSAs expressed by parasite isolates associated with severe malaria (VSA(SM)) are frequently recognized by IgG. METHODS: We analyzed levels of anti-VSA IgG in 57 individuals in Daraweesh, Sudan, before and after the transmission season. IgG responses to 79 Plasmodium falciparum isolates from children with defined malaria syndromes and exposed to high transmission in a different part of Africa were also analyzed. RESULTS: After the transmission season, individuals with malaria had an increase in IgG recognition to 25.8% (95% confidence interval [CI], 19.9%-31.7%) and a decrease in IgG recognition to 7.6% (95% CI, 4.4%-10.8%) of 79 parasite isolates, and individuals without malaria had an increase in IgG recognition to 8.1% (95% CI, 6.0%-10.2%) and a decrease in IgG recognition to 11.9% (95% CI, 7.0%-16.8%) of 79 parasite isolates. Most newly acquired IgG responses were against parasite isolates expressing VSAs(SM) that are frequently recognized by IgG. CONCLUSIONS: Anti-VSA IgG levels decrease in the absence of infection, and an episode of clinical malaria induces IgG against a range of VSAs, particularly VSAs(SM).

Animals↗
PubMed2005

Expression of Plasmodium falciparum erythrocyte membrane protein 1 in experimentally infected humans.

BACKGROUND: Parasites causing severe malaria in non-immune patients express a restricted subset of variant surface antigens (VSA), which are better recognized by immune sera than VSA expressed during non-severe disease in semi-immune individuals. The most prominent VSA are the var gene-encoded Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) family, which is expressed on the surface of infected erythrocytes where it mediates binding to endothelial receptors. Thus, severe malaria may be caused by parasites expressing PfEMP1 variants that afford parasites optimal sequestration in immunologically naive individuals and high effective multiplication rates. METHODS: var gene transcription was analysed using real time PCR and PfEMP1 expression by western blots as well as immune plasma recognition of parasite cultures established from non-immune volunteers shortly after infection with NF54 sporozoites. RESULTS: In cultures representing the first generation of parasites after hepatic release, all var genes were transcribed, but Group A var genes were transcribed at the lowest levels. In cultures established from second or third generation blood stage parasites of volunteers with high in vivo parasite multiplication rates, the var gene transcription pattern differed markedly from the transcription pattern of the cultures representing first generation parasites. This indicated that parasites expressing specific var genes, mainly belonging to group A and B, had expanded more effectively in vivo compared to parasites expressing other var genes. The differential expression of PfEMP1 was confirmed at the protein level by immunoblot analysis. In addition, serological typing showed that immune sera more often recognized second and third generation parasites than first generation parasites. CONCLUSION: In conclusion, the results presented here support the hypothesis that parasites causing severe malaria express a subset of PfEMP1, which bestows high parasite growth rates in individuals with limited pre-existing immunity.

Animals↗
PubMed2005

Naturally acquired immunity to Plasmodium falciparum malaria in Africa.

Infection by Plasmodium falciparum parasites can lead to substantial protective immunity to malaria, and available evidence suggest that acquisition of protection against some severe malaria syndromes can be fairly rapid. Although these facts have raised hopes that the development of effective vaccines against this major cause of human misery is a realistic goal, the uncertainty regarding the antigenic targets of naturally acquired protective immunity and the immunological mechanisms involved remain major vaccine development obstacles. Nevertheless, a coherent theoretical framework of how protective immunity to P. falciparum malaria is acquired following natural exposure to the parasites is beginning to emerge, not least thanks to studies that have combined clinical and epidemiological data with basic immunological research. This framework involves IgG with specificity for clonally variant antigens on the surface of the infected erythrocytes, can explain some of the difficulties in relating particular immune responses with specificity for well-defined antigenic targets to clinical protection, and suggests a radically new approach to controlling malaria-related morbidity and mortality by immunological means.

Africa↗
PubMed2005

Maternal vitamin A supplementation and immunity to malaria in pregnancy in Ghanaian primigravids.

BACKGROUND: Vitamin A supplementation is believed to enhance immune responses to infection but few studies have assessed its effects on anti-malarial immunity, especially during pregnancy when women are at increased risk from both vitamin A deficiency and pregnancy-associated malaria. The pathological effects of malaria in pregnancy are believed to be due to the sequestration of parasites in the placenta mediated via binding of variant surface antigens (VSA) expressed on the surface of P. falciparum infected red blood cells to placental chondroitin sulphate A (CSA). METHODS: We conducted a randomized double-blind controlled trial of vitamin A supplementation in 98 primigravid Ghanaian women to investigate the effects of vitamin A supplementation on levels of IgG antibodies binding to VSA of a clinical, P. falciparum placental isolate and to two isolates selected (or not) for adherence to CSA in vitro (anti-VSACSA IgG or anti-VSA IgG). Placental malarial infection was determined by placental blood smear and histology. RESULTS: Vitamin A supplementation was non-significantly associated with a decreased risk of active or chronic-active placental malarial infection compared to past, resolved infection at delivery, as determined by histology (OR=0.42, P=0.13--adjusted for level of education). After adjustment for differences in baseline values, levels of anti-VSACSA IgG to a placental, CSA-adherent isolate (EJ-24) but not to two isolates selected for CSA-adhesion in vitro (FCR3CSA and BusuaCSA), were significantly lower in women receiving vitamin A supplementation than in women receiving placebo (P=0.002). There was no apparent effect of vitamin A supplementation to levels of Ab to non-CSA-adherent parasite isolates. CONCLUSIONS: The data suggest that the reduction in the levels of anti-VSACSA antibodies to the known placental malaria isolate may reflect reduced intensity or duration of placental parasitaemia in women receiving vitamin A supplementation. These observations are of potential public health significance and deserve further investigation.

Adult↗
PubMed2005

Rapid acquisition of isolate-specific antibodies to chondroitin sulfate A-adherent plasmodium falciparum isolates in Ghanaian primigravidae.

Recent evidence suggests that pregnancy-associated malaria (PAM), associated with maternal anemia and low birth weight, results from preferential sequestration of parasitized red blood cells (pRBC) in the placenta via binding of variant surface antigens (VSA) expressed on the surface of pRBC to chondroitin sulfate A (CSA). The VSA mediating CSA binding (VSA(CSA)) and thus sequestration of pRBC in the placenta are antigenically distinct from those that mediate pRBC sequestration elsewhere in the body, and it has been suggested that VSA(CSA) are relatively conserved and may thus constitute an attractive target for vaccination against PAM. Using flow cytometry, levels of antibody to VSA and VSA(CSA) expressed on the surface of red blood cells infected with Plasmodium falciparum isolates were measured during pregnancy and lactation in Ghanaian primigravid women enrolled in a trial of maternal vitamin A supplementation. Antibody responses to VSA(CSA) were detected within the first trimester of pregnancy and increased with increasing duration of pregnancy, and they seemed to be isolate specific, indicating that different CSA-adherent parasite lines express antigenically distinct VSA and thus may not be as antigenically conserved as has been previously suggested. Levels of anti-VSA(CSA) were not significantly associated with placental malarial infection determined by histology, indicating that primary immune responses to VSA(CSA) may not be sufficient to eradicate placental parasitemia in primigravidae.

Animals↗
PubMed2005

Effects of pregnancy and intensity of Plasmodium falciparum transmission on immunoglobulin G subclass responses to variant surface antigens.

Placenta-sequestering Plasmodium falciparum involved in the pathogenesis of pregnancy-associated malaria (PAM) in otherwise clinically immune women expresses particular variant surface antigens (VSA(PAM)) on the surface of infected erythrocytes that differ from VSA found in parasitized nonpregnant individuals (non-PAM type VSA). We studied levels of immunoglobulin G (IgG) and IgG subclasses with specificity for VSA(PAM) and for non-PAM type VSA in pregnant and nonpregnant women from two sites with different endemicities in Cameroon. We found that VSA(PAM)-specific responses depended on the pregnancy status, parity, gestational age, and parasite transmission intensity, whereas only the parasite transmission intensity influenced the levels of IgG specific for non-PAM type VSA. For both types of VSA, the responses were dominated by the cytophilic subclass IgG1, followed by IgG3. In pregnant women, the levels of VSA(PAM)-specific antibodies either were very low or negative or were very high, whereas the levels of the antibodies specific for non-PAM type VSA were uniformly high. Interestingly, the levels of VSA(PAM)-specific IgG1 increased with increasing gestational age, while the levels of the corresponding IgG3 tended to decrease with increasing gestational age. The IgG subclass responses with specificity for non-PAM type VSA did not vary significantly with gestational age. Taken together, our data indicate that IgG1 and to a lesser extent IgG3 are the main subclasses involved in acquired VSA(PAM)-specific immunity to pregnancy-associated malaria.

Adolescent↗
PubMed2004

Evidence for the involvement of VAR2CSA in pregnancy-associated malaria.

In Plasmodium falciparum-endemic areas, pregnancy-associated malaria (PAM) is an important health problem. The condition is precipitated by accumulation of parasite-infected erythrocytes (IEs) in the placenta, and this process is mediated by parasite-encoded variant surface antigens (VSA) binding to chondroitin sulfate A (CSA). Parasites causing PAM express unique VSA types, VSAPAM, which can be serologically classified as sex specific and parity dependent. It is sex specific because men from malaria-endemic areas do not develop VSAPAM antibodies; it is parity dependent because women acquire anti-VSAPAM immunoglobulin (Ig) G as a function of parity. Previously, it was shown that transcription of var2csa is up-regulated in placental parasites and parasites selected for CSA binding. Here, we show the following: (a) that VAR2CSA is expressed on the surface of CSA-selected IEs; (b) that VAR2CSA is recognized by endemic plasma in a sex-specific and parity-dependent manner; (c) that high anti-VAR2CSA IgG levels can be found in pregnant women from both West and East Africa; and (d) that women with high plasma levels of anti-VAR2CSA IgG give birth to markedly heavier babies and have a much lower risk of delivering low birth weight children than women with low levels.

Africa↗
PubMed2004

Analysis of IgG with specificity for variant surface antigens expressed by placental Plasmodium falciparum isolates.

BACKGROUND: Pregnancy-associated malaria (PAM) is caused by Plasmodium falciparum-infected erythrocytes that can sequester in placental intervillous space by expressing particular variant surface antigens (VSA) that can mediate adhesion to chondroitin sulfate A (CSA) in vitro. IgG antibodies with specificity for the VSA expressed by these parasites (VSAPAM) are associated with protection from maternal anaemia, prematurity and low birth weight, which is the greatest risk factor for death in the first month of life. METHODS: In this study, the development of anti-VSAPAM antibodies in a group of 151 women who presented to the maternity ward of Albert Schweitzer Hospital in Lambaréné, Gabon for delivery was analysed using flow cytometry assays. Plasma samples from placenta infected primiparous women were also investigated for their capacity to inhibit parasite binding to CSA in vitro. RESULTS: In the study cohort, primiparous as well as secundiparous women had the greatest risk of infection at delivery as well as during pregnancy. Primiparous women with infected placentas at delivery showed higher levels of VSAPAM-specific IgG compared to women who had no malaria infections at delivery. Placental isolates of Gabonese and Senegalese origin tested on plasma samples from Gabon showed parity dependency and gender specificity patterns. There was a significant correlation of plasma reactivity as measured by flow cytometry between different placental isolates. In the plasma of infected primiparous women, VSAPAM-specific IgG measured by flow cytometry could be correlated with anti-adhesion antibodies measured by the inhibition of CSA binding. CONCLUSION: Recognition of placental parasites shows a parity- and sex- dependent pattern, like that previously observed in laboratory strains selected to bind to CSA. Placental infections at delivery in primiparous women appear to be sufficient to induce functional antibodies which can both recognize the surface of the infected erythrocytes as well as block their binding to CSA. The correlation between serum reactivities of placental field isolates from different geographic locations and collected at different times is indicative of the conserved nature of the antigen(s) mediating PAM.

Animals↗
PubMed2004

Plasmodium falciparum associated with severe childhood malaria preferentially expresses PfEMP1 encoded by group A var genes.

Parasite-encoded variant surface antigens (VSAs) like the var gene-encoded Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) family are responsible for antigenic variation and infected red blood cell (RBC) cytoadhesion in P. falciparum malaria. Parasites causing severe malaria in nonimmune patients tend to express a restricted subset of VSA (VSA(SM)) that differs from VSA associated with uncomplicated malaria and asymptomatic infection (VSA(UM)). We compared var gene transcription in unselected P. falciparum clone 3D7 expressing VSA(UM) to in vitro-selected sublines expressing VSA(SM) to identify PfEMP1 responsible for the VSA(SM) phenotype. Expression of VSA(SM) was accompanied by up-regulation of Group A var genes. The most prominently up-regulated Group A gene (PFD1235w/MAL7P1.1) was translated into a protein expressed on the infected RBC surface. The proteins encoded by Group A var genes, such as PFD1235w/MAL7P1.1, appear to be involved in the pathogenesis of severe disease and are thus attractive candidates for a vaccine against life-threatening P. falciparum malaria.

Amino Acid Sequence↗
PubMed2004

Variant surface antigen-specific IgG and protection against clinical consequences of pregnancy-associated Plasmodium falciparum malaria.

BACKGROUND: Pregnancy-associated malaria caused by Plasmodium falciparum adherence to chondroitin sulfate A in the placental intervillous space is a major cause of low birthweight and maternal anaemia in areas of endemic P falciparum transmission. Adhesion-blocking antibodies that specifically recognise parasite-encoded variant surface antigens (VSA) are associated with resistance to pregnancy-associated malaria. We looked for a possible relation between VSA-specific antibody concentrations, placental infection, and protection from low birthweight and maternal anaemia. METHODS: We used flow cytometry to measure VSA-specific IgG concentrations in plasma samples taken during child birth from 477 Kenyan women selected from a cohort of 910 women on the basis of HIV-1 status, gravidity, and placental histology. We measured VSA expressed by one placental P falciparum isolate and two isolates selected or not selected for chondroitin sulfate A adhesiveness in-vitro. FINDINGS: Concentrations of plasma IgG specific for VSA, expressed by chondroitin sulfate A-adhering parasites (VSA in pregnancy-associated malaria or vsa-pam), increased with gravidity and were associated with placental histological findings. Women with chronic pregnancy-associated malaria and low or absent VSA-PAM-specific IgG had lower haemoglobin values (reduced by 17 g/L; 95% CI 8.1-25.2) and delivered smaller babies (birthweight reduced by 0.26 kg; 0.10-0.55) than did corresponding women with high VSA-PAM-specific IgG. No such relation was shown for concentrations of IgG with specificity for non-pregnancy-associated malaria VSA. INTERPRETATION: VSA-PAM-specific IgG protects against low birthweight and maternal anaemia. Our data indicate an important mechanism of clinical protection against malaria and raise hope for the clinical effectiveness of a potential VSA-based vaccine against pregnancy-associated malaria.

Animals↗
PubMed2004

Malaria immunity in infants: a special case of a general phenomenon?

Newborn infants in endemic areas are markedly resistant to Plasmodium falciparum malaria. Consequently, severe disease is rare during the first few months of life, and infections tend to be low density and relatively asymptomatic during this period. Although this is generally ascribed to passively transferred immunity, attempts to identify the targets and mechanisms of this protection have been unsuccessful. The implications of the hypothesis that the progression from resistance through susceptibility and back to resistance during infancy and early childhood reflects the gradual acquisition of IgG to variant surface antigens (VSAs), while protection from maternal VSA-specific IgG steadily fades, are discussed here.

Adult↗
PubMed2004

Variant surface antigens, virulence genes and the pathogenesis of malaria.

The first Molecular Approaches to Malaria meeting was held 2-5 February 2000 in Lorne, Australia. Following the meeting, Brian Cooke, Mats Wahlgren and Ross Coppel predicted that research into the molecular details of the mechanisms behind sequestration of parasitized erythrocytes would "become increasingly more complicated, with further interactions, receptors, ligands and functional domains". Furthermore, they cautioned that "the challenge will be not to lose ourselves in the molecular detail, but remain focused on the role of [the var genes and other multigene families] in pathogenesis of malaria". We contemplate on these statements, following the recent second Molecular Approaches to Malaria meeting, which was held at the same venue on 2-5 February 2004.

Animals↗