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L Hviid

Publications and source records attributed to L Hviid.

At least 37 records · Page 2Linked to original sources
PubMed1999

Detection of antibodies to variant antigens on Plasmodium falciparum-infected erythrocytes by flow cytometry.

BACKGROUND: Naturally induced antibodies binding to surface antigens of Plasmodium falciparum-infected erythrocytes can be detected by direct agglutination of infected erythrocytes or by indirect immunofluorescence on intact, unfixed, infected erythrocytes. Agglutinating antibodies have previously been shown to recognise Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1). This protein is inserted by the parasite into the host cell membrane and mediates the adhesion to the venular endothelium of the host organism in vivo. METHODS: Erythrocytes infected at high parasitaemias with ethidium-bromide-labelled mature forms of P. falciparum parasites were sequentially exposed to immune plasma, goat anti-human immunoglobulin (Ig) G, and fluorescein-isothiocyanate-conjugated rabbit anti-goat Ig. Plasma antibodies recognising antigens exposed on the surface of parasitised erythrocytes were subsequently detected by two-colour flow cytometry. RESULTS: Binding of human antibodies to the surface of erythrocytes infected with adhesive strains of Plasmodium falciparum can be measured by the two-colour flow cytometry (FCM) assay described. In addition, we demonstrate that the adhesive capacity of a parasite isolate correlates with the capacity of human immune plasmas to label the isolate as detected by FCM. We also show that the antigens recognised by the labelling antibodies are strain specific and that their molecular weights are in the range previously described for PfEMP1 antigens. CONCLUSIONS: Our FCM assay predominantly detects antibodies that recognise PfEMP1 and thus constitutes a convenient assay for the analysis of acquisition, maintenance, and diversity of anti-PfEMP1-specific antibodies and for the examination of class and subclass characteristics.

Adult↗
PubMed1999

Overlapping antigenic repertoires of variant antigens expressed on the surface of erythrocytes infected by Plasmodium falciparum.

Antibodies against variable antigens expressed on the surface of Plasmodium falciparum-infected erythrocytes are believed to be important for protection against malaria. A target for these antibodies is the P. falciparum erythrocyte membrane protein 1, PfEMP1, which is encoded by around 50 var genes and undergoes clonal variation. Using agglutination and mixed agglutination tests and flow cytometry to analyse the recognition of variant antigens on parasitized erythrocytes by plasma antibodies from individuals living in Daraweesh in eastern Sudan, an area of seasonal and unstable malaria transmission, we show that these antibodies recognize different variant antigens expressed by parasites of different genotype. Comparing the levels and acquisition of antibody to variant antigens in pairs of parasite isolates expressing different variant types, there is a correlation between the acquisition of antibodies to some combinations of variant antigens but not to others. These results indicate that (1) a single infection will induce the production of antibodies recognizing several variants of surface-expressed antigens, (2) the repertoire of variable antigens expressed by different parasites is overlapping and the degree of overlap differs between isolates, and (3) the expression of at least some variant antigens is genetically linked.

Antibodies, Protozoan↗
PubMed1999

Leishmania-specific T cells expressing interferon-gamma (IFN-gamma) and IL-10 upon activation are expanded in individuals cured of visceral leishmaniasis.

Peripheral blood mononuclear cells (PBMC) from patients who have recovered from visceral leishmaniasis often respond to Leishmania antigens in vitro by production of both IL-4, IFN-gamma and IL-10. In order to establish the cellular sources of these cytokines, we activated cells from individuals with a history of visceral leishmaniasis with Leishmania antigen for 6 days in culture, and identified cytokine production at the single-cell level by flow cytometry. The cytokines were only found in CD3+ cells and among these mainly within the CD4+ subset. The percentage of cytokine-producing cells was compared in Leishmania-activated PBMC cultures from the previous patients and from individuals living in a village where leishmaniasis does not occur. The percentage of IL-10- and IFN-gamma-containing cells was significantly higher in the previous patients than in the controls, indicating that Leishmania-specific T cells producing IL-10 and/or IFN-gamma had been expanded as a result of the infection. The cytokine-producing cells in the previous patients could be divided into three types: (i) cells producing IFN-gamma only; (ii) cells producing IL-4 only; and (iii) cells producing IFN-gamma and IL-10 simultaneously. The first and second group of cells can be described as Th1- and Th2-type cells, respectively. The third group could be a regulatory subset of T cells important for maintaining a balance between Th1- and Th2-type cells in these individuals.

Animals↗
PubMed1999

Interferon-gamma- and tumour necrosis factor-alpha-producing cells in humans who are immune to cutaneous leishmaniasis.

Individuals infected with Leishmania major usually acquire immunity to cutaneous leishmaniasis. In this study we have investigated peripheral blood mononuclear cells (PBMC) stimulated by Leishmania antigens in two groups of Sudanese individuals, one with a history of cutaneous leishmaniasis and one living in an area without the disease. The production of interferon (IFN)-gamma, tumour necrosis factor (TNF)-alpha and interleukin (IL)-10 was investigated in culture supernatants, and the cellular sources of IFN-gamma and TNF-alpha were identified. Cells from individuals with a history of cutaneous leishmaniasis produced significantly higher levels of IFN-gamma and TNF-alpha than cells from individuals without a history of the disease. Similar levels of IL-10 were found in the two groups. Flow cytometric analysis revealed high numbers of CD3+ cells producing IFN-gamma and TNF-alpha, and only a few CD3+ cells containing IL-10, in the PBMC cultures from the individuals with a history of cutaneous leishmaniasis. Interferon-gamma and TNF-alpha were predominantly produced by CD4+ T cells rather than CD8+ T cells. The results suggest that cellular immunity against cutaneous leishmaniasis is mediated predominantly through antigen-specific CD4+ T cells in individuals with a history of cutaneous leishmaniasis.

CD4-Positive T-Lymphocytes↗
PubMed1999

Levels of antibody to conserved parts of Plasmodium falciparum merozoite surface protein 1 in Ghanaian children are not associated with protection from clinical malaria.

The 19-kDa conserved C-terminal part of the Plasmodium falciparum merozoite surface protein 1 (PfMSP119) is a malaria vaccine candidate antigen, and human antibody responses to PfMSP119 have been associated with protection against clinical malaria. In this longitudinal study carried out in an area of stable but seasonal malaria transmission with an estimated parasite inoculation of about 20 infective bites/year, we monitored 266 3- to 15-year-old Ghanaian children clinically and parasitologically over a period of 18 months. Blood samples were collected at the beginning of the study before the major malaria season in April and after the season in November. Using enzyme-linked immunosorbent assay, we measured antibody responses to recombinant gluthathione S-transferase-PfMSP119 fusion proteins corresponding to the Wellcome and MAD20 allelic variants in these samples. Prevalence of antibodies recognizing the Wellcome 19 construct containing both epidermal growth factor (EGF)-like motifs in Wellcome type PfMSP119 was about 30%. Prevalence of antibodies to constructs containing only the first EGF domain from either Wellcome or MAD20 type PfMSP119 was about 15%, whereas antibodies recognizing a construct containing only the second EGF domain of MAD20 type PfMSP119 was found in only about 4% of the donors. Neither the prevalence nor the levels of any of the antibody specificities varied significantly with season, age, or sex. Significantly, and in contrast to previous reports from other parts of West Africa, we found no evidence of an association between antibody responses to PfMSP119 and clinical protection against malaria.

Adolescent↗
PubMed1999

A longitudinal study of human antibody responses to Plasmodium falciparum rhoptry-associated protein 1 in a region of seasonal and unstable malaria transmission.

Rhoptry-associated protein 1 (RAP1) of Plasmodium falciparum is a nonpolymorphic merozoite antigen that is considered a potential candidate for a malaria vaccine against asexual blood stages. In this longitudinal study, recombinant RAP1 (rRAP1) proteins with antigenicity similar to that of P. falciparum-derived RAP1 were used to analyze antibody responses to RAP1 over a period of 4 years (1991 to 1995) of 53 individuals naturally exposed to P. falciparum malaria. In any 1 year during the study, between 23 and 39% of individuals who had malaria developed immunoglobulin G (IgG) antibodies detectable with at least one rRAP1 protein. However, the anti-RAP1 antibody responses were detected only during or shortly after clinical malarial infections. RAP1 antibody levels declined rapidly (within 1 to 2 months) following drug treatment of the infections. No anti-RAP1 antibodies were usually detected a few months after the end of malaria transmission, during the dry season, or by the start of the next malaria season. Thus, RAP1 IgG responses were very short-lived. The short duration of RAP1 antibody response may explain the apparent lack of response in a surprisingly high proportion of individuals after clinical malarial infections. For some individuals who experienced more than one malarial infection, a higher anti-RAP1 antibody response to subsequent infections than to earlier infections was observed. This suggested secondary responses to RAP1 and thus the development of immunological memory for RAP1.

Adult↗
PubMed1999

Nine-year longitudinal study of antibodies to variant antigens on the surface of Plasmodium falciparum-infected erythrocytes.

PfEMP1 is an antigenically variable molecule which mediates the adhesion of parasitized erythrocytes to a variety of cell types and which is believed to constitute an important target for naturally acquired protective immune responses in malaria. For 9 years we have monitored individuals living in an area of low-intensity, seasonal, and unstable malaria transmission in eastern Sudan, and we have used this database to study the acquisition, specificity, and duration of the antibody response to variant parasitized erythrocyte surface antigens. Both the levels and the spectrum of reactivity of these antibodies varied considerably among individuals, ranging from low levels of antibodies recognizing only few parasitized erythrocyte surface antigens to high levels of broad-specificity antibodies. In general, episodes of clinical malaria were associated with increases in the levels of parasitized erythrocyte surface-specific antibodies that subsided within months of the attack. This response was often, but not always, specific for the antigenic variants expressed by the parasite isolate causing disease. Our study provides evidence that Palciparum falciparum malaria is associated with a short-lived, variant-specific antibody response to PfEMP1-like antigens exposed on the surface of parasitized erythrocytes. Furthermore, our data suggest that the antigenic repertoires of variant antigens expressed by different parasite isolates show considerable overlapping, at least under Sahelian conditions of low-intensity, seasonal, and unstable malaria transmission. Finally, we demonstrate the existence of persistent differences among individuals in the capacity to mount antibody responses to variant surface antigens.

Adolescent↗
PubMed1998

A longitudinal study of type-specific antibody responses to Plasmodium falciparum merozoite surface protein-1 in an area of unstable malaria in Sudan.

Merozoite surface protein-1 (MSP-1) of Plasmodium falciparum is a malaria vaccine candidate Ag. Immunity to MSP-1 has been implicated in protection against infection in animal models. However, MSP-1 is a polymorphic protein and its immune recognition by humans following infection is not well understood. We have compared the immunogenicity of conserved and polymorphic regions of MSP-1, the specificity of Ab responses to a polymorphic region of the Ag, and the duration of these responses in Sudanese villagers intermittently exposed to P. falciparum infections. Recombinant Ags representing the conserved N terminus (Block 1), the conserved C terminus, and the three main types of the major polymorphic region (Block 2) of MSP-1 were used to determine the specificity and longitudinal patterns of IgG Ab responses to MSP-1 in individuals. Abs from 52 donors were assessed before, during, and after malaria transmission seasons for 4 yr. Ags from the Block 1 region were rarely recognized by any donor. Responses to the C-terminal Ag occurred in the majority of acutely infected individuals and thus were a reliable indicator of recent clinical infection. Ags from the polymorphic Block 2 region of MSP-1 were recognized by many, although not all individuals after clinical malaria infections. Responses to Block 2 were type specific and correlated with PCR typing of parasites present at the time of infection. Responses to all of these Ags declined within a few months of drug treatment and parasite clearance, indicating that naturally induced human Ab responses to MSP-1 are short lived.

Adolescent↗
PubMed1998

Clinical disease, immunity and protection against Plasmodium falciparum malaria in populations living in endemic areas.

Malaria remains one of the biggest health problems in large parts of the world. In sub-Saharan Africa alone, it is currently estimated that there are more than 150 million clinical cases annually, and that about 2 million people die from the disease every year. The bulk of malaria-related morbidity and mortality in an endemic setting (malaria is regularly found) is concentrated in children below the age of five years, and the increasing resistance to infection and disease with age is conventionally thought to reflect a slow and gradual acquisition of protective immunity. Many recent and comprehensive reviews of malarial immunity exist; rather than attempting to add another, this review summarises some of the recent evidence on how protective immunity is acquired in humans and what precipitates clinical disease, specifically as it relates to populations living in areas where the disease is endemic. It is becoming increasingly clear that naturally acquired protective immunity depends largely on responses directed against highly variable parasite antigens. This implies that a successful blood-stage vaccine against this disease must be able to either induce protective responses against many of these variants, or artificially initiate protective immune responses that do not normally occur following natural exposure to the parasites. Such protective immune responses might either rely on a different immunological mechanism, or be directed against other antigens, other than those seen in naturally acquired protective immunity. It remains to be seen whether it will be possible to induce protective responses such as these by 'artificially' immunising humans.

Journal Article↗
PubMed1998

Low plasma concentrations of interleukin 10 in severe malarial anaemia compared with cerebral and uncomplicated malaria.

BACKGROUND: Severe anaemia is a major complication of malaria but little is known about its pathogenesis. Experimental models have implicated tumour necrosis factor (TNF) in induction of bone-marrow suppression and eythrophagocytosis. Conversely, interleukin 10 (IL-10), which mediates feed-back regulation of TNF, stimulates bone-marrow function in vitro and counteracts anaemia in mice. We investigated the associations of these cytokines with malarial anaemia. METHODS: We enrolled 175 African children with malaria into two studies in 1995 and 1996. In the first study, children were classified as having severe anaemia (n=10), uncomplicated malaria (n=26), or cerebral anaemia (n=41). In the second study, patients were classified as having cerebral malaria (n=33) or being fully conscious (n=65), and the two groups were subdivided by measured haemoglobin as normal (>110 g/L), moderate anaemia (60-90 g/L), and severe anaemia (<50 g/L). IL-10 and TNF concentrations were measured by ELISA in plasma samples from all patients. FINDINGS: IL-10 concentrations were significantly lower in patients with severe anaemia than in all other groups. In 1995, geometric mean plasma IL-10 in patients with severe anaemia was 270 pg/mL (95% CI 152-482) compared with 725 pg/mL (465-1129) in uncomplicated malaria and 966 pg/mL (612-1526) in cerebral malaria (p<0.03). In 1996, fully conscious patients with severe anaemia also had significantly lower IL-10 concentrations than all other groups, including cerebral-malaria patients with severe anaemia and all patients with moderate anaemia (p<0.001). In both studies, TNF concentrations were significantly higher in cerebral malaria than in fully conscious patients (p<0.01). By contrast, the ratio of TNF to IL-10 was significantly higher in fully conscious patients with severe anaemia than in all other groups (p<0.001). INTERPRETATION: Our findings identify severe malarial anaemia as a distinct disorder in which insufficient IL-10 response to high TNF concentrations may have a central role.

Analysis of Variance↗
PubMed1998

Seasonal changes in the Plasmodium falciparum population in individuals and their relationship to clinical malaria: a longitudinal study in a Sudanese village.

Residents of Daraweesh village in Sudan were monitored for Plasmodium falciparum infection and malaria morbidity in 3 malaria seasons from 1993 to 1996. Malaria parasites were detected microscopically and by polymerase chain reaction (PCR) in a series of cross-sectional surveys. PCR revealed submicroscopical infections during the dry season, particularly among individuals who had recovered from a malaria episode following successful drug treatment. Clinical and subclinical infections were contrasted by assaying for allelic polymorphism at 2 gene loci, MSP-1 and GLURP and 2 hypotheses examined with reference to these data: that clinical malaria is associated with infection with novel parasite genotypes not previously detected in that host, or alternatively, that clinical malaria episodes are associated with an increased number of clones in an infection. We detected more mixed infections among clinical isolates, but people carrying parasites during the dry season were not found to have an increased risk of disease in the following malaria season. There was a clear association of disease with the appearance of novel parasite genotypes.

Adolescent↗
PubMed1998

Increased eosinophil activity in acute Plasmodium falciparum infection--association with cerebral malaria.

To assess the eosinophil response to Plasmodium falciparum infection a cohort of initially parasite-free Ghanaian children was followed for 3 months. Seven of nine children who acquired an asymptomatic P. falciparum infection showed increase in eosinophil counts, while a decrease was found in seven of nine children with symptomatic malaria, and no change was observed in 14 children who remained parasite-free. In a hospital-based study, paediatric patients with cerebral malaria (CM), severe anaemia (SA), or uncomplicated malaria (UM) had uniformly low eosinophil counts during the acute illness followed by eosinophilia 30 days after cure. Plasma levels of eosinophil cationic protein (ECP) and eosinophil protein X (EPX) were measured as indicators of eosinophil activation. In spite of the low eosinophil counts, ECP levels were increased on day 0 and significantly higher in patients with CM (geometric mean (95% confidence interval) 8.5 ng/ml (6.8-10.7 ng/ml)) than in SA (4.7 ng/ml (3.0-7.5 ng/ml)) and UM patients (4.3 ng/ml (3.6-5.3 ng/ml), P < 0.001). A similar pattern was found for EPX. It thus appears that the low eosinophil counts may be due to tissue sequestration and destruction rather than decreased production. The plasma levels of the granule proteins correlated with levels of tumour necrosis factor and soluble IL-2 receptor, implicating inflammatory responses and T cell activation as causes of the eosinophil activation. By contrast, the eosinophil induction did not appear to be part of a Th2-like response. Eosinophil granule proteins may be important in both control of malaria infection and the pathogenesis of severe malaria.

Animals↗
PubMed1998

Alkamides from Phyllanthus fraternus.

Two alkamides E,E-2,4-octadienamide and E,Z-2,4-decadienamide have been isolated from Phyllanthus fraternus, a plant used in Ghanaian traditional medicine to treat malaria. The compounds possess an alpha, beta, gamma, delta-unsaturated conjugated amide, a feature believed to enhance antiplasmodial activity. By means of an in vitro assay the two alkamides have been shown to possess moderate antiplasmodial activity.

Amides↗
PubMed1998

Seasonal variation in agglutination of Plasmodium falciparum-infected erythrocytes.

Agglutination and rosette formation are in vitro characteristics of Plasmodium falciparum-infected erythrocytes, which have been associated with host protective immune responses and also with parasite virulence. The present study was carried out in an area of seasonal and unstable malaria transmission in eastern Sudan. Plasma samples were obtained before, during, and after the transmission season from a volunteer cohort of 64 individuals seven years of age and older. These plasmas were assayed for their ability to agglutinate cultured parasitized erythrocytes originally obtained from acute malaria infection samples taken from five of the cohort members. Our data show that the capacity of donor plasma samples to agglutinate parasitized cells depended largely on the time of sampling relative to the transmission season, at least within this epidemiologic setting. Thus, although less than half of the pretransmission season samples could agglutinate any of the five lines of cultured parasites, all post-transmission season samples could agglutinate at least one of the parasite lines, with 74% agglutinating two or more lines. This increase in the agglutination capacity of individual plasma samples after the transmission season occurred essentially regardless of whether an individual had experienced a clinical malaria attack during the transmission season. The study thus confirms the acquisition of agglutinating antibodies following episodes of clinical malaria, but also demonstrates that such acquisition can take place in the absence of disease, presumably as a consequence of subclinical infection. This is the first demonstration of marked seasonal fluctuations in the capacity of individuals' sera to agglutinate parasitized red blood cells. Possible explanations for this effect include a decrease in the levels of agglutinating antibodies between seasons, or shifts in the antigens being recognized by such antibodies from one transmission season to the next. Finally, we showed the existence of marked seasonal fluctuation in the levels of agglutinating antibodies, either because levels of such antibodies are not sustained between seasons or because the antigens recognized change from one season to the next.

Adolescent↗
PubMed1997

Human T-cell recognition of synthetic peptides representing conserved and variant sequences from the merozoite surface protein 2 of Plasmodium falciparum.

Merozoite surface protein 2 (MSP2) is a malaria vaccine candidate currently undergoing clinical trials. We analyzed the peripheral blood mononuclear cell (PBMC) response to synthetic peptides corresponding to conserved and variant regions of the FCQ-27 allelic form of MSP2 in Ghanaian individuals from an area of hyperendemic malaria transmission and in Danes without exposure to malaria. PBMC from 20-39% of Ghanaians responded to each of the peptides by proliferation and 29-36% had PBMC which produced interferon-gamma (IFN-gamma) in response to peptide stimulation. In Danes, there was no proliferation to two of the peptides and only PBMC from 5% of the individuals proliferated to the other three peptides. IFN-gamma production was not detected to any peptide. In both Danes and Ghanaians in only a few instances was IL-4 detected in the PBMC cultures. Overall PBMC from 79% of the Ghanaians responded by proliferation and/or cytokine secretion to at least one of three peptides tested, whereas responses were only observed in 14% of Danes (P = 0.002). These data suggest that the Ghanaians had expanded peripheral blood T-cell populations recognizing the peptides as a result of natural infection. The findings are encouraging for the development of a vaccine based on these T-epitope containing regions of MSP2, as the peptides were broadly recognized suggesting that they can bind to diverse HLA alleles and also because they include conserved MSP2 sequences. Immunisation with a vaccine construct incorporating the sequences present in these peptides could thus be expected to be immunogenic in a high percentage of individuals and lead to the establishment of memory T-cells, which can be boosted through natural infection.

Adult↗