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Lysates of Plasmodium falciparum parasitized human erythrocytes stimulate U937 cells to secrete neopterin during a 48 hr co-culture period. Neopterin secretion by U937 cells was enhanced by the addition of human interferon gamma (IFN-gamma). Several P. falciparum antigens, 'FC27' (a synthetic 'S' antigen), Ag16 (a recombinant 'S' antigen) and Ag44/RHOP3 (a recombinant merozoite rhoptry protein), also activated U937 cells to neopterin secretion and produced a similar effect on peripheral blood mononuclear cells (PBMC) from 2 of 3 normal healthy donors cultured with the antigens for 7 days. Plasma from six Nigerian malaria patients contained high neopterin concentrations ranging from 5.06 to 14.17 ng/ml. This preliminary pilot study lends support for further investigation incorporating a larger number of malaria patients and further culture experiments with U937 cells and PBMC with the aim of defining the cause and source of the large quantities of plasma neopterin produced in this infection.

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The role of the erythrocyte skeleton in the invasion process of Plasmodium falciparum was evaluated using genetically variant erythrocytes containing well-defined molecular defects in alpha spectrin (alpha Sp) or protein 4.1 from eight unrelated families. Invasion into red cells from subjects of three black families with hereditary pyropoikilocytosis (HPP) due to inheritance of alpha I/74 mutant spectrin was significantly reduced in cells both from the patients and from the relatives of these who carried asymptomatic hereditary elliptocytosis (HE). Likewise, reduced invasion was also seen in red cells from two families with HE in which the alpha I/65 variant spectrin was present. Resistance to invasion was not absolute in any sample and varied between 38% and 71% of that seen in normal cells. The decreased invasion correlated with the percentage of spectrin dimers present within the membrane of variant cells. In contrast, invasion into elliptocytes from three families that had a partial deficiency in protein 4.1 (HE/4.1+) but a normal percentage of spectrin dimers was either unchanged or increased. The precise mechanism and molecular basis behind the reduced invasion into HPP and HE red cells bearing Sp alpha I domain variants remains to be elucidated but might relate to alterations in merozoite/red cell-receptor interactions and/or merozoite endocytosis. The occurrence of elliptocytosis with spectrin defects (in particular, Sp alpha I/65 and Sp alpha I/46 variants in West Africa) suggests that these mutations of the alpha Sp gene could be related to some protection against malaria.

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The majority (75%) of adult patients with uncomplicated Plasmodium falciparum and P. vivax malaria are positive for anti-phospholipid antibodies (aPLA) as demonstrated by ELISA using a panel of anionic and cationic phospholipids. The highest IgG and IgM binding was to the anionic phospholipids, phosphatidylserine (PS), phosphatidic acid (PA) and cardiolipin (CL), but excluding phosphatidylinositol (PI) to which only low antibody levels were found. Comparison of the mean IgG and IgM aPLA showed a trend for anti-PA > CL > PS > PC > PE > PI. Anti-PI levels were compared in two groups of African children, one group with non-severe and the other with severe (cerebral) falciparum malaria. Children with cerebral disease had significantly lower IgM anti-PI. The results are discussed with the view that serum-derived aPLA may have a role in 'anti-disease' immune responses. Their possible role in the opsonization and phagocytosis of parasitized erythrocytes and in thrombocytopenia is also considered.

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The plasma concentration of soluble P-selectin (GMP-140/CD62P/PADGEM), a selectin produced by activated platelets and endothelial cells, was quantitated in a group of adults and East African negro children presenting with either non-severe or severe (cerebral) malaria caused by Plasmodium falciparum. Sixty percent of adults with non-severe malaria had immunoreactive levels of P-selectin above 200 ng/ml (the maximum recorded for any normal healthy adult in the assay) and 86% of all African children with malaria had concentrations above normal irrespective of their clinical categorization, and most exceeded the maximum limits of the assay (> 640 ng/ml). There was no correlation between P-selectin levels and parasitemia. These results raise the possibility that elevated soluble P-selectin in malaria may have an important beneficial anti-inflammatory function.

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The addition of recombinant cytokines to Plasmodium falciparum in vitro cultures retarded the growth of the parasite with the effect of recombinant IL-2 (rIL-2) > interferon-gamma (IFN-gamma) > tumour necrosis factor-beta (TNF-beta). The process was concentration dependent, being greatest at 30,000 U/ml and required a 72-h period of continuous exposure for maximum effect. Growth inhibition, as determined morphologically and radiometrically, was a consequence of defective schizont maturation rather than inhibition of merozoite invasion. It was cumulative and detectable within one erythrocytic (48 h) growth cycle.

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Quantification of human peripheral blood NK subsets has been made in a group of Kenyan adults and children with acute P. falciparum malaria. Results were compared with data obtained from three age- and sex-matched control cohorts: parasitaemic but asymptomatic children; aparasitaemic children and adults; and adult Caucasians with no previous history of malaria. Separated NK subsets were tested in vitro for cytotoxicity to erythrocytic schizonts of P. falciparum in the presence and absence of cytokines. There was a statistically significant quantitative and qualitative depression of the CD3-CD56+ subset in patients with acute malaria and this was accompanied by an expansion of the 'non-functional' CD3-CD57+CD16-CD56- subset. Both CD3-CD16+ and CD3-CD56+ NK cells from all patients and donors lysed schizonts, and this cytotoxicity was enhanced by the addition of recombinant interferon-alpha and/or IL-2, notably with the CD3-CD56+ subset. Interestingly, asymptomatic donors had the highest levels of CD3-CD56+ NK cells, which also demonstrated an enhanced response to cytokine stimulation. Cytotoxicity to schizonts was accompanied by the release of soluble NK cell lytic factors. Neomycin suppressed cytotoxicity in a dose-dependent manner, indicating that the lysis of schizonts by NK cells involves phospholipase C-mediated phosphoinositide metabolism. Our findings define a role for NK cells in immunity to malaria through the lysis of infected erythrocytes as a first-line defence against the parasite.

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Peripheral blood lymphocytes from healthy, Epstein-Barr virus (EB-virus)-seropositive donors and from patients with acute Plasmodium falciparum malaria were tested for their cytotoxicity towards autologous EB-virus-infected B-cells using an in vitro regression assay. Of the 18 cultures from control donors, 88.8% showed the normal pattern of regression. Of the 20 malaria patients in the study, 40% failed to exhibit the normal pattern observed in the control group. Analysis of the lymphocyte subsets showed a high incidence of inverted CD4:CD8 ratios in the patient group due to an absolute rise in the CD8 population. This data suggests that the immunosuppressive effects of acute malaria extend to defective control over EB-virus. The relevance of the observations to the aetiology of EB-virus-associated, endemic Burkitt's lymphoma (eBL) is discussed.

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A series of peripheral blood films taken from Gambian children with either acute or low-grade Plasmodium falciparum infections were examined for abnormal features of the red and white cells. Hypochromia and polychromasia with cytoplasmic stippling were predominant features in both groups. Lymphocytosis, granulocytosis and plasmacytosis were common white cell abnormalities. An additional feature in films from patients with acute malaria was the presence of numerous atypical lymphocytes. A comparison of the features in the two groups indicated that some abnormalities are associated with an acute attack of malaria and that others have a nutritional or genetic aetiology.

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Supernatants from Plasmodium falciparum cultures containing soluble parasite material were mitogenic for normal human peripheral blood mononuclear cells (MNC) in vitro. This was evidenced by blast transformation and significant incorporation of 3H-thymidine and confirms earlier reports of the mitogenic potential of malaria parasites. Lymphocyte activation by these malaria derived products was polyclonal as demonstrated by increased secretion of IgA, IgG and IgM by the stimulated cells. Using rat tissues and Hep-2 cells as substrates, autoantibody activity was found in the IgM fraction of the secreted immunoglobulin. Speckled anti-nuclear (ANA) antibody, anti-globulins (rheumatoid factor) and anti-intermediate filament antibodies were produced by the stimulated lymphocytes. No significant immunoglobulin secretion with autoantibody specificity was found in control cultures in which normal MNC were incubated with supernatants from non-parasitized red cell cultures. The data supports the suggestion that polyclonal lymphocyte activation by parasite derived products occurs in vivo and, in addition, provides an explanation for the presence of autoantibodies in the serum of malaria patients.

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Supernatants obtained from the in vitro culture of Plasmodium falciparum infected erythrocytes induced prolonged lymphocyte survival in culture for more than 8 weeks in six cultures and permanent cell lines were established in four of these. The cells in the latter showed lymphoblastoid features similar to those seen in parallel cultures to which transforming Epstein-Barr (EB) virus instead of P. falciparum derived substances had been added. Cells from the same donors stimulated with other mitogens (pokeweed mitogen, Salmonella paratyphi culture supernatants) ceased to proliferate and died after 3-4 weeks. A 195 Kd polypeptide obtained from P. falciparum parasites also exhibited the potential to transform normal lymphocytes. Characterization of the cell lines indicated a B lymphocyte origin and the presence of EB virus in these lines suggests the possibility that P. falciparum products may activate latent EB virus genomes. These observations appear relevant to both the choice of P. falciparum derived antigens as vaccines, and to the interaction of EB virus and malaria in the aetiology of African Burkitt's lymphoma (BL).

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The importance of the red cell membrane sialoglycoproteins in the invasion of P. falciparum merozoites has been assessed. Human erythrocytes deficient in glycophorin A (En(a-)cells) or B (S-s-U-, S-s-U+ cells) showed significant resistance to invasion. Treatment of normal erythrocytes with trypsin and chymotrypsin also reduced invasion. These results indicate that determinants carried on glycophorins A, B and C play an essential role in the successful invasion into human red cells. Sugar components present on glycophorin, in particular N-acetyl glucosamine and N-acetyl galactosamine, as shown by specific sugar and antibody inhibition studies, appear to act as important determinants for attachment to the erythrocyte. This implicates a protein(s) on the merozoite surface membrane which has the properties of a lectin.

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