Immunoglobulins in the adhesion of Plasmodium falciparum-infected erythrocytes in malaria
Abstract: Characterisation of the delicate cell-to-cell interactions involved in the binding of Plasmodium falciparum- infected erythrocytes to host cells is central for the understanding of the development of severe malaria. Infected erythrocytes adhere in the deep vasculature of several organs, sequestration, leading to cerebral malaria, pulmonary oedema or renal failure. In this thesis, the main issue has been the role of normal non-immune immunoglobulins in the adhesion of infected erythrocytes to uninfected erythrocytes, in rosetting, and in placental binding. Immunofluorescence revealed that immunoglobulin G and M were bound on the infected erythrocyte surface. By adding IgM to immunoglobulin depleted infected erythrocytes, the rosettes were reformed dosedependently. The immunoglobulins were found to participate in the formation of fibrillar structures, as visualised by transmission electron microscopy, mediating binding to uninfected erythrocytes in rosettes. Immunoglobulin binding, rosetting, agglutination and endothelial cytoadhesion, were most prevalent with the mature parasite-infected erythrocytes. Several serum proteins i.e. immunoglobulin G and M fibrinogen, albumin, von Willebrand factor and trombospondin were important to secure stable rosetting. This was shown by several independent assays. The prevalence of immunoglobulin binding infected erythrocytes in fresh clinical isolates was established in a study in Gabon. Extensive immunoglobulin binding was found in a group of children with moderate anaemia. However, immunoglobulin binding was not correlated per se to severe anaemia. The high prevalence of those infected erythrocytes in the anaemic group might anyhow indicate immunoglobulins as one of several factors leading to malarial anaemia. It was suggested that immunoglobulins, bound to Plasmodium falciparum erythrocyte membrane protein 1, PfEMP1, is a co-receptor for adhesion of infected erythrocytes in the placenta. Immunoperoxidase staining of infected erythrocytes in Cameroonese malaria-infected placentas showed a high prevalence of immunoglobulin G binding infected erythrocytes. Also, infected erythrocytes of an immunoglobulin G binding clone were found to adhere directly to normal placenta tissue without prior enrichment. The binding could be dose-dependently inhibited with monomeric immunoglobulin G. This is consistent with the expression of immunoglobulin G receptors in the placenta, a structure the parasite seems to utilise. Rosetting, cytoadhesion and placental binding are intricate cell-to-cell interactions. Binding is mediated by PfEMP1, to which serum proteins attach to stabilise the interactions. The most important of these seems to be immunoglobulins. The attachment site for immunoglobulins on the PfEMP1 may serve as a target for therapeutic molecules preventing the development of severe disease.
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