Plasmodium falciparum: regional differences in lectin and cationized ferritin binding to the surface of the malaria-infected human erythrocyte

Cryo scanning electron microscopy of Plasmodium falciparum-infected erythrocytes

Plasmodium falciparum invades erythrocytes as an essential part of their life cycle. While living inside erythrocytes, the parasite remodels the cell's intracellular organization as well as its outer surface. Late trophozoite-stage parasites and schizonts introduce numerous small protrusions on the erythrocyte surface, called knobs. Current methods for studying these knobs include atomic force microscopy and electron microscopy. Standard electron microscopy methods rely on chemical fixation and dehydration modifying cell size. Here, a novel method is presented using rapid freezing and scanning electron microscopy under cryogenic conditions allowing for high resolution and magnification of erythrocytes. This novel technique can be used for precise estimates of knob density and for studies on cytoadhesion.

The sticky secrets of sequestration

Sequestration, the adherence of infected erythrocytes containing the more mature stages of parasite development (trophozoites and schizonts) to the endothelial cells lining the capillaries and post-capillary venules, is characteristic of Plasmodium falciparum infections. In this review, Irwin Sherman and his colleagues discuss recent advances in the characterization of the adhesive molecules on the surface of malaria-infected erythrocytes and the receptors on the endothelium to which they bind.

The malaria-infected red blood cell: structural and functional changes

The asexual stage of malaria parasites of the genus Plasmodium invade red blood cells of various species including humans. After parasite invasion, red blood cells progressively acquire a new set of properties and are converted into more typical, although still simpler, eukaryotic cells by the appearance of new structures in the red blood cell cytoplasm, and new proteins at the red blood cell membrane skeleton. The red blood cell undergoes striking morphological alterations and its rheological properties are considerably altered, manifesting as red blood cells with increased membrane rigidity, reduced deformability and increased adhesiveness for a number of other cells including the vascular endothelium. Elucidation of the structural changes in the red blood cell induced by parasite invasion and maturation and an understanding of the accompanying functional alterations have the ability to considerably extend our knowledge of structure-function relationships in the normal red blood cell. Furthermore, interference with these interactions may lead to previously unsuspected means of reducing parasite virulence and may lead to the development of novel antimalarial therapeutics.

Monoclonal antibodies that react with human band 3 residues 542-555 recognize different conformations of this protein in uninfected and Plasmodium falciparum infected erythrocytes

A monoclonal antibody generated against synthetic peptides patterned on amino acids 542-555 of human band 3, designated 1F4, specifically immunostained Plasmodium falciparum-infected erythrocytes and inhibited the cytoadherence of P. falciparum-infected erythrocytes to C32 amelanotic melanoma cells. 1F4 did not recognize intact band 3 protein on immunoblots, however it was reactive towards proteolytic fragments of band 3. The binding region of another murine monoclonal antibody previously reported to recognize the membrane spanning domain of human band 3, designated B6, was found to also recognize residues 542-555, however its properties differed from 1F4. Mab B6 recognized both infected and uninfected red cells, and reacted only with intact band 3 on immunoblots. Mab B6 was without effect on cytoadherence. These results demonstrate that monoclonal antibodies reactive against a common peptide sequence may bind to different conformations of the peptide sequence and suggest that the adherent competency of P. falciparum-infected erythrocytes may result from a change in the surface topography of human band 3 protein.

Antigens on the Plasmodium falciparum infected erythrocyte surface are not parasite derived

After invasion by Plasmodium falciparum a red blood cell undergoes marked alterations to its morphology and physiological properties. There are two schools of thought on the origin of these modifications: in the first of this pair of articles, Irwin Sherman and Enrique Winograd put the case for the changes to the surface proteins of the red cell being parasite-induced modifications to host proteins. They suggest that changes to naturally occurring host red cell proteins, especially band 3, result in subtle changes to the antigenicity, cytoadherence and to some extent to the morphology of infected red cells.

In vitro cytoadherence of Plasmodium falciparum-infected erythrocytes to melanoma cells: factors affecting adhesion

A reproducible and standardized assay for measuring the cytoadherence of knobby Plasmodium falciparum-infected red cells to amelanotic melanoma cells was developed. Adhesion was dependent on temperature, haematocrit, and parasitaemia. Addition of EDTA to the binding medium reduced adhesion. Removal of protease-sensitive molecules on the surface of the infected cell abolished cytoadherence, whereas removal of carbohydrate residues by treatment of cells with neuraminidase or galactosidase promoted adhesion. Calcium, magnesium, fibrinogen or fibronectin in the medium had no effect on adhesion nor was there any enhancement of adhesion by pre-loading infected cells with calcium. Serum was essential for good adhesion. Adhesion was species specific for target cells; human endothelial or amelanotic melanoma cells were suitable target cells whereas bovine cells were not. The amelanotic melanoma cell could be formalin-fixed and still retain its adhesion properties. The binding properties of formalin-fixed amelanotic melanoma cells were not identical to those of endothelial or unfixed target cells.

Characterization of a modified red cell membrane protein expressed on erythrocytes infected with the human malaria parasite Plasmodium falciparum: possible role as a cytoadherent mediating protein

Infections with the human malaria Plasmodium falciparum are characterized by the retention of parasitized erythrocytes in tissue capillaries and venules. Erythrocytes containing trophozoites and schizonts attach to the endothelial cells that line these vessels by means of structurally identifiable excrescences present on the surface of the infected cell. Such excrescences, commonly called knobs, are visible by means of scanning or transmission electron microscopy. The biochemical mechanisms responsible for erythrocyte adherence to the endothelial cell are still undefined. In an attempt to identify the cytoadhesive molecule on the surface of the infected cell, we have prepared monoclonal antibodies to knob-bearing erythrocytes infected with the FCR-3 strain of P. falciparum. One of these monoclonal antibodies, designed 4A3, is an IgM that reacts (by means of immunofluorescence) with the surface of unfixed erythrocytes bearing mature parasites of the knobby line; it does not react with knobless lines or uninfected erythrocytes. By immunoelectron microscopy the monoclonal antibody 4A3 was localized to the knob region. In an in vitro cytoadherence assay, the monoclonal antibody partially blocked the binding of knob-bearing cells (FCR-3 strain) to formalin-fixed amelanotic melanoma cells. The monoclonal antibody was used to immunoprecipitate a protein from extracts of knobby erythrocytes that had been previously surface iodinated. By a two-dimensional peptide mapping technique, the antigen recognized by the monoclonal antibody was found to be structurally related to band 3 protein, the human erythrocyte anion transporter.

Immunofluorescent and immunoelectron microscopic localization of protein antigens in red cells infected with the human malaria Plasmodium falciparum

Erythrocytes infected with the human malaria Plasmodium falciparum produce elevations of the surface membrane of the red cell called knobs. Through the use of transmission electron microscopy and a post-embedding protein A-immunogold technique, it was possible to show changes in the distribution of band 3, glycophorin A and spectrin in the region of the knob. These proteins appeared to be aggregated or condensed in the area of the knob, whereas the remainder of the red cell surface showed no such dense clusters; haemoglobin and the histidine-rich protein of P. lophurae could not be localized to the knobby protuberances. It was not possible to detect any changes in protein distribution using the light microscope and indirect immunofluorescence.