Induction of nucleoside transport sites into the host cell membrane of Babesia bovis infected erythrocytes

Normal bovine erythrocytes have negligible ability to transport adenosine and related nucleosides across their cell membrane. However, infection with the intraerythrocytic parasite Babesia bovis was found to induce a nucleoside permeation site into the host cell membrane. Transport experiments over periods of up to 30 s determined that the transport rate of 1 microM adenosine into the infected cell was 1.72 +/- 1.2 pmol incorporated (microliter cell water)-1s-1, a rate three times higher than for normal human erythrocytes. Incorporation studies over 6 h with labelled adenosine indicated that the purine moiety was incorporated into parasite nucleic acids. The mammalian nucleoside transport inhibitors, nitrobenzylthioinosine (NBMPR), nitrobenzylthioguanosine (NBTGR), dilazep and dipyridamole inhibited the induced nucleoside transport mechanism in the Babesia-infected erythrocytes, though at higher concentrations than those required to inhibit normal human erythrocyte transport. An ID50 value for NBMPR of 0.36 microM was determined. Phloretin and 5'-p-fluorosulphonyl benzoyl adenosine-HCl (5FSBA) were also shown to be inhibitory, with ID50 values of 0.11 and 0.18 microM, respectively, whilst phlorizin and verapamil at 1 microM had no effect. Binding studies with [3H]NBMPR indicated that high-affinity NBMPR binding sites could not be detected in either normal or B. bovis infected bovine erythrocytes. The results indicate that the induced nucleoside permeation site(s) in B. bovis infected erythrocytes has characteristics different from either human erythrocytes or erythrocytes infected with the malarial parasites Plasmodium falciparum or Plasmodium yoelii.     

Stage-specific alteration of nucleoside membrane permeability and nitrobenzylthioinosine insensitivity in Plasmodium falciparum infected erythrocytes

In human erythrocytes, the intracellular presence of malarial parasites (Plasmodium falciparum) markedly changed the permeation characteristics of the nucleosides, adenosine and tubercidin, an adenosine analogue. We report parasite-induced changes in the kinetics of cellular uptake of the nucleosides and in the appearance in infected cells of a nucleoside permeation route of low sensitivity to the classical inhibitor of erythrocytic nucleoside transport, nitrobenzylthioinosine (NBMPR). These changes and a diminution in NBMPR effectiveness during parasite maturation to the trophozoite or schizont stage, suggest the presence in the infected cells of an altered or new nucleoside permeation mechanism of low sensitivity to NBMPR. The incorporation of adenosine into polynucleotides was also of low sensitivity to 10 microM NBMPR. Binding studies of [3H]NBMPR with both normal erythrocytes and those harbouring parasites at each morphological stage indicated that fewer high affinity NBMPR binding sites were present on cells containing mature parasites than on the uninfected cells. The apparent low sensitivity to NBMPR of nucleoside permeation in erythrocytes containing P. falciparum forms may enable therapeutic measures with cytotoxic nucleosides to be directed with selectivity toward parasite-containing cells.     

D-lactate production in erythrocytes infected with Plasmodium falciparum

The production of D-lactate that accompanies the metabolism of glucose to L-lactate in Plasmodium falciparum was evaluated with erythrocytes that contained either young or mature parasites. Infected cells with ring-stage parasites release L-lactate and D-lactate at rates 1340 and 81 nmol h-1 (10(8) cells)-1, respectively. These rates increase to 2050 and 136 nmol h-1 (10(8) cells)-1, respectively, in infected cells with trophozoite/schizont-stage parasites. D-Lactate represents 6-7% of the total lactate. The formation of D-lactate is by way of a methylgloxal pathway in which methylglyoxal is formed nonenzymatically from dihydroxyacetone phosphate and is then converted into D-lactate by the sequential action of parasite glycoxalase I and glyoxalase II. The kinetic properties of parasite glyoxalase I and glyoxalase II allow these enzymes to be distinguished from those in the host cell. D-Lactate production by the parasite appears to be a defense mechanism to protect the parasite from the toxic effects of methylglyoxal.     

Water and urea transport in human erythrocytes infected with the malaria parasite Plasmodium falciparum

The permeability properties of the human red cell membrane to various solutes are altered by malarial infection. In the present work we show that the permeability of the red cell membrane to water is also affected by the intraerythrocytic growth of the malaria parasite Plasmodium falciparum, whereas urea permeability appears unchanged. The data from infected cells show decreases in membrane surface area, cell volume, the osmotically active water fraction (W_eff), and osmotic water permeability (P_f) as measured by stopped-flow spectroscopy. On the other hand, the data suggest an increase in diffusive water permeability (P_d) in infected cells with no change in urea permeability when measured by the continuous flow method. The decreased P_f/P_d ratio of infected cell membranes and its implications in the geometry of the red cell membrane water channel or pore are discussed.     

Nonopsonic phagocytosis of erythrocytes infected with ring-stage Plasmodium falciparum

Ring-stage parasitized erythrocytes (RPEs) were demonstrated to interact with effector cells of the innate immune system. With receptor blockade studies and CD36-null macrophages, human and murine macrophages were shown to phagocytose RPEs through the pattern recognition receptor CD36. These in vitro data implicate scavenger receptors in the clearance of RPEs.     

Complement activation by the surface of Plasmodium falciparum infected erythrocytes

The surface of trophozoite-stage Plasmodium falciparum infected erythrocytes will, in the presence of immune human or owl monkey serum, activate the classical complement pathway. This was demonstrated with a sensitive, enzyme-linked immunosorbent assay which detects the complex, C1s-C1 inhibitor, which is only generated when the classical pathway is activated. A second enzyme-linked immunosorbent assay, as well as Covaspheres coated with affinity-purified anti-C3, showed that immune activation of the classical pathway by infected erythrocytes resulted in the accumulation of significant amounts of C3b on the erythrocyte surface. During the development of the parasite to the trophozoite stage, the erythrocyte membrane is also transformed from a non-activator into a surface capable of activating complement by the alternative pathway. Erythrocytes infected with trophozoite-stage parasites directly activated the alternative complement pathway. This activation led to the specific binding of an average of 15,000 C3b molecules per infected cell. Alternative pathway activation was augmented by anti-parasite antibody. Such conditions mediated the accumulation of an average of 36,000 C3b molecules per infected erythrocyte. The amounts of C3b on the infected erythrocyte surface did not lead to cellular lysis. They are, however, likely to have a major impact on the total in vivo response to this parasite.     

Interactions of Plasmodium falciparum infected erythrocytes with ligand-coated agarose beads

The binding of normal and Plasmodium falciparum-infected squirrel monkey erythrocytes to columns of ligand-coated agarose beads was compared. Concanavalin A, ricin, soybean and peanut agglutinin-coated beads retain erythrocytes containing large developmental stages of the parasite preferentially to ring-containing erythrocytes or to normal erythrocytes. Binding is inhibited by the sugar corresponding to the lectin's specificity. Preferential binding does not occur with wheat germ or Ulex europaeus agglutinin-coated beads. When infected blood is preincubated in immune serum, infected erythrocytes are specifically retained by Protein A-coated beads. These peculiar binding properties reflect modifications of the infected erythrocyte membrane.     

New permeability pathways induced in membranes of Plasmodium falciparum infected erythrocytes

The permeability properties of the membrane of human erythrocytes infected with malaria parasites (Plasmodium falciparum) were studied by the method of osmotic hemolysis. At the trophozoite stage, the host membrane becomes permeable to substrates such as sorbitol and glucose. The new permeability pathway is insensitive to most inhibitors of the glucose carrier, but is highly susceptible to the membrane dipole modifier phloretin. It is blocked by disaccharides and oligosaccharides, both of which are impermeant to non-infected and infected cells. It has an enthalpy of activation of solute penetration of 10 +/- 1 kcal mol-1 (range of 5-37 degrees C). It appears that new permeability pathways with pore-like properties are induced in parasitized cells. The pore(s) admit(s) neutral and anionic substances of a discrete molecular volume, but exclude(s) cations. Apparently they play an essential role in parasite development.     

Plasmodium falciparum: surface modifications of infected erythrocytes from clinical isolates

 Infections of human erythrocytes with the mature asexual blood stages of Plasmodium falciparum result in antigenic changes in the host cell membrane that, by virtue of their position, length of exposure, and close association with functional changes critical to pathogenesis, are a potential important target for host effector mechanisms. These parasite-induced antigens expressed on the surface of infected erythrocytes have been shown to exhibit considerable polymorphism. An antibody-mediated agglutination assay using malaria serum samples from different regions of Venezuela has been developed to examine the extent of antigenic diversity of infected red blood cells (IRBC) taken from subjects with naturally acquired P. falciparum infections. An important humoral immune recognition of surface molecules from red blood cells infected with a wide variety of clinical isolates of P. falciparum was observed even when sera from individuals experiencing a single episode of malaria were used. A process of in vivo antigenic variation of surface molecules is postulated, since agglutination of IRBC was observed with acute heterologous but not autologous sera. When sera obtained from Amerindians inhabiting the Venezuelan Amazon were assayed, a strong immune response to different parasite isolates, including those of another geographic region, was observed, suggesting the recognition of highly conserved immunogenic parasitic epitopes in people exposed to multiple malaria infections. Received: 20 June 1995 / Accepted: 3 November 1995     

The movement of fluorescent endocytic tracers in Plasmodium falciparum infected erythrocytes.

The fluorescent lipophilic probe 1,1'-dihexadecyl-3-3'-3-3'- tetramethylindocarbocyanine (diIC16) inserted in the red cell surface, functioned as a non-exchangeable lipid marker which was not metabolised or toxic in plasmodial cultures. Invasion by Plasmodium falciparum resulted in the internalisation of the lipid, suggesting the uptake of red cell membrane components during parasite entry. The fluorescent lipid was not transported from red cell to parasite membranes at subsequent stages of development, but label in the erythrocyte-derived parasitophorous vacuole was eventually detected in daughter merozoites. Fluorescent dextrans of 10 kDa in the extracellular medium were also not internalised during intraerythrocytic parasite growth. The results support that with the exception of the invasion step, plasmodial infection does not induce endocytosis in the erythrocyte membrane. Despite the lack of endocytosis, both D and L stereoisomers of the head group blocked phospholipid analogue N-4-nitrobenzo-2-oxa-1,3-diazoledipalmitoyl phosphatidylethanolamine (N-NBD-PE) inserted in the erythrocyte membrane, were internalised by mature infected erythrocytes. Lipid internalisation occurred by a non head group dependent parasite mechanism, which could account for the stage-specific uptake of phospholipids observed in mature infected erythrocytes. We were unable to detect the transport of carbocyanine dyes and N-NBD-PE from intracellular parasites back to the erythrocyte membrane. Additionally, the carbocyanine dyes were not transferred between adjacent organisms in a double infected red cell. The data argue for the absence of bulk membrane lipid transport between individual parasites and their host cell bilayer in an infected erythrocyte.     

Plasmodium falciparum: a simplified technique for obtaining singly infected erythrocytes

We report the development of a simple technique involving 15 ml polypropylene tubes and a rotatory incubator for obtaining erythrocytes singly infected with Plasmodium falciparum. This technique will be useful for cloning of the parasite. Our finding that P. falciparum merozoite invasion is inhibited during rotation suggests that this method may also be useful for the study of parasite-erythrocyte interactions under dynamic circulatory conditions.     

Maturation of Plasmodium falciparum in multiply infected erythrocytes and the potential role in malaria pathogenesis

Erythrocytes containing two or more parasites, referred to here as multiply infected erythrocytes (MIEs), are common in the blood of humans infected by Plasmodium falciparum. It is necessary to study these cells closely because the excess numbers of parasites they contain suggest that they could be overloaded with virulence factors. Here, microscopic examinations of blood smears from patients showed that up to seven merozoites can successfully invade an erythrocyte and mature to ring stage. However, in vitro culture showed that only up to three parasites can mature to late schizont stage. These observations were made by culturing the parasites in erythrocytes containing hemoglobin AA (HbAA), HbAS, and HbSS. Biochemical analysis of saponin-concentrated culture suggests that more hemozoin is produced in a MIE than in a singly infected erythrocyte (SIE). Studies have shown that ingestion of excessive hemozoin destroys monocytes and neutrophils, which could impair the immune system. Cultured parasites were also examined by transmission electron microscopy, and it was found that the quantity of knobs was dramatically increased on the membranes of erythrocytes containing multiple schizonts, compared to those containing only one schizont. Knobs contain, among other things, P. falciparum erythrocyte membrane protein 1 (PfEMP1) complex which mediates sequestration and promotes severe malaria. These findings suggest that P. falciparum increases its virulence by producing MIEs. On sexual life cycle of the parasite, microphotographs are presented in this report showing, for the first time, that two gametocytes can develop in one erythrocyte; they are referred to here as twin gametocytes. It is not known whether they can infect mosquitoes.     

Secretion of a new spherical body protein of Babesia bovis into the cytoplasm of infected erythrocytes

A cDNA encoding a new Babesia bovis spherical body protein 4 (BbSBP-4) was reported to have no significant homology to other apicomplexan proteins or previously reported B. bovis spherical body proteins. In the present study, we further examined the molecular characteristics of BbSBP-4 including the expression and cellular localization of the BbSBP-4. An anti-rBbSBP-4 mouse serum specifically reacted to a 41-kDa native protein B. bovis in Western blot analysis. The immunoelectron microscopic examination confirmed the localization of BbSBP-4 in spherical bodies, but not in the nucleus, rhoptries, and micronemes. Interestingly, the protein was found to be localized not only in the spherical body of B. bovis but also in the cytoplasm of infected erythrocytes (iRBC) at the later stage of parasite development. The confocal laser microscopic examination and Western blot analysis demonstrated the increased accumulation of BbSBP-4 in the cytoplasm of iRBC and in the supernatant of cultivated B. bovis during the late developmental stage of the parasite. These results suggest that BbSBP-4 was secreted from spherical body into cytoplasm of iRBC during the late developmental stage of the parasite before the rupture of infected RBC. Taken together, BbSBP-4 might play an important role as a secreted protein in the intracellular development and/or survival of B. bovis.     

Babesia bovis: Vaccination of cattle against heterologous challenge with fractions of lysate from infected erythrocytes

The distilled water lysate of erythrocytes infected withBabesia bovis was separated by gel filtration on Sephadex G200. The void volume fraction or a pool of retained fractions which had immunodiffusion activity were injected into two groups of five cattle. These were challenged 2 weeks after the final vaccination with a heterologous strain ofB. bovis. A control group of five cattle was similarly challenged. Two of the five control animals died from the challenge, whereas none of the vaccinated animals died. There were significant differences in parasitaemia and pathophysiological parameters between the vaccinated groups and the control group.     

Characterization of a Plasmodium falciparum polypeptide associated with membrane vesicles in the infected erythrocytes

A Plasmodium falciparum polypeptide (46 kDa) associated with the infected erythrocytes of all asexual stages as well as immature gametocytes was identified by the monoclonal antibody (Mab) 30B8.3. The expression of this protein was not dependent upon the knobby phenotype and was detected in parasites grown either in human or Aotus erythrocytes. The antigen was heatstable, did not label with [14C]glucosamine, and was not sensitive to periodate oxidation. Immunofluorescent staining patterns of Mab 30B8.3 on in vitro cultured parasites varied from punctate (rings and trophozoites) to patchy (trophozoites and schizonts) fluorescence. The Mab 30B8.3 antigen was not detected on the infected erythrocyte surface by conventional wet-mount IFA procedure. However, when parasites were cultured in the presence of Mab 30B8.3, the epitope was detected by the monoclonal antibodies present in the culture medium. Differential extraction of the polypeptide from infected erythrocytes and immune electron microscopy of cryosectioned parasites localized the 30B8.3 epitope primarily on membranes of Maurer's clefts within the infected erythrocyte's cytosol. This 46 kDa polypeptide is unique because it seemed to be an integral membrane protein of the Maurer's clefts/vesicles and it was not secreted into the culture medium nor deposited on the infected erythrocyte membrane. Previous studies indicate that several parasite proteins, excreted extracellularly or deposited on infected erythrocyte membrane, are found to be associated with Maurer's cleft membranes and vesicles. The 46 kDa polypeptide described in this study may play an important role in the transport of the parasite antigens.     

Metabolism of glutamine in erythrocytes infected with the human malaria parasite: Plasmodium falciparum

The metabolism of glutamine was studied in erythrocytes infected with Plasmodium falciparum, comparatively to normal cells, in presence or not of DON (6-diazo-5-oxo-L-norleucine) or acivicin, two glutamine antagonists which have been shown to inhibit the growth of P. falciparum in vitro. Extracellular glutamine was partially converted into glutamate using two routes corresponding to gamma-glutamyl transpeptidase (GGT) and glutaminase activities. In cells infected with mature trophozoites, the observed enhancement of the glutamine influx and of the glutamate formation was consistent with the enhancement of GGT and glutaminase activities.     

Stage-specific antigens at the surface of erythrocytes infected by Plasmodium falciparum. Preliminary study

Immunofluorescence microscopy, and ultrastructural immunolabelling have been used to localize antigens determinants present on the surface of prefixed erythrocytes infected by P. falciparum. Two classes of antigens have been demonstrated: 1) one located on intact erythrocytes close to the site of invasion; 2) another detected after saponin treatment, and distributed over the entire surface of infected erythrocytes. Such antigens appeared to have a variable expression according to the stage of the parasite.     

Two approximately 300 kilodalton Plasmodium falciparum proteins at the surface membrane of infected erythrocytes

Two very large Plasmodium falciparum proteins are identified as constituents of the infected erythrocyte membrane. Sera were obtained from Aotus monkeys that had been repeatedly infected with asexual P. falciparum from one of four strains. The capacity of these sera to block in vitro cytoadherence of infected erythrocytes and agglutinate intact infected cells was determined. The sera were also used to immunoprecipitate protein antigens from detergent extracts of 125I-surface labeled or biosynthetically radiolabeled infected erythrocytes. For each serum/antigen combination, precipitation of only one protein correlated with the ability of the serum to interfere with cytoadherence and agglutinate infected cells. This malarial protein, denoted Pf EMP 1 (P. falciparum-erythrocyte-membrane-protein 1) bore strain-specific epitope(s) on the cell surface and displayed size heterogeneity (Mr approximately 220,000-350,000). Pf EMP 1 was strongly labeled by cell-surface radioiodination but was a quantitatively very minor malarial protein. Pf EMP 1 was distinguished by its size, surface accessibility and antigenic properties from a more predominant malarial protein in the same size range (Pf EMP 2) that is under the infected erythrocyte membrane at knobs. Monoclonal antibodies and rabbit antisera raised against Pf EMP 2 were used to show that this size heterogeneous antigen was indistinguishable from the previously described MESA (mature parasite infected erythrocyte surface antigen), identified by precipitation with rabbit antisera raised against the MESA hexapeptide repeats. Antibodies raised against Pf EMP 2/MESA did not precipitate Pf EMP 1. We conclude that Pf EMP 1 is either directly responsible for the cytoadherence phenomenon, or is very closely associated with another as yet unidentified functional molecule. Pf EMP 2/MESA must have a structural property/function that is important under the host cell membrane.