A monoclonal antibody to rhesus erythrocyte band 3 inhibits invasion by malaria (Plasmodium knowlesi) merozoites

The Plasmodium vivax MSP1P-19 is involved in binding of reticulocytes through interactions with the membrane proteins band3 and CD71

The parasite Plasmodium vivax preferentially invades human reticulocytes. Its merozoite surface protein 1 paralog (PvMSP1P), particularly the 19-kDa C-terminal region (PvMSP1P-19), has been shown to bind to reticulocytes, and this binding can be inhibited by antisera obtained by PvMSP1P-19 immunization. The molecular mechanism of interactions between PvMSP1P-19 and reticulocytes during P. vivax invasion, however, remains unclear. In this study, we analyzed the ability of MSP1P-19 to bind to different concentrations of reticulocytes and confirmed its reticulocyte preference. LC-MS analysis was used to identify two potential reticulocyte receptors, band3 and CD71, that interact with MSP1P-19. Both PvMSP1P-19 and its sister taxon Plasmodium cynomolgi MSP1P-19 were found to bind to the extracellular loop (loop 5) of band3, where the interaction of MSP1P-19 with band3 was chymotrypsin sensitive. Antibodies against band3-P5, CD71, and MSP1P-19 reduced the binding activity of PvMSP1P-19 and Plasmodium cynomolgi MSP1P-19 to reticulocytes, while MSP1P-19 proteins inhibited Plasmodium falciparum invasion in vitro in a concentration-dependent manner. To sum up, identification and characterization of the reticulocyte receptor is important for understanding the binding of reticulocytes by MSP1P-19.

K562 erythroleukemia line as a possible reticulocyte source to culture Plasmodium vivax and its surrogates

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miR-26a and miR-30a knockdowns promote differentiation in Fy-transduced K562 cell lines.

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miR-26a and miR-30a knockdowns promote enucleation in Fy-transduced K562 cell lines.

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Data denote an interplay in the mode of action of miR-26a and miR-30a in erythropoiesis.

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Plasmodium cynomolgi and P. knowlesi invade, albeit inefficiently, Fy-transduced K562 cells.

Establishing an in vitro “red blood cell matrix” that would allow uninterrupted access to a stable, homogeneous reticulocyte population would facilitate the establishment of continuous, long-term in vitro Plasmodium vivax blood stage cultures. In this study, we have explored the suitability of the erythroleukemia K562 cell line as a continuous source of such reticulocytes and have investigated regulatory factors behind the terminal differentiation (and enucleation, in particular) of this cell line that can be used to drive the reticulocyte production process. The Duffy blood group antigen receptor (Fy), essential for P. vivax invasion, was stably introduced into K562 cells by lentiviral gene transfer. miRNA-26a-5p and miRNA-30a-5p were downregulated to promote erythroid differentiation and enucleation, resulting in a tenfold increase in the production of reticulocytes after stimulation with an induction cocktail compared with controls. Our results suggest an interplay in the mechanisms of action of miRNA-26a-5p and miRNA-30a-5p, which makes it necessary to downregulate both miRNAs to achieve a stable enucleation rate and Fy receptor expression. In the context of establishing P. vivax-permissive, stable, and reproducible reticulocytes, a higher enucleation rate may be desirable, which may be achieved by the targeting of further regulatory mechanisms in Fy-K562 cells; promoting the shift in hemoglobin production from fetal to adult may also be necessary. Despite the fact that K562 erythroleukemia cell lines are of neoplastic origin, this cell line offers a versatile model system to research the regulatory mechanisms underlying erythropoiesis.

Multiple Plasmodium vivax proteins of Pv-fam-a family interact with human erythrocyte receptor Band 3 and have a role in red cell invasion

Elucidation of molecular mechanisms of receptor-ligand biology during host-parasite interaction helps in developing therapeutic targets. Several Pv-fam-a family proteins of Plasmodium vivax bind to host erythrocytes but their erythrocyte receptors remains to be explored. Here, we show that three merozoite proteins (PvTRAg36, PvATRAg74, and PvTRAg38) of this family interact with Band 3 on human erythrocytes through its three exofacial loops (loop 1, loop 3, and loop 6). These parasite proteins also interfered with the parasite growth in in-vitro, and the inhibition rate seems to be associated with their binding affinity to Band 3. This redundancy in receptor-ligand interaction could be one of the probable mechanism parasite utilizes to invade the host erythrocyte more efficiently.

Interaction of Plasmodium vivax Tryptophan-rich Antigen PvTRAg38 with Band 3 on Human Erythrocyte Surface Facilitates Parasite Growth

Plasmodium tryptophan-rich proteins are involved in host-parasite interaction and thus potential drug/vaccine targets. Recently, we have described several P. vivax tryptophan-rich antigens (PvTRAgs), including merozoite expressed PvTRAg38, from this noncultivable human malaria parasite. PvTRAg38 is highly immunogenic in humans and binds to host erythrocytes, and this binding is inhibited by the patient sera. This binding is also affected if host erythrocytes were pretreated with chymotrypsin. Here, Band 3 has been identified as the chymotrypsin-sensitive erythrocyte receptor for this parasite protein. Interaction of PvTRAg38 with Band 3 has been mapped to its three different ectodomains (loops 1, 3, and 6) exposed at the surface of the erythrocyte. The binding region of PvTRAg38 to Band3 has been mapped to its sequence, KWVQWKNDKIRSWLSSEW, present at amino acid positions 197-214. The recombinant PvTRAg38 was able to inhibit the parasite growth in in vitro Plasmodium falciparum culture probably by competing with the ligand(s) of this heterologous parasite for the erythrocyte Band 3 receptor. In conclusion, the host-parasite interaction at the molecular level is much more complicated than known so far and should be considered during the development of anti-malarial therapeutics.

Identification of Bartonella Trw host-specific receptor on erythrocytes

Each Bartonella species appears to be highly adapted to one or a limited number of reservoir hosts, in which it establishes long-lasting intraerythrocytic bacteremia as the hallmark of infection. Recently, we identified Trw as the bacterial system involved in recognition of erythrocytes according to their animal origin. The T4SS Trw is characterized by a multiprotein complex that spans the inner and outer bacterial membranes, and possesses a hypothetical pilus structure. TrwJ, I, H and trwL are present in variable copy numbers in different species and the multiple copies of trwL and trwJ in the Bartonella trw locus are considered to encode variant forms of surface-exposed pilus components. We therefore aimed to identify which of the candidate Trw pilus components were located on the bacterial surface and involved in adhesion to erythrocytes, together with their erythrocytic receptor. Using different technologies (electron microscopy, phage display, invasion inhibition assay, far western blot), we found that only TrwJ1 and TrwJ2 were expressed and localized at the cell surface of B. birtlesii and had the ability to bind to mouse erythrocytes, and that their receptor was band3, one of the major outer-membrane glycoproteins of erythrocytes, (anion exchanger). According to these results, we propose that the interaction between TrwJ1, TrwJ2 and band 3 leads to the critical host-specific adherence of Bartonella to its host cells, erythrocytes.

Identification of a cross-reacting, monoclonal anti-human CD233 antibody for identification and sorting of rhesus macaque erythrocytes

Erythroid biology research involving rhesus macaques has been applied to several topics including malaria, hemoglobinopathy and gene therapy research. However, analyses of the rhesus red blood cells are limited by the inability to identify and sort those cells in research blood samples using flow cytometry. Here it is reported that the BRIC 6 hybridoma clone raised to the human erythroid surface molecule (referred to as CD233, Band 3, AE1, or SLC4A1) produces cross-reactive and erythroid-specific antibodies for flow cytometric detection and sorting of rhesus macaque erythrocytes.

Host receptors in malaria merozoite invasion

The clinical manifestations of Plasmodium falciparum malaria are directly linked to the blood stage of the parasite life cycle. At the blood stage, the circulating merozoites invade erythrocytes via a specific invasion pathway often identified with its dependence or independence on sialic acid residues of the host receptor. The invasion process involves multiple receptor-ligand interactions that mediate a complex series of events in a period of approximately 1 min. Although the mechanism by which merozoites invade erythrocytes is not fully understood, recent advances have put a new perspective on the importance of developing a multivalent blood stage-malaria vaccine. In this review, we highlight the role of currently identified host invasion receptors in blood-stage malaria.

Malaria proteases and red blood cell invasion

Studies of malaria proteases have focused on two general groups, corresponding to activities specific to malaria parasites: (1) proteases involved in hemoglobin degradation which are active in the food vacuole and which exhibit optimal activity at low pH; and (2) proteases specific to schizonts and/or merozoites which are involved in merozoite maturation and red blood cell invasion and which exhibit optimal activity at neutral pH. In this paper, Catherine Braun Breton and Luis H. Pereira da Silva will focus on those activities necessary for the release of infectious merozoites and the entry of the parasite into its host cell.

Babesia divergens and Plasmodium falciparum use common receptors, glycophorins A and B, to invade the human red blood cell

Babesiosis has long been recognized as an economically important disease of cattle, but only in the last 30 years has Babesia been recognized as an important pathogen in humans. Invasion of erythrocytes is an integral part of the Babesia life cycle. However, very little information is available on the molecules involved in this process, in contrast to another hemoparasite, Plasmodium falciparum. Using invasion assays into normal red blood cells (RBCs), enzyme-treated cells, and clinically mutant cells, we showed that Babesia divergens uses neuraminidase- and trypsin-sensitive receptors to enter the RBCs, of which glycophorins A and B are the prominent ones. These results could have broad implications relating to evolutionarily conserved mechanisms of host cell entry in these related Apicomplexan parasites and pave the way toward a detailed molecular analysis of erythrocyte invasion in B. divergens.

Ability of Plasmodium falciparum to invade Southeast Asian ovalocytes varies between parasite lines

Plasmodium falciparum, the causative agent of the most lethal form of human malaria, uses multiple ligand-receptor interactions to invade host red blood cells (RBCs). We studied the invasion of P falciparum into abnormal RBCs from humans carrying the Southeast Asian ovalocytosis (SAO) trait. One particular parasite line, 3D7-A, invaded these cells efficiently, whereas all other lines studied invaded SAO RBCs to only about 20% of the extent of normal (non-SAO) cells. This result is consistent with the clinical observation that SAO individuals can experience high-density P falciparum infections and provides an explanation for previous discrepant results on invasion of SAO RBCs. Characterization of the invasion phenotype of 3D7-A revealed that efficient invasion of SAO RBCs was paralleled by relatively efficient invasion of normal RBCs treated with either neuraminidase, trypsin, or chymotrypsin and a novel capacity to invade normal RBCs treated sequentially with both neuraminidase and trypsin. Our results suggest that only parasites able to use some particular invasion pathways can invade SAO RBCs efficiently in culture. A similar situation might occur in the field.

Identification of a piroplasm protein of Theileria orientalis that binds to bovine erythrocyte band 3

Theileria orientalis infects cattle and causes various disease symptoms, including anaemia and icterus. The erythrocytic stages are responsible for these symptoms but the molecular events involved in these stages have not yet been fully elucidated. In this study, we identified a T. orientalis cDNA that encodes a polypeptide related to identity to the microneme-rhoptry protein of Theileria parva. Analysis of its recombinant product (ToMRP) by indirect fluorescent-antibody test revealed that it is specifically expressed at the early erythrocytic stage after invasion. This expression disappears during the intermediate stages of intra-erythrocytic development. Its expression then reappears at the late stages after the parasite has divided by binary fission into diad or tetrad forms and before these forms are released from the host erythrocyte. In vitro erythrocyte binding assays showed that ToMRP associates with the Triton X-insoluble fraction of erythrocytes membrane but not with intact erythrocytes. Cosedimentation and Western blot analyses revealed that ToMRP binds to band 3, a membrane component of bovine erythrocytes. These observations suggest that ToMRP may be involved in the parasite's egress from and/or invasion into the host erythrocytes by interacting with a protein in the membrane skeleton of the erythrocyte and thereby modifying the structure and function of the cell.

Band 3 is a host receptor binding merozoite surface protein 1 during the Plasmodium falciparum invasion of erythrocytes

We report the molecular identification of a sialic acid-independent host-parasite interaction in the Plasmodium falciparum malaria parasite invasion of RBCs. Two nonglycosylated exofacial regions of human band 3 in the RBC membrane were identified as a crucial host receptor binding the C-terminal processing products of merozoite surface protein 1 (MSP1). Peptides derived from the receptor region of band 3 inhibited the invasion of RBCs by P. falciparum. A major segment of the band 3 receptor (5ABC) bound to native MSP1(42) and blocked the interaction of native MSP1(42) with intact RBCs in vitro. Recombinant MSP1(19) (the C-terminal domain of MSP1(42)) bound to 5ABC as well as RBCs. The binding of both native MSP1(42) and recombinant MSP1(19) was not affected by the neuraminidase treatment of RBCs, but sensitive to chymotrypsin treatment. In addition, recombinant MSP1(38) showed similar interactions with the band 3 receptor and RBCs, although the interaction was relatively weak. These findings suggest that the chymotrypsin-sensitive MSP1-band 3 interaction plays a role in a sialic acid-independent invasion pathway and reveal the function of MSP1 in the Plasmodium invasion of RBCs.

Malaria parasite exit from the host erythrocyte: a two-step process requiring extraerythrocytic proteolysis

Intraerythrocytic malaria parasites replicate by the process of schizogeny, during which time they copy their genetic material and package it into infective merozoites. These merozoites must then exit the host cell to invade new erythrocytes. To better characterize the events of merozoite escape, erythrocytes containing Plasmodium falciparum schizonts were cultured in the presence of the cysteine protease inhibitor, l-transepoxy-succinyl-leucylamido-(4-guanidino)butane (E64). This treatment resulted in the accumulation of extraerythrocytic merozoites locked within a thin, transparent membrane. Immunomicroscopy demonstrated that the single membrane surrounding the merozoites is not erythrocytic but rather is derived from the parasitophorous vacuolar membrane (PVM). Importantly, structures identical in appearance can be detected in untreated cultures at low frequency. Further studies revealed that (i) merozoites from the PVM-enclosed merozoite structures (PEMS) are invasive, viable, and capable of normal development; (ii) PEMS can be purified easily and efficiently; and (iii) when PEMS are added to uninfected red blood cells, released merozoites can establish a synchronous wave of infection. These observations suggest that l-transepoxy-succinyl-leucylamido-(4-guanidino)butane (E64) causes an accumulation of an intermediate normally present during the process of rupture. We propose a model for the process of rupture: merozoites enclosed within the PVM first exit from the host erythrocyte and then rapidly escape from the PVM by a proteolysis-dependent mechanism.

Malaria parasite exit from the host erythrocyte: A two-step process requiring extraerythrocytic proteolysis

Intraerythrocytic malaria parasites replicate by the process of schizogeny, during which time they copy their genetic material and package it into infective merozoites. These merozoites must then exit the host cell to invade new erythrocytes. To better characterize the events of merozoite escape, erythrocytes containing Plasmodium falciparum schizonts were cultured in the presence of the cysteine protease inhibitor, l-transepoxy-succinyl-leucylamido-(4-guanidino)butane (E64). This treatment resulted in the accumulation of extraerythrocytic merozoites locked within a thin, transparent membrane. Immunomicroscopy demonstrated that the single membrane surrounding the merozoites is not erythrocytic but rather is derived from the parasitophorous vacuolar membrane (PVM). Importantly, structures identical in appearance can be detected in untreated cultures at low frequency. Further studies revealed that (i) merozoites from the PVM-enclosed merozoite structures (PEMS) are invasive, viable, and capable of normal development; (ii) PEMS can be purified easily and efficiently; and (iii) when PEMS are added to uninfected red blood cells, released merozoites can establish a synchronous wave of infection. These observations suggest that l-transepoxy-succinyl-leucylamido-(4-guanidino)butane (E64) causes an accumulation of an intermediate normally present during the process of rupture. We propose a model for the process of rupture: merozoites enclosed within the PVM first exit from the host erythrocyte and then rapidly escape from the PVM by a proteolysis-dependent mechanism.

The Primate Erythrocyte Complement Receptor (CR1) as a Privileged Site: Binding of Immunoglobulin G to Erythrocyte CR1 Does Not Target Erythrocytes for Phagocytosis

The primate erythrocyte (E) complement receptor, CR1, is a transmembrane glycoprotein located in clusters on the surface of E. In vivo studies have demonstrated that during processing and clearance of complement-opsonized immune complexes, large amounts of immunoglobulin G (IgG) can be bound to primate E via CR1 with no E loss or lysis. However, when comparable amounts of IgG are bound to other sites on E, in many cases the E are cleared from the circulation by the mononuclear phagocytic system. Therefore, due to its role in immune complex processing, CR1 may represent a privileged site on the primate E. To delineate further this property of E CR1, we performed in vitro phagocytosis assays in the absence of complement and examined the ingestion of E, opsonized at various sites with IgG, by peripheral blood monocytes. When either human or rhesus monkey E were opsonized at sites other than CR1, with between 1,000 and 15,000 IgG per E, substantial phagocytosis of E was evident. However, when comparable amounts of IgG were bound exclusively via CR1, little, if any, phagocytosis was observed. The key to the low phagocytic level of E opsonized via CR1 may be related to the requirements of a “zipper mechanism” for phagocytosis first annunciated by Griffin et al. Based on their findings, we suggest that due to the presence of preexisting clusters of CR1 on the E membrane, large amounts of IgG can be bound to E under conditions that preclude circumferential engagement (and phagocytosis) of the entire E by Fc receptors on the monocyte.

The origin of parasitophorous vacuole membrane lipids in malaria-infected erythrocytes

During invasion of an erythrocyte by a malaria merozoite, an indentation develops in the erythrocyte surface at the point of contact between the two cells. This indentation deepens as invasion progresses, until the merozoite is completely surrounded by a membrane known as the parasitophorous vacuole membrane (PVM). We incorporated fluorescent lipophilic probes and phospholipid analogs into the erythrocyte membrane, and followed the fate of these probes during PVM formation with low-light-level video fluorescence microscopy. The concentration of probe in the forming PVM was indistinguishable from the concentration of probe in the erythrocyte membrane, suggesting that the lipids of the PVM are continuous with and derived from the host cell membrane during invasion. In contrast, fluorescently labeled erythrocyte surface proteins were largely excluded from the forming PVM. These data are consistent with a model for PVM formation in which the merozoite induces a localized invagination in the erythrocyte lipid bilayer, concomitant with a localized restructuring of the host cell cytoskeleton.

In vitro assays for the study of erythrocyte invasion by malarial parasites

In vitro assays for the study o f erythrocyte invasion by merozoites are available for several primate and rodent malarial species. These assays are essential means by which potential anti-merozoite vaccine candidates are identified. John Dalton, John McNally and Susan O'Donovan describe the various types of invasion assays that are in current use, outline the procedures for performing these assays and add some pointers on interpretation of data derived from them.

Plasmodium chabaudi p68 serine protease activity required for merozoite entry into mouse erythrocytes

To define the role of malaria parasite enzymes during the process of erythrocyte invasion, we have developed an in vitro serum-free invasion assay of mouse erythrocytes by purified Plasmodium chabaudi merozoites. The sensitivity of a merozoite-specific serine protease (p68) to various inhibitors and the effect of these inhibitors on invasion indicate a crucial role for p68. The substrate specificity of the purified enzyme has been partially defined using fluorogenic peptides. Consistent with this, in vitro incubation of mouse erythrocytes with the merozoite enzyme led to the cleavage of band 3 protein. The possible implication of erythrocyte band 3 truncation for the successful entry of the merozoite into the erythrocyte is discussed.

Blocking of the receptor-mediated invasion of erythrocytes by Plasmodium knowlesi malaria with sulfated polysaccharides and glycosaminoglycans

Invasion of human erythrocytes by Plasmodium knowlesi requires the Duffy blood group antigen. P. knowlesi merozoites synthesize a 135-kDa polypeptide which binds to the Duffy antigen with receptor-like specificity. In this study, we show that the sulfated polysaccharide fucoidan and the glycosaminoglycan dextran sulfate inhibit the binding of the 135-kDa polypeptide to human Duffy-positive and rhesus erythrocytes while the chondroitin sulfates do not. Fucoidan and dextran sulphate also blocked the in vitro invasion of human Duffy b and rhesus erythrocytes cells by P. knowlesi merozoites. These inhibitors were more effective at blocking the binding of the 135-kDa polypeptide to human Duffy b erythrocytes than to rhesus erythrocytes, which correlated with them having a greater inhibitory effect on invasion of merozoites into human than into rhesus erythrocytes. The blocking by these sulfated sugars is not related to charge density on the polysaccharides; fucoidan with a relatively low charge density blocks binding of the 135-kDa polypeptide at 4 micrograms/ml, while the highly negatively charged chondroitin sulfates do not block binding even at the concentration of 1 mg/ml. Furthermore, fucoidan-Sepharose bound and removed the 135-kDa polypeptide from parasite culture supernatants with a selectivity equal to that of the Duffy blood group antigen. The negatively charged sulfate groups on fucoidan and dextran sulfate and the conformation in which they are held possibly mimic similarly charged groups on the Duffy antigen which bind the 135-kDa P. knowlesi polypeptide.

Release of merozoite dense granules during erythrocyte invasion by Plasmodium knowlesi

We used immunoelectron microscopy to study the fate of dense granules during the invasion of erythrocytes by Plasmodium knowlesi merozoites. When merozoites entered host cells, dense granules moved to the pellicle, released their contents into the parasitophorous vacuole space, and then moved into fingerlike channels of the vacuole membrane. This is the first report showing that the content of dense granules of P. knowlesi is different from the contents of rhoptries and micronemes and is associated with the formation of channels from the parasitophorous vacuole.

A study of red cell membrane properties in relation to malarial invasion

The shape and mechanical properties of human red cells were modified in several ways and the consequences for the efficiency of invasion by Plasmodium falciparum in culture were investigated. Inhibition of invasion by depletion of ATP was shown to be unrelated to cell shape or deformability changes. Treatment of cells with N-ethylmaleimide (NEM), which dissociates some 70% of the native spectrin tetramers into the dimer, grossly reduced deformation of the cells under shear and increased by a factor of two or more the shear elastic modulus, as measured by the micropipette aspiration technique. Cells thus treated were efficiently invaded by P. falciparum (ca. 75% of control). In a population of cells pretreated with chlorpromazine, parasites were found in stomatocytic cells which were highly undeformable under shear. There was also considerable invasion into cells from subjects with hereditary pyropoikilocytosis, and two types of elliptocytosis. Cells treated with wheat germ agglutinin showed a dose-dependent increase in rigidity; a fivefold increase in elastic modulus (with total loss of deformation under shear in our conditions) still permitted invasion at a level of 50% of the control. The results suggest that gross mechanical properties of the membrane per se, at least within any physiologically relevant range, are unlikely to be the primary determinant of malarial invasion; this may instead be linked to the freedom of membrane proteins to migrate in the course of entry of the parasite.

Malaria parasite invasion: interactions with the red cell membrane

The capacity to invade red cells is central to the biology of malaria parasites; both asexual multiplication and reinfection of the definitive mosquito host depend upon intraerythrocytic stages. The invasion process is complex. The briefly free merozoite specifically recognizes and adheres to ligands on the red cell surface, then alters the red cell membrane to produce an invagination into which it moves, and so becomes enclosed in a membrane-bound parasitophorous vacuole. Here we assess new evidence that bears on our understanding of this process. This has come from sources including biochemical and ultrastructural studies of the specialized surface and organelles of merozoites, from in vitro invasion studies using naturally refractory or artificially modified red cells, and from structural, chemical, and immunological analyses of the newly parasitized cell.

Erythrocyte receptor recognition varies in Plasmodium falciparum isolates

N-Acetylneuraminic acid (NeuNAc) is the terminal sugar residue of the O-linked tetrasaccharide linked to erythrocyte sialoglycoproteins, glycophorins. Erythrocytes lacking NeuNAc have been shown previously to be resistant to invasion by certain isolates of Plasmodium falciparum merozoites. We report here variation between different geographic isolates of P. falciparum in their dependency on NeuNAc for invasion of host erythrocytes. Seven different geographic isolates of P. falciparum were examined for their ability to invade neuraminidase treated erythrocytes. For all isolates invasion was reduced significantly, although considerable variation in NeuNAc dependency was apparent. Three isolates, FCR-3, FVO and It2, exhibited a very high dependence on NeuNAc residues for invasion (invasion reduced greater than 90%), whereas two isolates (Thai-Tn and FC-27) exhibited a moderately high dependence (invasion reduced 75%). Two other isolates (CDC-1 and 7G8) exhibited moderate dependence on NeuNAc (invasion reduced 50%). Cleavage of the complete O-linked tetrasaccharide by O-glycanase removes all carbohydrate from glycophorin A, B and C except the single N-linked oligosaccharide on glycophorin A and C. Invasion of FCR-3 and CDC-1 isolates into O-glycanase treated erythrocytes was not markedly different from that into neuraminidase treated cells indicating that NeuNAc is the important residue of the tetrasaccharide for both isolates. Invasion into endo-beta-galactosidase treated erythrocytes, in which the lactosaminoglycan side chain of band 3 and band 4.5 is cleaved, was not significantly reduced for either the CDC-1 or FCR-3 isolates. Additional results on the trypsin insensitivity of band 3 also suggest that this erythrocyte protein is not important in P. falciparum recognition. The greatest divergence in receptor specificity between FCR-3 and CDC-1 isolates was apparent in invasion into periodate-treated erythrocytes. Periodate oxidation results in cleavage of the exocyclic hydroxyl groups of the terminal NeuNAc but leaves its COOH group unaltered. These experiments also illustrated that the negatively charged COOH group of NeuNAc is not the important group in the interaction of the merozoite with the NeuNAc. Trypsin-treated erythrocytes were almost fully resistant to invasion by CDC-1 as well as the FCR-3 isolates suggesting that the CDC-1 isolate, in addition to interacting with NeuNAc, depends on a trypsin sensitive site for invasion. This site could involve the N-linked saccharide on glycophorin A and C or a protein on the erythrocyte surface unrelated to the glycophorins.

Identification of three stage-specific proteinases of Plasmodium falciparum

We have identified and characterized three stage-specific proteinases of Plasmodium falciparum that are active at neutral pH. We analyzed ring-, trophozoite-, schizont-, and merozoite-stage parasites by gelatin substrate PAGE and characterized the identified proteinases with class-specific proteinase inhibitors. No proteinase activity was detected with rings. Trophozoites had a 28 kD proteinase that was inhibited by inhibitors of cysteine proteinases. Mature schizonts had a 35-40 kD proteinase that also was inhibited by cysteine proteinase inhibitors. Merozoite fractions had a 75 kD proteinase that was inhibited by serine proteinase inhibitors. The stage-specific activity of these proteinases and the correlation between the effects of proteinase inhibitors on the isolated enzymes with the effects of the inhibitors on whole parasites suggest potential critical functions for these proteinases in the life cycle of malaria parasites.

Inhibition of malarial invasion by intracellular antibodies against intrinsic membrane proteins in the red cell

It has previously been shown that antibodies against the transmembrane proteins, band 3 and glycophorin A, inhibit entry of the merozoite into the red cell and, in the case of band 3, it was established that attachment of the parasite to the cell is not prevented. We have found that antibodies against the cytoplasmic domains of band 3 and of glycophorin A, when present in the interior of resealed ghosts of human red cells, also inhibit invasion by P. falciparum. It is inferred that attachment of the merozoite to the red cell causes structural effects that are transduced to the membrane cytoskeleton and the antibodies against transmembrane proteins interfere with the invasion sequence at this level.

Receptors for the malarial parasite

During the erythrocytic cycle of Plasmodium, the parasite develops within an enclosed space, the parasitophorous vacuole, formed by endocytosis of an invasive stage, the merozoite. Among the erythrocyte membrane proteins possibly acting as a receptor for the attachment of P. falciparum merozoites to human erythrocytes is glycophorin A. Isolated glycophorin inhibits merozoite entry in a competitive manner, perhaps via association with a 155 kDa surface protein. Another protein that competitively inhibits merozoite invasion, is band 3, the erythrocyte anion transport protein. The protein bearing Duffy blood group antigens may act to modulate invasion, but does not behave as a receptor.

Evidence for a malarial parasite interaction site on the major transmembrane protein of the human erythrocyte

Soluble oligosaccharides derived from the surface of human erythrocytes were tested for their ability to competitively inhibit invasion of erythrocytes by Plasmodium falciparum, a malarial parasite. Invasion was most effectively inhibited by erythroglycan, a carbohydrate component of the band 3 transmembrane protein. The lactosamine chains of erythroglycan contributed much of the inhibitory activity. This indication of a primary parasite interaction site on band 3 supports a role for this protein in mediating the radical alterations of the erythrocyte cytoskeleton that accompany invasion.

Plasmodium falciparum malaria: band 3 as a possible receptor during invasion of human erythrocytes

Human erythrocyte band 3, a major membrane-spanning protein, was purified and incorporated into liposomes. These liposomes, at nanomolar concentrations of protein, inhibited invasion of human erythrocytes in vitro by the malaria parasite Plasmodium falciparum. Liposomes containing human band 3 were ten times more effective in inhibiting invasion than those with pig band 3 and six times more effective than liposomes containing human erythrocyte glycophorin. Liposomes alone or liposomes containing erythrocyte glycolipids did not inhibit invasion. These results suggest that band 3 participates in the invasion process in a step involving a specific, high-affinity interaction between band 3 and some component of the parasite.

Identification of a strain-specific malarial antigen exposed on the surface of Plasmodium falciparum-infected erythrocytes

We have identified strain-specific antigens with Camp and St. Lucia strains of P. falciparum of Mr approximately 285,000 and approximately 260,000, respectively. These strain-specific antigens were metabolically labeled with radioactive amino acids, indicating that they were of parasite origin rather than altered host components. These proteins had the properties of a molecule exposed on the surface of infected erythrocytes (IE). First, the proteins are accessible to lactoperoxidase-catalyzed radioiodination of IE. Second, the radioiodinated proteins were cleaved by low concentrations of trypsin (0.1 microgram/ml). Third, these antigens were immunoprecipitated after addition of immune sera to intact IE. Fourth, the strain-specific immuno-precipitation of these proteins correlated with the capacity of immune sera to block cytoadherence of IE in a strain-specific fashion. Fifth, the strain-specific antigen had detergent solubility properties (i.e., insolubility in 1% Triton X-100, solubility in 5% sodium dodecyl sulfate) similar to the variant antigen of P. knowlesi, which has been proven to be a malarial protein exposed on the erythrocyte surface.

Identification of parasite proteins in a membrane preparation enriched for the surface membrane of erythrocytes infected with Plasmodium knowlesi

A subcellular fraction enriched in erythrocyte membranes has been isolated from rhesus monkey erythrocytes infected with Plasmodium knowlesi. Infected cells were lysed by centrifugation through a zone of hypotonic buffer and membranes isolated by equilibrium density gradient centrifugation in the same tube. The purified membrane fraction was shown to include the erythrocyte surface membrane by several methods: electron microscopy, identification of Coomassie Blue stained erythrocyte membrane proteins, identification of band 3 with a monoclonal antibody, and identification of radioiodinated cell surface proteins. The resulting ghosts were shown to be specifically reactive with monkey sera against the variant surface antigens of P. knowlesi by indirect immunofluorescence and membrane agglutination. No reactivity was seen with a monoclonal antibody (13C11) against the intracellular schizont surface. A number of metabolically labelled parasite proteins were enriched in this membrane function, including peptides of 277, 208, 173, 153, 134, 109, 80, 60 and 48 kDa and the variant surface antigens of variable molecular mass (180-207 kDa). These proteins were distinct from the major parasite proteins of total infected erythrocytes and isolated merozoites. The major glucosamine labelled glycoprotein of the internal schizont (230 kDa) was not found in this fraction. Moreover, no fragment of this parasite glycoprotein was found in this membrane fraction, indicating that no part of this molecule is transported to the erythrocyte surface. In contrast, the variant antigen of P. knowlesi, known to be on the erythrocyte surface, could be readily identified as peptides unique to specific cloned parasite lines. We propose that the other nine parasite proteins found within this membrane fraction represent a starting point for the identification of other parasite proteins transported to the surface membrane of the infected erythrocyte.

Processing of a major parasite surface glycoprotein during the ultimate stages of differentiation in Plasmodium knowlesi

A monoclonal antibody (13C11) was used to investigate the processing of a Plasmodium knowlesi plasma membrane protein during the late stages of schizogony. 13C11 bound to the surface of merozoites, blocked invasion of erythrocytes and immunoprecipitated a 230 kDa glycoprotein from metabolically labelled schizonts. This protein was a major parasite surface component inserted into the membrane of immature schizonts as shown through the study of saponin-freed schizonts which bound 13C11 to their surface (indirect immunofluorescence and immunoelectron microscopy); in addition, the 230 kDa protein on saponin-freed schizonts was susceptible to trypsin treatment. Cleavage of the protein in pulse-chase experiments was followed by immunoprecipitation with 13C11. As schizogony proceeded, the 230 kDa protein was cleaved to 200, 145 and 110 kDa polypeptides. However, this cleavage did not reflect processing but occurred in vitro during detergent extraction and was due to a proteolytic activity which appeared in the parasite during the later stages of schizogony. As schizonts reached maturity and infected erythrocytes lysed, the 230 kDa protein was processed to 75, 57, 50 kDa and 43 kDa polypeptides which were the surface labelled components on purified merozoites immunoprecipitated by 13C11.