Immunity to malarial antigens on the surface of Plasmodium falciparum-infected erythrocytes

Anti-Self Phosphatidylserine Antibodies Recognize Uninfected Erythrocytes Promoting Malarial Anemia

Plasmodium species, the parasitic agents of malaria, invade erythrocytes to reproduce, resulting in erythrocyte loss. However, a greater loss is caused by the elimination of uninfected erythrocytes, sometimes long after infection has been cleared. Using a mouse model, we found that Plasmodium infection induces the generation of anti-self antibodies that bind to the surface of uninfected erythrocytes from infected, but not uninfected, mice. These antibodies recognize phosphatidylserine, which is exposed on the surface of a fraction of uninfected erythrocytes during malaria. We find that phosphatidylserine-exposing erythrocytes are reticulocytes expressing high levels of CD47, a "do-not-eat-me" signal, but the binding of anti-phosphatidylserine antibodies mediates their phagocytosis, contributing to anemia. In human patients with late postmalarial anemia, we found a strong inverse correlation between the levels of anti-phosphatidylserine antibodies and plasma hemoglobin, suggesting a similar role in humans. Inhibition of this pathway may be exploited for treating malarial anemia.

Antigenic variation at the infected red cell surface in malaria

Many pathogens that either rely on an insect vector to complete their life cycle (e.g., Trypanosoma spp. and Borrelia spp.) or exist in a unique ecological niche where transmission from host to host is sporadic (e.g., Neisseria spp.) have evolved strategies to maintain infection of their mammalian hosts for long periods of time in order to ensure their survival. Because they have to survive in the face of a fully functional immune system, a common feature of many of these organisms is their development of sophisticated strategies for immune evasion. For the above organisms and for malaria parasites of the genus Plasmodium, a common theme is the ability to undergo clonal antigenic variation. In all cases, surface molecules that are important targets of the humoral immune response are encoded in the genome as multicopy, nonallelic gene families. Antigenic variation is accomplished by the successive expression of members of these gene families that show little or no immunological cross-reactivity. In the case of malaria parasites, however, some of the molecules that undergo antigenic variation are also major virulence factors, adding an additional level of complication to the host-parasite interaction. In this review, we cover the history of antigenic variation in malaria and then summarize the more recent data with particular emphasis on Plasmodium falciparum, the etiological agent of the most severe form of human malaria.

Growth of Plasmodium falciparum induces stage-dependent haemichrome formation, oxidative aggregation of band 3, membrane deposition of complement and antibodies, and phagocytosis of parasitized erythrocytes

Plasmodium falciparum-parasitized erythrocytes (RBCs) are progressively transformed into non-self cells, phagocytosed by human monocytes. Haemichromes, aggregated band 3 (Bd3) and membrane-bound complement fragment C3c and IgG were assayed in serum-opsonized stage-separated parasitized RBCs. All parameters progressed from control to rings to trophozoites to schizonts: haemichromes, nil; 0.64 +/- 0.12; 5.6 +/- 1.91; 8.4 +/- 2.8 (nmol/ml membrane); Bd3, 1 +/- 0.1; 4.3 +/- 1.5; 23 +/- 5; 25 +/- 6 (percentage aggregated); C3c, 31 +/- 11; 223 +/- 86; 446 +/- 157; 620 +/- 120 (mOD405/min/ml membrane); IgG, 35 +/- 12; 65 +/- 23; 436 +/- 127; 590 +/- 196 (mOD405/min/ml membrane). All increments in rings versus controls and in trophozoites versus rings were highly significant. Parasite development in the presence of 100 micromol/l beta-mercaptoethanol largely reverted haemichrome formation, Bd3 aggregation, C3c and IgG deposition and phagocytosis. Membrane proteins extracted by detergent C12E8 were separated on Sepharose CL-6B. Haemichromes, C3c and IgG were present exclusively in the high-molecular-weight fractions together with approximately 30% of Bd3, indicating the oxidative formation of immunogenic Bd3 aggregates. Immunoblots of separated membrane proteins with anti-Bd3 antibodies confirmed Bd3 aggregates that, in part, did not enter the gel. Immunoprecipitated antibodies eluted from trophozoites reacted preferentially with aggregated Bd3. Changes in parasitized RBC membranes and induction of phagocytosis were similar to oxidatively damaged, senescent or thalassaemic RBC, indicating that parasite-induced oxidative modifications of Bd3 were per se sufficient to induce and enhance phagocytosis of malaria-parasitized RBC.

Flow cytometry detection of surface antigens on fresh, unfixed red blood cells infected by Plasmodium falciparum

The immunofluorescence detection of parasite-specific antigens on the surface of red blood cells infected by Plasmodium falciparum parasites is usually performed by visual detection under a fluorescence microscope. We describe here a technique permitting the analysis of surface immunofluorescence labelling by flow cytometry. Infected red blood cells are selected on the basis of their parasitic DNA and RNA content by Hoechst and Thiazole Orange vital dyes. Cytometric analysis of these labels, as well as general erythrocyte characteristics assessed by analysis of forward and side scatter allows the selection of viable intact infected erythrocytes from other blood cells. The integrity of these selected erythrocytes was confirmed by the absence of labelling with antibodies directed against internal components such as spectrin. This technique permits the detection of specific surface immunofluorescence staining on red blood cells infected with mature stages of P. falciparum by antibodies in sera from hyperimmune Saimiri monkeys. Using Thiazole Orange dye for detection of parasitised cells, this analysis was performed on a FACSscan apparatus equipped with a single laser.

Rosetting Plasmodium falciparum-infected erythrocytes express unique strain-specific antigens on their surface

Spontaneous binding of uninfected erythrocytes to Plasmodium falciparum-infected erythrocytes (rosetting) has been suggested to have a critical role in the induction of cerebral malaria. We report here that rosetting can be mediated by several molecular mechanisms involving parasite polypeptides with M(r)s of 22,000 or 28,000, termed rosettins. Antibodies to either polypeptide disrupt rosettes in a strain-specific fashion. Rosettes of five of the seven isolates examined thus far are more easily disrpted by anti-22,000-M(r) rosettin antibodies than by anti-28,000-M(r) rosettin antibodies. Polyclonal anti-22,000-M(r) rosettin antibodies raised in mice or rabbits strongly and strain specifically stain the surface of nonfixed erythrocytes infected with late asexual stages of rosetting P. falciparum. Simultaneous antibody staining and rosetting are seen when the anti-22,000-M(r) rosettin antiserum is diluted so that only partial disruption of rosettes is obtained, confirming that the fluorescence-labelled infected erythrocytes are involved in rosetting. The 22,000-M(r) rosettin is accessible for surface iodination on erythrocytes infected with strains of rosetting parasites sensitive to anti-22,000-M(r) rosettin antibodies, whereas no labelling occurred on either normal erythrocytes or nonrosetting-P. falciparum-infected erythrocytes. Purified anti-22,000-M(r) rosettin serum immunoglobulin G immunoprecipitated three parasite-derived polypeptides with M(r)s of 22,000, 45,000 (doublet), and 50,000 from lysates of [35S]methionine-labelled, parasite-infected erythrocytes. Our results suggest that rosetting is mediated by strain-specific, antigenically distinct, P. falciparum-derived polypeptides.

Plasmodium falciparum: modulation of surface antigenic expression of infected erythrocytes as revealed by cell fluorescence ELISA

The surface reactivity of heterologous immune sera with erythrocytes infected with Plasmodium falciparum has been difficult to assess in quantitative terms because of the restricted accessibility of surface epitopes and the lack of sensitive methodologies. In a previous study we showed that cryptic antigens can become exposed on the surface of intact trophozoites if the sterol content of the membranes is increased by means conservative of cell integrity (D. Baruch and Z. I. Cabantchik, Molecular and Biochemical Parasitology 36, 127-138, 1990). In this work we introduce a novel and highly sensitive method of fluorescence cell ELISA for the quantitative estimation of immunoglobulin binding to the surface of P. falciparum-infected erythrocytes. We obtained that elevation of the membrane sterol content markedly increased the (external) surface accessibility of antigenic epitopes of trophozoites as well as rings of various strains of P. falciparum. This treatment induced exposure of similar epitope(s) on the surface of both rings and trophozoites insofar as preadsorption of sera on sterol-treated cells abolished immunoglobulin binding to either stage of infected erythrocytes (treated or not with sterol). These putative epitopes have relatively low but demonstrable accessibility on the surface of untreated rings but become virtually inaccessible at the trophozoite stage. Application of a large variety of sera (98) to sterol-treated infected cells revealed that almost 70% of the tested sera were found to give positive surface reactivity. Relatively higher intensity of binding was obtained with sera originating from clinically immune individuals. Binding of sera to cells infected with five different P. falciparum strains was essentially indistinguishable, strongly suggesting that elevation of membrane visocity induces surface exposure of cryptic epitopes common to different parasite strains.

Antigenic polymorphism in malaria: is it an important mechanism for immune evasion?

Malarial infections do not readily evoke an effective protective immunity against re-infection. Possible reasons for this include the ability of the parasites to interfere with the host's immune response and to evade the response in an immune host, by, for example, exploiting antigenic polymorphism or variation. Antigenic polymorphism undoubtedly exists in malaria parasite populations but does this polymorphism actually contribute to immune evasion by the parasite? Here, Kamini Mendis and colleagues examine the evidence for this and its implications for future malaria vaccines.

Erythrophagocytosis in malaria: Host defence or menace to the macrophage?

Macrophages in the host's bloodstream and tissue serve as a first line of defence during infection with Plasmodium. While the killing effect of these cells on parasites has been investigated extensively, relatively little is known about the phagocytosis of infected red blood cells. In this article, Paolo Arese and Franca Turrini have joined Hagai Ginsburg to address the perplexing relationships between the macrophage and the malaria-infected red blood cell. They suggest that the same molecular mechanisms that normally operate to remove senescent or damaged red blood cells also operate during malaria, although the parasite may indirectly cause the destruction of macrophages.

Passive modulation of antigenic expression in the surface of normal and malaria-infected erythrocytes

The membranes of Plasmodium falciparum-infected human red blood cells contain antigens of demonstrably cryptic character. We show here, by a cell surface radioimmunoassay using anti-human red cell membrane antisera, that raising the membrane microviscosity of intact cells leads to a marked increase in the cell surface antigen reactivity of normal cells, and even more so in cells infected in vitro with two strains of P. falciparum. A variety of sera from adults and children living in endemic areas and from malaria patients, all of which showed no detectable surface reactivity with either normal or infected red cells, were demonstrably surface-reactive to infected cells whose sterol membrane content has been raised by means conservative of cell integrity. New epitopes become exposed on the surface of infected cells after lipid modification. The present studies indicate that the reduced membrane viscosity reported in malaria-infected cells determines to a considerable extent the expression of cell surface antigens of both host and parasite, and could play a significant role in parasite immune evasion.

Field applications of agglutination and cytoadherence assays with Plasmodium falciparum from Papua New Guinea

Plasmodium falciparum isolates obtained directly from patients in Papua New Guinea were tested in their first cycle of growth in vitro for adherence to melanoma cells and for susceptibility to agglutination by immune serum. Binding varied among isolates and, in many cases, increased with further rounds of replication under optimal culture conditions. Binding inhibition assays and agglutination assays demonstrated extreme heterogeneity of surface antigens; apparently none of the sera from adult patients recognized all of the variants presented.

Plasmodium falciparum: characterization of a 33-kDa soluble antigen

A 33-kDa soluble antigen identified in the culture supernatant by patient serum and monoclonal antibodies was present in rings, trophozoites, schizonts, and merozoites of Plasmodium falciparum. The antigen which is released into the culture supernatant by growing parasites was also observed in the host cells of trophozoites and schizonts and could be localized on the host cell surface. Its specificity for the surface of trophozoites and schizonts was observed to decrease with increased duration without subculture. The antigen could then be detected on the surface of noninfected erythrocytes. The antigenicity of the 33-kDa antigen was destroyed by heating at 65 degrees C. Monoclonal and polyclonal specific antibodies weakly inhibited parasite growth in vitro. The antigen was present in both knob positive and knob negative parasites in all the P. falciparum isolates tested.

Variation in expression of antibody-dependent cell-mediated cytotoxicity in rodents with malaria

Antibody-dependent cell-mediated cytotoxicity (ADCC) against mouse erythrocytes sensitized with immunoglobulin G was studied in mice with malaria. Spleen cells from mice had enhanced cytotoxic activity early in Plasmodium berghei infection but not later in the disease. Sera from infected animals and partially purified malarial immune complexes inhibited ADCC. In addition, ADCC was diminished in spleen cells from mice infected with the lethal variant of P. yoelii 17x compared with that in mice infected with the nonlethal variant. P. berghei-infected erythrocytes did not release 51Cr when incubated with effector cells unless the erythrocytes were sensitized with antibodies against normal mouse erythrocytes.

Antigenic variation of cloned Plasmodium fragile in its natural host Macaca sinica. Sequential appearance of successive variant antigenic types

The course of infection of Plasmodium fragile in its natural host, the toque monkey Macaca sinica, consists of a primary peak of parasitemia followed by several distinct, successive peaks of lower parasitemia. In the S+ host, the late intraerythrocytic asexual developmental stages of P. fragile induce the expression of antigens on the surface of infected erythrocytes, which could be detected using the technique of surface immunofluorescence. Immunofluorescence using unfixed erythrocytes in suspension has shown that antigens are recognized by immune serum on the surface of the erythrocytes infected with more mature stages of the parasite. These antigens undergo variation, each successive peak of parasitemia being characterized by a different variant antigenic type (VAT). The appearance of the successive VATs occurs in a sequential manner, following the same order in different sets of animals. This constitutes the first example of a sequential expression of antigens in a malaria parasite; it indicates that, in P. fragile, antigenic variation is not the result of random mutations selected by antibody. Parasite-induced antigens on the surface of infected erythrocytes could not be detected in the S- host. However, when nonexpressing parasites from the S- host were transferred by blood passage into a naive S+ animal, they began to express antigens on the surface of infected erythrocytes within two erythrocytic cycles. We have demonstrated that the ability of S- parasites to switch to a particular VAT when passaged into a S+ animal changes during the course of an infection in the S- animal, indicating that, although surface antigens are not expressed, the processes leading to antigenic variation occurs even in the S- host. Antibodies directed against these surface antigens inhibit the growth of intra-erythrocytic parasites. The growth inhibition effects of antibodies are also variant specific, indicating that these variant surface antigens are functionally important for parasite survival.

Expression of senescent antigen on erythrocytes infected with a knobby variant of the human malaria parasite Plasmodium falciparum

Erythrocytes infected with a knobby variant of Plasmodium falciparum selectively bind IgG autoantibodies in normal human serum. Quantification of membrane-bound IgG, by use of 125I-labeled protein A, revealed that erythrocytes infected with the knobby variant bound 30 times more protein A than did noninfected erythrocytes; infection with a knobless variant resulted in less than a 2-fold difference compared with noninfected erythrocytes. IgG binding to knobby erythrocytes appeared to be related to parasite development, since binding of 125I-labeled protein A to cells bearing young trophozoites (less than 20 hr after parasite invasion) was similar to binding to uninfected erythrocytes. By immunoelectron microscopy, the membrane-bound IgG on erythrocytes infected with the knobby variant was found to be preferentially associated with the protuberances (knobs) of the plasma membrane. The removal of aged or senescent erythrocytes from the peripheral circulation is reported to involve the binding of specific antibodies to an antigen (senescent antigen) related to the major erythrocyte membrane protein band 3. Since affinity-purified autoantibodies against band 3 specifically bound to the plasma membrane of erythrocytes infected with the knobby variant of P. falciparum, it is clear that the malaria parasite induces expression of senescent antigen.

Parasite-infected-cell-agglutination and indirect immunofluorescence assays for detection of human serum antibodies bound to antigens on Plasmodium falciparum-infected erythrocytes

Two methods are described for detecting the binding of serum antibodies from adults in an endemic malarious area (The Gambia) to surface antigens on Plasmodium falciparum-infected erythrocytes. An antibody-mediated parasite-infected-cell-agglutination assay (without secondary antibody) and an indirect immunofluorescence assay employing an anti-Fc secondary reagent were used to detect bound antibody. The surface of erythrocytes containing mature parasites bound antibody, but the surface of uninfected cells or cells containing early parasite stages did not react. Serum from 'non-immune' Europeans did not agglutinate infected erythrocytes, however, in the immunofluorescence test with anti-Ig and anti-F(ab')2 secondary reagents we could detect the binding of IgG antibody from 'non-immune' European serum to a small proportion of infected cells. In contrast to the results with freshly collected isolates, antibodies from sera of Gambian adults did not bind to the surface of infected cells from five different culture-adapted isolates of P. falciparum. These assays are suitable for studies on the antigenic diversity of erythrocyte antigens in natural infections and specific antibody responses to these antigens in infected patients.

Anti-gamete antibodies block transmission of human vivax malaria to mosquitoes

Antibodies were raised in rabbits by immunizing against fresh unfixed or cryopreserved female gametes of the human malaria pathogen Plasmodium vivax. The antibodies were shown to react with the surface of gametes by the indirect immunofluorescent test. When parasite isolates from P. vivax infected individuals were fed through a membrane to Anopheles tessellatus mosquitoes in the presence of immune rabbit sera, they completely blocked the infectivity of the parasite isolates to the vector. Immunoglobulins separated from these sera also blocked infectivity to the same extent as did the immune sera indicating that antibodies were responsible for the transmission blocking effect of the sera. This study indicated that P. vivax like other malaria parasites is highly susceptible to anti gamete transmission blocking immunity.

The relationship to knobs of the 92,000 D protein specific for knobby strains of Plasmodium falciparum

A 92,000 D protein was identified associated with the membrane of host erythrocytes infected with the FCB1 Plasmodium falciparum strain from Colombia. The same protein was identified in the knob-forming Gambian (and the Malayan Camp) strain, but was not present in all the corresponding knobless strains. In the FCB1 strain as well as in the FCR3 strain the protein is synthesized during the ring-stage period. The cleavage products of the 92,000 D protein were investigated by peptide mapping following limited proteolytic digestion with Staphylococcus aureus V8 protease. The 92,000 D protein cleavage products from both the Colombian and the Gambian strains were identical. Moreover, both the proteins were sensitive to trypsin and chymotrypsin and also to treatment with neuraminidase. Enzymatic removal of the protein from the erythrocyte membrane by trypsin or chymotrypsin did not affect parasite maturation. The merozoites thus produced were fully invasive and the morphology of the knobs was unaltered. When the erythrocyte membrane was treated with trypsin before the time of synthesis of the 92,000 D protein, it was not possible to identify the protein in membranes of later stages of infected erythrocytes, indicating that the protein cannot be inserted into the membrane cytoskeleton compartment. Knobs, however, were formed more or less normally, suggesting that it is not the accumulation of this protein which products the knobs.

Antimalarial immunity in Saimiri monkeys. Immunization with surface components of asexual blood stages

Plasmodium falciparum polypeptides of 200 and 140 K mol wt exposed at the surface of merozoites and/or schizonts were purified by affinity chromatography and by electroelution from sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Monkeys were separated into three groups of four and immunized either with one of the two polypeptides or with saline (control). After intravenous challenge with 2.5 X 10(7) P. falciparum asexual blood stages, two monkeys of the control group had to be treated and two recovered spontaneously after peak parasitemia of 9 and 11%. The four monkeys immunized with the 140 K polypeptide recovered without treatment after peak parasitemia between 1.5 and 4.5%. Monkeys immunized with the 200 K polypeptide had similar peak parasitemia except one monkey who suffered from a large skin excoriation and who recovered spontaneously after a peak parasitemia of 11%. Prechallenge sera of the immunized monkeys reacted only with the polypeptide used for immunization except for one serum of the 140 K group, which precipitated an additional polypeptide of 39 K, and a polypeptide of 31 K weakly precipitated by the four sera of monkeys immunized with the 200 K polypeptide. The relatedness between the 200 and 140 K polypeptides was investigated using tryptic digestion and reverse phase chromatography. No clear analogy was found between the two polypeptides, which suggests that immunization with either of two independent surface components of P. falciparum asexual blood stages is able to induce at least a partial protective immunity in immunized hosts.

Vaccination against malaria: its plausibility and the present state of research

A vaccine for public health use against malaria is urgently required and is being actively researched into. The present review outlines the biology of malaria parasites and the immune response to them, with an emphasis on the worst of the human diseases, and considers the current analytical and molecular biological work on malaria parasite surfaces and antigens.