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

Kinetic Tracking of Plasmodium falciparum Antigens on Infected Erythrocytes with a Novel Reporter of Protein Insertion and Surface Exposure

Intracellular malaria parasites export many proteins into their host cell, inserting several into the erythrocyte plasma membrane to enable interactions with their external environment. While static techniques have identified some surface-exposed proteins, other candidates have eluded definitive localization and membrane topology determination. Moreover, both export kinetics and the mechanisms of membrane insertion remain largely unexplored. We introduce Reporter of Insertion and Surface Exposure (RISE), a method for continuous nondestructive tracking of antigen exposure on infected cells. RISE utilizes a small 11-amino acid (aa) HiBit fragment of NanoLuc inserted into a target protein and detects surface exposure through high-affinity complementation to produce luminescence. We tracked the export and surface exposure of CLAG3, a parasite protein linked to nutrient uptake, throughout the Plasmodiumfalciparum cycle in human erythrocytes. Our approach revealed key determinants of trafficking and surface exposure. Removal of a C-terminal transmembrane domain aborted export. Unexpectedly, certain increases in the exposed reporter size improved the luminescence signal, but other changes abolished the surface signal, revealing that both size and charge of the extracellular epitope influence membrane insertion. Marked cell-to-cell variation with larger inserts containing multiple HiBit epitopes suggests complex regulation of CLAG3 insertion at the host membrane. Quantitative, continuous tracking of CLAG3 surface exposure thus reveals multiple factors that determine this protein’s trafficking and insertion at the host erythrocyte membrane. The RISE assay will enable study of surface antigens from divergent intracellular pathogens.

Relationship between Antibody Levels, IgG Binding to Plasmodium falciparum-Infected Erythrocytes, and Disease Outcome in Hospitalized Urban Malaria Patients from Dakar, Sénégal

Background. Management of clinical malaria requires the development of reliable diagnostic methods and efficient biomarkers for follow-up of patients. Protection is partly based on IgG responses to parasite antigens exposed at the surface of infected erythrocytes (iRBCs). These IgG responses appeared low during clinical infection, particularly in severe disease. Methods. We analyzed the IgG binding capacity to the surface of live erythrocytes infected by knob positive FCR3 strain. Sera from 69 cerebral malaria (CM) and 72 mild malaria (MM) cases were analyzed by ELISA for IgG responses to five antigens from iRBC and by flow cytometry for IgG binding as expressed in labeling index ratio (LIR). The relationship between IgG levels, LIR, parasitemia, age, and the clinical outcomes was evaluated. Results. We found a significant decrease of LIR in adult CM fatal cases compared to surviving patients (p = 0.019). In MM, LIRs were correlated to IgG anti-iRBC and anti-PfEMP3/5 levels. In CM, no correlation was found between LIR, IgG levels, and parasitemia. Conclusion. The IgG binding assay was able to discriminate outcome of cerebral malaria cases and it deserves further development as a potential functional-associated assay for symptomatic malaria analysis.

Cytometry in malaria--a practical replacement for microscopy?

Malaria, caused by protozoan Plasmodium parasites, kills ~800,000 people each year. Exact figures are uncertain because presumptive diagnoses are often made without identifying parasites in patients' blood either by microscopy, using Giemsa's century-old stain, or by simpler tests that are ultimately dependent on microscopy for quality control. Microscopy itself relies on trained observers' ability to detect subtle morphological features of parasitized red blood cells, only a few of which may be present on a slide. Quantitative and objective flow cytometric measurements of cellular constituents such as DNA, RNA, and the malaria pigment hemozoin are now useful in research in malaria biology and pharmacology, and can provide more reliable identification of parasite species and developmental stages and better detection of low-density parasitemia than could microscopy. The same measurements can now be implemented in much smaller, simpler, cheaper imaging cytometers, potentially providing a more accurate and precise diagnostic modality.

Methodology and application of flow cytometry for investigation of human malaria parasites

Historically, examinations of the inhibition of malaria parasite growth/invasion, whether using drugs or antibodies, have relied on the use of microscopy or radioactive hypoxanthine uptake. These are considered gold standards for measuring the effectiveness of antimalarial treatments, however, these methods have well known shortcomings. With the advent of flow cytometry coupled with the use of fluorescent DNA stains allowed for increased speed, reproducibility, and qualitative estimates of the effectiveness of antibodies and drugs to limit malaria parasite growth which addresses the challenges of traditional techniques. Because materials and machines available to research facilities are so varied, different methods have been developed to investigate malaria parasites by flow cytometry. This review is intended to serve as a reference guide for advanced users and importantly, as a primer for new users, to support expanded use and improvements to malaria flow cytometry, particularly in endemic countries.

Quantitative measurement of Plasmodium-infected erythrocytes in murine models of malaria by flow cytometry using bidimensional assessment of SYTO-16 fluorescence

Flow cytometry is a powerful tool for measuring parasitemias in murine malaria models used to test new antimalarials. Measurement of the emission of the nonpermeable nucleic acid dye YOYO-1 (at 530 and 585 nm after excitation at 488 nm) allowed the unambiguous detection of low parasitemias (> or =0.01%) but required prolonged fixation and permeabilization of the sample. Thus, we tested whether this issue could be overcome by use of the cell-permeant dye SYTO-16 with this same bidimensional method. Blood samples from CD1 mice infected with Plasmodium yoelii, Plasmodium vinckei, or Plasmodium chabaudi or from NOD(scidbeta2m-/-) engrafted with human erythrocytes and infected with P. falciparum were stained with SYTO-16 in the presence or absence of TER-119 mAb (for engrafted mice) in 96-well plate format and acquired in Trucount tubes. Bidimensional analysis with SYTO-16 was quantitatively equivalent to YOYO-1. Moreover, by combining SYTO-16 with the use of TER-119-PE antimouse erythrocyte mAb and Trucount tubes, the measurement of the concentration of P. falciparum-infected erythrocytes over a range of five orders of magnitude was achieved. Bidimensional analysis using SYTO-16 can be used to accurately measure the concentration of Plasmodium spp.-infected erythrocytes in mice without complex sample preparation.

An early burst of IFN-gamma induced by the pre-erythrocytic stage favours Plasmodium yoelii parasitaemia in B6 mice

BACKGROUND

In murine models of malaria, an early proinflammatory response has been associated with the resolution of blood-stage infection. To dissect the protective immune mechanism that allow the control of parasitaemia, the early immune response of C57BL/6 mice induced during a non-lethal plasmodial infection was analysed.

METHODS

Mice were infected with Plasmodium yoelii 265BY sporozoites, the natural invasive form of the parasite, in order to complete its full life cycle. The concentrations of three proinflammatory cytokines in the sera of mice were determined by ELISA at different time points of infection. The contribution of the liver and the spleen to this cytokinic response was evaluated and the cytokine-producing lymphocytes were identified by flow cytometry. The physiological relevance of these results was tested by monitoring parasitaemia in genetically deficient C57BL/6 mice or wild-type mice treated with anti-cytokine neutralizing antibody. Finally, the cytokinic response in sera of mice infected with parasitized-RBCs was analysed.

RESULTS

The early immune response of C57BL/6 mice to sporozoite-induced malaria is characterized by a peak of IFN-gamma in the serum at day 5 of infection and splenic CD4 T lymphocytes are the major producer of this cytokine at this time point. Somewhat unexpected, the parasitaemia is significantly lower in P. yoelii-infected mice in the absence of IFN-gamma. More precisely, at early time points of infection, IFN-gamma favours parasitaemia, whereas helping to clear efficiently the blood-stage parasites at later time points. Interestingly, the early IFN-gamma burst is induced by the pre-erythrocytic stage.

CONCLUSION

These results challenge the current view regarding the role of IFN-gamma on the control of parasite growth since they show that IFN-gamma is not an essential mediator of protection in P. yoelii-infected C57BL/6 mice. Moreover, the mice parasitaemia is more efficiently controlled in the absence of an early IFN-gamma production, suggesting that this cytokine promotes parasite's growth. Finally, this early burst of IFN-gamma is induced by the pre-erythrocytic stage, showing the impact of this stage on the immune response taking place during the subsequent erythrocytic stage.

Monitoring Plasmodium falciparum growth and development by UV flow cytometry using an optimized Hoechst-thiazole orange staining strategy

The complex life cycle of Plasmodium falciparum (Pf) makes it difficult to limit infections and reduce the risk of severe malaria. Improved understanding of Pf blood-stage growth and development would provide new opportunities to evaluate and interfere with successful completion of the parasite's life cycle. Cultured blood stage Pf was incubated with Hoechst 33342 (HO) and thiazole orange (TO) to stain DNA and total nucleic acids, respectively. Correlated HO and TO fluorescence emissions were then measured by flow cytometry. Complex bivariate data patterns were analyzed by manual cluster gating to quantify parasite life cycle stages. The permutations of viable staining with both reagents were tested for optimal detection of parasitized RBC (pRBC). Pf cultures were exposed to HO and TO simultaneously to achieve optimal staining of pRBC and consistent quantification of early and late stages of the replicative cycle (rings through schizonts). Staining of Pf nucleic acids allows for analysis of parasite development in the absence of fixatives, lysis, or radioactivity to enable examination of erythrocytes from parasite invasion through schizont rupture using sensitive and rapid assay procedures. Investigation of the mechanisms by which anti-malarial drugs and antibodies act against different Pf lifecycle stages will be aided by this cytometric strategy.

A simple method for quantifying high density antigens in erythrocyte membrane by flow cytometry

RBC flow cytometric analysis is usually used to quantify antigen content. Calibration systems enable antigen content determination by relating mean fluorescence intensity with the number of bound antibody molecules (equivalent to the number of antigen molecules). For that reason, antibodies must be used at saturating concentration, which may lead to agglutination when working with high density antigens. Then, forward scattering, side scattering and fluorescence will be increased, thus obtaining wrong results. In this work, the simple Langmuir adhesion model was applied. Flow cytometry was used to quantify GPA, a transmembrane protein present at high density on RBC. The fluorescence intensity of samples at different anti-GPA sub-saturating concentrations was measured. Sometimes, agglutinates were present and two peaks of fluorescence were observed, the principal one corresponding to isolated cells and the secondary one corresponding to agglutinated cells. In those cases, the principal peak was taken into account for the analysis. The GPA antigen content obtained for nine analyzed samples ranged from 3 to 13 x 10(5) sites per cell, which is similar to those values found in literature. Therefore, the Langmuir adsorption model enables us to determine the antigen content for the anti-GPA/GPA system on RBC membrane. This model could be used to quantify high density antigens in RBC and in other cells.

Blood typing in Saimiri sciureus monkeys: influence of anti-red blood cell alloantibodies on Plasmodium falciparum parasitaemia in vivo

The Saimiri sciureus monkey is a well-established host for experimental studies with human malaria parasites. During the course of iterative inoculations with Plasmodium falciparum parasitised red blood cells (RBC), anti-RBC alloantibodies were detected in the sera of two of eight Saimiri monkeys. These anti-RBC antibodies were further used to investigate RBC phenotypes in 35 colony-reared Saimiri monkeys by flow cytometry. Three RBC phenotypes (named I-III) were observed. Their distribution was I (86%), II (11%) and III (3%). Using the Palo Alto FUP-2 strain, a variant P. falciparum line insensitive to hyperimmune serum and the passive transfer of anti-RBC alloantibodies, a dramatic drop in parasite growth was documented in an incompatible monkey.

Double staining of Plasmodium falciparum nucleic acids with hydroethidine and thiazole orange for cell cycle stage analysis by flow cytometry

BACKGROUND

Microarray analyses of stage-specific gene expression of Plasmodium falciparum require purification of RNAs from highly synchronized cultures. To date, no reliable method to control the quality of synchronization of P. falciparum cultures is available.

METHODS

A double-staining method using hydroethidine and thiazole orange for nucleic acid staining was carried out to compare by flow cytometric analysis the nucleic acid labeling of synchronized P. falciparum in cultures at different time points of the 48-h intraerythrocytic cycle.

RESULTS

With this method, we determined the quality of culture synchronization in schizont and ring stages. Nucleic acid analysis, based on thiazole orange fluorescence, clearly showed that low levels of schizonts in ring cultures results in a high contamination of ring nucleic acids by schizonts. Conversely, nucleic acids from trophozoite or schizont cultures containing ring stages did not present a significant contamination by ring nucleic acids.

CONCLUSION

The results demonstrated a very low nucleic acid content in the ring stage when compared with the high nucleic acid content of schizont-stage parasites. The rapid and reliable flow cytometric strategy using hydroethidine- and thiazole orange-stained parasite nucleic acids allows monitoring of the purity of the preparation, thus greatly improving the quality assessment of parasite cultures, a critical step to study gene expression patterns.

Relationship of binding of immunoglobulin G to Plasmodium falciparum-infected erythrocytes with parasite endemicity and antibody responses to conserved antigen in immune individuals

To investigate the potential for use of a well-established strain of Plasmodium falciparum as a reference strain for infected red blood cell (IRBC) surface reactivity, we monitored the binding of specific immunoglobulin G (IgG) from immune individuals to the reference Knob-positive FCR3 strain by flow cytometry. To permit interassay comparison for 162 plasma samples drawn after the rainy season, a labeling index (LI) was defined as the percentage of labeled parasites multiplied by the mean peak intensity. An LI ratio (LIR) was then calculated as the LI of the sample divided by the LI of the control. LIRs were calculated for individuals living in Dielmo and Ndiop, two Senegalese villages where P. falciparum is transmitted holoendemically and mesoendemically, respectively. The incidence (persons with an LIR of >3) observed in Dielmo was lower than that observed in Ndiop. Significantly higher LIRs were observed (i) for samples from Ndiop than for samples from Dielmo (P < 0.01) and (ii) in Ndiop, in subjects with hemoglobin AS (HbAS) than in those with hemoglobin AA (P = 0.03). No correlation with the cumulative age-associated immune status of the villagers was evidenced, contrary to antibody (Ab) responses against conserved IRBC-associated antigen (Ag) measured by enzyme-linked immunosorbent assay. These results are consistent with the notions that protection in HbAS individuals may relate to an increased IgG response to IRBC membrane Ags and that cell surface reactivity parallels IgG responses even though it is in itself a distinct indicator of the anti-P. falciparum Ab response. Measures of IgG binding to live IRBC are thus relevant for the functional screening of conserved IRBC-associated Ags that contribute to parasite destruction in vivo, as these Ags might be included in a multitarget vaccine.

Assay of spherical cell surface molecules by thermal lens microscopy and its application to blood cell substances

To detect and quantitate uneven cell surface molecules, such as blood group antigens on a blood cell and immunoglobulin molecules on a mast cell, an improved method of thermal lens microscopy was employed. The antigen molecules were immunologically stained with their antibodies, which were labeled with colloidal gold. Since the surface of the biological cells was not flat but spherical, the focal point of the probe laser beam inevitably deviated from the sample surface on the moving stage. Therefore, the deviation of the focal point of the probe beam was corrected by adjusting the phase of the signal. Using this technique, a three-dimensional antigen distribution on each cell surface was imaged. Despite the convex surface of cells, labeled colloidal gold was correctly quantified. In the measurement of erythrocyte antigens, a small quantity of Lewis antigens was successfully detected on the umbilical cord erythrocytes. Immunoglobulin E on a mast cell, derived from the allergic human mucosa fungus, was also observed by this method, and the distribution of IgE molecules on the cell surface was quantitatively imaged. A thermal lens microscope, which measures spherical samples correcting the deviation, made it possible for us to observe and assay the substances on biological specimens that have complicated forms, such as living cells in vivo or in situ.

Pfsbp1, a Maurer's cleft Plasmodium falciparum protein, is associated with the erythrocyte skeleton

Antibodies from hyperimmune monkey sera, selected by absorption to Plasmodium falciparum-infected erythrocytes, and elution at acidic pH, allowed us to characterize a novel parasite protein, Pfsbp1 (P. falciparum skeleton binding protein 1). Pfsbp1 is an integral membrane protein of parasite-induced membranous structures associated with the erythrocyte plasma membrane and referred to as Maurer's clefts. The carboxy-terminal domain of Pfsbp1, exposed within the cytoplasm of the host cell, interacts with a 35 kDa erythrocyte skeletal protein and might participate in the binding of the Maurer's clefts to the erythrocyte submembrane skeleton. Antibodies to the carboxy- and amino-terminal domains of Pfsbp1 labelled similar vesicular structures in the cytoplasm of Plasmodium chabaudi and Plasmodium berghei-infected murine erythrocytes, suggesting that the protein is conserved among malaria species, consistent with an important role of Maurer's cleft-like structures in the intraerythrocytic development of malaria parasites.

Evaluation of immunogenicity and protective efficacy of carrier-free Plasmodium falciparum R23 antigen in pre-exposed saimiri sciureus monkeys

We have reported previously that the recombinant Glutathione S-transferase GTR23, induced protection after immunisation of naive or previously exposed Saimiri monkeys. We investigated the immunogenicity of carrier-free R23 repeats in pre-exposed animals in two adjuvant formulations. Three of five monkeys immunised with alum-formulated repeats and one of two animals immunised with the Polyalphaolefine formulation produced high titres of cytophilic antibodies with a primary type kinetics, indicating that the anti-P. falciparum antibodies present on the day of challenge were R23-specific. The four responders in R23-specific antibodies were protected against a challenge infection with the virulent FUP/SP strain. The other three animals failed to respond to immunisation and experienced an infection that required drug treatment. Unlike the other three animals that experienced an infection requiring drug treatment. These experiments support further development of the R23 repeats as a vaccine candidate.

Detection and measurement of a single blood cell surface antigen by thermal lens microscopy

A highly sensitive method for detection of antigens on the surface of a single blood cell using thermal lens microscopy is described. Colloidal gold, coated with antibody, was used to stain membrane antigens of leukocytes. Human leukocyte antigens on the lymphocytes and mononuclear leukocytes were observed by new thermal lens microscopy, which involves spectrometry using a laser-induced thermal-lens effect. Antigens of HLA-A, -B, and -C loci on the lymphocytes were identified and quantitated using a single cell. The image of HLA-A, -B, and -C antigen distribution on a mononuclear leukocyte was obtained. Our laser microscope, newly devised for measuring convex surface cells, is a powerful analytical tool for detecting and quantitating localized antigens in a single cell and/or cell-surface-associated molecules.

Applications of flow cytometry to clinical microbiology

Classical microbiology techniques are relatively slow in comparison to other analytical techniques, in many cases due to the need to culture the microorganisms. Furthermore, classical approaches are difficult with unculturable microorganisms. More recently, the emergence of molecular biology techniques, particularly those on antibodies and nucleic acid probes combined with amplification techniques, has provided speediness and specificity to microbiological diagnosis. Flow cytometry (FCM) allows single- or multiple-microbe detection in clinical samples in an easy, reliable, and fast way. Microbes can be identified on the basis of their peculiar cytometric parameters or by means of certain fluorochromes that can be used either independently or bound to specific antibodies or oligonucleotides. FCM has permitted the development of quantitative procedures to assess antimicrobial susceptibility and drug cytotoxicity in a rapid, accurate, and highly reproducible way. Furthermore, this technique allows the monitoring of in vitro antimicrobial activity and of antimicrobial treatments ex vivo. The most outstanding contribution of FCM is the possibility of detecting the presence of heterogeneous populations with different responses to antimicrobial treatments. Despite these advantages, the application of FCM in clinical microbiology is not yet widespread, probably due to the lack of access to flow cytometers or the lack of knowledge about the potential of this technique. One of the goals of this review is to attempt to mitigate this latter circumstance. We are convinced that in the near future, the availability of commercial kits should increase the use of this technique in the clinical microbiology laboratory.

Applications of flow cytometry to clinical microbiology

Classical microbiology techniques are relatively slow in comparison to other analytical techniques, in many cases due to the need to culture the microorganisms. Furthermore, classical approaches are difficult with unculturable microorganisms. More recently, the emergence of molecular biology techniques, particularly those on antibodies and nucleic acid probes combined with amplification techniques, has provided speediness and specificity to microbiological diagnosis. Flow cytometry (FCM) allows single- or multiple-microbe detection in clinical samples in an easy, reliable, and fast way. Microbes can be identified on the basis of their peculiar cytometric parameters or by means of certain fluorochromes that can be used either independently or bound to specific antibodies or oligonucleotides. FCM has permitted the development of quantitative procedures to assess antimicrobial susceptibility and drug cytotoxicity in a rapid, accurate, and highly reproducible way. Furthermore, this technique allows the monitoring of in vitro antimicrobial activity and of antimicrobial treatments ex vivo. The most outstanding contribution of FCM is the possibility of detecting the presence of heterogeneous populations with different responses to antimicrobial treatments. Despite these advantages, the application of FCM in clinical microbiology is not yet widespread, probably due to the lack of access to flow cytometers or the lack of knowledge about the potential of this technique. One of the goals of this review is to attempt to mitigate this latter circumstance. We are convinced that in the near future, the availability of commercial kits should increase the use of this technique in the clinical microbiology laboratory.

Flow cytometric analysis of IgG reactive to parasitized red blood cell membrane antigens in Plasmodium falciparum-immune individuals

Antigens exposed at the surface of Plasmodium falciparum parasitized red blood cells (pRBCs) represent potential targets for protective antibodies involved in opsonization and immune phagocytosis of pRBCs. We measured the recognition of parasitized red blood cell membrane associated antigens by IgG in the plasma of clinically immune individuals by flow cytometry and ELISA. The plasmas were selected on the basis of preexisting IgG antibodies to pRBC membrane associated recombinant proteins. In every plasma sample IgG could bind the surface of live pRBCs in flow cytometry. In addition, there was a significant correlation between the level of IgG recognition of live pRBCs and of pRBC membrane ghost proteins or major identified antigens by ELISA. Flow cytometry thus represents a technique suitable to test for the accessibility and potential functionality of IgG antibodies directed to antigens expressed by the surface of pRBCs.

Plasmodium falciparum: analysis of the antibody specificity to the surface of the trophozoite-infected erythrocyte

Current opinion supports the view that immunity to the surface of the trophozoite-infected erythrocyte (IE) is to Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP-1). Here we provide further evidence using the mutant cell line 1776/C10 which no longer expresses PfEMP-1 at the IE surface, due to a subtelomeric deletion in chromosome 9. We have measured antibody reactivity to this mutant in comparison to it's intact isogenic parent line 1776, which does express PfEMP-1, using the sensitive technique of flow cytometry. IgG-specific antibodies (subclass IgG1) in the plasma of hyperimmune adults, reacted to 1776 but never to the 1776/C10 mutant. Antibody subclasses were also measured in individual plasma samples to the surface of trophozoite-IE. Predominantly IgG1 antibodies were detected, with a few individual plasma having additional IgG3 antibodies. Previous studies have used the agglutination assay to measure sero-conversion to PfEMP-1. Here we show that both agglutination and flow cytometric methods are comparable, suggesting that agglutination of trophozoite-IE is mediated by IgG antibodies. Comparison of the isogenic cell lines 1776 and 1776/C10 differing in expression of PfEMP-1 provides further evidence that IgG antibodies, in particular of the cytophilic subclasses, mediate recognition of PfEMP-1.

Plasmodium falciparum: immune pressure in Saimiri sciureus monkeys can select for a parasite population inducing a protective immunity that is not controlled by antibody

Protective immunity against a Plasmodium falciparum blood infection can be passively transferred by antibodies in humans and in the primate experimental malaria model Saimiri sciureus. We report here the emergence of a novel virulent parasite population after such passive transfer of hyperimmune serum in splenectomized monkeys. These FUP-2 parasites have been partially genotyped and phenotyped. Although no genotypic variation was detected for four polymorphic loci compared to the original FUP-1 parasite population, FUP-2-infected erythrocytes exhibit little or no detectable surface determinants, including those reacting with antibodies raised against FUP-1 surface antigens. In addition, FUP-2-infected erythrocytes exhibit no rosetting or autoagglutination. Interestingly, although Saimiri monkeys control efficiently FUP-2 parasites after repetitive infections, this protection cannot be passively transferred to naive recipients. Our results suggest that antibody-mediated and antibody-independent T-cell-mediated protective responses may cooperate in controlling P. falciparum infection in splenectomized Saimiri monkeys.

Plasmodium falciparum: altered expressions of erythrocyte membrane-associated antigens during antigenic variation

The O and R antigenic variants of the Plasmodium falciparum Palo Alto strain present differences in the morphology of the infected red blood cell membrane, in their adhesion properties, surface immunofluorescence, and agglutination specificities and importantly, induce a variant-specific protection after a primary infection in Saimiri sciureus monkeys. To identify potential targets of variant-specific immunity, we have compared the antigenic makeup of both variants by immunoblot. O-specific monkey sera generated similar profiles on both parasite types, while R-specific sera showed a consistent difference on a high-molecular-mass undefined antigen. Distinct antibody specificities were eluted from the surface of O- or R-infected erythrocytes, generating variant-specific agglutination, surface immunofluorescence, and immunoblot profiles. An antiserum raised to Pf60.1, predicted to cross-react with the cytoplasmic domain of PfEMP1, reacted with specific, SDS-soluble antigens in both variants. Antigens associated with the membrane of the infected red blood cells were further investigated using several specific antisera. The 85-kDa HRP1 gene product was more abundant in O than in R parasites, while the reverse was observed for the PfEMP3 protein. These data indicate that O and R parasites differ in the expression of several antigens associated with the membrane of the infected red blood cell.