Cross-Reactive Surface Epitopes on Chondroitin Sulfate A-Adherent Plasmodium falciparum-Infected Erythrocytes Are Associated with Transcript
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感染与免疫杂志 2005年第5期
Department of Medicine, University of Melbourne, Royal Melbourne Hospital, Victoria, Australia
ABSTRACT
Malaria in pregnancy is associated with placental accumulation of Plasmodium falciparum-infected erythrocytes (IE) that adhere to chondroitin sulfate A (CSA). Adhesion is mediated by P. falciparum erythrocyte membrane protein 1 (PfEMP1), a variant parasite protein expressed on the surface of IE and encoded by var genes. Rabbit antiserum was generated against the CSA-adherent P. falciparum line CS2, in which the dominant var transcribed is var2csa, a relatively conserved var gene that has been associated with CSA adhesion. Anti-CS2 recognized genetically distinct CSA-adherent P. falciparum lines but not CD36-adherent parent lines. Reactivity with anti-CS2 correlated with the level of adhesion to CSA. Fluorescence-activated cell sorting according to binding of anti-CS2 showed reactivity was associated with CSA adhesion and transcription of var2csa. These data are consistent with the hypothesis that var2csa encodes a PfEMP1 expressed on the surface of IE, which mediates adhesion to CSA and is relatively conserved between genetically distinct strains of P. falciparum.
INTRODUCTION
During pregnancy, women are more susceptible to the consequences of Plasmodium falciparum malaria infection. Malaria in pregnancy is associated with increased risk of maternal anemia and fetal intrauterine growth retardation (11, 54) and is characterized by accumulation of P. falciparum-infected erythrocytes (IE) in the placenta (59).
Adhesion of IE to multiple host receptors is mediated by P. falciparum erythrocyte membrane protein 1 (PfEMP1) (2, 12, 15, 16, 43; for a review see reference 40), a variant parasite protein expressed on the IE surface and encoded by the var multigene family (3, 51, 55). Transcriptional switching between var genes is associated with changes in parasite antigenic and adhesion phenotypes (51, 55). IE from nonpregnant individuals commonly bind to the endothelial receptors CD36 and intercellular adhesion molecule 1 (ICAM-1) (5, 26, 42). Placental IE have a unique phenotype, adhering to chondroitin sulfate A (CSA) and hyaluronic acid (HA), which are expressed on syncytiotrophoblasts (1, 5, 8, 26).
Partial immunity to pregnancy-associated malaria develops after several pregnancies. This is associated with acquisition of antibodies to variant parasite antigens expressed on the IE surface, which inhibit adhesion of placental IE to CSA (7, 28, 39, 45, 53). PfEMP1 is a potential vaccine candidate for pregnancy-associated malaria. There are approximately 60 var genes per parasite genome (30), and since var genes are highly polymorphic (55), the diversity of PfEMP1 molecules is likely to be large. However, the range of PfEMP1 molecules expressed by placental IE is probably restricted. Sera from malaria-exposed multigravid women can inhibit adhesion of placental IE to CSA even when IE and sera are obtained from geographically distinct locations, suggesting conservation of CSA binding domains (28).
Several var genes purported to encode CSA-binding domains have been described, including varCS2 (43) and FCR3varCSA (12, 50). Binding to CSA has been localized to DBL domains of these genes (12, 18, 32, 38, 43, 44). However, recent studies of FCR3varCSA failed to show a correlation between transcription and the CSA adhesion phenotype or specific transcription in placental-type parasites (21, 27, 35, 47). FCR3varCSA (12, 50) and varCS2 (43) were originally identified in CSA-adherent parasite lines using reverse transcription PCR (RT-PCR). Since single trophozoites can transcribe multiple var genes simultaneously (21), this approach might detect genes that do not encode the expressed PfEMP1 protein.
A recent study using quantitative RT-PCR showed that selection of laboratory lines for CSA adhesion was associated with increased transcription of the var gene var2csa (49). The var2csa gene was transcribed at higher levels in placental isolates than in isolates from children. Unlike many var genes, var2csa shows a high level of homology in genetically distinct isolates (33, 49). Recent mass spectrometric analysis failed to identify the var2csa protein in IE membrane extracts of CSA-selected or placental isolates (29). However, antisera generated against domains of 3D7 var2csa recognize CSA-selected IE, suggesting that the PfEMP1 encoded by var2csa is expressed on the infected erythrocyte surface (48).
In this study, we examined genetically distinct CSA-adherent P. falciparum lines, looking for an association between surface antigenic phenotype and var2csa transcription. Using rabbit antiserum generated against trophozoites of the CSA-adherent line CS2, in which the dominant var transcript is var2csa (22), we linked the presence of conserved or cross-reactive epitopes on the surface of CSA-adherent IE with transcription of var2csa.
MATERIALS AND METHODS
Parasite culture. Parasites were cultured as previously described (4, 57). Parasite lines were regularly tested for Mycoplasma contamination and genotyped (MSP1 and MSP2 alleles). Gelatin enrichment of knob-expressing parasites (31) and sorbitol synchronization (37) were performed every 1 to 2 weeks.
Parasite lines. Three genetically different P. falciparum lines were used (Fig. 1): ItG (shown to be identical to FCR3 [21, 46, 58]), 3D7, and HM (isolated from a traveler infected in Southeast Asia). FAF-EA8 (E8B) (Fig. 1A) was from a clone of ItG2F6 (FAF6) (9, 14) and bound predominantly to CD36. CS2 was obtained by repeated selection of FAF-EA8 on Chinese hamster ovary cells and purified CSA (17) (Fig. 1A). CS2 also binds HA (8). E8B-HA was derived from FAF-EA8 by selecting for adhesion to HA and also binds CSA (8) (Fig. 1A). E8B-ICAM was selected from FAF-EA8 by panning on ICAM-1 (Fig. 1A). 3D7 lines were selected for adhesion to CSA and ICAM-1 as described previously (41) (Fig. 1B). 3D7 lines selected for adhesion to CSA do not bind HA (4). 3D7-ICAM was obtained by repeated selection of 3D7 on purified ICAM-1 (Fig. 1B). HM was selected on purified CSA to generate HCS3 (6) (Fig. 1C), which binds HA as well (4).
Characterization of adhesion phenotype. Adhesion assays were performed as described previously (5). Receptors were diluted in phosphate-buffered saline (PBS) and coated in triplicate spots on the bases of 25-ml plastic petri dishes (Falcon 1058; Becton Dickinson) by overnight incubation at 4°C (CD36 purified from platelets [a gift from M. Berndt, Baker Institute, Australia]), soluble ICAM-1 [from transfected Chinese hamster ovary cells], CSA [from bovine trachea {Sigma}; 10 μg/ml), and HA [from bovine vitreous humor {Sigma}; 100 μg/ml). After blocking for 30 min with PBS/1% bovine serum albumin, plates were washed twice in RPMI-HEPES, and trophozoites (3 to 12% parasitemia) were resuspended at 7% hematocrit in adhesion medium (RPMI-HEPES supplemented with 50 μg/ml hypoxanthine, 2.5 μg/ml gentamicin, and 10% pooled human serum). Thirty-five microliters of this suspension was added per receptor spot and incubated at 37°C for 45 min. Four to six washes with RPMI-HEPES were used to remove unbound cells. Bound cells were fixed by incubation for at least 2 h in 2% glutaraldehyde in PBS and then stained with Giemsa stain. The IE bound in eight fields on each receptor spot were counted, and the mean and standard error of the mean for these 24 counts are given below.
Parasite selection for adhesion. Parasite selection for adhesion was performed as described previously (41). Receptors were diluted in PBS (CD36, ICAM-1, CSA, and HA as described above), filter sterilized, and added to a sterile 25-ml petri dish (Falcon 1058; Becton Dickinson) either as 10-μl spots covering the base of the dish (CD36, ICAM-1) or as 50 ml of diluted receptor (CSA, HA). After overnight incubation, the plate was blocked for 30 min using culture medium containing 10% human serum and then washed twice with RPMI-HEPES. Seventy to 100 ml of cultured parasites (5 to 10% mid-trophozoites) was Percoll purified, resuspended in 50 ml of sterile adhesion medium (as described above), and added to the receptor-coated dish. After 45 to 60 min of incubation at 37°C in 1% O2, 4% CO2, 95% N2, unbound cells were removed by four to six washes in RPMI-HEPES until no unbound IE were observed. Directly pipetting 10 ml of culture medium onto bound IE allowed the IE to be removed and transferred to a petri dish for culture with fresh red blood cells. After several selections to obtain the initial phenotype, repeated reselection was required for most parasite lines after 3 to 4 weeks in continuous culture to maintain optimum adhesion.
Generation of anti-CS2 rabbit antiserum (R1945). A rabbit was immunized with synchronized whole CS2 trophozoites as described previously (43) (approved by the Animal Ethics Committee, The Walter and Eliza Hall Institute of Medical Research). Rabbit antiserum was heat inactivated (45 min at 56°C) and preabsorbed against normal human erythrocytes for 1 h at room temperature.
Malaria-exposed human serum. Serum from malaria-exposed donors (from Malawi or Papua New Guinea) or serum from unexposed Australian controls was heat inactivated (45 min at 56°C). Ethical clearance was obtained from the Human Research Ethics Committee, Royal Melbourne Hospital, the College of Medicine Research Ethics Committee, Malawi, and the Medical Research Advisory Committee, Papua New Guinea.
Characterization of antigenic phenotype by flow cytometry. Trophozoites were incubated with anti-CS2 rabbit antiserum or malaria-exposed human serum as described previously (7). Anti-CS2 (1/20) was detected with fluorescein isothiocyanate (FITC)-conjugated goat anti-rabbit immunoglobulin (Ig) (1/20) (Silenus) or with goat anti-rabbit IgG (1/50) (Jackson ImmunoResearch) and FITC-conjugated anti-goat IgG (1/20) (Caltag). Human serum was detected by sequential incubation with goat anti-human IgG (Caltag) (1/50) and FITC-conjugated anti-goat IgG (1/20) (Caltag or Chemicon). Quadrants were positioned based on staining with serum from unimmunized rabbits or malaria-naive human serum performed in parallel for each experiment. The percentage of IE specifically recognized by anti-CS2 or human serum was calculated by subtracting the percentage of uninfected erythrocytes that stained positive.
Cell sorting. E8B-HA IE were enriched by gelatin sedimentation. After two washes in RPMI-HEPES and three washes in sterile PBS/1% fetal calf serum, staining with anti-CS2 was performed as described previously, with adjustment for the greater volume of cells (6 ml of cells at 0.1% hematocrit). All reagents were filter sterilized before use, and the FITC-conjugated anti-rabbit Ig (Silenus) was dialyzed against PBS overnight to remove sodium azide. Staining with ethidium bromide was omitted. Parasitized erythrocytes were sorted at 4°C, collected into culture medium with 10% human serum, and returned to culture. The phenotypes of sorted populations were characterized after 2 weeks of continuous culture (to generate sufficient numbers of IE).
Agglutination assay. Agglutination assays were performed as described previously (13). A parasite culture (5 to 10% trophozoites and 3% hematocrit) was spun down and resuspended in an equivalent volume of PBS with ethidium bromide (10 μg/ml). IE were incubated with serum at a 1/10 dilution on a rotating wheel at room temperature for 45 min. After gentle resuspension, aliquots were transferred to slides and examined by fluorescence microscopy. Samples were considered positive if more than three agglutinates consisting of more than five IE were observed in the absence of agglutinates of uninfected erythrocytes.
Adhesion inhibition. The ability of anti-CS2 to inhibit adhesion of CS2 trophozoites to CSA was determined as described previously (7). CS2 trophozoites were washed in RPMI-HEPES, resuspended at 0.5% hematocrit in RPMI-HEPES with anti-CS2 or normal rabbit serum (1/10 dilution), and incubated for 30 min at room temperature. Thirty microliters of the parasite suspension was then added to CSA spots for 20 min at 37°C. Otherwise the assay was performed as described above for the adhesion assays.
Nucleic acid isolation and manipulation. Standard phenol-chloroform extraction techniques were used to prepare CS2 genomic DNA (gDNA). RNA was extracted using Trizol reagent (Sigma) as previously described (52). Precipitated RNA was dissolved in either formamide (for Northern blots) or water (for reverse transcription reactions).
Northern blots. Northern blotting was performed as previously described (34, 56) using 0.9% agarose gels and random prime [-32P]dATP-labeled probes. Northern blots probed with the var exon 2 sequence were washed at low stringency with 2x SSC and 0.1% sodium dodecyl sulfate at 55°C (1x SSC is 0.15 M NaCl plus 0.015 M sodium citrate). Northern blots probed with the specific var2csa probe were washed at a higher stringency with 0.5x SSC and 0.1% sodium dodecyl sulfate at 60°C.
Quantitative RT-PCR (Q-RT-PCR). Contaminating gDNA in RNA from each parasite line was removed by repeated digestion with DNase I (DNA-free; Ambion). Ten to 20 μg of DNase I-digested RNA from each parasite line was made to 0.3 mg/ml in a solution containing 0.8 mM each of dATP, dCTP, dGTP, and dTTP and 20 μg/ml random hexamers. The solution was incubated at 65°C for 5 min and then chilled on ice, made to 20 μl containing 50 mM Tris-Cl (pH 8.3), 75 mM KCl, 3 mM MgCl2, 10 mM dithiothreitol, and 40 U of Rnasin (Promega), and incubated first at 25°C for 10 min and then at 42°C for 2 min. Two hundred units of SuperScript II RNase H reverse transcriptase was then added, and the solution was incubated first at 42°C for 50 min and then at 70°C for 15 min. The cDNA was diluted and stored as single-use aliquots at –80°C.
Forward and reverse primers to amplify var2csa (c10) (49) were used at a concentration of 2 μM. 18S rRNA primers (10) were used at concentrations of 0.7 μM (forward) and 6 μM (reverse). Each 10-μl PCR mixture contained 5 μl of SYBR Green PCR master mixture (PE Biosystems) and was amplified using the ABI Prism 7900HT sequence detection system. PCR mixtures were incubated at 95°C for 10 min and then subjected to 40 cycles of 95°C for 15 s and 60°C for 1 min. A final incubation of 95°C for 2 min was followed by a dissociation step at 60°C for 2 min with a 2% ramp rate to 95°C for 2 min. The specificity of each primer pair was determined by dissociation curve analysis.
DNA from var2csa was cloned into the pGEM-T EASY plasmid (Promega), and the concentration of the recombinant plasmid DNA was accurately determined using a fluorescent DNA quantitation kit (Bio-Rad). The plasmid was thawed and diluted 10-fold five times to generate a six-point standard curve, and 2 μl of each dilution was used per Q-RT-PCR mixture, enabling quantitation of 1 x 106 to 10 cDNA copies. CS2 gDNA at 50 μg ml–1 was also stored as single-use aliquots at –80°C and diluted 10-fold five times immediately prior to assaying to generate a six-point standard curve. Two microliters of each dilution was used per Q-RT-PCR mixture to generate a gDNA standard curve that spanned 50 μg ml–1 to 0.5 ng ml–1. This standard curve was used to quantitate the 18S rRNA levels that were used to normalize the var2csa data such that cDNA from equivalent numbers of cells were compared for all parasite lines. Absolute quantitation was performed by using the PE Biosystems instructions by interpolation of threshold cycle values for unknown samples onto standard curves of quantity plotted against threshold cycle.
RESULTS
Specificity of anti-CS2 rabbit antiserum. When flow cytometry was used, the majority of CS2 IE were recognized by anti-CS2 (mean ± standard deviation, 90.8% ± 5.7% [n = 10 experiments]) (Fig. 2A). There was minimal binding of serum from an unimmunized rabbit (2.5% ± 0.9% [n = 10]). Most IE of another ItG CSA-adherent line, E8B-HA (Fig. 1), also bound to anti-CS2 (83.3% ± 5.1% [n = 5]) (Fig. 2B), whereas few IE from the CD36-adherent parent line, FAF-EA8, were recognized (6.5% ± 3.4% [n = 8]) despite expressing variant surface antigens (demonstrated by the binding of malaria-exposed human serum) (Fig. 2C).
To exclude nonspecific binding of rabbit IgG to PfEMP1, CS2 trophozoites were incubated with purified rabbit IgG (10 mg/ml) in place of serum. The level of binding of purified rabbit IgG to CS2 trophozoites was similar to the level of binding of serum from unimmunized rabbits (results not shown). Antiserum against the conserved cytoplasmic acidic terminal sequence domain of PfEMP1 showed levels of binding to CS2 trophozoites similar to those of normal rabbit serum, confirming that only surface antigens were being detected by anti-CS2 (results not shown). A three-step staining protocol using anti-rabbit IgG secondary antibody gave results similar to the results of the two-step protocol with anti-rabbit Ig described previously (results not shown).
Anti-CS2 agglutinated CS2 trophozoites but not the CD36-binding parent line, FAF-EA8, and inhibited adhesion of CS2 to CSA (results not shown).
Binding of anti-CS2 to genetically different CSA-adherent parasite lines. Anti-CS2 bound to two CSA-adherent 3D7-derived parasite lines, 3C and 3CIC (Fig. 1), with a high proportion of IE recognized (for 3C, 83.7% ± 10.6% [n = 5]; for 3CIC, 64.5% ± 10.2% [n = 7]) (Fig. 3A and B). Few 3D7 IE, which adhered to CD36, were recognized by anti-CS2 (5.6% ± 3.1% [n = 6]) (Fig. 3C).
Expression of variant surface antigens by 3D7 was confirmed by showing binding of malaria-exposed human serum (Fig. 3C).
A third line, HCS3, derived by selection of the Southeast Asian isolate HM on CSA (Fig. 1), also had a high proportion of IE that bound to anti-CS2 (91.2% ± 5.6% [n = 4]) (Fig. 4A). Anti-CS2 did not recognize the majority of trophozoites from the parent line, HM, although there was low-level binding to a subpopulation (27.1% ± 4.6% [n = 3]) (Fig. 4B). In parallel assays, the mean fluorescence intensity for anti-CS2 binding to this subpopulation of HM IE was significantly lower than that for binding to HCS3 IE (75.3 ± 21.4 for HM, compared with 383.1 ± 100.3 for HCS3 [n = 3; P < 0.05]), suggesting that fewer epitopes on the surface of HM IE were recognized by anti-CS2. A subpopulation of HM (52.9% ± 5.4% [n = 2]) showed high-level binding of malaria-exposed human serum (mean fluorescence intensity, 590 ± 12.8) (Fig. 4B).
In a recent study, the dominant var transcripts in CSA-adherent lines derived from ItG and 3D7 hybridized with a var2csa-specific probe on Northern blots (22). In the present study, Northern blotting of HCS3 probed with a conserved var exon 2 sequence revealed two dominant var transcripts, which also hybridized to the var2csa probe (Fig. 4C). In contrast, the dominant var transcripts that hybridized to the conserved var exon 2 probe in the parent line, HM, did not hybridize to the var2csa probe.
Binding of anti-CS2 was related to CSA adhesion. Most parasite lines selected for adhesion lose their phenotype in continuous culture and require reselection. Reduced adhesion to CSA in selected lines derived from ItG, 3D7, and HM was associated with a decrease in the percentage of IE recognized by anti-CS2, which increased if parasite lines were reselected on the appropriate receptor (results not shown). After a period of continuous culture, E8B-HA showed reduced adhesion to CSA and HA, increased adhesion to CD36, and decreased recognition by anti-CS2 compared with the phenotype immediately after reselection on HA. This E8B-HA was reselected on CD36, CSA, or HA (Fig. 5). Reselection on either CSA or HA was associated with restoration of adhesion to both CSA and HA and enrichment of IE recognized by anti-CS2. There was no significant change in anti-CS2 binding associated with reselection on CD36.
To further establish that recognition by anti-CS2 was linked to CSA adhesion, fluorescence-activated cell sorting according to recognition by anti-CS2 was performed on the same E8B-HA IE with mixed adhesion phenotypes (Fig. 6A), and the sorted populations were characterized. The anti-CS2-positive population showed a dominant CSA and HA adherence phenotype (Fig. 6B). Adhesion to CD36 was also detected. In contrast, IE that were not recognized by anti-CS2 adhered predominantly to CD36, with minimal CSA or HA adhesion (Fig. 6C). Binding of malaria-exposed human serum to anti-CS2-negative IE confirmed that their lack of recognition by anti-CS2 was not related to an absence of variant surface antigens.
Binding of anti-CS2 was associated with transcription of var2csa. var2csa is the dominant var transcript detected on Northern blots in the ItG lines CS2 and E8B-HA (22). After continuous culture and before reselection of E8B-HA (as described above), low levels of the var2csa transcript were detected by Northern blotting (Fig. 7A) and Q-RT-PCR (Fig. 7B). Following reselection on CSA or HA, which increased the proportions of IE recognized by anti-CS2, var2csa was the dominant var transcript that hybridized with the conserved var exon 2 probe on Northern blots (Fig. 7A), and increased transcription of var2csa was detected by Q-RT-PCR (Fig. 7B). In IE reselected on CD36, which did not enrich for IE recognized by anti-CS2, transcription of var2csa was minimal (Fig. 7B).
Cell-sorted E8B-HA IE recognized by anti-CS2 showed increased transcription of var2csa by Q-RT-PCR. However, on Northern blots, in addition to var2csa, two other var transcripts were detected. Unlike var2csa, these var genes were not detected on Northern blots of the E8B-HA lines reselected on CSA or HA (Fig. 7A). var2csa transcription was decreased in anti-CS2-negative IE compared with the parent E8B-HA.
Taken together, these observations suggested that the major antigenic determinant on the trophozoite surface recognized by anti-CS2 rabbit antiserum was associated with adhesion to CSA and might be the protein product of var2csa.
Antiserum reactive with CSA-binding isolates may cross-react with surface antigens on IE having a different adhesion phenotype. Unexpectedly, two 3D7 lines selected for ICAM-1 adhesion, 3CI and 3D7-ICAM (Fig. 1), also showed high-level recognition by anti-CS2 (for 3CI, 64.8% ± 14.1% [n = 5]; for 3D7-ICAM, 74.8% ± 10.1% [n = 4]) without detectable adhesion to CSA (Fig. 8A and B). The proportion of IE recognized by anti-CS2 correlated with the level of adhesion to ICAM-1 (results not shown). The dominant var that hybridized with the var exon 2 probe in 3D7-ICAM and 3CI did not hybridize with the var2csa probe (Fig. 8C). A genetically distinct ICAM-1-adherent line, E8B-ICAM, was not recognized by anti-CS2 (13.4% ± 5.1% [n = 2]) (Fig. 8D).
DISCUSSION
Cross-reactive epitopes were detected on the surface of erythrocytes infected with genetically distinct CSA-adherent P. falciparum by rabbit antiserum generated against the CSA-adherent P. falciparum line CS2. Similar to placental isolates, CS2 is recognized by sera from pregnant women in a parity-dependent manner and is rarely recognized by sera from adult men (7). Recent work has shown that var2csa is the dominant var gene transcribed in CS2 (22); therefore, this antiserum should detect the PfEMP1 product of var2csa if it is expressed on the IE surface.
We and others have shown that infected erythrocytes of CSA-adherent P. falciparum lines derived from ItG, 3D7, and the patient isolate HM are recognized by serum antibodies from pregnant women in a parity-dependent manner (7, 48; Beeson et al., unpublished data). These observations suggest that infected erythrocytes of these CSA-selected lines express antigens similar to those expressed during pregnancy-associated malaria. In the current study, anti-CS2 recognized CSA-adherent IE from all three P. falciparum lines, ItG, 3D7, and HM. In contrast, anti-CS2 did not bind at high levels to CD36-adherent parent lines. We established that the parent lines expressed variant surface antigens that were not recognized by anti-CS2 by demonstrating binding of malaria-exposed human serum and adhesion to CD36. Binding of anti-CS2 was associated with adhesion to CSA. Reselection of the ItG line E8B-HA on either CSA or HA confirmed that this process selected for IE recognized by anti-CS2.
CSA-adherent IE have been shown to bind nonimmune human IgM (20) and IgG (25) by a mechanism that is not antigen specific. Immunoglobulins in nonimmune rabbit serum may also bind P. falciparum IE (19). We excluded nonspecific binding of rabbit IgM or IgG as an explanation for cross-reactivity of anti-CS2 with different CSA-adherent P. falciparum lines by showing that serum from unimmunized rabbits and purified rabbit IgG gave staining only marginally greater than the staining in the absence of serum. Comparable levels of staining with anti-Ig and anti-IgG secondary antibodies established that binding of nonspecific IgM did not contribute significantly to reactivity. Similar levels of staining with antiserum against the cytoplasmic acidic terminal sequence domain of PfEMP1 confirmed that anti-CS2 was binding to surface antigens and not to cytoplasmic proteins.
Binding of anti-CS2 was associated with transcription of var2csa in CSA-adherent lines. Recently, var2csa has been shown to be the dominant var transcript in CS2, E8B-HA, 3C, and 3CIC (22), and in the current study it was also found to be the dominant var gene transcribed in HCS3. All these lines were recognized by anti-CS2. This association was confirmed by cell sorting IE of the ItG line E8B-HA according to recognition by anti-CS2 and showing that var2csa was transcribed at high levels in anti-CS2-positive IE but not in anti-CS2-negative IE. Interestingly, anti-CS2-positive IE transcribed two var genes in addition to var2csa. Because sorted IE had to be cultured for several weeks before phenotyping, a subpopulation of anti-CS2-positive IE may have switched var gene expression. This could account for the anti-CS2-negative and CD36-adherent IE detected in the anti-CS2-positive population. Alternatively, PfEMP1 proteins encoded by these var genes might be antigenically cross-reactive with the var2csa PfEMP1, resulting in binding of anti-CS2.
A level of cross-reactivity between surface antigens expressed by CSA-adherent and non-CSA-adherent IE was evident. The CD36- and ICAM-1-adherent line HM contained a subpopulation that was recognized by anti-CS2, but the level of antibody binding (indicated by mean fluorescence intensity) was lower than the level of binding to CSA-selected HCS3 parasites. Anti-CS2 also bound to IE of the 3D7 ICAM-1-adherent lines 3CI and 3D7-ICAM but not to the genetically distinct line E8B-ICAM. Binding by anti-CS2 correlated with the level of adhesion to ICAM-1 in the 3D7 lines, suggesting that it was domains of PfEMP1 that were recognized. Finally, CD36-selected E8B-HA showed low-level reactivity with anti-CS2 in the absence of adhesion to CSA or transcription of var2csa. We suggest that our findings are explained by cross-reactivity between domains of PfEMP1 separate from the functional domains mediating parasite adhesion, although the possibility of binding to other variant proteins on the infected erythrocyte surface, such as rifins (24, 36), cannot be excluded.
In summary, using an antiserum against a P. falciparum line in which the dominant var gene transcribed is var2csa, we detected cross-reactive epitopes on the surface of genetically distinct CSA-adherent malaria parasite lines, the presence of which was associated with increased transcription of var2csa. These findings are consistent with the recently published observations of Salanti et al. (48). Both studies suggest that the 3D7 and ItG lines selected for adhesion to CSA express the protein product of var2csa, which is antigenically cross-reactive between these lines. Additionally, we showed that a third CSA-adherent line, HCS3, transcribes var2csa and expresses variant surface antigens that are cross-reactive with those of CSA-adherent 3D7 and ItG lines. Our observation that cell sorting infected erythrocytes with anti-CS2 reactivity selected for CSA-adherent IE that transcribed var2csa confirmed the close link between transcription of var2csa and the antigenic surface phenotype associated with CSA adhesion.
Given that women in malaria-endemic areas are susceptible to pregnancy-associated malaria, despite preexisting antimalarial immunity, a dedicated vaccine is required for this condition. Antibodies specific for variant surface antigens expressed by placental isolates are associated with protection against the consequences of pregnancy-associated malaria, namely, low birth weight and maternal anemia (53), and probably function by inhibiting parasite adhesion to CSA (23, 28). Ideally, a vaccine for pregnancy-associated malaria would induce antibodies specific for domains of surface proteins that mediate CSA adhesion. Together with recent observations (48), our data suggest that the PfEMP1 encoded by var2csa is expressed on the surface of CSA-adherent malaria-infected erythrocytes and is relatively conserved between genetically distinct P. falciparum lines, which is consistent with the high level of homology reported in the var2csa gene between isolates (33, 49). Recent data (48) indicate that antibodies specific for VAR2CSA are associated with a reduced risk of low birth weight. If VAR2CSA is the dominant protein responsible for CSA adhesion by placental parasites and is highly conserved, it might be a suitable vaccine candidate for malaria in pregnancy. Further studies using fresh placental isolates should be performed to establish the prevalence of VAR2CSA expression and its level of conservation in a natural setting.
ACKNOWLEDGMENTS
This work was supported by funding from the National Health and Medical Research Council of Australia (grant 215201).
We thank Aphrodite Caragounis for technical assistance.
Present address: The Walter and Eliza Hall Institute of Medical Research, Melbourne, Victoria, Australia.
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ABSTRACT
Malaria in pregnancy is associated with placental accumulation of Plasmodium falciparum-infected erythrocytes (IE) that adhere to chondroitin sulfate A (CSA). Adhesion is mediated by P. falciparum erythrocyte membrane protein 1 (PfEMP1), a variant parasite protein expressed on the surface of IE and encoded by var genes. Rabbit antiserum was generated against the CSA-adherent P. falciparum line CS2, in which the dominant var transcribed is var2csa, a relatively conserved var gene that has been associated with CSA adhesion. Anti-CS2 recognized genetically distinct CSA-adherent P. falciparum lines but not CD36-adherent parent lines. Reactivity with anti-CS2 correlated with the level of adhesion to CSA. Fluorescence-activated cell sorting according to binding of anti-CS2 showed reactivity was associated with CSA adhesion and transcription of var2csa. These data are consistent with the hypothesis that var2csa encodes a PfEMP1 expressed on the surface of IE, which mediates adhesion to CSA and is relatively conserved between genetically distinct strains of P. falciparum.
INTRODUCTION
During pregnancy, women are more susceptible to the consequences of Plasmodium falciparum malaria infection. Malaria in pregnancy is associated with increased risk of maternal anemia and fetal intrauterine growth retardation (11, 54) and is characterized by accumulation of P. falciparum-infected erythrocytes (IE) in the placenta (59).
Adhesion of IE to multiple host receptors is mediated by P. falciparum erythrocyte membrane protein 1 (PfEMP1) (2, 12, 15, 16, 43; for a review see reference 40), a variant parasite protein expressed on the IE surface and encoded by the var multigene family (3, 51, 55). Transcriptional switching between var genes is associated with changes in parasite antigenic and adhesion phenotypes (51, 55). IE from nonpregnant individuals commonly bind to the endothelial receptors CD36 and intercellular adhesion molecule 1 (ICAM-1) (5, 26, 42). Placental IE have a unique phenotype, adhering to chondroitin sulfate A (CSA) and hyaluronic acid (HA), which are expressed on syncytiotrophoblasts (1, 5, 8, 26).
Partial immunity to pregnancy-associated malaria develops after several pregnancies. This is associated with acquisition of antibodies to variant parasite antigens expressed on the IE surface, which inhibit adhesion of placental IE to CSA (7, 28, 39, 45, 53). PfEMP1 is a potential vaccine candidate for pregnancy-associated malaria. There are approximately 60 var genes per parasite genome (30), and since var genes are highly polymorphic (55), the diversity of PfEMP1 molecules is likely to be large. However, the range of PfEMP1 molecules expressed by placental IE is probably restricted. Sera from malaria-exposed multigravid women can inhibit adhesion of placental IE to CSA even when IE and sera are obtained from geographically distinct locations, suggesting conservation of CSA binding domains (28).
Several var genes purported to encode CSA-binding domains have been described, including varCS2 (43) and FCR3varCSA (12, 50). Binding to CSA has been localized to DBL domains of these genes (12, 18, 32, 38, 43, 44). However, recent studies of FCR3varCSA failed to show a correlation between transcription and the CSA adhesion phenotype or specific transcription in placental-type parasites (21, 27, 35, 47). FCR3varCSA (12, 50) and varCS2 (43) were originally identified in CSA-adherent parasite lines using reverse transcription PCR (RT-PCR). Since single trophozoites can transcribe multiple var genes simultaneously (21), this approach might detect genes that do not encode the expressed PfEMP1 protein.
A recent study using quantitative RT-PCR showed that selection of laboratory lines for CSA adhesion was associated with increased transcription of the var gene var2csa (49). The var2csa gene was transcribed at higher levels in placental isolates than in isolates from children. Unlike many var genes, var2csa shows a high level of homology in genetically distinct isolates (33, 49). Recent mass spectrometric analysis failed to identify the var2csa protein in IE membrane extracts of CSA-selected or placental isolates (29). However, antisera generated against domains of 3D7 var2csa recognize CSA-selected IE, suggesting that the PfEMP1 encoded by var2csa is expressed on the infected erythrocyte surface (48).
In this study, we examined genetically distinct CSA-adherent P. falciparum lines, looking for an association between surface antigenic phenotype and var2csa transcription. Using rabbit antiserum generated against trophozoites of the CSA-adherent line CS2, in which the dominant var transcript is var2csa (22), we linked the presence of conserved or cross-reactive epitopes on the surface of CSA-adherent IE with transcription of var2csa.
MATERIALS AND METHODS
Parasite culture. Parasites were cultured as previously described (4, 57). Parasite lines were regularly tested for Mycoplasma contamination and genotyped (MSP1 and MSP2 alleles). Gelatin enrichment of knob-expressing parasites (31) and sorbitol synchronization (37) were performed every 1 to 2 weeks.
Parasite lines. Three genetically different P. falciparum lines were used (Fig. 1): ItG (shown to be identical to FCR3 [21, 46, 58]), 3D7, and HM (isolated from a traveler infected in Southeast Asia). FAF-EA8 (E8B) (Fig. 1A) was from a clone of ItG2F6 (FAF6) (9, 14) and bound predominantly to CD36. CS2 was obtained by repeated selection of FAF-EA8 on Chinese hamster ovary cells and purified CSA (17) (Fig. 1A). CS2 also binds HA (8). E8B-HA was derived from FAF-EA8 by selecting for adhesion to HA and also binds CSA (8) (Fig. 1A). E8B-ICAM was selected from FAF-EA8 by panning on ICAM-1 (Fig. 1A). 3D7 lines were selected for adhesion to CSA and ICAM-1 as described previously (41) (Fig. 1B). 3D7 lines selected for adhesion to CSA do not bind HA (4). 3D7-ICAM was obtained by repeated selection of 3D7 on purified ICAM-1 (Fig. 1B). HM was selected on purified CSA to generate HCS3 (6) (Fig. 1C), which binds HA as well (4).
Characterization of adhesion phenotype. Adhesion assays were performed as described previously (5). Receptors were diluted in phosphate-buffered saline (PBS) and coated in triplicate spots on the bases of 25-ml plastic petri dishes (Falcon 1058; Becton Dickinson) by overnight incubation at 4°C (CD36 purified from platelets [a gift from M. Berndt, Baker Institute, Australia]), soluble ICAM-1 [from transfected Chinese hamster ovary cells], CSA [from bovine trachea {Sigma}; 10 μg/ml), and HA [from bovine vitreous humor {Sigma}; 100 μg/ml). After blocking for 30 min with PBS/1% bovine serum albumin, plates were washed twice in RPMI-HEPES, and trophozoites (3 to 12% parasitemia) were resuspended at 7% hematocrit in adhesion medium (RPMI-HEPES supplemented with 50 μg/ml hypoxanthine, 2.5 μg/ml gentamicin, and 10% pooled human serum). Thirty-five microliters of this suspension was added per receptor spot and incubated at 37°C for 45 min. Four to six washes with RPMI-HEPES were used to remove unbound cells. Bound cells were fixed by incubation for at least 2 h in 2% glutaraldehyde in PBS and then stained with Giemsa stain. The IE bound in eight fields on each receptor spot were counted, and the mean and standard error of the mean for these 24 counts are given below.
Parasite selection for adhesion. Parasite selection for adhesion was performed as described previously (41). Receptors were diluted in PBS (CD36, ICAM-1, CSA, and HA as described above), filter sterilized, and added to a sterile 25-ml petri dish (Falcon 1058; Becton Dickinson) either as 10-μl spots covering the base of the dish (CD36, ICAM-1) or as 50 ml of diluted receptor (CSA, HA). After overnight incubation, the plate was blocked for 30 min using culture medium containing 10% human serum and then washed twice with RPMI-HEPES. Seventy to 100 ml of cultured parasites (5 to 10% mid-trophozoites) was Percoll purified, resuspended in 50 ml of sterile adhesion medium (as described above), and added to the receptor-coated dish. After 45 to 60 min of incubation at 37°C in 1% O2, 4% CO2, 95% N2, unbound cells were removed by four to six washes in RPMI-HEPES until no unbound IE were observed. Directly pipetting 10 ml of culture medium onto bound IE allowed the IE to be removed and transferred to a petri dish for culture with fresh red blood cells. After several selections to obtain the initial phenotype, repeated reselection was required for most parasite lines after 3 to 4 weeks in continuous culture to maintain optimum adhesion.
Generation of anti-CS2 rabbit antiserum (R1945). A rabbit was immunized with synchronized whole CS2 trophozoites as described previously (43) (approved by the Animal Ethics Committee, The Walter and Eliza Hall Institute of Medical Research). Rabbit antiserum was heat inactivated (45 min at 56°C) and preabsorbed against normal human erythrocytes for 1 h at room temperature.
Malaria-exposed human serum. Serum from malaria-exposed donors (from Malawi or Papua New Guinea) or serum from unexposed Australian controls was heat inactivated (45 min at 56°C). Ethical clearance was obtained from the Human Research Ethics Committee, Royal Melbourne Hospital, the College of Medicine Research Ethics Committee, Malawi, and the Medical Research Advisory Committee, Papua New Guinea.
Characterization of antigenic phenotype by flow cytometry. Trophozoites were incubated with anti-CS2 rabbit antiserum or malaria-exposed human serum as described previously (7). Anti-CS2 (1/20) was detected with fluorescein isothiocyanate (FITC)-conjugated goat anti-rabbit immunoglobulin (Ig) (1/20) (Silenus) or with goat anti-rabbit IgG (1/50) (Jackson ImmunoResearch) and FITC-conjugated anti-goat IgG (1/20) (Caltag). Human serum was detected by sequential incubation with goat anti-human IgG (Caltag) (1/50) and FITC-conjugated anti-goat IgG (1/20) (Caltag or Chemicon). Quadrants were positioned based on staining with serum from unimmunized rabbits or malaria-naive human serum performed in parallel for each experiment. The percentage of IE specifically recognized by anti-CS2 or human serum was calculated by subtracting the percentage of uninfected erythrocytes that stained positive.
Cell sorting. E8B-HA IE were enriched by gelatin sedimentation. After two washes in RPMI-HEPES and three washes in sterile PBS/1% fetal calf serum, staining with anti-CS2 was performed as described previously, with adjustment for the greater volume of cells (6 ml of cells at 0.1% hematocrit). All reagents were filter sterilized before use, and the FITC-conjugated anti-rabbit Ig (Silenus) was dialyzed against PBS overnight to remove sodium azide. Staining with ethidium bromide was omitted. Parasitized erythrocytes were sorted at 4°C, collected into culture medium with 10% human serum, and returned to culture. The phenotypes of sorted populations were characterized after 2 weeks of continuous culture (to generate sufficient numbers of IE).
Agglutination assay. Agglutination assays were performed as described previously (13). A parasite culture (5 to 10% trophozoites and 3% hematocrit) was spun down and resuspended in an equivalent volume of PBS with ethidium bromide (10 μg/ml). IE were incubated with serum at a 1/10 dilution on a rotating wheel at room temperature for 45 min. After gentle resuspension, aliquots were transferred to slides and examined by fluorescence microscopy. Samples were considered positive if more than three agglutinates consisting of more than five IE were observed in the absence of agglutinates of uninfected erythrocytes.
Adhesion inhibition. The ability of anti-CS2 to inhibit adhesion of CS2 trophozoites to CSA was determined as described previously (7). CS2 trophozoites were washed in RPMI-HEPES, resuspended at 0.5% hematocrit in RPMI-HEPES with anti-CS2 or normal rabbit serum (1/10 dilution), and incubated for 30 min at room temperature. Thirty microliters of the parasite suspension was then added to CSA spots for 20 min at 37°C. Otherwise the assay was performed as described above for the adhesion assays.
Nucleic acid isolation and manipulation. Standard phenol-chloroform extraction techniques were used to prepare CS2 genomic DNA (gDNA). RNA was extracted using Trizol reagent (Sigma) as previously described (52). Precipitated RNA was dissolved in either formamide (for Northern blots) or water (for reverse transcription reactions).
Northern blots. Northern blotting was performed as previously described (34, 56) using 0.9% agarose gels and random prime [-32P]dATP-labeled probes. Northern blots probed with the var exon 2 sequence were washed at low stringency with 2x SSC and 0.1% sodium dodecyl sulfate at 55°C (1x SSC is 0.15 M NaCl plus 0.015 M sodium citrate). Northern blots probed with the specific var2csa probe were washed at a higher stringency with 0.5x SSC and 0.1% sodium dodecyl sulfate at 60°C.
Quantitative RT-PCR (Q-RT-PCR). Contaminating gDNA in RNA from each parasite line was removed by repeated digestion with DNase I (DNA-free; Ambion). Ten to 20 μg of DNase I-digested RNA from each parasite line was made to 0.3 mg/ml in a solution containing 0.8 mM each of dATP, dCTP, dGTP, and dTTP and 20 μg/ml random hexamers. The solution was incubated at 65°C for 5 min and then chilled on ice, made to 20 μl containing 50 mM Tris-Cl (pH 8.3), 75 mM KCl, 3 mM MgCl2, 10 mM dithiothreitol, and 40 U of Rnasin (Promega), and incubated first at 25°C for 10 min and then at 42°C for 2 min. Two hundred units of SuperScript II RNase H reverse transcriptase was then added, and the solution was incubated first at 42°C for 50 min and then at 70°C for 15 min. The cDNA was diluted and stored as single-use aliquots at –80°C.
Forward and reverse primers to amplify var2csa (c10) (49) were used at a concentration of 2 μM. 18S rRNA primers (10) were used at concentrations of 0.7 μM (forward) and 6 μM (reverse). Each 10-μl PCR mixture contained 5 μl of SYBR Green PCR master mixture (PE Biosystems) and was amplified using the ABI Prism 7900HT sequence detection system. PCR mixtures were incubated at 95°C for 10 min and then subjected to 40 cycles of 95°C for 15 s and 60°C for 1 min. A final incubation of 95°C for 2 min was followed by a dissociation step at 60°C for 2 min with a 2% ramp rate to 95°C for 2 min. The specificity of each primer pair was determined by dissociation curve analysis.
DNA from var2csa was cloned into the pGEM-T EASY plasmid (Promega), and the concentration of the recombinant plasmid DNA was accurately determined using a fluorescent DNA quantitation kit (Bio-Rad). The plasmid was thawed and diluted 10-fold five times to generate a six-point standard curve, and 2 μl of each dilution was used per Q-RT-PCR mixture, enabling quantitation of 1 x 106 to 10 cDNA copies. CS2 gDNA at 50 μg ml–1 was also stored as single-use aliquots at –80°C and diluted 10-fold five times immediately prior to assaying to generate a six-point standard curve. Two microliters of each dilution was used per Q-RT-PCR mixture to generate a gDNA standard curve that spanned 50 μg ml–1 to 0.5 ng ml–1. This standard curve was used to quantitate the 18S rRNA levels that were used to normalize the var2csa data such that cDNA from equivalent numbers of cells were compared for all parasite lines. Absolute quantitation was performed by using the PE Biosystems instructions by interpolation of threshold cycle values for unknown samples onto standard curves of quantity plotted against threshold cycle.
RESULTS
Specificity of anti-CS2 rabbit antiserum. When flow cytometry was used, the majority of CS2 IE were recognized by anti-CS2 (mean ± standard deviation, 90.8% ± 5.7% [n = 10 experiments]) (Fig. 2A). There was minimal binding of serum from an unimmunized rabbit (2.5% ± 0.9% [n = 10]). Most IE of another ItG CSA-adherent line, E8B-HA (Fig. 1), also bound to anti-CS2 (83.3% ± 5.1% [n = 5]) (Fig. 2B), whereas few IE from the CD36-adherent parent line, FAF-EA8, were recognized (6.5% ± 3.4% [n = 8]) despite expressing variant surface antigens (demonstrated by the binding of malaria-exposed human serum) (Fig. 2C).
To exclude nonspecific binding of rabbit IgG to PfEMP1, CS2 trophozoites were incubated with purified rabbit IgG (10 mg/ml) in place of serum. The level of binding of purified rabbit IgG to CS2 trophozoites was similar to the level of binding of serum from unimmunized rabbits (results not shown). Antiserum against the conserved cytoplasmic acidic terminal sequence domain of PfEMP1 showed levels of binding to CS2 trophozoites similar to those of normal rabbit serum, confirming that only surface antigens were being detected by anti-CS2 (results not shown). A three-step staining protocol using anti-rabbit IgG secondary antibody gave results similar to the results of the two-step protocol with anti-rabbit Ig described previously (results not shown).
Anti-CS2 agglutinated CS2 trophozoites but not the CD36-binding parent line, FAF-EA8, and inhibited adhesion of CS2 to CSA (results not shown).
Binding of anti-CS2 to genetically different CSA-adherent parasite lines. Anti-CS2 bound to two CSA-adherent 3D7-derived parasite lines, 3C and 3CIC (Fig. 1), with a high proportion of IE recognized (for 3C, 83.7% ± 10.6% [n = 5]; for 3CIC, 64.5% ± 10.2% [n = 7]) (Fig. 3A and B). Few 3D7 IE, which adhered to CD36, were recognized by anti-CS2 (5.6% ± 3.1% [n = 6]) (Fig. 3C).
Expression of variant surface antigens by 3D7 was confirmed by showing binding of malaria-exposed human serum (Fig. 3C).
A third line, HCS3, derived by selection of the Southeast Asian isolate HM on CSA (Fig. 1), also had a high proportion of IE that bound to anti-CS2 (91.2% ± 5.6% [n = 4]) (Fig. 4A). Anti-CS2 did not recognize the majority of trophozoites from the parent line, HM, although there was low-level binding to a subpopulation (27.1% ± 4.6% [n = 3]) (Fig. 4B). In parallel assays, the mean fluorescence intensity for anti-CS2 binding to this subpopulation of HM IE was significantly lower than that for binding to HCS3 IE (75.3 ± 21.4 for HM, compared with 383.1 ± 100.3 for HCS3 [n = 3; P < 0.05]), suggesting that fewer epitopes on the surface of HM IE were recognized by anti-CS2. A subpopulation of HM (52.9% ± 5.4% [n = 2]) showed high-level binding of malaria-exposed human serum (mean fluorescence intensity, 590 ± 12.8) (Fig. 4B).
In a recent study, the dominant var transcripts in CSA-adherent lines derived from ItG and 3D7 hybridized with a var2csa-specific probe on Northern blots (22). In the present study, Northern blotting of HCS3 probed with a conserved var exon 2 sequence revealed two dominant var transcripts, which also hybridized to the var2csa probe (Fig. 4C). In contrast, the dominant var transcripts that hybridized to the conserved var exon 2 probe in the parent line, HM, did not hybridize to the var2csa probe.
Binding of anti-CS2 was related to CSA adhesion. Most parasite lines selected for adhesion lose their phenotype in continuous culture and require reselection. Reduced adhesion to CSA in selected lines derived from ItG, 3D7, and HM was associated with a decrease in the percentage of IE recognized by anti-CS2, which increased if parasite lines were reselected on the appropriate receptor (results not shown). After a period of continuous culture, E8B-HA showed reduced adhesion to CSA and HA, increased adhesion to CD36, and decreased recognition by anti-CS2 compared with the phenotype immediately after reselection on HA. This E8B-HA was reselected on CD36, CSA, or HA (Fig. 5). Reselection on either CSA or HA was associated with restoration of adhesion to both CSA and HA and enrichment of IE recognized by anti-CS2. There was no significant change in anti-CS2 binding associated with reselection on CD36.
To further establish that recognition by anti-CS2 was linked to CSA adhesion, fluorescence-activated cell sorting according to recognition by anti-CS2 was performed on the same E8B-HA IE with mixed adhesion phenotypes (Fig. 6A), and the sorted populations were characterized. The anti-CS2-positive population showed a dominant CSA and HA adherence phenotype (Fig. 6B). Adhesion to CD36 was also detected. In contrast, IE that were not recognized by anti-CS2 adhered predominantly to CD36, with minimal CSA or HA adhesion (Fig. 6C). Binding of malaria-exposed human serum to anti-CS2-negative IE confirmed that their lack of recognition by anti-CS2 was not related to an absence of variant surface antigens.
Binding of anti-CS2 was associated with transcription of var2csa. var2csa is the dominant var transcript detected on Northern blots in the ItG lines CS2 and E8B-HA (22). After continuous culture and before reselection of E8B-HA (as described above), low levels of the var2csa transcript were detected by Northern blotting (Fig. 7A) and Q-RT-PCR (Fig. 7B). Following reselection on CSA or HA, which increased the proportions of IE recognized by anti-CS2, var2csa was the dominant var transcript that hybridized with the conserved var exon 2 probe on Northern blots (Fig. 7A), and increased transcription of var2csa was detected by Q-RT-PCR (Fig. 7B). In IE reselected on CD36, which did not enrich for IE recognized by anti-CS2, transcription of var2csa was minimal (Fig. 7B).
Cell-sorted E8B-HA IE recognized by anti-CS2 showed increased transcription of var2csa by Q-RT-PCR. However, on Northern blots, in addition to var2csa, two other var transcripts were detected. Unlike var2csa, these var genes were not detected on Northern blots of the E8B-HA lines reselected on CSA or HA (Fig. 7A). var2csa transcription was decreased in anti-CS2-negative IE compared with the parent E8B-HA.
Taken together, these observations suggested that the major antigenic determinant on the trophozoite surface recognized by anti-CS2 rabbit antiserum was associated with adhesion to CSA and might be the protein product of var2csa.
Antiserum reactive with CSA-binding isolates may cross-react with surface antigens on IE having a different adhesion phenotype. Unexpectedly, two 3D7 lines selected for ICAM-1 adhesion, 3CI and 3D7-ICAM (Fig. 1), also showed high-level recognition by anti-CS2 (for 3CI, 64.8% ± 14.1% [n = 5]; for 3D7-ICAM, 74.8% ± 10.1% [n = 4]) without detectable adhesion to CSA (Fig. 8A and B). The proportion of IE recognized by anti-CS2 correlated with the level of adhesion to ICAM-1 (results not shown). The dominant var that hybridized with the var exon 2 probe in 3D7-ICAM and 3CI did not hybridize with the var2csa probe (Fig. 8C). A genetically distinct ICAM-1-adherent line, E8B-ICAM, was not recognized by anti-CS2 (13.4% ± 5.1% [n = 2]) (Fig. 8D).
DISCUSSION
Cross-reactive epitopes were detected on the surface of erythrocytes infected with genetically distinct CSA-adherent P. falciparum by rabbit antiserum generated against the CSA-adherent P. falciparum line CS2. Similar to placental isolates, CS2 is recognized by sera from pregnant women in a parity-dependent manner and is rarely recognized by sera from adult men (7). Recent work has shown that var2csa is the dominant var gene transcribed in CS2 (22); therefore, this antiserum should detect the PfEMP1 product of var2csa if it is expressed on the IE surface.
We and others have shown that infected erythrocytes of CSA-adherent P. falciparum lines derived from ItG, 3D7, and the patient isolate HM are recognized by serum antibodies from pregnant women in a parity-dependent manner (7, 48; Beeson et al., unpublished data). These observations suggest that infected erythrocytes of these CSA-selected lines express antigens similar to those expressed during pregnancy-associated malaria. In the current study, anti-CS2 recognized CSA-adherent IE from all three P. falciparum lines, ItG, 3D7, and HM. In contrast, anti-CS2 did not bind at high levels to CD36-adherent parent lines. We established that the parent lines expressed variant surface antigens that were not recognized by anti-CS2 by demonstrating binding of malaria-exposed human serum and adhesion to CD36. Binding of anti-CS2 was associated with adhesion to CSA. Reselection of the ItG line E8B-HA on either CSA or HA confirmed that this process selected for IE recognized by anti-CS2.
CSA-adherent IE have been shown to bind nonimmune human IgM (20) and IgG (25) by a mechanism that is not antigen specific. Immunoglobulins in nonimmune rabbit serum may also bind P. falciparum IE (19). We excluded nonspecific binding of rabbit IgM or IgG as an explanation for cross-reactivity of anti-CS2 with different CSA-adherent P. falciparum lines by showing that serum from unimmunized rabbits and purified rabbit IgG gave staining only marginally greater than the staining in the absence of serum. Comparable levels of staining with anti-Ig and anti-IgG secondary antibodies established that binding of nonspecific IgM did not contribute significantly to reactivity. Similar levels of staining with antiserum against the cytoplasmic acidic terminal sequence domain of PfEMP1 confirmed that anti-CS2 was binding to surface antigens and not to cytoplasmic proteins.
Binding of anti-CS2 was associated with transcription of var2csa in CSA-adherent lines. Recently, var2csa has been shown to be the dominant var transcript in CS2, E8B-HA, 3C, and 3CIC (22), and in the current study it was also found to be the dominant var gene transcribed in HCS3. All these lines were recognized by anti-CS2. This association was confirmed by cell sorting IE of the ItG line E8B-HA according to recognition by anti-CS2 and showing that var2csa was transcribed at high levels in anti-CS2-positive IE but not in anti-CS2-negative IE. Interestingly, anti-CS2-positive IE transcribed two var genes in addition to var2csa. Because sorted IE had to be cultured for several weeks before phenotyping, a subpopulation of anti-CS2-positive IE may have switched var gene expression. This could account for the anti-CS2-negative and CD36-adherent IE detected in the anti-CS2-positive population. Alternatively, PfEMP1 proteins encoded by these var genes might be antigenically cross-reactive with the var2csa PfEMP1, resulting in binding of anti-CS2.
A level of cross-reactivity between surface antigens expressed by CSA-adherent and non-CSA-adherent IE was evident. The CD36- and ICAM-1-adherent line HM contained a subpopulation that was recognized by anti-CS2, but the level of antibody binding (indicated by mean fluorescence intensity) was lower than the level of binding to CSA-selected HCS3 parasites. Anti-CS2 also bound to IE of the 3D7 ICAM-1-adherent lines 3CI and 3D7-ICAM but not to the genetically distinct line E8B-ICAM. Binding by anti-CS2 correlated with the level of adhesion to ICAM-1 in the 3D7 lines, suggesting that it was domains of PfEMP1 that were recognized. Finally, CD36-selected E8B-HA showed low-level reactivity with anti-CS2 in the absence of adhesion to CSA or transcription of var2csa. We suggest that our findings are explained by cross-reactivity between domains of PfEMP1 separate from the functional domains mediating parasite adhesion, although the possibility of binding to other variant proteins on the infected erythrocyte surface, such as rifins (24, 36), cannot be excluded.
In summary, using an antiserum against a P. falciparum line in which the dominant var gene transcribed is var2csa, we detected cross-reactive epitopes on the surface of genetically distinct CSA-adherent malaria parasite lines, the presence of which was associated with increased transcription of var2csa. These findings are consistent with the recently published observations of Salanti et al. (48). Both studies suggest that the 3D7 and ItG lines selected for adhesion to CSA express the protein product of var2csa, which is antigenically cross-reactive between these lines. Additionally, we showed that a third CSA-adherent line, HCS3, transcribes var2csa and expresses variant surface antigens that are cross-reactive with those of CSA-adherent 3D7 and ItG lines. Our observation that cell sorting infected erythrocytes with anti-CS2 reactivity selected for CSA-adherent IE that transcribed var2csa confirmed the close link between transcription of var2csa and the antigenic surface phenotype associated with CSA adhesion.
Given that women in malaria-endemic areas are susceptible to pregnancy-associated malaria, despite preexisting antimalarial immunity, a dedicated vaccine is required for this condition. Antibodies specific for variant surface antigens expressed by placental isolates are associated with protection against the consequences of pregnancy-associated malaria, namely, low birth weight and maternal anemia (53), and probably function by inhibiting parasite adhesion to CSA (23, 28). Ideally, a vaccine for pregnancy-associated malaria would induce antibodies specific for domains of surface proteins that mediate CSA adhesion. Together with recent observations (48), our data suggest that the PfEMP1 encoded by var2csa is expressed on the surface of CSA-adherent malaria-infected erythrocytes and is relatively conserved between genetically distinct P. falciparum lines, which is consistent with the high level of homology reported in the var2csa gene between isolates (33, 49). Recent data (48) indicate that antibodies specific for VAR2CSA are associated with a reduced risk of low birth weight. If VAR2CSA is the dominant protein responsible for CSA adhesion by placental parasites and is highly conserved, it might be a suitable vaccine candidate for malaria in pregnancy. Further studies using fresh placental isolates should be performed to establish the prevalence of VAR2CSA expression and its level of conservation in a natural setting.
ACKNOWLEDGMENTS
This work was supported by funding from the National Health and Medical Research Council of Australia (grant 215201).
We thank Aphrodite Caragounis for technical assistance.
Present address: The Walter and Eliza Hall Institute of Medical Research, Melbourne, Victoria, Australia.
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