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Antigen-specific antibodies (Abs) to the 19-kDa carboxy-terminal region of Plasmodium falciparum merozoite surface protein 1 (MSP119) play an important role in protective immunity to malaria. Mouse monoclonal Abs (MAbs) 12.10 and 12.8 recognizing MSP119 can inhibit red cell invasion by interfering with MSP1 processing on the merozoite surface. We show here that this ability is dependent on the intact Ab since Fab and F(ab′)2 fragments derived from MAb 12.10, although capable of binding MSP1 with high affinity and competing with the intact antibody for binding to MSP1, were unable to inhibit erythrocyte invasion or MSP1 processing. The DNA sequences of the variable (V) regions of both MAbs 12.8 and 12.10 were obtained, and partial amino acid sequences of the same regions were confirmed by mass spectrometry. Human chimeric Abs constructed by using these sequences, which combine the original mouse V regions with human γ1 and γ3 constant regions, retain the ability to bind to both parasites and recombinant MSP119, and both chimeric human immunoglobulin G1s (IgG1s) were at least as good at inhibiting erythrocyte invasion as the parental murine MAbs 12.8 and 12.10. Furthermore, the human chimeric Abs of the IgG1 class (but not the corresponding human IgG3), induced significant NADPH-mediated oxidative bursts and degranulation from human neutrophils. These chimeric human Abs will enable investigators to examine the role of human Fcγ receptors in immunity to malaria using a transgenic parasite and mouse model and may prove useful in humans for neutralizing parasites as an adjunct to antimalarial drug therapy.
Plasmodium falciparum merozoite surface protein 1 (MSP1) is a major surface protein and a leading candidate antigen for a malaria vaccine. MSP1 is present on the surface of the merozoite as a polypeptide complex formed, in part, by the proteolytic cleavage, or primary processing, of the precursor protein. The complex is anchored to the merozoite by the C-terminal 42-kDa fragment attached to the membrane by a glycophosphoinositol anchor. At the time of erythrocyte invasion, the 42-kDa fragment undergoes a proteolytic cleavage, known as secondary processing, leaving a 19-kDa carboxy-terminal fragment (MSP119) attached to the merozoite and carried into the erythrocyte as the parasite invades (3).
Humans develop antibodies (Abs) against MSP119 through natural P. falciparum infection, and such Abs have been associated with protection from clinical malaria in a number of studies (5, 10, 11). There is also good evidence that Abs against MSP119 provide the major component of the invasion-inhibitory activity of human immune sera (26).
Secondary processing is essential for successful erythrocyte invasion and is vulnerable to interruption by some MSP119 specific Abs. For example, two murine monoclonal Abs (MAbs) specific for epitopes within MSP119, MAb 12.8 and MAb 12.10 (20) have been shown to prevent secondary processing of MSP1, and these MAbs also inhibit erythrocyte invasion (3, 4), suggesting that their biological activity, the inhibition of invasion, is due to the inhibition of secondary processing. It is therefore very important to understand why the antibodies are effective and which of their features are important. For example, their fine specificity of binding or their size may be key properties.
Determining the fine specificity of Abs against MSP119 is of crucial importance in understanding their biological activity. Significant associations have been demonstrated between the fine specificity of anti-MSP119 Abs and protection from subsequent malaria (27). Interestingly, MAbs 12.8 and 12.10 bind to epitopes that are conserved in both dimorphic forms of MSP119 and are found on parasites derived from all geographic locations examined (20). The structure of P. falciparum MSP119 has been determined (25, 28), and this information facilitates understanding the interaction between the antigen and the various Abs, which has been explored using binding studies with native or mutant forms of recombinant MSP119 (9, 21, 23, 24, 35) and by computational approaches (2). These studies have shown that MAbs 12.8 and 12.10 both bind to the same surface of MSP119 and their epitopes overlap, findings consistent with them having the same biological properties (reviewed in reference 16).
The size of the Ab binding to the antigen may also be an important factor in its activity. For example, Fab fragments of MAbs binding to apical membrane antigen 1 (AMA1) are more effective than the corresponding immunoglobulin Gs (IgGs) at inhibiting erythrocyte invasion (34). Furthermore, Fc-mediated effects have been shown to be an important component of the function of MSP119-specific antibodies in an in vivo transgenic model (21). We have prepared here F(ab′)2 and Fab fragments from MAb 12.10 and examined their biological activity compared to the intact antibody. In addition, we have determined the DNA and amino acid sequences of the variable regions of the heavy and light chains of MAbs 12.8 and 12.10 by cloning and mass spectrometry and used this information to create recombinant chimeric human IgG1 and IgG3 versions of these MAbs. When these sequences are expressed by mammalian cell lines, they retain both their ability to bind to MSP119 and to inhibit erythrocyte invasion by P. falciparum in vitro. The recombinant chimeric human IgG1 and IgG3 Abs may prove suitable for in vivo studies of protection from P. falciparum in both primate and rodent transgenic models (14, 21).
The mouse MAb 12.8 (IgG2b) and MAb 12.10 (IgG1) hybridomas were cultured in Dulbecco modified Eagle medium containing 10% fetal calf serum (depleted of bovine IgG) and Ab purified from culture supernatants by protein G-Sepharose affinity chromatography. F(ab′)2 fragments were generated from MAb 12.10 by pepsin digestion and gel filtration chromatography. Fab fragments were then derived from F(ab′)2 by mild reduction and alkylation, followed by further purification by gel filtration, essentially as described previously (19, 24). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis of the final intact MAb, F(ab′)2, and Fab showed that the preparations contained polypeptides of the expected molecular masses and were highly pure (see Fig. Fig.1A1A).
Two protocols, both described previously, were used to assess Ab-mediated inhibition of MSP1 secondary processing. In the first method, purified, naturally released merozoites were washed, incubated on ice in the presence of test Abs, transferred to 37°C for 1 h to allow processing to proceed, and then analyzed by Western blotting for de novo production of the MSP133 product of secondary processing (4, 15). Alternatively, mature schizonts were biosynthetically radiolabeled with [35S]methionine-cysteine and then placed back into culture with the addition of fresh erythrocytes, plus the Abs or Ab fragments being tested, and allowed to undergo merozoite release for 6 h. Cultures were then harvested; processing was quantified by immunoprecipitation of labeled MSP133 from the culture medium by using MAb X509 coupled to Sepharose, while ring-stage parasitemia in the cultures was assessed by microscopic examination of Giemsa-stained thin blood films (15).
Invasion assays were carried out essentially as previously described (3), using schizonts from the 3D7, FCB-1, or T9/96 lines of P. falciparum. Schizonts, collected and purified at 40 h postinvasion, were added to the culture with fresh erythrocytes. The plates were placed in a gassed box containing an atmosphere of 7% CO2, 5% O2, and 88% N2 and incubated at 37°C for 20 h.
For quantitative comparisons of binding by surface plasmon resonance (SPR) analysis, recombinant MSP119-glutathione S-transferase (GST) (7) or GST alone (negative control) were amine coupled to each flow cell of a CM5 sensor chip. Various concentrations (as indicated) of purified MAbs, their F(ab′)2 and Fab fragments, or human chimeric anti-MSP119 IgG1 and IgG3 were then injected over each antigen, and association and dissociation observed. The data from a BIAcore machine were analyzed by using BIAevaluation 3.0 software. For immunofluorescence (IF) studies, washed erythrocytes infected with the P. falciparum 3D7, T9/96, and FCB-1 parasite lines were fixed on slides in methanol-acetone (1:1 [vol/vol]) for 10 min. After being blocked in phosphate-buffered saline (PBS)-5% (vol/vol) goat serum, the slides were incubated with Abs at 5 μg ml−1 in blocking buffer for 1 h, washed, and then incubated for 1 h with fluorescein isothiocyanate (FITC)-conjugated goat F(ab′)2 anti-human IgG (Sigma or Caltag) diluted 1:500 in blocking buffer. After a washing step, the slides were mounted with DAPI (4′,6′-diamidino-2-phenylindole) antifade, and Ab binding was examined by IF microscopy on a Zeiss Axioscope 40 microscope.
IgG heavy and light chains were fractionated by SDS-PAGE, stained, excised from the gel, and then subjected to trypsin digestion. Peptides were analyzed on LCQ and Q-TOF tandem mass spectrometers at the National Institute for Medical Research and the Biopolymer Synthesis and Analysis Unit (University of Nottingham), and the sequences were determined by using an online database (Matrix Science). These sequences were used in a search of related murine immunoglobulin sequences using the basic local alignment search tool (BLAST) at the National Center for Biotechnology Information (1), and this information was used to design specific primers to amplify the V genes from the cDNA.
Variable heavy (VH) and light (VL) complementary DNAs (cDNAs) were obtained from the mouse hybridomas 12.10 and 12.8 by reverse transcriptase-PCR by using degenerate primers (5′-CAGGCGCGCACTCCSARGTNMARCTGCAGSAGTCWG-3′ and 5′-TGARGAGACGGTGACCGDGGTBCCTKGGCCCCA-3′ for VH and 5′-GAYATTGAGCTCACMCARWCTMCA-3′ and 5′-CCGTTTBAKCTCGAGCTTKGTSCC-3′ for the 12.8 VL or 5′-GACATTGAGCTCACMCAGTCTC-3′ and 5′-CCGTTTCAGCTCGAGCTTGGTCCC-3′) for the 12.10 VL. The VH and VL genes were cloned into pCR2.1-Topo or pGEM-T easy plasmids, respectively. The VH gene was subcloned as a BssHII-BstEII fragment into pVH-C1-γ1 or pVH-C1-γ3 vectors upstream of the IgG1 or IgG3 constant regions, respectively (21). The human IgG3 constant gene was amplified from human genomic DNA by overlap PCR using two sets of primers (G3FOR1, 5′-CCTGGATCCTCGTGGATAGACAAG-3′; G3REV1, 5′-GAAGATATCGAGTTACTCAGATCTGGG-3′; G3FOR2, 5′-CTCAGCTCAGACACCTTCTC-3′; G3REV2, 5′-TCCCTTCTAGAACTCAGGCCTCAGAC-3′), cloned into the pCR2.1 TOPO vector (17), sequenced, and subcloned as a BamHI-XbaI fragment into pVH-C1-γ1 to create pVH-C1-γ3. The VL gene was subcloned as an ApaLI-XhoI fragment into pVK-C1-Express upstream of the human κ gene as previously described (21, 29). Chinese hamster ovary (CHO-K1) cells were transfected with corresponding heavy- and light-chain constructs, and positive clones were selected (21, 29). Clones secreting MSP119-specific IgG1 or IgG3 were detected by enzyme-linked immunosorbent assay (ELISA) with recombinant MSP119 coated plates and by immunoblotting with goat anti-human IgG Abs conjugated to horseradish peroxidase. Recombinant MSP119 used in all ELISAs was generated as previously described (7). From large-scale cultures, human IgG1 or IgG3 were purified on HiTrap protein G-Sepharose (GE Healthcare) by fast performance liquid chromatography. The integrity and purity of the Abs was verified on SDS-PAGE gels.
Neutrophils were isolated from heparinized blood taken from healthy volunteers by sedimentation of erythrocytes in 6% (wt/vol) dextran T70 (GE Healthcare, United Kingdom) in 0.9% (wt/vol) saline at 37°C for 30 min, followed by leukocyte separation on a discontinuous density gradient of Lymphoprep (ρ = 1.077 g/cm3; Nycomed, Birmingham, United Kingdom) over Ficoll-Hypaque (ρ = 1.119 g/cm3) and centrifugation at 700 × g for 20 min at room temperature. Approval for the use of human cells was obtained from the local QMC ethical committee. Wells of chemiluminescence microtiter plates (Dynatech Laboratories, Billinghurst, Sussex, United Kingdom) were coated with 150 μl of MSP119 at 5 μg ml−1 in coating buffer (0.1 M carbonate buffer [pH 9.6]) and incubated overnight at 4°C. After three washes with PBS, 150-μl portions of anti-MSP119 Abs (MAbs 12.8, 12.10, and their chimeric human IgG1 and IgG3 derivatives at 50 μg ml−1) were added to the wells. In each case, triplicate wells were prepared and left for 2 h at room temperature. After a wash as described above, 100 μl of luminol (67 μg ml−1 in Hanks buffered saline solution [HBSS] containing 20 mM HEPES and 0.1 g of globulin-free bovine serum albumin [BSA]/100 ml [HBSS/BSA]) was added to each well. After the addition of 50 μl of purified neutrophils (106/ml in HBSS/BSA) to each well, the plates were transferred to a Microlumat LB96P luminometer, and the chemiluminescence was measured at 2-min intervals for 120 min at 37°C. The data were analyzed by using Excel software. Similarly coated wells not used for the chemiluminescence assay were tested by ELISA for reactivity with anti-mouse IgG (Pierce) or anti-human IgG (Sigma) reagents conjugated to peroxidase as appropriate, and coating levels for each Ab were found to be equivalent prior to entry into the respiratory burst assay.
To examine the contribution of the size of the antibody to inhibition of processing and red cell invasion, we removed the Fc region and generated purified bivalent F(ab′)2 and monovalent Fab fragments of MAb 12.10 (Fig. (Fig.1A).1A). Both F(ab′)2 and Fab fragments retained the specificity and binding properties of the parental MAb 12.10 as determined by SPR analysis, albeit with expected changes in affinity that are associated with the valency of mono- and bispecific reagents (Table (Table1).1). In addition, the F(ab′)2 and Fab fragments effectively competed with intact MAb 12.10 for antigen binding to live merozoites or MSP119 coated onto plates (data not shown). However, whereas MAb 12.10 inhibited MSP1 processing in vitro (Fig. (Fig.1B,1B, lane 4), when used at equimolar concentrations (2.6 μM each), neither the F(ab′)2 nor the Fab fragments inhibited MSP1 processing (Fig. (Fig.1B,1B, lanes 5 and 6). However, when merozoites were preincubated with the F(ab′)2 or Fab, the processing-inhibitory activity of MAb 12.10 was significantly reduced, presumably because they competed effectively for binding to the antigen (Fig. (Fig.1B,1B, lanes 7 and 8).
The next set of experiments was designed to investigate the ability of MAb 12.10 and its F(ab′)2 and Fab fragments to inhibit MSP1 processing and to interfere with erythrocyte invasion under conditions of active merozoite release and erythrocyte invasion in parasite cultures. For this purpose, an in vitro parasite invasion assay was used in which MSP1 secondary processing was measured in parallel with erythrocyte invasion. Mature T9/96 schizonts were biosynthetically radiolabeled with [35S]methionine-cysteine, and then merozoite release and red cell invasion was allowed to proceed in the presence of intact MAb 12.10, or the F(ab′)2 or Fab preparations. Erythrocyte invasion over the course of the experiment was then assessed by counting the number of new ring-stage parasites formed (Fig. (Fig.1C),1C), and MSP1 processing was assessed by direct immunoprecipitation of MSP133 from the culture supernatants (Fig. (Fig.1D).1D). Significant inhibition of both erythrocyte invasion and MSP1 processing was observed when either MAb 12.10 or EGTA (a potent inhibitor of MSP1 processing) was present in cultures. The F(ab′)2 and Fab preparations had no discernible effect on either erythrocyte invasion or MSP1 processing.
In competition experiments, in which increasing concentrations of F(ab′)2 or Fab were added to microcultures containing intact MAb 12.10 at 400 μg ml−1, it was observed that both fragments can interfere with the processing-inhibitory activity of MAb 12.10 in a dose-dependent manner and at high concentrations can effectively reverse both the invasion-inhibitory and the processing-inhibitory activity of MAb 12.10 (Fig. 1C and D).
Having established that intact IgG was necessary to inhibit invasion and MSP1 processing, we wanted to produce chimeric human IgG1 and IgG3 versions of 12.8 and 12.10 and examine their properties. As a first step, and in anticipation of nonproductive pseudogene expression in both hybridomas, to be certain of cloning the correct variable (V) gene sequences, we obtained amino acid sequence data for the heavy and light chains (separated by SDS-PAGE) from MAbs 12.8 and 12.10 by tryptic digestion and mass spectrometric analysis of the generated fragments (Fig. (Fig.2).2). For each of the heavy and light chains, at least two unique peptide sequences were derived from the variable regions, allowing assignment of the variable regions to specific families. Primers were then designed to amplify the correct sequences from cDNA.
The MAb heavy- and light-chain V regions were amplified by PCR from cDNA derived from each hybridoma and cloned into plasmid vectors by using restriction enzyme sites included in the amplification primers and sequenced (Fig. (Fig.2).2). Searches against the ImMunoGeneTics (IMGT) (http://imgt.cines.fr/) databases indicated that the heavy chain of MAb 12.8 is a member of the mouse IgHV9 family (IGHV9-1*02), whereas that of MAb 12.10 belongs to the IgHV1 family (IGHV1S81*02). Comparisons with existing mouse immunoglobulin light-chain databases at IMGT also found close matches for each of the light chains. Thus, the MAb 12.8 light chain is a member of the IgKV4 family (IGKV4-70*01), and MAb 12.10 is a member of the IgKV6 family (IGKV6-17*01). The V regions of both the heavy and light chains differ significantly in DNA sequence between the two MAbs, which is consistent with them belonging to different subclasses and V gene families, and with their binding to nonidentical epitopes on MSP119 (23).
We subcloned the murine variable region sequences derived from the two MAbs and linked them to human IgG1 and IgG3 constant domains in expression vectors used previously to generate fully human anti-P. falciparum or chimeric anti-P. yoelii MSP119 IgGs (21, 29). Both Abs, when purified from transfected CHO-K1 cell culture supernatants by protein G-Sepharose chromatography, contained polypeptides of the expected size on SDS-PAGE (Fig. (Fig.3,3, left panel) and the anticipated reactivity with anti-human IgG Fc-specific Abs (Fig. (Fig.3,3, right panel). As a consequence of the extended hinge region in human IgG3, the heavy chain has the expected higher apparent molecular mass compared to human IgG1.
Both recombinant IgG1 and IgG3 Abs bound to MSP119 in ELISAs and immunoblot analyses (data not shown) and produced a characteristic pattern of MSP119 reactivity with schizonts, merozoites, and ring-stage parasites in IF assays of P. falciparum 3D7, identical to that of both the parental murine MAbs (not shown) and the MSP119-specific human IgG1 C1 (JS1) (Fig. (Fig.3,3, lower panel). Importantly, the chimeric Abs interacted with MSP119 with very high affinity compared to that of the original parental mouse MAbs or the human IgG1 or IgG3 C1 (JS1) generated from a previous study (21) and did not bind to GST or irrelevant malaria antigens (e.g., AMA1 [data not shown]) when investigated by Biacore SPR analysis of antigen-coated sensor chips (Fig. (Fig.44).
The chimeric IgG1 antibodies were examined for their ability to inhibit erythrocyte invasion, using the parental murine MAbs 12.8 and 12.10 and EGTA as positive controls and the non-Plasmodium specific MAb BC1 as a negative control. The chimeric human IgG1s, 12.10 and 12.8, at concentrations of 560 and 1,200 μg ml−1, respectively, reduced the percentage of red cells invaded by P. falciparum by 57 and 54% (Fig. (Fig.5).5). Interestingly, greater inhibition of invasion was observed with the chimeric IgG1 12.10 than with the parental MAb 12.10 used at 4,130 μg ml−1, which may reflect variation in the activity of MAb 12.10 we observed in different batches prepared from the hybridoma on different occasions. We were unable to do these assays with the chimeric human IgG3 versions because of difficulties in generating sufficient quantities of Ab.
We next assessed the ability of these novel IgG reagents to interact with human Fcγ receptors (FcγRs) and induce NADPH oxidase activation in human blood neutrophils. For neutrophils, luminol chemiluminescence provides a readout of NADPH oxidase activation (respiratory burst) and myeloperoxidase release (22). In order to more directly compare the relative abilities of MSP119 specific Abs to induce functional responses, recombinant chimeric IgG1 and IgG3 with identical specificities for MSP119 and at equivalent concentrations, as determined by ELISA, were aggregated on MSP119-coated wells. Surprisingly, although both chimeric human IgG1s very effectively induced luminol chemiluminescence responses in resting neutrophils, the chimeric human IgG3 versions were without effect (Fig. (Fig.6).6). As anticipated from our findings with a mouse IgG2b specific to MSP119 from P. yoelii (29), murine MAb 12.8, which is also of the IgG2b subclass, was able to induce a weak burst, presumably through lower-affinity interactions with human FcγRs. Intriguingly, MAb 12.10, a mouse IgG1, was as effective as the human IgG1 at inducing bursts from human neutrophils.
MAbs 12.8 and 12.10 inhibit MSP1 processing and erythrocyte invasion by a mechanism that is independent of FcγRs (3). To better understand the contributing factors within the structure of an Ab involved in this process, we generated bivalent (Fab′)2 and monovalent Fab fragments from MAb 12.10. Removal of the Fc region abolished the processing-inhibitory activity of 12.10 without affecting the ability of both the (Fab′)2 and the Fab fragments to bind to MSP1. This result indicates that the Fc part of the molecule and perhaps the overall size of the antibody are essential for inhibition. This result is in contrast to what has been reported for antibodies that bind to AMA1 and inhibit erythrocyte invasion: (Fab′)2 and Fab fragments of MAbs specific for this protein are more effective than the intact IgG in mediating inhibition of invasion (8, 34). In this case, the smaller size may be important, and clearly in this case the Fc is not crucial to Ab function. On the other hand, recent work has shown that an anti-MSP119 human IgG1 (JS1) that cannot inhibit processing or invasion in vitro can protect animals from malaria in vivo by recruiting human FcγRs (21).
In order to understand the role of human Abs and their cognate FcγRs in immunity to malaria, we generated chimeric human IgG1 and IgG3 recognizing the candidate vaccine antigen MSP119. These reagents were generated through cloning of immunoglobulin variable (V) domains from murine hybridomas 12.8 and 12.10 and grafting them onto human constant region genes of the two most common IgG1 and IgG3 allotypes found in African populations (17) for expression in mammalian cell lines. The engineered chimeric human Abs recognized parasites in infected erythrocytes and bound specifically to MSP119 compared to a panel of control Abs. Reassuringly, SPR analysis revealed no reduction in affinity for MSP119 for the chimeric IgG1 or IgG3 compared to the parental murine MAbs. The chimeric human IgG1 Abs also very effectively inhibited red cell invasion and triggered potent FcγR-mediated respiratory bursts from human neutrophils. Human neutrophils express the activating FcγRIIA and the glycophosphoinositol-linked FcγRIIIB, and human IgG1 is known to bind to the former with very high affinity (6). Furthermore, FcγRIIIB cross-linking does not induce nuclear migration or Erk or MEK, suggesting that these two receptors mediate human IgG1 signaling through different pathways (13). Unexpectedly, these assays also indicated that human IgG3 may be a poor inducer of respiratory bursts despite possessing the highest affinity for FcγRs of all of the human IgG subclasses (6, 31). Since antigen-specific antimalarial Abs from clinically immune individuals can belong to the human IgG3 subclass (12, 30), these data suggest that the effectiveness of human IgG3 (and indeed human neutrophils) in controlling malaria parasites may not be mediated via respiratory bursts but through alternative modes of action. Neutrophils are abundant in blood, and products of their respiratory bursts have been shown to kill merozoites, although the exact mechanisms are unclear (18). Recent results with recombinant human Abs generated against P. falciparum AMA1 support these observations and show that human IgG3 may make up for this deficiency by being more efficient at promoting phagocytosis of malaria parasites than human IgG1 (unpublished data). It is known that human IgG3 immune complexes are unable to stimulate human neutrophil respiratory bursts, although they provide a strong stimulus for inducing neutrophil degranulation, presumably via FcγRIIIB triggering (36). Further work is therefore required with the human IgG3 to ascertain its mode of action in malaria.
Believing that antiparasite Abs that both inhibit processing and recruit FcγR mediated clearance may be more effective than Abs limited to a single mode of action, we postulate that the human chimeric Abs generated here would be more effective at clearing malaria parasites in vivo than the human MAb JS1 generated earlier (21). JS1, a human IgG1, does not inhibit MSP1 processing but nevertheless effectively cleared parasites when passively transferred into human FcγR1 transgenic mice infected with P. berghei parasites transgenic for P. falciparum MSP119. However, neither MAb 12.8 nor 12.10 could bind to the MSP119 transgenic P. berghei parasite (data not shown), probably because the first 7 amino acids (Gly-Ile-Asp-Pro-Lys-His-Val) of the transgenic MSP119 sequence following the secondary cleavage site are from P. berghei, which differs substantially from the corresponding P. falciparum sequence (Asn-Ile-Ser-Gln-His-Gln). In addition, passive immunization with MAb 12.8 and 12.10 had no effect on parasite development (data not shown), as expected if the antibodies do not bind to the parasite. In order to test the protective capacity of these human chimeric 12.8 and 12.10 antibodies in vivo prior to any clinical trial, it will be necessary to generate a new transgenic P. berghei parasite containing the complete MSP119 sequence, including the epitopes seen by both MAbs 12.8 and 12.10. For such a transgenic parasite to be viable the MSP119 cleavage site would need to be a suitable substrate for the P. berghei MSP1 processing enzyme, likely to be an orthologue of P. falciparum subtilisin 2. In the absence of an appropriate in vivo mouse model, Aotus monkeys have been used as a primate model for P. falciparum infection in passive immunization studies (14) and when testing MSP119-based vaccines (33). The same model could be used to examine the ability of the invasion-inhibitory Abs to ameliorate P. falciparum infection in vivo; the partly human chimeric Abs should have reduced immunogenicity in primates compared to the parental murine MAbs. Although nonhuman primates are widely used in malaria research, care must be taken in evaluating human IgG antibodies in these species. For example, although FcγRIII (CD16) from sooty mangabey's is capable of binding human IgG1 and IgG2, it cannot bind human IgG3 or IgG4 like the human CD16 orthologue, predicting difficulties in assessment of these reagents in the Aotus model (32). Thus, despite strong sequence conservation in the FcγRs, nonhuman primate and human FcγRs differ in their ability to bind human IgG subclasses. Clearly, differences in the interaction of human IgG with nonhuman primate FcγRs should be considered when these models are used for the in vivo evaluation of therapeutic antibodies. Nonetheless, chimeric human versions of MAbs 12.8 and 12.10 clearly directly inhibit invasion of human erythrocytes by parasites and therefore may be useful therapeutically when a malaria infection is nonresponsive to available drugs (31).
In conclusion, we have shown that intact antibody is essential for inhibition of MSP1 processing and red cell invasion, and have developed recombinant partly human chimeric IgG1/κ and IgG3/κ Abs targeting the malaria vaccine candidate antigen MSP119. These novel reagents will be useful in standardized immunological assays with which to rigorously examine their antiparasitic mechanisms of action, be they FcγR dependent or independent.
We thank the Medical Research Council (Career Establishment Award MRC G0300145 and Research Training Fellowship to R.M.J.), the European Union (Marie Curie Excellence Grants; Ab Immunotherapy for Malaria, MEXT-CT-2003-509670; and The European Malaria Vaccine Development Association, LSH-2005-037506), The Wellcome Trust (WT082915MA), and the Sir Halley Stewart Trust for funding.
We thank Brendan Crabb and Tania de Koning-Ward for provision of the P. berghei parasite transgenic for MSP119 from P. falciparum.
Approval for the use of human cells was obtained from the QMC Ethical Committee.
Editor: J. F. Urban, Jr.
Published ahead of print on 5 October 2009.