Three non-repeated transmission blocking epitopes recognized in the 21 kD surface antigen of zygotes-ookinetes of Plasmodium berghei

Summary Two-site and competitive ELISA have been developed against a surface antigen of zygotes-ookinetes of Plasmodium berghei using monoclonal antibodies (MoAbs) which block transmission of the parasite to the mosquito. Three such MoAbs have been studied, each of which recognized a protein of an M_r 21 kD (Pbs21) using immunoblot techniques. The assays showed that there are at least 3 single B-cell epitopes expressed in Pbs21. One epitope recognized by MoAb 17.9 is conformation dependent and antibodies bound to it interfered with other MoAbs (12.1 and 13.1) each recognizing a different, apparently linear epitope. Glycosylation might not be relevant to the binding of any of the antibodies tested to their respective epitopes.     

Properties of epitopes of Pfs 48/45, a target of transmission blocking monoclonal antibodies, on gametes of different isolates of Plasmodium falciparum

We have studied the properties of epitopes on Plasmodium falciparum gamete surface protein Pfs 48/45, a target antigen of malaria transmission blocking antibodies. Using a two site immunoradiometric assay we have defined three spacially separate, non-repeated, epitope regions on the peptides representing this antigen. Epitope region I is a target of monoclonal antibodies (MoAbs) which strongly suppress infectivity of gametocytes of P. falciparum to mosquitoes; the effect is complement independent and is mediated as effectively by the monovalent Fab fragments as by intact MoAb. Epitope region II consists of two spacially close subregions, IIa and IIb; variant forms of epitopes IIa and IIb occurred in different isolates of P. falciparum. Epitope region III also showed slight structural modification between isolates. MoAbs against regions II or III were relatively ineffective in suppressing gametocyte infectivity compared to MoAbs against region I. However, certain combinations of MoAbs against regions II and III together acted synergistically to suppress infectivity to mosquitoes. All these epitopes failed to react with MoAb when the antigen was presented in reduced form. A fourth epitope, however, was identified which reacted strongly with MoAb when the antigen was presented in reduced form. The MoAb against this epitope had no effect on the infectivity of gametocytes of P. falciparum to mosquitoes.     

Inhibition of the growth of Plasmodium falciparum and Plasmodium berghei in vitro by an extract of Cochlospermum angolense (Welw.).

An extract of Cochlospermum angolense (Welw.) is used in the traditional medicine of Angola for the therapy of icterus and for the prophylaxis of malaria. From the roots of this plant red crystalline substances have been isolated and tested for their effect on Plasmodium falciparum in vitro and on the DNA and protein synthesis of Plasmodium berghei. The multiplication of P. falciparum was decreased to 50% of the control in the presence of 10 μg/ml extracted material and there was a total inhibition at a concentration of 50 μ/ml. If mice erythrocytes infected by P. berghei were incubated for 6 h with 25 μg/ml of the extract DNA synthesis was depressed to nearly background level. And, even more important, this effect could be demonstrated immediately. On the contrary, protein synthesis continued for at least 90 min at a reduced rate and stopped then. The results obtained show the direct antiparasitic effect of the substances extracted from C. angolense. The activity seems to be directed against DNA synthesis.     

Fine specificities of monoclonal antibodies against the Plasmodium falciparum circumsporozoite protein: recognition of both repetitive and non-repetitive regions

The fine specificities of 6 monoclonal antibodies (MoAbs) raised against the circumsporozoite (CS) protein of the human malaria parasite, Plasmodium falciparum, were defined by their binding to a series of overlapping octapeptides corresponding to the 7G8 variant of the CS protein. The precise specificities of the MoAbs to the immunodominant NANP repeat region were elucidated by their binding to all possible 4, 5, 6, 7 and 8 amino acid peptides in this region. All 6 MoAbs recognized the NANP repeats. In addition all MoAb bound to nonrepetitive sites with 4 of the 6 MoAbs recognizing known functional sites outside the repeat region including sites required for T cell recognition and hepatocyte invasion. Antibody pressure may therefore be responsible for generating the epitope variation observed at T cell sites. The multiple specificities for all the MoAbs suggests that the repeat region may act as an internal immunological 'smokescreen' by competing more effectively for antibody binding compared to single epitope copy functional sites located outside the repeat region.     

Anti-idiotypic antibodies: preparation and identification; application to the detection of antigens of Plasmodium falciparum

Anti-idiotypic antibodies (anti 94D1Id and anti 94C3Id) were prepared by immunizing rabbits with designated monoclonal antibodies (MoAb) 94D1MoAb (94D1Id) and 94C3MoAb (94C3Id) specific for Plasmodium falciparum. The specificity of both was directed against idiotypes of the relevant MoAb. The interaction between 94D1Id and anti94D1Id or 94C3Id and anti94C3Id was almost completely inhibited when sufficient concentration of P. falciparum antigen was added, suggesting that the idiotypes recognized by anti94D1Id or anti94C3Id were located in the antigen binding site of the MoAbs or the construction of the V region of anti-idiotypic antibodies may be similar to that of P. falciparum antigenic determinants combining with MoAb. Each 4i assay (inhibition of idiotype-anti-idiotype interaction) had a different specificity and sensitivity in detecting P. falciparum asexual antigens. The 94C3Id-anti94C3Id system detected a minimal level of 0.001% parasitaemia and did not cross-react with other species, suggesting that 94C3Id-anti94C3Id system is species specific and may become a valuable tool for immunodiagnosis.     

Topography of epitopes on a polymorphic schizont antigen of Plasmodium falciparum determined by the binding of monoclonal antibodies in a two-site radioimmunoassay

The topographic distribution of common and variant epitopes on two divergent allelic forms of the 185-205K schizont glycoprotein of Plasmodium falciparum were studied by a two-site radioimmunoassay using monoclonal antibodies. Similarities in the conformation of the two molecules were apparent. On both antigens two distinct regions were mapped, each comprising of both strain-common and polymorphic epitopes. Epitopes common to the two PSAs were found to be closely associated with different variable epitopes in tertiary structure. It is suggested that this may contribute to parasite evasion of the host immune response.     

Expression of the Plasmodium berghei ookinete protein Pbs21 in a baculovirus-insect cell system produces an efficient transmission blocking immunogen

A surface protein of Plasmodium berghei ookinetes, Pbs21, was expressed in a baculovirus-insect cell system in cell culture and in Heliothis virescens larvae. Groups of BALB/c mice received two intraperitoneal inoculations of either i) Tris-buffer or homogenized H. virescens larvae infected with wild-type baculovirus; ii) enriched, homogenized ookinetes, or Hi) homogenizedH. virescens larvae expressing recombinant Pbs21 (rPbs21). All animals immunized with ookinetes or with rPbs21 had high litres of antibodies (IgG isotype) that bound to native Pbs21. The large majority of antibodies in immune sera of both groups recognized the antigen under non-reducing but not under reducing conditions. The predominant IgG-sub-classes in mice immunized with ookinetes was IgGl and in mice immunized with rPbs21, the subclasses were IgGl and IgG2a. Immunization with rPbs21 reduced the infec-tivity of P. berghei to mosquitoes by 91% compared to a 99% reduction following immunization with ookinetes. This preliminary data indicate that rPbs21 expressed in this eukaryotic system induces a transmission-blocking immunity which is more effective than that achieved using rPbsll expressed in Escherichia coli (Matsuoka et al. 1994).     

The roles of the glycosylphosphatidylinositol anchor on the production and immunogenicity of recombinant ookinete surface antigen Pbs21 of Plasmodium berghei when prepared in a baculovirus expression system

Malarial ookinetes express an immunodominant surface protein (P28) that is a priority candidate for the development of transmission-blocking vaccines. The full length P28 gene from Plasmodium berghei [Pbs21(1-213)] and a deletion construct [Pbs21(1-188)] encoding a protein that lacks the 25 C-terminal amino acids, including the glycosylphosphatidylinositol (GPI) anchor signal, were expressed in insect cells using baculovirus vectors. Pbs21(1-213) protein is strongly hydrophobic, found in the cytoplasm and on the surface of Spodoptera Sf21 cells, and in the culture medium. Pbs21(1-188) protein was largely found in the aqueous phase of the medium and in the cytoplasm of Sf21 cells, but was not detected on the cell surface. The presence of 25 C-terminal amino acids is therefore critical to the attachment of recombinant Pbs21 to the parasite plasma membrane. Mice were immunized subcutaneously or intramuscularly with affinity purified recombinant Pbs21(1-213), Pbs21(1-188) or native Pbs21 proteins. Following two immunizations, native Pbs21 induces higher titres when administered by either route, than the recombinant protein bearing an insect GPI anchor, which in turn is markedly more immunogenic than the recombinant polypeptide lacking a GPI anchor. When specific anti Pbs21 antibody titres exceeded 1 mg/ml all three antigens were capable of inducing transmission blockade > or = 90%, below 1 mg/ml blockade did not correlate with antibody concentration.     

Transmission blocking antibody of the Plasmodium falciparum zygote/ookinete surface protein Pfs25 also influences sporozoite development

The Plasmodium falciparum zygote/ookinete surface protein, Pfs25, persists in the oocyst wall throughout its development. Anti-25 kD transmission blocking antibody, given to infected Anopheles stephensi or A. gambiae mosquitoes in an additional bloodmeal, 3-6 days after being fed gametocyte infected blood, penetrated the oocyst and reacted with the 25 kD protein within it. This reaction caused a significant reduction in the number of developing sporozoites. Mouse serum containing antibodies raised by immunization with a recombinant 25 kD yeast product showed a similar effect.     

Characterization of two monoclonal antibodies raised in Btkxid mice that recognize phosphorylcholine-bearing antigens from Trichinella and other helminths

This study investigated the binding properties of two monoclonal antibodies (mAbs US1 and US2) raised in (CBA/n x BALB/c)F1 (NBF1) Btk(xid) male mice. Both mAbs show unusual specificity for phosphorylcholine (PC)-containing TSL4 antigens of Trichinella. Specifically, and in contrast to mAbs raised in normal mice, US1 and US2 mAbs do not bind to artificial PC-protein conjugates and are not inhibited by either free PC or NPPC, although US2 was partially inhibited by NPPC at high concentration (10(-2) M). However, both mAbs completely abrogate the binding to Trichinella antigens of other anti-PC mAbs (e.g. BH8 and Mab-2). These results suggest that both US1 and US2 recognize complex PC-containing epitopes. The patterns of recognition of PC-bearing antigens from different helminths by US1, US2, Mab-2 and BH8 were broadly correlated with phylogenetic proximity. The closest similarities were observed between the members of the Trichinelloidea superfamily (Trichinella spiralis and Trichuris muris) and among the ascarids (Toxocara canis, Anisakis simplex, Hysterothylacium aduncum and Ascaris lumbricoides). However, US1 did not react with the filarial nematode Onchocerca volvulus and reacted only weakly with Onchocerca gibsoni, while US2 reacted only weakly with both species. Only BH8 recognized PC-bearing antigens from the trematode Fasciola hepatica and the cestode Bothriocephalus scorpii. These results suggest that PC is attached to identical or very similar structures on most different nematode species, although major differences exist with respect to helminth species from groups such as the trematodes and cestodes that are phylogenetically distant from the nematodes.     

Epitopes in the 19kDa fragment of the Plasmodium falciparum major merozoite surface protein-1 (PfMSP-119) recognized by human antibodies

The antibody response to two different epitopes located in the C-terminal 19 kDa fragment of the Plasmodium falciparum merozoite surface protein-1 (MSP-1₁₉) has been studied using a competitive ELISA based on the inhibition of monoclonal antibody (MoAb) binding by serum samples. Sera from children aged three to eight years who suffered clinical symptoms of malaria, or were partially immune with an asymptomatic infection, and from adults all living in The Gambia, West Africa were tested. The results suggest that the antibody response to MSP-1 iQ has a role in naturally-acquired immunity in Gambian individuals.,     

Studies on the immunogenicity of a recombinant ookinete surface antigen Pbs21 from Plasmodium berghei expressed in Escherichia coli

Plasmodium berghei ookinete surface antigen (Pbs21), was produced as a fusion product with maltose binding protein (MBP) in Escherichia coli and used to induce transmission-blocking immunity in mice. Specificity of induced antibody was confirmed by Western blotting with native ookinete Pbs21, and by the indirect immunofluorescent antibody test on ookinete bloodfilms. Immunized mice were infected with P. berghei and transmission to Anopheles stephensi mosquitoes determined by both the intensity and prevalence of oocyst infections. Compared with a control group immunized with MBP alone the maximum blockade of oocyst intensity was 66% in the mice immunized with recombinant MBP-Pbs21. Over nine experiments blockade averaged only 33%. By comparison with native Pbs21 protein, which usually induces 90% blockade, our data suggests the recombinant protein produced in this bacterial system is a less effective immunogen despite expressing epitopes recognized by known transmission-blocking monoclonal antibodies.     

In-vitro exoerythrocytic development of Plasmodium cynomolgi bastianellii inhibitory activity of monoclonal antibodies against sporozoites of different P. cynomolgi strains and of P. knowlesi

The inhibitory effect of anti-sporozoite monoclonal antibodies (MoAb) on the in-vitro development of liver stages of Plasmodium cynomolgi bastianellii (NIH strain) was evaluated using primary cultures of rhesus monkey hepatocytes. MoAbs against the circumsporozoite proteins of five strains of P. cynomolgi (NIH, London, Gombak, Ceylon, Berok), and of P. knowlesi (H strain) were used. Incubation of sporozoites of P. cynomolgi bastianellii with the anti-NIH strain MoAbs entirely prevented liver-stage development; MoAbs produced against the other four strains had no apparent activity. The anti-P. knowlesi MoAbs had a partially inhibitory effect on parasite development. These functional studies complement previous immunological studies on P. cynomolgi strain specificity, and confirm the cross-reactivity observed previously between sporozoites of P. cynomolgi bastianellii and P. knowlesi (H strain).     

Protective immunity to malaria: studies with cloned lines of Plasmodium chabaudi chabaudi in CBA/Ca mice. III. Protective and suppressive responses induced by immunization with purified antigens

The protective effect of affinity purified antigen has been investigated in an experimental model for malaria which shows a well marked recrudescence of parasitaemia, a feature of the disease in man. A monoclonal antibody (MoAb) recognizing an epitope common to two genetically distinct cloned lines of Plasmodium chabaudi (AS and CB), was used to purify a Mr250,000 polymorphic schizont antigen (PSA) from these parasites. The purified preparations were then examined for the presence of specific and cross-reactive epitopes by immunoprecipitation with a panel of MoAb raised against P. chabaudi AS. When tested previously on smears of parasitized blood by immunofluorescence, or against lysates of parasitized erythrocytes by immunoprecipitation, most of these MoAb had been found to be AS specific. When either AS or CB affinity purified Mr250,000 PSA was used as the target, these same MoAb immunoprecipitated both antigens, and in some cases, a number of associated polypeptides (AP) which copurify with the Mr250,000 PSA. Subsequently, mice were immunized with either the purified AS or CB antigens in Freund's complete adjuvant (FCA). Prechallenge sera were compared by indirect immunofluorescence and immunoprecipitation. Sera from mice immunized with AS antigen reacted strongly with AS and cross-reacted with CB parasite preparations. Pre-challenge serum from CB antigen immunized mice reacted well with CB, but only faintly with AS preparations. In mice immunized with the AS antigen and then challenged with either AS or CB parasites, the initial parasitaemias were delayed in appearance and the height of the peak parasitaemia reduced, an effect which was most pronounced after challenge with homologous parasites. Only homologous challenge of the mice immunized with CB antigen produced statistically significant modification of the initial parasitaemia. In the immunized mice challenged with homologous parasites, the delayed appearance and slightly reduced peak of the primary parasitaemia was associated with delayed resolution of the patent parasitaemia and significant enhancement of the recrudescence.     

Fine epitope mapping of monoclonal antibodies to the NH2-terminal part of von Willebrand factor (vWF) by using recombinant and synthetic peptides: interest for the localization of the factor VIII binding domain

. Two different approaches were used in order to define the epitope of three monoclonal antibodies (MoAbs) against the NH₂-terminal part of the mature subunit of von Willebrand factor (vWF) which contains its factor VIII (FVIII) binding site. First, a vWF cDNA fragment library using the bacteriophage λgt11 expression vector was screened with radiolabelled MoAbs. The epitope of each MoAb was defined, following sequence analysis, by the overlapping DNA sequence of immunoreactive clones. MoAb 32B12, a potent inhibitor of FVIII/vWF interaction, binds within the Glu35-Ile81 sequence of vWF subunit. MoAb 14A12, a non-inhibitory antibody, recognizes a sequence within Thr141-Val220. MoAb 31H3, a partial inhibitory antibody, gives no positive clone. In the second method, a panel of 24 synthetic pentadecapeptides corresponding to the first NH₂-terminal 105 amino acid residues was used to block the binding of inhibitor MoAbs to immobilized vWF in an ELISA system. The localization of MoAb 32B12 epitope was confirmed and restricted to the Met51-Ala60 sequence, The MoAb 31H3 binding to vWF is inhibited by two synthetic peptides with the overlapping sequence Cys66-Gly76. All these data confirm that the FVIII binding site of vWF is not limited to the binding area (Thr78-Thr96) of the previously described MoAbs inhibiting FVIII/vWF interaction but is composed of several key sequences.