Structure of an immunodominant epitope of the circumsporozoite surface protein of Plasmodium knowlesi

Polyvalent synthetic vaccines: relationship between T epitopes and immunogenicity

Three different synthetic polyvalent vaccines have been constructed by conjugating four synthetic peptides without any carrier protein. The peptides were copy fragments of two bacterial antigens (Streptococcus pyogenes M protein and diphtheria toxin), two parasitic antigens (circumsporozoite protein of Plasmodium falciparum and Plasmodium knowlesi), and one viral antigen (hepatitis B surface antigen). Outbred guinea-pigs immunized with polyvalent vaccine containing streptococcal, diphtheric, P. knowlesi and hepatitis peptides raised high specific antibody response against the four specificities. Individual T cell analysis demonstrated that hepatitis peptide bears T dominant epitope. A similar immune response was obtained with a second polyvalent vaccine where the P. knowlesi peptide had been replaced by the P. falciparum peptide. In both experiments the malarial peptides behave like pure B epitopes. Prediction of immunodominant helper T-cell antigenic sites were performed with the five peptides using computer algorithm. Hepatitis and diphtheric peptides were selected whereas the streptococcal peptide was rejected although it can experimentally contain a T epitope. To confirm this result animals were immunized with a third polyvalent vaccine which does not contain the hepatitis peptide. No T cell proliferation or antipeptide antibodies were detected. These results demonstrate that the cooperative immune response requires a certain degree of antigenic complexity for the induction of antibody response.

Conformational analysis of the immunodominant epitopes of the circumsporozoite protein of Plasmodium falciparum and knowlesi

All of the coat proteins of the sporozoite and merozoite stages of Plasmodium, determined to date, contain tandem repeats and most of these contain at least one proline residue. These tandemly repeated segments of the circumsporozite (CS) proteins of P. falciparum and P. knowlesi have been shown to constitute an immunodominant epitope. Antibodies to these peptide segments have been shown to be protective and cause the shedding of the CS protein, known as the CSP reaction. In this study, four synthetic peptides were prepared by solid-phase peptide synthesis. The first peptide corresponds to the tetrapeptide tandem repeat in the CS protein of P. falciparum, repeated eight times, (NANP)8. The second peptide is an analogue of the first in which glycine is substituted for proline, (NANG)8. The third peptide corresponds to the tandem repeat of P. knowlesi, PK(1-24), which is repeated twice (QAQGDGANAGQP)2. The fourth peptide is a tetrapeptide repeat, corresponding to the C-terminal tetrapeptide of PK(1-24) and is repeated eight times, (AGQP)8. It is shown by CD measurements that the presence of proline in these repeats induces an increase in beta-sheet (beta-turn) content in the (NANP)8 peptide relative to the repeat of (NANG)8 and PK(1-24) peptide in aqueous media. The (AGQP)8 peptide has the highest beta-sheet (beta-turn) content of all peptides studied. The Chou-Fasman predictive algorithm indicates a high beta-turn content in the synthetic peptides. It is concluded that this increase in defined structure correlates well with and hence may contribute to the increased antigenicity in these repeats.

Enhanced epitopic response to a synthetic human malarial peptide by preimmunization with tetanus toxoid carrier

Successful human vaccination by synthetic malarial sporozoite peptides may depend on the choice of an appropriate carrier. Tetanus toxoid (TT) has been proposed because of its safe and widespread use in humans. Paradoxically, however, prior exposure to this toxoid vaccine could produce specific epitopic suppression against synthetic malarial peptides conjugated to this same protein as carrier. Indeed, we have previously reported that such a phenomenon can occur in the case of a synthetic vaccine made with a streptococcal peptide conjugated to TT. Our present study shows that similar results can be observed in mice preimmunized with TT 1 month before the administration of a conjugate containing TT and a Plasmodium knowlesi peptide. Analysis of the isotypic pattern of the antipeptide response showed that the immunoglobulin G1 (IgG1) subclass and especially the IgG2a and IgG2b subclasses were suppressed. In contrast, when a sporozoite peptide from Plasmodium falciparum was coupled to TT, the total antipeptide antibodies and particularly the IgG1 subclass were enhanced by preimmunization by TT. This increase of antipeptide antibodies was correlated with a greater ability of the sera to neutralize sporozoite infectivity. These results indicate that prior exposure to TT does not systematically impair the antibody response against a peptide administered as a peptide-TT conjugate.

Induction of biologically active antibodies by a polyvalent synthetic vaccine constructed without carrier

Four synthetic peptides that copy fragments of two bacterial antigens (Streptococcus pyogenes M protein and diphtheria toxin), one viral antigen (hepatitis B surface antigen), and one parasitic antigen (circumsporozoite protein of Plasmodium knowlesi) were covalently bound within the same construct. This totally synthetic polyvalent administered to mice with Freund complete adjuvant or in saline with murabutide (an adjuvant-active muramyl peptide) elicited high levels of antibodies which, in certain cases, were shown to be biologically active. The results indicated that these antibodies recognized specifically the four peptides. None of the epitopes were immunodominant. It was also demonstrated that the association of several peptides enhanced their respective immunogenicities as compared with those of their homopolymers. Finally, this study shows that a totally synthetic vaccine administered in saline with a synthetic adjuvant can be immunogenic in the absence of a protein carrier.

Expression in yeast of a Plasmodium vivax antigen of potential use in a human malaria vaccine

DNA coding for 234 amino acids of the circumsporozoite (CS) protein of Plasmodium vivax was incorporated into yeast expression vectors. The DNA encoded all the repeat domain and codons for a highly conserved sequence, KLKQP, found in CS proteins from all malaria parasites. Yeast cells transformed with these autonomously replicating plasmids expressed, upon induction, high levels of the CS polypeptide. The malaria antigen was purified in good yields from yeast extracts and was injected into mice using alum as adjuvant. The antibodies recognized the authentic CS protein, and at high dilutions, they inhibited the invasion of hepatocytes by sporozoites in vitro.

Isolation of decay accelerating factor (DAF) by a two-step procedure and determination of its N-terminal sequence

Decay-accelerating factor (DAF) from human red cell membranes was purified by a two-step procedure involving anion exchange and immunoaffinity chromatography. The DAF preparations were purified to homogeneity as judged by silver staining. In several experiments, the final product yields were approximately 23% of the total DAF present in the initial membrane extracts. The purified DAF retained its ability to inhibit the classical pathway C3-convertase and to reincorporate into cell membranes. An amino-terminal sequence was obtained by gas-phase sequencing. Rabbit antibodies to a synthetic peptide representing part of this sequence reacted with purified reduced membrane DAF by Western blotting and by a solid-phase immunoradiometric assay.

Detection of human antibodies against Plasmodium falciparum sporozoites using synthetic peptides

A large peptide consisting of about 40 (Asn-Ala-Asn-Pro) repeats of Plasmodium falciparum circumsporozoite protein, (NANP)40, was synthesized. It was recognized specifically by monoclonal antibodies produced against P. falciparum sporozoites. Moreover, this peptide strongly inhibited the binding of such monoclonal antibodies to antigens present in a sporozoite extract. The (NANP)40 peptide was employed without any carrier to develop an enzyme-linked immunosorbent assay to detect sporozoite-specific serum antibodies arising after natural malaria infections. Antibodies were detected in a high percentage (43.1%) of European patients suffering from acute P. falciparum malaria and in Africans living in an area of Gabon endemic for malaria. In the latter group, the frequency of antisporozoite antibodies increased with age, reaching 65.9% in individuals more than 40 years old. There was a significant correlation between the results obtained with an immunofluorescence assay with glutaraldehyde-fixed sporozoites and those obtained by enzyme-linked immunosorbent assay with (NANP)40. Therefore, such synthetic peptides representing the repetitive epitope of P. falciparum circumsporozoite protein can be used for the detection of antisporozoite antibodies and for the epidemiological studies required to obtain base-line data concerning the immune status of individuals before their participation in a sporozoite vaccine trial.

Predicted conformations for the immunodominant region of the circumsporozoite protein of the human malaria parasite Plasmodium falciparum

The circumsporozoite protein of the human malaria parasite Plasmodium falciparum contains multiple tandem repeats of the amino acid sequence Asn-Ala-Asn-Pro. The repeated sequence encompasses the immunodominant region of the protein, and antibodies raised against it are potent inhibitors of invasion and development of sporozoites in cultured hepatocytes. Using a modified build-up procedure, we have explored a large number of possible helical and near-helical conformations of a terminally blocked tetraicosapeptide, consisting of six repeats of the sequence Asn-Ala-Asn-Pro, and conclude that two helical conformations are energetically favored to the exclusion of all others. One of these conformations is longer, thinner, and left-handed and is likely to be adopted in nonpolar environments, while the other is shorter, broader, and right-handed and should be favored in aqueous solutions. We propose that the immunodominant region of the circumsporozoite protein of P. falciparum adopts one of these conformations in vivo.

Rabbit and human antibodies to a repeated amino acid sequence of a Plasmodium falciparum antigen, Pf 155, react with the native protein and inhibit merozoite invasion

The Plasmodium falciparum-derived antigen of Mr 155,000 designated Pf 155, deposited in the membrane of infected erythrocytes, contains at least two blocks of tandemly repeated amino acid sequences. The peptide Glu-Glu-Asn-Val-Glu-His-Asp-Ala, which corresponds to a subunit of a C-terminally located repeat, was synthesized. Rabbits immunized with the octapeptide conjugated with either keyhole limpet hemocyanine or tetanus toxoid formed antibodies against the octapeptide. These antibodies reacted with Pf 155 as detected by immunoblotting or a modified immunofluorescence assay. Sera from humans exposed to P. falciparum also contained antibodies binding to the octapeptide in a dot-blot immunoassay. Their anti-octapeptide titers were correlated with their immunofluorescence titers in the assay detecting Pf 155 and other parasite antigens in the membrane of infected erythrocytes. Human octapeptide-reactive antibodies were isolated on an affinity column with the octapeptide conjugated to bovine serum albumin as ligand. These human antibodies reacted with Pf 155 in immunoblotting and strongly stained the surface of infected erythrocytes in the modified immunofluorescence assay. Approximately 20% of this immunofluorescence activity in a high-titered human serum could be recovered from the octapeptide column, indicating that a significant fraction of these anti-parasite antibodies react with epitopes associated with the octapeptide. Furthermore, the human octapeptide-reactive antibodies very efficiently inhibited merozoite reinvasion into erythrocytes in vitro. Similarly purified rabbit antibodies also significantly inhibited reinvasion. Our results suggest that the C-terminal segment of repeated peptides in Pf 155 is a major antigenic region of the molecule and may contain target sites for protective immunity in P. falciparum malaria.

Human-human hybridomas secreting monoclonal antibodies to the Mr 195,000 Plasmodium falciparum blood stage antigen

Using the human lymphoblastoid cell line, GM 4672, and PBL of Gambian adults immune to Plasmodium falciparum (Pf) malaria, we have produced human-human hybridomas and selected those that produce mAb against Pf antigens. The fusion frequency, using PWM-stimulated donor lymphocytes was between 6.8 X 10(-5) and 1.5 X 10(-6). Using immune fluorescence, immune precipitation, and Pf in vitro growth inhibition, we cloned four hybridomas that reacted with the Pf Mr 195,000 schizont/merozoite protein. The differences in proteins immune precipitated and in growth inhibition indicate that, during development of protective immunity against Pf malaria, a spectrum of antibodies is produced reacting with different epitopes on the same antigen. Only a portion of these antibodies exhibits biological activity, suggesting that the recognition of certain epitopes is required for the development of a protective immune response.

Production of anti-sporozoite antibodies in absence of response to carrier by coupling an MDP derivative to a malaria peptide-tetanus toxoid conjugate

A synthetic peptide (pep) representing a portion of the Plasmodium knowlesi circumsporozoite protein attached to a tetanus toxoid (TT) carrier, has been shown to be immunogenic when delivered in saline with derivatives of the synthetic adjuvant, muramyl dipeptide (MDP). The present study was designed to determine if the degree of substitution of pep and of MDP derivatives on the tetanus toxoid (TT) carrier, as well as the choice of MDP derivative used play a role in determining anti-pep and anti-TT antibody levels. One of the MDP derivatives used in the conjugates was epsilon-amino-caproic Murabutide, since Murabutide which is currently in clinical trials cannot be conjugated. The results show that low doses of this derivative coupled with pep on TT can be used to stimulate high levels of circulating anti-pep antibodies without augmenting the anti-carrier response. In addition, anti-pep antibodies elicited in response to one of the conjugates were biologically active since they produced shedding of the circumsporozoite coat of live parasites.

Rationale for development of a synthetic vaccine against Plasmodium falciparum malaria

Protective immunity against malaria can be obtained by vaccination with irradiated sporozoites. The protective antigens known as circumsporozoite (CS) proteins, are polypeptides that cover the surface membrane of the parasite. The CS proteins contain species-specific immunodominant epitopes formed by tandem repeated sequences of amino acids. Here it is shown that the dominant epitope of Plasmodium falciparum is contained in the synthetic dodecapeptide Asn-Ala-Asn-Pro-Asn-Ala-Asn-Pro-Asn-Ala-Pro or (NANP)3. Monoclonal antibodies and most or all polyclonal human antibodies to the sporozoites react with (NANP)3, and polyclonal antibodies raised against the synthetic peptide (NANP)3 react with the surface of the parasite and neutralize its infectivity. Since (NANP)3 repeats are present in CS proteins of P. falciparum from many parts of the world, this epitope is a logical target for vaccine development.

Multiple non-repeated epitopes on the circumsporozoite protein of Plasmodium knowlesi

The Plasmodium knowlesi circumsporozoite (CS) protein contains a repetitive immunodominant epitope. Here we show that the serum of rabbits repeatedly immunized with P. knowlesi sporozoites contains antibodies which bind to immobilized synthetic peptides ('C2', 'N2', and 'charged') representing two different polar regions of the CS polypeptide. These reactions are specific since the binding is inhibited only by the homologous peptides. Antisporozoite antibodies were isolated from the rabbit serum by affinity chromatography on Sepharose beads coupled to two synthetic peptides, 'C2' and 'charged'. Both purified antibodies recognized the CS protein and the intracellular precursors as shown by Western blotting analysis using sporozoite extracts. These results demonstrate that the corresponding areas of the native CS molecule are immunogenic, accessible to interaction with antibody, and therefore constitute potential targets for vaccine development. In addition, the present findings confirm the published amino acid sequence of a large portion of the CS protein which has been deduced from the nucleotide sequence of the corresponding gene.

Structure and organization of genes for sporozoite surface antigens

The major surface antigen (circumsporozoite-protein) of the sporozoite stage of Plasmodium knowlesi has been cloned and characterized. The gene is notable for the presence of a 36 base pair unit repeated in tandem 12 times. These repeats may function at the DNA level in regulating gene expression and at the protein level in providing multiple copies of a single epitope (repitope) as part of a protein designed to evade and decoy the immune system.

Development of sporozoite vaccines

Protective immunity against malaria has been achieved in hosts ranging from birds to man by repeated inoculation of irradiated sporozoites. The main antigens involved in protective immunity to sporozoites are the circumsporozoite (CS) proteins, which are part of a family of proteins, covering the whole surface membrane of the parasite, and which have similar physico-chemical and antigenic properties. Monovalent fragments of monoclonal antibodies to CS proteins neutralize sporozoite infectivity. All monoclonal antibodies recognize a single immunodominant region within the various CS proteins, and this region contains repetitive epitopes. The recurrent immunodominant epitope of the CS protein of P. knowlesi has been identified, and shown to consist of 12 tandemly repeated subunits of 12 amino acids. The dimer of the dodecapeptide was coupled to protein carriers, emulsified in Freund's complete adjuvant, and injected into rodents and monkeys. All animals made anti-peptide antibodies, and most of the antisera reacted with P. knowlesi CS protein.