Malaria vaccines: immunogen selection and epitope mapping

Enzyme Immunoassay (EIA)/Enzyme-Linked Immunosorbent Assay (ELISA)

This brief note addresses the historical background of the invention of the enzyme immunoassay (EIA) and enzyme-linked immunosorbent assay (ELISA). These assays were developed independently and simultaneously by the research group of Peter Perlmann and Eva Engvall at Stockholm University in Sweden and by the research group of Anton Schuurs and Bauke van Weemen in The Netherlands. Today, fully automated instruments in medical laboratories around the world use the immunoassay principle with an enzyme as the reporter label for routine measurements of innumerable analytes in patient samples. The impact of EIA/ELISA is reflected in the overwhelmingly large number of times it has appeared as a keyword in the literature since the 1970s. Clinicians and their patients, medical laboratories, in vitro diagnostics manufacturers, and worldwide healthcare systems owe much to these four inventors.

Protein design: on the threshold of functional properties

The ultimate goal in protein de novo design is the creation of novel macromolecules with tailor-made receptor, sensory, and catalytic functions. Despite considerable progress in understanding basic rules of secondary structure formation and protein stability, the well-known protein folding problem is still far from being solved and, in general, only a limited number of designed proteins are folded uniquely. In this article the state-of-the-art in protein design is demonstrated on some selected examples, indicating that the construction of protein-like macromolecules mimicking some essential features of natural proteins seems to be within reach. Thus, protein design and mimicry has become an interdisciplinary challenge with most intriguing perspectives.

Potentiation of immune response against the RESA peptides of Plasmodium falciparum by incorporating a universal T-cell epitope (CS.T3) and an immunomodulator (polytuftsin), and delivery through liposomes

Synthetic peptides representing repeat sequences of ring-infected erythrocyte surface antigen (RESA) of Plasmodium falciparum have shown poor immunogenicity and protection. In this study, the RESA peptides [(EENVEHDA)2 and (DDEHVEEPTVA)2] were chemically linked to a universal T-cell determinant, CS.T3, derived from the CS protein of P. falciparum. Polytuftsin (TKPR)40, a polymer of naturally occurring immunomodulator "tuftsin," was physically mixed with these conjugates. These preparations in alum and liposomes were immunized in four inbred strains of mice with different genetic backgrounds to study the humoral response. In the case of liposome-entrapped preparations, a 10 microg dose of antigen showed the optimum antibody response. Mice immunized with liposome containing RESA peptide(s)-CS.T3 conjugate along with polytuftsin showed the highest antibody levels in all the strains, whereas the RESA peptide(s) alone, adsorbed on alum or entrapped in liposomes, showed either poor or moderate antibody levels. The antibodies raised against liposome-entrapped preparations in both high-responder strain (SJL/J H-2s) and low-responder strain (FVB/J H-2q) showed 2 4-fold lower Kd values as compared to the alum adsorbed preparations, suggestive of high affinity antibodies. All the antigen preparations predominantly induced IgG2a and IgG2b isotype response, suggesting that the T-helper response involved is of the CD4 Thl type. The in vitro merozoite reinvasion inhibition assay showed 50-92% inhibition with sera raised against different antigen formulations. The highest percentage inhibition was observed with the RESA peptide-CS.T3 conjugate containing polytuftsin in liposomes. Thus, the incorporation of peptide antigens inside liposomes not only reduced the antigen dose by 5-fold but also elicited a high titre with high affinity antibodies and the inhibition of merozoites to RBC in vitro. Therefore, we conclude that the incorporation of these synthetic constructs in liposomes could be a useful strategy for the development of a subunit immunogen against malaria.

Influence of immunoadjuvants and a promiscous T-cell determinant on the immunogenicity of RESA peptide antigen of P. falciparum

Synthetic peptide antigens representing the repeat sequences of malarial antigens showed poor immunogenicity and protection in clinical trials. In the present study, RESA, an asexual blood stage antigen, containing (EENVEHDA)2 and (DDEHVEEPTVA)2 sequences were chemically linked to a promiscous T-cell determinant (CS.T3) of the circumsporozoite protein of P. falciparum. The synthetic constructs either alone or coentrapped with immunoadjuvants (nor muramyl dipeptide/lauroyl tetrapeptide) were administered in liposomes to mice of varying genetic background and the immunogenicity of different formulations were compared under identical experimental conditions. The RESA peptide formulations containing the T-cell determinant and the adjuvants generated high titre and affinity antibodies in all the strains, as compared to peptide(s) alone. The booster immunization induced a strong anamnestic response in each group. Though the major IgG isotype is of IgG1 and IgG2b interestingly, formulations containing CS.T3 have a higher proportion of cytophilic IgG2b isotype. There was a significant fall in the levels of IgG2b isotype while IgG1 levels were maintained same in the third bleed (day 60, without booster immunization). The mixed peptide group preparation containing the adjuvant is found to be a better immunogen than that of respective peptides itself. The in vitro merozoite reinvasion inhibition assay showed 76-96% inhibition with the formulations containing RESA peptide(s)-CS.T3 and the adjuvant, while with peptides alone the inhibition was 50-56%. This study highlights the importance of an alternative approach for developing peptide based immunogen against malaria.

Characterization of immune responses to experimental polyvalent subunit vaccines assembled in iscoms

Immune responses to experimental polyvalent subunit vaccines assembled in a particulate adjuvant/delivery system, iscoms, are described. The fusion protein ZZ-M5 comprises structures of staphylococcal protein A (ZZ) and the Plasmodium falciparum malaria antigen Pf155/RESA (M5). MHC congenic mice were immunized with ZZ-M5 conjugated to iscoms containing human influenza virus antigen (flu ag, M5-flu-isc) or to iscom matrix (iscom particles without flu ag, M5-isc). Comparison of antibody and T-cell responses to M5-isc and M5-flu-isc demonstrated that the flu ag in M5-flu-isc exhibits carrier-related helper functions and that the assembly of immunogens in M5-flu-isc did not result in any apparent antigenic competition. In addition, assembly of ZZ-M5 and flu ag in iscoms induced an alteration of the IgG subclass profile of the antibody response to M5. The results suggest that assembly of immunogens in iscoms may be a useful approach to the design of subunit vaccines but that both quantitative and qualitative aspects of the immunogenic properties of such constructs should be scrutinized.

A malariometric survey in a rural community in the Muheza district, Tanzania: age profiles in the development of humoral immune responses

A malariometric survey was carried out in a rural community situated in a malaria holoendemic endemic area of Tanzania. A random sample (n = 228) of different age groups was taken to elucidate the association between anti-Pf155/RESA and anti-Pf332 antibody responses and classical malaria indices. Parasitaemia, fever, splenomegaly, haematocrit and antimalarial consumption were assessed. Antibody responses against Pf155/RESA and Pf332 peptides were determined by ELISA. The age profiles of parasite density, splenomegaly, fever, haematocrit values and prevalence of antibody responses indicated intensive malaria exposure and the highest impact of malaria in small children. Forty-five percent of the study population had detectable chloroquine and desethyl-chloroquine blood levels, and the highest frequency and concentrations were recorded in the 12-23 months old. There was no significant association between the presence of drug and parasite density in the different age groups, although in the < 15 years old there was lower parasite prevalence among the children positive for drug in their blood (P < 0.05). High prevalence of antibody responses to all antigens was observed already at an early age, but the mean anti-Pf155/RESA and anti-Pf332 antibody levels increased significantly only in the adult group (P < 0.01). Significantly lower mean parasite densities were observed in high responders to Pf155/RESA and Pf332 peptides for the > or = 10 years old. For the 1-9 years, a similar difference was only observed in the high responders to Pf332. For the whole material, anti-Pf155/RESA and anti-Pf332 antibody levels correlated positively with age. When the effect of age was allowed for in analysing the relationship between parasite density and antibody level against the different antigens, a significant negative correlation was found only with regard to Pf332 in the > = 10 years age group. These results suggest that anti-Pf332 antibodies appear to be a better indicator for antiparasitic immunity, but both antigens are important for immune protection.

Asymptomatic malaria parasitaemia and seroreactivities to Plasmodium falciparum antigens in blood donors from Ibadan, south-western Nigeria

Malaria parasite rates, parasite densities and seroreactivities to two Plasmodium falciparum antigens (Pf155/RESA and circumsporozoite protein) were investigated in a random sample of 416 blood donors attending the Blood Transfusion Unit of the University College Hospital in Ibadan, south-western Nigeria: 224 in October-November 1991 and 192 in March 1992. The incidence of malaria parasitaeia observed in 1991 was significantly higher than that seen in 1992 (41% v. 19%; P < 0.001). In contrast, the geometric mean parasite density in 1992 was significantly higher than in 1991 (440 v. 191) parasites/microliters blood; P < 0.001). Although parasite rates were highest in the group aged 25-31 years in both surveys, there was no apparent correlation between age of donor and parasite density in either survey. Parasite density was significantly higher in AA- than in AS-haemoglobin individuals only in the 1992 survey (P = 0.050). All the blood donors were seropositive for antibodies to crude parasite antigens, indicating heavy exposure to malaria infection. Seroreactivity to Pf155/RESA was similar in the two surveys but that to circumsporozoite protein (CSP) was significantly higher in 1991 than in 1992 (P < 0.001). The seropositivity rates were generally similar to malaria-positive and -negative blood donors. In 1992, however, all the blood donors with high reactivities to Pf155/RESA, as detected by erythrocyte membrane immunofluorscence, were negative for malaria parasites, indicating that this group was relatively protected against malaria parasitaemia. It is recommended that blood samples from prospective blood donors be examined for malaria parasites and that recipients of malaria-infected blood samples be given a curative regimen of antimalarials.

A longitudinal study of seroreactivities to Plasmodium falciparum antigens in Nigerian infants during their first year of life

The kinetics of passively transferred maternal antibodies to antigens of Plasmodium falciparum and the dynamics of acquisition of these antibodies during the first year of life was investigated in infants born in a malaria endemic area of south-western Nigeria. Blood samples were collected from the infants at bi-monthly follow-up visits for the analysis of total serum immunoglobulin G, IgM, IgA and antibodies to the antigen Pf155/RESA and against synthetic peptides representing antigenic sequences of the blood stage antigen Pf155/RESA and Ag332 or the circumsporozoite protein (CSP). IgG levels fell from birth till 4 months and a steady rise was observed thereafter till ten months of life. On the contrary mean IgM and IgA levels increased throughout the first year of life. Generally the number of infants positive for antibodies to the antigens under investigation fell from birth and between 4-6 months of age was either low or absent. None of the infants were positive for antibodies to the peptide representing Ag332 during the first year of life. The earliest seroconversion was detected at 6 months of age involving the Pf155/RESA and (NANP)6 antigens. The results indicate a high level of exposure in this study area to malaria infection early in life. The finding of an active antibody response to malarial antigens in infancy encourages the hope that a malaria vaccine administered early in life may accelerate the development of naturally acquired immunity and thus protect the population most at risk.

Karyotype analysis of virulent Plasmodium falciparum strains propagated in Saimiri sciureus: strain adaptation leads to deletion of the RESA gene

The squirrel monkey, Saimiri sciureus, is an important experimental model for the study of the human malaria parasite Plasmodium falciparum. A detailed karyotype analysis of four different P. falciparum strains propagated in S. sciureus was done using various subtelomeric antigen gene probes. We observed deletion of the complete RESA gene from chromosome 1 in all four strains. Interestingly, a loss of RESA was observed immediately upon adaptation to the squirrel monkey, suggesting that this DNA rearrangement is fundamental for the P. falciparum infection of S. sciureus erythrocytes. However, a RESA-specific gene probe hybridized with chromosome 1 of wild isolates from 28 different patients, indicating that this gene is maintained during infection of humans.

The virulent Saimiri-adapted Palo Alto strain of Plasmodium falciparum does not express the ring-infected erythrocyte surface antigen

The Palo Alto strain of Plasmodium falciparum is highly virulent for the Saimiri sciureus monkey. We have observed that these parasites do not express the Ring-infected erythrocyte surface antigene (RESA) gene. Immunoblots indicated that the Pf155/RESA protein was absent. The RESA mRNA could not be detected. Polymerase chain reaction and Southern blot analysis demonstrated that this lack of expression is due to gene rearrangements. The majority of the Palo Alto parasites have a deletion of the entire RESA gene, whereas in a minor fraction the RESA sequences remain detectable, but the 5' miniexon 1 is inverted. These data show that the RESA protein is dispensable for in vivo parasite growth, at least in Saimiri monkeys.

Malaria vaccines

The development of an effective malaria vaccine is a feasible goal. Most of the vaccines being developed today are subunit vaccines derived from selected parasite antigens or their immunologically active fragments. The precise characterization of protective immune responses against Plasmodium parasites remains a fundamental part of present research aimed at obtaining a malaria vaccine(s).

Epitopes of the human malaria parasite P. falciparum carried on the surface of HBsAg particles elicit an immune response against the parasite

The development of recombinant subunit vaccines against pathogenic organisms requires not only the identification of epitopes eliciting a protective immune response but also suitable carriers with adjuvant function. B- and T-cell epitopes of the malaria vaccine candidate gp190 were selected on the basis of a systematic search along the gp190 molecule and by computer prediction based on the amino acid sequence. Using some of the epitopes identified, we have redesigned the surface of the hepatitis B surface antigen lipoprotein particles by replacing the major antigenic determinants with malaria-specific sequences of up to 61 amino acids in length. Upon expression via vaccinia virus the hybrid particles elicit an anti-gp190 immune response in animals. Monoclonal antibodies derived from such infections recognize the native parasite.

Passive immunization of Aotus monkeys with human antibodies to the Plasmodium falciparum antigen Pf155/RESA

In order to assess the protective effects of anti-Pf155/RESA antibodies of different specificities in vivo, passive immunizations of Aotus monkeys were performed. Antibodies reactive with the Pf155/RESA repeat sequences (EENV)2 and EENVEHDA were isolated from the immunoglobulin G (IgG) fraction of a pool of plasmas from Liberia by affinity chromatography on synthetic peptides. The two fractions of antibodies differed in specificity but displayed similar capacities to inhibit merozoite invasion in Plasmodium falciparum in vitro cultures. Four groups of monkeys (named groups I to IV) were injected with (i) 160 mg of total control IgG, (ii) 2 mg of IgG affinity purified on (EENV)2, (iii) 2 mg of IgG affinity purified on EENVEHDA, and (iv) 160 mg of total immune IgG, respectively. The monkeys were then challenged with P. falciparum-infected erythrocytes, and the levels of parasitemia and hematocrits as well as other serological parameters were determined daily. Although all groups developed parasitemia, groups II and IV tended to show lower mean daily levels. Three monkeys of group II and two monkeys (each) of groups III and IV self cured the infections, but so did one monkey from the group treated with control IgG (group I). The serum levels of transfused antibodies were low at the peak of parasitemia, suggesting that clearance of parasites was mediated by immune responses mounted by the monkeys. The results indicate that antibodies to epitopes formed by repeats of Pf155/RESA may depress P. falciparum parasitemias and thus that immunogens based on such repeats should be suitable components in a subunit vaccine against asexual stages of P. falciparum.

Trypanosoma cruzi: cellular and antibody response against the parasite in mice immunized with a 19-amino acid synthetic peptide

Several monoclonal antibodies were prepared against the flagellar fraction of Trypanosoma cruzi epimastigotes (Tulahuén strain, stock Tul 2). One of them, FCH-F8-4, has previously shown biologic activity against the parasite (complement-mediated lysis and neutralization of the trypomastigote infectivity). Immunopurified antigens using this monoclonal antibody elicited a protective immune response in mice. Two recombinant cDNA clones were detected with this anti-flagellar fraction monoclonal antibody on a lambda gt11 expression library prepared from T. cruzi epimastigote mRNA. The insert of one of these cDNA clones, lambda(FCH-F8-4)1 (150 bp) coded for a 19-amino acid peptide (PAFLGCSSRFSGSFSGVEP). This insert hybridized with a 5.0-kb mRNA from epimastigotes. The beta-galactosidase fusion protein was produced in lysogenic bacteria. The monoclonal antibody recognized the epitope present in the fusion protein after western blotting of the crude lysate. A synthetic peptide (SP4) containing the complete sequence of lambda(FCH-F8-4)1 was constructed on solid phase. This peptide was able to inhibit the ELISA reactivity (in a range from 13 to 52%) of flagellar fraction immunized mouse sera and when administered (coupled to KLH or alone) to BALB/c mice with Bordetella pertussis as adjuvant, it induced a humoral and cellular immune response which was detected by ELISA, immunofluorescence, blotting, and DTH reactions against T. cruzi antigens. The immune response obtained indicates that this synthetic peptide resembles the parasite antigen conformation and could be useful for diagnosis purposes or be able to elicit immunoprotection against T. cruzi infection.

Functional characterization of the antibody-mediated protection against blood stages of Plasmodium falciparum in the monkey Saimiri sciureus

The squirrel monkey Saimiri sciureus is one of the World Health Organization recommended experimental models of Plasmodium falciparum blood stage infection. Anti-malaria antibodies developed by this host after a drug-controlled infection play an important part in the acquired protection against the P. falciparum blood stages. Furthermore, the use of two anti-Saimiri immunoglobulin (Ig) monoclonal antibodies (mAb) has permitted the differentiation between protective (mAb 3A2/G6) and non-protective (mAb 3E4/H8) antibodies, as shown by transfer experiments to recipient monkeys infected with blood stage parasites. In the present study we have established that protection conferred by the 3A2/G6+ protective Ig preparation is strictly associated with an in vitro opsonic activity. Such an opsonic activity is not detectable in the 3E4/H8+ non-protective Ig population. In addition, results indicate that the 3E4/H8+ non-protective Ig population competes with protective opsonic 3A2/G6+ Ig antibodies when co-incubated with parasitized red blood cells. Thus, it follows that protection can be directly correlated to the quantitative and qualitative fluctuation of the two Ig populations. When challenged with 1 x 10(8) P. falciparum-infected Saimiri red blood cells parasitemia occurred in 5 out of 12 Saimiri who were lacking detectable 3A2/G6+ opsonic antibodies in their sera. By employing antibodies against the human Fc receptor for IgG (Fc gamma R) in an in vitro phagocytic assay, we have been able to show that the principal receptor is Fc gamma RIII. Finally, we also show that in contrast to the situation in man, this receptor is present on circulating monocytes. These findings could lead to a different strategy in designing malaria vaccine candidates and also allow the possibility of predicting the outcome of immunization trials in Saimiri monkeys.

Plasmodium falciparum: differential parasite reactivity of rabbit antibodies to repeated sequences in the antigen Pf155/RESA

For selection of immunogens capable of inducing high levels of antibodies reactive with the Plasmodium falciparum antigen Pf155/RESA, rabbits were immunized with synthetic peptides corresponding to sequences based on the repeat subunits EENVEHDA and (EENV)2 from the C-terminus of this antigen. The antibodies obtained were analyzed with regard to binding to synthetic peptides in ELISA and to reactivity with parasite antigens by immunofluorescence or immunoblotting. All antisera reacted with both the peptides EENVEHDA and (EENV)2 as well as with Pf155/RESA. Antibody fractions specific for each of the two peptides were prepared by affinity chromatography on insolubilized peptides. Strong reactivity with antigens in the membrane of erythrocytes infected with early stages of the parasite as well as reactivity with Pf155/RESA in immunoblotting correlated with reactivity of antibody with (EENV)2. Antibody preparations reactive with EENVEHDA and depleted of (EENV)2 reactivity showed only a weak reactivity with Pf155/RESA but reacted also with P. falciparum polypeptides of 250, 210, and 88 kDa. In immunofluorescence, these antibodies stained mainly the intraerythrocytic parasite. Both EENVEHDA- and (EENV)2-specific antibodies inhibited merozoite reinvasion in P. falciparum in vitro cultures, the latter antibodies being the most efficient. This study defines the specificity and cross-reactivity with other P. falciparum antigens of antibodies to the C-terminal repeats of Pf155/RESA.

Anti-idiotypic antibodies counteract the invasion inhibition capacity of antibodies to major epitopes of the Plasmodium falciparum antigen Pf155/RESA

Rabbits were immunized with the synthetic peptide EENVEHDA or (EENV)2, corresponding to a tandemly repeated sequence in the C-terminal part of the Plasmodium falciparum antigen Pf155/RESA, or with Escherichia coli-derived fusion proteins containing the corresponding repeats. For all sera, the capacity of the total immunoglobulin G fractions to inhibit P. falciparum merozoite invasion in vitro was similar and relatively low. Affinity purification of Pf155/RESA-specific antibodies on parasite-infected erythrocyte monolayers or on peptide columns increased the inhibitory capacity 50 to 5,000 times, whereas the immunofluorescence titers were increased only 10 times. The addition of small amounts of total immunoglobulin G to the affinity-purified antibodies gave a marked and dose-dependent reduction of the inhibitory capacity of the purified antibodies. However, this reduction was only seen in combinations where the immunoglobulin G fraction was from the same serum as the affinity-purified antibodies, suggesting that it was mediated by anti-idiotypic antibodies reacting with non-cross-reacting idiotopes of the invasion-inhibiting antibodies.

A general strategy for polymerization, assembly and expression of epitope-carrying peptides applied to the Plasmodium falciparum antigen Pf155/RESA

Polymerization of DNA fragments in a head-to-tail arrangement provides a convenient way to obtain multimeric expression of a specific gene product, e.g., epitope-carrying peptides for immunological studies. A novel technique for the polymerization and assembly of peptides has been developed, involving the use of the class-IIS restriction enzyme BspMI which enables unidirectional insertion of the DNA fragments to be polymerized [Kim and Szybalski, Gene 71 (1988) 1-8]. One or several DNA fragments are polymerized in subsequent steps, using in vitro DNA polymerization, and the obtained gene constructs containing several repeats are screened and sequenced using polymerase chain reaction techniques. Using a two-step polymerization strategy a peptide, comprising two repetitive sequences from the Plasmodium falciparum malaria blood-stage antigen Pf155/RESA, was assembled and subsequently synthesized in Escherichia coli. Two different fusion proteins suitable for affinity purification were produced using a dual affinity system. Rabbits were immunized with one of the fusion proteins and the antibody response was analyzed by the enzyme-linked immunosorbent assay and immunofluorescence using the second fusion protein.

A longitudinal study of antibodies to the Plasmodium falciparum antigen Pf155/RESA and immunity to malaria infection in adult Liberians

118 adult Liberians from 2 villages were studied prospectively for one year with monthly blood examinations for malaria parasites. The crude parasite rate was 41.5% and the crude gametocyte rate was 6.1%. The inoculation rate varied between 0.075 in the dry season and almost 0.4 in the rainy season, which is in accordance with other data from holoendemic areas. 47.5% (56) had a titre to the Pf155/RESA antigen less than or equal to 1/50 ('low responders') and 52.5% (62) had a titre of greater than or equal to 1/250 ('high responders'). The response was not age-dependent in this adult population, which may suggest that genetic factors are determining whether the individual become a high or low responder. Antibodies against the Pf155/RESA antigen were measured in 2 surveys 8 months apart, and the mean antibody response to Pf155/RESA and its EENV sequence was constant without seasonal variation. Pf155/RESA high responders had lower parasite densities during all 3 seasons surveyed, and Pf155/RESA high responders, with high antibody reactivity against the (EENV)6 sequence from the 3' repeat region of Pf155/RESA, had significantly lower parasite densities in the rainy season of 1987. The data suggest that high titres of antibodies to the Pf155/RESA antigen, and especially to its EENV sequence, might play a role in protective immunity in adults.

Immunogenicity and antigenicity in rabbits of a repeated sequence of Plasmodium falciparum antigen Pf155/RESA fused to two immunoglobulin G-binding domains of staphylococcal protein A

A synthetic gene encoding a tetramer of the repeated subunit EENVEHDA of the Plasmodium falciparum antigen Pf155/RESA was expressed in a dual-expression system. The resulting fusion proteins, designated ZZ-M1 and BB-M1, comprised the EENVEHDA repeats and either two immunoglobulin G-binding domains from staphylococcal protein A or the human serum albumin-binding domains from streptococcal protein G, respectively. The soluble fusion proteins were affinity purified to homogeneity in one-step procedures. ZZ-M1 was used for immunization of rabbits. The rabbit antisera reacted with BB-M1 in an enzyme-linked immunosorbent assay and with Pf155/RESA in immunofluorescence of infected erythrocytes and immunoblotting. Inhibition studies revealed that the antibodies mainly recognized epitopes formed by two or more EENVEHDA subunits and were remarkably specific for Pf155/RESA. Importantly, the antibodies also inhibited P. falciparum merozoite reinvasion in vitro efficiently, indicating that they reacted with biologically important epitopes exposed on the native antigen. Immunization with Freund complete adjuvant resulted in high levels of specific immunoglobulin G antibodies over a 1-year period, whereas the antibody response obtained after immunization without adjuvant was generally weaker, immunoglobulin G and M mediated, and not sustained for longer periods. However, these titers were restored after booster injection. Taken together, the results support the usefulness of recombinant gene constructs of this type as immunogens for malaria vaccines.

A dual expression system for the generation, analysis and purification of antibodies to a repeated sequence of the Plasmodium falciparum antigen Pf155/RESA

A novel dual expression system for the generation and analysis of immune responses to recombinant protein is described. The two expression systems are based on the IgG-binding domains (ZZ) of staphylococcal protein A (SpA) and the human serum albumin (HSA) binding domains (BB) of streptococcal protein G, respectively. Products of fusions with the ZZ region are used to generate an immune response against the recombinant peptide and the corresponding peptide fused to the BB region is used for analysis and purification of the specific antibodies. The protein A and protein G expression systems were used to produce fusion proteins with the repeated C terminal octapeptide subunit EENVEHDA of the Plasmodium falciparum merozoite derived protein Pf155/RESA. Rabbits were immunized with the protein A-derived fusion protein (designated ZZ-M1) and the antibody response was analyzed using the protein G-derived fusion protein (designated BB-M1). The rabbit antisera reacted with BB-M1 in both ELISA and immunoblotting. In addition, BB-M1 proved to be an efficient ligand for affinity purification of antibodies specific for the malaria peptide. Furthermore, the rabbit antisera reacted with Pf155/RESA both in merozoite extracts and when deposited in the membrane of parasite infected erythrocytes.

Malaria vaccines: results of human trials and directions of current research

Recent human trials of candidate vaccines against the sporozoite and asexual blood stages of the malaria parasite have produced encouraging evidence that synthetic peptides and recombinant proteins can stimulate an antibody response and provide at least limited protection from malaria infection. Current research is focused on the continued evaluation of asexual blood stage antigens and on the improvement of the immunogenicity of candidate antigens through stimulation of more effective cellular and humoral responses. Efforts are also being directed toward the development of transmission-blocking vaccines based on antigens of sexual stages of the parasite.

Enhancement or inhibition of Plasmodium falciparum erythrocyte reinvasion in vitro by antibodies to an asparagine rich protein

A clone encoding a recombinant protein which reacted strongly with human antibodies from a donor clinically immune to malaria, was isolated from a genomic Plasmodium falciparum library. Mice injected with this protein, designated 10b, produced antibodies which reacted with all developmental stages of erythrocytic asexual parasites in indirect immunofluorescence. In immunoblotting, the same antibodies recognized two P. falciparum polypeptides of 36 kDa and 33 kDa. Of three monoclonal antibodies raised against the 10b recombinant protein, two inhibited parasite reinvasion of erythrocytes in an isolate specific manner. Surprisingly, however, the third was found to significantly enhance reinvasion of erythrocytes and also to induce a more rapid maturation of intraerythrocytic parasites in all isolates tested. Nucleotide sequence analysis of the 1124 bp insert revealed that it encodes a protein which consists of 30% asparagine and contains three asparagine rich, imperfect tandem repeats: Lys-Lys-Asn-Asn (3x), Met-Asn-His/Gln-Pro-Asn-Asn (14x), and Lys-Asn-Asn-Asn-Asn (7x).

Development and application of synthetic peptides as vaccines

Vaccination against bacterial and viral diseases has been one of the major achievements in medicine and immunology since the beginning of this century. Extensive vaccination programs have been able to control or, in the case of smallpox, virtually wipe out some of the most dangerous infectious diseases e.g. poliomyelitis, measles, whooping cough, diphtheria and tetanus. However, as this success has been limited mainly to the developed, affluent countries, infectious diseases still remain the worlds largest health problem. Furthermore, vaccines against human parasites are non-existent. Recent advances in immunology and molecular biology including recombinant DNA technology have provided the basis for new approaches to vaccine development.