New B cell epitopes in the Plasmodium falciparum malaria circumsporozoite protein

Most antibodies directed against the Plasmodium falciparum circumsporozoite (CS) protein react with its central domain, which contains about 40 repeats of the tetrapeptide Asn-Ala-Asn-Pro (NANP). To search for new epitopes in the non-repetitive part of the CS protein, we expressed the non-repetitive regions of the protein in E. coli as fusion proteins with mouse dihydrofolate reductase linked to six adjacent histidine residues. These fusion proteins were obtained at greater than 70% purity by a single Ni-chelate affinity chromatography step. Of the new epitopes defined in the C-terminal portion of the CS protein, three are located in a stretch of 65 amino acids immediately C-terminal of the protein's central repetitive domain. Pooled sera from inhabitants of a malaria-endemic area reacted with epitopes in this region of the molecule, and four mouse monoclonal antibodies to this region also reacted with the native CS protein on sporozoites. Two of the monoclonal antibodies reacted with a peptide PNDPNRNVD derived from a conserved region of the CS protein. The other two antibodies showed different reactivities to sporozoites of the NF54 and Ro59 parasite isolates. One, which reacted with a peptide ENANANNAV, recognized Ro59 but not NF54 sporozoites, while the other reacted with a small percentage of NF54 but not Ro59 sporozoites. Antibodies which react with non-repetitive regions of the CS protein could contribute to maintaining its genetic variability.

Consecutive determinations of seroreactivities to Pf 155/RESA antigen and to its different repetitive sequences in adult men from a holoendemic area of Liberia

Sera from 32 adult men residing in a malaria holoendemic area of Liberia were investigated for seroreactivities to different asexual blood-stage malaria antigens on five consecutive occasions from 1984 to 1986. The seroreactivities to crude parasitic antigens and to Pf 155/RESA (EMIF) were determined by immunofluorescence and to repetitive sequences of Pf 155/RESA by enzyme-linked immunosorbent assay (ELISA). All sera were highly reactive against the crude parasitic antigens with reciprocal titres varying from 5000 to 100,000. The EMIF titres showed a wider variation from negative (less than 10) to 25,000, and when the same individuals were re-examined on subsequent surveys similar EMIF titres were found. The ELISA seroreactivities to three different repetitive sequences of Pf 155/RESA also showed different individual profiles which were rather consistent on consecutive surveys. High EMIF titres appeared to be correlated mainly to one of the peptide sequences, namely (EENV)2. The consistent individual profiles of the seroreactivities to Pf 155 and its repetitive sequences suggest genetic restriction of the humoral immune response. Although no significant correlation was found between EMIF titres and parasitic densities in the adult hyperimmune men the specific peptides, however, offer new possibilities of further investigating protective capacities of different immune responses to specific epitopes of the malaria parasite.

Cytotoxic T cells recognize a peptide from the circumsporozoite protein on malaria-infected hepatocytes

Irradiated malaria sporozoites can induce CD8+ T cells that are required for protection against infection. However, the parasite antigens targeted by this immune response are unknown. We have discovered a 16-amino acid epitope from the Plasmodium yoelii circumsporozoite (CS) protein that is recognized by cytotoxic T cells from immune mice. Lymphocytes stimulated with this peptide can kill P. yoelii liver stage parasites in vitro in an MHC-restricted, antigen-specific manner. Thus, epitopes from the CS protein are presented on the surface of infected hepatocytes and can be targets for T cells, even though intact CS protein has not been detected on the surface of the infected hepatocyte. A vaccine that induced CTL to parasite antigens might protect humans against malaria by eliminating liver stage parasites.

Genetic complexity of Plasmodium vivax parasites in individual human infections analyzed with monoclonal antibodies against variant epitopes on a single parasite protein

Monoclonal antibodies against variant epitopes of a highly polymorphic protein (PV200) in schizonts of Plasmodium vivax have been used to analyze the variety of genetically distinct populations of parasites present in the peripheral blood of individual P. vivax infections in Sri Lanka. In 9 out of 10 isolates of freshly drawn P. vivax infected blood from different individuals, parasites of only 1 PV200 serotype was found within each individual infection, even though parasites were serotypically distinct between individuals. In 1 isolate parasite population, 3 distinct PV200 serotypes were identified. Thus, most P. vivax infections appeared to consist of a single genetically homogeneous population of parasites within the detection limits of the technique. The prevalence of P. vivax infections in an area of malaria transmission in southern Sri Lanka and the densities of oocysts in mosquitoes fed on P. vivax infected individuals indicated that parasite populations would be transmitted many times before encountering parasites of other origins, and that individual populations would tend to reduce to genetic homogeneity during transmission. These expectations are consistent with the high proportion of genetically homogeneous P. vivax isolates observed.

Chemical crosslinking of Plasmodium falciparum glycoprotein, Pf200 (190-205 kDa), to the S-antigen at the merozoite surface

Merozoites were isolated from Plasmodium falciparum cultures labeled with [3H]mannose and [35S]methionine and treated with a cleavable homobifunctional crosslinker, dithiobis(succinimidyl) propionate. The crosslinked complexes were immunoprecipitated with Mab.5B1 directed against the major merozoite surface glycoprotein. Pf200 (MW 190-205), and reduced with dithiothreitol. Crosslinked immunocomplexes did not contain the second major merozoite surface glycoprotein, Pf50 (MW 45-55 kDa), or other major [35S]methionine-labeled proteins, except for a weakly labeled protein of 150 kDa. Crosslinked complexes immunoprecipitated with Mab.5B1 and then reduced with DTT were immunoblotted with antibody directed against three soluble P. falciparum antigens, a serine-rich antigen known as Pf126 or SERA, the S-antigen, and GBP-130. The 150-kDa S-antigen was readily detected in crosslinked immunocomplexes with Pf200. The SERA antigen, although crosslinked under these conditions, was not detected in association with Pf200 nor was GBP-130.

Interferon-alpha and synthetic peptide malaria sporozoite vaccine in non-immune adults: antibody response after 40 weeks

Adults with no known immunity to sporozoites received, i.m., at weeks 0 and 8 two single 200 micrograms doses of a peptide Plasmodium falciparum sporozoite vaccine conjugated to tetanus toxoid ((NANP)3-TT) plus placebo (group 1) or interferon-alpha (IFN-alpha) (group 2) and were followed for antibody responses at weeks 4, 12 and 40. Peak antibody responses were observed at week 12. At week 40, a greater than or equal to 4-fold increase in antibody titre to sporozoites in IFA, or to (NANP)50 in ELISA was still detectable in 6 of 12 (50%) volunteers in group 1 and in 16 of 25 (64%) in group 2. Peak antibody titres in IFA and ELISA decreased with a rate of 0.8% and 0.5% per week, respectively.

Expression, partial purification and immunogenicity of fragments of the knob protein of Plasmodium falciparum

Three structural domains of the histidine-rich knob protein (KP) of Plasmodium falciparum were expressed in Escherichia coli. A single-step purification scheme was devised to obtain great enrichment of expressed polypeptides for use in subsequent experiments. Immune human sera from Africa, South-East Asia and South America were tested for reactivity with each of the expressed fragments. While the two fragments which represented the central and C-terminal regions of KP showed a strong reactivity with all the antisera which were tested, the N-terminal fragment which contains the repetitive histidine-rich sequences showed almost no reactivity.

Human studies with synthetic peptide sporozoite vaccine (NANP)3-TT and immunization with irradiated sporozoites

The synthetic peptide Plasmodium falciparum circumsporozoite (CS) protein conjugate vaccine (NANP)3-TT was safe when given parenterally to 202 volunteers. However, with a few notable exceptions, antibody responses were low and could not be boosted. Vaccinees' lymphocytes did not proliferate when exposed in vitro to (NANP)3. The tetanus toxoid (TT) carrier immunomodulated the response to the CS peptide in that both epitopic suppression and immune enhancement were demonstrated during the course of the clinical trials. During efficacy challenge studies, 1 of 7 vaccinees was protected against sporozoite challenge and in other vaccinees the prepatent period was significantly delayed. P. falciparum-infected mosquitos were irradiated with 20,000 rad (200 Gy). Five volunteers were immunized with 54, 55, 224, 663, and 715 total infective bites of irradiated mosquitos in an attempt to immunize with attenuated sporozoites. Four of these volunteers had significant humoral and cellular immune responses. Two volunteers (who received the largest immunizing doses) were challenged by the bites of infective mosquitos and both developed parasitaemia. In the volunteer with the highest antibody titre there was a marked delay in patency as determined by serial plasmodial cultures. T-cell clones are being obtained and characterized.

Conformation and immunogenicity of engineered repeating segment of the circumsporozoite surface protein of Plasmodium falciparum

Peptide vaccines based on units of the immunodominant tetrapeptide repeats, Asn-Ala-Asn-Pro and Asn-Val-Asp-Pro, of the circumsporozoite surface protein of the parasite Plasmodium falciparum are presently being developed as potential malaria vaccines. The N-terminal fusion of a hydrophobic protein to units of the tetrapeptide repeat affected the immunogenicity and conformational stability of the peptide, and also induced a secondary structure in the peptide. Peptide antigenicity, as well as conformational stability, was significantly increased.

Major population differences in T cell response to a malaria sporozoite vaccine candidate

Using a complete series of overlapping peptides, we have identified the T cell epitopes of a malaria vaccine candidate, the circumsporozoite (CS) protein, that are recognized by sporozoite-exposed residents of a non-endemic country. This protein and subunits from it are being considered as malaria sporozoite vaccine candidates, as CS-specific antibodies and cytotoxic T lymphocytes have been shown to have a role in protection. The rationale for developing an antibody-based vaccine is that in Plasmodium falciparum the immunodominant B cell epitope of the protein, (Asn-Ala-Asn-Pro)n [(NANP)n], is invariant. However, the ideal vaccine must contain CS protein-derived T cell antigenic epitopes to allow natural boosting of the antibody response following sporozoite exposure. Here, we show that major differences occur between the CS-specific T cell responses of non-endemic Caucasians and an endemic African population. HLA differences between the populations are, in part, responsible. Subunit malaria vaccines for one population may be ineffective in a different population.

[Biotechnological approaches to the creation of antimalarial vaccines]

The authors analyze the latest data on antigen pattern of malaria Plasmodium at various stages of development and extraction of immunogenic determinants contributing to creation of malaria vaccine. Knowledge of the spectra of Plasmodium antigens and stimulants of humoral and cellular immune response in the body will simplify the search for effective biotechnological vaccines able to provide reliable protection against malaria.

Chemical characterization of the parasitophorous vacuole membrane antigen QF 116 from Plasmodium falciparum

On the basis of amino acid sequencing and immunological cross-reactivity, the Plasmodium falciparum parasitophorous vacuole antigens QF116 and exp-1/CRA are apparently identical. The epitope recognized by an inhibitory monoclonal antibody directed against QF116 is located proximal to the C-terminus of the protein. The QF116 protein is processed during maturation by the cleavage of a 22-amino-acid signal peptide and acylated as measured by labeling with myristic acid.

Biochemical characterization, localization and immunostimulating properties of a soluble glycoprotein, Ag1, isolated from in vitro cultures of Plasmodium falciparum

The soluble amphiphilic glycoprotein, Ag1 (gp60), purified from supernatants of in vitro cultures of Plasmodium falciparum has a molecular mass of 60 kDa and did not exhibit size variation in the different P. falciparum isolates tested by immunoblotting. Ag1 was shown to interact with the lectin Erythrina christagalli agglutinin, which is specific for carbohydrates bearing beta-D-galactose(1-4)-D-N-acetylglucosamine. Indirect immunofluorescence studies showed that Ag1 is located on the surface of trophozoites and schizonts but not on the surface of merozoites. Ag1 is recognized by human immune sera from six different malaria-endemic regions. Ag1 induces in vitro proliferation of lymphocytes from malaria-immune individuals in an antigen-specific manner.

The PSA-2 glycoprotein complex of Leishmania major is a glycosylphosphatidylinositol-linked promastigote surface antigen

Polyclonal rabbit antiserum to the Triton X-114 phase material of Leishmania major, which comprises the surface and internal integral membrane proteins of the parasite, was used to screen a lambda gt11 genomic expression library. A recombinant clone producing a Mr 123,000 beta-galactosidase fusion protein was isolated. Antibodies affinity-purified on this fusion protein recognized a complex of three surface-oriented proteins of promastigotes of L. major of Mr 94,000, 90,000, and 80,000 that we have termed the promastigote surface Ag 2 (PSA-2) complex. The DNA sequence of the insert in this clone predicted the 3' end of an open reading frame encoding a hydrophobic C-terminus. The inferred C-terminal sequence was suggestive of a glycosylphosphatidyl-inositol membrane anchoring mechanism. Phosphatidylinositol-specific phospholipase C treatment of the native PSA-2 proteins caused a shift in their electrophoretic mobility with an apparent reduction in the molecular weight of the PSA-2 complex. After phospholipase C treatment these proteins also displayed the cryptic cross-reacting determinant recognized by antibodies to the Trypanosoma brucei variant surface Ag. Moreover, PSA-2, which previously partitioned in the detergent phase after Triton X-114 phase separation, became water-soluble after phospholipase C treatment. Immunoprecipitation of the PSA-2 proteins with sera directed to lectin-binding proteins indicated that these polypeptides may be differentially glycosylated. Finally, these PSA-2 proteins were recognized by sera from some patients with cutaneous leishmaniasis.

The seroreactivity against Pf155 (RESA) antigen in villagers from a mesoendemic area in Somalia

Pf155 (RESA), a malaria antigen in the membrane of infected red cells, was recently identified as a possible future malaria vaccine candidate. In this study the seroreactivities against this antigen were compared with those against crude parasitic antigens in 195 subjects from a Somali village with mesoendemic malaria. The seroreactivities were determined with immunofluorescence. With age, there was an increased seroreactivity to both Pf155 and the crude parasitic antigens. However, the acquisition of seroreactivity was much slower against Pf155. Hence in the age group 15-24 years, only half of the subjects had detectable antibodies against Pf155.

Development of the human immune response against the major surface protein (gp190) of Plasmodium falciparum

The 190-kilodalton glycoprotein (gp190) of Plasmodium falciparum, the precursor of the major surface proteins of merozoites, is considered a promising candidate for a blood stage malaria vaccine. DNA sequences specific for the gp190 of the two isolates K1 and MAD20 were subcloned and expressed in Escherichia coli. The panel of fusion proteins obtained represents about 80% of the polymorphic sequences observed so far within various isolates of P. falciparum. Sera from individuals living in a malaria-endemic area of West Africa were tested in immunoblots against the gp190 fusion proteins, and antibody reactivity was mapped to defined regions of the gp190. Depending on the age of the individual and on the presence of parasites in the blood, distinct regions of gp190 were differentially recognized by the respective antibodies. Similarly, the analysis of sera from German patients with acute malaria revealed a distinct pattern. When grouped according to age and to parasitemia, the reactivity of the sera of people living in malaria-endemic areas may indicate a correlation between certain gp190 regions and protective immune response.

Flagellar membrane and paraxial rod proteins of Leishmania: characterization employing monoclonal antibodies

Flagella-specific proteins of Leishmania have been identified employing the monoclonal antibody technique. Six monoclonal antibodies recognized 3 different proteins. A doublet of protein of Mr 69,000 and 74,000 Da identified by monoclonal antibodies F-3, F-4 and F-6 is continuously distributed along the flagellum by immunofluorescence. Immunocytochemical electron microscopic studies localize these molecules to the paraxial rod of the flagellum. A single protein of Mr 13,200 Da is recognized by monoclonal antibodies F-1, F-2 and F-5. The distribution of the Mr 13,200 protein appears irregular, occurring in localized patches along the length of the flagellum, especially at the flagellar tip. Immunocytochemical electron microscopic experiments show that the Mr 13,200 molecule is associated with the membrane of the flagellum. Indirect immunofluorescence experiments demonstrated these monoclonal antibodies cross-reacted with members of the Kinetoplastida family (Endotrypanum, trypanosoma, Leishmania) suggesting that these molecules may be evolutionarily conserved.

Identification of major Trypanosoma cruzi antigenic determinants in chronic Chagas' heart disease

To identify Trypanosoma cruzi target antigens in overt Chagas' heart disease, a parasite lambda gt11 cDNA library was screened with the serum of a patient with a severe chagasic heart involvement (JL). Using a phage dot array immunoassay, 5 highly antigenic clones, JL1, JL5, JL7, JL8, and JL9, were probed with sera from clinically characterized T. cruzi infected subjects. The correlation of cloned T. cruzi antigen recognition with the clinical status of the subjects led to the identification of a recombinant antigen, JL5, that reacted predominantly with sera from patients with Chagas' heart disease. The antigenic determinant of the JL5 recombinant was a small 35 amino acid peptide. The nucleotide and the deduced amino acid sequence, together with other experimental data, allowed identification as the C-terminal portion of a T. cruzi P ribosomal protein. The C-terminal undecapeptide in JL5, EDDDMGFGLFD, was highly homologous to the same region of the human P protein SD(D/E)DMGFGLFD. The latter sequence has been identified as the P protein epitope in systemic lupus erythematosus (SLE). Positive SLE sera reacted with the JL5 recombinant phage, suggesting that the T. cruzi P protein might induce antibodies with a similar specificity to that of P antibodies in SLE.

A circumsporozoite-like protein is present in micronemes of mature blood stages of malaria parasites

We demonstrate for the first time the presence of a circumsporozoite (CS)-like protein in invasive blood stages of malaria parasites. Immunogold electron microscopy using antisporozoite monoclonal antibodies localized these antigens in the micronemes of merozoites. Western immunoblot and two-dimensional gel electrophoresis of mature blood stage extracts of Plasmodium falciparum, P. berghei, P. cynomolgi, and P. brasilianum identified polypeptides having the same apparent molecular mass and isoelectric points as the corresponding sporozoite (CS) proteins. The CS-like protein of merozoites is present in relatively minor amounts, compared to the CS protein of sporozoites. Mice with long-term P. berghei blood-induced infections develop antibodies which react with sporozoites.

Entamoeba histolytica: molecules involved in the target cell-parasite relationship

To detect molecules of Entamoeba histolytica involved in the trophozoite-target cell interaction, three different antisera were generated: (a) two rabbit antisera, one against total amebic proteins and another directed specifically to the 112-kDa adhesin; and (b) a mouse antiserum against amebic molecules adhering to the red blood cell (RBC) surface after incubation of RBCs with total soluble protein from trophozoites (anti-adhesion serum). All three antisera recognized the 112-kDa adhesion. Adhesion of this molecule to the RBC surface was temperature-dependent. More of the 112-kDa adhesion was found on the surface of RBCs incubated with trophozoites at 37 degrees C than on RBCs incubated at room temperature or at 0 degree C. Experiments using both anti-adhesin and anti-total ambebic protein sera revealed the presence of 210, 160, 112, 90, 70, 50, and 24-kDa proteins on RBC incubated with trophozoites. Surface proteins obtained from iodinated MDCK cells recognized amebic proteins of 112, 90, and 48-50 kDa. Virulence-deficient mutants presented a similar amount of the 112-kDa adhesin to the wild-type strain. However, in mutants, the adhesion was not functional, since they did not adhere to RBCs. 90- and 24-kDa proteins were also found to be altered in mutants.

An epidemiological study of humoral and cell-mediated immune response to the Plasmodium falciparum antigen PF155/RESA in adult Liberians

We investigated the seroreactivity and T cell reactivity against the Plasmodium falciparum antigen Pf155/RESA, different oligopeptides from the 3' and 5' repeat regions of the Pf155/RESA antigen, and crude Plasmodium falciparum antigens in 164 adult Liberians. We compared 2 long-term residential groups with high and low exposure to malaria. The seropositive rate to the peptides was significantly higher with increased exposure. There was no significant difference in response rates to the Pf155/RESA. This may indicate the level of persistent T cell memory in previously primed donors. The seropositive rates to 3 Pf155/RESA peptides and the rates measured by either 3H-thymidine incorporation or IFN-gamma release after stimulation with Pf155/RESA and the peptides were all lower in parasite positive individuals. Even low grade, asymptomatic parasitemia can impair the T cell response in vitro. The lower antibody response in parasite positive subjects may be explained by either antibody consumption or lower protection against malaria parasitemia in subjects with low concentrations of antibodies against the Pf155/RESA antigen.

A heat shock-like protein from the human malaria parasite Plasmodium falciparum induces autoantibodies

The humoral immune response to a 72-kDa heat shock-like protein of Plasmodium falciparum has been analyzed using mouse monoclonal antibodies (mAb) and human immune sera. Three regions of the molecule containing B cell epitopes were identified by screening a sublibrary encoding the COOH-terminal half of the antigen with the mAb. One B cell epitope mapped to a region poorly conserved between the parasite 72-kDa polypeptide and mammalian heat-shock proteins (Hsp 70). Another mAb, G10C9, reacted with an amino acid region that has a high degree of homology with mouse (87.5%) and human (81.2%) Hsp 70. Both mouse and human cells were recognized by this mAb when analyzed by indirect immunofluorescence and by two-dimensional immunoblots. Sera from humans infected with malaria also recognized the human Hsp 70. Thus, our results indicate that autoantibodies directed against host Hsp 70 can be induced by the homologous parasite protein.

Recombinant polypeptides for serology of malaria

We have evaluated 3 molecularly defined polypeptides encoded by encloned Plasmodium falciparum genes for their ability to serve as antigens for detecting antimalaria antibodies. The recombinant proteins correspond to (i) a conserved part of 190-200 kDa schizont merozoite surface component, (ii) the carboxy terminal part of the P. falciparum aldolase, and (iii) the 5.1 antigen. Antibodies were detected using enzyme-linked immunosorbent assays (ELISA) in a high percentage of sera from individuals from a malaria endemic area in The Gambia (up to 99% for some adult groups). These results were further improved, especially for detection of antimalaria antibodies in children, when a pool of all 3 polypeptides (ELISA MIXT) was used as antigen. This ELISA MIXT improves presently available assays for the detection of antimalaria antibodies directed against asexual blood stages in respect of standardization, sensitivity and specificity.

Can malaria-associated polyclonal B-lymphocyte activation interfere with the development of anti-sporozoite specific immunity?

To study the relevance of polyclonal B cell activation (PBA) associated with malaria in the development of specific anti-sporozoite immunity, we used a reverse haemolytic plaque assay and an immunoradiometric assay employing the synthetic peptide (NANP)3, the main epitope of the circumsporozoite (CS) protein of Plasmodium falciparum, to assess respectively the degree of activation of IgG and IgM secreting cells and the level of anti-sporozoite antibodies in 95 subjects with malaria and 21 non-infected individuals. A positive correlation was observed between the anti-(NANP)3 antibody levels and the number of past attacks of malaria but not between the former and the age of individuals or the number of months of residence in the endemic region. Individuals with high numbers of IgG or IgM secreting cells (SC) had lower levels of anti-(NANP)3 antibodies; those with levels of antibodies above the mean for malaria-infected individuals had lower numbers of IgGSC and higher haematocrit and haemoglobin values. These data show the existence of a negative relationship between malaria-induced PBA and anti-sporozoite immunity, and it is suggested that either PBA blocks the development of anti-sporozoite immunity or, alternatively, the latter protects individuals against malaria and malaria-associated PBA.

Age-related prevalence of antibody response against three different, defined Plasmodium falciparum antigens in children from the Haut-Ogooué province in Gabon

The kinetics of the humoral response to defined Plasmodium falciparum antigens was studied in 543 children, 1 month to 15 years old, living in an area endemic for malaria. The antigens used for enzyme-linked immunosorbent assay were (i) the synthetic peptide (NANP)40 representing the immunodominant repeated region of the circumsporozoite protein, and (ii) the fusion peptide 31.1, representing the N-terminal portion of the 83 kDa polypeptide expressed at the surface of merozoites which is a processed product of the 190-200 kDa glycoprotein. In addition, glutaraldehyde-fixed infected red blood cells (RBC) were used to detect ring-infected erythrocyte surface antigen (RESA) and unfixed infected RBC to detect intra-erythrocytic asexual form (IEF) antigens by immunofluorescence. In the 1 to 2 months age group, 50%, 26% and 21% of the children had antibodies for IEF, (NANP)40 and 31.1 respectively, but none had anti-RESA antibodies. The proportions of positive subjects decreased until 3 to 6 months and then increased progressively for the 4 antigens, approaching, but not reaching, adult values by the age of 15 years. Antibodies against specific antigens were acquired concomitantly. Children born from (NANP)40-positive mothers showed enhanced anti-(NANP)40 IgG responses.

Asexual blood stage vaccines: from merozoites to peptides

Asexual blood stage proliferation is responsible for the morbidity and mortality associated with malaria infection in man. These developmental stages are therefore obvious targets for the development of malaria vaccines. Several asexual blood stage components have been identified as potential candidates for the development of vaccines and some of them have been shown, following immunization, to induce at least partial protection in a variety of Plasmodium-host combinations. Studies on defined parasite components and on synthetic peptides derived from them have revealed new insights at the molecular level into parasite mechanisms involved in propagation and survival in the infected host, and into the interaction between parasite components and the host immune system. Practical application of these findings is likely to provide the basis for the design of more appropriate antigens for the development of vaccines.

Role of the proline residues on the immunogenic properties of a P. falciparum circumsporozoite peptide linked to a carrier protein

The circumsporozoite (CS) protein of P. falciparum contains an immunodominant epitope, NADP, that is repeated 37 times in the native molecule. The presence of proline in the coat proteins of the Plasmodium parasite at various developmental stages and strains is a frequent occurrence. In this study we evaluate the influence of substitution of proline residues by glycine on the immunogenic behavior of two tandemly repeated peptides linked via glutaraldehyde to a protein carrier: The (NANP)4 P. falciparum circumsporozoite peptide and its glycine-substitute analog, (NANG)4. The results obtained show that the (NANP)4 induces antibodies which recognize the peptide free in solution, bound on a solid phase, and linked to a carrier protein. It has been previously reported that such antibodies recognize the antigenic sites of the peptide in the native protein on the surface of the sporozoite. Antibodies raised against (NANG)4 in the same experimental conditions as (NANP)4, cannot recognize the peptide free in solution or bound to the solid phase. However, these antibodies can react with the peptide when it is linked to a protein carrier. The coupling of a glycine-containing analog to a carrier results in a significant shift in its conformation, allowing it to be recognized by the antibodies.

A portion of the Pf155/RESA antigen of Plasmodium falciparum is accessible on the surface of infected erythrocytes

An investigation of antigens accessible to lactoperoxidase-catalysed cell surface iodination on intact Plasmodium falciparum-infected red blood cells (RBC) has identified a 125I-labelled antigen with an apparent size of about 155 kD. This labelled protein was specifically immunoprecipitated by the following antibodies: a rabbit antiserum and a mouse monoclonal antibody raised against a synthetic peptide comprising the 3',8-mer repeat EENVEHDA of the Pf155/RESA protein; a rabbit antiserum raised against a synthetic octapeptide comprising two copies of the 3',4-mer repeat EENV of the Pf155/RESA protein; and rabbit antisera against another synthetic peptide C(MYSNNNVED)2. The last antibody shows a strong reaction in asexual blood stage parasites with the Pf155/RESA antigen. While this antigen has been described previously as a submembrane component of the outer membrane of infected RBC, this report shows that at least part of its is accessible to the surface of both ring and late trophozoite-infected erythrocytes.