The synthetic Plasmodium falciparum circumsporozoite peptide PfCS102 as a malaria vaccine candidate: a randomized controlled phase I trial.

BACKGROUND: Fully efficient vaccines against malaria pre-erythrocytic stage are still lacking. The objective of this dose/adjuvant-finding study was to evaluate the safety, reactogenicity and immunogenicity of a vaccine candidate based on a peptide spanning the C-terminal region of Plasmodium falciparum circumsporozoite protein (PfCS102) in malaria naive adults. METHODOLOGY AND PRINCIPAL FINDINGS: Thirty-six healthy malaria-naive adults were randomly distributed into three dose blocks (10, 30 and 100 microg) and vaccinated with PfCS102 in combination with either Montanide ISA 720 or GSK proprietary Adjuvant System AS02A at days 0, 60, and 180. Primary end-point (safety and reactogenicity) was based on the frequency of adverse events (AE) and of abnormal biological safety tests; secondary-end point (immunogenicity) on P. falciparum specific cell-mediated immunity and antibody response before and after immunization. The two adjuvant formulations were well tolerated and their safety profile was good. Most AEs were local and, when systemic, involved mainly fatigue and headache. Half the volunteers in AS02A groups experienced severe AEs (mainly erythema). After the third injection, 34 of 35 volunteers developed anti-PfCS102 and anti-sporozoite antibodies, and 28 of 35 demonstrated T-cell proliferative responses and IFN-gamma production. Five of 22 HLA-A2 and HLA-A3 volunteers displayed PfCS102 specific IFN-gamma secreting CD8(+) T cell responses. Responses were only marginally boosted after the 3(rd) vaccination and remained stable for 6 months. For both adjuvants, the dose of 10 microg was less immunogenic in comparison to 30 and 100 microg that induced similar responses. AS02A formulations with 30 microg or 100 microg PfCS102 induced about 10-folds higher antibody and IFN-gamma responses than Montanide formulations. CONCLUSIONS/SIGNIFICANCE: PfCS102 peptide was safe and highly immunogenic, allowing the design of more advanced trials to test its potential for protection. Two or three immunizations with a dose of 30 microg formulated with AS02A appeared the most appropriate choice for such studies. TRIAL REGISTRATION: Swissmedic.ch 2002 DR 1227.

Localization of HLA-A2.1-restricted T cell epitopes in the circumsporozoite protein of Plasmodium falciparum.

Localization of human MHC class I-restricted T cell epitopes in the circumsporozoite (CS) protein of the human parasite Plasmodium falciparum is an important objective in the development of antimalarial vaccines. To this purpose, we synthesized a series of overlapping synthetic 20-mer peptides, spanning the entire sequence of the 7G8 CS molecule except for the central repeat B cell domain. The P.f.CS peptides were first tested for their ability to bind to the human MHC class I HLA-A2.1 molecule on T2, a human cell line. Subsequently, the use of a series of shorter peptide analogues allowed us to determine the optimal A2.1 binding sequence present in several of the 20-mers. Binding P.f.CS peptides were further tested for their capacity to activate PBL from HLA-A2.1+ immune donors living in a malaria-endemic area. Specific IFN-gamma production was detected in the supernatant of cultures of PBL from exposed individuals. Cytotoxic T cell lines and clones were derived from the PBL of one responder, and their activity was shown to be HLA-A2.1-restricted and specific for the peptide 334-342 of the CS protein. In addition, double transgenic HLA-A2.1 x human beta 2-microglobulin mice were immunized with peptide 1-10 of the CS protein. T cells derived from immune lymph nodes displayed a peptide-specific HLA-A2.1-restricted cytolytic activity after one in vitro stimulation.

Heterologous prime-boost regimen adenovector 35-circumsporozoite protein vaccine/recombinant Bacillus Calmette-Guérin expressing the Plasmodium falciparum circumsporozoite induces enhanced long-term memory immunity in BALB/c mice.

BACKGROUND: Sustained antibody levels are a hallmark of immunity against many pathogens, and induction of long-term durable antibody titers is an essential feature of effective vaccines. Heterologous prime-boost approaches with vectors are optimal strategies to improve a broad and prolonged immunogenicity of malaria vaccines. RESULTS: In this study, we demonstrate that the heterologous prime-boost regimen Ad35-CS/BCG-CS induces stronger immune responses by enhancing type 1 cellular producing-cells with high levels of CSp-specific IFN-γ and cytophilic IgG2a antibodies as compared to a homologous BCG-CS and a heterologous BCG-CS/CSp prime-boost regimen. Moreover, the heterologous prime-boost regimen elicits the highest level of LLPC-mediated immune responses. CONCLUSION: The increased IFN-γ-producing cell responses induced by the combination of Ad35-CS/BCG-CS and sustained type 1 antibody profile together with high levels of LLPCs may be essential for the development of long-term protective immunity against liver-stage parasites.

The C-terminal domain of Plasmodium falciparum merozoite surface protein 3 self-assembles into alpha-helical coiled coil tetramer.

Proteins located on the surface of the pathogenic malaria parasite Plasmodium falciparum are objects of intensive studies due to their important role in the invasion of human cells and the accessibility to host antibodies thus making these proteins attractive vaccine candidates. One of these proteins, merozoite surface protein 3 (MSP3) represents a leading component among vaccine candidates; however, little is known about its structure and function. Our biophysical studies suggest that the 40 residue C-terminal domain of MSP3 protein self-assembles into a four-stranded alpha-helical coiled coil structure where alpha-helices are packed "side-by-side". A bioinformatics analysis provides an extended list of known and putative proteins from different species of Plasmodium which have such MSP3-like C-terminal domains. This finding allowed us to extend some conclusions of our studies to a larger group of the malaria surface proteins. Possible structural and functional roles of these highly conserved oligomerization domains in the intact merozoite surface proteins are discussed.

Virosome-Formulated Plasmodium falciparum AMA-1 & CSP Derived Peptides as Malaria Vaccine: Randomized Phase 1b Trial in Semi-Immune Adults & Children.

Background: This trial was conducted to evaluate the safety and immunogenicity of two virosome formulated malaria peptidomimetics derived from Plasmodium falciparum AMA-1 and CSP in malaria semi-immune adults and children.Methods: The design was a prospective randomized, double-blind, controlled, age-deescalating study with two immunizations. 10 adults and 40 children (aged 5-9 years) living in a malaria endemic area were immunized with PEV3B or virosomal influenza vaccine Inflexal (R) V on day 0 and 90.Results: No serious or severe adverse events (AEs) related to the vaccines were observed. The only local solicited AE reported was pain at injection site, which affected more children in the Inflexal (R) V group compared to the PEV3B group (p = 0.014). In the PEV3B group, IgG ELISA endpoint titers specific for the AMA-1 and CSP peptide antigens were significantly higher for most time points compared to the Inflexal (R) V control group. Across all time points after first immunization the average ratio of endpoint titers to baseline values in PEV3B subjects ranged from 4 to 15 in adults and from 4 to 66 in children. As an exploratory outcome, we found that the incidence rate of clinical malaria episodes in children vaccinees was half the rate of the control children between study days 30 and 365 (0.0035 episodes per day at risk for PEV3B vs. 0.0069 for Inflexal (R) V; RR = 0.50 [95%-CI: 0.29-0.88], p = 0.02).Conclusion: These findings provide a strong basis for the further development of multivalent virosomal malaria peptide vaccines.

Protection against Plasmodium falciparum challenge induced in Aotus monkeys by liver-stage antigen-3-derived long synthetic peptides.

The vaccine potential of Plasmodium falciparum liver stage antigen-3 (LSA3) was investigated in Aotus monkeys using two long synthetic peptides corresponding respectively to an N-terminal non-repeat peptide (NRP) and repeat 2 (R2) region of the LSA3, adjuvanted by ASO2. Both 100-222 (NRP) and 501-596 repeat peptides induced effector B- and T-cell responses in terms of antigen-driven antibodies and/or specific IFN-gamma secretion. Animals challenged with P. falciparum sporozoites were protected following immunization with either the NRP region alone or the NRP combined with the R2 repeat region, as compared with controls receiving the adjuvant alone. These results indicate that the NRP may be sufficient to induce full, sterile protection and confirm the vaccine potential of LSA3 previously demonstrated in chimpanzees and in Aotus.

Synthesis and immunological characterization of 104-mer and 102-mer peptides corresponding to the N- and C-terminal regions of the Plasmodium falciparum CS protein.

We investigated the immunogenicity and the conformational properties of the non-repetitive sequences of the Plasmodium falciparum circumsporozoite (CS) protein. Two polypeptides of 104 and 102 amino acids long, covering, respectively, the N- and C-terminal regions of the CS protein, were synthesized using solid phase Fmoc chemistry. The crude polypeptides were purified by a combination of size exclusion chromatography and RP-HPLC. Sera of mice immunized with the free polypeptides emulsified in incomplete Freund's adjuvant strongly reacted with the synthetic polypeptides as well as with native CS protein as judged by ELISA and IFAT assays. Most importantly, these antisera inhibited the sporozoite invasion of hepatoma cells. In addition, sera derived from donors living in a malaria endemic area recognized the CS 104- and 102-mers. Conformational studies of the CS polypeptides were also performed by circular dichroism spectroscopy showing the presence of a weakly ordered structure that can be increased by addition of trifluoroethanol. The obtained results indicate that the synthetic CS polypeptides and the natural CS protein share some common antigenic determinants and probably have similar conformation. The approach used in this study might be useful for the development of a synthetic malaria vaccine.

The N-terminal domain of Plasmodium falciparum circumsporozoite protein represents a target of protective immunity.

The N-terminal domain of the circumsporozoite protein (CSP) has been largely neglected in the search for a malaria vaccine in spite of being a target of inhibitory antibodies and protective T cell responses in mice. Thus, in order to develop this region as a vaccine candidate to be eventually associated with other candidates and, in particular, with the very advanced C-terminal counterpart, synthetic constructs representing N- and C-terminal regions of Plasmodium falciparum and Plasmodium berghei CSP were administered as single or combined formulations in mice. We show that the antisera generated against the combinations inhibit sporozoite invasion of hepatocytes in vitro better than antisera against single peptides. Furthermore, two different P. falciparum CSP N-terminal constructs (PfCS22-110 and PfCS65-110) were recognized by serum samples from people living in malaria-endemic regions. Importantly, recognition of the short N-terminal peptide (PfCS65-110) by sera from children living in a malaria-endemic region was associated with protection from disease. Taken together, these results underline the potential of using such fragments as malaria vaccine candidates.

A decrease of plasma macrophage migration inhibitory factor concentration is associated with lower numbers of circulating lymphocytes in experimental Plasmodium falciparum malaria.

Macrophage migration inhibitory factor (MIF) has recently been implicated in the pathogenesis of malarial anaemia. However, field studies have reported contradictory results on circulating MIF concentrations in patients with clinically overt Plasmodium falciparum malaria. We determined plasma MIF levels over time in 10 healthy volunteers during experimental P. falciparum infection. Under fully controlled conditions, MIF levels decreased significantly during early blood-stage infection and reached a nadir at day 8 post-infection. A decrease in the number of circulating lymphocytes, which are an important source of MIF production, paralleled the decrease in MIF levels. Monocyte/macrophage counts remained unchanged. At MIF nadir, the anti-inflammatory cytokine interleukin (IL)-10, which is an inhibitor of T-cell MIF production, was detectable in only 2 of 10 volunteers. Plasma concentrations of the pro-inflammatory cytokines IL-8 and IL-1beta were only marginally elevated. We conclude that circulating MIF levels decrease early in blood-stage malaria as a result of the decline in circulating lymphocytes.

Design, synthesis, and biological and crystallographic evaluation of novel inhibitors of Plasmodium falciparum enoyl-ACP-reductase (PfFabI).

Malaria, a disease of worldwide significance, is responsible for over one million deaths annually. The liver-stage of Plasmodium's life cycle is the first, obligatory, but clinically silent step in malaria infection. The P. falciparum type II fatty acid biosynthesis pathway (PfFAS-II) has been found to be essential for complete liver-stage development and has been regarded as a potential antimalarial target for the development of drugs for malaria prophylaxis and liver-stage eradication. In this paper, new coumarin-based triclosan analogues are reported and their biological profile is explored in terms of inhibitory potency against enzymes of the PfFAS-II pathway. Among the tested compounds, 7 and 8 showed the highest inhibitory potency against Pf enoyl-ACP-reductase (PfFabI), followed by 15 and 3. Finally, we determined the crystal structures of compounds 7 and 11 in complex with PfFabI to identify their mode of binding and to confirm outcomes of docking simulations.

Transgenic tomato plants expressing the antigen gene PfCP-2.9 of Plasmodium falciparum

The objective of this work was to obtain transgenic tomato plants expressing the PfCP-2.9 protein (a chimera of the antigens MSP1 and AMA1 of Plasmodium falciparum). Cotyledons of seven-day-old tomatoes, cultivar Summers, were transformed via Agrobacterium tumefaciens. Transgenic expression in the T0 plants was verified in the DNA extracted from fruits. PCR analysis was used to test the presence of the gene of interest in the T1 generation. Reverse transcriptase PCR provided evidence of gene expression at the RNA level, and Western blot analysis confirmed the presence of the protein of interest in the T1 plants. This is the first report of successful transformation with the expression of a malaria antigen (PfCP-2.9) in transgenic tomato plants from the T0 and T1 generations.

Longitudinal analyses of immune responses to Plasmodium falciparum derived peptides corresponding to novel blood stage antigens in coastal Kenya

We have recently described 95 predicted alpha-helical coiled-coil peptides derived from putative Plasmodium falciparum erythrocytic stage proteins. Seventy peptides recognized with the highest level of prevalence by sera from three endemic areas were selected for further studies. In this study, we sequentially examined antibody responses to these synthetic peptides in two cohorts of children at risk of clinical malaria in Kilifi district in coastal Kenya, in order to characterize the level of peptide recognition by age, and the role of anti-peptide antibodies in protection from clinical malaria. Antibody levels from 268 children in the first cohort (Chonyi) were assayed against 70 peptides. Thirty-nine peptides were selected for further study in a second cohort (Junju). The rationale for the second cohort was to confirm those peptides identified as protective in the first cohort. The Junju cohort comprised of children aged 1-6 years old (inclusive). Children were actively followed up to identify episodes of febrile malaria in both cohorts. Of the 70 peptides examined, 32 showed significantly (p<0.05) increased antibody recognition in older children and 40 showed significantly increased antibody recognition in parasitaemic children. Ten peptides were associated with a significantly reduced odds ratio (OR) for an episode of clinical malaria in the first cohort of children and two of these peptides (LR146 and AS202.11) were associated with a significantly reduced OR in both cohorts. LR146 is derived from hypothetical protein PFB0145c in PlasmoDB. Previous work has identified this protein as a target of antibodies effective in antibody dependent cellular inhibition (ADCI). The current study substantiates further the potential of protein PFB0145c and also identifies protein PF11_0424 as another likely target of protective antibodies against P. falciparum malaria

Human Gene Expression in Uncomplicated Plasmodium falciparum Malaria.

To examine human gene expression during uncomplicated P. falciparum malaria, we obtained three samples (acute illness, treatment, and recovery) from 10 subjects and utilized each subject's recovery sample as their baseline. At the time of acute illness (day 1), subjects had upregulation of innate immune response, cytokine, and inflammation-related genes (IL-1β, IL-6, TNF, and IFN-γ), which was more frequent with parasitemias >100,000 per μL and body temperatures ≥39°C. Apoptosis-related genes (Fas, BAX, and TP53) were upregulated acutely and for several days thereafter (days 1-3). In contrast, the expression of immune-modulatory (transcription factor 7, HLV-DOA, and CD6) and apoptosis inhibitory (c-myc, caspase 8, and Fas Ligand G) genes was downregulated initially and returned to normal with clinical recovery (days 7-10). These results indicate that the innate immune response, cytokine, and apoptosis pathways are upregulated acutely in uncomplicated malaria with concomitant downregulation of immune-modulatory and apoptosis inhibitory genes.

Immunogenicity of dimorphic and C-terminal fragments of Plasmodium falciparum MSP2 formulated with different adjuvants in mice.

BACKGROUND: Plasmodium falciparum MSP2 is a blood stage protein that is associated with protection against malaria. It was shown that the MSP2 dimorphic (D) and constant (C) regions were well recognized by immune human antibodies, and were characterized by major conserved epitopes in different endemic areas and age groups. These Abs recognized merozoite-derived proteins in WB and IFA. Here, the goal was to determine in mice the immunogenicity of the two allelic MSP2 D and C domains formulated with different adjuvants, for their possible use in future clinical studies. METHOD: Female A/J, C3H, and ICR mice were immunized subcutaneously 3 times at 3-week interval with a mixture of allelic and conserved MSP2 long synthetic peptides formulated with different adjuvants. One week after the third injection, sera from each group were obtained and stored at -20°C for subsequent testing. RESULTS: Both domains of the two MSP2 families are immunogenic and the fine specificity and intensity of the Ab responses are dependent on mouse strains and adjuvants. The major epitopes were restricted to the 20-mer peptide sequences comprising the last 8aa of D and first 12aa of C of the two allelic families and the first 20aa of the C region, this for most strains and adjuvants. Strong immune responses were associated with GLA-SE adjuvant and its combination with other TLR agonists (CpG or GDQ) compared to alhydrogel and Montanide. Further, the elicited Abs were also capable of recognizing Plasmodium-derived MSP2 and inhibiting parasite growth in ADCI. CONCLUSION: The data provide a valuable opportunity to evaluate in mice different adjuvant and antigen formulations of a candidate vaccine containing both MSP2 D and C fragments. The formulations with GLA-SE seem to be a promising option to be compared with the alhydrogel one in human clinical trials.

Characterization of Plasmodium falciparum glutamate dehydrogenase-soluble antigen

The major aim of this study was to characterize a soluble Plasmodium falciparum antigen from the plasma of malaria-infected humans and Plasmodium falciparum culture supernatants, using immunoabsorbent techniques and Western blotting. An Mr 60-kDa protein was isolated from the plasma of patients with Plasmodium falciparum malaria by affinity chromatography using rabbit anti-Proteus spp GDH(NADP+) serum as ligand. This protein, present in plasma of patients with acute Plasmodium falciparum infection, in Plasmodium falciparum culture supernatants, and in immune complexes, was tested with Plasmodium falciparum malaria hyperimmune serum from patients living in hyperendemic areas and rabbit anti-Proteus spp GDH(NADP+) serum prepared in the laboratory. In this report, we describe the results of a study showing that parasite GDH(NADP+) can be used to detect the presence of Plasmodium falciparum. It appears that this technique permits the chromatographic detection of a Plasmodium falciparum excretion antigen that may be used in the production of monoclonal antibodies to improve immunodiagnostic assays for the detection of antigenemia, and opens the possibility of its use as a non-microscopic screening method.