Synthesis, Conformational Properties, and Immunogenicity of a Cyclic Template-Bound Peptide Mimetic Containing an NPNA Motif from the Circumsporozoite Protein of Plasmodium falciparum

Peptides for Vaccine Development

This review discusses peptide epitopes used as antigens in the development of vaccines in clinical trials as well as future vaccine candidates. It covers peptides used in potential immunotherapies for infectious diseases including SARS-CoV-2, influenza, hepatitis B and C, HIV, malaria, and others. In addition, peptides for cancer vaccines that target examples of overexpressed proteins are summarized, including human epidermal growth factor receptor 2 (HER-2), mucin 1 (MUC1), folate receptor, and others. The uses of peptides to target cancers caused by infective agents, for example, cervical cancer caused by human papilloma virus (HPV), are also discussed. This review also provides an overview of model peptide epitopes used to stimulate non-specific immune responses, and of self-adjuvanting peptides, as well as the influence of other adjuvants on peptide formulations. As highlighted in this review, several peptide immunotherapies are in advanced clinical trials as vaccines, and there is great potential for future therapies due the specificity of the response that can be achieved using peptide epitopes.

Strategies for Fine-Tuning the Conformations of Cyclic Peptides

Cyclic peptides are promising scaffolds for drug development, attributable in part to their increased conformational order compared to linear peptides. However, when optimizing the target-binding or pharmacokinetic properties of cyclic peptides, it is frequently necessary to "fine-tune" their conformations, e.g., by imposing greater rigidity, by subtly altering certain side chain vectors, or by adjusting the global shape of the macrocycle. This review systematically examines the various types of structural modifications that can be made to cyclic peptides in order to achieve such conformational control.

Structure-Based Design of Inhibitors of Protein-Protein Interactions: Mimicking Peptide Binding Epitopes

Protein-protein interactions (PPIs) are involved at all levels of cellular organization, thus making the development of PPI inhibitors extremely valuable. The identification of selective inhibitors is challenging because of the shallow and extended nature of PPI interfaces. Inhibitors can be obtained by mimicking peptide binding epitopes in their bioactive conformation. For this purpose, several strategies have been evolved to enable a projection of side chain functionalities in analogy to peptide secondary structures, thereby yielding molecules that are generally referred to as peptidomimetics. Herein, we introduce a new classification of peptidomimetics (classes A-D) that enables a clear assignment of available approaches. Based on this classification, the Review summarizes strategies that have been applied for the structure-based design of PPI inhibitors through stabilizing or mimicking turns, β-sheets, and helices.

Malaria vaccine development using synthetic peptides as a technical platform

The review covers the development of synthetic peptides as vaccine candidates for Plasmodium falciparum- and Plasmodium vivax-induced malaria from its beginning up to date and the concomitant progress of solid phase peptide synthesis (SPPS) that enables the production of long peptides in a routine fashion. The review also stresses the development of other complementary tools and actions in order to achieve the long sought goal of an efficacious malaria vaccine.

Structure of the Plasmodium falciparum circumsporozoite protein, a leading malaria vaccine candidate

The Plasmodium falciparum circumsporozoite protein (CSP) is critical for sporozoite function and invasion of hepatocytes. Given its critical nature, a phase III human CSP malaria vaccine trial is ongoing. The CSP is composed of three regions as follows: an N terminus that binds heparin sulfate proteoglycans, a four amino acid repeat region (NANP), and a C terminus that contains a thrombospondin-like type I repeat (TSR) domain. Despite the importance of CSP, little is known about its structure. Therefore, recombinant forms of CSP were produced by expression in both Escherichia coli (Ec) and then refolded (EcCSP) or in the methylotrophic yeast Pichia pastoris (PpCSP) for structural analyses. To analyze the TSR domain of recombinant CSP, conformation-dependent monoclonal antibodies that recognized unfixed P. falciparum sporozoites and inhibited sporozoite invasion of HepG2 cells in vitro were identified. These monoclonal antibodies recognized all recombinant CSPs, indicating the recombinant CSPs contain a properly folded TSR domain structure. Characterization of both EcCSP and PpCSP by dynamic light scattering and velocity sedimentation demonstrated that both forms of CSP appeared as highly extended proteins (R(h) 4.2 and 4.58 nm, respectively). Furthermore, high resolution atomic force microscopy revealed flexible, rod-like structures with a ribbon-like appearance. Using this information, we modeled the NANP repeat and TSR domain of CSP. Consistent with the biochemical and biophysical results, the repeat region formed a rod-like structure about 21-25 nm in length and 1.5 nm in width. Thus native CSP appears as a glycosylphosphatidylinositol-anchored, flexible rod-like protein on the sporozoite surface.

A virosomal malaria peptide vaccine elicits a long-lasting sporozoite-inhibitory antibody response in a phase 1a clinical trial

OBJECTIVES

Peptides delivered on the surface of influenza virosomes have been shown to induce solid humoral immune responses in experimental animals. High titers of peptide-specific antibodies were also induced in a phase 1a clinical trial in volunteers immunized with virosomal formulations of two peptides derived from the circumsporozoite protein (CSP) and the apical membrane antigen 1 (AMA-1) of Plasmodium falciparum. The main objective of this study was to perform a detailed immunological and functional analysis of the CSP-specific antibodies elicited in this phase 1a trial.

METHODOLOGY/PRINCIPAL FINDINGS

46 healthy malaria-naïve adults were immunized with virosomal formulations of two peptide-phosphatidylethanolamine conjugates, one derived from the NANP repeat region of P. falciparum CSP (designated UK-39) the other from P. falciparum AMA-1 (designated AMA49-C1). The two antigens were delivered in two different concentrations, alone and in combination. One group was immunized with empty virosomes as control. In this report we show a detailed analysis of the antibody response against UK-39. Three vaccinations with a 10 microg dose of UK-39 induced high titers of sporozoite-binding antibodies in all volunteers. This IgG response was affinity maturated and long-lived. Co-administration of UK-39 and AMA49-C1 loaded virosomes did not interfere with the immunogenicity of UK-39. Purified total IgG from UK-39 immunized volunteers inhibited sporozoite migration and invasion of hepatocytes in vitro. Sporozoite inhibition closely correlated with titers measured in immunogenicity assays.

CONCLUSIONS

Virosomal delivery of a short, conformationally constrained peptide derived from P. falciparum CSP induced a long-lived parasite-inhibitory antibody response in humans. Combination with a second virosomally-formulated peptide derived from P. falciparum AMA-1 did not interfere with the immunogenicity of either peptide, demonstrating the potential of influenza virosomes as a versatile, human-compatible antigen delivery platform for the development of multivalent subunit vaccines.

TRIAL REGISTRATION

ClinicalTrials.gov NCT00400101.

A randomized placebo-controlled phase Ia malaria vaccine trial of two virosome-formulated synthetic peptides in healthy adult volunteers

Background and Objectives

Influenza virosomes represent an innovative human-compatible antigen delivery system that has already proven its suitability for subunit vaccine design. The aim of the study was to proof the concept that virosomes can also be used to elicit high titers of antibodies against synthetic peptides. The specific objective was to demonstrate the safety and immunogenicity of two virosome-formulated P. falciparum protein derived synthetic peptide antigens given in two different doses alone or in combination.

Methodology/Principal Findings

The design was a single blind, randomized, placebo controlled, dose-escalating study involving 46 healthy Caucasian volunteers aged 18–45 years. Five groups of 8 subjects received virosomal formulations containing 10 µg or 50 µg of AMA 49-CPE, an apical membrane antigen-1 (AMA-1) derived synthetic phospatidylethanolamine (PE)-peptide conjugate or 10 ug or 50 ug of UK39, a circumsporozoite protein (CSP) derived synthetic PE-peptide conjugate or 50 ug of both antigens each. A control group of 6 subjects received unmodified virosomes. Virosomal formulations of the antigens (designated PEV301 and PEV302 for the AMA-1 and the CSP virosomal vaccine, respectively) or unmodified virosomes were injected i. m. on days 0, 60 and 180. In terms of safety, no serious or severe adverse events (AEs) related to the vaccine were observed. 11/46 study participants reported 16 vaccine related local AEs. Of these 16 events, all being pain, 4 occurred after the 1^st, 7 after the 2^nd and 5 after the 3^rd vaccination. 6 systemic AEs probably related to the study vaccine were reported after the 1^st injection, 10 after the 2^nd and 6 after the 3^rd. Generally, no difference in the distribution of the systemic AEs between either the doses applied (10 respectively 50 µg) or the synthetic antigen vaccines (PEV301 and PEV302) used for immunization was found. In terms of immunogenicity, both PEV301 and PEV302 elicited already after two injections a synthetic peptide-specific antibody response in all volunteers immunized with the appropriate dose. In the case of PEV301 the 50 µg antigen dose was associated with a higher mean antibody titer and seroconversion rate than the 10 µg dose. In contrast, for PEV302 mean titer and seroconversion rate were higher with the lower dose. Combined delivery of PEV301 and PEV302 did not interfere with the development of an antibody response to either of the two antigens. No relevant antibody responses against the two malaria antigens were observed in the control group receiving unmodified virosomes.

Conclusions

The present study demonstrates that three immunizations with the virosomal malaria vaccine components PEV301 or/and PEV302 (containing 10 µg or 50 µg of antigen) are safe and well tolerated. At appropriate antigen doses seroconversion rates of 100% were achieved. Two injections may be sufficient for eliciting an appropriate immune response, at least in individuals with pre-existing anti-malarial immunity. These results justify further development of a final multi-stage virosomal vaccine formulation incorporating additional malaria antigens.

Trial Registration

ClinicalTrials.gov NCT00400101

Structure-activity-based design of a synthetic malaria peptide eliciting sporozoite inhibitory antibodies in a virosomal formulation

The circumsporozoite protein (CSP) of Plasmodium falciparum is a leading candidate antigen for inclusion in a malaria subunit vaccine. We describe here the design of a conformationally constrained synthetic peptide, designated UK-39, which has structural and antigenic similarity to the NPNA-repeat region of native CSP. NMR studies on the antigen support the presence of helical turn-like structures within consecutive NPNA motifs in aqueous solution. Intramuscular delivery of UK-39 to mice and rabbits on the surface of reconstituted influenza virosomes elicited high titers of sporozoite crossreactive antibodies. Influenza virus proteins were crucially important for the immunostimulatory activity of the virosome-based antigen delivery system, as a liposomal formulation of UK-39 was not immunogenic. IgG antibodies elicited by UK-39 inhibited invasion of hepatocytes by P. falciparum sporozoites, but not by antigenically distinct P. yoelii sporozoites. Our approach to optimized virosome-formulated synthetic peptide vaccines should be generally applicable for other infectious and noninfectious diseases.

Synthesis of 10- and 9-membered dilactams derived from aspartic and glutamic acids

9- and 10-membered bridged dipeptides derived from L-aspartic acid and L- or D-glutamic acid were synthesized using aminoacyl incorporation reaction. Key intermediates containing internal pyroglutamyl moiety were prepared via side chain to backbone cyclization of related protected dipeptide derivatives of glutamic acid.

A new strategy for improved nicotine vaccines using conformationally constrained haptens

Constrained nicotine analogues were synthesized and coupled to the carrier protein KLH. The immunogenic effects were compared to those using our previously designed flexible nicotine hapten. Immunization of mice with the constrained hapten conjugates resulted in highly increased antibody titers and affinity for nicotine.

Non-covalent stabilization of a beta-hairpin peptide into liposomes

An oligopeptide modified on both the N- and C-termini with hydrophobic moieties was prepared on a solid phase and anchored into a liposome, stabilizing the fold of the peptide into a beta-hairpin, which would otherwise be a random coil.

Structure of a malaria parasite antigenic determinant displayed on filamentous bacteriophage determined by NMR spectroscopy: implications for the structure of continuous peptide epitopes of proteins

The NANP repeating sequence of the circumsporozoite protein of Plasmodium falciparum was displayed on the surface of fd filamentous bacteriophage as a 12-residue insert (NANP)(3) in the N-terminal region of the major coat protein (pVIII). The structure of the epitope determined by multidimensional solution NMR spectroscopy of the modified pVIII protein in lipid micelles was shown to be a twofold repeat of an extended and non-hydrogen-bonded loop based on the sequence NPNA, demonstrating that the repeating sequence is NPNA, not NANP. Further, high resolution solid-state NMR spectra of intact hybrid virions containing the modified pVIII proteins demonstrate that the peptides displayed on the surface of the virion adopt a single, stable conformation; this is consistent with their pronounced immunogenicity as well as their ability to mimic the antigenicity of their native parent proteins.

Structure of the YSPTSPS repeat containing two SPXX motifs in the CTD of RNA polymerase II: NMR studies of cyclic model peptides reveal that the SPTS turn is more stable than SPSY in water

The carboxyl-terminal domain of RNA polymerase II, which is rich in phosphorylation sites, contains 17--52 tandem repeats with the consensus sequence of the heptapeptide, YSPTSPS. The repeat unit of the heptapeptide has two SPXX motifs showing potential beta-turns, SPTS and SPSY. NMR studies were performed in water at pH 4.0 for two cyclic peptides containing one and two repeat units, cyclo-[C(1)R(2)D(3)Y(4)S(5)P(6)T(7)S(8)P(9)S(10)Y(11)S(12)R(13)D(14)C(15)] (peptide 1) and cyclo-[C(1)R(2)D(3)Y(4)S(5)P(6)T(7)S(8)P(9)S(10)Y(11)S(12)P(13)T(14)S(15)P(16)N(17)Y(18)S(19)R(20)D(21)C(22)] (peptide 2), which are cyclized with a disulfide bridge of two Cys residues at the N- and C-termini. SP in 1 and 2 are predominantly in trans form. The following NMR parameters were detected: (1) lower temperature coefficients of amide proton chemical shifts of T7 and S8 in 1, and Tx (T7 or T14), Sx (S8 or S15), Tz (T14 or T7) and Sz (S15 or S8) in 2, (2) significantly large deviation of H(alpha) chemical shifts from its random coil value (Delta H(alpha)) of Pro preceding the Thr (P6 in 1, and Px and Pz in 2), (3) relatively large (3)J(HNH alpha) coupling constants (>8.7 Hz) of T7 in 1 and Tx and Tz in 2, and (4) NOE (d(NN) (i, i+1)) connectivities between the amide protons of T7-S8 and S10-Y11 in 1, and Tx-Sx, S10-Y11, Tz-Sz, and N17-Y18 in 2, although two Pro-Thr-Ser segments in 2 (each of these are annotated by 'x' and 'z') in the first and second repeat units were not distinguishable. Comparison of the NMR parameters between the cyclic peptides and the corresponding linear peptides indicates that cyclization promotes structural stabilization in water. The present NMR data were consistent with the presence of a beta-turn at both SPTS and SPSY: S(5)P(6)T(7)S(8) and S(8)P(9)S(10)Y(11) in 1, and SPxTxSx, SPzTzSz, SP(9)S(10)Y(11), SP(16)N(17)Y(18) in 2. However, the structure of the SPTS segment is more stable than that of the SPSY segment. Conformations consistent with NMR parameters including NOE distances were obtained through molecular dynamics and energy minimization methods. These calculations yielded two stable conformers for the SPTS segment. One of the two corresponds to a type I beta-turn.

Structural features of a chimeric peptide inducing cytotoxic T lymphocyte responses in saline

Little information is available correlating the structural properties of peptides with their immunogenicity in terms of responses via cytotoxic T lymphocytes (CTLs). The TT-NP6 chimeric peptide, consisting of two copies of a promiscuous T-helper epitope (T: residues 288-302 from the fusion protein of the measles virus) linked to the NP6 T-cytotoxic epitope (NP6: residues 52-60 from the nucleoprotein of measles virus) was able to induce virus-specific CTL responses in the absence of any adjuvant and hydrophobic component. The present work was undertaken to gain insight into structural features of the TT-NP6 peptide that may be important in optimizing the CTL immunogenicity of the peptide. Circular dichroism data, obtained in a buffer of physiological ionic strength and pH, strongly suggest a self-associated state for the peptide, which was confirmed by a sedimentation velocity experiment. However, helix association is accompanied by loss of overall helical content. Thermal-dependence studies show that the unfolding of self-associated alpha-helices is significantly more pronounced than the unfolding of isolated alpha-helices. Circular dichroism data, together with tryptic limited proteolysis, suggest the presence of a charged amino acid within the hydrophobic core. This study should provide a basis for engineering more effective immunogenic peptides against the measles virus by increasing the stability of the TT-NP6 peptide.