Fabiola Espejo

The first field trials of the chemically synthesized malaria vaccine SPf66: safety, immunogenicity and protectivity

This paper reports the results of the first field study performed to assess the safety, immunogenicity and protectivity of the synthetic malaria vaccine SPf66 directed against the asexual blood stages of Plasmodium falciparum. Clinical and laboratory tests were performed on all volunteers prior to and after each immunization, demonstrating that no detectable alteration was induced by the immunization process. The vaccines were grouped as high, intermediate or low responders according to their antibody titres directed against the SPf66 molecule. Two of the 185 (1.08%) SPf66-vaccinated and nine of the 214 (4.20%) placebo-vaccinated volunteers developed P. falciparum malaria. The efficacy of the vaccine was calculated as 82.3% against P. falciparum and 60.6% against Plasmodium vivax.

Genetic control of the immune response to a synthetic vaccine against Plasmodium falciparum

Summary Two independent vaccination trials using a hybrid synthetic poly pep‐tide containing epitopes from four proteins of Plasmodium falciparum were performed. In the first trial 63 and in the second 122 volunteers were vaccinated, using different immunization schedules. The analysis of the humoral response to the vaccine, measured by IgG antibody titres to the polypeptide showed a bimodal distribution in both cases suggesting genetic control of the immune response to this protein. There was a small group of low or non‐responders and a large group of good responders. HLA phenotyping of the two groups disclosed an association of the low responders to HLA‐DR4 antigens with chi‐square P value of 0.00039 when compared with the good responders group. These findings provide evidence for the genetic control of the immune response to the synthetic vaccine by the association of this response with particular alleles of the HLA class II antigens; such findings may lead to an explanation of the mechanism involved in disease susceptibility and need to be used in the design of a totally effective vaccine.

Alpha helix shortening in 1522 MSP‐1 conserved peptide analogs is associated with immunogenicity and protection against P. falciparum malaria

1522 is a nonimmunogenic conserved high‐activity binding peptide (HABP) belonging to Plasmodium falciparum MSP‐1 protein N‐terminal fragment. The key amino acids in binding to red blood cells (RBC) were identified and replaced by others having similar mass but different charge. Because conserved HABPs are not antigenic nor immunogenic, immunogenicity and protectivity studies were then conducted on them in the Aotus monkey. 1H‐NMR studies included the lead peptide 1522 as well as the analogs 9782, 13446, 13448, and 13442 to relate their structure to biological function. All the peptides presented α‐helical structure, with differences observed in helix location and extension. The nonprotective 1522 peptide was totally helical from the N‐ to the C‐terminus, very similar to nonprotective 13442 and 13448 peptides whose extension was almost totally helical. The 9782 and 13446 protective peptides, however, possessed a shorter helical region where modified critical binding residues were not included. A more flexible region was generated at the C‐terminus in those peptides with a shorter helical region, leading to a greater number of conformers. These data suggest that peptide flexibility results in increased interaction with immune system molecules, generating protective immunity. Proteins 2003;50:400–409.

Orientating peptide residues and increasing the distance between pockets to enable fitting into MHC-TCR complex determine protection against malaria.

The erythrocyte binding antigen EBA-175 is a 175-kDa Plasmodium falciparum protein, which has been shown to be involved in the process of invasion of erythrocytes. It has been found that conserved peptide 1818 belonging to this protein has high red blood cell binding capacity and plays an important role in the invasion process. This peptide is neither immunogenic nor protective. Peptide 1818 analogues had some of their previously recognized critical red blood cell binding residues substituted for amino acids having similar volume or mass but different polarity to make them fit into HLA-DRbeta(1)*1101 molecules; these 1818 peptide analogues were then synthesized and inoculated into Aotus nancymaae monkeys, generating different immunogenic and/or protective immune responses. Short structures such as 3(10)-helix, classical, or distorted type-III beta-turns were found in the immunogenic and protective peptides once the secondary structure had been analyzed by NMR and its structure correlated with its immunological properties. These data suggest that peptide flexibility may lead to better fitting into immune system molecules, therefore making them excellent candidates for consideration as components of a subunit-based, multicomponent synthetic antimalarial vaccine.

Reduced amide bond pseudopeptides induce antibodies against native proteins of Plasmodium falciparum

The malaria peptide coded 1513 whose primary structure is was designed from the amino acid sequence of Plasmodium falciparum protein MSP-1. The primary sequence of the binding motif KeKMVL of 1513 to RBCs is also contained in the structure of the SPf-66 synthetic malaria vaccine[l]. In order to explore the immunogenicity of 1513 peptide analogues containing reduced amide bonds, a set of peptide mimetics was synthesized and biochemically characterized. The analogues were synthesized by systematically replacing one CO-NH peptide bond at a time by a reduced amide isostere according to the Coy strategy [2]. To determine the bidimensional structure of each 1513 analogue studies were performed in a mixture of in a ratio 90/10 as well as in aqueous 30%TFE. NMR analysis performed for the Pse-437 analogue containing the bond between and showed features of secondary structure not yet detected for the parent non modified 1513 peptide. The recorded Pse-437 NMR data were processed for molecular modeling on an Indigo-II computer provided with a graphic package from Biosym/MSI. In the present work we propose a preliminary tridimensional molecular model for Pse-437. Polyclonal antibodies were obtained after immunization of BALB/c mice and New Zealand rabbits with oxidized pseudopeptide analogues of 1513 containing a cysteine residue at the N and C termini. Monoclonal antibodies to Pse-437 were obtained by standard cellular fusion of mice spleen cells to X-63Ag8 myeloma cells [3]. Seven immunoglobulin producing hybridomas to Pse-437 showed strong reactivity by ELISA and Western blot against Plasmodium falciparum protein MSP-1 as well as against the SPf-66 vaccine. Each reactive hybrid cell was cloned to obtain cross-reacting mAbs to 195 kDa, and 83 kDa proteins and SPf-66. We propose these second generation novel peptides as possible tools for the development of chemically synthesized Plasmodium falciparum malaria vaccines.