Peptide-Protein Conjugate Vaccines

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Bacillus anthracis: A recombinant PA was isolated from an uncapsulated strain. Several formaldehyde treated and/or alum adsorbed formulations were immunogenic in mice. These formulations were safe in adult volunteers. Local and systemic reactions were rare and minor. Their serum IgG anti PA levels are being analyzed. Serum IgG anti PA was measured in 246 sera of recruits injected with the Anthrax Vaccine Adsorbed (AVA) stored at the Department of Defense Serum Repository. Paired sera were analyzed by ELISA. Serum conversion rates of greater than or equal to a 4-fold increase in antibody levels were: pre-post 3rd 85.3%, pre 4th-post 4th 67.9% and pre 6th-post 6th 45%. Geometric mean levels of all individuals were 59.9 mcg/mL following the 3rd injection, 157.4 mcg/mL following the 4th and 277 mcg/mL following the 6th. The capsule has been isolated from a non toxic strain and it or corresponding synthetic peptides were bound to BSA, rEPA, rPA or tetanus toxoid. Thioether, hydrazone and oxime linkages between the PGA and the proteins, with active groups at the C or N termini, yielded conjugates immunogenic in mice, with no statistical difference between them. The induced antibodies were opsonophagocytic. Peptides 10 to 20-mer long, and 10-15 mole PGA per mole protein were the most immunogenic. Dose response experiments of an rPA-PGA conjugate, using between 0.31 to 20 mcg PGA/mouse showed 1.25 mcg to be optimal for a PGA response, while PA antibody levels increased with higher immunizing dosages. The use of alum increased PA antibody levels while having little effect upon anti PGA levels. Chimpanzees were immunized, 10 mcg PGA/animal, sc, one with PA-PGA another with TT-PGA, with the goal of preparing humanized monoclonal antibodies to PGA. Both chimps responded with antibodies to both vaccine components. Higher anti PGA levels were obtained in the TT-PGA injected chimp. PGS specific IgG1 and IgG3 were prepared. The glycosyl part of BclA is an oligosaccharide composed of 2-O-methyl-4-(3-hydroxy-3-methylbutanamido)-4,6-dideoxy-D-glucose referred to as anthrose, and three rhamnose residues. Similar structures to anthrose, were found in (1) the side-chain of the capsular polysaccharide (CP) of Shewanella spp. MR-4: 4-(3-hydroxy-3-methylbutanamido)-4,6-dideoxy-D-glucose. Under certain growth conditions the bacteria produce a variant CP lacking one methyl group on the hydroxybutyrate: 4-(3-hydroxybutanamido)-4,6-dideoxy-D-glucose. Contrary to anthrose, neither of the Shewanella CPs is 2-O-methylated. Both Shewanella CPS variants reacted with anti-B. anthracis spore sera. (2) flagellae of Pseudomonas syringae, reported to be glycosylated with a similar terminal saccharide, 4-(3-hydroxybutanamido)-4,6-dideoxy-2-O-methyl-D-glucose. Sera produced by immunization with Shewanella or P. syringae cells bound to B. anthracis spores but not to B. cereus spores in a fluorescent microscopy assay. Protein conjugates of the two variants of Shewanella CP induced antibodies that bound to both Shewanella CP variants, by ELISA, and to B. anthracis spores, detected by fluorescent microscopy. We propose the use of Shewanella CP conjugates as a component of an anthrax vaccine. The structure of Shewanella MR-4 LPS carbohydrate backbone was elucidated. No anthrose-like sugar was found, confirming the presence of such sugar in the CP only. Plasmodium falciparum: The most studied experimental malaria vaccine is the circumsporozoite protein (CSP), expressed extracellularly on the sporozoite, and various forms of its synthesized repeat unit, NANP. These vaccines were safe, immunogenic but poorly protective and of limited duration, even when administered with adjuvants. We used two approaches to provide experimental malaria vaccines: (1) directed to the asexual, human stage of the parasite; CSP was cloned into e.coli, isolated and several formulations, including adsorption onto alum, were evaluated in young outbred mice. Based on our studies with peptides of the B. anthracis capsule, peptides of 4-5 repeat units of NANP were synthesized and boundnanp- to carrier proteins at different molar ratios and end groups. Injected into general purpose mice, all tested preparations induced high levels of antibodies that bound to circumsporozoites in IFA. NANP-Pfs25 conjugates induced long lasting antibodies to both vaccine components; 3 months serum levels higher than 1 week after the last injection. Alum adsorbed CSP induced higher antibody levels than the non-adsorbed protein. The terminal amino acid of the CSP-derived peptides was shown to be an important determinant, with NPNA-protein being the best immunogen. (2) directed to the sexual, mosquito parasite stage, to provide a transmission blocking vaccine. Pfs25, a low molecular mass protein, non immunogenic by itself, was bound onto itself or to carrier proteins by amide, hydrazone or thioether linkages. Injected into mice, all conjugates were immunogenic with booster responses upon reinjection. Remarkably, the serum antibody levels 3 and 7 months after immunization were higher than 1 week after the last injection. The best immunogens used adipic acid dihydrazide as the linker. Adsorption of the conjugates onto alum increased further the antibody levels. Transmission blocking activity of immune sera correlated with antibody levels measured by ELISA.