High-level production and purification of P30P2MSP1(19), an important vaccine antigen for malaria, expressed in the methylotropic yeast Pichia pastoris.

P30P2MSP1(19) is a recombinant subunit vaccine derived from merozoite surface protein 1 (MSP1) of Plasmodium falciparum, the causative agent of malaria. P30P2MSP1(19) consists of two universal T-cell epitopes fused to the most C-terminal 19-kDa portion of MSP1, and this protein has previously shown promising potential as a vaccine for malaria. However, previous attempts at producing this molecule in Saccharomyces cerevisiae resulted in the production of a truncated form of the molecule missing most of the universal T-cell epitopes. Here, we report the production of full-length P30P2MSP1(19) in Pichia pastoris. As salt precipitation is a common problem during P. pastoris high-density fermentation, we utilized an alternative low-salt, fully defined medium that did not reduce growth rates or biomass yields to avoid precipitation. A total of 500 mg/L of secreted purified protein was produced in high cell density fermentation and the protein was purified in one step utilizing nickel-chelate chromatography. P30P2MSP1(19) produced in Pichia was reactive with monoclonal antibodies that recognize only conformational epitopes on correctly folded MSP1. Rabbits immunized with this molecule generated higher and more uniform antibody titers than rabbits immunized with the protein produced in Saccharomyces. P30P2MSP1(19) produced in Pichia may prove to be a more efficacious vaccine than that produced in Saccharomyces and Pichia would provide a system for the cost-effective production of such a vaccine.

Efficacy of two alternate vaccines based on Plasmodium falciparum merozoite surface protein 1 in an Aotus challenge trial.

In an attempt to produce a more defined, clinical-grade version of a vaccine based on Plasmodium falciparum merozoite surface protein 1 (MSP1), we evaluated the efficacy of two recombinant forms of MSP1 in an Aotus nancymai challenge model system. One recombinant vaccine, bvMSP1(42), based on the 42-kDa C-terminal portion of MSP1, was expressed as a secreted protein in baculovirus-infected insect cells. A highly pure baculovirus product could be reproducibly expressed and purified at yields in excess of 8 mg of pure protein per liter of culture. This protein, when tested for efficacy in the Aotus challenge model, gave significant protection, with only one of seven monkeys requiring treatment for uncontrolled parasitemia after challenge with P. falciparum. The second recombinant protein, P30P2MSP1(19), has been used in previous studies and is based on the smaller, C-terminal 19-kDa portion of MSP1 expressed in Saccharomyces cerevisiae. Substantial changes were made in its production process to optimize expression. The optimum form of this vaccine antigen (as judged by in vitro and in vivo indicators) was then evaluated, along with bvMSP1(42), for efficacy in the A. nancymai system. The new formulation of P30P3MSP1(19) performed significantly worse than bvMSP1(42) and appeared to be less efficacious than we have found in the past, with four of seven monkeys in the vaccinated group requiring treatment for uncontrolled parasitemia. With both antigens, protection was seen only when high antibody levels were obtained by formulation of the vaccines in Freund's adjuvant. Vaccine formulation in an alternate adjuvant, MF59, resulted in significantly lower antibody titers and no protection.

Structural conformers produced during malaria vaccine production in yeast.

A recombinant protein expression system based on Saccharomyces cerevisiae has been used to express malarial vaccine candidate antigens. The antigens so produced have been used in three Phase 1 clinical trials and numerous preclinical non-human primate trials. Further Phase I trials are planned using these candidate vaccine antigens. These molecules were identified as attractive candidates for antimalarial vaccines, as they are all surface-exposed at some stage in the parasite's life cycle. They all share an unusual structural feature: epidermal growth factor (EGF)-like motifs. When these proteins are expressed in our S. cerevisiae expression system, they are produced as a series of stable structural conformers, each with a different disulphide bonding pattern. This leads to both biochemical and, more importantly, antigenic differences between the conformers (e.g. presence or absence of an antibody B cell epitope). These findings have important ramifications for other EGF-domain-containing proteins expressed in S. cerevisiae, or for proteins which contain other cysteine-folding motifs not normally expressed by this organism, both for vaccine production or for research/reagent purposes.