Purification, characterization, and immunogenicity of a disulfide cross-linked Plasmodium vivax vaccine candidate antigen, merozoite surface protein 1, expressed in Escherichia coli

The Plasmodium vivax merozoite surface protein 1 (MSP-1) 42-kDa fragment (P vMSP-1 p42) is a promising vaccine candidate antigen against the blood stag e of the malarial parasite. We have developed a process for the production of this vaccine target, keeping in mind its use in human volunteers. A nove l strain, Origami (DE3), of Escherichia coli with mutations in the glutathi one and thioredoxin reductase genes yielded 60% more soluble PvMSP-1 p42 th an the conventional E. coli BL21(DE3) strain. Recombinant PvMSP-1 p42 was p urified to greater than or equal to 99% purity with a rapid two-step protoc ol designed for easy scaling up. The final product had a low endotoxin cont ent and was stable in its lyophilized form. PvMSP-1 p42 was found to have t he predicted primary and tertiary structures and consisted of a single conf ormer containing one free cysteine, as predicted. The product was recognize d by conformational monoclonal antibodies against P. vivax MSP-1. Immunogen icity studies of PvMSP-1 p42 were carried out with two strains of mice and the adjuvants Montanide ISA51 and Montanide ISA720. Both formulations were found to induce high levels of immunoglobulin G1 (IgG1), IgG2b, and IgG2a a ntibodies along with low levels of IgG3. Lymphocytes from animals in all th e PvMSP-1 p42-immunized groups showed proliferative responses upon stimulat ion with PvMSP-1 p42; the cytokines interleukin 2 (IL-2), gamma interferon, IL-4, and IL-10 were detected in the culture supernatants. These results i ndicate that PvMSP-1 p42 in combination with both of the adjuvants elicited cellular and humoral responses in mice.