A DNA vaccine encoding the 42 kDa C-terminus of merozoite surface protein 1 of Plasmodium falciparum induces antibody, interferon-γ and cytotoxic T cell responses in rhesus monkeys: immuno-stimulatory effects of granulocyte macrophage-colony stimulating factor

Abstract

We have constructed a DNA plasmid vaccine encoding the C-terminal 42-kDa region of the merozoite surface protein1 (pMSP1 42) from the 3D7 strain of Plasmodium falciparum (Pf3D7). This plasmid expressed recombinant MSP1 42 after in vitro transfection in mouse VM92 cells. Rhesus monkeys immunized with pMSP1 42 produced antibodies reactive with Pf3D7 infected erythrocytes by IFAT, and by ELISA against yeast produced MSP1 19 (yMSP1 19). Immunization also induced antigen specific T cell responses as measured by interferon-γ production, and by classical CTL chromium release assays. In addition, immunization with pMSP1 42 primed animals for an enhanced antibody response to a subsequent boost with the recombinant yMSP1 19. We also evaluated Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF) as an adjuvant for pMSP1 42. We tested both rhesus GM-CSF expressed from a DNA plasmid, and E. coli produced recombinant human GM-CSF. Plasmids encoding rhesus GM-CSF (prhGM-CSF) and human GM-CSF (phuGM-CSF) were constructed; these plasmids expressed bio-active recombinant GMCSF. Co-immunization with a mixture of prhGM-CSF and pMSP1 42 induced higher specific antibody responses after the first dose of plasmid, but after three doses of DNA monkeys immunized with or without prhGM-CSF had the same final antibody titers and T cell responses. In comparison, rhuGM-CSF protein did not lead to accelerated antibody production after the first DNA dose. However, antibody titers were maintained at a slightly higher level in monkeys receiving GM-CSF protein, and they had a higher response to boosting with recombinant MSP1 19. The GM-CSF plasmid or protein appears to be less potent as an adjuvant in rhesus monkeys than each is in mice, and more work is needed to determine if GM-CSF can be a useful adjuvant in DNA vaccination of primates.