Immune responses induced by gene gun or intramuscular injection of DNA vaccines that express immunogenic regions of the serine repeat antigen from Plasmodium falciparum

Abstract

The liver- and blood-stage-expressed serine repeat antigen (SERA) of Plasmodium falciparum is a candidate protein for a human malaria vaccine. We compared the immune responses induced in mice immunized with SERA-expressing plasmid DNA vaccines delivered by intramuscular (i.m.) injection or delivered intradermally by Gene Gun immunization. Mice were immunized with a pcdna3 plasmid encoding the entire 47-kDa domain of SERA (amino acids 17 to 382) or the N-terminal domain (amino acids 17 to 110) of SERA. Minimal antibody responses were detected following DNA vaccination with the N-terminal domain of SERA, suggesting that the N-terminal domain alone is not highly immunogenic by this route of vaccine delivery. Immunization of mice by Gene Gun delivery of the 47-kDa domain of SERA elicited a significantly higher serum antibody titer to the antigen than immunization of mice by i.m. injection with the same plasmid did. The predominant isotype subclass of the antibodies elicited to the SERA protein following i.m. and Gene Gun immunizations with SERA plasmid DNA was immunoglobulin G1. Coimmunization of mice with SERA plasmid DNA and a plasmid expressing the hepatitis B surface antigen (pCMV-s) by the i.m. route resulted in higher anti-SERA titers than those generated in mice immunized with the SERA DNA plasmid alone. Vaccination with DNA may provide a viable alternative or may be used in conjunction with protein-based subunit vaccines to maximize the efficacy of a human malaria vaccine that includes immunogenic regions of the SERA protein.

FIG. 1

Antibody titers from mice immunized with pcdna3 SERA 17-382 or pcdna3 SERA 17-110 DNA vaccines by i.m. injection. Where indicated, mice groups were coimmunized with the pCMV-s DNA plasmid, which was used as a positive control. Mice were immunized at 0, 5, and 8 weeks, and serum samples were collected at week 10. Serial dilutions were used to measure the end point-positive titers. The results presented are the averages of two separate ELISAs. The titer of each serum sample is the inverse of the largest dilution at which the substrate colorometric development is above that of a negative control. The variation in the end point-positive titers was always within one “twofold serial dilution.” Preimmune serum titers for the SERA protein were <80 in this assay.

FIG. 2

Antibody titers from mice immunized with pcdna3 SERA 17-382 or pcdna3 SERA 17-110 DNA vaccines by intradermal Gene Gun injection. Serum samples were collected at weeks 4, 8, 12, and 23 from mice immunized at weeks 0 and 4. Serial dilutions were used to measure the end point-positive titers. The results presented are the averages of two separate ELISAs. The titer of each serum sample is the inverse of the largest dilution at which the substrate colorometric development is above that of a negative control. The variation in the end point-positive titers was always within one “twofold serial dilution.” Preimmune serum titers for the SERA protein were <80 in this assay.

FIG. 3

Antibody titers from mice immunized with the pcdna3 SERA 17-382 DNA vaccine. Mice were immunized by i.m. injection or Gene Gun, as indicated. Serum was collected at weeks 6 and 8; all mice were immunized at weeks 0 and 4. Serial dilutions were used to measure the end point-positive titers. The results presented are the averages of two separate ELISAs. The titer of each serum sample is the inverse of the largest dilution at which the substrate colorometric development is above that of a negative control. The variation in the end point-positive titers was always within one “twofold serial dilution.” Preimmune serum titers for the SERA protein were <80 in this assay.