Genetic vaccination against malaria infection by intradermal and epidermal injections of a plasmid containing the gene encoding the Plasmodium berghei circumsporozoite protein

Abstract

The circumsporozoite protein (CSP) from the surface of sporozoite stage Plasmodium sp. malaria parasites is among the most important of the malaria vaccine candidates. Gene gun injection of genetic vaccines encoding Plasmodium berghei CSP induces a significant protective effect against sporozoite challenge; however, intramuscular injection does not. In the present study we compared the immune responses and protective effects induced by P. berghei CSP genetic vaccines delivered intradermally with a needle or epidermally with a gene gun. Mice were immunized three times at 4-week intervals and challenged by a single infectious mosquito bite. Although 50 times more DNA was administered by needle than by gene gun, the latter method induced significantly greater protection against infection. Intradermal injection of the CSP genetic vaccine induced a strong Th1-type immune response characterized by a dominant CSP-specific immunoglobulin G2a (IgG2a) humoral response and high levels of gamma interferon produced by splenic T cells. Gene gun injection induced a predominantly Th2-type immune response characterized by a high IgG1/IgG2a ratio and significant IgE production. Neither method generated measurable cytotoxic T lymphocyte activity. The results indicate that a gene gun-mediated CS-specific Th2-type response may be best for protecting against malarial sporozoite infection when the route of parasite entry is via mosquito bite.

FIG. 1

Gene gun immunization induces a superior antibody response compared to needle injection into the same tissue. Immunizations were performed four times at 4-week intervals. BALB/c mice (four mice/group) received either 100 μg of pCMV-TPA/CS in saline by needle injection or 2 μg of pCMV-TPA/CS by the gene gun method. Sera were collected from individual mice and analyzed for anti-CS repeat epitope IgG antibody by ELISA. Data are shown as the mean ± the SEM of endpoint titers of each group.

FIG. 2

Effect of the route of immunization on the subclass distribution of antibodies in serum after needle (a) and gene gun (b) immunization. PbCSP-repeat epitope-specific IgG1 and IgG2a levels were measured at weeks 4, 6, 10, 12, and 14. The results are presented as the mean ± the SEM of each group (n = 4 mice), expressed in kilophoton counts/second.

FIG. 3

CS-specific IgE (a) and IgM (b) titers of the respective immunization groups. Sera taken at weeks 0, 4, 6, 8, 10, and 14 for IgE and 0, 3, 4, 5, 9, and 13 for IgM were evaluated by ELISA. The results are presented as the mean ± the SEM of each group (n = 4 mice), expressed in kilophoton counts/second.