Genetic immunization with the region encoding the alpha-helical domain of PspA elicits protective immunity against Streptococcus pneumoniae

Abstract

Pneumococcal surface protein A (PspA) is a pneumococcal virulence factor capable of eliciting protection against pneumococcal infection in mice. Previous studies have demonstrated that the protection is antibody mediated. Here we examined the ability of pspA to elicit a protective immune response following genetic immunization of mice. Mice were immunized by intramuscular injections with a eukaryotic expression vector encoding the alpha-helical domain of PspA/Rx1. Immunization induced a PspA-specific serum antibody response, and immunized mice survived pneumococcal challenge. Survival and antibody responses occurred in a dose-dependent manner, the highest survival rates being seen with doses of 10 microg or greater. The ability of genetic immunization to elicit cross-protection was demonstrated by the survival of immunized mice challenged with pneumococcal strains differing in capsule and PspA types. Also, immunized mice were protected from intravenous and intratracheal challenges with pneumococci. Similar to the results seen with immunization with PspA, the survival of mice genetically immunized with pspA was antibody mediated. There was no decline in the level of protection 7 months after immunization. These results support the use of genetic immunization to elicit protective immune responses against extracellular pathogens.

FIG. 1

Immunoblot of HeLa cell lysates tested with anti-PspA monoclonal antibody XiR278. The indicated cell lysates or purified full-length PspA/Rx1 (rightmost lane) were reacted with XiR278. The pJB100 lane shows a band of the expected size of about 43 kDa. A 43 kDa band in the pJB10 lane was visible only in the original blot.

FIG. 2

Immunization with pJB100. CBA/N mice were immunized with 50 μg of either pJB100 or pNGVL3 (control). Mice were challenged i.v. with pneumococcal strain WU2. Data represent the results for five or more mice per group.

FIG. 3

Dose-dependent antibody responses. CBA/N mice were immunized with the indicated amounts of pJB100 or 50 μg of pNGVL3 (control). Serum samples collected on day 28 were analyzed by ELISA to determine the antibody responses. Antibody concentrations were calculated based on a standard curve generated using an anti-PspA antiserum of known concentration. Data represent the results for four mice per group. Error bars show standard errors of the means.

FIG. 4

Dose-dependent survival. CBA/N mice were immunized with the indicated amounts of pJB100 or 50 μg of pNGVL3 (control). Mice were challenged on day 35 with pneumococcal strain WU2. Data represent the results for four mice per group.

FIG. 5

Anti-PspA antibody responses following immunization with pJB100. CBA/N mice were immunized on days 0, 14, and 28 with pJB100 (10 [triangles] or 50 [squares] μg) or pNGVL3 (50 μg) (circles). Data represent the results for seven or more mice per group. Error bars show standard errors of the means.

FIG. 6

i.t. challenge of immunized mice. Mice were immunized with 10 μg of pJB100 or pNGVL3 (control) and challenged on day 35 with pneumococcal strain A66. Data represent the results for six mice per group.