Protective Effects of Gonococcal OpcA Protein

Background: Gonorrhoea is one of the most common STDs worldwide.  Currently there is no effective vaccine for protection against gonococcal infections.  Many vaccine candidates have proven ineffective, because of phase and antigenic variations, thus failing to provide protection against diverse bacterial strains.

Objectives: To evaluate vaccine potential of gonococcal OpcA protein by determining its sequence diversity and protective effects.

Methods: 210 diverse gonococcal strains were screened for presence of an opcA gene.  The opcA gene was further sequenced and compared with previously described opcA sequences in the databases.  Recombinant gonococcal OpcA protein was prepared and used for mouse immunizations.  The OpcA-induced antibody responses were evaluated by ELISA and bactericidal assays.

Results: The opcA gene was detected in all gonococcal strains examined in this study (100%).  Among 792 bp coding region of the opcA, only 14 sites were identified as polymorphic sites (1.77%).  This low level sequence variation suggests that gonococcal OpcA protein has less antigenic variation than other surface proteins and may therefore be more suitable as a vaccine candidate.  The mouse immunization study showed that gonococcal OpcA protein was highly immunogenic and induced strong systematic and mucosal antibody responses.  OpcA-specific antibody was found bactericidal when the OpcA protein was administrated intranasally with suitable adjuvant.

Conclusions: This study demonstrated that the gonococcal opcA gene is highly conserved in the gonococcal population.  Moreover, gonococcal OpcA protein induced a bactericidal antibody response in mice, suggesting that the OpcA protein is an excellent vaccine candidate for protection against this widespread disease.

Implications for Programs, Policy, and Research: An effective vaccine could be developed based on gonococcal OpcA protein for prevention and control of gonococcal infections.