In silico Analysis of Surface Proteins of Streptococcus pneumoniae Serotype 19F for Identification of Immunoprotective Epitopes

Abstract

Pneumococcal conjugate vaccines (PCVs) were developed through chemical coupling of polysaccharide capsules of pneumococci to immunogenic carrier proteins and World Health Organization recommends inclusion of these vaccines in national immunization programs for children. However, the PCVs implementation in developing countries can be prevented by the high manufacturing costs. This issue can be overcome by construction of protein based vaccines against pneumococci. We already identified three pneumococcal surface proteins including D-alanyl-D-alanine-carboxy peptidase (DDCP), choline binding protein D (CBPD), and cell wall surface anchor family protein (CWSAP) as appropriate protein candidates for eliciting protection against S. pneumoniae serotype 19F. The protein protective antigenicity, the absence of autoimmunity induction, and the amino acid sequence conservancy in serotype 19F pneumococcal strains were used as selection criteria. However, regarding the requirement of both antibody and cellular immune responses for protection against pneumococci, analysis of protective B and T-cell epitopes of these proteins is necessary to examine their usefulness in new vaccine formulations. In the present study, therefore, we aim to identify protective epitopes of these proteins via widely used bioinformatic tools. The Bepipred program was used for identification of linear B-cell epitopes. The conformational B-cell epitopes were predicted using the CBTope program. T-cell epitopes were identified using the Immune Epitope Database tool. The immunoprotective abilities of epitopes were evaluated using VaxiJen. Our results showed that all of the three studied proteins included protective epitopes. However, the greatest number of epitopes was identified in a truncated form of CWSAP. Moreover, the most probable immunoprotective epitopes reside in this protein and these epitopes were highly conserved in CWSAPs of the most common pneumococcal serotypes in the world. Therefore, the truncated CWSAP was an appropriate candidate for development of protein based vaccines against the most common pneumococcal serotypes.