PCV13 (13-valent pneumococcal conjugate vaccine) containing capsular polysaccharides of Streptococcus pneumoniae serotypes 6A and 6B confers cross-protection against newly discovered serotypes 6C and 6D

Objective To investigate whether capsular polysaccharides of Streptococcus pneumoniae serotypes 6A and 6B contained in 13-valent pneumococcal conjugate vaccine (PCV13) could induce cross-protective antibodies against newly discovered serotypes 6C and 6D and the differences between them. Methods New Zealand rabbits were radomly divided into three groups and respectively muscularly administrated with three doses of PCV13, PCV6A and PCV6B on days 0, 14 and 28. PCV6A and PCV6B were conjugates of capsular polysaccharides of serotypes 6A and 6B chemically coupled with diphtheria toxin mutant CRM197. Serum samples were collected on days 0 and 35. Enzyme-linked immunosorbent assay (ELISA) recommended by World Health Organization (WHO) was used to quantitatively measure serotype-specific antibodies to capsular polysaccharides of serotypes 6A, 6B, 6C and 6D. Opsonophagocytosis assay (OPA) of WHO pneumococcal serology reference laboratory was used to determine antibody functional activities targeting serotypes 6A, 6B, 6C and 6D. Results Immunization rabbits with PCV13 induced the secretion of antibodies to capsular polysaccharides of serotypes 6A and 6B. These antibodies were able to not only cross-react with capsular polysaccharides of serotypes 6C and 6D but also recognize and bind to target Streptococcus pneumoniae serotypes 6A, 6B, 6C and 6D, resulting in the activation of complements and further phagocytosis of target bacteria by differentiated HL60 cells. Bactericidal titers were largely even among these serotypes except for serotype 6D which was slightly lower. PCV6A could induce antibody against capsular polysaccharide of serotype 6A, which was able to cross-react with capsular polysaccharides of serotypes 6B, 6C and 6D and showed higher bactericidal titers to serotypes 6A, 6B and 6C over serotype 6D. PCV6B could induce antibody against capsular polysaccharide of serotype 6B, which was able to cross-react with capsular polysaccharides of serotypes 6A, 6C and 6D and showed higher bactericidal titers to serotypes 6A, 6B and 6C over serotype 6D. Antibody concentrations and bactericidal titers specific to serotypes 6A, 6B, 6C and 6D were significantly increased following immunization with PCV13, PCV6A or PCV6B (P<0.01). Conclusion PCV13 containing pneumococcal serotypes 6A and 6B induced antibodies against capsular polysaccharides of serotypes 6A and 6B in New Zealand rabbits, which were able to cross-react with capsular polysaccharides of serotypes 6C and 6D and provide cross-protection to bacteria of serotypes 6C and 6D. Both serotypes of 6A and 6B contained in PCV13 contributed to the induction of cross-protective antibodies, especially to serotype 6C. Key words: 13-valent pneumococcal conjugate vaccine (PCV13); Serotype 6C; Serotype 6D; Enzyme-linked immunosorbent assay (ELISA); Opsonophagocytosis assay (OPA); Cross-protective antibody