Serotype-independent protection against pneumococcal infections elicited by intranasal immunization with ethanol-killed pneumococcal strain, SPY1.

The 23-valent polysaccharide vaccine and the 7-valent pneumococcal conjugate vaccine are licensed vaccines that protect against pneumococcal infections worldwide. However, the incidence of pneumococcal diseases remains high in low-income countries. Whole-cell vaccines with high safety and strong immunogenicity may be a favorable choice. We previously obtained a capsule-deficient Streptococcus pneumoniae mutant named SPY1 derived from strain D39. As an attenuated live pneumococcal vaccine, intranasal immunization with SPY1 elicits broad serotype-independent protection against pneumococcal infection. In this study, for safety consideration, we inactivated SPY1 with 70% ethanol and intranasally immunized BALB/c mice with killed SPY1 plus cholera toxin adjuvant for four times. Results showed that intranasal immunization with inactivated SPY1 induced strong humoral and cellular immune responses. Intranasal immunization with inactivated SPY1 plus cholera toxin adjuvant elicited effective serotype-independent protection against the colonization of pneumococcal strains 19F and 4 as well as lethal infection of pneumococcal serotypes 2, 3, 14, and 6B. The protection rates provided by inactivated SPY1 against lethal pneumococcal infection were comparable to those of currently used polysaccharide vaccines. In addition, vaccine-specific B-cell and T-cell immune responses mediated the protection elicited by SPY1. In conclusion, the 70% ethanol-inactivated pneumococcal whole-cell vaccine SPY1 is a potentially safe and less complex vaccine strategy that offers broad protection against S. pneumoniae.