A Nontoxic Adjuvant for Mucosal Immunity to Pneumococcal Surface Protein A

In this study, we demonstrated that pneumococcal surface protein A (PspA) nasally administered with a nontoxic A subunit mutant of cholera toxin (mCT) S61F elicited a protective immune response. Immunization with PspA and mCT elicited higher levels of PspA-specific IgG and IgA Abs in serum and of IgG and IgA anti-PspA Ab-forming cells in spleens, cervical lymph nodes (CLN), and lung tissue when compared to nonimmunized mice. Furthermore, significant PspA-specific IgA Abs were induced in saliva and nasal secretions. These responses were dependent on the use of mCT as a mucosal adjuvant. The PspA-specific Ab responses induced by mCT S61F were comparable with those induced by native CT (nCT). Analysis of cytokine responses showed that nasal PspA plus mCT S61F enhanced the induction of PspA-specific CD4+ T cells producing IL-4 but not IFN-γ in CLN at both the protein and mRNA levels. Importantly, significant numbers of mice intranasally immunized with PspA plus mCT S61F were protected from lethal challenge with capsular serotype 3 Streptococcus pneumoniae A66. These results show that intranasal administration of PspA together with mCT S61F is an effective mucosal vaccine against pneumococcal infection and induces CD4+ Th2-type cells, which provide help for both mucosal and systemic Ab responses.