Mucosal Immunization With An Unadjuvanted Vaccine That Targets Streptococcus Pneumoniae Pspa To Human Fc Gamma Receptor Type I Protects Against Pneumococcal Infection Through Complement- And Lactoferrin-Mediated Bactericidal Activity

Targeting an antigen to Fc receptors (FcR) can enhance the immune response to the antigen in the absence of adjuvant. Furthermore, we recently demonstrated that intranasal immunization with an Fc gamma R-targeted antigen enhances protection against a category A intracellular mucosal pathogen, Francisella tularensis. To determine if a similar strategy could be applied to the important pathogen Streptococcus pneumoniae, we used an improved mucosal FcR-targeting strategy that specifically targets human Fc gamma R type I (hFc gamma RI). A humanized single-chain antibody component in which the variable domain binds to hFc gamma RI [anti-hFc gamma RI (H22)] was linked in a fusion protein with the pneumococcal surface protein A (PspA). PspA is known to elicit protection against pneumococcal sepsis, carriage, and pneumonia in mouse models when administered with adjuvants. Anti-hFc gamma RI-PspA or recombinant PspA (rPspA) alone was used to intranasally immunize wild-type (WT) and hFc gamma RI transgenic (Tg) mice in the absence of adjuvant. The hFc gamma RI Tg mice receiving anti-hFc gamma RI-PspA exhibited elevated S. pneumoniae-specific IgA, IgG2c, and IgG1 antibodies in serum and bronchoalveolar lavage fluid. Neither immunogen was effective in protecting WT mice in the absence of adjuvant, but when PspA was targeted to hFc gamma RI as the anti-hFc gamma RI-PspA fusion, enhanced protection against lethal S. pneumoniae challenge was observed in the hFc gamma RI Tg mice compared to mice given nontargeted rPspA alone. Immune sera from the anti-hFc gamma RI-PspA-immunized Tg mice showed enhanced complement C3 deposition on bacterial surfaces, and protection was dependent upon an active complement system. Immune serum also showed an enhanced bactericidal activity directed against S. pneumoniae that appears to be lactoferrin mediated.