Mucosal Immunization with an Unadjuvanted Vaccine That Targets Streptococcus pneumoniae PspA to Human Fcγ Receptor Type I Protects against Pneumococcal Infection through Complement- and Lactoferrin-Mediated Bactericidal Activity

Abstract

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␥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␥R type I (hFc␥RI). A humanized single-chain antibody component in which the variable domain binds to hFc␥RI [antihFc␥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␥RI-PspA or recombinant PspA (rPspA) alone was used to intranasally immunize wild-type (WT) and hFc␥RI transgenic (Tg) mice in the absence of adjuvant. The hFc␥RI Tg mice receiving anti-hFc␥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␥RI as the anti-hFc␥RI-PspA fusion, enhanced protection against lethal S. pneumoniae challenge was observed in the hFc␥RI Tg mice compared to mice given nontargeted rPspA alone. Immune sera from the anti-hFc␥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.

Pertussis Toxin Improves Immune Responses to a Combined Pneumococcal Antigen and Leads to Enhanced Protection against Streptococcus pneumoniae

“…This result supports the importance of the complement system in the protection elicited by PspA5-wP. Similarly, a nasal vaccine composed of a hybrid protein that targets PspA to the human Fc␥ receptor type I (anti-hFc␥RI-PspA) induced high levels of anti-PspA antibodies and complement-dependent protection against a challenge with a serotype 3 pneumococcal strain in transgenic mice expressing hFc␥RI (40).…”

“…Cobra venom factor (CVF) from Naja naja kaouthia (Quidel Corporation, Darmstadt, Germany) was used for the depletion of complement in vivo, as described previously (40,41), and depletion was confirmed by the absence of rabbit erythrocyte hemolytic activity in the sera of mice treated with CVF (41). BALB/c mice (14 animals) were immunized with the PspA5-wP formulation using the sixdose schedule.…”

Modified Opsonization, Phagocytosis, and Killing Assays To Measure Potentially Protective Antibodies against Pneumococcal Surface Protein A

“…These properties of PspA make it likely that the mediation of phagocytosis in vivo is a major protective mechanism of immunity to PspA. Antibodies to PspA also enhance the killing of pneumococci by the antibacterial peptides of apolactoferrin (22,37).…”

Controlled Inflammatory Responses in the Lungs Are Associated with Protection Elicited by a Pneumococcal Surface Protein A-Based Vaccine against a Lethal Respiratory Challenge with Streptococcus pneumoniae in Mice

“…Broad-coverage vaccines based on PspA would thus depend on the use of more than one molecule or on the choice of specific PspA molecules (37). In animal models, protection elicited by vaccines composed of PspA is often accompanied by the induction of high levels of specific antibodies (10,22,24,44) which, upon binding to pneumococcal surface, promote the deposition of complement (9,12,47,57) and enhance killing by lactoferrin (9,48). In addition, the use of adjuvants that elicit Th1 immune responses against PspA seems to optimize protection (4,19,20).…”