Lack of safe and effective mucosal adjuvants has severely hampered the development of mucosal subunit vaccines. In this regard, we have previously shown that immunogenicity of vaccine antigens can be improved by targeting the antigens to the antigen-presenting cells. Specifically, groups of mice immunized intranasally with a fusion protein (Bivalent-FP) containing a fragment of pneumococcal-surface-protein-A (PspA) as antigen and a single-chain bivalent antibody raised against the anti-human Fc-gamma-receptor-I (hFcγRI) elicited protective immunity to pulmonary Streptococcus pneumoniae infection. In order to further enhance the immunogenicity, an additional hFcγRI-binding moiety of the single chain antibody was incorporated. The modified vaccine (Trivalent-FP) induced significantly improved protection against lethal pulmonary S. pneumoniae challenge compared to Bivalent-FP. In addition, the modified vaccine exhibited over 85% protection with only two immunizations. Trivalent-FP also induced S. pneumoniae-specific systemic and mucosal antibodies. Moreover, Trivalent-FP also induced IL-17-and IL-22-producing CD4 + T cells. Furthermore, it was found that the hFcγRI facilitated uptake and presentation of Trivalent-FP. In addition, Trivalent-FP also induced IL-1α, MIP-1α, and TNF-α; modulated recruitment of dendritic cells and macrophages; and induced CD80/86 and MHC-II expression on antigen presenting cells.With over 90 known serotypes, Streptococcus pneumoniae remains the leading cause of community acquired pneumoniae. In addition, younger and immunocompromised populations are more vulnerable to infection. Furthermore, S. pneumoniae causes over 1 million deaths per year in children aged less than 5 years [10]. To date, two polysaccharide-based subunit vaccines are available to combat select serotypes. However, use of these 13 and 23 serotype vaccines cause serotype replacement in the vaccinated population [11]. This results in a surge of non-vaccine serotypes within the vaccinated population. Thus, new approaches to pneumococcal vaccines are required, which can generate protection over multiple serotypes. The pneumococcal surface protein A (PspA) has been known to induce cross-serotype protection against S. pneumoniae [12]. Thus, not only is S. pneumoniae a significant global health problem, it also expresses a well-defined protective protein antigen, making it a particularly suitable model to test and optimize our mucosal subunit vaccine platform.Previously we have shown that targeting vaccine antigens to antigen-presenting cells (APCs) eliminates the need for adjuvants.By genetically fusing a bivalent single-chain variable fragment-based antibody (Bivalent anti-human-Fc-gamma-receptor-I (FcγRI)), which specifically recognizes human-FcγRI, to a pneumococcal antigen (PspA), a fusion protein named Bivalent-FP was obtained ( Figure 1A). Bivalent-FP induces systemic and mucosal antibodies and protection against pulmonary S. pneumoniae infection by intranasal immunization. However, this vaccine requires at least three i...