Streptococcus pneumoniae persists as a leading cause of bacterial pneumonia despite the widespread use of polysaccharide-based vaccines. The limited serotype coverage of current vaccines has led to increased incidence of nonvaccine serotypes, as well as an increase in antibiotic resistance among these serotypes. Pneumococcal infection often follows a primary viral infection such as influenza virus, which hinders host defense and results in bacterial spread to the lungs. We previously isolated human monoclonal Abs (mAbs) against the conserved surface Ag pneumococcal histidine triad protein D (PhtD), and we demonstrated that mAbs to this Ag are protective against lethal pneumococcal challenge prophylactically and therapeutically. In this study, we elucidated the mechanism of protection of a protective anti-pneumococcal human mAb, PhtD3, which is mediated by the presence of complement and macrophages in a mouse model of pneumococcal infection. Treatment with mAb PhtD3 reduced blood and lung bacterial burden in mice, and mAb PhtD3 is able to bind to bacteria in the presence of the capsular polysaccharide, indicating exposure of surface PhtD on encapsulated bacteria. In a mouse model of secondary pneumococcal infection, protection mediated by mAb PhtD3 and another mAb targeting a different epitope, PhtD7, was reduced; however, robust protection was restored by combining mAb PhtD3 with mAb PhtD7, indicating a synergistic effect. Overall, these studies provide new insights into anti-pneumococcal mAb protection and demonstrate, to our knowledge, for the first time, that mAbs to pneumococcal surface proteins can protect against secondary pneumococcal infection in the mouse model.

Streptococcus pneumoniae persists as a leading cause of bacterial pneumonia despite the widespread use of polysaccharide-based vaccines. The limited serotype coverage of current vaccines has led to increased incidence of non-vaccine serotypes, as well as an increase in antibiotic resistance among these serotypes. Pneumococcal infection often follows a primary viral infection such as influenza virus, which hinders host defense and results in bacterial spread to the lungs. We previously isolated human monoclonal antibodies (mAbs) against the conserved surface antigen pneumococcal histidine triad protein D (PhtD), and we demonstrated that mAbs to this antigen are protective against lethal pneumococcal challenge prophylactically and therapeutically. In this study, we elucidated the mechanism of protection of a protective anti- pneumococcal human mAb, PhtD3, which is mediated by the presence of complement and macrophages in a mouse model of pneumococcal infection. Treatment with mAb PhtD3 reduced blood and lung bacterial burden in mice, and mAb PhtD3 is able to bind to bacteria in the presence of the capsular polysaccharide, indicating exposure of surface PhtD on encapsulated bacteria. In a mouse model of secondary pneumococcal infection, protection mediated by mAb PhtD3 and another mAb targeting a different epitope, PhtD7, was reduced, however, robust protection was restored by combining mAb PhtD3 with mAb PhtD7, indicating a synergistic effect. Overall, these studies provide new insights into anti-pneumococcal mAb protection and demonstrate for the first time that mAbs to pneumococcal surface proteins can protect against secondary pneumococcal infection in the mouse model.Author SummaryThe persistence of Streptococcus pneumoniae as a leading cause of bacterial pneumonia despite numerous approved pneumococcal vaccines is a serious threat to public health globally. Currently, prophylactic and therapeutic options for Streptococcus pneumoniae are constrained by the limited serotype coverage of vaccines and the emergence of antibiotic resistant strains. An additional hurdle to overcome is the incidence of secondary pneumococcal infection following a viral infection, which leads to increased mortality. Here, we determined the mechanism of action of a monoclonal antibody (mAb) that targets Streptococcus pneumoniae. We found that mAb PhtD3 operates through macrophage and complement mediated functions. mAb PhtD3 was also discovered to reduce bacterial titers in the lungs and blood and bind to a related antigen PhtE. We also tested additional mAbs and discovered that two unique mAbs to the antigen PhtD conferred protection in a pneumococcal-influenza virus co-infection model. Our study provides new insights into the mechanisms and therapeutic potential of mAbs targeting conserved proteins of Streptococcus pneumoniae.