Protection against Pneumococcal Colonization and Fatal Pneumonia by a Trivalent Conjugate of a Fusion Protein with the Cell Wall Polysaccharide

Cell wall polysaccharide (CWPS), pneumolysin, and surface adhesin A (PsaA) are antigens common to virtually all serotypes of Streptococcus pneumoniae (pneumococcus), and all have been studied separately for use in protection. Previously we showed that protection against nasopharyngeal (NP) colonization by intranasal vaccination of mice with killed pneumococci is mediated by TH17 cells and correlates with interleukin-17A (IL-17A) expression by T cells in vitro; we have also shown that CWPS and other species-common antigens protect against colonization by a similar mechanism. Here we made a fusion protein of PsaA with the pneumolysin nontoxic derivative PdT and then coupled CWPS to the fusion protein, aiming to enhance immune responses to all three antigens. When given intranasally with cholera toxin adjuvant, the fusion conjugate induced higher serum antibody titers and greater priming for IL-17A responses than an equimolar mixture of the three antigens. The conjugate administered intranasally protected mice against experimental NP colonization by a strain of serotype 6B, while mice immunized with the mixture or with bivalent conjugates were not protected. Subcutaneous immunization with the conjugate and alum adjuvant likewise induced higher antibody titers than the mixture, primed for IL-17A responses, and reduced colonization. The conjugate, but not the antigen mixture, fully protected mice from fatal pneumonia caused by a highly virulent serotype 3 strain. Thus, a covalent construct of three antigens common to all serotypes exhibits protection with both mucosal and systemic administration.