Mucosal Immunization with Spore-Based Vaccines against <i>Mannheimia haemolytica</i> Enhances Antigen-Specific Immunity

Background: <i>Mannheimia haemolytica</i> is a bovine respiratory pathogen commonly associated with bacterial bronchopneumonia. Current vaccine strategies have shown variable efficacy in feedlot cattle, and therefore novel vaccines are needed. <i>Bacillus subtilis</i> spores have been investigated as a mucosal vaccine platform, due to their ability to bind and present antigens to the mucosa and act as an adjuvant. The aim of this study was to develop two spore-based mucosal vaccines targeting <i>M. haemolytica</i> and evaluate their immunogenicity in mice. Methods: Two antigen constructs composed of cholera toxin B subunit, <i>M. haemolytica</i> leukotoxin, and either the <i>M. haemolytica</i> outer membrane protein PlpE (MhCP1) or GS60 (MhCP2) were synthesized, purified and then bound to spores as vaccines. In two separate mice trials, the spore-bound vaccines (Spore-MhCP1 and Spore-MhCP2) were administered to mice through intranasal and intragastric routes, while free antigens were administered intranasally and intramuscularly. Unbound spores were also evaluated intranasally. Antigen-specific serum IgG and mucosal IgA from bronchoalveolar lavage, feces, and saliva were measured after vaccination. Mice sera from all treatment groups were assessed for their bactericidal activity against <i>M. haemolytica</i>. Results: In both mice experiments, intramuscular immunization induced the strongest serum IgG antibody response. However, the intranasal administration of Spore-MhCP1 and Spore-MhCP2 elicited the greatest secretory IgA-specific response against leukotoxin, PlpE, and GS60 in bronchoalveolar lavage, saliva, and feces (<i>p</i> < 0.05). Compared to the intranasal administration of free antigen, spore-bound antigen groups showed greater bactericidal activity against <i>M. haemolytica</i> (<i>p</i> < 0.05). Conclusions: Since intranasally delivered Spore-MhCP1 and Spore-MhCP2 elicited both systemic and mucosal immune responses in mice, these vaccines may have potential to mitigate lung infection in cattle by restricting <i>M. haemolytica</i> colonization and proliferation in the respiratory tract. The efficacy of these mucosal spore-based vaccines merits further assessment against <i>M. haemolytica</i> in cattle.