Effect of a spore-based Mannheimia haemolytica vaccine on immune responses and respiratory microbiota in sheep.

Mannheimia haemolytica is an opportunistic pathogen associated with respiratory disease in ruminants. Current vaccines provide incomplete protection, highlighting the need for improved immunization strategies. In this study, a mucosal vaccine was developed using Bacillus subtilis spores as an adjuvant and evaluated for its effects on immune responses and respiratory microbiota in sheep. A chimeric protein (MhCP) containing neutralizing epitopes from leukotoxin A (NLKT) and outer membrane protein PlpE was expressed and adsorbed onto spores to produce Spore-MhCP, which was administered via two mucosal routes: intranasal and intragastric. Unbound MhCP was delivered intranasally and intramuscularly, while unbound spores and saline were used as controls. Intranasal Spore-MhCP generated the strongest secretory IgA-specific responses against PlpE and NLKT in nasal swab, bronchoalveolar lavage, and fecal samples. It also elicited earlier and sustained serum IgG responses among mucosal immunization groups. Notably, intragastric vaccination also increased PlpE- and NLKT-specific antibodies in lung and fecal samples. From nasopharyngeal samples, 16S rRNA gene sequencing revealed 27 genera altered in the intranasal Spore-MhCP group, including a decrease in Mannheimia (days 14-35; p < 0.01). These findings indicate that intranasal Spore-MhCP enhances immunity and may reduce M. haemolytica proliferation in the upper respiratory tract, reducing lung infection risk.