Comparative innate immune responses of bison and cattle to Mannheimia haemolytica wildtype and LPS sialylation-deficient mutant strains.

Cattle macrophages possess an enhanced ability to phagocyte some type of bacteria compared to American bison (Bison bison) macrophages. However, it remains unclear whether observed phagocytic differences between the two related ruminant species are restricted to E. coli. Although not frequently reported, bison are susceptible to pneumonia caused by Mannheimia haemolytica. Previously, we showed that M. haemolytica LPS sialylation-deficient mutant (ΔneuA) was more sensitive to phagocytic (neutrophils and monocytes) and complement-mediated killing than were the wildtype parent strain. Therefore, the goal of this study was to compare the phagocytic- and complement-mediated killing of M. haemolytica wildtype and ΔneuA strains between bison and cattle. Relatively higher percentages of reactive oxygen species positive neutrophils and monocytes were found in cattle compared to bison. Significant uptake of M. haemolytica wildtype and ΔneuA mutant strains by both monocytes and neutrophils in cattle were observed, compared to bison as assessed by flow cytometry (p < 0.05). However, both species showed similar phagocytic- and complement-mediated killing in which ΔneuA mutant was more sensitive to killing than the wildtype strain (p < 0.0001). Complement-mediated ΔneuA mutant killing disappeared in both species when the serum was heat inactivated. Taken together, these findings suggested that despite differences in phagocytosis efficiencies and uptake, there is no significant difference between phagocytic- and complement-mediated killing of M. haemolytica ΔneuA mutant between two related ruminant species.