Bovine respiratory tract disease is a multi-factorial disease complex involving several viruses and bacteria. Viruses that play prominent roles in causing the bovine respiratory disease complex include bovine herpesvirus-1, bovine respiratory syncytial virus, bovine viral diarrhea virus and parinfluenza-3 virus. Bacteria that play prominent roles in this disease complex areMannheimia haemolyticaandMycoplasma bovis. Other bacteria that infect the bovine respiratory tract of cattle areHistophilus(Haemophilus)somniandPasteurella multocida. Frequently, severe respiratory tract disease in cattle is associated with concurrent infections of these pathogens. Like other pathogens, the viral and bacterial pathogens of this disease complex have co-evolved with their hosts over millions of years. As much as the hosts have diversified and fine-tuned the components of their immune system, the pathogens have also evolved diverse and sophisticated strategies to evade the host immune responses. These pathogens have developed intricate mechanisms to thwart both the innate and adaptive arms of the immune responses of their hosts. This review presents an overview of the strategies by which the pathogens suppress host immune responses, as well as the strategies by which the pathogens modify themselves or their locations in the host to evade host immune responses. These immune evasion strategies likely contribute to the failure of currently-available vaccines to provide complete protection to cattle against these pathogens.

Significance: Contrary to the paradigm, the signal peptide of CD18, an adhesive protein expressed on the surface of leukocytes of cattle and other ruminants, is not cleaved. Intriguingly, leukotoxin produced byMannheimia haemolyticabinds to the intact signal peptide and causes lysis of ruminant leukocytes, resulting in acute pneumonia. In this proof-of-principle study, we used precise gene editing to induce a single amino acid substitution in CD18, which resulted in the cleavage of the signal peptide and abrogation of leukotoxin-induced cytolysis of leukocytes of the gene-edited bovine fetus. This report demonstrates the feasibility of developing lines of cattle genetically resistant toM. haemolytica-caused pneumonia, which will have significant impact on the sustainability of food-animal production in the United States and elsewhere.

Mannheimia(Pasteurella)haemolyticais the only pathogen that consistently causes severe bronchopneumonia and rapid death of bighorn sheep (BHS;Ovis canadensis) under experimental conditions. Paradoxically,Bibersteinia(Pasteurella)trehalosiandPasteurella multocidahave been isolated from BHS pneumonic lungs much more frequently thanM. haemolytica. These observations suggest that there may be an interaction between these bacteria, and we hypothesized thatB. trehalosiovergrows or otherwise inhibits the growth ofM. haemolytica. Growth curves (monoculture) demonstrated thatB. trehalosihas a shorter doubling time (∼10 min versus ∼27 min) and consistently achieves 3-log higher cell density (CFU/ml) compared toM. haemolytica. During cocultureM. haemolyticagrowth was inhibited whenB. trehalosientered stationary phase (6 h) resulting in a final cell density forM. haemolyticathat was 6 to 9 logs lower than expected with growth in the absence ofB. trehalosi. Coculture supernatant failed to inhibitM. haemolyticagrowth on agar or in broth, indicating no obvious involvement of lytic phages, bacteriocins, or quorum-sensing systems. This observation was confirmed by limited growth inhibition ofM. haemolyticawhen both pathogens were cultured in the same media but separated by a filter (0.4-μm pore size) that limited contact between the two bacterial populations. There was significant growth inhibition ofM. haemolyticawhen the populations were separated by membranes with a pore size of 8 μm that allowed free contact. These observations demonstrate thatB. trehalosican both outgrow and inhibitM. haemolyticagrowth with the latter related to a proximity- or contact-dependent mechanism.

Signal peptides of membrane proteins are cleaved by endoplasmic reticulum-resident signal peptidase, and thus, are not present on mature membrane proteins. Here, we report that, contrary to the paradigm, the signal peptide of ruminant CD18, the β-subunit of β₂-integrins, is not cleaved. Intriguingly, the intact signal peptide of CD18 is responsible for the susceptibility of ruminant leukocytes toMannheimia(Pasteurella)haemolyticaleukotoxin (Lkt). Inhibition of Lkt-induced cytolysis of ruminant leukocytes by CD18 peptide analogs revealed that the Lkt-binding site is formed by amino acids 5–17 of CD18, which, surprisingly, comprise most of the signal sequence. Flow cytometric analysis of ruminant leukocytes indicated the presence of the signal peptide on mature CD18 molecules expressed on the cell surface. Analysis of transfectants expressing CD18 containing the FLAG epitope at the putative cleavage site confirmed that the signal peptide of bovine CD18 is not cleaved. Analysis of the signal sequence of CD18 of eight ruminants and five nonruminants revealed that the signal sequence of CD18 of ruminants contains “cleavage-inhibiting” Q, whereas that of nonruminants contains “cleavage-conducive” G at position −5 relative to the cleavage site. Site-directed mutagenesis of Q to G at position −5 of the signal peptide of bovine CD18 resulted in the cleavage of the signal peptide and abrogation of cytolysis of transfectants expressing bovine CD18 carrying the Q(−5)G mutation. We propose that engineering cattle and other ruminants to contain this mutation would provide a novel technology to render them less susceptible to pneumonic pasteurellosis and concomitant economic losses.

This report describes the genome sequences of Mannheimia haemolytica serotype A2 isolated from pneumonic lungs of two different ruminant species, one from Ovis aries , designated ovine (O), and the other from Bos taurus , designated bovine (B).