Bovine pneumonic pasteurellosis: Evaluation of interactions between bovine pulmonary lavage cells and Pasteurella haemolytica (biotype A, serotype 1), using a luminol-dependent chemiluminescence assay
Authors: Richards Alan B. , Renshaw Harland W. 
Published: February 2025

SUMMARY
In vitro interactions of bovine pulmonary lavage cells (plc) and pathogenic isolates of Pasteurella haemolytica biotype A, serotype 1, were examined, using a luminol-dependent chemiluminescence (ldcl) assay. The plc containing high concentrations of bovine alveolar macrophages were incubated with living and heat-killed Ρ haemolytica at bacteria to plc ratio of approximately 1:1. Kinetics of the mean ldcl response of bovine plc to heat-killed Ρ haemolytica cells were characterized by a gradual increase in the amount of light emitted over 150 minutes followed by a slight decrease at 180 minutes. In contrast, the ldcl responses of reaction mixtures containing living Ρ haemolytica were characterized by the development of a maximal response at 60 minutes followed by a continued precipitous decrease in light emission to background values by 150 minutes. Differences were not noticed in the ldcl response of plc suspensions from the same cow to 3 Ρ haemolytica isolates. In each instance, reaction mixtures containing heat-killed bacteria had a similar ldcl profile that was characterized by continuous production of light over 180 minutes, whereas all reaction mixtures containing living bacteria underwent a precipitous decrease in light emission, which eventually resulted in a complete cessation of chemiluminescence. The plc suspensions from different cattle did not respond to bacterial stimuli uniformly, with respect to the amplitude or detailed nature of the ldcl profile. The time that lapsed between the addition of living Ρ haemolytica to plc suspensions and the complete cessation of chemiluminescence varied for different cows. The differences in amounts of ldcl produced by individual bovine plc suspensions may reflect innate differences in the functional activity of the oxidative antimicrobial capacity of alveolar macrophages from different cattle.
The initial kinetics of the mean ldcl response of plc suspensions to 3 concentrations of living and heat-killed Ρ haemolytica were also determined. The ldcl profiles indicated that plc responded to all concentrations of heat-killed bacteria in a similar manner. However, increasing the concentration of living Ρ haemolytica caused a decrease in the maximal ldcl values, and total mean integrated values of ldcl were significantly (P < 0.005) less for cultures phagocytically stimulated with the 2 higher concentrations of living bacteria than those for corresponding counterpart cultures stimulated with heat-killed bacteria. During the initial few minutes of interaction, those reaction mixtures containing living Ρ haemolytica developed a significantly steeper slope of ldcl (P < 0.05) than did those containing heat-killed bacteria. The total cessation of ldcl from bovine alveolar macrophages after continued in vitro exposure indicated that the microorganism may have the capability to render an important pulmonary defense system inoperative in vivo.