Cattle requiring multiple treatments for bovine respiratory disease exhibit decreased capacity to protect against histone cytotoxicity
Bovine respiratory disease (BRD) is the leading cause of morbidity and mortality in feedlot cattle. Pneumonia associated with BRD causes significant inflammation and lesions in lung tissue of infected cattle. During acute inflammatory responses, circulating histones increase and contribute to mortality in rodents and humans, yet serum proteins provide protection against histone cytotoxicity in some cases. We hypothesized that cattle experiencing fatal cases of BRD have reduced ability to protect against histone cytotoxicity. Bovine kidney cells (MDBK) were exposed to 0, 50 µg/mL and 100 µg/mL of histones from calf thymus for 18 h without serum. To assess cell viability, Resazurin was added (0.5%) and cells were incubated for an additional 6 h followed by fluorescent quantification. Because both doses exhibited cytotoxic effects, and work in humans suggests that serum histone levels rise to 50 µg/mL during sepsis, 50 µg/mL was chosen for subsequent studies. At feedlot arrival, serum samples were collected from 37 bull calves followed by castration and normal feedlot processing procedures. Animals were retrospectively assigned to either Controls (never treated for BRD; CONT; n = 12), Recovery (treated once for BRD and recovered; RECOV; n = 9), Dead (treated once for BRD and subsequently died; DEAD; n = 8), or Chronic (treated 4 times for BRD; CHRON; n = 8). Duplicate wells containing MDBK cells were cultured in 96-well plates as described above except were supplemented with 1% serum from individual animals plus 50 µg/mL histones and duplicate wells with 1% serum alone. Fluorescent values from serum alone were subtracted from values obtained for histone treatment for each animal and analyzed using the GLM procedure of SAS. Results showed that histone treatment reduced cell viability in all groups and treatment group affected serum protective capacity (P = 0.05). Serum from CONT, RECOV, and DEAD calves all exhibited a similar (P > 0.50) response to histone treatment with values of -591.8 ± 549.9, -1086.9 ± 634.9, and -1193.8 ± 634.9, respectively. However, CHRON calves demonstrated an impaired capacity to protect against histones (-3054.4 ± 673.4) and were reduced (P < 0.05) when compared to each of the other groups. Results suggest that calves which require multiple treatments for BRD have reduced ability to protect against cytotoxicity of histones. Understanding the underlying mechanism responsible for protecting against histone cytotoxicity could lead to better identification of animals susceptible to severe cases of BRD.
Keywords: BRD, histones, feedlot, health