PS2-4. Expression of β -defensins in Nasal Mucosal Secretions in Calves Affected with Bovine Respiratory Disease Indicated by Elevated Neutrophils or Lymphocytes.

Abstract Defensins are naturally occurring antimicrobial peptides that play a vital role in the innate immune system, primarily through immunomodulatory effects. β-defensins function in mucosal immunity and are secreted from epithelial cells of the skin, eye, bronchial tree, and nose. Bovine respiratory disease (BRD) is common within the U.S. beef industry that results in significant economic losses. Prevention and treatment of BRD relies heavily on antibiotic use, and their overuse has contributed to rising concerns regarding antimicrobial resistance. Characterizing β-defensin expression against pathogens could potentially lead to more effective strategies for mitigating BRD. Therefore, the objectives of this study were to 1) identify β-defensin expression in bovine mucosal secretions and 2) compare β-defensin expression in healthy and BRD infected calves. Post-weaned crossbred beef calves (n = 9; average BW 278.1 ± 36 kg) were used for analysis of complete blood count (CBC) to determine lymphocyte and neutrophil concentrations and β-defensin gene expression (DEFB401, DEFB402, and DEFB403) in nasal swabs. Blood and nasal swabs, were taken on D 0, 1, 2, and 3 at 0800 h. Calves were given a BRD status by two analyses. In analysis one, if neutrophil concentrations were ≥ 4.0 109 /L, then the calf was determined sick, otherwise (neutrophil concentrations < 4.0 109 /L) the calf was determined healthy. In analysis two, if lymphocyte concentrations were ≥ 7.5 109 /L, then calves were determined sick, otherwise (lymphocyte concentrations < 7.5 109 /L) the calves were healthy. Relative expression of the target genes were determined by qPCR using the 2-ΔΔCt method. Data were analyzed using The GLIMMIX procedures of SAS and significance was declared at P ≤ 0.05. The animals' BRD status, determined by neutrophils, showed no significant differences in DEFB401 or DEFB402 expression in mucosal secretions (P = 0.80, P = 0.62, respectively). However, DEFB403 differed significantly (P = < 0.01) in sick versus healthy animals; where sick animals had greater expression of DEFB403 compared to healthy animals. No differences were observed in animals' immune status, determined by lymphocytes, and DEFB401, DEFB402, DEFB403 expression (P = 0.30, P = 0.21, P = 0.25, respectively). These data indicate that DEFB403 expression in nasal mucosal secretions shares an association with neutrophils, but not with lymphocytes. These differences in expression suggest that DEFB403 may be involved in the innate immune response influenced by neutrophils. These data also suggest that DEFB401 and DEFB402 in nasal mucosal secretions were not associated with either immune response to BRD in this study. Overall, the results demonstrate that β-defensin expression in mucosal secretion exists, and that there are differences in β-defensin expression during BRD pathogen exposure. Further characterizing which defensins are upregulated when exposed to various BRD pathogens could help reduce antibiotic use.