Responses of toll-like receptors 2 and 4 in bucks post-challenge with Mannheimia haemolytica A2 and its lipopolysaccharide and outer membrane

Background: Pneumonic mannheimiosis is one of the most commonly occurring respiratory inflammatory diseases among small ruminants worldwide. It is caused by Mannheimia haemolytica A2, which induces the production of pro-inflammatory cytokines in infected animals. Toll-like receptors (TLRs) function as pattern recognition receptors, triggering signalling pathways upon detecting pathogen-associated molecular patterns, such as lipopolysaccharide (LPS) and outer membrane proteins (OMPs). Despite known roles of these TLRs in bacterial recognition, their diagnostic potential in pneumonic mannhemiosis remains unclear. This study aimed to quantify serum TLR2 and TLR4 levels in goats post-challenged with M. haemolytica A2 and its LPS and OMP at pre-determined intervals and evaluating their potential as diagnostic biomarkers for pneumonic mannheimiosis in small ruminants. Methods: Sixteen male goats were equally divided into four groups and treated with PBS pH 7.0 (Group 1), whole-cell suspension of M. haemolytica A2 at 1.2×10^9 CFU/mL (Group 2), LPS endotoxin (Group 3), and OMP (Group 4). Blood sera collected at pre-determined intervals were subjected to quantitative ELISA assays to determine TLR2 and TLR4 responses. Results: Serum concentrations of TLR2 and TLR4 were elevated post-challenge in Groups 2, 3 and 4 compared with Group 1. The highest serum concentrations of TLRs were recorded in Group 4 with a 1.7-fold increase in TLR2 and Group 3 with a 2.1-fold increase in TLR4 in comparison to Group 1. A significant (p < 0.05) increase in serum concentrations of TLRs, similar to that observed in natural field infections, was noted in Group 2 bucks. Hence, these results demonstrated that serum TLR2 and TLR4 were elevated in bucks post-challenge with a significant (p<0.05) change in TLR4 in groups 2 and 3. Conclusion: This finding suggests that elevation of serum TLR4 levels occurs during pneumonic mannheimiosis and could potentially be used as a biomarker for clinical diagnosis of the disease in small ruminants.