Understanding the Pathophysiology of Bovine Digital Dermatitis
Creators: name: Lahiri, Priyoshi
Publisher: name: Graduate Studies
Year: 2023
Subjects: subject: bovine digital dermatitis
subject: pathophysiology
subject: monocytes and macrophages
subject: proteases
subject: matrix metalloproteases
subject: Education--Health
subject: Education--Sciences

description: This thesis studied digital dermatitis (DD), a leading disease affecting cattle health, characterized by painful ulcerative lesions in the feet, for which there is no fully effective treatment. This thesis focused on why DD lesions, mostly colonized by T. phagedenis, perpetuate in the acute (M2) stage or progress to the chronic (M4) stage, that may eventually revert to active inflammation. This demonstrated that classical (CD14highCD16low) monocytes/ macrophages were abundant in M2 DD lesions and persisted after oxytetracycline (OTC) treatment. The pro-inflammatory characteristics of classical macrophages may contribute to persistent ulcerative inflammation in DD and impair wound healing. In fact, T. phagedenis, isolated from DD lesions, when injected subcutaneously into the dorsal skin of mice, induced abscesses with local recruitment of neutrophils that persisted until 7 days post-challenge, and pro-inflammatory (Ly6Chigh CCR2+) monocytes/macrophages, which appeared at disease onset (4 days post-challenge) and persisted for at least 7 days post-challenge. When exploring the ability of macrophages exposed to DD-associated T. phagedenis to mount inflammation, we showed that bovine blood-derived macrophages challenged with sonicated T. phagedenis secreted IL-1 via mechanisms dependent on NLRP3 inflammasome and caspase-1. Upon proteomic analysis of DD lesions, this thesis demonstrated high enrichment of neutrophil degranulation-related pathways in affected lesions. Topical OTC treatment promoted cornification in DD lesions. However, OTC-treated lesions retained certain inflammatory proteomic signatures, as depicted by the persistent enrichment of NF- transcription factor activity pathway, along with the neutrophil aggregation-stimulating and ROS-generating molecule, S100A8. This thesis also demonstrated high activity of matrix metalloproteinases (MMP) in DD lesions which likely aggravated the skin inflammation and ulcer formation. An MMP-specific inhibitor, CMC2.24, conferred partial protection against DD. CMC2.24 suppressed IL-1 responses in both bovine and murine macrophages, and when topically applied to M2-DD lesions in cattle, reduced inflammatory cell infiltration was observed, as depicted in histologic evaluation, along with low enrichment of ROS generation pathways as shown by the proteomic analysis. However, clinical observations of CMC2.24 treatment did not show alleviation of the disease symptoms. Taken together, this thesis confirmed that persistent infiltration of neutrophils and pro-inflammatory macrophages dominates the active non-healing ulcerative lesions in M2-DD. We provide proof-of-concept data that antibiotic-free therapeutic alternatives, such as the use of MMP inhibitors (CMC2.24), could potentially alter the inflammatory setting in DD toward a pro-healing tissue-remodeling environment.