ABSTRACT: Purpose, aims and objectives: Bovine digital dermatitis (BDD) is a painful and ulcerative, inflammatory, infectious disease of digital skin in dairy and beef cattle and is one of the most economically important causes of severe cattle lameness worldwide. A polytreponemal aetiology has been consistently implicated in BDD pathogenesis with three key phylogroups implicated globally: Treponema medium, Treponema phagedenis and Treponema pedis. This study aimed to investigate the hypothesis that the three Treponema phylogroups implement distict mechanisms of pathogenesis within cells of the bovine foot skin tissue during infection. Using a next generation RNA sequencing (RNA-Seq) approach, the dysregulation of global mRNA expression was investigated in primary bovine foot skin fibroblasts following challenge with representative strains of the three predominant BDD Treponema phylogroups (T19, T320A and T3552B respectively). Transcriptome profiles were compared to that of a commensal treponeme, Treponema ruminis strain Ru1, in attempts to identify pathogenic signatures. Results and conclusions: Here, for the first time, we implicate fibroblasts as a likely source of interleukin-8 dysregulation in BDD lesions. Despite treponemes reportedly having atypical LPS, a strong pro-inflammatory response was elicited by the BDD treponemes in a similar manner to the Gram-negative LPS control, which appeared to be mediated through interleukin-17 signalling. Along with interleukin-17, several interesting gene targets were identified which may help to inform and drive future vaccine development and therapeutic approaches; particularly those of RGS16, GRO1, MAFF and ZC3H12A, which were only stimulated by BDD treponemes. Interestingly, the three BDD treponemes were found to elicit very distinct mechanisms of pathogenesis in fibroblasts. T. phagedenis and T. pedis spirochaetes were found to upregulate several genes associated with apoptosis. Both T. medium and T. pedis spirochaetes, but notably not T. phagedenis, stimulated genes associated with actin rearrangement and a loss of cell adhesion, which may promote their persistence and deeper invasion within host tissues. The upregulation of antimicrobial peptide precursor, DEFB123, by T. phagedenis spirochaetes may present a microbial ecological advantage to all treponemes within BDD infected tissue, explaining their dominance within lesions. Interestingly, T. medium phylogroup was the least stimulatory and appeared to induce immunosuppression via the dysregulation of genes such as TSC22D3. Together with findings that the commensal, T. ruminis, significantly dysregulated over three times the number of host genes compared to each BDD treponeme; this suggests that, alike the syphilis pathogen (Treponema pallidum), BDD treponemes have evolved to become “stealth pathogens” and avoid triggering a substantial host immune/inflammatory response to promote persistence and facilitate invasion of host tissues. Overall, this study has highlighted the complexities of BDD pathogenesis; three key Treponema phylogroups appear to elicit very distinct mechanisms of pathogenesis in bovine foot skin cells. Several interesting gene targets have been identified from this study which may help to inform and drive future vaccine development and therapeutic approaches.
