Toll-like receptor 3–activated allogeneic umbilical cord blood–derived mesenchymal stromal cells as an adjunct for the treatment of synovial sepsis in 3 horses

Objective: To describe the case presentation, treatment, and outcome of horses treated with Toll-like receptor 3 (TLR-3)-activated allogeneic umbilical cord blood-derived mesenchymal stromal cells (CB-MSCs). Animals: 3 horses presenting with chronic or antimicrobial-resistant (AMR) septic synovitis. Clinical Presentation: All horses presented with clinical signs associated with septic synovitis including marked grade 4/5 lameness, joint effusion, heat/swelling of the limb, and synovial fluid parameters consistent with sepsis. Case 1 presented 3 weeks following the onset of clinical signs. Cases 2 and 3 developed infections 4 and 5 weeks after elective arthroscopy and third phalanx fracture fixation, respectively. In cases 2 and 3, culture and sensitivity identified AMR bacteria. All cases had ultrasonographic and radiographic evidence of septic synovitis in addition to osteitis (n = 2) and presumptive septic tendinitis (1). Results: All cases were treated with systemic and regional antimicrobials and synovial lavage. In 2 cases, TLR-3-activated CB-MSCs were used following a lack of response to conventional treatments and in the third case because of infection chronicity and the presence of septic tendinitis. Twenty million TLR-3-activated CB-MSCs were administered by regional limb perfusion or direct IA injection as dictated by the horse's clinical presentation. In all cases, the septic synovitis was resolved following combination treatment. Clinical Relevance: Because of the increasing emergence of AMR infections in both veterinary species and humans, the advancement of alternative and synergistic therapeutics is imperative. This case series highlights the use of TLR-3-activated CB-MSCs as an adjunctive treatment in cases of synovial sepsis involving chronic or AMR infections.