The versatile translational LPS-based joint inflammation model in the horse: Refinement and proof of its value in generating evidence-based medicine

Inflammation is central to many joint diseases, such as osteoarthritis (OA). OA is highly prevalent in humans and horses, imposing major welfare and economic burdens. Current intra-articular therapies - such as corticosteroids, platelet-rich plasma (PRP), and mesenchymal stem cells (MSCs) - offer inconsistent benefits and uncertain mechanisms. A deeper understanding of joint inflammation and refined translational models are needed to guide therapeutic development. This thesis refines and applies the equine lipopolysaccharide (LPS)-induced joint inflammation model as a translational tool for testing intra-articular treatments. LPS reliably induces transient joint inflammation without permanent joint damage, but its acute, self-limiting nature limits relevance to chronic diseases like OA. The primary aim of this thesis was to enhance the model’s translational potential by developing modified protocols that mimic recurrent, subclinical inflammation while safeguarding animal welfare. A secondary aim was to demonstrate its utility for therapeutic evaluation. The early chapters focused on model refinement. Repeated low-dose intra-articular LPS administrations induced consistent inflammatory responses without evidence of sensitisation, were tolerated well by horses, though reducing levels of synovial glycosaminoglycans suggested potential cartilage effects. Wearable inertial sensors were piloted for objective welfare monitoring, with behaviour switching identified as a sensitive pain indicator. Dose-response comparisons revealed that measurable biochemical inflammation can occur without overt lameness, supporting the use of subclinical models for therapeutic testing. The later chapters applied these refined models to evaluate intra-articular therapeutics. A nanocomplex containing salmon calcitonin, hyaluronic acid, and chitosan demonstrated feasibility for scale-up to a large-animal model, and showed some biochemical but not obvious clinical benefits. Corticosteroid (triamcinolone acetonide) treatment reduced some inflammatory markers but paradoxically increased cartilage-associated metabolites, highlighting potential adverse effects. MSC-secretome reduced joint circumference and increased synovial glycosaminoglycans, with efficacy comparable to MSCs, suggesting a paracrine mechanism of action, and positioning secretome as a promising, standardisable alternative. In contrast, PRP failed to reduce inflammation and transiently increased prostaglandin E2, challenging assumptions of its universal benefit and emphasising the importance of product standardisation and context-specific use. Collectively, these findings advance the LPS model as a flexible tool for investigating inflammatory cascades and testing acute interventions. By implementing repeated, low-dose, and subclinical protocols, the model more closely reflects features of chronic disease while minimising harms to experimental animals. Objective outcome measures, including wearable technologies, further strengthen study design. While the observed therapeutic effects were modest, the thesis discusses the importance of aligning model choice with expected drug mechanisms and of employing complementary models to capture the multifactorial nature of OA. In conclusion, the refined equine LPS joint inflammation model provides a translationally relevant tool for preclinical testing of intra-articular therapeutics. It may be particularly suited for early-stage screening of anti-inflammatory interventions, while complementary models of degeneration may be required for evaluating long-term cartilage effects. Future directions could include molecular comparisons between LPS-induced inflammation and naturally occurring OA, and incorporation with complementary models. Ultimately, this work positions the equine LPS model as a valuable component of a multimodal strategy to accelerate the development of effective joint therapies for both human and equine patients.