Equine Peripheral Gene Expression Changes in Response to Dose-Dependent Lipopolysaccharide-Induced Synovitis

•Decreasing doses of lipopolysaccharide in an acute synovitis model fails to decrease lameness severity while preserving an mRNA response in horses•Some genes demonstrate dose-dependent mRNA expression changes•Lower lipopolysaccharide doses introduce increased mRNA expression and lameness variability between horses The use of lipopolysaccharide to induce a localized source of inflammation (acute synovitis) and allow for monitoring of changes in systemic mRNA expression has been recently reported. Here, the goal was to maintain a significant systemic mRNA response while limiting the severity of lameness such that this model can be used to examine the effects of various anti-inflammatory treatment modalities on mRNA expression. Three mixed breeds, four-year-old geldings were utilized for this study. One milliliter of phosphate-buffered saline containing 1,000 ng or less of lipopolysaccharide from E. coli O111:B4 was aseptically injected into alternating radiocarpal joints following washout periods. Blood for complete blood cell count, serum amyloid A concentration, and mRNA analysis via RT-qPCR for 23 different genes were collected before each injection, as well as at multiple times post-injection. Lameness severity was also graded at each time point. Two-way, repeated measures analysis of variance was used for statistical analysis (P < .05). Results largely replicated those previously reported, with multiple genes exhibiting significant expression changes during the acute inflammatory period (including increases in CD14, TLR4, IL-1β, IL1RN, MMP1, and MMP9 expression) while some demonstrated dose-dependent changes; significant increases in complete blood cell count parameters and serum amyloid A concentrations were also noted. Attempts to temper the severity of lameness were not successful as nonweight bearing lameness was noted at doses of 10ng or higher, while a dose of 1ng elicited neither a detectable lameness nor a significant change in mRNA expression.