GEOTranscriptomicsEquus caballusExpression profiling by high throughput sequencinghttps://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE312304GEOGSEfalseCD8 T-cells, CD86+ macrophages, and TNF-α signaling pathways are correlated with fetlock osteoarthritis in racehorsesBackground: There is emerging evidence for the role of the immune system in osteoarthritis (OA) pathophysiology; however, little is known about how immune cells and the synovial transcriptome are altered in naturally occurring equine OA. Objectives: To evaluate synovial fluid (SF) and synovial membrane (SM) immune cell populations and the SM transcriptome in racehorses with fetlock osteoarthritis. Methods: Fetlock joints from racehorses euthanized on NY racetracks were considered for inclusion. SF and SM were analyzed by flow cytometry using T-cell (CD3, CD4, CD8) and macrophage (CD14, CD86, CD206) surface markers. OA severity was characterized by gross joint evaluation and SM histology. Bulk RNA-sequencing of SM was performed to identify differentially expressed genes. Flow cytometry data was analyzed with mixed linear modeling, including OA severity as a fixed effect and horse as a random effect. RNA-sequencing was evaluated with mixed linear and quadratic modeling using DREAMSeq method to capture differential gene expression over the range of OA severity. Results: Twenty-four fetlocks from 12 horses met inclusion criteria. CD8+ T-cells (R2 = 0.25 for SM, 0.35 for SF) and CD86+ macrophages (R2 = 0.14 for SM) were positively correlated with OA severity, while CD4+ T-cells (R2 = 0.17 for SM, 0.31 for SF) were negatively correlated. Macrophages co-expressing CD86 and CD206 correlated with OA severity in SM (R2 = 0.31). Pathways including those associated with TNF-α signaling, connective tissue development, and cell and neuron projection organization/development were differentially regulated in SM with greater OA scores. Main Limitations: Limited sample size, particularly of horses with severe OA. Conclusions: CD8+ T-cells, CD86+ macrophages, and macrophages co-expressing CD206 and CD86 are enriched in horses with more advanced OA. Gene set analysis suggests the importance of TNF-α signaling and disinhibition of neuron and cell projection development in OA pathogenesis.2026/04/21GSE312304GSM9343034GSM9343023GSM9343022GSM9343033GSM9343021GSM9343032GSM9343020GSM9343031GSM9343030GSM9343040GSM9343019GSM9343018GSM9343029GSM9343039GSM9343028GSM9343027GSM9343038GSM9343037GSM9343026GSM9343025GSM9343036GSM9343024GSM934303521401312304Equus caballus