Effects of repeated arthrocentesis on systemic cytokine expression and leukocyte population in young horses challenged with intra-articular lipopolysaccharide.

Osteoarthritis (OA) is a prevalent and economically costly source of lameness in the athletic horse. Previous studies investigating OA pathology have focused on localized trauma to the articular cartilage of a joint, largely ignoring the systemic immune status of the animal. In this study, yearling Quarter Horses were used to evaluate systemic cytokine gene expression and circulating leukocytes following a localized intra-articular inflammatory insult of the endotoxin, lipopolysaccharide (LPS). Treatments for the 35-d experiment included an intra-articular injection of 0.25 ng (n = 7) or 0.50 ng (n = 6) of LPS obtained from Escherichia coli O55:B5 or sterile lactated Ringer's solution (n = 6; control) into the radial carpal joint. Blood and synovial fluid samples were collected at preinjection hour 0 and 2, 6, 12, and 24 h postinjection. Synovial fluid was obtained for a companion study. Total RNA was isolated from plasma leukocytes and real-time PCR was used to determine relative gene expression of the cytokines interleukin (IL)-1beta (β), IL-6, IL-8, IL-10, and tumor necrosis factor-alpha (TNF-α). Total leukocyte subpopulations and differentials were performed using a cell counter. Data were analyzed using the PROC MIXED procedure of SAS. Gene expression of all cytokines were unaffected by intra-articular treatment. However, IL-1β increased above baseline beginning at hour 6 and remained elevated to 24 h (P = 0.04). In contrast, IL-6 decreased from hours 6 to 12 and then increased to 24 h (P = 0.02). Levels of TNF-α increased at 6 and 12 h (P = 0.01) postinjection. Only IL-8 exceeded a 2-fold change in expression (P = 0.01), peaking at 12 h and indicating greater responsiveness to arthrocentesis when compared with other cytokines. No treatment effects on the leukocyte population were observed; however, total circulating leukocytes increased over time (P = 0.04), peaking at 6 h postinjection. Similarly, an increase over time was observed in monocytes (P = 0.02) and in platelets (P = 0.01) at 24 h postinjection. The results indicate that regardless of treatment, a mild immune response was elicited, which may be due to repeated arthrocentesis. Future experiments should consider the effects of arthrocentesis and potential systemic inflammatory response, even in control animals, when administering intra-articular LPS to young horses.