February 04, 2019

Interpretive Summary: Articular injection with lipopolysaccharides

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

By: Dr. Caitlin Vonderohe

Osteoarthritis is the most common cause of lameness in equine athletes. The causes of osteoarthritis are complex, and multiple models of osteoarthritis have been developed to evaluate the efficacy of treatments. Articular injection of lipopolysaccharide (LPS), a strongly pro-inflammatory molecule associated with E. Coli bacteria, has been used to transiently induce an inflammatory reaction within the joint to model the inflammatory aspect of joint disease.

Most research involving the LPS model of osteoarthritis has focused on inflammation and damage within individual joints, and less has explored the effect of LPS on the entire animal. A recent study, “Effects of repeated arthrocentesis on systemic cytokine expression and leukocyte population in young horses challenged with intra-articular lipopolysaccharide,” published in the Journal of Animal Science by Hunt et al., examined the effect of articular injection with LPS on systemic markers of inflammation: circulating leukocyte population and gene expression of proinflammatory cytokines.

Nineteen yearling quarter horses were assigned one of three treatments; 0.25 ng LPS, 0.5 ng LPS, or sterile saline. Horses were acclimated for 21 days and housed in stalls. Heart rate, respiratory rate, degree of lameness at the trot and circumference of the carpal joint was assessed before the joint injection procedure. Injections were performed by a licensed veterinarian, where synovial fluid was first aspirated for later analysis, followed by the injection of LPS or saline. Plasma was collected from a jugular catheter and additional synovial fluid samples were collected post injection. White blood cells were enumerated using a hematology analyzer. RNA was extracted from the collected blood and quantitative real-time PCR was performed to detect the expression of Beta-2 microglobulin, interleukin-6 (IL-6), interleukin-8 (IL-8), interleukin-10 (IL-10), tumor necrosis factor-alpha (TNF-α) and interleukin 1-beta (IL1-β).

All horses were sound prior to injection; however, all were lame 24 hours post injection, regardless of LPS level. Horses that received intra-articular injections of LPS were found to have a greater degree of lameness, based on the American Association of Equine Practitioners scale, compared to the saline-injected animals. Carpi circumference increased after injection in all treatments. LPS level did not affect circulating leukocytes. However, there was a greater number of circulating white blood cells in all horses at 6 hours post joint-injection.  There were minor increases in monocytes (associated with inflammation and tissue necrosis) and platelets relative to other types of white blood cells. Injections of LPS into the joint did not result in any changes in IL1-β, TNF-α, IL-10, or IL-6 expression, but the expression of IL-1β. TNF-α and IL-6 increased, and IL-10 tended to increase over time in all horses post injection.

Overall, this study showed that intra-articular injection of LPS did not result in a different systemic immune response compared to an injection of sterile saline. However, the fact that a mild systemic inflammatory response, including increases in the expression of pro-inflammatory cytokines and mild increases in circulating white blood cells occurred after injection of both sterile saline and LPS indicates that intra-articular injections themselves are pro-inflammatory. Therefore, when intra-articular injection is used as part of an experimental model for a disease process, it is necessary to use an appropriate control.

To view the full article, visit the Journal of Animal Science.