Acute Effects of Intra-Articular Liposomal IDO-1 following Anterior Cruciate Ligament Injury

Joint injuries, such as rupture of the anterior cruciate ligament (ACL), is associated with the development of post-traumatic osteoarthritis (PTOA). It is known that ACL rupture can lead to disruption of metabolic pathways, including the conversion of the essential amino acid tryptophan to kynurenine, which is associated with a sustained inflammatory response. An in vivo study was undertaken to determine the acute effects of intra-articular administration of liposomes loaded with the tryptophan-catabolizing enzyme indoleamine 2,3-dioxygenase-1 (IDO-1) following ACL rupture. Using an established rat model of non-surgical ACL injury, male and female Lewis rats underwent a single intra-articular injection of empty liposomes, or liposomes loaded with IDO-1 and were subsequently randomized to 1- or 2-week endpoints. IDO-1 treatment after ACL injury was associated with a significant reduction in synovial fluid concentration of tryptophan at both 1-and 2-week endpoints. In addition to a reduction in tryptophan, IDO-1 treatment led to significantly lower synovial fluid concentrations of IL-1b and TNF-a. Intra-articular administration of IDO-1-loaded liposomes also increased the ratio of regulatory T lymphocytes (Tregs) to IL-17-secreting helper T lymphocytes (Th17 cells). Similarly, IDO-1 treatment increased the number of CTLA4+ cells relative to IL-17A+ cells that infiltrated joint tissues at a 2-week endpoint. Contrast-enhanced micro-computed tomography (CE-uCT) was used to quantify treatment-based effects on articular cartilage thickness and surface roughness at at 2-week endpoint. In addition to sex-based differences, IDO-1-loaded liposome treatment was associated with increased cartilage thickness, with no significant effects on surface roughness. Histologic characterization is needed to determine whether this increased cartilage thickness represents a chondroprotective effect, or a degenerative effect of IDO-1-treatment.