Delayed lubricin injection improves cartilage repair in an in vivo rabbit osteochondral defect model

Introduction: High quality repair of osteochondral (OCL) defects in articular cartilage has been an enduring challenge. Previous studies have shown that lower interfacial strains and slopes correlate with better cartilage repair, and that therapies that effectively lubricate the joint decrease shear strains and prevent cell death. These data suggest mechano-targeted therapies that alter the mechanical environment of the joint could improve repair cartilage integration. Lubricin, a mucinous glycoprotein in the synovial fluid, has been well characterized as a cartilage boundary lubricant that effectively lowers interfacial shear strains. Lubricin has been shown to have an anti-inflammatory role, but has not been evaluated in preclinical cartilage repair models. Recently, a bioengineering approach using codon scrambling was developed to produce a full-length recombinant human lubricin (rhLubricin). While preliminary characterization has revealed that rhLubricin can lubricate cartilage, little is known about its ability to alter the quality of repaired cartilage. The goal of this study is to determine whether a single injection of rhLubricin can improve cartilage repair in an established New Zealand rabbit OCL defect model. We hypothesize that lubricin injections will: (A) improve the quality of the total repair of the tissue, and (B) reduce the surface roughness of the repaired cartilage.

Materials and

Methods: In vivo study design: The NYU Institutional Animal Care and Use Committee approved all surgical procedures and experimental designs (IACUC 2017-0025). 4 mm diameter osteochondral (OCL) defects along with bone marrow stimulation (BMS) were performed on the medial femoral condyle of 18 New Zealand rabbits. rhLubricin and saline injections were performed 4-weeks after surgery. Control limbs received no surgical defect or injection. Animals were euthanized at 12 weeks and the femoral condyles were collected (Fig 1A). Two rabbits had surgical complications and were excluded from the analysis (rhLubricin: n = 9 joints; saline: n = 15 joints; unoperated control: n = 8 joints). Surface Profilometry: The root mean square surface roughness (Rq) was measured in the central region of the defect site on the medial femoral condyle using a Keyence VK-X260 optical profilometer. Macroscopic evaluation of repair tissue: To assess the quality of the repair, modified ICRS scores and Goebel scores were collected from 4-5 observers for the saline and rhLubricin group on the OCL repair site. Histological analyses: Histology was performed to assess proteoglycan content in the repaired cartilage tissue using Safranin-O/Fast Green staining. The ratio of GAG area per total area of repaired tissue was quantified for each sample. Statistical analyses: Statistical differences in surface roughness and GAG content in repair tissue were calculated using a one-way ANOVA, and repair tissue quality scores were compared using a Wilcoxon rank sum test.

Results, Conclusions, and Discussions: rhLubricin treated joints had enriched GAG staining in the repair tissue compared to saline (p < 0.01, Fig 1BC). Repair tissue from the rhLubricin treated group (Rq = 8.1 ± 2.7 μm) trended towards lower surface roughness compared to the saline treated group (Rq = 11.5 ± 5.6 μm, p = 0.056, Fig 1D). Gross condyle scorings of total modified ICRS scores and and Goebel scores indicated that samples from the rhLubricin treated group had higher median repair scores relative to the saline group, however these differences was not significant (rhLubricin = 10 ± 2, saline = 8 ± 2; p = 0.11 and rhLubricin = 14 ± 2, saline = 11 ± 2; p = 0.26, respectively, Fig 1EF).

In this study, we show that rhLubricin injections 4 weeks after surgery decreased surface roughness and increased GAG content of the repair tissues. Lubricin supplementation has been shown to inhibit integrative repair of cartilage due to its anti-adhesive properties, but lubricin is critical for proper joint lubrication, maintaining low friction, and preserving cell viability. Previously, lubricin injections performed immediately after the creation of OCL defects inhibited poor integration between repair and native cartilage. However, the timing of the injection was immediately after the creation of the defect. In contrast, our study suggests that allowing a moderate level of infill before injection can improve the surface roughness of the defect and as well as improve the quality of the repair tissue. Notably, the rhLubricin injected joints showed decreased surface roughness compared to saline injected joints, which is concomitant with improved lubricating ability of the cartilage. Additionally, there was increased GAG content in the repair tissue for rhLubricin treated joints compared to saline, which could result in improvements in compressive mechanical properties. This study is the first to quantify the quality of repair in a OCL defect in a preclinical model after a single injection of a recombinant lubricin. The rhLubricin treated limbs showed increased GAG content and trends of reduced surface roughness and higher repair tissue quality scores, which may improve the lubricating ability and mechanical properties of the repair tissue.

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