Could rheumatoid arthritis result from an abnormal T cell response towards lubricin/superficial zone protein?

Distribution of lubricin in the ruptured human rotator cuff and biceps tendon: a pilot study

BACKGROUND

Lubricin is a lubricant for diarthrodial joint tissues and has antiadhesion properties; its presence in the (caprine) rotator cuff suggests it may have a role in intrafascicular lubrication.

QUESTIONS/PURPOSES

To preliminarily address this role, we asked: (1) What is the distribution of lubricin in human ruptured supraspinatus and biceps tendons? (2) What are the potential cellular sources of lubricin?

METHODS

We obtained seven torn rotator cuff samples and four torn biceps tendon samples from 10 patients; as control tissues, we obtained the right and left supraspinatus tendons from each of six cadavers. Specimens were fixed in formalin and processed for immunohistochemical evaluation using a monoclonal antibody for lubricin.

RESULTS

We found lubricin as a discrete layer on the torn edges of all of the ruptured supraspinatus and biceps tendon samples. None of the transected edges of the tissues produced during excision of the tissues showed the presence of lubricin. Lubricin was found in 3% to 10% of the tendon cells in the cadaveric controls and in 1% to 29% of the tendon cells in the torn supraspinatus and biceps tendon samples.

CLINICAL RELEVANCE

The lubricin layer on the torn edges of ruptured human supraspinatus and biceps tendons may interfere with the integrative bonding of the torn edges necessary for repair.

The presence and distribution of lubricin in the caprine intervertebral disc

Lubricin is a large, multifunctional glycoprotein that is known to play a role as a boundary lubricant in diarthrodial joint articulation. The hypothesis of this study was that lubricin is present in the intervertebral disc in a distribution consistent with serving to facilitate interlamellar tribology. The objectives were to: (1) determine the distribution of lubricin in the normal caprine disc; and (2) investigate the synthesis of lubricin by caprine annulus fibrosus (AF) and nucleus pulposus (NP) cells in vitro, using immunohistochemical methods. Caprine lumbar intervertebral discs from five levels and four animals were studied. Positive staining revealed the presence of the lubricin in the outer AF of nearly all samples. No staining was present in the inner AF or the NP. Within the outer AF, lubricin was prominent in the layers separating lamellae and in the extracellular matrix of the lamellae. Some of the AF cells within the lubricin-positive regions demonstrated intracellular lubricin staining, suggesting that these cells may be synthesizing the lubricin protein observed. Immunohistochemistry performed on monolayer cultures of primary AF and NP cells demonstrated intracellular lubricin staining in both cell types. Thus, lubricin is selectively present in the outer caprine intervertebral disc AF, and its distribution suggests that it may play a role in interlamellar tribology. Cells from both the annulus and nucleus were found capable of synthesizing lubricin in vitro, suggesting that these cells may be a potential source of the glycoprotein under some conditions.

Lubricin distribution in the goat infraspinatus tendon: a basis for interfascicular lubrication

BACKGROUND

This study was motivated by the need to better understand the tribology of the rotator cuff, as it could provide insights into degenerative processes and suggest therapeutic approaches to cuff disorders. The objective was to evaluate the distribution of a known lubricating protein in the infraspinatus tendon of the rotator cuff in adult goats. The hypothesis was that lubricin, also known as superficial zone protein or proteoglycan 4, serves as an interfascicular lubricant.

METHODS

Eight infraspinatus tendons were resected from eight Spanish goats, and five patellar tendons and articular cartilage samples were also resected from five of the goats. Samples were processed for immunolocalization of lubricin with use of a purified monoclonal antibody and for histological analysis with Masson trichrome staining for collagen. The locations of lubricin within the tendon were documented, and measurements were made of the distance to which lubricin was detected in the tendon, relative to the humeral insertion site, and the depth of lubricin staining into the fibrocartilage of the fascicle bordering the humeral joint space. Images from polarized light microscopy were used to measure the fascicle diameter and the crimp length.

RESULTS

Lubricin was prominent in layers separating the fascicles in the infraspinatus tendon, with occasional intrafascicular staining. The fibrocartilaginous portion of the fascicle bordering the humeral joint space displayed diffuse lubricin staining. The Masson trichrome staining of the collagen in this lubricin-containing fibrocartilage indicated that it was not being tensioned at the time of fixation, whereas the collagen contained in the body of the tendon was under tension. The crimp of the fascicles near the humeral joint side of the tendon displayed a shorter peak-to-peak length than the crimp of the fascicles in the superior region of the tendon. Only the surface of two of the five patellar tendon samples stained for lubricin; there was no staining within these ligaments.

CONCLUSIONS

The sheaths of the fascicles of the infraspinatus tendon near the bone insertion site contain lubricin, indicating that this lubricating protein may be facilitating interfascicular movement. The fact that the crimp pattern of fascicles changes with location in the tendon provides support for the supposition that fascicles move relative to one another as the tendon is loaded, underscoring the importance of a lubricating protein in the layer separating the fascicles.

Cartilaginous deposits in subchondral bone in regions of exposed bone in osteoarthritis of the human knee: histomorphometric study of PRG4 distribution in osteoarthritic cartilage

The objective of this study was to identify and characterize cartilaginous deposits aggregates in the subchondral bone in areas of the human osteoarthritic knee with exposed bone. A specific aim was to determine the distribution of the joint lubrication molecule, lubricin/superficial zone protein [referred to by its gene, proteoglycan4 (PRG4)], in these cartilaginous deposits and in osteoarthritic cartilage. This work was carried out in the context of assessing the potential contribution of these chondrocyte aggregates for joint resurfacing in certain cartilage repair procedures. The discarded bone cuts of femoral condyles and tibial plateaus were collected from 11 patients with advanced osteoarthritis (OA) of the knee during total knee arthroplasty; 9 women and 2 men with a mean age of 68 years. Sections of paraffin-embedded tissue were stained with Safranin-O, and with antibodies to type II collagen, alpha-smooth muscle actin (SMA), and PRG4. Chondrocyte aggregates were found in the subchondral bone of regions of exposed bone in sections from five individuals. The average diameter of cartilaginous aggregates was 152 microm, and the average depth of the aggregates below the surface was about 475 microm. Most aggregates were fibrocartilaginous and stained positive for type II collagen. Of interest was the finding that the cartilaginous deposits and osteoarthritic cartilage contained PRG4. Only a small percentage of chondrocytes stained positive for SMA. Cartilaginous deposits containing chondrocyte aggregates exist in subchondral bone in regions of exposed bone in some patients with advanced OA of the knee. These cells may be able to contribute to the resurfacing of the joint in certain cartilage repair procedures.