ARTHRITIS & RHEUMATISM Vol. 52, No. 4, April 2005, pp 1091–1099 DOI 10.1002/art.20986 © 2005, American College of Rheumatology Inhibition of Integrative Cartilage Repair by Proteoglycan 4 in Synovial Fluid Carsten Englert, 1 Kevin B. McGowan, 2 Travis J. Klein, 2 Alexander Giurea, 3 Barbara L. Schumacher, 2 and Robert L. Sah 2 Objective. To determine the effects of the articular cartilage surface, as well as synovial fluid (SF) and its components, specifically proteoglycan 4 (PRG4) and hyaluronic acid (HA), on integrative cartilage repair in vitro. Methods. Blocks of calf articular cartilage were harvested, some with the articular surface intact and others without. Some of the latter types of blocks were pretreated with trypsin, and then with bovine serum albumin, SF, PRG4, or HA. Immunolocalization of PRG4 on cartilage surfaces was performed after treat- ment. Pairs of similarly treated cartilage blocks were incubated in partial apposition for 2 weeks in medium supplemented with serum and 3 H-proline. Following culture, mechanical integration between apposed carti- lage blocks was assessed by measuring adhesive strength, and protein biosynthesis and deposition were determined by incorporated 3 H-proline. Results. Samples with articular surfaces in appo- sition exhibited little integrative repair compared with samples with cut surfaces in apposition. PRG4 was immunolocalized at the articular cartilage surface, but not in deeper, cut surfaces (without treatment). Carti- lage samples treated with trypsin and then with SF or PRG4 exhibited an inhibition of integrative repair and positive immunostaining for PRG4 at treated surfaces compared with normal cut cartilage samples, while samples treated with HA exhibited neither inhibited integrative repair nor PRG4 at the tissue surfaces. Deposition of newly synthesized protein was relatively similar under conditions in which integration differed significantly. Conclusion. These results support the concept that PRG4 in SF, which normally contributes to carti- lage lubrication, can inhibit integrative cartilage repair. This has the desirable effect of preventing fusion of apposing surfaces of articulating cartilage, but has the undesirable effect of inhibiting integrative repair. Many therapies used to treat defects in articular cartilage result in the apposition of cartilage or cartilag- inous surfaces where integrative repair is desirable (1,2). These therapies, including microfracture (3–5), implan- tation of allografts or autografts (6–9), articular fracture reduction in trauma (10,11), and implantation of tissue- engineered constructs (12,13), create either cartilage– cartilage or cartilage–implant interfaces. Inadequate in- tegration at these interfaces, such as that remaining in the long term after experimentally produced tissue lac- erations, may ultimately lead to fibrillation and degen- eration of the surrounding tissue (14). The mechanisms underlying cartilage integration have been studied under controlled conditions in vitro. The extent of such integration depends on the presence of viable cells (15) and collagen synthesis, crosslinking, and transport (16,17). Also, several processes associated with cartilage integration can be enhanced by treatment with certain enzymes. Treatment with glycosaminogly- can (GAG)–releasing enzymes, such as trypsin or chon- droitinase ABC, does not appear to affect integrative repair by endogenous chondrocytes, but can facilitate cell attachment at treated surfaces (18,19). In vitro studies have typically been performed on cartilage in which the articular surface has been removed. Thus, the Supported by the Arthritis Foundation, the DFG, the Max Kade Foundation, NASA, the NSF, and the NIH. 1 Carsten Englert, MD: University of California, San Diego, and University of Regensburg, Regensburg, Germany; 2 Kevin B. McGowan, MS, Travis J. Klein, MS, Barbara L. Schumacher, BS, Robert L. Sah, MD, ScD: University of California, San Diego; 3 Alexander Giurea, MD: University of Vienna, Vienna, Austria. Address correspondence and reprint requests to Robert L. Sah, MD, ScD, Department of Bioengineering, 9500 Gilman Drive, Mail Code 0412, University of California San Diego, La Jolla, CA 92093-0412. E-mail: rsah@ucsd.edu. Submitted for publication August 17, 2004; accepted in revised form January 11, 2005. 1091