Studies have shown that collagen type II (CII) on the intact articular surface of cartilage is partially protected from binding to anti-CII antibodies by material proteinaceous in nature, not present in synovial fluid, synthesized by resident chondrocytes, and exquisitely sensitive to polymorphonuclear (PMN) attack and neutrophil elastase digestion. Thus, anti-CII antibody differential binding to articular cartilage surfaces before and after brief neutrophil elastase digestion may be used as a sensitive marker of cartilage damage.
We measured binding of anti-CII antibodies to the pannus-free articular surfaces of 4 normal, 11 rheumatoid (RA), and 10 osteoarthritic (OA) cartilage specimens, before and after brief digestion with PMN elastase. In addition, antibody binding was quantitated with an antiserum against a 4 M guanidine extract of human cartilage surface.
Whereas anti-CII binding increased 59.0% +/- 2.8 after 1 h incubation of normal cartilage with elastase, both the RA and OA specimens failed to show significant increases (RA: 1.0 +/- 0.1; p < 0.001; OA: 27.2% +/- 1.6, p < 0.05). Moreover, anti-CII antibody binding to untreated cartilage specimens was highest for the RA group (Normal: 189.2.1 +/- 38.7 pg anti-lg/mg tissue; RA: 407.5 +/- 80.6, p < 0.05; OA: 243.6 +/- 50.6, NS). Concomitant binding studies with antiserum against cartilage surface material showed greater antibody binding to the articular surfaces than to the cut cartilage surfaces in normal and OA specimens. RA cartilage samples exhibited somewhat smaller antibody binding to the articular surfaces.
Our studies suggest that in human inflammatory and noninflammatory arthritides the articular cartilage surface undergoes alterations that can be detected by differential binding with anti-CII antibodies, before and after brief digestion with PMN elastase.