Short term exposure of articular cartilage to blood induces chondrocyte apoptosis

Recurrent joint bleedings lead to irreversible cartilage damage. Also short term bleedings, due to trauma or sports injuries, may lead to cartilage damage (Roosendaal et al.. Arthritis&Rheum, 1999). The permanent loss of metabolically active chondrocytes is directly linked to exposure to both mononuclear cells (MC) and red blood cells (RBC). It remains unclear what triggers the chondrocytes in the affected cartilage to become drastically altered. The objective of this study was to investigate the contribution of chondrocyte apoptosis in cartilage damage after a short term exposure of articular cartilage to blood. Human cartilage expiants were cocultured for 4 days with MC and RBC. In control cultures chondrocyte apoptosis was induced by adding staurosporine (0-10 μM). Instable DNA, a marker of apoptosis, was evaluated by the immunohistochemical staining of single-strand DNA with F7-26 mAb after heat treatment of human cartilage slices. Prevention of apoptosis was induced by addition of a specific caspase-3 inhibitor (acDEVDcho, 0-500 |iM) as well as a pancaspase inhibitor (zVADfmk 0-500 (lM) during the first 4 days of culture. Cartilage matrix proteoglycan synthesis (¹⁵SO₄ ^2- incorporation) as a measure of chondrocyte metabolism was determined after 4 days as well as 16 days (recovery period) of culture. Short term exposure of articular cartilage to both 50% whole blood as well as the combined MC+RBC had permanent inhibited proteoglycan synthesis (26% and 22% on day 16, /"<0.05). Immunohistochemical staining of ssDNA after 16 days showed apoptotic chondrocytes in cartilage exposed to blood cell and staurosporine treated cartilage samples, whereas control cartilage did not. Both acDEVDcho as well as zVADfmk reversed proteoglycan synthesis up to respectively 65% and 76% (P<0.05) of control cartilage and reduced the staining of apoptotic chondrocytes. These in vitro results suggest that a short term exposure of articular cartilage to blood, as occurs during a single intraarticular bleeding, results in irreversible alterations in cartilage metabolism in which apoptosis plays an important role and ultimately leads to joint damage.