Expression of thrombospondin-1 and its receptor CD36 in human osteoarthritic cartilage

Objective-Thrombospondin-1 (TSP-1), a trimeric glycoprotein, is involved in cell-matrix interactions of various tissues, particularly in cartilage. Bi ochemical analyses show expression of TSP-l in human cartilage, but its cel lular source as web as the presence of its main surface receptors CD36 and CD51 in normal and osteoarthritic cartilage remain unknown. Therefore, to l ocalise TSP-1 and its receptors immunohistochemistry and in situ hybridisat ion were used. Methods-Radioactive in situ hybridisations with an RNA probe that encodes T SP-1 combined with immunostaining were carried out to investigate the expre ssion patterns of TSP-1, CD36, and CD51 in seven normal and 23 osteoarthrit ic human cartilage samples. Results-In normal cartilage TSP-1 was present mainly in the middle and uppe r deep zone. RNA expression was predominantly seen over chondrocytes of the middle zone. CD36 was found in chondrocytes of the superficial and upper m iddle zone. In mild and moderate osteoarthritic cartilage an increased numb er of TSP-1 expressing chondrocytes were seen and an increased pericellular staining close to the surface. In severe osteoarthritic cartilage a decrea se in the number of TSP-1 synthesising chondrocytes and a strong reduction in matrix staining were observed, Most of these severe osteoarthritic sampl es showed a strongly enhanced number of CD36 positive chondrocytes. Conclusion-The cellular source of TSP-1 in normal cartilage is mainly mid-z one chondrocytes, which also express CD36. In early osteoarthritic cartilag e lesions an increase of TSP-1 uas seen, whereas reduced TSP-1 synthesis is paralleled by a strong decrease in TSP-1 protein staining in severe osteoa rthritis. Furthermore, in severe osteoarthritic cartilage the number of CD3 6 immunostained chondrocytes is significantly increased.