The serine‐protease inhibitor of cartilage matrix is not a chondrocytic gene product

Human articular cartilage contains significant amounts of antileukoprotease, a cationic low‐molecular‐mass serine‐protease inhibitor, which was originally purified from mucous secretions (synonym: secretory leukocyte proteinase inhibitor). As it was not known whether the inhibitor molecule is also synthesized locally, we investigated antileukoprotease gene expression in chondrocytes. No antileukoprotease‐specific mRNA was detected in adult or foetal human chondrocytes by in situ hybridization, Northern‐blot analysis or polymerase chain reaction. Concurrently, the chondrocytesremained unstained on immunohistology, whereas immunoreactive antileukoprotease was demonstrated in the cartilage matrix. By Northern‐blot analysis, the antileukoprotease message was detected in the promyelocytic cell line HL60, the myelomonocytic cell line U937 and even in mature polymorphonuclear leukocytes from the peripheral blood of healthy donors. Immunoperoxidase staining of polymorphonuclear leukocytes for the antileukoprotease protein indicated that this cell is likely to be the physiological source of the inhibitor in serum. The results further suggest an accumulation of the inhibitor in the cartilage matrix.