Degradation of cartilage proteoglycan and collagen by synovial cells. Stimulation by macrophages under activation by phagocytosis, lymphocyte factors, bacterial products or other inflammatory stimuli.

When cultured together with dead 35S-labelled cartilage discs or at the surface of [3H]proteoglycan/[14C]collagen-coated plates, synovial cells from either arthritic or normal rabbit joints digested both the proteoglycan and the collagen of the substrates after a lag-period of 1-2 days. These digestions were inversely related to the age (number of subculture passages) of the synovial cells and they could be modulated by serum components that were either inhibitory or stimulatory. They were dependent on a protein synthesis by the cells and were paralleled, in young cultures, by the release of collagenase and of a proteoglycan-degrading neutral proteinase. The co-culture of synovial cells with macrophages or their culture with macrophage-conditioned culture media caused a more rapid and more extensive degradation of collagen and proteoglycan due to the stimulation of the synovial cells by a nondialysable macrophage factor. The production of this synovial cell-activating 'matrix regulatory monokine' by the macrophage was enhanced by several immunological or inflammatory stimuli such as lymphocyte factors, phagocytosis, asbestos fibres, endotoxin, adjuvant muramyl dipeptide or chemotactic formyl-methionyl peptide, as well as by other membrane-active agents (phorbol myristate acetate, concanavalin A). It is presumed that these interactions are of importance in the development of cartilage destruction in rheumatoid and other chronic inflammatory arthritis.