Macrophage cathepsin L, a factor in the erosion of subchondral bone in rheumatoid arthritis

Objective. To test the hypothesis that the proteinase cathepsin L is involved in the subchondral bone lesions found in chronic rheumatoid arthritis (RA). Methods. The medial tibial plateaus from 4 control cases and 30 patients diagnosed as having end-stage RA were examined immunochemically for cathepsin L. Results. RA lesions include large groups of mononuclear cells, many of which are rich in cathepsin L, Since these mononuclear cells contained the CD68 glycoprotein and, in the electron microscope, displayed an irregular cell surface, cytoplasmic vacuoles, lysosomes, and phagosomes, they were identified as belonging to the macrophage family. The lesions were classified into 2 main patterns, both displaying these cathepsin L-rich cells, which, in at least 1 of the 2, were closely associated with bone degradation. Conclusion. The cathepsin L-rich macrophages are sufficiently numerous to be considered a major factor in producing the erosion of subchondral bone found in chronic RA lesions.