A feature common to all forms of chronic inflammatory arthritis, irrespective of the possible underlying cause, is the persistent exudation of large numbers of polymorphonuclear leucocytes (PMNL) into synovial fluid. These cells possess potent degradative enzymes and proinflammatory mediators, and their removal is vital to normal inflammatory resolution. A major route of disposal of extravasated PMNL appears to be programmed cell death (apoptosis), followed by their rapid recognition, and intact phagocytosis, by mature tissue macrophages. Such macrophages, containing PMNL (cytophagocytic mononuclear cells (CPM)), long recognised in synovial fluid as Reiter cells, are commonly found in reactive arthritis, spondyloarthritis, and crystal arthritides, but only rarely in rheumatoid disease. In a retrospective analysis of 187 knee synovial fluid cytospins, the relation between the formation of CPM and the presence of apoptotic (pyknotic) PMNL was investigated. As long as the synovial fluid examined was fresh there was a high correlation between numbers of CPM (as a percentage of macrophages) and pyknotic numbers of PMNL in fluids containing CPM. This suggests that the formation of CPM occurs in vivo and is involved in the disposal of PMNL. Numbers of pyknotic PMNL increased rapidly in stored synovial fluid without a significant change in numbers of CPM, and were highest in synovial fluid which did not contain CPM. The presence or absence of CPM, or their disease associations, could not be explained simply by limiting numbers of macrophages, or apoptotic PMNL in synovial fluid. These findings are consistent with a regulatory role for CPM in synovial fluid, where they may be important in preventing autolysis of PMNL, and thus local tissue damage.
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