Inflammatory cytokine production induced by an analogue of muramyl dipeptide MDP-Lys(L18) in rat macrophage cultures and dog synovial fluid.

To examine the involvement of cytokines in the mechanisms of N2-[(N-acetylmuramoyl)-L-alanyl-D-isoglutaminyl]-N6-stearoyl-L-lysine, MDP-Lys(L18)-induced arthritis, we analyzed interleukin-1 (IL-1), tumor necrosis factor (TNF), colony-stimulating factor (CSF), and neutrophil chemotactic factor (NCF) by bioassays in the rat macrophage-conditioned medium (Mluminal diameter-CM) stimulated by MDP-Lys(L18) in vitro and the synovial fluid from dogs treated subcutaneously with MDP-Lys(L18) for 14 days in vivo. The dog showed arthritis characterized by swelling of the knee joint, increased synovial fluid and thickened synovial membrane, and a single subcutaneous injection of MDP-Lys(L18) was previously shown to induced synovitis in rat tarsal joint. IL-1, TNF, CSF, and NCF activities in Mluminal diameter-CM were increased by MDP-Lys(L18), while only NCF activity was detected in the dog synovial fluid. Partial purification procedures revealed that NCF in Mluminal diameter-CM was not leukotriene B4 but a protein having heparin-affinity, and, in addition, immuno-reactive IL-8 was evident to be in Mluminal diameter-CM. The NCF activity in the dog synovial fluid was not inhibited by dialysis, showing that NCF is a protein substance, possibly a chemokine. These results suggest that MDP-Lys(L18) produces a chemokine, such as IL-8, which recruits neutrophils to the synovial membrane for subsequent development of synovitis in rats and dogs.