Objective: Since many cytokines have been identified in chronically inflamed human synovium, it is possible that particular cytokines or combinations of cytokines play dominant roles in driving or inhibiting metabolic processes important to inflammation. To assess these possibilities, we compared selected effects of individual cytokines and their binary, ternary, and higher combinations in human synovial cell cultures.
Methods: Cytokines studied known to occur in human synovial tissue included: interleukin 1beta (IL-1beta), IL-6, tumor necrosis factor-alpha, granulocyte macrophage colony stimulating factor, interferon-gamma, acidic fibroblast growth factor (aFGF), basic FGF (bFGF), platelet derived growth factor, transforming growth factor-beta1, connecting tissue activating peptide-III, and epidermal growth factor. The growth related effects of these agents singly and in combinations were assessed by measuring newly synthesized [3H]DNA and [14C]GAG (glycosaminoglycan) in human synovial cell cultures. Cytokine induced synthesis of prostaglandin E2 (PGE2) was measured by ELISA.
Results: Most cytokine combinations resulted in additive/synergistic anabolic effects, except when IL-1beta was present; IL-1beta was markedly antagonistic to the mitogenic effects of other cytokines tested. Combinations of platelet derived cytokines were the most potent stimulators of DNA synthesis, while combinations of synovial derived cytokines were more active in stimulating GAG synthesis. Synovial cells exposed simultaneously to both platelet and synovial derived cytokines produced large quantities of [14C]GAG and showed a modest increase in [3H]DNA synthesis. IL-1beta, alone or in combinations, was dominant with respect to stimulation of PGE2 synthesis. Acetylsalicylic acid substantially interfered with all the effects of cytokine combinations measured.
Conclusion: Quantitative alterations in synovial cell synthesis of GAG and DNA varied greatly depending on the ambient mixture of cytokines. Virtually all combinations of cytokines tested gave rise to large increases in synovial cell synthesis of GAG. Four platelet derived cytokines, a "physiologic combination," appeared to be dominant agents in stimulating DNA synthesis. This effect was profoundly reduced by the antagonistic effect of IL-1beta, mediated in part by PGE2. The patterns of cytokine combination induced metabolic effects suggest that the "cytokine network" has a significant measure of redundancy with respect to control of synovial cell metabolism.