AbstractHuman fibroblasts grown in vitro as monolayer cultures were “activated” by syngeneic lymphocytes, polymorphonuclear leukocytes, and fibroblasts as well as by allogeneic leukocytes, human embryonic kidney cells, fibroblasts, and thrombocytes. “Activation” was characterized by increased medium acidity, increased glucose uptake, increased lactate formation, and marked stimulation of hyaluronic acid formation. The “activation” process was blocked by preincubation with actinomycin D, but not by incubation with 2,4 dinitrophenol. “Activated” fibroblasts continued to exhibit increased mucopolysaccharide synthesis for 5 to 28 days following a single exposure to stimul
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