An Interleukin 1 Like Factor Stimulates Bone Resorption in Vitro

Many activities are now ascribed to the monokine interleukin 1 including enhancement of immune responses, stimulation of thymocyte proliferation, activation of B cells, stimulation of proteinase and prostaglandin production by connective tissue cells, stimulation of the production of acute phase proteins, induction of fever and the induction of neutrophilia. These activities were thought to be due to various different factors, but are now considered probably due to very similar, if not identical, molecules. The term interleukin 1 (IL-1) was coined to describe the factor released by monocyte/macrophages which acts on T and B lymphocytes. Only after this definition had been accepted was it shown that target cells other than lymphocytes were affected by IL-1. Products of human blood monocytes (mononuclear cell factor, MCF) have been implicated in the pathogenesis of inflammatory diseases such as rheumatoid arthritis and periodontal disease. Bone resorption is often a feature of such diseases, and monocytes are frequently found at sites of localized bone resorption. Preliminary experiments with monocyte-conditioned medium indicated that MCF could stimulate bone resorption. We therefore undertook this study to verify these observations and to determine whether purified IL-1 could stimulate connective tissue breakdown in vitro.