We demonstrate, using a new rosette method of determination of interleukin 1 activity that macrophages secrete Il-1 upon contact with syngeneic thymocytes or with thymocytes homologous in the Ia region of MHC complex. The phenomenon takes place in the absence of foreign antigen. Blocking of class II MHC antigens on macrophages with monoclonal antibodies against structures of the T-cell receptor complex (alpha/beta T-cell receptor-TCR, CD3 and L3T4) inhibits production of Il-1 to the background level. We conclude that secretion of Il-1 from macrophages, upon contact with syngeneic thymocytes, is triggered by a T-cell signal following interaction: TCR-Ia molecules.
