Identification of a membrane-associated interleukin 1 in macrophages

We have found a surface membrane-associated interleukin 1 (IL-1) with potent thymocyte and T-cell stimulatory activity on peptone-elicited peritoneal macrophages. The IL-1 activity was demonstrated on both fixed macrophage monolayers and on isolated membranes from unfixed macrophages. Membrane IL-1 was induced by adherence and/or by adding heat-killed Listeria monocytogenes to macrophage cultures. The macrophage membrane IL-1 was similar functionally and antigenically to soluble IL-1, but its expression could be temporally dissociated from IL-1 secretion; membrane IL-1 was induced earlier and persisted longer than IL-1 secretion during in vitro macrophage culture. Moreover, when cultured macrophages that had ceased both secretion and membrane expression of IL-1 were restimulated by adding heat-killed Listeria, substantial membrane IL-1 was induced in the absence of detectable IL-1 secretion. Membrane IL-1 appears to be an integral membrane protein since it was solubilized by detergent but was not eluted by EDTA, high salt, or low pH treatment of the membranes.