Ultrastructural localization of Interleukin 1 in human peripheral blood monocytes; evidence for IL-1β in mitochondria

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Abstract
The pathway of interleukin 1 (IL-1) secretion from the cell remains unclear. IL-1β is the major form produced by human monocytes, and is synthesized as a precursor of 35kDa which is processed to the extracellular biologically active 17kDa form. We have examined the intracellular localization of IL-1β in lipopolysaccharide (LPS)-stimulated human peripheral blood monocytes, by immunocytochemistry and immunoprecipitation of subcellular fractions. LPS treatment slightly damaged the cells. Unstimulated cells showed very little immunolabelling. In contrast, there was heavy immunolabelling on LPS stimulated cells. Immunolabelling occured within the cytoplasm, nucleus and mitochondria. There was no immunolabelling on the membranous secretory organelles and the plasma membrane. Blebs of cytoplasm budding from the cell surface were immunolabelled, suggesting an alternative route of secretion of IL-1β from the cell. Immunoprecipitation studies confirmed these results.