Lipid bodies: Cytoplasmic organelles important to arachidonate metabolism in macrophages and mast cells

Much has been learned about the biochemical nature and pharmacologic activity of the products of arachidonic acid (AA) oxidation, but relatively little is known about the structures in nucleated cells into which AA is incorporated and from which it is initially mobilized. To address this question, we administered ³H-AA or other ³H-fatty acids in vitro to human lung mast cells and alveolar macrophages as well as to mouse and guinea pig peritoneal macrophages. The subcellular distribution of ³H label was assessed by electron microscopic autoradiography, and the nature of cell-associated ³H-lipids was determined by thin layer chromatography. Autoradiographic analysis of human lung mast cells localized virtually all of the ³H-AA to cytoplasmic lipid bodies. Lipid bodies are roughly spherical, variably osmiophilic, nonmembrane-bound structures that appear in the cytoplasm of a wide variety of cells, but we have found that these lipid bodies occur with increased frequency in granulocytes, macrophages, and mast cells at sites of inflammatory, immunologic, or neoplastic processes. Macrophages also incorporated ³H-AA predominantly into cytoplasmic lipid bodies. In contrast to mast cells, however, macrophages incorporated ³H-AA into the plasma membrane as well. Stimulation of macrophage phagocytosis resulted in striking alterations of the relationship of lipid bodies to intracellular membranes, so that many lipid bodies appeared adjacent to phagolysosomes. In addition, some phagolysosomes contained ³H label, which along with other morphologic evidence suggested that lipid bodies may discharge their contents into these structures. Mast cell and macrophage cytoplasmic lipid bodies appear to represent a major site of intracellular storage and metabolism of products of AA and perhaps other fatty acids taken up from the external milieu. These heretofore neglected organelles may thus influence cellular function in a wide variety of adaptive or pathologic processes.