Kinetics and ultrastructural studies of the induction of rat alveolar macrophage fusion by mediators released from mitogen-stimulated lymphocytes

Treatment of F344 rat alveolar macrophages (AMs) in vitro with cell-free supernatant fluids obtained from concanavalin-A (Con-A)-stimulated syngeneic lymphocytes induced extensive fusion. The lymphokine responsible for the fusion of AMs (but not other cells) is here referred to as AM fusion factor (Con-A-MFF). Fusion is dependent on the dose of Con-A-MFF and the population density of AM cultures and occurred 10 hours after Con-A-MFF was added to cultures of normal AMs. Con-A-MFF must interact with AMs for more than 8 hours before full expression of fusion is reached at 24 hours. Using a technique allowing for sequential scanning to transmission electron microscopy analysis of cells, the authors determined the relationship of the morphologic characteristics of the surface and the internal structure of cells fusing to form multinucleate giant cells (MGCs). The process of AM fusion begins with the aggregation of AMs, followed by interdigitation of cell processes. Serial sections of MGCs showed lysosomes associated with remnants of plasma membrane in the cytoplasm. The MGCs contained numerous organelles associated with increased secretory activity of cells.