Fusion of mammalian cells by lipid vesicles

The fusion of different types of cultured mammalian cells to produce hybrid cells and heterokaryons is now an established research tool which is widely used in many areas of cell biology and virology. The use of viruses, Sendai virus in particular, to fuse cells was a major factor in the successful development of cell fusion techniques in the 1960s, and viruses remain the the most commonly used agents for inducing cell fusion. Although viruses are undoubtedly highly efficient agents for inducing cell fusion in vitro, there are certain practical difficulties associated with their use. First, the virus components responsible for inducing cell fusion are not fully characterized, and therefore the 'fusion potential' of different virus preparations cannot be standardized. The second major difficulty is that the virus used to fuse cells may itself produce functional changes in the newly fused cells, including: alterations in cellular macromolecular synthesis. After the observations that lysolecithin was able to fuse cells, a wide range of similar lipolytic agents have been evaluated as possible alternatives to viruses for routine use in the fusion and hybridization of mammalian cells. Potential progress has been made with the recent demonstration that small vesicles (approximately 250-500 Å diameter) prepared from a variety of phospholipids are able to induce fusion of mammalian cells in vitro. The mechanism by which lipid vesicles induce cell fusion is not known, they compare favorably with inactivated Sendai virus in their ability to fuse cells, yet are devoid of the cytotoxic properties associated with lysolecithin. An outline of the methods to produce lipid vesicles for inducing fusion and hybridization of cultured mammalian cells, as used by the authors, is presented.