Horizontal Transfer of DNA by the Uptake of Apoptotic Bodies

The control of cell number in multi-cellular organisms depends on the fine balance between cell proliferation and cell death. Cell death by apoptosis plays a key role in the elimination of cells during embryonic development as well as in adults. It is, for example, involved in the negative selection of neural cells, removing redundant cells during organ formation. In adults, apoptosis plays an important role in the maintenance of tissue homeostasis; in the human small intestine alone, approximately 1 O9 cells apoptose every hour and are sloughed off into the lumen of the gut. Apoptosis is also of importance in tumor development. It may serve as an anti-cancer mechanism by restricting the expansion of cells with unleashed proliferative potential. Tumors remain constant in size or dormant when proliferation is balanced by an equal level of cell death. This state of dormancy may be controlled at the genetic level such as induction of apoptosis by the p53 tumor suppressor gene or by the inability of the tumor cells to recruit vessels from the micro-environment. Loss of p53 or the induction of angiogenesis results in lowered incidence of apoptosis and escape from dormancy. Thus there exists stages during tumor development where there is a high turnover of genetic material that is fragmented during apoptosis and subsequently taken up by neighboring cells. The recognition of apoptotic bodies by macrophages, dendritic or neighboring cells is mediated by several different pathways. Translocation of phosphatidylserine from the inner to outer side of the cell membrane serves as a recognition signal for phagocytosis. Surface receptors such as the phosphatidylserine receptor, vitronectin receptor (of,,I3,) and thrombospondin receptor (CD36) located on the macrophages may recognize apoptotic cells via the binding of thrombospondin. Not only macrophages are capable of phagocytosing apoptotic bodies. Injection of apoptotic liver cells in the portal vein in the mouse liver results in uptake of apoptotic bodies in the liver endothelial cells. In vitro, apoptotic macrophages are rapidly phagocytosed by human fibroblast cells within 1 h. Horizontal transfer of genes has been reported in bacteria and fungi and plays an important role in the generation of resistance to antibiotic drugs as well as adaptation to new environments. Horizontal (or 'lateral') gene transfer is an effective mechanism for the exchange of genetic information in bacteria. Classical examples include genes for antibiotic resistance, virulence and enzymes involved in various metabolic processes. These genes are often carried by plasmids or transposable elements that quite readily shuttle between bacteria. Horizontal gene transfer therefore represents a powerful avenue for bacterial diversification and adaptation to new environments. In previous studies, we showed that DNA could be transferred from apoptotic cells to recipient cells after phagocytosis. Co-cultivation of cell lines containing integrated copies of Epstein-Barr virus (EBV) resulted in rapid uptake and transfer of EBV-DNA as well as genomic DNA to the nucleus of the phagocytosing cell. This is an efficient mode of gene transfer since fluorescence in situ hybridization analysis (FISH) of bovine aortic endothelial cells showed uptake of apoptotic DNA in the nuclei of 15% of the phagocytosing cells. Once transferred, expression of EBV-encoded genes was detected at protein and mRNA levels. We have also shown that HIVDNA is transferred in a similar fashion to cells that are resistant to virus infection indicating a novel pathway of receptor-independent transfer of HIV. The transfer of drug