The apoptotic cells are rapidly engulfed and digested by phagocytes such as macrophages and immature dendritic cells. The swift engulfment of cell corpses by phagocytes prevents the release of noxious or immunogenic debris from dying cells into the circulation. In the process of apoptosis, the dying cells expose phosphatidylserine on their external membrane in a caspase-dependent manner. This externalization of phosphatidylserine is one of the hallmarks of apoptosis and acts as an “eat me” signal for phagocytes HDAC inhibitor 3. Recently, several molecules
that recognize phosphatidylserine have been identified 4–7. Systemic lupus erythematosus (SLE) is a chronic autoimmune disease caused by multiple genetic and environmental factors 8. Patients with SLE develop a broad spectrum of clinical manifestations affecting the skin, kidney, lungs, blood vessels, and/or nervous system. SLE is also characterized by the presence in sera of autoantibodies against nuclear components (anti-RNP
and anti-DNA antibodies). Unengulfed apoptotic cells can be found in the germinal centers of the lymph nodes of some SLE patients, and macrophages from these patients show a reduced ability to engulf apoptotic cells 9. Furthermore, circulating DNA or nucleosomes can also be found in the sera of SLE patients 10, 11. These results suggest that a deficiency in the clearance of apoptotic cells is one of the causes of SLE. Milk fat globule-EGF factor 8 (MFG-E8) is a glycoprotein. At the N-terminus, it has a EGF-like selleck chemicals repeat(s), and at the C-terminus, there are two discoidin domains that bind phosphatidylserine. It was originally identified as a component of milk fat globules that bud from the mammary epithelia during lactation. But it is now known to play
important roles in various systems such as involution of mammary glands, adhesion between sperm and egg, repair of intestinal mucosa, and angiogenesis 12. MFG-E8 is secreted by activated macrophages and immature dendritic cells 13, and it promotes the engulfment of apoptotic cells by working as a bridging molecule between apoptotic cells and phagocytes 7. In MFG-E8-knockout mice, many apoptotic Ribonucleotide reductase cells are left unengulfed in the germinal centers of the spleen 14. The MFG-E8−/− mice produce autoantibodies including anti-cardiolipin and anti-dsDNA antibodies and suffer from an SLE-type autoimmune disease. Human MFG-E8 is maintained at the optimal concentration to support the engulfment of apoptotic cells; in excess, MFG-E8 inhibits phagocytosis and causes autoimmune diseases 15, 16. In this report, we analyzed the human MFG-E8 gene of SLE patients, and found in two female patients an intronic mutation that caused aberrant splicing of intron 6, resulting in the inclusion of a cryptic exon in the transcript.