Efferocytosis and Genomic Polymorphism in Autoimmune Diseases

Over the past few years, growing evidences revealed that clearance of apoptotic cells by phagocytosis can result in powerful anti-inflammatory and immunosuppressive effects. In vivo, apoptotic cells are cleared rapidly by neighboring cells, macrophages and related scavengers. Defective clearance of apoptotic cells has been linked closely to autoimmunity and persistent inflammatory disease. Several phagocytic receptors, bridging molecules produced by phagocytes and 'eat-me' signals on apoptotic cells are coordinately involved in mediating clearance of apoptotic cells. Complement receptors (CR3, CR4), collection, CD14, CD36 (Class B scavenger receptor), class A scavenger receptor, asialoprotein receptor, Mer receptor kinase were reported to recognize apoptotic cells. The best characterized system for clearance of apoptotic cells is the recognition of phosphatidylserine (PS) on apoptotic cells by phosphatidylserine receptor (PSR). Milk fat globule- epidermal growth factor 8 (MFG-E8) is an opsonin that bridges phagocytes (by interacting with α vβ3, αvβ5 integrins via RGD motif) and apoptotic cells (by binding PS through Factor V/VIII-C domain). Activated macrophages produce and secret MFG-E8. MFG-E8 is a critical component in PSR-mediated phagocytosis of apoptotic cells. The dominant negative mutant MFG-E8, D89E, that carried a mutated RGD motif inhibited phagocytosis of apoptotic cells in vitro. Injection of D89E into wild type mice induced autoantibodies and IgG deposition on glomeruli. Macrophages from MFG-E8 deficiency (MFG-E8-/-) mice were impaired in engulfment of apoptotic cells, which can be restored by adding recombinant MFG-E8. The female MFG-E8-/- mice spontaneously produced high titer of autoantibodies and developed lupus-like glomerulonephritis at the age of week 40. Defective clearance of apoptotic cells is closely related to development of autoimmunity. In the past 4 years, a growing number of molecules were recognized as receptors for the PS exposed on t