Complement regulator loss on apoptotic neuronal cells causes increased complement activation and promotes both phagocytosis and cell lysis.

In neuroinflammatory disease complement (C) activation and neuronal apoptosis occur in areas of active pathology. C has a role in clearing apoptotic debris, but is also known to cause necrotic cell death by insertion of the membrane attack complex (MAC). It is therefore unclear whether C is protective or injurious in this context. Here we examine C regulator expression and susceptibility to C activation, lysis and phagocytosis in human neuronal cells undergoing apoptosis in order to model the in vivo situation. We demonstrate that apoptotic neuronal lines lose the C regulators CD46 and CD59. Regulator loss occurred only on cells positive for apoptotic markers, and was caspase dependent. Both CD46 and CD59 were shed from cells, CD46 as a soluble form following MMP cleavage, and CD59 on apoptotic blebs and as a soluble form. Apoptotic cells activated C and were opsonised more readily than control cells; as a consequence they were more readily phagocytosed by macrophages than non-apoptotic cells. Susceptibility to C-mediated lysis was complicated in that early cells were more sensitive while late apoptotic cells were more resistant to killing. MMP inhibition protected against the increased lysis seen in early apoptotic cells, but had no effect on susceptibility of non-apoptotic cells to C-mediated lysis. Our studies suggest that C activation on apoptotic neuronal cells is delicately balanced between enhancing their safe disposal through phagocytosis, and triggering necrosis by C-mediated lysis. The data suggest that therapeutic MMP inhibition, by restricting loss of CD46, may limit neuronal damage in neurological disease.