Macrophages induce apoptosis in normal cells in vivo

It is well established that macrophages have a function in scavenging apoptotic bodies from cells undergoing programmed cell death. Here we show that macrophages can also induce apoptosis of normal cells. Using injected toxic liposomes to eliminate macrophages in the anterior chamber of the rat eye, we provide direct evidence that, in vivo, macrophages induce apoptosis in normal vascular endothelial cells during programmed capillary regression. Macrophage elimination resulted in the survival of endothelial cells that normally would die and the persistence of functional capillaries. Furthermore, replacement of eliminated macrophages with bone-marrow-derived macrophages 'rescued' lack of capillary regression. Viability of the persistent target cells was demonstrated through their lack of apoptotic morphology, expression of intracellular esterases and synthesis of DNA. These results uncover a new function for macrophages in the remodeling of tissues through the induction of programmed cell death and provide direct evidence of a key role for macrophages in capillary regression.