An auxiliary mode of apoptotic DNA fragmentation provided by phagocytes

CAD (caspase-activated DNase) can cause DNA fragmentation in apoptotic cell s. Transgenic mice that ubiquitously express a caspase-resistant form of th e CAD inhibitor (ICAD) were generated. Thymocytes prepared from the mice we re resistant to DNA fragmentation induced by a variety of stimuli. However, similar numbers of TUNEL-positive cells were present in adult tissues of t ransgenic and wild-type mice. Exposure to gamma-irradiation caused a striki ng increase in the number of TUNEL-positive cells in the thymus of wild-typ e, but not transgenic, mice. TUNEL-positive nuclei in transgenic mice were confined to thymic macrophages. When apoptotic thymocytes from the transgen ic mice were cocultured with macrophages, the thymocytes underwent phagocyt osis and their chromosomal DNA underwent fragmentation. This DNA fragmentat ion was sensitive to inhibitors that block the acidification of lysosomes. Hence, we conclude that the DNA fragmentation that occurs during apoptosis not only can result cell-autonomously from CAD activity but can also be att ributed to a lysosomal acid DNase(s), most likely DNase II, after the apopt otic cells are engulfed.