Clustering of apoptotic cells via bystander killing by peroxides

Clustering of apoptotic cells is a characteristic of many developing or ren ewing systems, suggesting that apoptotic cells kill bystanders. Bystander k illing can be triggered experimentally by inducing apoptosis in single cell s and may be based on the exchange of as yet unidentified chemical cell dea th signals between nearby cells without the need for cell-to-cell communica tion via gap junctions. Here we demonstrate that apoptotic cell clusters oc curred spontaneously, after serum deprivation or p53 transfection in cell m onolayers in vitro. Clustering was apparently induced through bystander kil ling by primary apoptotic cells. Catalase, a peroxide scavenger, suppressed bystander killing, suggesting that hydrogen peroxide generated by apoptoti c cells is the death signal. Although p53 expression increased the number o f apoptoses, clustering was found to be similar around apoptotic cells whet her or not p53 was expressed, indicating that there is no specific p53 cont ribution to bystander killing. Bystander killing through peroxides emitted by apoptotic cells may propagate tissue injury in different pathological si tuations and be relevant in chemo-, gamma-ray, and gene therapy of cancer.