Macrophage-mediated bystander effect triggered by tumor cell apoptosis.

Restoration of apoptosis is an important therapeutic strategy for cancer, but bystander effects may be crucial to treatment success. We examined the involvement of bystander effects in the outcome of pro-apoptotic treatments and investigated the role of macrophages. Using a murine N202 breast cancer chamber model and intravital microscopy, we observed bystander apoptosis in vivo in mixed spheroids consisting of bystander N202 cells plus modified N202 cells overexpressing the p14ARF N-terminal region, which promotes p53-mediated apoptosis. The effect was not observed in cocultures in vitro, and could not be transferred through conditioned medium from modified N202 cells. However, if macrophages were also included in the N202 co-cultures, bystander apoptosis was restored, and correlated with elevated surface expression of phosphatidyl serine, a macrophage recognition molecule, on the modified N202 cells. Bystander killing was not observed in cocultures of N202 cells plus macrophages plus cisplatin-treated or 5-fluorouracil-treated N202 cells, where apoptosis induction in the target cell population was inefficient, suggesting that specific activation of macrophages by apoptotic tumor cells was required. The results suggest that pro-apoptotic therapies benefit both from the intrinsic vulnerability of cancer cells to apoptosis and from an innate immune response that amplifies the therapeutic effect.