The bystander effect in photodynamic inactivation of cells

Treatment of MDCK II cells with the lipophilic photosensitizer tetra(3-hydroxyphenyl)porphyrin and light was found to induce a rapid apoptotic response in a large fraction of the cells. Furthermore, the distribution of apoptotic cells in microcolonies of eight cells was found to be different from the binomial distribution, indicating that the cells are not inactivated independently, but that a bystander effect is involved in cell killing by photodynamic treatment. The observation of a bystander effect disagrees with the common view that cells are inactivated only by direct damage and indicates that communication between cells in a colony plays a role in photosensitized induction of apoptosis. The degree of bystander effect was higher for cells dying by necrosis than for cell dying by apoptosis.