Detection of apoptotic cells using propidium iodide staining

Flow cytometry assays are often used to detect apoptotic cells in in vitro cultures. Depending on the experimental model, these assays can also be useful in evaluating apoptosis in vivo. In this protocol, we describe a propidium iodide (PI) flow cytometry assay to evaluate B-cell lymphomas that have undergone apoptosis in vivo. B-cell lymphoma cells are injected into recipient mice and, on tumor formation, the mice are treated with the apoptosis inducer vorinostat (a histone deacetylase inhibitor). Tumor samples collected from the lymph nodes and/or the spleen are used to prepare a single-cell suspension that is exposed to a hypotonic solution containing the fluorochrome PI. The DNA content of the cells, now labeled with PI, is analyzed by flow cytometry. Nuclear DNA content is lost during apoptosis, resulting in a hypodiploid (or sub-G₁) DNA profile during flow cytometry. In contrast, healthy cells display a sharp diploid DNA profile.