Evaluation of Cell Proliferation and Apoptosis in Immunotoxicity Testing

Immunotoxicity testing is important in determining the toxic effects of various chemicals on the immune system. The immune system is a direct target of numerous toxicants, and the adverse effects include serious health complications such as susceptibility to infections, cancer, allergic reactions, and autoimmune diseases. One way to investigate the harmful effects of different chemicals is to study apoptosis and/or proliferation in immune cells. Apoptosis is defined as programmed cell death, and in general, this process helps in development and maintenance of tolerance and homeostasis. However, in the case of an insult by a toxicant, enhanced apoptosis of immune cells may cause immunosuppression resulting in the development of cancer and the inability to fight infections. Apoptosis is characterized by cell shrinkage, nuclear condensation, changes in cell membrane and mitochondria, DNA fragmentation, and protein degradation by caspases. Various methods are employed to investigate apoptosis, including direct measurement of apoptotic cells with flow cytometry and in situ labeling, as well as RNA, DNA, and protein assays that are indicative of apoptotic molecules. In addition to apoptosis, quantification of cell proliferation can provide important additional information about the effect of a toxicant upon various immune cell populations. In some cases, a toxicant may act as a mitogen pushing the immune cell into the different stages of the cell cycle. There are four stages of the active cell cycle: G1, S, G2, and M, with cell division occurring in M stage. Proliferation can be quantified by numerous methods, including staining with ki-67 or CFSE, BrdU labeling, MTT assay, and/or ATP quantification.