Cytotoxicity assays using cryopreserved target cells pre-labeled with the fluorescent marker europium

We describe a method for standardizing cytotoxicity assays by the use of cryopreserved, fluorescently labeled target cells. The cells are labeled in batches with the fluorogenic element europium (Eu) and frozen in multiple aliquots. Replicate aliquots can be thawed on different days and used for cytotoxicity assays. Thawed pre-labeled cells from the same batch are killed reproducibly, allowing cytotoxicity assays to be standardized. The target cells need only be thawed, washed, counted for viability and used. The availability of pre-labeled, cryopreserved targets facilitates the assessment of cytotoxic activity of individual donors at different times. The method also simplifies the use of large panels of target cell types. By using this procedure it is not necessary to maintain active cultures of target cells, nor is it necessary to use and dispose of radioisotopes. These pre-labeled target cells can be used in various assays where the result is a destroyed target cell such as cell-mediated cytotoxicity, complement mediated cytotoxicity, or antibody-dependent cellular cytotoxicity.