APPLICATION OF THE DIRECT BETA-COUNTER MATRIX-96 FOR CYTOTOXIC ASSAYS- SIMULTANEOUS PROCESSING AND READING OF 96 WELLS USING A CR-51 RETENTION ASSAY

To assess the cytotoxic activity of immune cells, we have developed a Cr-51-retention assay in which the radioactivity retained by Cr-51-lab eled target cells, following coincubation with cytotoxic cells, is mon itored using the automated Matrix 96 beta counter. The Matrix 96 is de signed for simultaneously counting 96 samples isolated from a 96-well microplate. It uses 96 uniform and independent detectors operating on the principle of avalanche gas ionization in the Geiger-Muller mode. S amples must be dry because the detectors are of the open-window type. Therefore, samples from the 96 wells of the microplate are simultaneou sly harvested onto a filter using the MicroMate 196, a 96-well cell ha rvester, dried and quantified in the Matrix 96. Usually the Cr-51 isot ope is measured by the detection of gamma radiation in gamma counters. The Matrix 96, however, monitors Auger electrons, which are also emit ted by Cr-51. We have shown that the retention assay can be used to mo nitor the cytotoxic activity of activated lymphocytes including lympho kine-activated killer cells and tumor-infiltrating lymphocytes against various tumor cell lines. This assay is most suitable for experiments in which low E/T ratios are sufficient to detect highly cytotoxic cel ls, such as clone screening in cloning assays or in limiting-dilution analysis assays. These assays involve processing and reading large num bers of microplates. In this case, the retention assay monitored in th e Matrix 96 will improve the work flow and decrease the amount of radi oactive waste.