Comparison of benchtop microplate beta counters with the traditional gamma counting method for measurement of chromium-51 release in cytotoxic assays.

The most traditional method used to measure the lytic activity of cytotoxic T lymphocytes or natural killer (NK) cells is the chromium release assay (CRA). No study has been reported that systematically compares the traditional gamma counting method with various benchtop microplate scintillation formats to measure chromium release. Here we investigated the utilization of microplate beta counters in comparison with the traditional gamma counting method to quantitate antigen-specific cytolysis, lymphokine-activated killer (LAK) activity, and NK activity in the CRA. Supernatants from standard CRA (n = 7) were directly transferred to a 96-well microplate containing either a solid scintillant (Lumaplate) or a liquid scintillant (flexible beta plate). Samples were quantified by using two benchtop microplate beta counters, Wallac Microbeta Trilux (Lumalux and Trilux methods, respectively) and Packard TopCount instruments (TopCount method). These results were then compared with data from an identical assay run in parallel using the traditional gamma counting method (LKB). The lytic activity for influenza virus-stimulated effectors measured in the benchtop microplate beta counters using Lumalux and Trilux methods exhibited excellent correlations with the one measured in the traditional LKB (r = 0.967 and 0.968, respectively). The TopCount method demonstrated a similar correlation (r = 0.966). Similar findings were observed for LAK and NK activity. The 96-well microplate format, specifically the dry-scintillant Lumaplates, offers several advantages over the traditional gamma counting format. Most notable are the reductions in sample volume needed and in the total sample preparation and counting time. Furthermore, this system reduces the amount of dry and mixed radioactive waste generated while using the same instrument for gamma- and beta-emitting isotopes.