Irradiation of platelet components: inhibition of lymphocyte proliferation assessed by limiting‐dilution analysis

BACKGROUND: Using a limiting-dilution analysis (LDA) assay that measures clonigenic T cells, it has been demonstrated that, with 2500 cGy, there is no T-cell growth in red cell components irradiated in blood bags. In the current study, the LDA assay was used to investigate the effect of gamma radiation on the proliferative capacity of T cells in plateletpheresis components. STUDY DESIGN AND METHODS: Platelets were collected by using an apheresis instrument and settings that provided sufficient mononuclear cells for the LDA assay. Platelet components (n = 8) were irradiated in 1-L plastic bags 24 hours after collection with 500, 1500, and 2500 cGy of gamma radiation in a stepwise manner. Mononuclear cells were isolated after each irradiation dose by the use of ficoll-hypaque. A density separation medium was used to reduce the platelet numbers. T cells were enumerated by fluorescence-activated cell sorter and functionally assessed by LDA assay, which quantified T cells proliferating in the presence of polyclonal stimuli and cytokines. The frequency of T-cell growth (f) was visually scored after 4 weeks of incubation at 37°C. Data were calculated as fexperimental/fcontrol and expressed as log10 reduction. RESULTS: The T-cell content of the mononuclear cell population was 17 ± 10.5 percent, which was unaltered by irradiation. After 500-cGy irradiation, functional T cells were reduced by 2.09 log10. Irradiation with 1500 cGy resulted in a 3.96 log10 reduction, but viable clonable T cells were detected in all experiments. With 2500-cGy irradiation, no T-cell growth was detected; this represented a greater than 4.86 log10 reduction. CONCLUSION: As the dose of gamma radiation delivered to plateletpheresis components increased, the number of residual functional T cells decreased exponentially. Irradiation with 2500 cGy inactivates T cells in apheresis platelets, as measured by an LDA assay.