The effect of the ratio of CD4+ to CD8+ T-cells on radiation-induced apoptosis in human lymphocyte subpopulations

Purpose: Apoptosis occurs spontaneously in cultured human peripheral blood lymphocytes but is enhanced by exposure to ionizing radiation. Subpopulations of lymphocytes are known to have varying radiosensitivities to radiation-induced apoptosis. The purpose of this study was to examine the radiation-induced apoptotic response of CD4+ and CD8+ T-cells incubated as a complete lymphocyte population. Materials and Methods: Using a four-colour flow-cytometry method, which measures annexin-V binding to phosphatidyl serine and propidium iodide, spontaneous and radiation-induced apoptosis was measured in the total lymphocyte fraction and in CD4+ and CD8+ T-cell subpopulations. Results: It was found that CD8+ T-cells were more sensitive to radiation-induced apoptosis than CD4+ T-cells at doses up to 2Gy. The yield of radiation-induced apoptosis in the total lymphocyte fraction decreased with increasing ratios of CD4+ to CD8+ T-cells (CD4/CD8 ratio). By manipulating the CD4/CD8 ratio within lymphocyte cultures, it was found that the CD4/CD8 ratio had a dramatic effect on the yield of spontaneous apoptosis of total lymphocytes fraction and CD4+ T-cells but not CD8+ T-cells. Conclusion: The CD4/CD8 ratio affects the apoptotic response of human lymphocytes and CD4+ T-cells