Frequency of splenic Ery.A is a function of their Fas expression. (A) The frequency of splenic Ery.A is a function of their Fas expression. Each data point is derived from the mean values of Ery.A frequency and Fas expression presented for each erythropoietic stress model in Figures 5, 6 and Tables 1, 2. The same data are presented with either linear axes (left panel) or logarithmic axes (right panel). Ery.A frequency is expressed as a ratio of mean Ery.A frequency in the stressed mice to mean Ery.A frequency in matched controls. The fraction of Fas-positive Ery.A is calculated as the ratio of mean Fas-positive Ery.A in the stressed mice to the mean Fas-positive cells in matched controls. The slope of the line in the left panel is (-1.9), and the data fit the equation y = 0.82x^-1.9 (R²  = 0.912), where y is Ery.A frequency and x is Ery.A Fas expression. If only chronic erythropoietic stress models are considered (ie, pregnancy and Epo injection are omitted), the equation describing the data becomes y = 1.00x^-1.9 (R²  = 0.9999). (B) The homeostatic role of Fas in splenic early erythroblasts and in T cells. Splenic early erythroblasts coexpress Fas and FasL in the basal state, resulting in continuous apoptosis. Erythropoietic stress leads to Epo-mediated down-regulation of Fas and FasL expression and enhanced erythroblast survival, increasing erythropoietic rate. In contrast, naive T cells do not express Fas and FasL in the basal state. Fas/FasL expression is induced following T-cell activation and clonal expansion, when Fas-mediated apoptosis is key in activation-induced cell death.