Fas Stabilizes and Accelerates Erythropoiesis hematocrit in the gld-Rag12/2 mice represents approximately a third of the erythropoietic output in high altitude hypoxia. Unlike the lpr-Rag12/2, plasma Epo in the gld-Rag12/2 mice was not significantly different to that of controls (Fig 3A), possibly because of the already low basal Epo in control Balb/C mice which was ,40% lower than in control mice on the C57BL/6 background (Fig 3A, lower panels, black symbols). Of note, responses by both gld-Rag12/2 (Balb/C background) and the lpr-Rag12/2 (C57BL/6 background) mice have in common a higher erythropoietic rate per unit plasma Epo than in matched control mice, consistent with the absence of a negative regulator of erythropoiesis. Increased splenic CFU-e, ProE and EryA in gld-Rag12/2 and lpr-Rag12/2 mice A representative flow-cytometric histogram in Figure 4A shows that the frequency of the ProE and EryA subsets within gld- Rag12/2 spleen erythropoietic tissue (Ter119+ cells) was increased. A similar analysis in 11 to 40 mice per strain/sex combination is summarized in Figures 4B and 4D. An increase in subset frequency does not necessarily reflect a corresponding increase in cell number, since it may also arise as a result of decreased cell number in other subsets. We therefore also examined the absolute number of cells in each of the erythroid precursor subsets, computed by multiplying the subset frequency Figure 4. Increased frequency of spleen, but not bone-marrow, erythroid progenitors and precursors in mice deficient in the Fas pathway. The Legend in panel B also applies to C,D. (A) Representative flow-cytometric analysis of spleen erythroid subsets in gld-Rag12/2 and matched Rag12/2 controls, showing increased frequency of ProE and EryA within Ter119+ cells. (B) Frequency of erythroblast subsets in spleen erythropoietic tissue, measured as in A, expressed as fraction of all spleen Ter119+ cells. F = female M = male. Box and whiskers delineate the central 50% and 95% of readings, respectively, with the median indicated with a horizontal line and arithmetic mean with a ‘+’. Data points are individual mice (11–40 mice per sex/genotype combination). Data was pooled from several independent experiments. (C) CFU-e progenitors in spleen and bone-marrow. Data pooled from two independent experiments for females, and one experiment for males, for each genotype. (D) Frequency of erythroblast subsets in bone-marrow expressed as fraction of all spleen Ter119+ cells. For all panels, *p,0.05 **p,0.02, ***p,0.002 (ANOVA, for difference in means). f = p,0.05 ,ff = p,0.02, fff = p,0.002 (F test, for difference in variance). doi:10.1371/journal.pone.0021192.g004 PLoS ONE | www.plosone.org 6 July 2011 | Volume 6 | Issue 7 | e21192