Changes in early erythropoietin receptor–positive (EPOR⁺) erythroid progenitors determined by FACS analysis and in vitro EPO-stimulated proliferation assay. (A) Numbers of early EPOR-positive erythroid progenitors per spleen. (B) In vitro EPO-stimulated proliferation of unfractionated total splenocytes. Data are means ± SEM for 3 to 5 mice per group. N indicates untreated naive controls; N+EPO, naive mice treated with EPO; I, untreated infected controls; I+EPO, infected mice treated with EPO. When 2 sets of data are labeled with superscripts of different lowercase letters, it indicates that these 2 sets of data are statistically different (P < .05). Similar results are representative of 3 independent experiments.