CNTO 530 uniquely induces the sustained expansion of a bone marrow–resident Kit^negCD71^highTer119^neg E2 progenitor pool. (A) Based in part on recent ex vivo analyses of murine bone marrow erythroid progenitor pools,^32,34,35  a course of stepwise development can be defined as a Kit^pos CD71^highTer119^neg → Kit^neg CD71^highTer119^neg → Kit^neg CD71^highTer119^pos cell progression (ie, stages E1 → E2 → E3). (B) At steady state, stage-E3 bone marrow progenitors were predominant as assayed via costaining for Kit, CD71, and Ter119 markers, and gating-off red blood cells (RBCs) and reticulocytes. E1 and E2 progenitors were assayed by gating off Ter119^pos cells and on Kit^pos vs Kit^neg CD71^high cells. Each symbol in each set represents the value for an independently treated and analyzed mouse. (C) CNTO 530 uniquely and markedly expands stage-E2 bone marrow progenitors: C57BL/6 mice were dosed with either CNTO 530 (0.2 μg/mouse gram-weight) or darbepoietin (0.015 μg/mouse gram-weight). At days 2.5 and 4.5, levels of E1, E2, and E3 progenitors were then determined via flow cytometry. Graphed values (top panel) are means ± SE (n = 3). (D) Primary flow cytometry data illustrate CNTO 530 expansion of stage-E2 erythroid progenitors within bone marrow (here at day 4.5 after dosing with either CNTO 530, darbepoietin, or saline).