Abstract
The development of committed erythroid progenitors and their continued maturation into erythrocytes requires the cytokine erythropoietin (Epo). Here, we describe the immunophenotypic identification of a CD34- colony-forming unit-erythroid (CFU-E) progenitor subtype, termed late CFU-E (lateC), that arises in an Epo-dependent manner during human early erythropoiesis (EE). LateC cells lack CD235a (glycophorin A) but have high levels of CD71 and CD105, characterized as Lin-CD123-CD235a-CD49d+CD117+CD34-CD71hiCD105hi. Analysis of ex vivo cultures of bone marrow (BM) CD34+ cells showed that acquisition of the CD71hiCD105hi phenotype in lateC occurs through the formation of four other EE subtypes. Of these, two are CD34+ burst-forming unit-erythroid (BFU-E) cells, distinguishable as CD71loCD105lo early BFU-E (earlyB) and CD71hiCD105lo late BFU-E (lateB), and two are CD34- CFU-E, also distinguishable as CD71loCD105lo early CFU-E (earlyC) and CD71hiCD105lo mid CFU-E (midC). The EE transitions are accompanied by a rise in CD36 expression, such that all lateC cells are immunophenotypically CD36+. Patterns of CD34, CD36, and CD71 indicate two differentiation routes-in one earlyB lose CD34 to form earlyC, and in another, earlyB gain CD36 and CD71hi expression prior to losing CD34 to form midC, bypassing the earlyC stage. Regardless of the route, the transition from midC to lateC requires Epo. All five EE subtypes could be prospectively detected in human BM cells and, upon isolation and reculture, exhibited the potential to continue differentiating along the erythroid trajectory. Finally, we find that all five EE populations can also be detected in cultures of cord blood-derived CD34+ cells at levels similar to those observed in BM CD34+ cell cultures.