The fetal liver counterpart of adult common lymphoid progenitors gives rise to all lymphoid lineages, CD45(+)CD4(+)CD3(-) cells, as well as macrophages

We identified an IL-7R(alpha+)Sca-1(low)c-Kit(low) population in E14 fetal liver, which is the phenotypical analog of common lymphoid progenitors (CLP ) in adult bone marrow. After transfer into newborn mice, the IL-7R alpha ( +)Sca-1(low)c-Kit(low) population rapidly differentiated into CD45(+)CD4(+) CD3(-) cells, which are candidate cells for initiating lymph node and Peyer 's patch formation. In addition, this population also gave rise to B, T, NK , and CD8 alpha (+) and CD8 alpha (-) dendritic cells. The fetal liver prec ursors expressed a significantly lower level of the myeloid-suppressing tra nscription factor Pax-5, than adult CLP, and retained differentiation activ ity for macrophages in vitro. We propose that the transition from fetal liv er IL-7R alpha (+)Sca-1(low)Kit(low) cells to adult CLP involves a regulate d restriction of their developmental potential, controlled, at least in par t, by Pax-5 expression.