Isolation and characterization of early B lymphoid progenitor cell populations

The isolation and functional characterization of phenotypically defined populations of hematopoietic stem and progenitor cells have enabled the study of the early stages in blood cell development. Using a combination of cell surface markers and fluoresence-activated cell sorting, we isolated a cell population having the phenotype Thy-1.1<[neg][c-]kit[pos]Sca-1[ pos]Lin[neg] (Thy-1.1[neg]) from lineage-depleted bone marrow of adult mice. In vivo transplant studies of Thy-1.1[neg]) cells showed that this cell population mediated rapid bone marrow reconstitution predominantly of lymphoid lineage cells with some myeloid lineage cells. However, no engraftment of erythroid and megakaryocyte lineages was seen. Thy-1.1[neg]) cells share the c-kit[pos]Sca-1[pos]Lin[neg] phenotype of hematopoietic stem cells (HSCs), but differ in that HSCs express low levels of Thy-1.1. Evaluation of other cell surface markers expressed in Thy-1.1[neg]) cells shows that these cells are predominantly B220[neg] but express CD43 and CD24. Compared to Thy-1.1[low] HSCs, Thy-1.1[neg]) cells have started downregulating CD43 and upregulating CD24 suggesting that Thy-1.1[neg]) cells are at a transitional stage towards becoming B220[pos] lymphoid progenitor cells. The studies presented here describe the in vitro characterization of Thy-1.1[neg]) cells. Bulk cultures of Thy-1.1[neg]) cells show that these cells proliferate and differentiate to a B220[pos] stage in response to cytokine stimulation. Using clonogenic assays we show that although the Thy-1.1[neg]) cell population is enriched for lymphoid-committed progenitors, it also contains two other progenitor populations--a myeloid-committed progenitor and a bipotent lymphoid-myeloid progenitor. Moreover, defining the Thy-1.1[neg]) cells further by the level of expression of c-kit and AA4.1 delineated two functionally distinct populations: c-kit[high] AA4.1[neg] cells which are predominantly committed myeloid progenitors and c-kit[low] AA4.1[neg/pos] cells which are predominantly lymphoid-committed progenitors. The AA4.1[ neg] and AA4.1[pos] subsets of c-kit[low] Thy-1.1[neg]) cells form a lineage, with AA4.1[neg cells giving rise to AA4.1[pos] cells. Analysis of T cell potential of these two c-kit[low] Thy-1.1[neg]) subsets showed that only AA4.1[neg] cells retain some T cell potential. However, the frequency of T cell progenitors in AA4.1[neg] cells is 32-fold lower compared to Thy-1.1[neg]) HSCs. Furthermore, gene expression studies showed that both AA4.1<super>neg