Lin-Scal+Kit- Bone Marrow Cells Contain Early Lymphoid-Committed Precursors That Are Distinct from Common Lymphoid Progenitors

The significance of a population in mouse bone marrow of lineage-negative (Lin(-)), Sca1-positive, c-kit-negative (LSK-) Cells, which is reported to be devoid of long-term repopulation capacity or myeloid potential, is unknown. In this study, we show that the LSK- population is composed or several subsets defined by the expression of flt3 CD25, and IL-7R alpha. The first subset was CD25(-) and more than 90% expressed either flt3, IL-7R alpha, or both. The CD25(-)LSK(-) population had T cell, B cell, and NK cell potential in vivo, and most of this activity was localized in the flt3(+) subset, irrespective of the expression of IL-7R alpha. Although lymphoid potential of flt(3+)LSK(-) cells in vivo was 3-fold lower than that of lin(-)Sca1(low)kit(low)IL7R alpha(+) common lymphoid progenitors (CLPs), their cloning efficiency in vitro was 10-fold lower than that of CLPs. Furthermore, although the myeloid potential of flt(3+)LSK(-) cells was 10-fold lower than that of CLPs in the absence of M-CSF, the relative myeloid potential of both populations was similar in its presence. These observations suggest differential growth factor requirements of both populations. The second subset of LSK- cells was homogeneously CD25(+)flt3(-)IL7R alpha(+) and could be generated from both CD25(-)LSK(-) cells and from CLPs, but did not engraft in immunodeficient Rag1(-/-) or Rag1(-/-)gamma(-/-)(c) hosts. This population, of which the significance is unclear, was increased in Rag1(-/-) mice and in old mice. Thus, the LSK- population is phenotypically and functionally heterogeneous and contains early lymphoid-committed precursors. Our findings imply that the early stages of lymphoid commitment are more complex than was thus far assumed. The Journal of Immunology, 2008, 181: 7507-7513.