Variable self-renewal of reconstituting stem cells in long-term bone marrow cultures

Long-term bone marrow cultures (LTBMC) serve as a valuable in vitro model of the bone marrow microenvironment. The stromal layer supports the growth of immature and mature cell populations through production of colony-stimulating factors and cell:cell interactions. LTBMC are devoid of mature lymphoid cells but contain stem cells capable of restoring lymphoid and myeloid function in suitable recipients. Severe combined immune deficiency (scid) mice provide a useful environment to study lymphocyte development, as their autosomal recessive mutation on chromosome 16 leaves them with a severe deficiency of B and T lymphocytes. To determine the presence of different classes of stem cells in LTBMC, adherent cells from the cultures were grafted into sublethally irradiated scid mice and lineage reconstitution was evaluated 6 weeks to 3 months postengraftment. Self-renewal of donor stem cells was tested by serial transfer of scid bone marrow to secondary C.B-17 and scid recipients. Mature lymphoid and myeloid cells were isolated from reconstituted mice and a restriction fragment length polymorphism (RFLP) at the C mu immunoglobulin locus was used to distinguish donor and host cells. We found that LTBMC contained both long-term and short-term reconstituting stem cells. The long-term stem cells had significant self-renewal potential and fully reconstituted all lineages in both primary and secondary recipients. The short-term stem cells produced mostly lymphoid progeny at the time of analysis and their limited self-renewal capacity led to partial reconstitution of only the primary recipients. The short-term reconstituting cells may be lymphoid-restricted stem cells.