Marrow repopulation in mice treated with busulphan or isopropyl methane sulphonate and bone marrow.

By using karyotypic analysis of female mice treated with busulphan or isopropyl methane sulphonate (IMS), and injected with male bone marrow the donor contribution to both total marrow cellularity and spleen colony forming cells (CFU-S) was assessed for up to 6 months after transplant. In the mice treated with busulphan the marrow cells yielded metaphases of which between 40% and 83% were of donor type. Between 60% and 97% of metaphases in spleen colonies formed in irradiated mice were of donor type during the 24-week study period. In contrast, mice prepared for the transplant with IMS showed no cells of donor type at any time after transplant, neither did they possess CFU-S of donor type. We were therefore led to conclude that the donor cells made no contribution to longterm engraftment in mice prepared with IMS, whilst in those prepared with busulphan they were the predominantly active haemopoietic cells. These results are consistent with a model of haemopoiesis in which the most primitive cells reside in a 'niche' where they are resistant to the effects of IMS but susceptible to the action of busulphan. Busulphan may vacate some niches to allow engraftment by transplanted marrow, whilst IMS yields no unoccupied niches for grafted cells to occupy, and cannot therefore lead to a stable chimaerism.