Reduction of marrow hematopoietic progenitor and stem cell content is not sufficient for enhanced syngeneic engraftment

The mechanisms regulating long-term engraftment of primitive stem cells are largely unknown. Most conditioning strategies use myeloablative agents for experimental or clinical hematopoietic stem cell transplantation. Host con ditioning regimens, in part, have been designed on the assumption that tran s planted cells home to specific marrow sites and if these sites are occupi ed by host stem cells, engraftment will not take place. However, there is n ow evidence that stable long-term syngeneic engraftment may occur in the ab sence of host marrow stem cell depletion. To further study the association of engraftment with stem cell depletion, we investigated whether the marked egress of hematopoietic progenitor and stem cells from the marrow into the peripheral blood in C57BL6 mice following a single dose of cyclophosphamid e (day 1) and four days of G-CSF (days 3-6) afforded an increased opportuni ty for long term syngeneic donor engraftment. During and after mobilization , glucose phosphate isomerase (GPT)-1(b) mice received 30 x 10(6) GPI-1(n) marrow cells without further myeloablation. The level of donor/recipient ch imerism was assessed in cell lysates after six months. Increased long-term syngeneic donor engraftment was observed prior to mobilization (before day 6), during a period of active hematopoietic regeneration following the admi nistration of cyclophosphamide. Hematopoietic regeneration was evidenced by a reduced but rapidly increasing marrow cellularity and an increased propo rtion of hematopoietic progenitors in S phase. In contrast, long-term synge neic donor engraftment was not increased over controls during the period of maximum progenitor and stem cell mobilization (after day 5), At this time there were minimal numbers of progenitor and stem cells in the marrow. Thes e data suggest that in the absence of host stem cell ablation, maximal engr aftment does not occur during mar row progenitor or stem cell depletion, su ggesting that the presence of "open" marrow sites is not a prerequisite for engraftment, The mechanisms for increased engraftment during progenitor ce ll regeneration following cyclophosphamide need further investigation. Unde rstanding the mechanisms for engraftment without host stem cell ablation ma y allow strategies for improved long-term engraftment of syngeneic or autol ogous stem cells with reduced post transplant toxicity.