THIOTEPA IMPROVES ALLOGENEIC BONE-MARROW ENGRAFTMENT WITHOUT ENHANCING STEM-CELL DEPLETION IN IRRADIATED MICE

Thiotepa (TT) has long been considered for inclusion in clinical bone marrow transplant (BMT) conditioning regimens in an attempt to prevent allograft rejection and leukemia relapse, These studies have been enc ouraged by initial murine experiments showing a clear improvement in a llogeneic bone marrow engraftment with addition of TT to total body ir radiation (TBI) where it was assumed that TT enhances donor-type chime rism via ablation of competing stem cells in the recipient, The aim of the present study was to re-evaluate the hematological toxicity of TT among different stem cell subsets that included primitive cells capab le of long-term repopulation and to assess how the combination of TT w ith TBI influences the development of donor engraftment in both syngen eic (B6-Gpi-1(a) --> B6-Gpi-1(b)) and H-2 compatible allogeneic (BALB. B10 --> B6) BMT models. At 24 h after TT (20 mg/kg) the femoral conten t of different stem cell subsets was determined from the frequency of transient repopulating, and the more primitive cobblestone area-formin g, cells (CAFCs) growing in stroma-supported cultures, This assay show ed a large TT-induced depletion (2% survival) of early clones developi ng at day 7 in culture but survival recovered towards normal for later appearing clones developing from more primitive CAFC subsets, The spa ring of these primitive stem cells was reflected as undetectable level s of donor marrow repopulation in recipients given TT followed by syng eneic BMT. Addition of TT to TBI did not significantly improve long-te rm engraftment of syngeneic marrow while this combination had a dramat ic effect in allogeneic BMT by preventing allograft rejection. In this respect TT shares similar properties with cyclophosphamide and sugges ts that the large improvement of allogeneic stem cell engraftment is a ttributable to the immune suppressive properties of TT rather than to its toxicity against host primitive stem cells.