Bone Marrow Transplantation, (1998) 21, 345–354  1998 Stockton Press All rights reserved 0268–3369/98 $12.00 Primitive stem cells alone mediate rapid marrow recovery and multilineage engraftment after transplantation WE Nibley 1 and GJ Spangrude 1,2 1 Department of Medicine, Division of Hematology/Oncology and 2 Department of Pathology, Division of Cell Biology and Immunology, University of Utah, Salt Lake City, UT, USA Summary: but do not make a sustained contribution to hematopo- iesis. 5–7 Other cells contribute little to early engraftment, but are very active in long-term blood and bone marrow The engraftment of hematopoietic stem and progenitor cells in lethally irradiated mice was evaluated following repopulation. 3,8–10 The functional activity of these various stem and progenitor cell populations during early transplants of enriched hematopoietic cell populations which were defined by surface antigen and rhodamine- engraftment following bone marrow transplants is only beginning to be described, and the details of lineage 123 staining. Phenotypically defined long-term repopul- ating stem cells, short-term pluripotent progenitors, and relationships between stem cell subsets, the regulation of engraftment by mature blood components, and the kinetics committed myeloerythroid progenitors all rapidly reconstituted splenic cellularity and peripheral red of post-transplant hematopoietic organ repopulation remain unclear. blood cells after transplant into myeloablated animals. In contrast, marrow cellularity was reconstituted only A subset of engraftable hematopoietic stem and progeni- tor cells has been defined in certain mouse strains by the after transplant of long-term repopulating stem cells. In addition, peripheral blood platelet and lymphocyte cell surface antigen phenotype Thy-1.1 low Lin neg Sca-1 + . 11,12 These cells represent approximately 0.05% of total bone counts increased only after transplantation of the long- term repopulating population. Transplantation of marrow and are 500- to 2000-fold enriched for stem and progenitor cell functions as measured by various assays. 13 highly enriched multipotent progenitors resulted in a transient increase in peripheral blood myeloid cells that Significant heterogeneity exists within the Thy-1.1 low Lin neg- Sca-1 + population, based on analysis using additional mark- occurred with kinetics similar to that seen after trans- plant of the primitive stem cell population. Erythroid ers. 5,14–16 In particular, the fluorescent mitochondrial dye rhodamine-123 (Rh-123) discriminates a spectrum of stain- reconstitution was similar in all groups, suggesting that the effect of myeloerythroid progenitor cells in mouse ing intensities which correlates with cell cycle and cellular activation status as well as p-glycoprotein-mediated efflux marrow transplants is primarily on reconstitution of the erythroid lineage due to splenic hematopoiesis. Collec- of the dye. 4,16–18 It has been shown that cell populations characterized by bright Rh-123 staining (Rh high ) can tively, these results suggest that the cells which function to rapidly reconstitute the nucleated blood cells in a mediate early engraftment of lethally irradiated animals, but that this engraftment is transient. 6,19 In contrast, cells which transplant setting are more closely related to primitive, marrow-homing stem cells than to committed stain at a low level with Rh-123 (Rh low ) contribute to early engraftment but also mediate long-term repopulation of all progenitor cells. Keywords: engraftment kinetics; hematopoiesis; progeni- hematopoietic lineages. 2,4,6,20,21 In addition, limiting dilution experiments have revealed a subset of Rh low cells tor cells; flow cytometry that mediates late, sustained engraftment without an early contribution to hematopoietic function, 22 consistent with reports of delayed kinetics of long-term hematopoietic The sustained production of blood cells following bone reconstitution by quiescent stem cells. 9,10,23 marrow transplantation is mediated by hematopoietic stem To clarify the roles played by various hematopoietic cell and progenitor cells contained within the bone marrow populations in post-transplant recovery, we compared the graft. While it is a common perception that hematopoietic repopulation kinetics of bone marrow, spleen, and periph- stem cells can be strictly defined as a functionally homo- eral blood in mice following lethal irradiation and intra- geneous population, the bone marrow stem cell compart- venous injection of each of three hematopoietic cell popu- ment has in fact been demonstrated to consist of a complex lations which reflect the heterogeneity of the mouse stem hierarchy of multipotent cells. 1–4 Some multipotent cells and progenitor cell pool. All three populations are charac- give rise to differentiated progeny rapidly after transplant terized by low cell surface expression of the Thy-1.1 anti- gen (Thy-1.1 low ) and the absence of a panel of antigens characteristic of differentiated hematopoietic lineages Correspondence: Dr GJ Spangrude, Department of Pathology, Division of (Lin neg ). 24,25 This phenotype includes .99% of trans- Cell Biology and Immunology, University of Utah, Salt Lake City, UT plantable hematopoietic stem cells in the mouse strain util- 84132, USA Received 27 May 1997; accepted 29 August 1997 ized here (C57BL) 12 within a cell fraction representing less