PATTERNS OF ORGAN-SPECIFIC ENGRAFTMENT BY STEM-CELL SUBSETS AND COMMITTED PROGENITORS

The kinetics of blood and organ engraftment following transplants of d efined populations of hematopoietic stem/progenitor cells were investi gated utilizing cell populations defined by surface antigen and rhodam ine-123 staining, While long-term repopulating stem cells, short-term multipotent progenitors and committed progenitors all reconstituted pe ripheral blood red cells and splenic cellularity, only the population of cells that includes highly enriched long-term repopulating stem cel ls (Thy-1.1(low)Lin(neg)Sca-1(+)Rh123(low)) reconstituted marrow cellu larity. In addition, peripheral blood platelet and nucleated cell coun t increased only after transplant of the long-term repopulating popula tion. These results argue that the major cell population that function s to reconstitute hematopoiesis after bone marrow transplantation is a primitive, marrow-homing stem cell, Transplantation of highly enriche d multipotent progenitors that lack long-term reconstituting potential had no impact on hematopoietic recovery, apart from a transient incre ase in circulating erythrocytes. These results suggest that the primar y cell population that functions to reconstitute hematopoiesis in a tr ansplant setting is the long-term repopulating stem cell. This observa tion is discussed in the contest of the normal hematopoietic process.