HUMAN ALLOGENEIC STEM-CELL MAINTENANCE AND DIFFERENTIATION IN A LONG-TERM MULTILINEAGE SCID-HU GRAFT

The ability to determine the functional capacity of putative human hem atopoietic stem cell (HSC) populations requires in vivo assays in whic h long-term multilineage differentiation can be assessed. We hypothesi zed that if human fetal bone was transplanted adjacent to a fetal thym us fragment in severe combined immunodeficient (SCID) mice, a conjoint organ might form in which HSC in the human bone marrow (BM) would mim ic human multilineage differentiation into progenitor cells, B cells, and myeloid cells; undergo self-renewal; and migrate to and differenti ate into T cells within the thymic microenvironment. To test this poss ibility, SCID mice were transplanted subcutaneously with HLA class I m ismatched fetal bone, thymus, and spleen fragments (SCID-hu BTS). We f ound that the BM of SCID-hu BTS grafts maintained B cells, myeloid cel ls, CD34(+) cells for at least 36 weeks posttransplant. Assayable hema topoietic progenitors colony-forming units-granulocyte-macrophage were present in 100% (66/66) of grafts over a period of 28 weeks. Cells wi th a HSC phenotype (CD34(+)Thy-1(+)Lin(-)) were maintained for 20 week s in SCID-hu BTS grafts. These CD34(+)Thy-1(+)Lin(-) cells had potent secondary multilineage reconstituting potential when isolated and inje cted into a secondary HLA mismatched SCID-hu bone assay and analyzed 8 weeks later. In addition, early progenitors within the BM of SCID-hu BTS grafts were capable of migrating to the human thymus and undergoin g differentiation through immature CD4(+)CD8(+) double-positive T cell s and produce mature T cells with a CD4(+)CD8(-) or CD8(+)CD4(-) pheno type that could be detected for at least 35 weeks. Phenotypically defi ned human fetal liver (FL) and umbilical cord blood (UCB) hematopoieti c stem cell populations were injected into irradiated SCID-hu BTS graf ts to assess their multilineage repopulating capacity and to assess th e ability of the BTS system to provide an environment where multiple l ineages might differentiate from a common stem cell pool. Injection of irradiated grafts with FL HSC or UCB HSC cells resulted in donor-deri ved B cells, myeloid cells, immature and mature T cells, and CD34(+) c ells in individual grafts when analyzed 8 weeks postreconstitution, fu rther showing the multipotential nature of these stem cell populations . In addition, a strong correlation was observed between maintenance o f host graft-derived CD8(+) cells and failure of donor stem cell engra ftment. Therefore, SCID-hu BTS grafts may be capable of rejecting an a llogeneic stem cell graft if not sufficiently T-cell depleted, indicat ing that this model may be useful for studying the functional capacity of HSC as well as factors or cells that are capable of promoting or p reventing allogeneic HSC engraftment in vivo. (C) 1995 by The American Society of Hematology.