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 hematopoietic stem cell (HSC) populations requires in vivo assays in which long-term multilineage differentiation can be assessed. We hypothesized that if human fetal bone was transplanted adjacent to a fetal thymus fragment in severe combined immunodeficient (SCID) mice, a conjoint organ might form in which HSC in the human bone marrow (BM) would mimic human multilineage differentiation into progenitor cells, B cells, and myeloid cells; undergo self-renewal; and migrate to and differentiate into T cells within the thymic microenvironment. To test this possibility, SCID mice were transplanted subcutaneously with HLA class I mismatched fetal bone, thymus, and spleen fragments (SCID-hu BTS). We found that the BM of SCID-hu BTS grafts maintained B cells, myeloid cells, CD34⁺ cells for at least 36 weeks posttransplant. Assayable hematopoietic progenitors colony-forming units- granulocyte-macrophage were present in 100% (66/66) of grafts over a period of 28 weeks. Cells with a HSC phenotype (CD34⁺Thy-1⁺Lin^-) were maintained for 20 weeks in SCID-hu BTS grafts. These CD34⁺Thy-1⁺Lin^- cells had potent secondary multilineage reconstituting potential when isolated and injected 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 undergoing differentiation through immature CD4⁺CD8⁺ double-positive T cells and produce mature T cells with a CD4⁺CD8^- or CD8⁺CD4^- phenotype that could be detected for at least 36 weeks. Phenotypically defined human fetal liver (FL) and umbilical cord blood (UCB) hematopoietic stem cell populations were injected into irradiated SCID-hu BTS grafts to assess their multilineage repopulating capacity and to assess the ability of the BTS system to provide an environment where multiple lineages might differentiate from a common stem cell pool. Injection of irradiated grafts with FL HSC or UCB HSC cells resulted in donor-derived B cells, myeloid cells, immature and mature T cells, and CD34⁺ cells in individual grafts when analyzed 8 weeks postreconstitution, further showing the multipotential nature of these stem cell populations. In addition, a strong correlation was observed between maintenance of host graft-derived CD8⁺ cells and failure of donor stem cell engraftment. Therefore, SCID-hu BTS grafts may be capable of rejecting an allogeneic stem cell graft if not sufficiently T-cell depleted, indicating 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 preventing allogeneic HSC engraftment in vivo.