The understanding of human stem cell biology has undergone explosive growth in the past decade, due largely to improvements in immunodeficient mouse xenotransplantation models. Aspects of human stem cell biology such as homing after intravenous infusion and the subsequent differentiation of transplanted stem cells are best studied in vivo. The use of the immunodeficient mouse as a recipient of the human stem cell graft is now a common and cost-effective strategy, and the number of new and improved strains that are available are increasing each year. The most common immunodeficient mouse xenograft strain used to date is the NOD/LtSz-scid/scid (nonobese diabetic–severe combined immunodeficient [NOD/SCID]).1,2 NOD/SCID/beta-2-microglobulin null (NOD/SCID/B2M null) mice have a more absolute immunodeficiency than the NOD/SCID strain and have virtually no natural killer (NK) cell function, so they are highly permissive for acceptance of human stem cells.3-5 Newer strains such as the NOD/LtSz-Rag-1 null mouse,6 the Rag-2 null/common gamma chain null mouse,7-9 and the NOD/SCID/MPSVII (NOD/SCID/mucopolysaccharidosis type VII) mouse10 have all been recently characterized, and each offers a different benefit to the xenotransplantation field.
However, a common theme that runs through all studies involving human hematopoiesis in mice is that the murine growth factors are not suitably cross-reactive to allow development of all human blood cell lineages to maturity. The seminal observation that human cell engraftment in immunodeficient mice was augmented by human-specific cytokines was made by Kamel-Reid and Dick in 1988.11 In this report, human interleukin-3 (IL-3), which is not cross-reactive from mouse to human, was injected intraperitoneally into each mouse every 48 hours, for the duration of each experiment. Although innovative and elegant, this technique was relatively costly and time consuming. Therefore, our group developed a method to deliver human IL-3 and then other cytokines in vivo from engineered marrow stromal cells/mesenchymal stem cells (MSCs) cotransplanted with human hematopoietic stem cells into the mice.12,13 Next, Bock et al generated SCID–transgenic mice expressing the genes for human interleukin-3, granulocyte-macrophage colony-stimulating factor (GM-CSF), and stem cell factor, and showed an increase in the success of human hematopoietic engraftment14 compared with control SCID mice. This study was accomplished by the generation of human cytokine-transgenic mice in standard strains that are appropriate for conventional plasmid microinjection, followed by extensive backcrossing to move the transgenes onto the SCID strain. It would be very advantageous to now have a method to test production of novel cytokines in the newer, NK-cell–deficient strains of immunodeficient mice that allow enhanced human stem cell engraftment over the traditional SCID strains.
In the current issue, Punzon et al (page 580) report a simplified method for generating human growth factor transgenic immunodeficient mice. They had previously analyzed the capacity of immature human dendritic cells to support HIV-1 infection in immunodeficient mice and found that productive infection is dependent on the presence of exogenously administered recombinant human GM-CSF (rhGM-CSF). Therefore, to provide hGM-CSF in the most convenient manner in the mice, they used direct lentivirus-mediated transgenesis of fertilized oocytes from NOD/SCID mice. This technique proved to be highly efficient in this strain, eliminating the need for microinjection into the conventional strains used for transgenesis and subsequent extensive backcrossing, which is lengthy in a strain such as NOD/SCID where multiple loci are involved in the phenotype. Due to its relative ease and cost effectiveness in comparison with conventional backcrossing, the single cell embryo transduction method should significantly improve the efficiency with which the murine microenvironment can be humanized, allowing improved studies of human stem cell biology in immunodeficient mice.
