Abstract 3524 Poster Board III-461 Several strains of immunodeficient mice have been employed in the effort to model both normal and aberrant growth and differentiation of human hematopoietic cells in vivo. Despite numerous successes in this field, room remains for improvement. For example, robust models of in vivo differentiation and expansion of human erythroid and T cells are lacking or challenging at best. With the goal to further improve in vivo human hematopoietic cell study, we have crossed the NOD/SCID IL2Rγ knockout mice (NSG) with the NOD/SCID-SGM3 mice (NS-SGM; transgenic expression of human cytokines SCF, GM-CSF, and IL-3) to create a new strain of immunodeficient mice (NSG-SGM3). Uncultured umbilical cord blood CD34+ cells (UCB) were used to initiate xenografts in these three strains as well as NOD/SCID (NS) mice for comparison. Although xenografts with high human composition can be set up in NS mice, the graft, which is B-lymphoid biased without any measurable T cell component, tends to fade over time. In comparison, NSG mice offer longer sustained engraftment with a predominantly B-lymphoid bias and an inconsistent minor T cell component that is absent in NS mice. These mice also display increased levels of human cells in the periphery, allowing for easier measurement of the graft. Similar to NS grafts, NS-SGM3 grafts are initiated at high levels of chimerism, decrease significantly over time, lack T cells, and have a relatively low peripheral involvement. However, the graft consists of a more balanced myeloid/lymphoid distribution. The NSG-SGM3 combines some of the useful properties of the parent strains. Grafts remain at high, sustained levels through 16 weeks while maintaining a fairly balanced myeloid/lymphoid contribution. Chimerism in the NSG-SGM3 mice is readily detectable in the periphery as well as the BM. Remarkably, as early as 8 weeks post-transplant, we observed consistently high human T cell engraftment and expansion far superior to what we observed in NSG mice in parallel experiments using the same UCB samples. Flow cytometric analysis revealed the presence of numerous T cell sub-populations, suggesting significant recapitulation of human T cell development and function in NSG-SGM3 mice. Although it remains unclear as to why the unique combination of SGM3 cytokine expression and IL2Rγ knockout promotes enhanced human T cell engraftment and expansion, this mouse presents an improved opportunity to examine human T cell development and function in a xenograft setting as well as modeling T cell associated diseases including HIV infection, GVHD, and T cell leukemia. Disclosures: No relevant conflicts of interest to declare.