NSGS Mice Develop a Progressive, Myeloid Cell Dependent Aplastic Anemia and Bone Marrow Failure Upon Engraftment With Human Umbilical Cord Blood CD34+ Cells

One key limitation of hematopoietic xenograft mouse models is the relative lack of effective myelopoiesis compared with human hematopoiesis. NSGS mice expressing human myelo-supportive cytokines allow for improved myeloid cell output upon human hematopoietic cell transfer. Surprisingly, we found that NSGS mice developed a severe, eventually fatal anemia upon humanization with umbilical cord blood (UCB) CD34+ cells. Anemia was characterized by a consistent, progressive drop in red blood cell numbers and hemoglobin that was correlated with higher MCV and an increased reticulocyte production at early time points. Eventually total white blood cell and platelet counts were also affected and reticulocyte numbers declined to or below normal. These effects were only apparent in mice with human grafts, and were also not observed in NSG mice with human grafts. Bone marrow cellularity was 40% of control humanized NSG mice and spleens were significantly larger, implying bone marrow dysfunction and extra medullary hematopoiesis. Bone marrow sections from anemic mice clearly showed a general hypocellularity with abundant fatty tissue filling the majority of the intramedullary space. We were able to recover red blood cell counts and hemoglobin to some degree upon transfusion of non-humanized mouse blood, however, the effect lasted only 7 days. CFSE labeling of anemic blood allowed us to detect a decreased half-life of red blood cells of 20 days compared to 30 days for humanized NSG mice. Treatment of mice with Campath, a CD52 antibody, completely reversed the phenotype at the expense of the entire human graft, demonstrating clearly that this was a graft-mediated process. However, specific ablation of B and T cells with rituximab and OKT3 did not alter the phenotype, even when administered soon after CD34+ cell transplant to suppress development of lymphocytes. Additionally, dexamethasone and IV/IG were ineffective in reversing the disease. These data, coupled with negative findings from tests to detect mouse reactive human antibodies, point towards myeloid cell involvement. Indeed, further experiments revealed that specific ablation of human myeloid cells effectively reversed the anemia. In summary, these data highlight a potential pitfall for effective myelopoiesis in the xenograft setting. More broadly, these findings demonstrate the possibility for myeloid driven bone marrow dysfunction and may warrant investigation of myeloid cells in cases of idiopathic aplastic anemia that fail to respond to therapies directed at autoimmune lymphocyte activity.