Spatio-Temporal Bone Remodeling after Hematopoietic Stem Cell Transplantation.

The interaction of hematopoietic cells and the bone microenvironment to maintain bone homeostasis is increasingly appreciated. We hypothesized that the transfer of allogeneic T lymphocytes has extensive effects on bone biology and investigated trabecular and cortical bone structures, the osteoblast reconstitution, and the bone vasculature in experimental hematopoietic stem cell transplantations (HSCT). Allogeneic or syngeneic hematopoietic stem cells (HSC) and allogeneic T lymphocytes were isolated and transferred in a murine model. After 20, 40, and 60 days, bone structures were visualized using microCT and histology. Immune cells were monitored using flow cytometry and bone vessels, bone cells and immune cells were fluorescently stained and visualized. Remodeling of the bone substance, the bone vasculature and bone cell subsets were found to occur as early as day +20 after allogeneic HSCT (including allogeneic T lymphocytes) but not after syngeneic HSCT. We discovered that allogeneic HSCT (including allogeneic T lymphocytes) results in a transient increase of trabecular bone number and bone vessel density. This was paralleled by a cortical thinning as well as disruptive osteoblast lining and loss of B lymphocytes. In summary, our data demonstrate that the adoptive transfer of allogeneic HSCs and allogeneic T lymphocytes can induce profound structural and spatial changes of bone tissue homeostasis as well as bone marrow cell composition, underlining the importance of the adaptive immune system for maintaining a balanced bone biology.

Novel pre-clinical mouse models for chronic Graft-versus-Host Disease.

Despite considerable progress in allogeneic hematopoietic cell transplantation (allo-HCT) has been achieved over the past years, chronic Graft-versus-Host Disease (cGvHD) still contributes to high morbidity rates, thus remaining a major hurdle in allo-HCT patients. To understand the complex pathophysiology of cGvHD and to develop refined prophylaxis and treatment strategies, improved pre-clinical models are needed. In this study, we developed two murine cGvHD models, which display high long-term morbidity but low mortality and depict the heterogeneous clinical manifestations of cGvHD seen in patients. We established a haploidentical C57BL/6→B6D2F1 allo-HCT model that uses myeloablative radiation and G-CSF-mobilized splenocytes as stem cell source and a sub-lethally irradiated Xenograft model, which utilizes the transfer of human peripheral blood mononuclear cells (PBMCs) into NOD scid gamma (NSG)-recipients. We characterized both mouse models to exhibit diverse clinical and histopathological signs of human cGvHD as extensive tissue damage, fibrosis/sclerosis, inflammation and B cell infiltration in cGvHD target organs skin, liver, lung and colon and found a decelerated immune cell reconstitution in the late phase after HCT. Our pre-clinical models can help to gain a deeper understanding of the target structures and mechanisms of cGvHD pathology and may enable a more reliable translation of experimental findings into the human setting of allo-HCT.

The splenic marginal zone shapes the phenotype of leukemia B cells and facilitates their niche-specific retention and survival.

Microenvironmental regulation in lymphoid tissues is essential for the development of chronic lymphocytic leukemia. We identified cellular and molecular factors provided by the splenic marginal zone (MZ), which alter the migratory and adhesive behavior of leukemic cells. We used the Cxcr5-/-Eµ-Tcl1 leukemia mouse model, in which tumor cells are excluded from B cell follicles and instead accumulate within the MZ. Genes involved in MZ B cell development and genes encoding for adhesion molecules were upregulated in MZ-localized Cxcr5-/-Eµ-Tcl1 cells. Likewise, surface expression of the adhesion and homing molecules, CD49d/VLA-4 and CXCR7, and of NOTCH2 was increased. In vitro, exposing Eµ-Tcl1 cells or human CLL cells to niche-specific stimuli, like B cell receptor- or Toll-like receptor ligands, caused surface expression of these molecules characteristic for a follicular or MZ-like microenvironment, respectively. In vivo, inhibition of VLA-4-mediated adhesion and CXCL13-mediated follicular homing displaced leukemic cells not only from the follicle, but also from the MZ and reduced leukemia progression. We conclude that MZ-specific factors shape the phenotype of leukemic cells and facilitate their niche-specific retention. This strong microenvironmental influence gains pathogenic significance independent from tumor-specific genetic aberrations.