Frédérique Hubin

The study of the human hematopoietic system would be facilitated by availability of a relevant animal model. Because the medullar microenvironment is made of different types of cells, interactions between hematopoietic cells and stromal cells are difficult to analyze in detail. As an approach for establishing an in vivo model to dissect these interactions, we grafted murine bone marrow fibroblastic cells (MS-5 cell line) with hematopoietic cells into the kidney capsule of syngenic mice. To identify the origin of cells present in the graft, we used green fluorescent protein-stable transfected MS-5 cells for the transplantation. To analyze the evolution of stromal cells and identify hematopoietic cells able to develop in these conditions, we performed morphology, histochemistry, and immunohistology on tissue sections at different times after transplantation. When injected alone, MS-5 cells differentiate into adipocytes. When injected with a bone marrow suspension or with isolated CD45+ cells (leukocytes), the stromal cells keep their fibroblastic morphology and their alkaline phosphatase expression and sustain granulopoiesis. When injected with hematopoietic stem cells called c-kit+ Sca-1+ Lin- suspension, clusters of hematopoietic cells are also observed: They do not present any granulopoietic activity and do not belong to B or T population nor to erythroid lineage. They are quiescent, induce bone marrow recovery and survival of lethally irradiated recipients, are able to form macroscopic colonies in the spleen, and are able to form very few colonies in vitro, suggesting that they are hematopoietic stem cells. In conclusion, our results show that reticular fibroblastic stromal cells MS-5 sustain the survival of stem cells and are not able to induce their differentiation. However, they can control differentiation, proliferation, and/or survival of hematopoietic cells engaged in myeloid lineage.

Attempts were made to establish models to study interactions between marrow stromal cells and hematopoietic cells in vivo. The approach was to create a NOD-SCID-hu murine model of long-term human hematopoiesis by implantation of a human adult bone fragment. Nine to 12 weeks posttransplantation, human CD45+ cells were detected in the blood and the spleen of some mice. The histology of the human transplant showed that human bone fragment was viable at 9 weeks. Moreover, vessels of human origin, as assessed by immunohistochemical detection of human β₂-microglobulin, were observed in the mouse tissue surrounding the transplanted human fragment.