Analysis of tumor‐associated stromal cells using SCID GFP transgenic mice: contribution of local and bone marrow‐derived host cells

The green fluorescence protein (GFP) from the UBI-GFP/BL6 transgenic line was bred into C57BL/6J-scid and C.B-17-scid mice for investigating host-tumor cell interactions. These mice express high levels of GFP under the control of the ubiquitin promoter in virtually all cells examined. In tumor tissue generated by implanting tumor cells in the GFP transgenic SCID mice, the tumor cells and tumor-associated murine host cells were clearly distinguished by GFP expression. A population of cells expressing the endothelial cell marker VEGFR-2/Flk-1, and the progenitor markers c-Kit and Sca-1, were incorporated into tumor tissue. The majority of the Flk-1-positive cells were hematopoietic-derived cells that coexpressed CD45. To investigate the contribution of bone marrow-derived cells to the formation of tumor vessels and stroma, tumor cells were implanted in nontransgenic SCID mice that received a bone marrow transplant from GFP-expressing SCID mice. Although GFP-positive cells were readily detected by histology in tumors taken from bone marrow transplanted animals, they were spatially isolated and lacked organization. In contrast, if tumors were implanted in nontransgenic SCID mice adjacent to a patch of transplanted GFP-expressing skin, these tumors recruited GFP-positive cells that organized into tumor vessels. The results demonstrate that hematopoietic-derived cells, including Flk-1+/CD45+ cells, readily colonized the tumor stroma but were minimally incorporated in the tumor vasculature. The majority of the tumor vessels were instead recruited from tissue adjacent to the tumor. The expression of Flk-1 on nonendothelial, tumor-associated host cells raises the possibility that VEGF antagonists, such as Avastin, could inhibit tumor growth by a mechanism involving hematopoietic-derived CD45+/Flk-1+ cells, in addition to direct suppression of endothelial cell function.