Chemoattraction of femoral CD34+ progenitor cells by tumor-derived vascular endothelial cell growth factor.

Patients and animals with GM-CSF-producing tumors have an increased number of mobilized CD34+ progenitor cells within their peripheral blood and tumor tissue. These CD34+ cells are inhibitory to the activity of intratumoral T-cells. The present study used the murine Lewis lung carcinoma (LLC) model to assess mechanisms that could lead to the accumulation of CD34+ cells within the tumor tissue. In vitro analyses showed that LLC tumor explants released chemoattractants for normal femoral CD34+ cells. The LLC tumor cells contributed to the production of this activity since CD34+ cell chemoattractants were also released by cultured LLC cells. Antibody neutralization studies showed that most, although not all, of the chemotactic activity that was produced by LLC cells could be attributed to VEGF. In vivo studies with fluorescent-tagged CD34+ cells showed their accumulation within the tumor tissue, but not within the lungs, spleen or bone marrow, suggesting a selective accumulation within the tumor. Whether or not VEGF could chemoattract CD34+ cells in vivo was measured with a VEGF-containing Matrigel plug assay. Infusion of fluorescent-tagged CD34+ cells into mice after the plugs became vascularized revealed the accumulation of fluorescent-tagged cells within the plugs. However, these CD34+ cells failed to accumulate within the VEGF-containing Matrigel plugs when they were infused together with neutralizing anti-VEGF antibody. Through a combination of in vitro and in vivo analyses, the LLC cells were shown to be capable of chemoattracting CD34+ cells, with most of the tumor-derived chemotactic activity being due to tumor release of VEGF.