Surveillance of spontaneous breast cancer metastasis by TRAIL-expressing CD34⁺ cells in a xenograft model.

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), delivered as a membrane-bound molecule expressed on the surface of adenovirus-transduced CD34(+) cells (CD34-TRAIL(+)), was analyzed for its apoptotic activity in vitro on 12 breast cancer cell lines representing estrogen receptor-positive, HER2(+) and triple-negative (TN) subtypes and for its effect on tumor growth, vascularization, necrosis, and lung metastasis incidence in NOD/SCID mice xenografted with the TN breast cancer line MDA-MB-231. Mesenchymal TN cell lines, which are the richest in putative tumor stem cells among the different breast cancer cell subtypes, were the most susceptible to apoptosis induced by CD34-TRAIL(+) cells. Indeed, tumor cell "stemness", assessed based on the proportion of CD44(+)/CD24(-/low) cells, was significantly correlated with susceptibility to TRAIL. Moreover, in vitro cytotoxicity experiments showed that CD34-TRAIL(+) cells selectively targeted CD44(+)/CD24(-/low) cells. Although in vivo treatment with CD34-TRAIL(+) cells did not lead to tumor growth inhibition, treated mice revealed significantly larger areas of necrosis associated with damage of tumor vasculature than did control mice. Moreover, lungs from MDA-MD-231 tumor-bearing mice were completely free of metastases at 12 days after the last injection of CD34-TRAIL(+) cells, whereas metastases were present in all control mouse lungs. An anti-metastatic effect of CD34-TRAIL(+) cells was also observed in a model of experimental lung metastases. The correlation between in vitro susceptibility to membrane-bound TRAIL and tumor stem cell content, together with CD34-TRAIL(+) cell-induced inhibition of the metastatic process, points to the selective targeting of cancer stem cells by CD34-armed cells and the potential value of such cells in eradicating tumor stem cells before the onset of overt metastases.