Human CD34+ Cells Expressing Membrane-Bound Tumor Necrosis Factor-Related Apoptosis-Inducing Ligand (TRAIL) Exert a Potent Anti-Lymphoma Effects by Targeting Tumor Vasculature.

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is expected to play a key role in cancer therapy due to its high cancer cell-specificity and potent antitumor activity. We have previously demonstrated that CD34+ cells transduced with an adenovirus encoding the full-length human TRAIL gene (CD34-TRAIL+) exert a potent anti-lymphoma effect in NOD/SCID mice. To investigate the mechanism of action of CD34-TRAIL+ cells, in vivo experiments were perfomed to analyze

1. the tumor homing capacity of CD34-TRAIL+ cells and

2. the effects of transduced cells on tumor vasculature.

Tumor homing of CD34-TRAIL+ cells was investigated in NOD/SCID mice bearing subcutaneous lymphoid tumors. Following a single intravenous injection of CD34-TRAIL+ cells (5 x 10⁶), nodules were excised at different time-points and immunostained with an anti-human CD45 antibody. Sections were digitally recorded and the total number of CD45+ cells per tissue section was then counted using the imaging software ImageJ (http://rsb.info.nih.gov/ij/). Tumor homing of CD34-TRAIL+ cells peaked 24 hours after cell injection when a mean of 200 CD34-TRAIL+ cells was recorded per 1 x 10⁵ tumor cells (0.2% CD45+ cells per 1 x 10⁵ tumor cells). Mice pretreatment with the anti-VCAM-1 antibody or the CXCR4 antagonist AMD3100 significantly reduced tumor homing of CD34-TRAIL+ cells, with 0.09% and 0.05% CD45+ cells being recorded per 1 x 10⁵ tumor cells, respectively. To analyze the effects of CD34-TRAIL+ cells on tumor vasculature, mice were injected with sulfo-biotin (0.1 mg, IV, 15 min) and tumor endotelial cells (TEC) were then revealed by staining frozen sections with horseradish peroxidase (HRP)-conjugated streptavidin. As compared with CD34-mock- or soluble (s)TRAIL-treated mice, treatment with CD34-TRAIL+ cells induced within 24 hours a significant (P ≤.001) increase of the thickness of the vessell wall (3.7 ± 1 μm vs 3.4 ± 1 μm vs 6 ± 1 μm, respectively) as well as the endothelial surface (10 ± 4% vs 7 ± 4% vs 21 ± 6% of total tissue section, respectively), suggesting that CD34-TRAIL+ cells induce an early vascular disruption. Confocal microscopic imaging of tumor sections double-stained with anti-CD31 and anti-TRAIL-R2 showed that this receptor was expressed by 8–12% of large tumor vessels. Interestingly, upon treatment with CD34-TRAIL+ cells, but not sTRAIL, TUNEL staining revealed an extensive apoptosis of CD31+/TRAIL-R2+ TEC. Forty-eight hours following injection of CD34-TRAIL+ cells, a 21-fold increase of apoptotic index was detected, which was associated with extensive necrotic areas (20% to 25% of tissue section). These data show that:

1. tumor homing of CD34-TRAIL+ cells induces extensive vascular damage, hemorrhagic necrosis and tumor destruction;

2. the antitumor effect of CD34-TRAIL+ cells is mediated by both indirect vascular-disrupting mechanisms and direct tumor cell killing.