PARENT SESSION

Experimental Therapeutics: Normal Tissue Response

(MS018) Endothelial stem/progenitor cells are X-ray sensitive with limited repair capacity.

Mendonca, Marc^{*,1, 2}, Chin-Sinex, Helen¹, Mead, Laura³, Yoder, Merv³, Ingram, David³, ¹ Radiation Oncology / Radiation and Cancer Biology Labs, Indianapolis, IN, USA² Medical and Molecular Genetics, Indianapolis, IN, USA³ Pediatrics, Herman B. Wells Center for Pediatric Research, Indianapolis, IN, USA

ABSTRACT- Introduction: A better understanding the role of the endothelial stem/progenitor cells in normal tissue injury and tumor response after radiation and/or chemotherapy treatment will open up new strategies to further improve therapeutic gain. The ability to clearly identify and isolate these stem/progenitor cell populations in vivo and to maintain these high proliferative potential cells in culture is critical for quantitative in vitro analysis. We have recently reported the identification and isolation of a novel hierarchy of endothelial progenitor cells isolated from human peripheral and umbilical cord blood, & vessel walls (Blood 2004, 2005). Objective: To complete the first in vitro characterization of the radiation response of three human primary endothelial stem/progenitor isolates CBM4, CBF10, and CBM12. These high proliferative potential cells were isolated from cord blood and are CD31, 141, 105, 146, and 144 positive and CD45 and 14 negative. Results: The plating efficiencies (PEs) of the three cell isolates were 0.34± 0.02, 0.20 ± 0.04, and 0.23 ± 0.06 respectively and were quite stable over the five to six weeks of culture. Population doubling times were 18.3± 2.6, 14.6 ±2.5, and 11.7 ± 3.4 hours respectively. The cells grew well in vitro and control and irradiated cells formed large visible colonies one week after single cell plating. The radiation sensitivity of the three endothelial stem / progenitor isolates were investigated with 160 and 250 kVp X-rays. Surviving Fractions (SF_2Gys) for CBM4, CBF10, and CBM12 were 0.34 ± 0.03, 0.26 ± 0.07, 0.24 ± 0.02 respectively. The differences in SF-2Gys were not statistically significant. Construction of delayed plating X-ray survival curves revealed exponential cell killing with little if any shoulder evident. D₀s for CBM4, CBF10, and CBM12 were 1.00 ± 0.1, 1.10 ± 0.1, and 1.40 ± 0.04 Gy respectively. These data suggest that endothelial stem/progenitor cells are relatively radiosensitive with limited repair capacity. Preliminary fractionation studies (5 Gy versus 2.5 Gy +T + 2.5 Gy) confirm modest split dose recovery. Conclusions: Taken together, the data suggest that endothelial stem cells are relatively radiosensitive compared to many tumor cells of epithelial origin and may play an important role in radiation-induced tumor response and normal tissue injury.

Key words: endothelial stem / progenitor cells, 160 & 250 kVp X-ray, In Vitro, Surviving Fraction