353. Retrovirally-Marked Human Bone Marrow Derived Mesenchymal Stem Cells Attenuate Radiation Induced Pneumonitis in a Xenotransplant Model

Top of pageAbstract Thoracic radiation is used to treat patients with malignancies to improve survival and decrease symptoms. Radiation induced lung toxicity limits thoracic radiation doses and volumes and can restrict therapeutic use of radiation. Clinical radiation pneumonitis can be life-threatening, despite aggressive steroid treatment, especially in those patients with pre-existing pulmonary disease. We hypothesize that bone marrow derived mesenchymal stem cells (BMSC) assist in repair of irradiated lung tissue, and will preferentially localize to tissue with radiation-induced DNA damage. We have developed a xenotransplant model of radiation-induced pulmonary pneumonitis that will allow transplantation of adult human bone marrow mesenchymal stem cells for support of endogenous repair mechanisms and attenuation of pro-fibrotic inflammatory cytokine cascades. We have previously shown that BMSC have functional CXCR4 and c-met receptors, two molecules involved in trafficking and homing of various cell populations to areas of injury. Ionizing radiation was delivered to NOD-SCID MPS7 in increasing amounts either as total body irradiation (TBI) or thoracic-limited irradiation. Dose rate was controlled as well as total radiation administered. BMSC retrovirally engineered with an MND-eGFP-SN retroviral vector were administered at the time of irradiation and post-radiation serum collections were performed at hourly intervals for 12 hours. Serum elevation of hepatocyte growth factor (HGF), transforming growth factor beta-1 (TGF-b1), and interferon-gamma (IFN-g) were measured by ELISA; HGF in particular was upregulated in a dose-response fashion by total radiation dose and rate of dose administration. In addition to serum collections, lungs of irradiated mice were excised, inflated, and sectioned for immunohistochemistry against phosphorylated gamma-H2Ax, and TUNEL, both early markers of DNA damage and predictive of apoptosis. Parallel cohorts were allowed to develop pulmonary pneumonitis and blinded histologic analysis was performed against non-transplanted controls for fibrosis. Finally, transplanted mice were screened by viral sequence specific PCR for persistence of BMSC at up to 75 days post-transplant, and explanted GFP+BMSC cold be explanted and cultured in G418 media at all timepoints. Total human cell engraftment was determined simultaneously with retroviral insertions per cell by multiplex QPCR specific for murine Rapsyn, human beta-Globin, and eGFP respectively. In summary, BMSC in this model of radiation-induced pneumonitis served to reduce total serum amounts of pro-inflammatory cytokines, as well as reduce overall fibrosis. The cells maintained transgene expression for greater than 75 days and were viable on explant. We conclude that mesenchymal stem cells are particularly well suited for this application due to their immune-privileged status and ability to extravasate in response to chemotactic injury signals, and may hold promise in an autologous transplant setting to alleviate radiation-induced lung toxicity.