Differential Response of Mouse Thymic Epithelial Cell Types to Ionizing Radiation-Induced DNA Damage

Abstract

Thymic epithelial cells (TECs) are the main components of the thymic stroma that support and control T-cell development. Preparative regimens using DNA-damaging agents, such as total body irradiation and/or chemotherapeutic drugs, that are necessary prior to bone marrow transplantation (BMT) have profound deleterious effects on the hematopoietic system, including the thymic stroma, which may be one of the main causes for the prolonged periods of T-cell deficiency and the inefficient T cell reconstitution that are common following BMT. The DNA damage response (DDR) is a complex signaling network that allows cells to respond to all sorts of genotoxic insults. Hypoxia is known to modulate the DDR and play a role affecting the survival capacity of different cell types. In this study, we have characterized in detail the DDR of cortical and medullary TEC lines and their response to ionizing radiation, as well as the effects of hypoxia on their DDR. Although both mTECs and cTECs display relatively high radio-resistance, mTEC cells have an increased survival capacity to ionizing radiation (IR)-induced DNA damage, and hypoxia specifically decreases the radio-resistance of mTECs by upregulating the expression of the pro-apoptotic factor Bim. Analysis of the expression of TEC functional factors by primary mouse TECs showed a marked decrease of highly important genes for TEC function and confirmed cTECs as the most affected cell type by IR. These findings have important implications for improving the outcomes of BMT and promoting successful T cell reconstitution.

The Transcription Factor Hif-1 Enhances the Radio-Resistance of Mouse MSCs

“…However, total body irradiation (TBI) and other preparative regimens required prior to BMT have important destructive effects on the supportive cell types that orchestrate recovery of the haematopoietic process (Williams and Gress, 2008 ; Cao et al, 2011 ). Indeed, reduction of the intensity of the preparative regimens has been shown to improve bone marrow HSC engraftment as well as the subsequent reconstitution of the immune system, probably due to the better preservation of the bone marrow and thymic microenvironments (Basara et al, 2002 ; Jiménez et al, 2005 ; Calvo-Asensio et al, 2017 ). Yet, while the protection of endogenous MSCs and/or the administration of exogenous MSCs are likely to have beneficial effects improving the outcomes of allogeneic BMT and in treating GVHD (Sugrue et al, 2017 ), MSC are also known to be recruited into the tumor microenvironment (Hanahan and Coussens, 2012 ; Kidd et al, 2012 ) where they are transformed into CAFs and promote tumorigenesis through various mechanisms (Kraman et al, 2010 ; Hanahan and Weinberg, 2011 ; Cuiffo and Karnoub, 2012 ; Hanahan and Coussens, 2012 ; Kidd et al, 2012 ; Barcellos-de-Souza et al, 2013 ).…”

DN2 Thymocytes Activate a Specific Robust DNA Damage Response to Ionizing Radiation-Induced DNA Double-Strand Breaks

“…Our group has previously shown that the capacity of irradiated mouse MSCs and TECs to repair DNA DSBs is modulated by hypoxia, correlating with an effect on their intrinsic radioresistance ( 34 , 40 ). Similar to the bone marrow, the thymus also consists of a hypoxic environment and therefore we were interested in investigating whether hypoxia may affect the radiobiology of DN2 thymocytes.…”

“…Previous studies ( 69 ) have indicated that ckit is a target gene for Notch signaling. Therefore, the decrease in CD117 expression may be the result of decreased Notch ligand expression by TECs following irradiation ( 34 ).…”

“…Similar to DN2 thymocytes, mesenchymal stromal cells (MSCs) that support hematopoiesis in the bone marrow, and thymic epithelial cells (TECs), which support thymopoiesis in the thymus, are also relatively radioresistant ( 32 – 34 ). In previous studies, we have demonstrated that the activation of the DDR plays important roles in enabling in vitro irradiated MSCs and TECs to quickly respond to IR-induced DNA damage and to engage molecular pathways that promote rapid DNA DSB repair, DNA damage checkpoint activation, and cell survival ( 34 , 35 ). Therefore, using the methods we have previously applied to investigate the DDR of irradiated MSCs, we aimed herein to investigate the role of the DDR in mediating the radioresistance of DN2 thymocytes.…”

Influence of Ionizing Radiation on Development of Thymus and Thymocytes

“…Clinical studies also suggest that a lower-intensity cytoreduction regimen leads to increased T cell lymphopoiesis, thus suggesting a direct link between thymus tissue damage and T cell formation. The studies described above have led to a more indepth examination of processes aimed at the mechanisms of protecting and restoring thymus and thymopoiesis tissue (38). Greater attention is drawn to this subject area within today's dynamically developing field of cancers immunotherapy, which primarily is directed to supporting and activating the natural immune response to eliminate tumours in the body.…”