Purpose: The lung is the major dose-limiting organ for radiotherapy of cancer in the thoracic region. Immediate cellular damage after irradiation is supposed to result in cytokine-mediated multicellular interactions with induction and progression of inflammatory and fibrotic tissue reactions. Pentoxifylline (PTX) down-regulates the production of proinflammatory cytokines, particularly TNF-alpha, in response to noxious stimuli and may therefore provide protection against radiation-induced, cytokine-mediated cellular damage. The purpose of this study was to investigate the temporal and spatial release of TNF-alpha in the lung tissue after thoracic irradiation with 12Gy. In addition, we evaluated the ability of PTX to reduce the radiation-induced TNF-alpha release in this animal model of thoracic irradiation.
Materials and methods: C57BL/6J mice were exposed to either sham irradiation or single fraction of 12Gy delivered to the thorax. Four study groups were defined: those that received neither irradiation nor PTX (NT group), those that received PTX but no irradiation (PTX group), those that underwent irradiation without PTX (XRT group) and those that received both PTX and irradiation (PTX/XRT group). Treated and sham-irradiated mice were sacrificed corresponding to the latent period and the pneumonic phase. The TNF-alpha mRNA expression in the lung tissue was quantified by 'real-time' quantitative reverse transcriptase polymerase chain reaction (RT-PCR). Immunohistochemical detection methods (alkaline phosphatase anti-alkaline phosphatase (APAAP)) and automated image analysis were used for objective quantification of TNF-alpha protein expression.
Results: Following thoracic irradiation with a single dose of 12Gy (XRT group), radiation-induced TNF-alpha mRNA release in the lung tissue was significantly increased during the acute phase of pneumonitis (P<0.05). The elevated levels of TNF-alpha mRNA during the pneumonic phase correlate with a significant increase of positive inflammatory cells, predominantly macrophages, in the lung parenchyma (P<0.05). In contrast to the radiation-only group (XRT-group), the lung tissue of the PTX-treated mice (PTX/XRT group) revealed only a minor radiation-mediated TNF-alpha response on mRNA and protein level.
Conclusions: This study demonstrates a significant radiation-induced increase of TNF-alpha (on mRNA and protein level) in the lung tissue during the pneumonic phase. The predominant localisation of TNF-alpha in areas of inflammatory cell infiltrates suggests involvement of this cytokine in the pathogenesis of radiation-induced lung injury. In addition, we observed a pronounced reduction of the TNF-alpha mRNA and protein production in the study group that received both PTX and radiation (PTX/XRT group) as compared to the radiation-only group (XRT group). Therefore our results indicate that PTX down-regulates the TNF-alpha mRNA and protein production in the lung tissue in response to radiation.