Purpose/Objective: The development of pulmonary fibrosis limits the dose of therapeutic lung radiation. The pleiotrophic cytokine TGFb is thought to be central to this process and to other fibrotic diseases. The integrin avb6 activates TGFb from its latent form, and plays an important role in localizing TGFb effects; it is dramatically upregulated on lung epithelial cells before fibrosis develops after injury. Furthermore, knockout mice that lack avb6 are protected from lung fibrosis after radiation exposure, as well as from other fibrotic stimuli such as bleomycin. This makes inhibition of the avb6 integrin a possible therapeutic target and, to that end, a monoclonal antibody has been developed. Blockade at another level in this pathway by soluble TGFb receptor, which inhibits all TGFb activity, prevents early radiation pneumonitis and bleomycin-induced fibrosis. We compared the effects of anti-avb6 mAb and soluble TGFb receptor on the late pulmonary fibrotic response after thoracic radiation in a mouse model. Materials/Methods: We injected 149 C57BL/6 mice with various doses of anti-avb6 antibodies (0.3 mg/kg, 1 mg/kg and 10mg/kg), soluble TGFb receptor (5 mg/kg), or appropriate controls (PBS and IgG), starting 15 weeks after 14Gy thoracic radiation. Mice were monitored daily and sacrificed if in respiratory distress or moribund. We assessed fibrosis in mice sacrificed at 26 weeks by calculating the percent fibrosis area of paraffin-embedded, formalin-fixed tissue sections that were stained with Masson trichrome. Expression of avb6 was assessed by immunohistochemistry. Results: We found that the antibody caused a dose-dependent attenuation of fibrosis. Soluble TGFb receptor also inhibited fibrosis, but did not block the upregulation of avb6. Kaplan-Meier analysis showed no survival benefit as a result of avb6 blockade; in fact, there was a trend to excess mortality in the high-dose group (p=0.08). Conclusions: The prevention of pulmonary fibrosis by the late administration of either soluble TGFb receptor or antibody to avb6 clearly demonstrate that radiation-induced pulmonary fibrosis is a TGFb-dependent process. Although TGFb upregulates the avb6 integrin in vitro, we did not observe that blockade of TGFb activity altered avb6 expression in vivo. The 1 mg/kg dose of anti-avb6 but not the 0.3 mg/kg dose blocked fibrosis. However the trend to higher mortality with treatment with the higher dose antibody (10 mg/kg), if confirmed in further experiments, may indicate that partial rather than high-level inhibition of avb6-mediated TGFb activation may yield optimal results. These results support further investigation into blockade of avb6 as a therapeutic modality.
Read full abstract
Nov 16, 2010
A L Tatler + 5
Open Access
