The systemic delivery of bleomycin (BLM) to mice through subcutaneously implanted osmotic minipumps may be used to experimentally mimic the typical features of systemic sclerosis and related interstitial lung diseases. The published studies on this model principally focused on induced dermal modifications, probably because lung lesions are typically mild, subpleurally localized and difficult to analyze. The use of high BLM doses to increase their severity has been proposed, but is ethically questionable because of compromising animal welfare. We propose a tailored histomorphometric method suitable to detect and quantify this type of mild lung lesions. Using a two steps automated image analysis, a peripheral region of interest with a depth of 250 µm from the pleural edge was defined on whole slide images and the fibrotic foci were histomorphometrically characterized. The effects of different BLM doses on lung alterations were evaluated in C57BL/6 mice and 60 U/kg resulted in a fair compromise between fibrotic lesions and animal welfare. This dose was also tested in time-course experiments. The analysis revealed a peak of histologic fibrotic-like alterations, cytokine expression, metalloprotease and macrophagic activation, between the 21th and 28th day post pump implant. The induced dermal fibrosis was characterized by the progressive loss of the white dermal adipose layer, an increase of dermal thickness, dermal hyperplasia and more compacted collagen fibers. Despite the trend to spontaneous resolution, our model allowed a double organ readout of the BLM effect and the identification of a therapeutic window for testing pharmacological compounds without using life threatening doses.
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