Parabiosis and transplantation models show no evidence of circulating dermal fibroblast progenitors in bleomycin-induced skin fibrosis

To test the hypothesis of an extra-dermal origin of dermal fibroblasts, parabiosis, and transplantation models were developed utilizing a collagen promoter green fluorescent protein (GFP) reporter transgene expressed in dermal fibroblasts. Parabiotic pairs were treated with bleomycin to induce the skin fibrosis that was evaluated for a dense deposition of collagen and inflammatory cell infiltrates in the thickened dermis in comparison with parabiotic pairs treated with saline. Although, in all cases, repeated injection of bleomycin for 4 weeks induced skin fibrosis, only a few GFP positive cells were detected in skin samples from some of the treated non-transgenic mice. Unexpectedly, similar results were observed in saline treated controls. Furthermore, bone marrow chimeras were created in which non-transgenic recipient mice received injections of bone marrow cell preparations isolated from pOBCol3.6GFP transgenic mice. After bone marrow chimerism had been successfully established, fibrotic lesions in the skin were induced by local bleomycin injections. Donor GFP expressing cells were observed in the skin from all recipient mice. However, no difference in the presence of GFP expressing cells was observed between non-treated mice or mice treated with bleomycin or saline. A large number of GFP expressing cells were observed in the lung preparations from all chimeric mice. Mac-3 antibody immunostaining confirmed a macrophage phenotype for these GFP expressing cells suggesting the expression of the pOBCol3.6GFP transgene in a non-collagen producing cell. Based on these observations, we found no evidence of circulating dermal fibroblast progenitors that participate in the development of bleomycin-induced skin fibrosis.