Extracellular matrix remodeling in equine sarcoid: an immunohistochemical and molecular study

Equine sarcoids are locally invasive, fibroblastic benign skin tumors. Bovine papillomavirus type-1 (BPV-1) and/or Bovine papillomavirus type-2 (BPV-2) are believed to be the causative agent of sarcoids, although the mechanisms by which the virus induce the tumor are still poorly understood. We hypothesized that in genetically predisposed equines latent BPV infection may be reactivated by immunosoppression and/or mechanical injury leading to a form of pathologic wound which may transform into a sarcoid. In this study, we investigated in 25 equine sarcoids and in five normal skin samples the histological features and evaluated the immunohistochemical and molecular expression of type I and type III Collagen, vimentin (VIM), alfa Smooth Muscle Actin (α-SMA), Matrix Metalloproteinase (MMPs) -2, 9, 14 and tissue inhibitor of metalloproteinase 2 (TIMP-2).

In 64% of investigated sarcoids, type I collagen staining was stronger than that of type III collagen. In 80% of sarcoids, SFs were strongly positive for vimentin and negative for α-SMA; the remaining sarcoid samples (20%) showed 70-80% of SFs labeled for vim and approximately 20-30% labeled for α-SMA. Moreover, all sarcoid specimen showed a variable staining pattern (weak to moderate) for MMP-9 and MMP-14, and a moderate to strong staining for MMP-2 and TIMP-2. Biochemical analysis confirmed immunohistochemical results and showed in sarcoids, for the first time, the cleaved form of MMP9, the 35 KDa active species for MMP-9.

This study revealed that in equine sarcoids exhibit an altered turnover of the Extracellular Matrix (ECM) deposition and degradation, as result of an altered expression of MMPs and TIMPs. Therefore, these observations seem to confirm that the basic mechanism for growth of equine sarcoids could be a neoplastic transformation during wound healing.