High numbers of CD163-positive macrophages in the fibrotic region of exuberant granulation tissue in horses

Simple Summary Horses are prone to develop a wound healing disorder on their limbs called exuberant granulation tissue (EGT). The exact mechanism for the formation of this tissue remains unknown but the inflammatory response is supposed to be an important contributing factor. In this article, we investigated this inflammatory response in both EGT wounds as well as in control horse wounds. In biopsies, we detected two types of immune cells: (1) immune cells involved in early inflammation (MAC387+ cells) and (2) immune cells important in the later phases of inflammation (CD163+ cells). We detected a higher number of immune cells in EGT wounds compared with the control wounds of 19 days old. This suggests that EGT wounds may not be able to proceed through further phases in the wound healing process or that the inflammation phase is prolonged. Exuberant granulation tissue (EGT) is a frequently encountered complication during second intention healing in equine distal limb wounds. Although it is still unknown what exactly triggers the formation of this tissue, previous research has revealed a persistent inflammatory response in these wounds. In this preliminary study we examined this inflammatory response in EGT-developing wounds as well as in experimental induced wounds. Immunohistological stainings were performed to detect primary inflammatory immune cells (MAC387 staining) as well as pro-resolution immune cells (CD163 staining). Our results show a significantly higher amount of MAC387+ and CD163+ cells in the fibrotic regions of EGT compared with the 19-day-old experimental wounds. This persistent high amount of fibrosis-promoting CD163+ cells in EGT suggests that the wound healing processes in EGT-developing wounds are arrested at the level of the proliferation phase