In vivo observations of cell trafficking in allotransplanted vascularized skin flaps and conventional skin grafts

Abstract

The problem of allogeneic skin rejection is a major limitation to more widespread application of clinical composite tissue allotransplantation (CTA). Previous research examining skin rejection has mainly studied rejection of conventional skin grafts (CSG) using standard histological techniques. The aim of this study was to objectively assess if there were differences in the immune response to CSG and primarily vascularized skin in composite tissue allotransplants (SCTT) using in vivo techniques in order to gain new insights in to the immune response to skin allotransplants. CSG and SCTT were transplanted from standard Lewis (LEW) ad Wistar Furth (WF) to recipient transgenic green fluorescent Lewis rats (LEW-GFP). In vivo confocal microscopy was used to observe cell trafficking within skin of the transplants. In addition, immunohistochemical staining was performed on skin biopsies to reveal possible expression of class II major histocompatibility antigens. A difference was observed in the immune response to SCTT compared to CSG. SCTT had a greater density cellular infiltrate than CSG (p<0.03) that was focused more at the center of the transplant (p<0.05) than at the edges, likely due to the immediate vascularization of the skin. Recipient dendritic cells were only observed in rejecting SCTT, not CSG. Furthermore, dermal endothelial class II MHC expression was only observed in allogeneic SCTT. The immune response in both SCTT and CSG was focused on targets in the dermis, with infiltrating cells clustering around hair follicles (CSG and SCTT; p<0.01) and blood vessels (SCTT; p<0.01) in allogeneic transplants. This study suggests that there are significant differences between rejection of SCTT and CSG that may limit the relevance of much of the historical data on skin graft rejection when applied to composite tissue allotransplantation. Furthermore, the use of novel in vivo techniques identified characteristics of the immune response to allograft skin not previously described, which may be useful in directing future approaches to overcoming allograft skin rejection.