Allogeneic versus xenogeneic immune reaction to bioengineered skin grafts

There are conflicting reports on the survival and immune reaction to allografts and xenografts of cultured skin substitutes (CSS). In this study, we investigated the allogeneic and xenogeneic responses to CSS of human keratinocytes and genetically engineered CSS expressing keratinocyte growth factor (KGF) that forms a hyperproliferative epidermis. CSS (control and KGF modified) and neonatal human foreskins were evaluated by immunohistochemistry for the expression of MHC class I and II. To study allograft rejection, grafts were transplanted to human peripheral blood mononuclear cell (huPBMC)-reconstituted SCID mice. To study xenograft rejection, grafts were transplanted to immunocompetent mice. Graft survival and immune reaction were assessed visually and microscopically. After transplantation, control CSS formed a normal differentiated epidermis, whereas KGF CSS formed a hyperproliferative epidermis. Control and KGF CSS expressed class I similar to neonatal foreskin, but did not express class II. In the allograft model, rejection of neonatal foreskins was between 5 and 9 days. In contrast, neither control nor KGF CSS was rejected by huPBMC-SCID mice. Histology showed dense mononuclear cell infiltration in human foreskins, with few, if any, mononuclear cells in control or KGF CSS. In contrast to the allogeneic reaction, CSS (control and KGF) were rejected in the xenograft model, but rejection was delayed (9-21 days) compared with neonatal skin (5-8 days). Humanized SCID mice rejected allografts of human neonatal foreskins, but did not reject control CSS or KGF CSS, even though the KGF CSS formed a hyperproliferative epidermis. Rejection of control and KGF CSS by immunocompetent mice in a xenograft model was comparable and their survival was significantly prolonged compared with neonatal skin. These results demonstrate that control CSS and hyperproliferative KGF CSS are less immunogenic than normal human skin and that sustained hyperproliferation of the epidermis does not accelerate rejection.