Immunological function of the endothelial cell within the setting of organ transplantation

In organ transplantation, development of immunosuppressive treatment and improved diagnosis of allograft rejection has resulted in increased allograft survival in recent years. Nevertheless, rejection remains a major cause of graft loss and a better understanding of the characteristics of the allo-immune response is required to identify new diagnostic and therapeutic tools. The allogeneic immune response depends upon a major family of antigenic targets: the Major Histocompatibility Complex molecules (MHC) which are present on donor cells. These molecules are targets of both the humoral and cellular arms of the graft recipient's immune system: T lymphocytes which are implicated in acute cellular rejection and antibodies which are implicated in antibody-mediated rejection (AMR). Allo-recognition of allograft MHC antigens by either T cells or allo-antibodies is the primary event which can ultimately lead to graft rejection. Although immunosuppressive strategies have mainly focused on the T cell response and acute cellular rejection has therefore become relatively rare, antibody mediated rejection (AMR) remains resistant to conventional immunosuppressive treatment and results in frequent graft loss. Damage to the endothelium is a prominent histological feature of AMR underlining the involvement of endothelial cells in initiating the allo-immune response. Furthermore, endothelial cells express both HLA class I and class II molecules in the context of organ transplantation endowing them with the capacity to present antigen to the recipient T cells. The endothelium should therefore be viewed both as a stimulator of, and as a target for allo-immune responses. In this review, we will summarize current knowledge about the implication of endothelial cells in the allo-immune response in the context of organ transplantation.