Alloreactive cytotoxic CD4+ responses elicited by cytomegalovirus-infected endothelial cells: role of MHC class I antigens.

Cytomegalovirus (CMV) has been associated with chronic graft rejection in solid organ transplant patients. To elucidate the mechanism by which CMV leads to graft rejection, we hypothesized that CMV infection of endothelial cells could stimulate alloreactive cytotoxic T lymphocytes (CTL). This hypothesis was explored using the following experimental model: peripheral blood mononuclear cells (MNC) obtained from normal hosts were grown on monolayers of umbilical vein endothelial cells (UVEC) infected with CMV (CMV-UVEC) or not (control) and tested for CTL activity against uninfected UVEC. We showed that CMV-UVEC-stimulated MNC have significant CTL activity against uninfected UVEC. The CTL activity elicited by CMV-UVEC stimulation was significantly higher compared with that stimulated by uninfected UVEC or by ganciclovir-treated CMV-UVEC, indicating the critical role of productive CMV infection. The CTL activity was specific for the UVEC used as stimulators and did not affect MHC-unrelated UVEC. However, lymphoblastoid lines (LBL) major histocompatibility complex (MHC)-identical with the stimulator UVEC were also killed by the CMV-UVEC-stimulated MNC. CTL killed identical UVEC and LBL in a competitive fashion. Blocking experiments with monoclonal antibodies (mAbs) identified CD4 cells as the main effector of CTL activity and MHC class I as the antigenic target of CTL. Although natural killer (NK) cells did not significantly contribute to the CTL activity of CMV-UVEC-stimulated MNC, their presence in the MNC cultures during the stimulation process was critical for the development of CTL. This model offers a framework for understanding the role of CMV infection in graft rejection and for devising preventative strategies.