CMV as an exacerbating agent in transplant vascular sclerosis: Potential mechanisms modelled in vitro

CMV has been associated with transplant vascular sclerosis; however mechanisms explaining this association remain elusive. We have approached this issue by focussing on the endothelium (EC), first by demonstrating by proliferation assay and limiting dilution analysis that CMV-infected EC (EC/CMV) powerfully activate allogeneic T cells. Hypothesizing that cytokines elaborated by CMVresponsive host T cells might enhance graft endothelial immunogenicity, we have shown that EC monolayers incubated beneath trans-well culture inserts containing T cells+EC/CMV exhibit elevated levels of HLA class I & II, ICAM, and VCAM, effects not induced by T cells alone, EC/CMV alone, or T cells + uninfected EC (ECuN). Importantly, this induction persists in the presence of concentrations of Cyclosporine A sufficient to suppress allo responses. Postulating that cytolytic activity may also play a role, T cells were stimulated in vitro with allogeneic EC/CMV or ECUN in the presence of various concentrations of IL2, then tested for cytolysis against EC targets by s'Cr release assay. At high IL2 concentrations, ECUN-stimulated T cells lysed ECUN but not EC/CMV, while EC/CMV-stimulated T cells lysed both targets. Interestingly, at concentrations of IL2 reduced to eliminate cytolytic activity of ECUN-stimulated T cells, substantial levels of cytolysis were still exhibited by EC/CMV-stirnulated T cells against both infected and uninfected targets. Collectively these studies suggest two mechanisms by which CMV might perturb equilibrium at the graft/host interface, both triggered by CMV-infected EC within the graft or elsewhere, but ultimately targeting uninfected graft endothelia either by promiscuous cytolysis, or by cytokine-mediated endothelial activation.