Long-Term Survival of Hamster Hearts in Presensitized Rats

We transplanted hamster hearts into rats that had been sensitized to hamster cardiac grafts 5 days earlier as a model for discordant xenotransplantation. Sensitized rats had high serum levels of elicited anti-donor IgM and IgG that caused hyperacute rejection. Transient complement inhibition with cobra venom factor (CVF) plus daily and continuing cyclosporin A (CyA) prevented hyperacute rejection. However, grafts underwent delayed xenograft rejection (DXR). DXR involved IgG and associated Ab-dependent cell-mediated rejection, because depletion of IgG or Ab-dependent cell-mediated rejection-associated effector cells prolonged graft survival and the serum-mediated Ab-dependent cell-mediated cytotoxicity in vitro. Blood exchange in combination with CVF/CyA treatment dramatically decreased the level of preexisting Abs, but DXR still occurred in association with the return of Abs. Splenectomy and cyclophosphamide acted synergistically to delay Ab return, and when combined with blood exchange/CVF/CyA facilitated long-term survival of grafts. These grafts survived in the presence of anti-donor IgM, IgG, and complement that precipitated rejection of naive hearts, indicating that accommodation (survival in the presence of anti-graft Abs and complement) had occurred. We attribute the long-term survival to the removal of preexisting anti-donor Abs and therapy that attenuated the rate of Ab return. Under such conditions, the surviving hearts showed expression in endothelial cells and smooth muscle cells of protective genes and an intragraft Th2 immune response. Th2 responses and protective genes are associated with resistance to IgM- and IgG-mediated, complement-dependent and -independent forms of rejection.