Complement activation by anti-endothelial cell antibodies in MHC-mismatched and MHC-matched heart allograft rejection: Anti-MHC-, but not anti non-MHC alloantibodies are effective in complement activation

Classical complement activation is a major effector mechanism in the develo pment of vascular lesions contributing to allograft rejection. We investiga ted complement activation by alloantibodies reactive with graft endothelial cell (EC) alloantigens in settings of MHC-mismatched and MHC-matched (non- MHC-mismatched) rat heart transplantation (Tx). Allosera and heart allograf ts were collected at the day of rejection (day 7-8 and clay 28-35 in MHC-mi smatched and non-MHC-mismatched Tx respectively) or earlier. Allosera react ivity was studied in vitro using rat-heart-endothelial-cell (RHEC) lines ex pressing the appropriate donor MHC and non-MHC alloantigen profile. Immunoh istochemical analysis of rejected heart allografts showed deposition of all oantibodies in both MHC-mismatched and MHC-matched heart allografts, but ex pression of C3 was only seen in the vasculature of MHC-mismatched grafts. F AGS analysis showed that anti MHC as well as anti non-MHC allosera were rea ctive with donor EC cell surface antigens. Both sera had similar IgG subcla ss profiles of anti-endothelial cell antibodies. Complement activation by a nti MHC and anti non-MHC alloantibodies on EC was measured by FAGS analysis of C3 and C5b-9 (MAC) expression. Distinct expression of C3 was noticed fo r EC incubated with anti-MHC allosera, but hardly for EC incubated with ant i non-MHC allosera. C5b-9 was low but showed no difference between the two allosera. However, complement-mediated cytotoxicity experiments showed that functional (lytic) MAC was induced with anti MHC allosera but hardly with anti non-MHC allosera. Our data show that in settings of MHC-matched heart transplantation alloantibodies against endothelial non-MHC alloantigens are generated, but, in contrast to alloantibodies to MHC alloantigens, these a lloantibodies have only poor complement-activating and lytic potentials. Wh ether anti non-MI-IC allolantibodies effect other biological processes rele vant for heart allograft vasculopathy, including development of graft arter iosclerosis, needs further elucidation.