Effects of antibody reactivity to major histocompatibility complex (MHC) and non-MHC alloantigens on graft endothelial cells in heart allograft rejection

Background. To gain insight in the pathogenesis of vascular lesions in heart allograft rejection, we investigated effects of allosera reactive with major histocompatibility complex (MHC) or non-MHC alloantigens on graft endothelial cells (EC) in a rat transplantation model. Methods. Anti-MHC and anti-non-MHC allosera were obtained from Brown Norway (RT.1(n)) recipients of a Lewis (RT.1(1)) or congenic LEW.1N (RT.1(n)) heart allograft respectively. Reactivity with endothelial alloantigens was studied in vitro using a series of three rat heart endothelial cell (RHEC) lines of Lewis origin. Phenotypic studies of MHC and non-MHC alloantigen expression, and adhesion molecule induction on EC were performed by immunostaining and fluorescence-activated cell sorting analysis, Complement-mediated cytotoxicity of allosera was studied using a Cr-51 re lease assay, Results. Both anti-MHC allosera and anti-non-MHC allosera showed reactivity with all three RHEC lines. EC stimulation with tumor necrosis factor-alpha and interferon-gamma resulted in increased reactivity of anti-MHC but not of anti-non-MHC allosera, Anti-MHC allosera showed complement-mediated cytotoxicity for EC, which was strongly increased when cytokine-stimulated EC were used, With anti-non-MHC allosera, only minor cytotoxicity was measured, irrespective of the activation of EC, Anti-MHC and anti-non-MHC allosera from the day of rejection (days 7-8 and days 29-35, respectively) had similar subclass profiles of allospecific IgG, except for allospecific IgM, which was only detected in anti-MWC allosera, Complement-mediated cytotoxicity of anti-MHC allosera from the day of rejection was effected mainly by IgM alloantibodies, whereas, in allosera taken 4 days after rejection, a predominance of cytotoxic alloantibodies of the IgG class was observed. No indications were found that either alloantibody reactivity alone or in combination with complement activation led to EC activation processes relevant to intercellular adhesion molecule-1 or vascular cell adhesion molecule-1 induction. Conclusions. Our data show that, in heart allograft rejection, MHC but also non-MHC alloantigens on EC are target structures in the alloantibody response. Alloantibodies reactive with endothelial MHC, but not those reactive with non-MHC alloantigens, may significantly contribute to vasculopathy by complement-mediated cytotoxicity, Although no evidence was found that alloantibodies reactive with graft EC induce adhesion molecule expression, they may trigger other EC mechanisms relevant to graft vasculopathy.