Hyperacute rejection of vascularized heart transplants in BALB/c Gal knockout mice

Pig-to-primate vascularized xenografts undergo hyperacute rejection (HAR). This results from pre-formed xenoreactive antibodies directed against galac tose-alpha1,3-galactose (alpha Ga1) in the donor organ and activation of th e complement cascade. We describe an in vivo murine model of HAR using a BA LB/c mice system devoid of histocompatibility or complement differences bet ween donor and recipient to investigate in isolation, the effects of alpha Gal epitope and anti-alpha Gal antibody interactions in causing rejection o f vascularized heart transplants. Gal KO mice were immunized with rabbit re d blood cell membranes to induce high anti-alpha Gal antibody titers that w ere predominantly IgM by ELISA (enzyme-linked immunosorbent assay). When al pha Gal-expressing mice hearts were transplanted heterotopically into these recipients (n=12), 67% of grafts rejected within 24 h, the majority within 16 h with histological features of HAR. In contrast, none of the grafts in the non-immunized Gal KO recipient control group (n=11) underwent HAR. Int erestingly, approximately 50% of the remaining grafts in both the immunized and non-immunized Gal KO recipient group were rejected between 7 and 27 da ys by a rejection process characterized by a dense infiltrate of macrophage /monocytes, perivascular cuffing and tissue destruction similar to recent d escriptions of delayed xenograft rejection (DXR). In addition, some grafts (21.5%) continued to survive in the immunized Gal KO recipients despite the presence of anti-alpha Gal antibody and normal complement activity and the se showed well-preserved myocardium when harvested whilst still functioning well at days 30 or 90. No rejection was seen when Gal KO donors were used in this system (n=4), nor when alpha Gal-expressing BALB/c hearts were tran splanted into alpha Gal-expressing BALB/c recipients (n=5). This in vivo sm all animal model offers the opportunity to test a variety of strategies to overcome HAR prior to more resource intensive pig-to-primate studies, and m ay provide insights into the processes similar to DXR and accommodation.