Immunohistological studies of complement activation after xenogeneic perfusion of a working heart model

Transplantation of organs from one species to another leads to immediate hyperacute rejection. Activation of complement is one important factor involved in this process. Whether complement activation is induced by preformed natural antibodies (PNAbs) via the classical pathway or by an activator surface via the alternative pathway is unclear. In order to simulate the relevant clinical situation of animal donor/human recipient, we perfused working porcine hearts ex vivo with human blood. This also offered the possibility to study the process of complement activation in a precisely defined system with human complement proteins. PNAb titer and complement lytic activity of the plasma were measured. Immunohistological stainings for IgG, IgM, C1q, C4, C3d, C5-9, factor B, and properdin were performed on tissue sections of the left ventricle. PNAb titer almost totally disappeared within the first 5 min of perfusion. Complementlytic activity of the classical pathway decreased similarly within the first 3 h of xenogeneic and autologous perfusion from 70% to 40%. More detailed immunohistological studies revealed positive staining for C3d on endothelium and myocardium of ex vivo perfused xenogeneic hearts. Complement-induced cytotoxicity was proven by the presence of C5-9 (membrane attack complex). However, hardly and C1q and C4 could be found in the ex vivo xenogeneic perfused hearts. Staining for factor B was positive and proved activation via the alternative pathway. Beyond that, the presence of properdin binding even indicated an upregulation of the alternative pathway C3 convertase. In contrast to the prevaling opinion, we found that the alternative pathway plays the dominant role in complement activation in hyperacute xenograft rejection, as shown ex vivo using pig hearts perfused with human blood.