Acute humoral xenograft rejection: Destruction of the microvascular capillary endothelium in pig-to-nonhuman primate renal grafts

The major cause of xenograft loss beyond hyperacute rejection is a form of injury, traditionally termed delayed xenograft rejection (DXR), whose patho genesis is unknown. Here we analyze the immunologic and morphologic feature s of DXR that develops in pig kidney xenografts transplanted into nonhuman primates. Kidneys from miniature swine were transplanted into cynomolgus mo nkeys (n = 14) or baboons (n = 11) that received regimens aimed to induce m ixed chimerism and tolerance. No kidney was rejected hyperacutely. Morpholo gic and immunohistochemical studies were performed on serial biopsies, and an effort was made to quantify the pathologic features seen. The early phas e of DXR (Days 0-12) was characterized by focal deposition of IgM, IgG, C3, and scanty neutrophil and macrophage infiltrates. The first abnormality re cognized was glomerular and peritubular capillary endothelial cell death as defined by in situ DNA nick-end labeling (TUNEL). Damaged endothelial cell s underwent apoptosis and, later, frank necrosis. The progressive phase dev eloped around Day 6 and was characterized by progressive deposition of IgM, IgG, C3, and prominent infiltration of cytotoxic T cells and macrophages, with a small number of NK cells. Thrombotic microangiopathy developed in th e glomeruli and peritubular capillaries with TUNEL+ endothelial cells, plat elet aggregation, and destruction of the capillary network. Only rare damag ed arterial endothelial cells and tubular epithelial cells were observed, w ith rare endothelialitis and tubulitis. in the advanced phase of DXR, inter stitia[ hemorrhage and infarction occurred. During the development of DXR, the number of TUNEL+ cells increased, and this correlated with progressive deposition of antibody. The degree of platelet aggregation correlated with the number of TUNEL+ damaged endothelial cells. We conclude that peritubula r and glomerular capillary endothelia are the primary targets of renal DXR rather than tubular epithelial cells or arterial endothelium and that the e arliest detectable change is endothelial cell death. DXR was characterized by progressive destruction of the microvasculature (glomeruli and peritubul ar capillaries) and formation of fibrin-platelet thrombi. Both cytotoxic ce lls and antibodies potentially mediate the endothelial damage in DXR; howev er, in this model, DXR is largely humorally mediated and is better termed " acute humoral xenograft rejection."