Resistance of established porcine islet xenografts to humoral rejection by hyperimmune sera

Background. Although preformed natural antibodies cause hyperacute rejection of primarily vascularized xenografts, tissue grafts such as skin or islets are revascularized by in-growth of host, capillaries and therefore might be resistant to circulating antibodies. We examined the effect of hyperimmune serum and primed T cells on the survival of long-term porcine islet xenografts in diabetic nude mice., Methods. Porcine islets were transplanted beneath the kidney capsule of streptozotocin-induced diabetic BALB/c athymic mice. Hyperimmune serum and sensitized splenocytes were prepared by repeated immunization of BALB/c mice with porcine lymph node cells, Splenic T cells were enriched by nylon wool column separation. Tissues were examined by immunohistology using murine- and porcine-specific monoclonal antibodies. Results. Porcine islets survived in nude mice for >100 days with high levels of circulating porcine C-peptide and maintenance of normoglycemia. Injection of the hyperimmune sera (IgG) into normoglycemic nude mice bearing porcine islets for >70 days failed to induce rejection despite the continued presence of circulating anti-porcine cytotoxic antibody. Injection of sensitized T cells caused acute rejection of long-term (>140 days) porcine islets, whereas injection of naive T cells had no effect. Histologically, porcine islets removed from mice treated with hyperimmune serum showed no staining for IG. Long-surviving porcine islet grafts showed strong staining for interleukin (IL)-10 and a lesser amount of IL-4 but no staining for IL-2 or interferon-gamma. Although fresh porcine islets were positive for swine leukocyte antigen class I antigen and intercellular adhesion molecule (ICAM)-1 but negative for mouse platelet endothelial cell adhesion molecule and ICAM-2, long-surviving porcine islets showed positive endothelial staining for mouse platelet endothelial cell adhesion molecule and ICAM-2. Conclusions. Established islet xenografts are resistant to hyperimmune serum as a result of a lack of target endothelial antigens, whereas they remain susceptible to rejection caused by primed T cells. Local production of Th2 cytokines may explain the inability of long-surviving islet xenografts to activate injected naive T cells.