Characterization of humoral human anti‐porcine xenoreactivity

Abstract

The presence and specificity of natural xenoreactive human anti‐pig antibodies was studied. Titers of cytotoxic antibodies reactive with porcine blood lymphocytes were similar in sera from healthy controls, patients with high titers of HLA alloantibodies, uremic patients, diabetic uremic patients, kidney allografted patients on maintenance immunosuppressive therapy, patients who had been splenectomized and in bone marrow transplanted patients 6 months after transplantation. Similar titers were also recorded in patients with various autoimmune disorders, such as SLE and RA; there was no correlation to autoantibody titers. Furthermore, we found no correlation to natural isohemagglutinins; absorption with adequate human red cells did not reduce activity. However, absorption with pig red blood cells reduced the titer significantly and absorption with pig platelets completely abolished reactivity. Finally, antibodies were also present in cord blood samples indicating an endogenous ligand as an activating substance for antibody production. Most sera contained only IgM cytotoxic antibodies, but some also had a weak non‐IgM cytotoxic titer. Fluorescence studies with pig blood lymphocytes indicated that even sera that did not contain non‐IgM cytotoxic antibodies had IgG and IgA antibodies reactive with pig cells. Direct immunofluorescence experiments were also carried out to demonstrate the presence of target structures for naturally reactive antibodies on fetal pig islet cells. Pig blood lymphocytes were sensitive to ADCC‐mediated killing by IgG natural antibodies, whereas fetal pig islet cells were resistant. However, using adequate positive controls, such as anti‐pig MHC class I antibodies, it was shown that fetal pig islet cells can be sensitive to ADCC‐mediated killing. Finally, lysostrip and fluorostrip experiments were performed to test whether pig MHC antigens contain target determinants, possibly as oligosaccharide side chains, for natural antibodies. We found that pig MHC class I antigens do not contain such determinants.