Xenotransplant Histocompatibility Assays: Improving Detection of Antibodies Which Bind Class II Swine Leukocyte Antigens While Avoiding Artifacts

BACKGROUND: Human xenotransplantation has begun, but antibody-mediated rejection remains a major barrier to success despite reducing donor xenoantigenicity through genetic engineering. Consequently, as in allotransplantation potential recipients must be carefully tested to determine whether they have pre-formed anti-pig antibodies. These antibodies target glycans and major histocompatibility complex proteins, known as swine leukocyte antigens (SLA). Predicting donor-specific humoral immunity remains challenging because the xenotransplant histocompatibility assays are in early phases of development. METHODS: We studied the binding of human antibodies to porcine peripheral blood mononuclear cells (PBMCs) and renal endothelial cells. We examined whether in vitro conditions affected the detection of antibodies by the endothelial cells. We also tested these cells' ability to detect antibodies to class II SLA proteins. Finally, we studied whether beads, coated with SLA proteins, enabled anti-SLA immunoglobulin to be identified. RESULTS: Renal endothelial cells can benefit from and suffer from in vitro manipulations. These cells can be made to express class II SLA proteins increasing their ability to detect anti-pig antibodies. However, endothelial cells also acquire Neu5Gc from the environment when in culture which increases background signal as anti-Neu5Gc antibodies bind these targets. Although not experiencing in vitro acquisition of xenoantigens, PBMC struggled to detect IgG and IgM toward class II SLA proteins. Beads, coated with individual class II SLA proteins, did detect anti-pig IgG and IgM that was missed by the PBMC. CONCLUSIONS: Our findings suggest that complementary assays may be essential to reliably identify xenotransplant candidates at minimal risk of antibody-mediated rejection.