Genetic control of the humoral immune response to xenografts. I. Functional characterization of rat monoclonal antibodies to hamster heart xenografts.

The rejection of cardiac xenografts in the hamster-to-rat combination is characterized by the production of IgM antibodies that result in the rapid loss of the graft. We have recently produced rat monoclonal antibodies (mAb) to hamster heart xenografts in an attempt to develop reagents for use in identifying the target antigens for this reaction and to study the nature of the genetic control of the humoral response. The monoclonals were created by the fusion of myeloma cells with splenic lymphocytes from LEW rat recipients of hamster cardiac xenografts. The hybridomas were screened for antibody production, reactivity to hamster cell surface antigens, and the ability to mediate hyperacute rejection of hamster heart xenografts. A panel of monoclonal antibodies has been identified that are capable of inducing hyperacute rejection. All of these mAbs are IgM and bind strongly to hamster vascular endothelium. None of the mAbs were lymphocytotoxic or bound to hamster lymphocytes or erythrocytes. Immunopathologic studies demonstrated that these mAbs react specifically with hamster vascular endothelium and mediate a complement-dependent humoral reaction leading to the destruction of the cardiac xenografts. One of the mAbs (designated as HAR-1) has been characterized in detail. HAR-1 detects antigens distributed in the vascular endothelium, epithelium of bronchi in the lung, small intestine, tubules of kidney, and selective components of lymphoid organs--e.g., the stromal cells of the spleen and thymic medullary epithelium. Western blot analysis of hamster heart proteins with HAR-1 showed multiple bands with two major bands migrating at 80 kDa and 48 kDa. Absorption of the HAR-1 antibody with 48 individual carbohydrate molecules demonstrated that the strongest reactivity of the antibody is with a sialyl-Lea carbohydrate antigen.