Serum protein immunogenicity: Implications for liver xenografting

The objectives of this study were threefold: (i) assess immunogenicity of d onor plasma proteins following hepatic xenotransplantation, (ii) identify p otential immunogens, and (iii) consider the implications of antibody format ion against these plasma proteins in xenograft survival. We studied liver a nd heart xenografts in a concordant combination, hamster to rat. All grafts were examined at necropsy for evidence of rat immunoglobulin G (IgG) depos ition. Cardiac xenografts were placed in recipients who had, or had tin G ( IgG) deposition. Cardiac xenografts were placed In recipients who naa, or n aa not, been sensitized with hamster serum. Hepatic xenografts were placed in naive recipients to see if antibodies to hamster serum proteins could be eluted from the Department of Chemistry, rejecting organ. Sera of immunize d rats were examined for the presence of anti-hamster antibodies by immunoe lectrophoresis and by Western blotting following sodium dodecyl sulfate-pol yacrylamide gel electrophoresis (SDS-PAGE) separation of hamster serum. Ant ibodies in sera of immunized rats were compared with those eluted from reje cting livers. Candidate antigens were identified by tandem mass spectrometr y, sequence analysis, and reference to protein databases. Results showed th at sera of immunized rats recognized a minimum of four different antigens i n hamster serum by immunoelectrophoresis, and a minimum of seven by the mor e sensitive SDS-PAGE Western blot. IgG eluted from rejecting livers bound t hree of seven candidate antigens recognized by sera of the immunized animal s. Sequence analysis searches revealed proteinase inhibitors in each of the three SDS-PAGE bands common to the above samples. All of these candidate p roteinase inhibitor immunogens share a common catabolic fate, uptake via th e lipoprotein-related protein (LRP/alpha 2-macroglobulin receptor CD91). Se nsitization to hamster serum proteins hastened cardiac xenograft rejection in 30-50% of recipients (depending on sensitization protocol). Vascular dep osition of rat IgG occurred in all rejecting xenografts. Antibody binding t o proteinase inhibitors could disturb their functional activity and contrib ute to the pathogenesis of delayed xenograft rejection.