Alpha-Gal on bioprostheses: xenograft immune response in cardiac surgery

The alpha-Gal (Galalpha1,3-Galbeta1-4GlcNAc-R) epitope is the major xenoantigen causing hyperacute rejection of pig organs transplanted into primates. Porcine bioprostheses are utilized in cardiac surgery. However, premature degeneration of bioprostheses has limited utilization in younger patients and the immune response remains elusive. We sought to investigate whether a specific alpha-Gal immune response may play a role in this clinical scenario.

We investigated the presence of alpha-Gal-epitope on native and fixed porcine valves by means of confocal laser scanning microscope (CLSM). ELISA was utilized to evidence whether implantation of bioprostheses elicits augmentation of pre-existing cytotoxic anti alpha-Gal IgM antibodies within 10 days of surgery. Patients who underwent coronary artery bypass grafting (CABG) or mechanical valve replacement served as controls (each group, n = 12). To corroborate the clinical relevance of the alpha-Gal immune response in vivo, we studied serum obtained before and after implantation of bioprostheses and its potency to lyse porcine alpha-Gal-bearing PK15 cells.

We found the immunogenic alpha-Gal-epitope on fibrocytes interspersed in the connective tissue of porcine valves as determined by vimentin/IB4 lectin binding. Moreover, patients who were provided with a bioprostheses had developed a significant increase of naturally occurring cytotoxic IgM antibodies directed towards alpha-Gal after surgical intervention as compared with control patients (P < 0.0001, respectively). Sera obtained from the patients after the implantation of bioprostheses demonstrated an increased cytotoxicity against alpha-Gal-bearing PK-15 cells as compared with preoperative sera (P < 0.001). The specificity of the cytotoxic effects was proven as soluble Galalpha1-3Galbeta1-4GlcNAc markedly inhibited cell death of alpha-Gal-bearing PK15 cells (P < 0.001).

Our data suggest that implantation of bioprostheses in cardiac surgery induces a xenograft-specific immune response. Procedures diminishing the presence of alpha-Gal on bioprostheses, such as utilization of genetically manipulated alpha-Gal-deficient xenograft or pretreatment with alpha-Galactosidase, might diminuate the immune response against bioprostheses and extend durability.