SELECTIVE IGM DEPLETION PROLONGS ORGAN SURVIVAL IN AN EX-VIVO MODEL OF PIG-TO-HUMAN XENOTRANSPLANTATION

In the pig-to-primate model, xenograft hyperacute rejection (HAR) is m ediated by antibody and complement. Previous studies have implicated x enoreactive IgM natural antibody (nAb) as the predominant immunoglobul in involved in HAR. To further evaluate the role of IgM, we selectivel y reduced IgM levels in human blood, without changing IgG and IgA leve ls, and then used this blood to perfuse porcine hearts ex vivo. Specif ic IgM depletion was accomplished with an immunoabsorption column cont aining sheep anti-human IgM (mu-chain specific) conjugated to Sepharos e beads. Human blood was separated into plasma and cellular components . For control experiments, those components were unmodified and recomb ined in the perfusion system. For experiments with IgM reduced blood, the plasma was passed through the IgM column. Immunoabsorption resulte d in similar to 90% reduction in xenoreactive IgM levels, as measured by ELISA. Porcine hearts perfused with unmodified human blood survived 25+/-5.6 min (n=5). Porcine hearts perfused with human blood containi ng reduced levels of IgM survived 229+/-45.2 min (n=4; P<0.01). Organ survival was negatively associated with xenoreactive IgM nAb levels me asured immediately before perfusion (r=-0.83; P=0.01), and not with Ig G nAb levels (r=-0.21; P=0.62). The ability of plasma from IgM-deplete d blood to elicit complement activation, measured by iC3b binding to p orcine aortic endothelial cells in vitro, was also strongly associated with IgM xenoreactive nAb levels (r=0.92; P<0.0001), control hearts p erfused with unmodified human blood showed typical widespread histolog ic features of HAR, while porcine hearts perfused with IgM-reduced blo od demonstrated milder and less uniform changes. Immunopathological an alysis of heart tissues obtained at the completion of each study showe d similar deposition of IgG; between groups but markedly less IgM, C3, C4, and C9 in the IgM reduction group, These results suggest that sel ective IgM reduction delays HAR with prolongation of survival and that xenoreactive IgM may be the predominant immunoglobulin involved in HA R in the pig-to-human combination.