Hyperacute Lung Rejection in the Pig-to-Human Model 4: Evidence for Complement and Antibody Independent Mechanisms

Background. We assessed whether the combination of complement regulation and depletion of xenoreactive antibodies improves the outcome of pulmonary xenografts compared with either strategy alone. Methods. Lungs from pigs heterozygous (hDAF±) or homozygous (hDAF+/+) for the human decay accelerating factor transgene (hDAF) or their nontransgenic litter mates (hDAF−/−) were perfused with heparinized whole human blood. In additional groups, xenoreactive natural antibodies (XNA) were depleted by pig lung perfusion (hDAF−/−/AbAbs, hDAF±/AbAbs) before the experiment. This combined approach was augmented by adding soluble complement receptor 1 (sCR1) to the perfusate in one further group (hDAF±/AbAbs/sCR1). Results. HDAF−/− lungs perfused with unmodified human blood were rejected after 32.5 min (interquartile range, IQR 5 to 210). HDAF± lungs survived for 90 min (IQR 10 to 161, P=0.54). Both groups showed a rapid rise in pulmonary vascular resistance (PVR), which is a characteristic feature of hyperacute rejection (HAR). This phenomenon was blunted in the hDAF+/+ group, although survival (48 min, IQR 14 to 111) was not further prolonged. Antibody depletion (AbAbs) led to a significant increase in survival time (hDAF−/−/AbAbs: 315 min, IQR 230 to 427; hDAF±/AbAbs: 375 min, IQR 154 to 575), reduced PVR and less complement production. Addition of sCR1 reduced complement elaboration but did not further improve survival (200 min, IQR 128 to 580) and surprisingly tended to increase PVR. Conclusions. Depletion of xenoreactive antibodies is more effective than membrane-bound complement regulation to blunt hyperacute rejection of pulmonary xenografts, but even the combined approach including soluble-phase complement inhibition is not sufficient to reliably prevent organ failure within hours. It therefore seems likely that other factors independent of antibody and complement contribute to HAR in this model.