J. R. Butler1,2, M. R. Greg2, J. Ladowski2, Z. Wang2, J. L. Estrada2, M. Tector2, A. Tector2 1Indiana University School Of Medicine,Indianapolis, IN, USA 2University Of Alabama at Birmingham,Birmingham, Alabama, USA
Introduction: Historically, animal models of renal xenotransplantation have been challenged by acute humoral rejection. When studied in pig-to-primate models, there have been inconsistent reports of postoperative proteinuria; because proteinuria is a hallmark of complement-mediated graft damage, it remains unclear whether renal handling of serum macromolecules represents a physiologic barrier to clinical application in the absence of immune-mediated injury. Recently, serum albumin has been identified as a renal filtrate; serum proteostasis is dependent on post-glomerulus albumin absorption. In particular, the neonatal Fc receptor (FcRn) has been identified as a primary driver of renal proximal tubule albumin absorption in humans. The object of this study was to query the potential for porcine FcRn to promote absorption of human albumin in the absence of immunologic injury.
Methods: Intravital 2-photon microcopy has been instrumental in the study of renal albumin handling. Though this process has identified albumin as a renal filtrate in rat, mouse, and human, this is the first reported use of dual photon microcopy to assess the porcine kidney. After mapping the path of albumin across the filtration barrier in the porcine kidney, we established a fluorescence-based in vitro model of cellular uptake to query the ability of porcine proximal tubule cells to effectively absorb human albumin. With an established model to asses cross-species albumin-epithelial interactions at the cellular level, we sought to better understand the relationship at the molecular level. To this end we used a Biacore sensorgram to establish kenetics of the FcRn-albumin interaction. Finally, at the organismal level, we utilized a rejection free pig-to-primate model of life-sustaining renal transplant to analyze the potential loss of serum protein macromolues.
Results: Intravital 2-photon microcopy successfully followed labeled human albumin through the porcine kidney. Similar to results from other animal models, albumin is filtered at the porcine glomerulus. An in vitro cellular model of albumin uptake supports the ability of porcine renal proximal tubal cells to absorb human albumin. At the molecular level, porcine FcRn binds human albumin with the same affinity it does autologous porcine albumin; KD values of 7.22e-7 vs 3.92e-7 respectively (p= 0.21). Finally, when studied in vivo in a porcine to primate model free of immune-mediated graft damage the porcine kidney does not produce significant proteinuria.
Conclusion: Despite historical concern that renal loss of albumin would cause clinically significant proteinuria in porcine-to-primate xenotransplantation, this is not observed in the absence of immune-mediated graft injury. At the organ, cellular, and molecular level the porcine kidney process human albumin in a physiologic manner. Proteinuria does not represent an independent barrier to the clinical application of porcine-to-human xenotransplantation.