2024 Banff Meeting Report: Xenotransplantation

This white paper provides a comprehensive and clear overview of the challenges and standards that need to be established for xenotransplantation. I particularly like the included tables and the proposed scoring criteria. While there are similarities to allotransplantation, introducing a non-human organ into a human body presents unique challenges. Both allo- and xenotransplantation share mechanisms driving rejection, but there are distinct differences. For instance, human CD4+ T cells can respond directly to pig antigens, potentially eliciting a stronger response compared to human antigens in allotransplantation (Yin IH, et al., doi: 10.4285/ctr.24.0056), indicating greater challenges in xenotransplantation.

As a complement biologist, I believe that the complement system plays a critical role in xenotransplantation by inducing innate and adaptive immune mechanisms that, if uncontrolled, can lead to organ rejection. Complement activation occurs in both acute and long-term settings during xenotransplantation, from initial ischemia-reperfusion injury to later events involving anti-pig antibodies. Although transgenic organs expressing complement regulatory proteins such as CD46, CD55, and CD59 are used, anti-pig antibodies or other complement-triggering events can sometimes overcome this protection, resulting in complement-mediated damage and recruitment of innate (monocytes, macrophages, NK cells) and adaptive immune cells (CD4+ and CD8+ T cells), causing further inflammation and organ damage.

I agree fully with the notion that in future studies investigators should be strongly encouraged to save a pre-transplantation sample from the organ for comparison with post-transplant biopsies, especially in kidney xenografts. This approach provides valuable information on immune cell infiltration, complement activation, and molecular pathways activated post-transplant that contribute to acute and chronic rejection. Additionally, taking serum and plasma samples from the recipient at the time of transplantation and sequentially in the following weeks and months can help measure systemic complement activation and proinflammatory cytokine increases along with clinical parameters.

In the glomerulonephritis section, much can be learned from human kidney disease, which is often driven by complement system dysregulation. The mechanisms that drive kidney damage in xenografted organ including proteinuria increase and eGFR reduction are likely to be similar to human kidney disease (pre- and post- allotransplantation) as it pertains to complement activation. Clinicians typically use both anti-C3c and anti-C4d biopsy staining to classify diseases driven by the alternative pathway (e.g., C3 glomerulopathy, IgA nephropathy, membranous nephritis) or those with mixed classical and alternative pathway activation (e.g., immune complex membranoproliferative glomerulonephritis, glomerulonephritis associated with monoclonal gammopathies, focal segmental glomerulosclerosis). Plasma measurement of total C3 levels is also important to determine if complement activation is local or systemic. Implementing C3c kidney staining and systemic C3 level measurements can better identify which part of the complement system is driving kidney damage in xenografts, as it may not always be anti-xenograft antibody classical pathway driven. For example, the lack of C4d staining while C3c staining is present has been shown by Foote et al. (doi: 10.1111/xen.12715). While pig organs express complement regulatory proteins such as CD46 and CD55, these regulators are typically not expressed on the glycocalyx of the glomerular basement membrane in human kidneys (Heiderscheit, A. K., et al., doi: 10.1002/ajmg.c.31986). Pig kidneys have a similar expression pattern to humans, with CD46 primarily located on endothelial and epithelial cells (Diamond L et al., DOI: 10.1097/00007890-200101150-00021). Thus, protection from complement-mediated damage will only come from systemic factor H or administration of a complement-inhibiting drug.

In the TMA kidney xenograft section, TMA can have various causes, but one relevant to xeno- and allotransplantation is ischemia-reperfusion injury, which can trigger lectin and alternative pathway activation and C3b and membrane attack complex (MAC) deposition on the transplanted organ (Avila A et al., doi: 10.3389/fmed.2021.642864). Therefore, early TMA cases might benefit from immunofluorescence staining for C3c and MAC along with C4d staining.

In the anti-porcine antibody typing section, consider developing or using off-the-shelf complement-dependent assays to determine classical pathway complement activation by anti-porcine antibodies. These assays have been used successfully in kidney allotransplantation as biomarkers to help predict graft outcomes (Lan JH et al., DOI: 10.1097/TP.0000000000001819).

In Table 2. I would recommend considering adding C3c staining above the C4d staining. Perhaps the scoring criteria should be similar to either allograft or C3G where intensity level of 1-3 is applied.

Very nice paper - just for awareness, in light of the progress in Xeno transplantation, The Banff Foundation has adjusted its name from Banff Foundation for Allograft Pathology to Banff Foundation for Transplant Pathology to be inclusive of the emerging Xenotransplantation pathology community. Maybe the following sentence in the introduction can be modified to reflect this change?

"As the field makes a brisk step towards clinical application, the Banff Foundation for Allograft Pathology has established a Working Group for Xenotransplantation Pathology8 to initiate recommendations for xenograft tissue handling and evaluation, based on published literature and field expertise from experimental and preclinical work."

change to

As the field makes a brisk step towards clinical application, the Banff Foundation for Allograft Pathology has established a Working Group for Xenotransplantation Pathology8 to initiate recommendations for xenograft tissue handling and evaluation, based on published literature and field expertise from experimental and preclinical work. To be inclusive to this emerging field of pathology the Banff Foundation adjusted its name to the Banff Foundation for Transplant Pathology.

Also it might be helpful to add a few pictures of xeno specific pathology to the report? For example a photomicrograph of the unusual C4d staining patterns and/or the frequent TMAs, and examples of the proposed different glomerulitis scores.