From Proteopedia

Xenotransplantation, Xenografts

What is Xenotransplantation?

According to the United States Food & Drug Administration, "Xenotransplantation is any procedure that involves the transplantation, implantation or infusion into a human recipient of either (a) live cells, tissues, or organs from a nonhuman animal source, or (b) human body fluids, cells, tissues or organs that have had ex vivo [Have been removed from a living creature] contact with live nonhuman animal cells, tissues or organs."^[1] In other words, xenotransplantation is the process of transferring body fluids, cells, organs, or tissues from a non-human animal into a human. Currently, genetically modified porcine (pig) donors are thought to be the most promising.

Why Xenotransplantation?

Patient mortality due to lack of availability of donor organs remains a growing concern. Patient mortality due to lack of availability of donor organs remains a growing concern. According to Sykes and Sachs, as of 2020, “the Organ Procurement and Transplantation Network reported that 116,577 patients in the USA were on waiting lists for organ transplantation, including 97,541 who were waiting for kidney allografts. On average, 20 patients on transplant waiting lists die every day because they do not obtain an organ. One promising solution to this developing problem is xenotransplantation, using organs from non-human sources in place of human organs. ^[2] There has been significant progress in the development of viable models for organ production for xenotransplantation using triple knockout porcine donors as new technology such as CRISPR has become available. However, antibody mediated rejection (AMR), rejection of the organ due to immune response, remains an issue that has not been solved.

Rejection of Xenografts

Hyperacute Rejection

Hyperacute rejection is graft (pig organ) destruction which occurs quickly (within minutes to hours). It is caused by human antibodies binding antigens present on the porcine endothelial cells leading to activation of the complement system, part of the human immune response. In other words, the pig organ transplanted into the human patient is recognized as an invader by the human immune system and is destroyed in a manner similar to that of destruction of harmful bacteria. ^[3]

Associated Genes

hCD46

is a human gene which encodes the protein "Membrane cofactor protein" (MCP). MCP plays a regulatory role in the complement system. ^[4]

hCD55

HCD55 is a human gene that encodes the protein, “Decay accelerating factor,” which is involved in regulation of the complement cascade. ^[5]

hCD59

HCD59 is a human gene that encodes the protein “MAC inhibitory protein” which is involved in complement regulation through inhibition of the complement membrane attack complex as well as in signal transduction used in T cell activation. ^[6]

GGTA1 (Sus scrofa)

GGTA1 encodes the protein α-1,3-galactosyltransferase and is present in most mammals including pigs. ^[7] With reference to xenograft rejection, α-1,3-galactosyltransferase is responsible for the presence of galactose- α -1,3-galactose on the endothelium of the porcine xenograft. Antibodies present in the recipient respond to the presence of the galactose-α-1,3-galactose xenoantigen, activating the recipient’s immune response through complement activation leading to the rejection of the xenograft. ^[8]

A was created using ICM Pro to examine the proposed structure of porcine GGTA1 protein products. NCBI data set obtained for GGTA1. Fasta sequence was put into NCBI blast set to search PDB to find template. Search returned 73 results. The top three results showed 87.85% alignment to the query sequence. Alignment was conducted between query and 1GWV using Uniprot align resulting in 87.54% identity.

PDB search using ICM Pro was conducted for 1GWV. Object was created and sequence was extracted giving results for 1GWV_a and 1GWV_b. Alignment with ICM pro was conducted between query and both IGWV isomers. This alignment resulted in 70% identity. There is a 81 amino acid chain present in the porcine protein that is not present in the bovine sequence. Protein Health analysis was conducted in ICM Pro, and the first 75 amino acids were removed due to lack of secondary structure.

Acute Humoral Rejection

Associated Genes

CMAH

The CMAH gene is responsible for the expression of CMP-N-acetylneuraminic acid hydroxylase which plays a role in conversion of N-acetyl-neuraminic acid (Neu5Ac) to N-acetyl-neuraminic acid (Neu5Ac), a xenoantigen, through a CMAH generated cofactor. ^[9]

β4GalNT2

The β -1,4-N-acetylgalactosaminyltransferase 2 gene is responsible for the production of the SDa antigen, a carbohydrate found on the surface of some blood cells. Porcine endothelial cells of the xenograft will express SDa leading to a reaction with human anti-SDa antibodies causing xenograft failure following immune response. ^[10]

Innate Cellular Rejection

Associated Genes

hCD47

HCD47, also called integrin associated protein, is a transmembrane receptor involved in signal regulation. Under normal conditions, HCD47 binds SIRP-α (a macrophage inhibiting protein) inhibiting macrophage activity. However, human SIRP-α and porcine CD47 are incompatible, therefore macrophages respond to xenograft endothelial cells leading to graft rejection. ^{[11] [12]}

hCD200

CD200 is expressed by endothelial cells (and others) and bind to macrophages altering cytokine production. Through this action, the immune response of the macrophages is hindered. Lei et al note when porcine endothelial cells express hCD200, xenograft survival is increased. ^{[13] [14]}

hHLA-E

hB2M

hHLA-G1

CIITA

B2M (Sus Scrofa)

Adaptive Cellular Rejection

Associated Genes

Coagulation Disorder

ASGR1

Systemic Inflammation

Cross Species Infection

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References

1. ↑ Xenotransplantation. https://www.fda.gov/vaccines-blood-biologics/xenotransplantation. Accessed 9 September 2024.
2. ↑ Sykes, M., & Sachs, D. H. (2022). Progress in xenotransplantation: Overcoming immune barriers. Nature Reviews Nephrology, 18(12), 745–761. https://doi.org/10.1038/s41581-022-00624-6
3. ↑ Thuan, P. Q., & Dinh, N. H. (2024). Cardiac Xenotransplantation: A Narrative Review. Reviews in Cardiovascular Medicine, 25(7), 271. https://doi.org/10.31083/j.rcm2507271
4. ↑ PubChem. “CD46 - CD46 Molecule (Human).” Accessed October 11, 2024. https://pubchem.ncbi.nlm.nih.gov/gene/CD46/human.
5. ↑ PubChem. “CD55 - CD55 Molecule (Cromer Blood Group) (Human).” Accessed October 11, 2024. https://pubchem.ncbi.nlm.nih.gov/gene/CD55/human.
6. ↑ PubChem. (n.d.). CD59—CD59 molecule (CD59 blood group) (human). Retrieved October 23, 2024, from https://pubchem.ncbi.nlm.nih.gov/gene/CD59/human
7. ↑ GGTA1 glycoprotein alpha-galactosyltransferase 1 (inactive) [Homo sapiens (human)]—Gene—NCBI. (n.d.). Retrieved October 23, 2024, from https://www.ncbi.nlm.nih.gov/gene/2681
8. ↑ Lei, T., Chen, L., Wang, K., Du, S., Gonelle-Gispert, C., Wang, Y., & Buhler, L. H. (2022). Genetic engineering of pigs for xenotransplantation to overcome immune rejection and physiological incompatibilities: The first clinical steps. Frontiers in Immunology, 13, 1031185. https://doi.org/10.3389/fimmu.2022.1031185
9. ↑ Park, J.-Y., Park, M.-R., Kwon, D.-N., Kang, M.-H., Oh, M., Han, J.-W., Cho, S.-G., Park, C., Kim, D.-K., Song, H., Oh, J.-W., & Kim, J.-H. (2011). Alpha 1,3-Galactosyltransferase Deficiency in Pigs Increases Sialyltransferase Activities That Potentially Raise Non-Gal Xenoantigenicity. BioMed Research International, 2011(1), 560850. https://doi.org/10.1155/2011/560850
10. ↑ Byrne, G., Ahmad-Villiers, S., Du, Z., & McGregor, C. (2018). B4GALNT2 and xenotransplantation: A newly appreciated xenogeneic antigen. Xenotransplantation, 25(5), e12394. https://doi.org/10.1111/xen.12394
11. ↑ Lei, T., Chen, L., Wang, K., Du, S., Gonelle-Gispert, C., Wang, Y., & Buhler, L. H. (2022). Genetic engineering of pigs for xenotransplantation to overcome immune rejection and physiological incompatibilities: The first clinical steps. Frontiers in Immunology, 13, 1031185. https://doi.org/10.3389/fimmu.2022.1031185
12. ↑ Kaur, S., Isenberg, J. S., & Roberts, D. D. (2021). CD47 (Cluster of Differentiation 47). Atlas of Genetics and Cytogenetics in Oncology and Haematology, 25(2), 83.
13. ↑ Lei, T., Chen, L., Wang, K., Du, S., Gonelle-Gispert, C., Wang, Y., & Buhler, L. H. (2022). Genetic engineering of pigs for xenotransplantation to overcome immune rejection and physiological incompatibilities: The first clinical steps. Frontiers in Immunology, 13, 1031185. https://doi.org/10.3389/fimmu.2022.1031185
14. ↑ PubChem. (n.d.). CD200—CD200 molecule (human). Retrieved November 7, 2024, from https://pubchem.ncbi.nlm.nih.gov/gene/CD200/human