Xenotransplantation: Current Challenges and Emerging Solutions

Abstract

To address the ongoing shortage of organs available for replacement, xenotransplantation of hearts, corneas, skin, and kidneys has been attempted. However, a major obstacle facing xenotransplants is rejection due to a cycle of immune reactions to the graft. Both adaptive and innate immune systems contribute to this cycle, in which natural killer cells, macrophages, and T-cells play a significant role. While advancements in the field of genetic editing can circumvent some of these obstacles, biomarkers to identify and predict xenograft rejection remain to be standardized. Several T-cell markers, such as CD3, CD4, and CD8, are useful in both the diagnosis and prediction of xenograft rejection. Furthermore, an increase in the levels of various circulating DNA markers and microRNAs is also predictive of xenograft rejection. In this review, we summarize recent findings on the advancements in xenotransplantation, with a focus on pig-to-human, the role of immunity in xenograft rejection, and its biomarkers.

Keywords: diagnostic biomarkers; genetic editing; immune rejection; predictive biomarkers; tolerance induction; xenoantigens; xenotransplantation.

Figure 1.

Cellular rejection in xenotransplants. (A) Neutrophil-mediated rejection. Upon activation, neutrophils release tubular networks known as NETs. NETs induce damage to xenograft cells through ROS and are recognized by macrophages as DAMPs. Binding of DAMPs to macrophages triggers the release of cytokines and inflammatory markers. (B) NK cell-mediated rejection via the direct pathway. Stimulating receptors, such as NKG2D and pULBP-1, bind to NKp44 and an unidentified molecule, respectively. Once activated, NK cells release granzymes and perforin, which induce damage in xenograft cells. The inhibiting receptors, KIR, ILT2, and CD94, do not recognize SLA-1 well in porcine cells. Therefore, there is a lack of inhibitory feedback of NK cells in xenografts. (C) NK cell-mediated rejection via the ADCC pathway. Interactions between FcRs and xenoantibodies lead to activation of NK cells. Among the antibodies recognized by NK cells is anti-SLA1. Binding on NK cells to anti-SLA1 induces the release of granzymes and perforin. (D) Macrophage-mediated rejection. Macrophages recognize anti- α1,3Ga antibodies through FcR, inducing the release of inflammatory cytokines, such as TNF-α, IL-1, and IL-6. Macrophages activate circulating T-cells, which further activate more macrophages. CD47 is an important receptor in the pathway of macrophage inhibition. However, it does not readily recognize its ligand, SIRP-α, in porcine cells, causing ineffective inhibition. (E) T-cell-mediated rejection via the direct pathway. SLA1 and SLA2 bind to T-cell receptors, triggering the release of cytokines and mediating direct cytotoxic effects. (F) T-cell-mediated rejection via the indirect pathway. Recipient antigen-presenting cells (APC) express xenogeneic antigens, activating CD4+ T-cells. Activated T-cells induce a cascade of antibody production and B-cell activation. NET: neutrophil extracellular traps; ROS: reactive oxidative species; DAMP: damage-associated molecular patterns; NK: natural killer; NKG2D: natural killer group-2D; pULBP-1: porcine UL16-binding protein-1; KIR: Killer Ig-like Receptor; ILT2: Ig-like transcript-2; ADCC: antibody-dependent cellular cytotoxicity; TNF-α: tumor necrosis factor; IL: interleukin; SIRP-α: signaling regulatory protein; CD94: cluster of differentiation-94; SLA-1: swine leukocyte antigen-1.