Complement networks in gene-edited pig xenotransplantation: enhancing transplant success and addressing organ shortage

Abstract

The shortage of organs for transplantation emphasizes the urgent need for alternative solutions. Xenotransplantation has emerged as a promising option due to the greater availability of donor organs. However, significant hurdles such as hyperacute rejection and organ ischemia-reperfusion injury pose major challenges, largely orchestrated by the complement system, and activated immune responses. The complement system, a pivotal component of innate immunity, acts as a natural barrier for xenotransplantation. To address the challenges of immune rejection, gene-edited pigs have become a focal point, aiming to shield donor organs from human immune responses and enhance the overall success of xenotransplantation. This comprehensive review aims to illuminate strategies for regulating complement networks to optimize the efficacy of gene-edited pig xenotransplantation. We begin by exploring the impact of the complement system on the effectiveness of xenotransplantation. Subsequently, we delve into the evaluation of key complement regulators specific to gene-edited pigs. To further understand the status of xenotransplantation, we discuss preclinical studies that utilize gene-edited pigs as a viable source of organs. These investigations provide valuable insights into the feasibility and potential success of xenotransplantation, offering a bridge between scientific advancements and clinical application.

Keywords: Clinical trials; Complement systems; Genetically modified pigs; Xenotransplantation; α-1,3-galactosyltransferase gene-knockout.

Fig. 1

Simplified overview of the active complement cascade. Foreign surface-bound antigen–antibody (Ag/Ab) complexes initiate the classic pathway, while polysaccharides, lipopolysaccharides, and/or IgA activate the alternative complement pathway. Damage-associated molecular patterns (DAMPs) like mannose-binding lectin (MBL), ficolins (Fcns), and certain collections (CLs) can directly trigger the classic pathway or initiate the lectin pathway. The three complement activation pathways collectively cleave C3 into C3b and C3a, triggering terminal pathway activation, mainly involving C5-C9, which assembles to form the membrane attack complex (MAC). Key complement regulatory factors include C1 inhibitor (C1-INH), factor H (FH), factor I (FI), CD46, C4BP, and CD59. C1-INH inhibits C1r activation of C1s, preventing C4 and C2 cleavage. Simultaneously, C1-INH can inhibit the binding of MBL to MASP-1 or MASP-2. Factor I and factor H, aided by C4BP and CD46, can phagocytize C3 from the alternative pathway, inhibiting its activation. CD59 prevents C9 from binding to C5b678 to form MAC

Fig. 2

Xenograft activates the complement system. The binding of IgG/IgM to the protein α-Gal on the graft surface activates the classical pathway of complement, while the interaction of IgA with α-Gal activates the alternative pathway. The combined activation of classical and alternative pathways leads to the generation of C5 convertase, ultimately resulting in the formation of the membrane attack complex (MAC) composed of C5b, C6, C7, C8, and C9. This MAC complex functions to attack the xenograft

Fig. 3

Gene-edited pig & xenotransplantation. Organs cultivated from genetically modified cloned pigs, such as heart, liver, lung, kidney, etc., can be transplanted into patients

Fig. 4

Preclinical and clinical trials of complement therapy. Complement therapy progresses through different stages, spanning from preclinical work to market authorization. These stages include laboratory research, animal models, clinical phases I, II, and III, and final clinical implementation. The therapeutic goals are categorized into four quadrants, representing major complement categories: anaphylatoxins, active pathways, amplification and terminal pathways, and effectors. Each arrow denotes a specific agent and its development stage. Drugs targeting C1r/s and MASP include Cinryze (Shire), Berinert (CSL Behring), Cetor (Sanquin), and Ruconest (Pharming), which are already being used in clinics; drugs targeting C1q include ANX005 (Annexon); drugs targeting C1s include TNT003 (True North), TNT009 (True North), and BIVV020 (Sanofi); TP10 (CDX-1135; Celldex Therapeutics) targets the soluble form of complement receptor type 1 (CR1); OMS721 (Narsoplimab, Omeros) targets the MASP-2 target; Drugs targeting MASP-3 include OMS906 (Omeros); drugs targeting Properdin include CLG561 (Novartis) and NM9401 (Novelmed); drugs targeting C3 include AMY-101 (Amyndas), APL-1 (Apellis), APL-2 (Apellis), CB2782 (Catalyst), Cp40 (Amyndas); drugs targeting C3b and convertases include AMY-201 (Amyndas), and Mirococept (MRC); drugs targeting FB include Bikaciomab (Novelmed); drugs targeting FD include Lampalizumab (Genentech), ACH-4471 (Achillion), and “Compound 6”(Novartis); drugs targeting C5 include Eculizumab (Soliris, Alexion), ALXN1210 (Alexion), ALXN5500 (Alexion), LFG316 (Novartis), Coversin (Akari), RA101495 (Ra Pharma), ALN-CC5 (Alnylam), RA101348 (RaPharma), ARC 1905 (Zimura; Ophthotech), and the affibody SOBI002 (Swedish, Orphan Biovitrum) targets C5 (programme recently terminated); drugs targeting C5a include IFX-1 (InflaRx), ALXN-1007 (Alexion), NOX-D21 (Noxxon Pharma); drugs targeting C5aR include CCX168 (Chemocentryx); C6 target drug includes Regenemab (Regenesance); CR2–FH target drug includes TT30 (ALXN 1102; Alexion)