Emerging roles for complement in lung transplantation

Abstract

The complement system is an evolutionarily conserved host defense system that has evolved from invertebrates to mammals. Over time, this system has become increasingly appreciated as having effects beyond purely bacterial clearance, with clinically relevant implications in transplantation, particularly lung transplantation. For many years, complement activation in lung transplantation was largely focused on antibody-mediated injuries. However, recent studies have highlighted the importance of both canonical and noncanonical complement activation in shaping adaptive immune responses, which influence alloimmunity. These studies, together with the emergence of FDA-approved complement therapeutics and other drugs in the pipeline that function at different points of the cascade, have led to an increased interest in regulating the complement system to improve donor organ availability as well as improving both short- and long-term outcomes after lung transplantation. In this Review, we provide an overview of the when, what, and how of complement in lung transplantation, posing the questions of: when does complement activation occur, what components of the complement system are activated, and how can this activation be controlled? We conclude that complement activation occurs at multiple stages of the transplant process and that randomized controlled trials will be necessary to realize the therapeutic potential of neutralizing this activation to improve outcomes after lung transplantation.

Figure 1. Canonical activation of the complement cascade.

The complement cascade can be viewed as one that can be triggered by a series of proteases, causing it to assemble into convertases that cleave key proteins such as C3 and C5, thus amplifying the cascade and, ultimately, attacking the target. The cascade is triggered via the CP (primarily by antigen-antibody complexes) or the lectin pathway (by carbohydrates on surfaces binding to pattern recognition molecules such as mannose-binding lectin [MBL] or ficolins, and C2 and C4 cleavage by mannose-associated serine proteases [MASPs] or small MBL-associated proteins [sMAP]), to form an enzyme that cleaves C3, called the C3 convertase (C4bC2b). The alternative pathway can be initiated in the fluid phase by the conversion of C3 to C3(H₂O), which binds to factor B, and in the presence of factor D, can generate C3b from C3. C3b binds to hydroxyl (-OH) or amine (-NH2) groups on carbohydrates or proteins on cellular surfaces via its thioester bond. Alternatively, C3b deposits directly on a surface and binds to factor B, which is then cleaved into Bb by factor D to form the (alternative pathway) C3 convertase, C3bBb. C3 convertases cleave C3 to C3a and C3b, facilitating the formation of a C5 convertase, which cleaves C5 to form C5a and C5b. C3a and C5a serve as anaphylatoxins, promoting vasodilation and chemotaxis by binding to their cognate receptors (C3aR, and C5aR1, although the role of C5aR2 continues to be clarified). C3b facilitates opsonophagocytosis and can also bind to factor B to amplify the alternative pathway. C5b binds to C6, C7, and C8 and subsequently C9 to form the membrane attack complex (MAC, C5b-9). At each step, a series of membrane regulators and fluid-phase regulators keep this system in check.

Figure 2. Proposed scenarios and consequences of local complement activation in lung transplantation.

(A) Antibody-mediated rejection. Antibodies against HLA antigens and neoepitopes activate the classical complement pathway. Antibodies also activate immune cells, promoting cytotoxicity. Recent studies suggest that complement component deposition in the endothelium and signaling through the C3a and C5a receptors (especially C5aR1) can disrupt the integrity of the endothelial barrier and increase recruitment of immune cells. Additionally, signaling through noncomplement receptors can affect endothelial proliferation and thrombosis and promote resistance to complement-mediated damage. (B) Primary graft dysfunction. Local complement activation can damage the donor tissue, resulting in acute lung injury. Sources of complement proteins can include the tissue-resident cells, such as epithelial cells, fibroblasts, endothelial cells, and myeloid cells, but these proteins can also be sourced from the circulation in the setting of alveolar-capillary barrier disruption. (C) Chronic lung allograft dysfunction. Complement activation in donor lung contributes to persistent inflammation and immune dysregulation, culminating in CLAD. The effects of complement in CLAD can be attributed to the effects of activation fragments influencing B cells and/or ongoing inflammation resulting in the downregulation of regulatory proteins. For example, increased TGF-β in the lung downregulates CD46 and CD55 in the epithelium. CD59 can also be cleaved, in addition to being downregulated. MAC, membrane attack complex.

Figure 3. Therapies for systemic and local complement inhibition.

(Left) Currently available FDA-approved complement therapeutics function via systemic inhibition of complement activation. These include inhibitors of the initiation stage (C1 esterase inhibitors [such as berinert, cinryze, haegarda, or ruconest]) or those targeting C1s (such as sutimlimab), central component C3 (pegceptacoplan), the amplification loop (iptacopan, targeting factor B; danicopan, targeting factor D), or at terminal effector pathways (eculizumab, ravulizumab, or zilucoplan, targeting C5) and C5a signaling (avacopan) (Table 3). (Right) However, given that activation of complement is a local event, there is potential to inhibit complement at the level of the graft to modulate it locally without affecting host systemic complement functions. To date, these approaches have been explored only in experimental transplantation and/or cell culture models. APT070 (mirococept), is a membrane-localizing C3 convertase inhibitor that has been explored in kidney transplantation (120). Due to its unique membrane-interacting synthetic peptide, which mediates binding to phospholipids on the cell surface, it can be perfused into the donor graft to precoat the endothelium prior to transplantation. Recombinant protein (134, 137) and natural antibody single-chain fragment (53) targeting moieties have been used to target complement inhibitors to the graft via binding to complement opsonins or exposure of damage-associated molecular patterns and/or neoantigens that are exposed by ischemia/reperfusion in the graft, respectively. Given the unique structure of the lung, direct targeting of complement can also be achieved by nebulization. Preclinical studies directly nebulizing C3aR antagonist to the donor lung have shown efficacy in reducing IRI and rejection onset (63). While not specifically tested in lung transplantation, epithelial intracellular factor B inhibition (28) and C5aR1 antagonism (33) have shown promise in reducing lung epithelial injury.