Membrane attack complexes, endothelial cell activation, and direct allorecognition

Abstract

Endothelial cells (ECs) form a critical immune interface regulating both the activation and trafficking of alloreactive T cells. In the setting of solid organ transplantation, donor-derived ECs represent sites where alloreactive T cells encounter major and minor tissue-derived alloantigens. During this initial encounter, ECs may formatively modulate effector responses of these T cells through expression of inflammatory mediators. Direct allorecognition is a process whereby recipient T cells recognize alloantigen in the context of donor EC-derived HLA molecules. Direct alloresponses are strongly modulated by human ECs and are galvanized by EC-derived inflammatory mediators. Complement are immune proteins that mark damaged or foreign surfaces for immune cell activation. Following labeling by natural IgM during ischemia reperfusion injury (IRI) or IgG during antibody-mediated rejection (ABMR), the complement cascade is terminally activated in the vicinity of donor-derived ECs to locally generate the solid-phase inflammatory mediator, the membrane attack complex (MAC). Via upregulation of leukocyte adhesion molecules, costimulatory molecules, and cytokine trans-presentation, MAC strengthen EC:T cell direct alloresponses and qualitatively shape the alloimmune T cell response. These processes together promote T cell-mediated inflammation during solid organ transplant rejection. In this review we describe molecular pathways downstream of IgM- and IgG-mediated MAC assembly on ECs in the setting of IRI and ABMR of tissue allografts, respectively. We describe work demonstrating that MAC deposition on ECs generates 'signaling endosomes' that sequester and post-translationally enhance the stability of inflammatory signaling molecules to promote EC activation, a process potentiating EC-mediated direct allorecognition. Additionally, with consideration to first-in-human xenotransplantation procedures, we describe clinical therapeutics based on inhibition of the complement pathway. The complement cascade critically mediates EC activation and improved understanding of relevant effector pathways will uncover druggable targets to obviate dysregulated alloimmune T cell infiltration into tissue allografts.

Keywords: allorecognition; antibody-mediated rejection; complement; endothelial cell; transplant.

Figure 1

Three pathways of complement activation. The complement cascade may be activated by donor endothelial cells via 3 separable pathways, the classical (left), alternative (middle), and lectin (right) pathways, to mediate rejection of tissue allografts.

Figure 2

Three pathways of allorecognition. Donor-derived alloantigens may activate allogeneic T cells via three separable pathways. Human endothelial cells may act as antigen presenting cells to participate in direct allorecognition responses (left), while murine hosts primarily participate in semidirect and indirect allorecognition (middle, right) through interactions involving professional antigen presenting cells.

Figure 3

A model for autologous alloantibody-induced MAC assembly. 'High' panel reactive antibody (PRA) sera is obtained from transplant candidates and contain high titers of alloantibodies binding to many HLA specificities. PRA mediates binding of donor specific alloantibody to surface MHC I/II molecules on human ECs. This activates complement components within PRA sera, causing terminal activation of the classical complement pathway and formation of non-cytolytic MAC on target ECs. Human-derived MAC on human ECs causes EC activation without inducing cell lysis.

Figure 4

MAC-induced signaling endosomes cause EC activation. Non-cytolytic MAC assembled on EC surfaces rapidly undergoes clathrin-mediated endocytosis to form MAC+Rab5+ signaling endosomes that sequester signal-activating elements. In a Rab5-dependent manner, signaling endosomes post-translationally stabilize ZFYVE21, a Rab5 effector. (A) ZFYVE21 recruits SMURF2, an E3 ubiquitin ligase, to mediate degradative removal of PTEN from signaling endosomes, causing enrichment for PI(3,4,5)P3 and recruitment of phosphorylated Akt (pAkt, B). This process sequentially activates non-canonical NF-κB [marked by NIK (NF-kB Inducing Kinase)], NLRP3 inflammasomes, and IL-1β- mediated canonical NF-kB (C).