Notch Signaling and Alloreactivity

Abstract

Solid organ and allogeneic hematopoietic cell transplantation have become standard therapeutic interventions that save patient lives and improve quality of life. Our enhanced understanding of transplantation immunobiology has refined clinical management and improved outcomes. However, organ rejection and graft-versus-host disease remain major obstacles to the broader successful application of these therapeutic procedures. Notch signaling regulates multiple aspects of adaptive and innate immunity. Preclinical studies identified Notch signaling as a promising target in autoimmune diseases, as well as after allogeneic hematopoietic cell and solid organ transplantation. Notch was found to be a central regulator of alloreactivity across clinically relevant models of transplantation. Notch inhibition in T cells prevented graft-versus-host disease and organ rejection, establishing organ tolerance by skewing CD4 T helper polarization away from a proinflammatory response toward suppressive regulatory T cells. Notch ligand blockade also dampened alloantibody deposition and prevented chronic rejection through humoral mechanisms. Toxicities of systemic Notch blockade were observed with γ-secretase inhibitors in preclinical and early clinical trials across different indications, but they did not arise upon preclinical targeting of Delta-like Notch ligands, a strategy sufficient to confer full benefits of Notch ablation in T cell alloimmunity. Because multiple clinical grade reagents have been developed to target individual Notch ligands and receptors, the benefits of Notch blockade in transplantation are calling for translation of preclinical findings into human transplantation medicine.

Figure 1. The Notch signaling pathway

Four Notch receptors (Notch1–4) and 5 Notch ligands of the Jagged (Jagged1–2) and Delta-like (Dll1,3,4) families have been identified in mammals. Notch receptors are modified by Fringe glycosyltransferase, which modulates their interaction with Notch ligands. Receptor-ligand binding leads to extracellular cleavage of the receptor by the ADAM10 metalloprotease, followed by the intramembrane proteolysis via γ-secretase. Cleaved intracellular Notch (ICN) translocates into the nucleus. ICN associates with CSL (CBF1/Suppressor of Hairless/LAG-1; also known as RBPJ), a Mastermind-like (MAML) family coactivator, and other transcriptional co-activators (CoA) to displace co-repressors (CoR) and build a large multiprotein complex that activates target gene transcription.

Figure 2. Effects of Notch signaling in the alloresponse

Inflammation and use of conditioning regimens in allo-HCT activate dendritic cells (DC) and nonhematopoietic cells, which express Dll1 and Dll4 Notch ligands. In concert with TCR and costimulatory molecules, Notch activates naïve T cells, leading to proliferation, Th polarization, proinflammatory cytokine production, and acquisition of effector functions, thus promoting GVHD or solid organ rejection (left). In addition, T cell help and Notch engagement in B cells facilitate alloreactive antibody production, contributing to tissue deposition and enhanced alloimmune tissue injury. Upon genetic or biochemical inhibition of Notch signaling (right), cytokine production is decreased and Treg expansion enhanced, despite preserved overall T cell activation and proliferation. Decreased antibody production by B cells adds to an environment of enhanced tolerance characterized by diminished GVHD and long term solid organ transplant survival.