Specific Targeting of Notch Ligand-Receptor Interactions to Modulate Immune Responses: A Review of Clinical and Preclinical Findings

Abstract

Understanding and targeting Notch signaling effectively has long been valued in the field of cancer and other immune disorders. Here, we discuss key discoveries at the intersection of Notch signaling, cancer and immunology. While there is a plethora of Notch targeting agents tested in vitro, in vivo and in clinic, undesirable off-target effects and therapy-related toxicities have been significant obstacles. We make a case for the clinical application of ligand-derived and affinity modifying compounds as novel therapeutic agents and discuss major research findings with an emphasis on Notch ligand-specific modulation of immune responses.

Keywords: Notch signaling; Notch therapeutics; T cells; antigen presenting cells; cancer immunotherapy; engineered Notch ligands; immunosurveillance; tumor escape.

FIGURE 1

Notch interactions between antigen-presenting cells and T cells influence helper and effector T cell activity. T cells express T cell receptor (TCR) complex and Notch receptors. Antigen-presenting cells (APCs) express costimulatory molecules and Notch ligands. During T cell activation, the identity of Notch ligand present on the cell surface of APCs can influence T cell polarization and differentiation. Changes in expression levels of fringe glycosyl transferases can influence the process by modifying Notch receptor affinity to different ligands. Notch signaling in T cells regulates expression of transcription factors and cytokines (indicated within []) involved in helper and cytotoxic T cell functions. APCs with high expression levels of DLL1 or DLL4 can polarize CD4⁺ T cells into aTh1 phenotype and drive CD8⁺ T cell differentiation into memory cells. Increase (↑) in LFNG and MFNG expression and downregulation/loss (↓) of RFNG expression can enhance Th1 differentiation; identity of ligands involved in fringe-mediated Th1 differentiation are yet to be investigated (represented by ?ligand?). APCs with high JAG2 and low DLL1,4 expression drive helper T cell differentiation into Th2 or Th17 phenotypes. Expression of MFNG and downregulation of RFNG can block Th2 differentiation. Loss of LFNG in uncommitted T cells as well as Th2 polarized cells inhibits Notch interactions with DLL4 and attenuates Th2 responses. APCs with high JAG1 expression can induce T cell polarization into regulatory T cells (T_reg). CD40 blockade together with JAG1 expression on APCs enhances immunosuppressive functions of T_reg cells. APC, antigen presenting cell; DLL, Delta-like ligand; JAG, Jagged ligand; LFNG, lunatic fringe; MFNG, manic fringe; MHC, major-histocompatibility complex; TCR,; Th1, T helper type 1, Th2: T helper type 2; Th17, T helper type 17; T_reg, T regulatory cell; T_EM, effector-memory T cell; T_CM, central-memory T cell; RFNG, radical fringe.

FIGURE 2

Mechanistic basis for therapeutic targeting of ligand-specific Notch signaling. Agents targeting Notch signaling can be grouped by the step or process in the Notch signaling pathway that is being affected. Soluble decoys comprise of extracellular portions of Notch ligands or receptors that can competitively inhibit multivalent receptor-ligand interactions. Soluble multivalent ligands comprise of clustered ligands that provide and/or augment ligand-specific Notch activation. Blocking antibodies block receptor interactions with ligands and are paralog-specific antagonists with high selectivity. γ-secretase inhibitors prevent NICD release by inhibiting S3 cleavage of Notch receptors at the juxtamembrane domain. L-fucose analogs (solid red triangles) are taken up by cells from media and incorporated into receptor extracellular domains. Fucose analogs on Notch receptors alter ligand-binding affinities and can be used to block selective ligand interactions. Blocking peptides target protein-protein interface in the nuclear Notch transcriptional complex and prevent transcription of Notch target genes. In vitro, pre-clinical and clinical studies demonstrating Notch-modulatory activities and anti-tumor efficacy of various classes of Notch therapeutics are presented in Table 1.