Sweet Immune Checkpoint Targets to Enhance T Cell Therapy

Abstract

Despite tremendous success against hematological malignancies, the performance of chimeric Ag receptor T cells against solid tumors remains poor. In such settings, the lack of success of this groundbreaking immunotherapy is in part mediated by ligand engagement of immune checkpoint molecules on the surface of T cells in the tumor microenvironment. Although CTLA-4 and programmed death-1 (PD-1) are well-established checkpoints that inhibit T cell activity, the engagement of glycans and glycan-binding proteins are a growing area of interest due to their immunomodulatory effects. This review discusses exemplary strategies to neutralize checkpoint molecules through an in-depth overview of genetic engineering approaches aimed at overcoming the inhibitory programmed death ligand-1 (PD-L1)/PD-1 axis in T cell therapies and summarizes current knowledge on glycoimmune interactions that mediate T cell immunosuppression.

Figure 1.. Engineering strategies to target the PD-1/PD-L1 axis and enhance CAR T-cell performance.

Researchers have interfered with PD-1 expression via shRNA or siRNA-mediated knockdown or CRISPR/Cas-9-mediated knockout. CAR T-cells have also been engineered to secrete full length antibodies or single chain variable fragments (scFvs) that block the binding of PD-1 with its cognate ligand PD-L1. Alternatively, groups have generated PD-L1-targeting CAR T-cells as well as CAR T-cells co-expressing PD-1 dominant-negative receptors with no intracellular signaling domains, or PD-1 switch receptors containing the intracellular domain of the costimulatory molecule CD28.

Figure 2.. Aberrantly glycosylated tumor cells can engage lectins to inhibit the activity of CD8+ T-cells.

a) Examples of tumor-associated sialoglycan engagement of sialic acid-binding immunoglobulin-like lectins (Siglecs) to modulate the immune response. Specific axes that modulate myeloid and natural killer cells have been identified, such as the binding of MUC1 aberrantly glycosylated with short, sialylated O-glycans (MUC1-ST) to Siglec-9 expressed on macrophages to promote a tumor-associated macrophage-like phenotype. Although Siglec-9 upregulation on T-cells and Siglec-15 expression on macrophages and tumor cells have been implicated in the inhibition of CD8+ T-cells, the specific ligands that mediate their effects have not been identified. b.) Galectin-3 regulates proliferation and cytokine production by CD8+ T-cells. Galectin-3 binding to β-galactoside glycan structures, N-acetyllactosamine (LacNAc), causes an increase in tumor cell proliferation and immune escape. The galectin-3 inhibitor GB1107 reduced mouse and human lung adenocarcinoma growth and caused an increased expression of cytotoxic (IFNγ, granzyme B, perforin-1) and apoptotic effector molecules, recruitment and activation in CD8+ T-cells, and decreased tumor cell proliferation. c.) Examples of C-type Lectin modulation of adaptive immune function. Macrophage galactose-type lectin (MGL) interacts with terminal α-GalNAc residues (Tn antigen) on tumor cells to induce an immunosuppressive phenotype as well as on CD45 on effector T-cells to directly inhibit activity. NKG2A/CD94 recognizes HLA-E on tumor cells, leading to immunosuppression through increased TGF-β and decreased IL-15 secretion. This effect has been prevented through lentiviral transduction to produce shRNA against NKG2A transcripts, leading to increased NK and T-cell cytotoxicity. The interaction of DC-SIGN expressed on dendritic cells with Lewis X antigens on the tumor surface causes adaptive immunosuppression through many pathways, including increased PD-1 expression on T-cells, leading to apoptosis.