Targeting Siglec-Sialylated MUC1 Immune Axis in Cancer

Abstract

Siglecs play a key role in mediating cell-cell interactions via the recognition of different sialylated glycoconjugates, including tumor-associated MUC1, which can lead to the activation or inhibition of the immune response. The activation occurs through the signaling of Siglecs with the cytoplasmic immunoreceptor tyrosine-based activation motif (ITAM)-containing proteins, while the inhibition signal is a result of the interaction of intracellular immunoreceptor tyrosine-based inhibition motif (ITIM)-bearing receptors. The interaction of tumor-associated MUC1 sialylated glycans with Siglecs via ITIM motifs decreases antitumor immunity. Consequently, these interactions are expected to play a key role in tumor evasion. Efforts to modulate the response of immune cells by blocking the immune-suppressive effects of inhibitory Siglecs, driving immune-activating Siglecs, and/or altering the synthesis and expression of the sialic acid glycocalyx are new therapeutic strategies deserving further investigation. We will highlight the role of Siglec's family receptors in immune evasion through interactions with glycan ligands in their natural context, presented on the protein such as MUC1, factors affecting their fine binding specificities, such as the role of multivalency either at the ligand or receptor side, their spatial organization, and finally the current and future therapeutic interventions targeting the Siglec-sialylated MUC1 immune axis in cancer.

Keywords: MUC1; Siglecs; glycocode; immune evasion; sialoglycans.

Figure 1

MUC1 transformation from normal to malignant phenotype in human cancers. In normal cells, MUC1 is covered with branched Galβ1-3[GlcNAcβ1-6]GalNAcα1-O-Ser/Thr (core 2) O-glycans with lactosamine extensions. The sugars form a protective and selective barrier, undertake receptor-ligand interactions, and communicate information about external cell conditions through signal transduction. The increased expression and altered density of shorter glycoforms of mucin, such as O-linked N-acetylgalactosamine (Tn: αGalNAc-), sialic acid capped Tn (sTn: αNeuNAc-2,6-αGalNAc-), Thomsen-Friedenreich (T: βGal-1,3-αGalNAc-) antigen, and sialic acid-capped T [sT: Galβ1–3(Neu5Acα2–6)-GalNAcα- and Neu5Acα2-3(Galβ1-3)-GalNAcα-] are commonly observed changes in malignant and premalignant epithelia. The expression of these TACAs is usually associated with cancer aggressiveness and poor prognosis.

Figure 2

(a) Transmembrane lectins expressed by cells of the innate and adaptive immune systems. The C-type lectins include selectins, macrophage galactose lectin (MGL), and dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN). (b) Human Siglec family. Shown are the two major Siglec subgroups: the evolutionary conserved Siglecs (Siglec-1, -2, -4, and -15), and the CD33-related Siglecs (Siglec-3, -5, -6, -7, -8, -9, -10, -11, -12, -14, and -16).

Figure 3

(a) Siglecs ligands with α2,6- (sTn, sT), α2,3- (sT), and α2,8-linkages (PSA-linear homopolymers of N-acetylneuraminic acid). (b) Cis (when present on the same cell) and trans (when present on other cells) interactions of Siglecs with sialic acid ligands lead to downstream immune inhibitory signaling.

Figure 4

The engagement of Siglecs (-7, -9, 15) expressed on the surface of immune cells (neutrophils, macrophages, NK cells, T cells, and dendritic cells) and sialoglycans of MUC1 expressed on cancer cells results in immunosuppression in the tumor microenvironment via impaired maturation and activation of macrophages, deactivation of natural killer (NK) cells, and the formation of regulatory T cells. Thus, it is thought that these interactions are novel immune checkpoints in a similar fashion to PD-1/PD-L1 checkpoint inhibitors. The disruption of Siglec–sialylated MUC1 immune axis is a potential new target for cancer immunotherapy approaches.

Figure 5

(a) Carbon groups (C-) in the sialic acid backbone that could be substituted to design high-affinity SAMs are highlighted in blue; (b) nanoparticle carriers decorated with sialic acid mimetics for high-affinity binding to Siglecs; (c) glycopolymers that target membranes and can bind to Siglec on immune cells; (d) creation of a highly reactive cell glycocalyx (clickable sialic acid-azide approach) for Siglecs.

Figure 6

Antibody-based approaches. (a) Anti-Siglecs antibodies block the Siglec–ligand interaction and activate the immune cell attack on the cancer cell; anti-Siglecs antibody–drug(toxin) conjugates are comprised of two parts; the anti-Siglecs antibody portion of the drug targets Siglecs on the surface of cancer cells, leading to the internalization of the antibody and the drug and successive release of the cytotoxic payload within the cancer cell. (b) Anti-Siglecs antibody against HER2–sialidase conjugates can remove sialic acid from surface of glycans and restore the immune cell response. (c) Chimeric antigen receptors (CARs); (left) Siglec-7/-9 CARs can recognize and eradicate cancer cells through binding to sialylated glycans. (Right) Activated T cells are linked to cancer cells via anti-Siglec BiTEs, which cause the cancer cell to be eradicated.

Figure 7

The use of small-molecule metabolic inhibitors, such as the fluorinated sialic acid analogue 3Fax-Neu5Ac, to obstruct the synthesis of sialylation pathway. As a result, the cell surface-expressed glycans are not capped with sialic acid, and their interaction with Siglecs is blocked.