Current Status on Therapeutic Molecules Targeting Siglec Receptors

Abstract

The sialic acid-binding immunoglobulin-type of lectins (Siglecs) are receptors that recognize sialic acid-containing glycans. In the majority of the cases, Siglecs are expressed on immune cells and play a critical role in regulating immune cell signaling. Over the years, it has been shown that the sialic acid-Siglec axis participates in immunological homeostasis, and that any imbalance can trigger different pathologies, such as autoimmune diseases or cancer. For all this, different therapeutics have been developed that bind to Siglecs, either based on antibodies or being smaller molecules. In this review, we briefly introduce the Siglec family and we compile a description of glycan-based molecules and antibody-based therapies (including CAR-T and bispecific antibodies) that have been designed to therapeutically targeting Siglecs.

Keywords: CAR; Siglec; antibody; glycan; sialic acid.

Figure 1

Schematic representation of human Siglec receptors. Siglecs contain one N-terminal V-type Ig-like domain that mediates sialic-acid recognition and varying numbers of constant (C)-type Ig-like domains at the extracellular domain. Siglecs can be divided into two groups (classic and CD33/Siglec-3 related) based on sequence similarity and evolutionary conservation. Siglec-13 is present in baboons and chimpanzees and is specifically deleted in humans. Siglec-12 in humans has lost the ability to bind sialic acids. The cell-expression patterns are shown (Mø, macrophages; DC, dendritic cell; B, B cells; MC, mast cells; Schw, Schwann cells; OD, oligodendrocytes; Ocl, osteoclasts; Myp, myeloid progenitor; Mo, monocytes; Mic, microglia; N, neutrophils; Troph, trophoblasts; NK, natural-killer cells; T, T cells; Eo, eosinophils; Ba, basophils; Lum epi, lumen epithelia cells).

Figure 2

Most common sialic acids in mammals. (A) Chemical representation of the most common type of sialic acids in mammals and their linkage to the subterminal glycan. (B) Sialic acids are found at the outer most exposed non-reducing end of glycan chains on glycoproteins or glycolipids on the cell surface.

Figure 3

Glycan binding specificities of human Siglecs.

Figure 4

The crystal structures of CD22_d1-d3 (PDB ID: 5VKJ), myelin-associated glycoprotein (MAG)_d1-d5 (PDB ID: 5LFU), CD33_d1-d2 (PDB ID: 5IHB), and Siglec-5_d1-d2 (PDB ID: 2ZG2) in cartoon representation. Domain d1 (in grey) adopts de V-type Ig-like domain and contains the sialic acid binding pocket (indicated with an arrow) with the conserved Arg (in sticks). The N-linked glycans are represented with sticks and spheres. The disulfide bonds are also depicted with sticks. The secondary structure differences between the V- (strands A(A′)B(B′)ED and CC′FG(G′)), C1- (strands ABED and CFG) and C2- (ABE and C(C′)FG(G′) strands) type Ig folding are shown with a diagram (inside the box).

Figure 5

Superposition of the unliganded (grey) and liganded (orange) structures of d1 from Sialoadhesin (PDB ID: 1QFP and 1QFO), CD22 (PDB ID: 5VKJ and 5VKM), MAG (PDB ID: 5LFR and 5LF5), CD33 (PDB ID: 5IHB and 5J06), Siglec-5 (PDB ID: 2ZG2 and 2ZG3), Siglec-7 (PDB ID: 1O7S and 2HRL) and Siglec-8 (PDB ID: 2N7A and 2N7B).

Figure 6

Antibody based molecules targeting Siglecs. (A) mAbs against Siglecs. (Left) Anti Siglec-8-Ab, such as AK002, deplete eosinophil activation and mast cell degranulation upon binding. (Middle) Isotope/drug conjugated Abs targeting CD22 on B cells are endocyted and can deliver the isotope or drug at the cytoplasm to allow cell killing. (Right) Anti-Siglec-15 Abs (e.g., NC318) can block interaction of cancer cells expressing Siglec-15 and T cells. (B) Targeting Siglecs with chimeric antigen receptors (CARs) and bi-specific Abs. (Left) CAR-T cells targeting CD22 have being designed to specifically target and kill malignant B cells. (Right) The bi-specific T-cell engaging Ab (BITE) targeting CD33 and CD3 on the surface of T cells is able to activate a cytotoxic response against leukemic cells without requiring the prior activation of T cells. (C) Targeting cancer-associated glycans of tumor cells. (Left) Siglec-7/-9 derived CAR T-cells can recognize and kill cancer cells through binding to sialylated glycans. (Right) The tumor-targeted mAbs against HER2 fused with neuraminidase, is able to degrade Siglec-7/9 ligands and restore the immune cell response.

Figure 7

Modified glycan ligands targeting Siglecs. (A) General structure of sialic acid and examples of chemical substituents (R₁–R₄) used to generate specific and high-affinity modified glycans against Siglecs. (B) Development of sialic acid mimetics of high-affinity for CD22. (C) High-affinity ligands for Siglec-8. (D) Examples of nanocarriers coated with modified sialic acids.