Therapeutic Targeting of Siglecs using Antibody- and Glycan-Based Approaches

Abstract

The sialic acid-binding immunoglobulin-like lectins (Siglecs) are a family of immunomodulatory receptors whose functions are regulated by their glycan ligands. Siglecs are attractive therapeutic targets because of their cell type-specific expression pattern, endocytic properties, high expression on certain lymphomas/leukemias, and ability to modulate receptor signaling. Siglec-targeting approaches with therapeutic potential encompass antibody- and glycan-based strategies. Several antibody-based therapies are in clinical trials and continue to be developed for the treatment of lymphoma/leukemia and autoimmune disease, while the therapeutic potential of glycan-based strategies for cargo delivery and immunomodulation is a promising new approach. Here we review these strategies with special emphasis on emerging approaches and disease areas that may benefit from targeting the Siglec family.

Keywords: Siglecs; antibodies; glycans; sialic acid; synthetic ligands.

Figure 1

The family of human Siglecs. Differences between family members include the number of extracellular Ig domains, the number of intracellular ITIM motifs, the presence of a positively charged intramembrane residue (Siglecs 14-16), and loss of sialic acid recognition (Siglec-12). Inhibitory-type and activatory-type Siglecs are noted. Expression patterns for each Siglec in normal individuals are indicated below: Mac, macrophage; B, B cell; Mon, monocyte; MyP, myeloid precursor; OligD, oligodendrocyte; Neu, neutrophil; NK, natural killer cell; Eos, eosinophil; Bas, basophil; Mast, mast cell; DC, dendritic cell; Epi, epithelial cell; Osclast, osteoclast.

Figure 2

Antibody-based approaches under development for therapeutic targeting of the Siglecs. (a) Unconjugated antibodies, antibody-drug conjugates, and antibody-protein toxin conjugates (also known as immunotoxin) are traditional antibody-based targeting approaches, but the clinical properties of these therapeutic molecules continue to be improved, such as the stability/cleavability of the linker between antibody and drug or toxin as well as the antigenicity of the protein toxin. Not represented, but also under development, are radionuclide-conjugated antibodies. (b) Advancements in the design and production of bispecific antibodies have yielded a few cases of Siglec-targeting bispecific antibodies that are in early phase clinical development. (c) A chimeric antigen receptor (CAR) consists of single-chain variable fragment (scFv) of an antibody, followed by a linker peptide and a combination of transmembrane/intracellular signaling domains of T cell receptor (CD3ζ) and co-stimulatory receptor (e.g., CD28 or CD137/4-1BB). Primary T cells from the patient are virally transduced with the construct to express CAR, propagated, and returned to the patient's body. CAR-positive T cells are stimulated upon encounter with the antigen-expressing target cells in vivo, leading to their expansion and the cytolytic elimination of the target cell population. A few cases of early phase clinical trials of Siglec-targeting CAR have been reported.

Figure 3

General structure of sialic acid analogs used as high affinity Siglec ligands. Different approaches have been employed to develop high affinity Siglec ligands by introducing various substituents at C-9, C-5, and C-4 of sialic acid. Varied substituents at C-2 of sialic acid have also been used including aromatic aglycones (A) and glycans where the modified sialic acid is linked α2-6 (B) or α2-3 (C) to galactose (Gal) of lactose or LacNAc.

Figure 4

Overview of sialoside analog based high affinity Siglec ligands. Numbering based on discussion of compounds in text. Ph, phenyl. Lactose, Galβ1-4Glc. LacNAc, Galβ1-4GlcNAc.

Figure 5

Glycan ligand-based approaches with therapeutic potential. The therapeutic potential of high affinity Siglec ligands have been shown in the area of delivering drugs or toxins to cancer cells (upper left), delivering antigens to APCs (upper right), and inducing antigen-specific B cell tolerance (lower). For delivery of chemotherapeutic agents to cancer cells, high affinity Siglec ligands have been attached to liposomes encapsulated with drug or directly conjugated to a protein toxin. As has been demonstrated for CD22, following endocytosis into endosomes Siglec-ligand interactions dissociate, allowing the cargo to be dropped off and the Siglec to recycle to the cell surface for another round of delivery. For delivery of antigens to APCs, protein and lipid antigens have been delivered to APCs (macrophages and dendritic cells) via incorporation into Siglec-targeted liposomes. Once inside the cell, antigens are loaded onto the appropriate complex (MHC I, CD1d, or CD1b) and are presented for activation of T cells, NKT cells, or CD1b-restricted T cells, respectively. For induction of antigen-specific B cell tolerance, Siglec-engaging tolerance-inducing antigenic liposomes (STALs) present both antigen and high affinity CD22 or Siglec-G ligand, which co-engage the Siglecs with the BCR to deliver a pro-apoptotic effect to B cells that recognize the antigen.