Mol Aspects Med: A New Immune Checkpoint for Cancer Treatment: the Siglec Receptor

Over the past decade, cancer immunotherapy, in the form of immune checkpoint inhibitors and cell therapy, has improved the treatment and outcomes
of many patients.
Despite this, most cancers remain resistant to currently approved cancer immunotherapies
.
New methods and reasonable combinations are needed to overcome these resistances
.

Recent research suggests that the interaction between sialic acid, which contains sialic acid glycans in the tumor microenvironment, and sialic acid-binding immunoglobulin-like lectin (Siglec) receptors on tumor-infiltrating immune cells may represent a new immune checkpoint and a potential new target for cancer immunotherapy
.

Siglec receptors and signaling pathways

Siglecs is a family
of receptors that bind to sialic acid-containing glycans.
Most Siglec receptors are inhibitory, with 15 human-derived and 9 murine-derived Siglec molecules
currently identified.
Siglec receptors can be further divided into sequence-conserved receptors and rapidly evolving receptors
associated with CD33.
Siglec-1, Siglec-2 (CD22), Siglec-4, and Siglec-15 belong to the conservative family; Siglec-3 (CD33), Siglec-5, Siglec-6, Sigleg-7, Siglec8, Siglec-9, Sigle-c-11, Siglec-XII, Siglec-14, and Siglec-16 are among the rapidly evolving CD33-related Siglecs receptors
.

According to the different signaling domains in the cell, Siglec receptors can also be divided into inhibitory, activated and non-signaling
.
Siglec-11, Siglec-14, and Siglec-15 are activated Siglec receptors, while Siglec-1 and Siglec-4 do not have direct immunomodulatory intracellular domains
.
All other human Siglec receptors are inhibitory in
nature.
The Siglec family belongs to the type I membrane protein that is once permeable, which has very typical and conserved structural characteristics in structure, and its transmembrane region is composed of 2 ~17 extracellular Ig domains, and the N-terminal is composed
of a V-set Ig domain bound to sialic acid and a certain number of C2-set Ig domains.

Inhibitory Siglec receptor cells contain domains of immune receptor tyrosine inhibitory motif (ITIM) and immune receptor switch motif (ITSM), which can regulate intracellular signaling
through the participation of SHP1 and SHP2 phosphatases.
Thus, inhibitory Siglecs can inhibit immune cell activation
in a similar mode of action to PD-1/PD-L1.
The activated Siglec receptor has a transmembrane domain of positively charged amino acids that mediates the recruitment of DAP12 when CRD binds to sialic acid glycan ligands, which contains the immune receptor tyrosine group activation motif (ITAM) and transmits activation signals
.

Expression of cancer-associated Siglec ligands

Many studies have reported changes in the expression of sialic acid glycans in the cancer and tumor microenvironment, and tumor cells are typically hypersialylated, producing ligands
that suppress Siglec receptors on immune cells.
For example, pancreatic ductal adenocarcinoma (PDAC) tumor cells show increased
sialylacidation due to overexpression of the sialyltransferases ST3GAL1 and ST3GAL4.
Sialylation of PDAC cells is recognized by Siglec-7 and Siglec-9 on myeloid cells and polarizes
monocytes towards tumor-promoting macrophages.
Similarly, upregulation of the Siglec-9 ligand has been demonstrated
in human colorectal, prostate, breast, and non-small cell lung cancers.

Through genome-wide screening, sialylated CD43 has been determined to be a highly specific ligand for Siglec-7, inhibiting NK cell-mediated K562 leukemia cell killing
.
LGALS3BP has been shown to be a secretory carcinoma-associated ligand for several Siglecs, including Siglec-9, inhibiting neutrophil activation
.
CD24 is overexpressed in many cancers and has become a major mechanism
of immune evasion in some ovarian and breast cancers through interaction with Siglec-10 on tumor-associated macrophages (TAMs).
In addition, soluble CD52 also binds to Siglec-10 on T cells and has been reported to inhibit T cells
in autoimmune diseases.

Effects of Siglec receptors on cancer immune cells

Siglec receptors are widely expressed on different cells of
the immune system.
Functionally correlated interactions between sialic acid glycan ligands and Siglec receptors on different immune cells have been shown to help establish an immunosuppressive microenvironment
in the context of cancer.

Innate immune cells, especially macrophages, are highly expressed in several different Siglec receptors, including Siglec-3, Siglec-5, Siglec-7, Siglec9, Siglec-10, Siglec/14, and Siglec-15
.
It has recently been shown that sialic acid glycan binding to Siglec-7 and Siglec-9 in pancreatic cancer cells induces pro-tumor macrophage phenotypes
.

。 In addition, it has been shown that sialylated CD24 on cancer cells interacts with Siglec-10 on TAM to inhibit phagocytosis
.
Siglec-15 on macrophages has also been shown to inhibit T cell-mediated anti-tumor immunity
.

Dendritic cells are important mediators of the anti-tumor immune response and are closely related to
the success of immunotherapy.
Recent studies further demonstrate the role
of Siglec receptors on classical dendritic cells (cDCs).
Mouse Siglec-G on DC has been shown to modulate antigen processing; In addition, Siglec-E in mice was shown to be involved in antigen uptake and presentation
to CD4+ T cells.
In mouse model systems, sialylation of antigens can induce tolerance regulatory T cells
by Siglec-E.
Sialic acid glycans on human monocyte-derived dendritic cells inhibit immune cell activation
via Siglec-7 and Siglec-9.
Sialic acid glycans also induce high-affinity interactions
between DC and CD8+ T cells.

NK cells are important congenital lymphocytes
.
Several sets of studies have shown that Siglec-7 and Siglec-9 on NK cells can interact with cancer-associated sialic acid glycans and participate in inhibiting anti-tumor immune activation
.
Insertion of synthetic sialic acid glycans into the cell membranes of tumor cells enables dose-dependent inhibition of NK cell-mediated killing and degranulation
.
Recent work further demonstrates that Siglec-7 interacts with sialylated PSGL-1 on multiple myeloma cells and is able to inhibit NK cell-mediated myeloma cell killing
.
In addition, in renal cancer cells, overexpression of the ganglioside Siglec-7 ligand inhibits NK cell activation; Sialylated MUC16 binds to human Siglec-9 and inhibits NK cells
in ovarian cancer.

In addition to affecting myeloid cells and other innate immune cells, sialic acid-Siglec interactions also affect cancer's adaptive immune system
.
The study found that Siglec-9 was upregulated in blood and tumor-invasive T cells in cancer patients, and it was expressed
on tumor-specific depleted PD-1+ T cells.
Reduction of Siglec-9 ligands on tumor cells significantly induces T cell-mediated effector function and tumor cell killing
.
Siglec-5 and Siglec-10 were also found to be upregulated after acute activation of T cells and may affect anti-tumor immunity
.

In conclusion, the interaction of sialic acid glycans with Siglec receptors has been shown to contribute to immunosuppressing the tumor microenvironment
by inducing the oncogenic phenotype of tumor-associated macrophages, inhibiting NK cell and neutrophil activation, reducing DC maturation and antigen presentation, and inhibiting T cell responses.

Drug development for sialic acid glycans-Siglec

In recent years, Siglec receptors, as tumor antigens, have been an important target for the treatment of cancer
.
Siglec-2 (CD22) is expressed in many B-cell malignancies, and antibody conjugates (ADCs) have been successfully used to target tumor cells such as inotuzumab ozogamicin in the treatment of recurrent acute lymphoblastic leukemia
.
CAR-T cells targeting Siglec-2 and CD19 have also been developed to treat patients with
recurrent diffuse large cell B-cell lymphoma or acute lymphoblastic leukemia.
ADCs targeting Siglec-3 are also being
developed and tested.

In addition to acting as a direct tumor-associated antigen, the Siglec receptor and its sialic acid glycan ligand can also act as targeted activation of immune cells against tumors
.
Siglec receptors can be blocked by high-affinity antibodies, similar to immune checkpoint inhibitors
of PD-1/PD-L1 and CTLA-4.
The blocking antibody NC318 targets Siglec-15 and is currently undergoing a clinical study of the combined use of pembrolizumab in patients with advanced non-small cell lung cancer (NCT04699123).

。 Siglec-7 and Siglec-9 are potential targets to enhance the anti-tumor activity of NK cells, and a preclinical study using Siglec-7 and -9 blocking antibodies demonstrated anti-tumor efficacy in mouse models, blocking inhibitory Siglec receptors can support the repolarization of TAM in the immunosuppressive microenvironment and increase the phagocytosis
of anti-tumor macrophages.
In addition, the interaction between CD24 on tumor cells and Siglec-10 on tumor-associated macrophages is considered a new therapeutic target to enhance
macrophagocytosis.

Another method is to use blocking antibodies
that target specific sugars.
A recent study showed that antibodies against the ganglioside GD2 can improve anti-tumor immunity by inhibiting the binding of GD2 to Siglec-7 on macrophages, thereby enhancing the effect
of CD47 blockade by further increasing phagocytosis.
In addition, reducing sialcan density in tumor cells and tumor microenvironment is an alternative strategy
.
New inhibitors of sialic acid biosynthesis can be developed to improve cancer immunotherapy
.
In a mouse model, inhibition of N-glycosylation of tumor cells using 2-deoxy-D-glucose increases CAR-T cell-mediated killing and efficacy
.
The use of enzymes to reduce the density of sialic acid glycans in tumors has also been shown to enhance anti-tumor immunotherapy, a bacterial sialidase conjugated to the anti-HER2 antibody trastuzumab, which is currently in first-in-human clinical trials
.

brief summary

Improvements in cancer immunotherapy require the development of new approaches
.
The Siglec receptor and its interaction with sialic acid glycan ligands are a potential novel immune checkpoint
for improving cancer immunotherapy.
Over the past few years, several preclinical and clinical studies have been conducted to support drug development
targeting the Siglec receptor.
Cancer immunotherapy targeting the sialic acidlycan-Siglec axis has shown good potential
.

References:

1.
Siglec receptors as new immune checkpoints in cancer.
Mol Aspects Med.
2022 Aug 7; 101112.