Siglecs as the new immunecheckpoint target

Due to their involvement as checkpoints in immune cell responses, Sialic-acid-binding ImmunoGlobulin-like Lectins (Siglecs) have gained more and more interest as therapeutic targets in Immuno-Oncology and Immune regulation. You’ll be interested to learn that BPS Bioscience have developed and released a number of Siglec proteins and stable cell lines, which you can obtain in Europe through tebu-bio.

What are Siglecs?

Cells of the immune system express a multitude of complex glycans on their surfaces, making them highly recognizable by diverse glycan-binding proteins. Siglecs (Sialic-acid-binding immunoglobulin-like lectins) are type I transmembrane proteins that bind to glycans containing sialic acid. These highly polymorphic proteins have very distinct distribution patterns among different subsets of the immune cells. For example, Siglec 1 is found only on macrophages, while Siglec 7 is expressed on Natural Killer Cells and Siglec 8 is expressed on eosinophils. 

Not surprisingly, these transmembrane proteins are involved in regulating multiple pathways of the immune response, such as:

• Distinguishing pathogens from self,
• Cell trafficking to sites of inflammation
• Balancing immune responses leading to activation or tolerance (1)

Because of their restricted cell type expression and their roles as checkpoints in immune cell responses to infectious pathogens and diseases such as cancer, asthma, allergy, and autoimmune disorders, Siglecs have recently gained a lot of attention as potential therapeutic targets (2). 

Structure and evolution

Most Siglecs have a similar structure, including an N–terminal lectin domain for binding to sialic acid-containing glycans, followed by a variable number of ‘C2-set’ domains, which are domains that are similar in sequence and structure to the constant region of immunoglobulins (Fig.1). 

The longer Siglecs usually interact with a sialic acid on a different cell (a trans interaction), while the shorter ones often interact with sialic acids present on the same cell that is expressing the Siglec (cis interaction). Since sialic acids are present on all cells, the recognition of self-associated glycans by Siglecs is thought to keep resting immune cells in a quiescent state (3) (Fig.2).

In humans, Siglecs can be divided into two major groups. The one which are strongly conserved as: sialoadhesin (Siglec-1), CD22 (Siglec-2), MAG (Sigelc-4), and Siglec-15, and a group of CD33-related Siglecs that are more variable (4) (Fig.1).

Most CD33-related Siglecs contain immunoreceptor tyrosine-based inhibition motifs (ITIMs) in their cytoplasmic tail that can transmit inhibitory signals upon phosphorylation (Fig. 1). The binding of different sialylated glycans induces the phosphorylation of intracellular tyrosine residues in the Siglec by Src-family members. Phosphorylation initiates the recruitment of SHP1 and SHP2, which bind to the Siglec and inhibit kinase-dependent pathways (5).  

Siglecs function and Immune Response

While a few of the Siglecs are acting as activators , most of them are inhibitors, downregulating the immune response.  Sialic acid-dependent ligation of various Siglecs provides a mechanism that helps set the appropriate thresholds for immune cell activation. By recognizing sialic acid–containing glycans, Siglecs help the immune system distinguish between self and nonself (6). Since sialic acids are mainly restricted to vertebrates, the immune system can be activated to permit effective killing of non-sialylated pathogens, while preventing undesirable self-reactivity and tissue damage.

Several pathogens take advantage of sialic acids to dampen the immune system and give them a survival advantage. For example, the protozoa Trypanosoma cruzi, which causes Chagas disease, scavenges host sialic acids with a trans-sialidase enzyme and “expresses” them on its surface to avoid an immune response (7). The parasitic sialylated glycans interact with human Siglecs 7 and 9 in dendritic cells to suppress the host immune response (8).  

Similarly, Pseudomonas aeruginosa or Group B streptococcus bacteria have glycoconjugates that mimic sialic acid and trigger Siglec-5 and−9 on neutrophils, thereby inhibiting their ability to respond to the bacteria (9,10).  Mycobacterium tuberculosis does not express sialic acids, yet infection with M. tuberculosis activates both inhibitory Siglec-5 and Siglec-15 and the activating Siglec-4, which lacks the cytoplasmic ITIM sequence that mediates inhibitory signaling. 

This delicate balance of regulatory Siglec signals to modulate the T cell response to pathogens may relate to the severity of disease (11,12). Sialic acids are used by many different pathogens to infect host cells (e.g. influenza A virus) and dampen the immune response. Understanding how pathogens exploit Siglecs to evade immune surveillance can help in the design of new therapeutic strategies.

Siglecs and Cancer

Upon malignant transformation, many types of cancer cells express high levels of sialic acids on the cell membrane or by secretion, and can therefore take advantage of the sialic acid-Siglec interaction to induce immune tolerance.  Increased sialylation has been demonstrated in renal cell carcinoma, prostate, colon, breast and oral cancers, head and neck squamous cell carcinoma, and oral cancer. In breast cancer, many tumors secrete mucins that engage Siglec-9 on monocytes and macrophages as part of an evasion strategy (13). 

Similarly, many melanomas express high levels of the ganglioside GD3, which interacts with Siglec 7 on NK cells and suppresses the NK cell killing activity (14). Likewise, Siglec-6 expression is upregulated when mast cells are stimulated by colon cancer cells, suppressing mast cell activity and implicating Siglec-6 in immune evasion in the tumor microenvironment (15).

Not surprisingly, antibodies against Siglecs are being explored for the treatment of different cancers. The FDA approved Besponsa (Pfizer), a monoclonal antibody against Siglec2, to target Siglec-positive B cells in Acute Lymphoblastic Lymphoma (ALL) (16). Another Siglec that is being actively targeted is Siglec-3.  Siglec3, also known as CD33, is expressed on nearly all acute myeloid leukemia (AML) cells, making it an ideal target for therapy (17, 18).  A CD33/CD3-bispecific T-cell Engaging (BiTE) antibody construct (AMG330, Amgen) can re-direct T cells to eradicate CD33+ myeloid derived suppressor cells (19).

Another approach is to increase anti-tumor immunity by targeting a monoclonal antibody to a sialidase. Xiao et al. (20) fused a HER2 monoclonal to a sialidase that specifically cuts off the sialic acid ligands that are bound by Siglec-7 and Siglec-9.  In vitro the HER2 Ab / sialidase increased NK cell-mediated killing of HER2 positive tumor cells. Since most breast cancer patients are HER2 positive, targeting HER2 with this sialidase-fused antibody could be an effective treatment strategy.

Siglecs are also for autoimmune disorders.  For example, anti-Siglec-8 antibodies have been proposed for treating diseases mediated by eosinophils and/or mast cells, such as bronchial asthma (19). Similarly, Siglec-15 is known to be involved in osteoclast differentiation, and is considered to be a potential therapeutic target for osteoporosis (12).  

Because of their cell type-specific expression patterns,  high expression on cancer cells, and ability to modulate receptor signaling (21), Siglecs are attractive therapeutic targets .  Pioneering efforts to develop  glycan-based and antibody-based therapies (including CAR-T and bispecific antibodies) are now reaching clinical trials for the treatment of numerous infectious diseases, autoimmune disorders, and cancer.  As knowledge of the biochemistry and cellular roles for Siglecs continues to expand, so will the opportunities for impacting disease by targeting Siglecs. 

Take a look at some of our other posts on the topics of Immuno Oncology, mRNA and CAR-T cells research tools:

• Your complete tool box to facilitate your Immunotherapy research
• Immuno-Oncology: 6 must haves to boost your projects
• CAR-T cell therapy: New hope for Immunotherapy and cancer treatment
• PROTACs: A promising strategy in Drug Development
• Hit-and-run cell-based therapy research with Trilink mRNA

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