Researchers are increasingly interested in the intersection of immunotherapy development with glycobiology, the study of carbohydrates. This is because glycans, or carbohydrates, have been implicated in many illnesses including inflammation and cancer. Immunotherapy is a medical treatment, typically associated with cancer, that harnesses an individual’s immune system to effectively eliminate the disease.
This blog will discuss the various overlaps between glycobiology and immunotherapy, and how advancement in understandings of glycobiology directly impacts the quality of therapeutics for many pathological states.
Glycans are abundant carbohydrates that form the essential “building blocks of life” in organisms and cover the surface of human cells and pathogens. They are often covalently bound to other types of molecules and serve as an important part in biological protein folding, cell-cell adhesion, and immune recognition. In addition to regulating cell functions and proteins, glycans have recently been demonstrated as impacting noncoding RNAs, also known as GlycoRNA (1).
Glycosylation is an important enzyme-driven molecular process involving glycans in which they are attached to proteins, lipids, or other carbohydrates. This occurs in the endoplasmic reticulum of essentially all cells, and dysregulation can contribute to the development of many diseases (2).
To learn more about glycans and glycosylation, check out our blog post “An Introduction to the Universe of Glycans”.
Cancer is one of the many malignancies that irregular glycosylation can cause. Glycans expressed on tumors, called tumor-associated carbohydrate antigens (TACAs), can be used to monitor the progression of the disease.
For example, it has been noted that the expression of TACA markers on cancer cells is associated with cell survival, metastasis, and also with cell attachment preference to certain organs or tissues (3). Additional changes to normal glycan structures in cancer include overexpression of certain glycans like incomplete or truncated glycans, or even the appearance of new structures of glycans (4).
Therapeutic monoclonal antibodies (mAbs) are glycoproteins produced from hybridomas harvested from immunized mice and are currently widely used as cancer treatment. The fact that they are “glycoproteins” means that there are many glycan segments attached to the antibody structure, and it’s been demonstrated that modifications to these glycan entities can majorly impact antibody stability, efficacy, and immunogenicity (5).
For example, mAbs produced in Chinese Hamster Ovary cells are glycosylated much like human IgG, but those derived from murine myeloma cells contain glycan residues that can be highly immunogenic in human and thus, are potentially less efficacious and safe (6).
Other recent studies have demonstrated how therapeutic monoclonal antibodies have high sensitivity and specificity for immunostaining of glycan antigens, such as Lewis X antigen, a glycan-blood type antigen in exfoliated cells from voided urine samples from bladder cancer patients (7).
Lectins are glycan-binding proteins found within many organisms, including both plant and animal tissues, that can recognize specific carbohydrates without having their molecular properties altered (8). Because they can bind glycans, and glycans can contribute to pathogenesis of cancer, lectins have been studied as a potential anticancer therapy.
The mistletoe plant species in particular has been demonstrated as a lectin source that’s effective against tumor cells, but research does indicate some issues with dosing including pro-apoptosis effects at some dosages but anti-apoptotic contraindications at others (9).
The molecular mechanism of action for these lectins is still the subject of major studies. For example, it’s been determined that Chinese mistletoe lectin-1 (CM-1) induces apoptosis in colorectal cancer via down-regulating miR-135a&b and up-regulating the adenomatous polyposis coli (APC) gene causing dampening of Wnt signaling, thus inhibiting cancer cell proliferation (10).
To learn more about lectins, check out our blog post “Everything We Know About Lectin Structure, Classification, and Function”.
Many immunotherapies involve a combination treatment for enhanced anticancer efficacy. For example, a widely used commercially available combination therapy for a broad array of metastatic solid tumors is anti–PD-1/CTLA-4, nivolumab plus ipilimumab, to target the immune checkpoints on T cells or their ligands on the tumor cell. These particular proteins, which are processed via the cell’s glycosylation apparatus, contain glycans motifs on their structures that have been demonstrated to impact the efficacy of these combination therapies (11).
Additionally, Gangliosides, sialic acid-containing glycosphingolipids (a glycolipid subclass), that are involved in tumor cell metastasis have shown to be effectively targeted by combination treatments such as anti-GD2 mAb ch14.18 (dinutuximab) administered with IL-2, GM-CSF, and isotretinoin (12).
More recently, the advancement of CAR (chimeric antigen receptor) T cells in the treatment of cancers has expanded to the field of glycobiology since many glycan antigens can be used to design CARs. These type of CAR-T cells, dubbed “sweet” CARs due to the fact that they are based on carbohydrates, have shown promise in targeting solid tumors because glycans are abundantly present on the cancer stromal cells (13).
CAR-T cells have also been successfully developed to target glycan epitopes of glycolipids and glycoproteins expressed in various cancers, including TAG72 (the sialyl Tn O-glycan epitope), the Lewis Y antigen, the disialoganglioside GD2, and others (14).
Glycans are currently used in several commercially available vaccines including those for Haemophilus influenzae type b (Hib), streptococcus pneumoniae, and Neisseria meningitides. These particular vaccines, consisting of glycans coupled to carrier proteins, have proven to be highly effective in comparison to polysaccharide vaccines which do not illicit strong immune responses due to their T-cell independent mechanism of action (15).
TACAs are also a desirable target of anticancer vaccines and many are being studied that target the mucin-related Tn, STn, and T antigens; the gangliosides GM2 and GD3; and the glycosphingolipid Globo-H.
Glycan-based vaccines have demonstrated efficacy and many are currently in clinical trials for breast, ovarian, prostate, and lung cancers, but immunogenicity of these vaccine are the subject of many ongoing studies including novel experiments for displaying vaccine glycans in a multivalent manner (16).
Glycans have been demonstrated to have diverse roles in the development and action of various types of immunotherapies. Glycans play a role in the advancement of disease states like cancer but are also important features of existing therapies that can be fine-tuned to produce greater therapeutic effects. Future directions of research include trying to understand the underlying biology of glycosylation with novel methods of carbohydrate chemistry. Additionally, recently uncovered areas like the connection between O-linked glycosylation and immunity as well as glycolipids offer new avenues for developing immunotherapies.