IgE Wars: A NEU Hope

The term “allergy” describes a large category of disease conditions in which our bodies mount inappropriate immune responses to otherwise harmless stimuli, such as peanuts or dust particles. Symptoms range from difficulty breathing, to painful gut and skin manifestations, and even to anaphylaxis—a rapid, systemic, life-threatening response. The causes are incredibly complex, as many different genetic and environmental factors have been found to be at play. However, the global burden of allergic diseases is rising, and the discovery of appropriate treatments has only grown more and more important.

One of the main unifying themes in allergy is the importance of an antibody called IgE. IgE plays a key role in the triggering, development, and chronicity of inflammatory responses in allergic diseases. Interestingly, IgE constitutes but a small portion of total antibodies in circulation due to the rarity of IgE-producing cells and its short half-life compared to other antibody types1. Despite this, circulating IgE readily binds its high-affinity cell surface receptor FcεRI. Upon binding of molecules called antigens (such as portions of peanut proteins) to IgE on the cell surface, FcεRI receptors undergo cross-linking. This activates certain cell signaling pathways that lead various immune cell subsets to rapidly produce and release molecules involved in the allergic response. 

Although IgE is known to be absolutely required in allergic diseases, there is no direct correlation between levels of total and antigen-specific IgE and severity of disease. Recently, a research study led by Dr. Kai-Ting Shade at Harvard Medical School discovered a structural feature of IgE, glycan composition, that modulates its role in allergy2. The addition of sugar chains called glycans, or glycosylation is a posttranslational modification that can heavily influence the activity of proteins. In addition, the composition of the glycans themselves impart various functions. Although the composition of glycans attached to another antibody type, IgG, has been found to regulate its role in allergic disease, this had not been known for IgE. 

The authors first sought to answer whether differences in the glycan composition of IgE was different between human subjects who were (a) non-atopic, or individuals with low IgE levels and low reactivity to various allergens (peanut, birch pollen, dust mite, and cat) or (b) allergic. Using a technique called mass spectrometry to determine the molecular composition of IgE, they found that the allergic group had increased numbers of sialic acid residues per IgE molecule. Sialic acids are simple sugar chains that are located at the end of glycan molecules attached to proteins like IgE. Previously, differences in the sialic content of glycans in IgG had been found to be important in its role in anaphylaxis3. 

In order to test the role of sialylation in IgE, the authors treated mouse IgE protein with the enzyme neuraminidase (NEU) to remove its sialic acid molecules. They then used this “asialylated” IgE in two mouse models of allergy: passive systemic anaphylaxis and passive food allergy. Compared to sialylated (or normal) IgE, administration of asialylated IgE resulted in reduced allergic reactions in both models, as measured by factors such as temperature change, vascular leakage, and histamine release. Similarly, asialylated human IgE also reduced degranulation—or release of inflammatory molecules—in a human mast cell line (LAD2). Removal of sialic acids from IgE did not seem to alter its half-life or ability to bind to either FcεRI receptor and allergens. The authors then asked whether sialylation alters the signaling pathways involved in allergic responses after receptor and allergen binding. They found that asialylated human IgE exhibited dampened phosphorylation (activation) of the kinase protein Syk, which initiates the signaling cascade downstream of crosslinking, as well as calcium uptake, which is important in the process of degranulation. 

Due to these promising results, the authors asked whether asialylation of IgE had potential as a therapeutic in allergic disease. To test this, they engineered a version of human IgE that contained neuraminidase to target removal of sialic acids on IgE-bearing cells. By exposing previously IgE-laden LAD2 mast cells to their new fusion protein, they were able to not only significantly decrease degranulation in response to IgE bound to a model protein (ovalbumin) but also to IgE from patients with peanut allergies. Importantly, their fusion protein did not cause degranulation of mast cells on its own, suggesting no inherent activation. In addition, exposure to heat inactivates their fusion protein and attenuates its inhibitory activity. They also found that treatment with their fusion protein significantly reduces allergic responses in a mouse model. 

Using a combination of mouse and human experiments, the authors were able to show that the sialic content of IgE was important in modulating its activity in not just mouse cells and models, but also in a human cell line and relevant samples. In addition, the authors found that two other molecules, biGlcNAc and galactose, were also altered—in this case, these residues were found to be reduced in allergic individuals. It would be interesting to see whether these residues possibly impart a “protective” effect on IgE-mediated responses. 

We interviewed the primary author, Dr. Kai-Ting Shade, regarding their findings. Currently, the anti-IgE antibody Xolair (Omalizumab) has been successful at targeting IgE in circulation, but Shade’s approach is focused on targeting IgE already bound to cells involved in the allergic response. Shade said, “The fusion IgE-NEU experiment is our proof-of-concept approach to show that IgE sialic acid can be targeted in allergic disease…We are hopeful that targeting sialic acid on IgE using neuraminidase [NEU] will be translated into [a] future allergy treatment.”

By Geil Merana

References:

1.    Cyster, J. G., & Allen, C. (2019). B Cell Responses: Cell Interaction Dynamics and Decisions. Cell, 177(3), 524–540. https://doi.org/10.1016/j.cell.2019.03.016

2.    Shade KC, Conroy ME, Washburn N, et al. Sialylation of immunoglobulin E is a determinant of allergic pathogenicity. Nature. 2020;582(7811):265-270. https://doi.org/10.1038/s41586-020-2311-z

3.    Silva SR, Casabuono A, Jacysyn JF, et al. Sialic acid residues are essential for the anaphylactic activity of murine IgG1 antibodies. J Immunol. 2008;181(12):8308-8314. https://doi.org/10.4049/jimmunol.181.12.8308

IgE Wars: A NEU Hope.