Epigenetics and Allergies: What is Really Responsible for one of the Most Common Dysfunctions of the Immune System?

Epigenetic Regulation via Altered Histone Acetylation Results in Suppression of Mast Cell Function and Mast Cell-Mediated Food Allergic Responses

Approximately 3-6% of individuals have some type of food allergy, and that percentage is currently rising, especially in western countries.¹ Previous studies have linked environmental stimuli, antibiotic treatment, previous infection and more to the origins of allergies.^2,3 However, the mechanisms underlying the development of allergy has been poorly understood, especially the differences in sensitization, or induction of an adaptive response, from person to person. A study from Western New England University sought to connect epigenetics, a hot field in biology, with mast cells, which are cells that are primarily responsible for allergic reactions. We tend to think that all of our characteristics, allergies included, are ingrained in our genes. However, this study shows that this may not always be the case. Epigenetics refers to changes in gene expression that result from anything other than altering the genetic code. For example, an environmental stimulus may cause the need for certain proteins to be produced. Our DNA exists as condensed chromatin in the tiny nuclei of our cells, tightly wrapped around proteins called histones. In order for the genes to be transcribed, and those proteins to be produced, certain modifications of these histones must take place to open up the DNA where the genes are located. This experiment observed certain epigenetic changes in mast cells, which are immune cells that reside in our tissues. They remain ready to release toxic granules upon exposure to antigens. In the case of allergies, antigens can be specific proteins on peanuts, eggs, pet dander, or any other allergen. The toxic granules normally act as weapons that can kill infected cells for the benefit of the host. However, in the case of an allergic reaction they can be released in dangerous quantities, and this is the cause of symptoms such as rash, bronchoconstriction, intestinal problems, and even shock. The researchers therefore hypothesized that mast cell function during food allergy may be epigenetically regulated, resulting in the development or suppression of allergic reaction.

In order to test this, the researchers used bone marrow-derived mast cells (BMMCs) and exposed them to trichostatin A (TSA). TSA is a treatment that inhibits histone deacetylases. Acetylation is a process where certain molecules called acetyl groups, are added to histones to open up the chromatin. By inhibiting histone deacetylation, the DNA remains uncoiled in certain locations causing differences in gene transcription. Specifically, they were looking for differences in genes expressed by mast cells treated with TSA. The antibody IgE is responsible for for telling mast cells to degranulate, which can cause a response as severe as anaphylaxis. During the process of sensitization, certain antigens may be ingested or inhaled. They are taken up by specialized immune cells that process and present them to helper T cells which then signal for B cells to produce these IgE antibodies that are specific to that particular antigen. IgE is used throughout this study to simulate an allergic response.

They first found that treatment of unaltered mast cells with TSA resulted in reduced cytokine expression. Cytokines are signaling molecules that tell other cells how to respond and in this study, the cytokines mentioned are those that are closely associated with the activation of an allergic response. This therefore meant that altering acetylation of the DNA resulted in the suppression of mast cell signals that are normally used to communicate with other immune cells during a reaction. They then used IgE to activate mast cells that were treated with TSA to discover the effect of TSA on cells that were “experiencing an allergic response.” They found that the activated, TSA-treated cells had suppressed cytokine production here as well. Without these cytokines, the mast cells remained unable to send signals to other immune cells to react how they normally would during an allergic response.

Figure 1. TNF-α, IL-6, IL-4 and IL-13 are all cytokines (signals) that are released during an allergic reaction. Activated/unactivated refers to whether the mast cells were exposed to IgE. It is shown that activated bone marrow-derived mast cells produce large quantities of these cytokines when activated by IgE. When TSA was introduced, cytokine levels decreased.

The researchers then wondered if TSA had any effect on the mast cells' release of cytotoxic granules, which is what causes most of the damage during an allergic response. They tracked the amount of β-hexosaminidase, which is a protein present in the granules and as they expected, its levels were lower with TSA treatment. Because activation of mast cells is largely dependent on the binding of IgE to its receptor, they also chose to track levels of this receptor in TSA-treated mast cells. It was found that at 24 hours after treatment, there was a significant decrease in the amount of receptor, suggesting that TSA prevents binding of IgE. This provides evidence for another mechanism that TSA uses to regulate the activation of mast cells. After they experimented with IgE, they wanted to determine whether TSA could produce similar results when mast cells were activated by innate cytokines. IgE is part of the adaptive response, meaning that a specific form of IgE is produced that can only interact with a specific antigen. IL-33 however, is a cytokine that is part of the innate immune response. It is produced in response to pathogen-associated molecular patterns (PAMPs), or molecules present on an invader that are highly conserved and can be quickly recognized by immune cells. They tested the effect of TSA on IL-33-stimulated cells and found that cytokine production decreased here as well. With sufficient evidence for both innate and adaptive effects of TSA, they turned to the pathway that actually causes mast cells to release cytokines: the NF-κB signaling pathway. While this pathway is complex, the main point here is that NF-κB is a transcription factor that binds to the DNA and recruits other molecules to help transcribe the genes needed for cytokine production. They found that NF-κB levels significantly decreased upon treatment with TSA, which is what caused the suppression of cytokine secretion mentioned previously. Finally, the researchers tested the effects of TSA in vivo. Mice were sensitized to chicken egg ovalbumin (OVA), a protein in egg white, in order to simulate an egg allergy, which is common among many people. As for physical symptoms, the mice that received TSA treatment experienced less allergy-induced diarrhea than the mice that did not receive the treatment. At the cellular and molecular levels, there were fewer mast cells present in the intestines of TSA-treated mice, and like what was found in vitro, cytokine production and NF-κB levels were significantly reduced.

Figure 2. Levels of cytokines associated with an allergic response (IL-4, IL-5, IL-13, IL-10, IL-33) are decreased with TSA treatment in mice with OVA (egg protein) allergy. Transcription factor NF-κB, and OVA-specific IgE levels are also lower. Jejunal mast cells are mast cells that reside in the middle part of the small intestine. TSA treatment causes the number of these cells to decrease, indicating that the response had been suppressed.

What does it all mean?

Although this experiment had many complex parts, the main take-away is that regulation of mast cell activation can occur by changes in how our DNA is differentially “opened up” for transcription. TSA acts at the transcriptional level to suppress the signaling that normally causes allergy-associated cytokine secretion and harmful degranulation of mast cells. Allergies are not completely determined by our genetic code, and finding the right ways to make epigenetic changes may soon be very important for the future of allergy treatments. While the researchers found much evidence in vitro, and were able to confirm their findings with a model of egg allergy in vivo, perhaps future steps could consist of testing the effects of TSA on different allergy models, or even using different epigenetic modifiers. Interestingly, the researchers previously carried out a study using curcumin (more commonly known as turmeric) where they found that the common household spice decreased mast cell activity.⁶ This may suggest that dietary supplements may even able to act as epigenetic modifiers that help make allergic responses less severe!

References

1.     Dunlop JH, Keet CA. Epidemiology of Food Allergy. Immunology and Allergy Clinics of North America. 2018;38(1):13-25. doi:10.1016/j.iac.2017.09.002.

2.     Sicherer SH, Sampson HA. Food allergy: a review and update on epidemiology, pathogenesis, diagnosis, prevention, and management. J Allergy Clin Immunol. (2018) 141:41–58. doi: 10.1016/j.jaci.2017.11.003

3.     Renz H, Allen KJ, Sicherer SH, Sampson HA, Lack G, Beyer K, et al. Food allergy. Nat Rev Dis Primers (2018) 4:17098. doi: 10.1038/nrdp. 2017.98

4.     Immunoglobulin E (IgE) | AAAAI. The American Academy of Allergy, Asthma & Immunology. https://www.aaaai.org/conditions-and-treatments/conditions-dictionary/immunoglobulin-e-(ige). Accessed November 24, 2018.

5.     ABOUT MAST CELLS. MastCellAware :: About Mast Cells. http://www.mastcellaware.com/mast-cells/about-mast-cells.html. Accessed November 24, 2018.

6.     Kinney SR, Carlson L, Ser-Dolansky J, Thompson C, Shah S, Gambrah A, et al. Curcumin ingestion inhibits mastocytosis and suppresses intestinal anaphylaxis in a murine model of food allergy. PLoS ONE (2015) 10:e0132467. doi: 10.1371/journal.pone.0132467

7.     Krajewski D, Kaczenski E, Rovatti J, et al. Epigenetic Regulation via Altered Histone Acetylation Results in Suppression of Mast Cell Function and Mast Cell-Mediated Food Allergic Responses. Frontiers in Immunology. 2018;9. doi:10.3389/fimmu.2018.02414.