The Role of Regulatory T Cells in Epicutaneous Immunotherapy for Food Allergy

Abstract

In recent decades, a rapid increase in the prevalence of food allergies has led to extensive research on novel treatment strategies and their mechanisms. Mouse models have provided preliminary insights into the mechanism of epicutaneous immunotherapy (EPIT)-induced immune tolerance. In EPIT, antigen applied on the skin surface can be captured, processed, and presented in the lymph nodes (LNs) by Antigen-presenting cells (APCs). In the LNs, induction of regulatory T cells (Treg cells) requires both direct contact during antigen presentation and indirect mechanisms such as cytokines. Foxp3⁺CD62L⁺ Treg cells can exhibit the characteristics of hypomethylation of Foxp3 TSDR and Foxp3^-LAP⁺ Treg cells, which increase the expression of surface tissue-specific homing molecules to exert further sustained systemic immune tolerance. Studies have shown that EPIT is a potential treatment for food allergies and can effectively induce immune tolerance, but its mechanism needs further exploration. Here, we review Treg cells' role in immune tolerance induced by EPIT and provide a theoretical basis for future research directions, such as the mechanism of EPIT and the development of more effective EPIT treatments.

Keywords: allergen-specific immunotherapy (AIT); epicutaneous immunotherapy (EPIT); food allergy; immune tolerance; regulatory T cell (Treg cell).

Figure 1

Treg cells suppressive mechanisms. Treg cells inhibit the proliferation and effects of NKs through membrane-bound TGF-β, mainly including inhibiting the expression of the latter’s surface protein NKG20 and the production of IFN-γ. Treg cells can inhibit the secretion of IL-5 and IL-13 by ILC2 in an ICOS/ICOSL-dependent manner, thereby inhibiting its function. Treg cells can inhibit conventional T cells (Tcons) action in many ways. 1) Treg cells can produce anti-inflammatory cytokines (IL-10, IL-35, and TGFβ) affecting Tcons; 2) they can release perforin and granzyme, which damage the target cell membrane leading to apoptosis; 3) Treg cells can also sequester, by the high expression of CD25, IL-2 from the microenvironment reducing effector Tcons proliferation; 4) Treg cells can quickly inhibit TCR-induced Ca²⁺, NFAT, and NF-κB signaling; 5) indirectly inhibiting Tcons by reducing the expression of CD80/CD86 on DCs through CTLA-4 (inhibit DC antigen presentation function) or disrupting the microenvironment in the immunological synapse provided by DCs (essential for T cell proliferation); 6) The expression of CD39 on Treg cells mediates the conversion of ATP to adenosine and reduces the proliferation of Tcons. Treg cells can directly affect B cells via PDL1/PD-1 interaction and DCs via CTLA-4 and LAG-3. CTLA-4 blocks co-stimulation, reducing CD80/CD86 expression, and it induces upregulation of IDO. Treg cells can inhibit B cells action and release granzyme B and perforin through the PD-1 signaling pathway to kill B cells. Treg cells can also bias monocytes to M2 macrophages, enhancing CD163 and CD206 on their surface molecules. They can similarly induce the suppressive phenotype of neutrophils and basophils and reduce the secretion of ILC2 cytokines.

Figure 2

The possible mechanism of EPIT treatment of food allergy to induce immune tolerance. First, Antigen applied on the skin surface can be captured, processed, and presented in the LNs by Antigen-presenting cells (APCs), such as LCs in the epidermis, and macrophages, DCs in the dermis. Besides, they can promote more APCs aggregation by secreting TGF-β or IL-10. Second, APCs migrate to lymph nodes and promote naive T cells distinguish into Treg cells by secreting TGF-β, reducing Foxp3 TSDR methylation level or direct contact. Most importantly, EPIT will exert its immune tolerance by inducing CD4⁺CD25⁺Foxp3⁺ Treg cells of a specific phenotype, especially for Foxp3+CD62L⁺ Treg. Besides, Foxp3^-LAP⁺ Treg may exert local effects by expressing intestinal homing molecules CCR6, CCR9, and skin-homing molecules CLA, CCR4, related to the higher safety EPIT treatment.

Figure 3

Different allergen-specific immunotherapy can induce the production of different regulatory T cell subtypes. CD4⁺CD25⁺Foxp3⁺ Tregs were induced by the three treatment routes but with more significant numbers induced by EPIT. This difference is due to an increase in naive Tregs in EPIT because the induction of effector Tregs was similar in EPIT, OIT, and SLIT, and only EPIT induced naive Tregs. EPIT and OIT also increased the level of CD4⁺LAP⁺ cells (Th3), whereas SLIT induced IL-10⁺ Tr1 cells. The suppressive activity of EPIT-induced Tregs did not depend on IL-10 but required CTLA-4, whereas OIT acted through both mechanisms, and SLIT was strictly dependent on IL-10. Furthermore, whereas OIT- or SLIT-induced Tregs lost their suppressive activities after treatment was discontinued, the suppressive activities of EPIT-induced Tregs were still effective eight weeks after the end of treatment, suggesting the induction of a more long-lasting tolerance. Moreover, Allergen-specific immunotherapy is the only option for the long-term cure of allergic diseases. It has been used for food allergy treatment research in recent years. The latest developments in the specific field of allergy and immunology aim to improve the efficacy, applicability, and patient compliance with treatment while reducing side effects and duration. Novel administration routes, the definition of biomarkers for better monitorization of therapy success, novel adjuvants for increased AIT efficacy, and the development of allergen preparations with increased antigenicity and decreased allergenicity allergoids all serve these crucial goals. The final result of AIT is called allergen-specific tolerance, which is a state of active immune response due to changes in immune cell function. A successful AIT’s clinical outcome can reduce drug demand, allergen reactivity, and allergic symptoms and improve life quality.