Immunological approaches for tolerance induction in allergy.

Allergy is the consequence of an inappropriate inflammatory immune response generated against harmless environmental antigens. In allergic disorders such as asthma and rhinitis, the Th2 mediated phenotype is a result of loss of peripheral tolerance mechanisms. In cases such as these, approaches such as immunotherapy attempt to treat the underlying cause of allergic disease by restoring tolerance. Immunotherapy initiates many complex mechanisms within the immune system that result in initiation of innate immunity, activation of both cellular and humoral B cell immunity, as well as triggering T regulatory subsets which are major players in the establishment of peripheral tolerance. Though studies clearly demonstrate immunotherapy to be efficacious, research to improve this treatment is ongoing. Investigation of allergenicity versus immunogenicity, native versus modified allergens, and the use of adjuvant and modality of dosing are all current strategies for immunotherapy advancement that will be reviewed in this article.

Immune responses in healthy and allergic individuals are characterized by a fine balance between allergen-specific T regulatory 1 and T helper 2 cells.

The mechanisms by which immune responses to nonpathogenic environmental antigens lead to either allergy or nonharmful immunity are unknown. Single allergen-specific T cells constitute a very small fraction of the whole CD4+ T cell repertoire and can be isolated from the peripheral blood of humans according to their cytokine profile. Freshly purified interferon-gamma-, interleukin (IL)-4-, and IL-10-producing allergen-specific CD4+ T cells display characteristics of T helper cell (Th)1-, Th2-, and T regulatory (Tr)1-like cells, respectively. Tr1 cells consistently represent the dominant subset specific for common environmental allergens in healthy individuals; in contrast, there is a high frequency of allergen-specific IL-4-secreting T cells in allergic individuals. Tr1 cells use multiple suppressive mechanisms, IL-10 and TGF-beta as secreted cytokines, and cytotoxic T lymphocyte antigen 4 and programmed death 1 as surface molecules. Healthy and allergic individuals exhibit all three allergen-specific subsets in different proportions, indicating that a change in the dominant subset may lead to allergy development or recovery. Accordingly, blocking the suppressor activity of Tr1 cells or increasing Th2 cell frequency enhances allergen-specific Th2 cell activation ex vivo. These results indicate that the balance between allergen-specific Tr1 cells and Th2 cells may be decisive in the development of allergy.

GATA3-driven Th2 responses inhibit TGF-beta1-induced FOXP3 expression and the formation of regulatory T cells.

Transcription factors act in concert to induce lineage commitment towards Th1, Th2, or T regulatory (Treg) cells, and their counter-regulatory mechanisms were shown to be critical for polarization between Th1 and Th2 phenotypes. FOXP3 is an essential transcription factor for natural, thymus-derived (nTreg) and inducible Treg (iTreg) commitment; however, the mechanisms regulating its expression are as yet unknown. We describe a mechanism controlling iTreg polarization, which is overruled by the Th2 differentiation pathway. We demonstrated that interleukin 4 (IL-4) present at the time of T cell priming inhibits FOXP3. This inhibitory mechanism was also confirmed in Th2 cells and in T cells of transgenic mice overexpressing GATA-3 in T cells, which are shown to be deficient in transforming growth factor (TGF)-beta-mediated FOXP3 induction. This inhibition is mediated by direct binding of GATA3 to the FOXP3 promoter, which represses its transactivation process. Therefore, this study provides a new understanding of tolerance development, controlled by a type 2 immune response. IL-4 treatment in mice reduces iTreg cell frequency, highlighting that therapeutic approaches that target IL-4 or GATA3 might provide new preventive strategies facilitating tolerance induction particularly in Th2-mediated diseases, such as allergy.

IL-10 suppresses CD2-mediated T cell activation via SHP-1.

Interleukin (IL)-10 is an essential suppressive cytokine and plays a key role in peripheral T cell tolerance to allergens, autoantigens, transplantation antigens and tumor antigens. However, the molecular mechanisms of direct T cell suppression by IL-10 are not fully understood. Here, we demonstrate that IL-10 directly inhibits CD2 signaling in T cells. T cell stimulation via CD2 alone induces activation and proliferation, when endogenous IL-10 sources are eliminated from cultures. IL-10 utilizes the src-homology-2 domain containing tyrosine phosphatase (SHP-1) to directly suppress T cell activation. The role of SHP-1 in IL-10-mediated suppression of CD2 co-stimulation on T cells is demonstrated by using dominant-negative SHP-1 over-expressing T cells and silencing endogenous SHP-1 by small inhibitory RNA. Findings are confirmed using both SHP-1-deficient mice and IL-10-deficient mice. CD2-induced proliferation is suppressed by exogenous IL-10 in IL-10-deficient, but not SHP-1-deficient murine T cells. In conclusion, SHP-1-mediated inhibition of CD2 signaling represents a novel mechanism for direct T cell suppression by IL-10.

Clinical and immunologic effects of H1 antihistamine preventive medication during honeybee venom immunotherapy.

BACKGROUND: H1 antihistamines increase safety during allergen-specific immunotherapy and might influence the outcome because of immunoregulatory effects. OBJECTIVE: We sought to analyze the influence of 5 mg of levocetirizine (LC) on the safety, efficacy, and immunologic effects of ultrarush honeybee venom immunotherapy (BVIT). METHOD: In a double-blind, placebo-controlled study 54 patients with honeybee venom allergy received LC or placebo from 2 days before BVIT to day 21. Side effects during dose increase and systemic allergic reactions (SARs) to a sting challenge after 120 days were analyzed. Allergen-specific immune response was investigated in skin, serum, and allergen-stimulated T-cell cultures. RESULTS: Side effects were significantly more frequent in patients receiving placebo. Four patients receiving placebo dropped out because of side effects. SARs to the sting challenge occurred in 8 patients (6 in the LC group and 2 in the placebo group). Seven SARs were only cutaneous, and 1 in the placebo group was also respiratory. Difference of SARs caused by the sting challenge was insignificant. Specific IgG levels increased significantly in both groups. Major allergen phospholipase A(2)-stimulated T cells from both groups showed a slightly decreased proliferation. The decrease in IFN-gamma and IL-13 levels with placebo was not prominent with LC, whereas IL-10 levels showed a significant increase in the LC group only. Decreased histamine receptor (HR)1/HR2 ratio in allergen-specific T cells on day 21 in the placebo group was prevented by LC. CONCLUSIONS: LC reduces side effects during dose increase without influencing the efficacy of BVIT. LC modulates the natural course of allergen-specific immune response and affects the expression of HRs and cytokine production by allergen-specific T cells.

Regulation and role of transforming growth factor-beta in immune tolerance induction and inflammation.

Transforming growth factor-beta (TGF-beta) is known to mediate pleiotropic functions both inside and outside the immune system. Recent progress in this field underlines the role of TGF-beta in regulatory T (Treg) cells, where it participates in both suppression and differentiation. In addition, recent information highlights the role of TGF-beta in repair responses that lead to matrix deposition and tissue remodelling. Many chronic inflammatory diseases, such as asthma, profit from the suppression of specific immune responses by TGF-beta; however, TGF-beta-mediated tissue remodelling can be a serious complication in such diseases.

Apoptosis in tissue inflammation and allergic disease.

Genetic predisposition and environmental instructions tune thresholds for the activation, effector functions and lifespan of T cells, other inflammatory cells and resident tissue cells. Defects in apoptosis and peripheral tolerance in T cells define different allergic phenotypes. In individuals with atopic allergic disease, activated allergen-specific T cells expressing high levels of IFN-gamma predominantly undergo apoptosis in the circulation, skewing the immune response to surviving T helper type 2 (Th2) cells. In affected tissues, these cells switch on effector cytokines and induce the activation and apoptosis of epithelial cells. In individuals with non-atopic monoallergic disease, by contrast, a disturbed balance towards allergen-specific Th2 cells instead of T regulatory type 1 (Tr1) cells characterizes the T cell response.

Immune regulation by histamine.

Histamine is a potent bioamine with multiple activities in various pathological and physiological conditions. In addition to its well-characterised effects in the acute inflammatory and allergic responses, histamine regulates several aspects of antigen-specific immune response development. Histamine affects the maturation of dendritic cells and alters their T cell-polarising capacity. Histamine also regulates antigen-specific T helper 1 and T helper 2 cells, as well as related antibody isotype responses. Apparently, diverse effects of histamine on immune regulation are because of differential expression of four types of histamine receptors and their distinct intracellular signals.

The role of histamine in regulation of immune responses.

Histamine is not only the major mediator of the acute inflammatory and immediate hypersensitivity responses, but has also been demonstrated to affect chronic inflammation and regulate several essential events in the immune response. It can influence numerous functions of the cells involved in the regulation of immune response and hematopoiesis including macrophages, dendritic cells, T lymphocytes, B lymphocytes and endothelial cells. These cells express histamine receptors and also secrete histamine, which can selectively recruit the major effector cells into tissue sites and affect their maturation, activation, polarization, and effector functions leading to chronic inflammation. Histamine regulates antigen-specific Th1 and Th2 cells, as well as related antibody isotype responses. Histamine acting through its receptor (HR) type 2, positively interferes with the peripheral antigen tolerance induced by T regulatory (T(Reg)) cells in several pathways. The diverse effects of histamine on immune regulation are due to differential expression and regulation of 4 histamine receptors and their distinct intracellular signals. In addition, differences in affinities of these receptors are highly decisive on the biological effects of histamine and agents that target histamine receptors. Although substantial evidence has been accumulated about histamine metabolism, receptors, signal transduction, physiological and pathological effects, the complex interrelationship and cross-talk by histamine, its receptors and other G-protein coupled receptors remain to be understood.

T regulatory cells in allergy.

Activation-induced cell death, anergy and/or immune response modulation by T regulatory cells (T(Reg)) are essential mechanisms of peripheral T-cell tolerance. There is growing evidence that anergy, tolerance and active suppression are not entirely distinct, but rather, represent linked mechanisms possibly involving the same cells and multiple suppressor mechanisms. Skewing of allergen-specific effector T cells to T(Reg) cells appears as a crucial event in the control of healthy immune response to allergens and successful allergen-specific immunotherapy. The T(Reg) cell response is characterized by abolished allergen-induced specific T-cell proliferation and suppressed T helper 1 (Th1)- and Th2-type cytokine secretion. The increased levels of interleukin-10 (IL-10) and transforming growth factor-Beta (TGF-Beta) that are produced by T(Reg) cells, potently suppress IgE production, while simultaneously increasing production of noninflammatory isotypes IgG4 and IgA, respectively. In addition, T(Reg) cells directly or indirectly suppress effector cells of allergic inflammation such as mast cells, basophils, and eosinophils. In conclusion, peripheral tolerance to allergens is controlled by multiple active suppression mechanisms. It is associated with regulation of antibody isotypes and effector cells to the direction of a healthy immune response and opens a window for novel therapies of allergic diseases.

Mechanisms of allergen-specific immunotherapy.

Allergen-specific immunotherapy (SIT) is the only treatment, which leads to a life-long tolerance against allergens due to restoration of normal immunity. The induction of a tolerant state in peripheral T cells represents an essential step in allergen-SIT. Peripheral T-cell tolerance is characterized mainly by suppressed proliferative and cytokine responses against the major allergens and its T-cell recognition sites. It is initiated by autocrine action of IL-10 and/or TGF-Beta, which are increasingly produced by the antigen-specific T Regulatory (T(Reg)) cells. Tolerized T cells can be reactivated to produce either of the distinct Th1 or Th2 cytokine patterns, thus directing allergen-SIT towards successful or unsuccessful treatment. T(Reg) cells directly or indirectly influence effector cells of allergic inflammation, such as mast cells, basophils and eosinophils. In addition, there is accumulating evidence that they may suppress IgE production and induce IgG4 and IgA production against allergens. By the application of the recent knowledge in mechanisms of allergen-SIT, more rational and safer approaches are awaiting in the future for the prevention and cure of allergic diseases.

The role of TGF-beta in allergic inflammation.

The transforming growth factor beta (TGF-beta) plays a dual role in allergic disease. It is important in suppressing T cells and also mediates repair responses that lead to unwanted remodeling of tissues. Advances in the immunology of allergy indicate that allergens cause overreactions in the lymphocyte compartment because of the lack or decreased number of suppressive, regulatory T cells. TGF-beta was shown to induce regulatory T cells and participate directly in suppression of effector T cells. Therefore, TGF-beta may help return reactivity to allergens to normal subsymptomatic activity. Whether chronic inflammatory diseases such as asthma profit from TGF-beta-mediated suppression of specific immune responses or whether the TGF-beta-mediated tissue remodeling aggravates diseases more than it helps control immune reactions is unclear. This article addresses these issues and future strategies in this field.

Mechanisms of allergen-specific immunotherapy: T-regulatory cells and more.

Activation-induced cell death, anergy, or immune response modulation by regulatory T cells (Treg cells) are essential mechanisms of peripheral T-cell tolerance. Genetic predisposition and environmental instructions tune thresholds for the activation of T cells, other inflammatory cells, and resident tissue cells in allergic diseases. Skewing allergen-specific effector T cells to a Treg-cell phenotype seems to be crucial in maintaining a healthy immune response to allergens and successful allergen-specific immunotherapy. The Treg-cell response is characterized by an abolished allergen-specific T-cell proliferation and the suppressed secretion of T-helper 1- and T-helper 2-type cytokines. Suppressed proliferative and cytokine responses against allergens are induced by multiple suppressor factors, including cytokines such as interleukin-10 (IL-10) and transforming growth factor beta (TGF-beta), and cell surface molecules such as cytotoxic T-lymphocyte antigen-4, programmed death-1, and histamine receptor 2. The increased levels of IL-10 and TGF-beta produced by Treg cells potently suppress IgE production while simultaneously increasing the production of noninflammatory isotypes IgG4 and IgA, respectively. In addition, Treg cells directly or indirectly suppress the activity of effector cells of allergic inflammation, such as mast cells, basophils, and eosinophils. In conclusion, peripheral tolerance to allergens is controlled by multiple active suppression mechanisms on T cells, regulation of antibody isotypes, and suppression of effector cells. The application of current knowledge of Treg cells and related mechanisms of peripheral tolerance may soon lead to more rational and safer approaches to the prevention and cure of allergic disease.

Histamine receptors in immune regulation and allergen-specific immunotherapy.

The cells involved in the regulation of immune responses and hematopoiesis express histamine receptors and secrete histamine. Histamine acting through four types of its receptors has been shown not only to affect chronic inflammatory responses but also to regulate several essential events in the immune response. Histamine signals have a role in the mechanisms of tolerance induced during allergen-specific immunotherapy (SIT), acting mainly through its receptor (HR) type 2. It positively interferes with the peripheral antigen tolerance induced by T regulatory cells in several pathways. The rationale for the concomitant use of H1 antihistamines during SIT is diverse and includes reduction of its immediate side effects as well as enhancement of mechanisms of specific tolerance and anti-inflammatory effects of vaccination.

T regulatory cells in allergen-specific immunotherapy.

A dramatic increase in the prevalence of allergy and asthma has occurred during the past few decades. Although the symptoms of many allergic disorders can be suppressed quite effectively by pharmacological interventions, these do not provide a curative solution and therefore involve lifelong use of medication. Allergen-specific immunotherapy (SIT) on the other hand provides a long-lasting effect on the immune response to common environmental antigens, therefore allowing cessation of the therapy after several years. The changes in the immune response brought about by allergen-SIT are slowly being unveiled and explained. Mechanisms underlying allergen-SIT and in particular the role of regulatory T cells will be discussed in this review, based on recent findings and current concepts.

New insights into the mechanisms of allergen-specific immunotherapy.

PURPOSE OF REVIEW: Specific immunotherapy represents the only curative treatment of a specific allergy, and is therefore of great interest in terms of immunological mechanisms and therapeutic developments. RECENT FINDINGS: Allergen-specific regulatory T cells are induced after the initiation of specific immunotherapy, and are assumed to suppress effector T cells directly mediating allergic inflammation. Therefore regulatory T cells may play a key role in the re-induction of allergen tolerance. Multiple pathways in many systems are described to promote or enhance regulatory T cells. This review summarizes the current view on immunological mechanisms leading to and coming from regulatory T cells. SUMMARY: The highlighted mechanisms may not only contribute to improved specific immunotherapy, but also give insight into a clinically relevant therapy targeting regulatory T cells. The approach of addressing endogenous regulatory mechanisms specifically controlling inflammation instead of targeting inflammation itself is relevant for future pharmacological developments.

Are regulatory T cells the target of venom immunotherapy?

PURPOSE OF REVIEW: Allergen-specific immunotherapy is the only treatment that leads to lifelong tolerance to previously disease-causing allergens by restoring normal immunity against allergens. T-regulatory (TReg) cells are involved in preventing sensitization to allergens and represent a major target for venom- or other allergen-specific immunotherapy. RECENT FINDINGS: Induction of peripheral tolerance in T cells, which is characterized mainly by suppressed proliferative and cytokine responses against the T-cell epitopes of major allergens, is an essential step in specific immunotherapy. It is initiated by the autocrine action of interleukin-10 and/or transforming growth factor-beta, which are produced by antigen-specific TReg cells. Tolerized T cells can be reactivated to produce distinct T-helper-1 or T-helper-2 cytokine patterns, thus directing allergen-specific immunotherapy toward successful or unsuccessful outcomes. TReg cells directly or indirectly influence effector cells of allergic inflammation, such as mast cells, basophils and eosinophils. In addition, there is accumulating evidence that they may suppress IgE production and induce IgG4 and IgA production against allergens. In addition, histamine released from mast cells and basophils may efficiently contribute to immunoregulation during specific immunotherapy, and affect TReg cells and the production of their cytokines via histamine receptor 2. SUMMARY: By applying recent knowledge in TReg-cell-dependent mechanisms of peripheral tolerance, more rational and safer approaches to the prevention and cure of venom hypersensitivity may be developed in the future.

T helper (Th) 2 predominance in atopic diseases is due to preferential apoptosis of circulating memory/effector Th1 cells.

T cells constitute a large population of cellular infiltrate in atopic/allergic inflammation and a dysregulated, Th2-biased peripheral immune response appears to be an important pathogenetic factor. In atopic dermatitis, circulating cutaneous lymphocyte-associated antigen-bearing (CLA+) CD45RO+ T cells with skin-specific homing property represent an activated memory/effector T cell subset. They express high levels of Fas and Fas ligand and undergo activation-induced apoptosis. The freshly purified CLA+ CD45RO+ T cells of atopic individuals display distinct features of in vivo-triggered apoptosis such as pro-caspase degradation and active caspase-8 formation. In particular, the Th1 compartment of activated memory/effector T cells selectively undergoes activation-induced cell death, skewing the immune response toward surviving Th2 cells in atopic dermatitis patients. The apoptosis of circulating memory/effector T cells was confined to atopic individuals whereas non-atopic patients such as psoriasis, intrinsic-type asthma, contact dermatitis, intrinsic type of atopic dermatitis, bee venom allergic patients, and healthy controls showed no evidence for enhanced T cell apoptosis in vivo. These results define a novel mechanism for peripheral Th2 response in atopic diseases.

Microarrayed allergen molecules: diagnostic gatekeepers for allergy treatment.

Type I allergy is an immunoglobulin E (IgE)-mediated hypersensitivity disease affecting more than 25% of the population. Currently, diagnosis of allergy is performed by provocation testing and IgE serology using allergen extracts. This process defines allergen-containing sources but cannot identify the disease-eliciting allergenic molecules. We have applied microarray technology to develop a miniaturized allergy test containing 94 purified allergen molecules that represent the most common allergen sources. The allergen microarray allows the determination and monitoring of allergic patients' IgE reactivity profiles to large numbers of disease-causing allergens by using single measurements and minute amounts of serum. This method may change established practice in allergy diagnosis, prevention, and therapy. In addition, microarrayed antigens may be applied to the diagnosis of autoimmune and infectious diseases.