Allergens

Allergens are almost always proteins, but not all proteins are allergens. For a protein antigen to display allergenic activity, it must induce IgE production, which must lead to a type 1 hypersensitivity response upon subsequent exposure to the same protein. Biochemical properties of the allergen, stimulating factors of the innate immune response around the allergen substances at the time of exposure, stability of the allergen in the tissues, digestive system, skin, or mucosa, and the dose and time of stay in lymphatic organs during the interaction with the immune system are all factors hat may cause an antigen to become an allergen. This is distinguished from general antigen responses, which induce a state of immune responsiveness without associated IgE production.

Most allergens are usually proteins of 10-70 kd molecular weight; molecules < 10 kd do not bridge adjacent IgE antibody molecules on the surfaces of mast cells or basophils; most molecules > 70 kd do not pass through mucosal surfaces, a feature needed to reach antigen-presenting cells (APCs) for stimulation of the immune system. Allergens frequently contain proteases, which promote barrier dysfunction and increase allergen penetration into host tissues. Low molecular weight moieties such as drugs can become allergens by reacting with serum proteins or cell membrane proteins to be recognized by the immune system. Carbohydrate structures have also been identified as allergens. This finding is most relevant with the increasing use of biologics in clinical practice; patients with cetuximab-induced anaphylaxis have IgE antibodies in specific for galactose-α-l,3-galactose (Chapter 146).

T Cells

Everyone is exposed to potential allergens. Atopic individuals respond to allergen exposure with rapid expansion of T helper type 2 (Th2) cells that secrete cytokines, such as interleukins IL-4, IL-5, and IL-13, favoring IgE synthesis and eosinophilia. Allergen-specific IgE antibodies associated with atopic response are detectable by serum testing or positive immediate reactions to allergen extracts on prick skin testing (Chapter 135). The Th2 cytokines IL-4 and IL-13 play a key role in immunoglobulin isotype switching to IgE (Fig. 134-1). IL-5 and IL-9 are important in differentiation and development of eosinophils. The combination of IL-3, IL-4, and IL-9 contributes to mast cell activation. Th2 cytokines are important effector molecules in the pathogenesis of asthma and allergic diseases; acute allergic reactions are characterized by infiltration of Th2 cells into affected tissues. In addition, IL-25 and IL-33 contribute to Th2 response and eosinophilia.

Figure 134-1 Role of Th2 cytokines in allergic cascade. DC, dendritic cell; EOS, eosinophil; GM-CSF, granulocyte-macrophage colony-stimulating factor; IL, interleukin; Th2, T helper type 2 cell.

A fraction of the immune response to allergen results in proliferation of T helper type 1 (Th1) cells. Th1 cells are typically involved in the eradication of intracellular organisms, such as mycobacteria, because of the ability of Th1 cytokines to activate phagocytes and promote the production of opsonizing and complement-fixing antibodies. The Th1 component of allergen-specific immune response contributes to chronicity and the effector phase in allergic disease. Activation and apoptosis of epithelial cells induced by Th1 cell–secreted interferon-γ (IFN-γ), tumor necrosis factor-α (TNF-α), and Fas-ligand constitute an essential pathogenetic event for the formation of eczematous lesions in atopic dermatitis and bronchial epithelial cell shedding in asthma.

Chronic allergic reactions are characterized by infiltration of Th1 and Th17 cells. This is important because Th1 cytokines such as IFN-γ can potentiate the function of allergic inflammatory effector cells such as eosinophils and thereby contribute to disease severity. Cytokines in the IL-17 family act on multiple cell types, including epithelial cells and APCs, to cause the release of chemokines, antimicrobial peptides, and pro-inflammatory cytokines to enhance inflammation and antimicrobial responses. In addition, recently identified Th9 cells produce IL-9 but not other typical Th1, Th2 and Th17 cytokines and constitute a distinct population of effector T cells that promotes tissue inflammation. Figure 134-2 depicts the complex cytokine cascades involving Th1, Th2, Th9, and Th17 cells.

Figure 134-2 Effector T-cell subsets. Following antigen presentation by dendritic cells (DCs), naïve T cells differentiate into Th1, Th2, Th9, and Th17 effector subsets. Their differentiation requires cytokines and other cofactors that are released from dendritic cells and also expressed in the micromilieu. T-cell activation in the presence of interleukin-4 (IL-4) enhances differentiation and clonal expansion of Th2 cells, perpetuating the allergic response. IFN-γ, interferon-gamma; TGF-β, transforming growth factor-beta.

(From Akdis CA, Adkis M: Mechanisms and treatment of allergic disease in the big picture of regulatory T cells, J Allergy Clin Immunol 123:735–746, 2009.)

T regulatory (Treg) cells are a subset of T cells thought to play a critical role in expression of allergic and autoimmune diseases. These cells have the ability to suppress effector T cells of either the Th1 or Th2 phenotypes (Fig. 134-3). Treg cells express CD4⁺CD25⁺ surface molecules and immunosuppressive cytokines such as IL-10 and transforming growth factor-β (TGF-β1). The forkhead box/winged-helix transcription factor gene FOXP3 is expressed specifically by CD4⁺CD25⁺ Treg cells and programs their development and function. Adoptive transfer of Treg cells inhibits the development of airway eosinophilia and protects against airway hyperreactivity in animal models of asthma. T cell response to allergens in healthy individuals shows a wide range, from no detectable response to involvement of active peripheral tolerance mechanisms mediated by different subsets of Treg cells. Individuals who are not allergic even though they are exposed to high doses of allergens, such as beekeepers and cat owners, show a detectable allergen-specific IgG4 response accompanied by IL-10–producing Treg cells. It is thought that CD4⁺CD25⁺ Treg cells play an important role in mitigating the allergic immune response and that the lack of such cells may predispose to the development of allergic diseases. Patients with mutations in the human FOXP3 gene lack CD4⁺CD25⁺ Treg cells and develop severe immune dysregulation, with polyendocrinopathy, food allergy, and high serum IgE levels (XLAAD/IPEX disease) (Chapter 120).

Figure 134-3 FoxP3⁺ CD4⁺CD25⁺ and Tr1 cells contribute to the control of allergen-specific immune responses in several major ways: Suppression of dendritic cells that support the generation of effector T-cells; suppression of Th1, Th2, and Th17 cells; suppression of allergen-specific immunoglobulin (Ig) E, and induction of IgG4 and/or IgA; suppression of mast cells, basophils, and eosinophils; interaction with resident tissue cells and remodeling, and suppression of effector T-cell migration to tissues. IL-10, interleukin 10.

(From Akdis CA, Adkis M: Mechanisms and treatment of allergic disease in the big picture of regulatory T cells, J Allergy Clin Immunol 123:735–746, 2009.)

Antigen-Presenting Cells

Dendritic cells, Langerhans cells, monocytes, and macrophages have the ability to present allergens to T cells and thereby modulate allergic inflammation by controlling the type of T-cell development. Antigen-presenting cells are a heterogeneous group of cells that share the property of antigen presentation in the context of the major histocompatibility complex (MHC) and are found primarily in lymphoid organs and the skin. Dendritic cells (DCs) and Langerhans cells are unique in their ability to prime naïve T cells and are responsible for the primary immune response, or the sensitization phase of allergy. Monocytes and macrophages are thought to contribute to activating memory T-cell responses upon re-exposure to allergen, which characterizes the elicitation phase of allergy.

Peripheral DCs residing in sites such as the skin, intestinal lamina propria, and lung are relatively immature. These immature DCs take up antigens in tissues and then migrate to the T-cell areas in locally draining lymph nodes. The DCs undergo phenotypic and functional changes during migration, characterized by increased expression of MHC class I, MHC class II, and co-stimulatory molecules that react with CD28 expressed on T cells. In the lymph nodes, they directly present processed antigens to resting T cells to induce their proliferation and differentiation.

Mature DCs have been designated as DC1 or DC2 on the basis of their ability to favor Th1 or Th2 differentiation, respectively. The critical factor for polarization to Th1 cells is the level of IL-12 produced by DC1 cells. By contrast, DC2 dendritic cells have low levels of IL-12, resulting in Th2 cell predominance. Local histamine and prostaglandin (PG) E₂ inhibit IL-12 synthesis and thereby promote the development of DC2 phenotype. There is considerable interest in the role of thymic stromal lymphopoietin (TSLP), which is overexpressed in the mucosal surfaces and skin of atopic individuals. TSLP enhances Th2 cell differentiation by inducing expression of OX40L on immature myeloid DCs in the absence of IL-12 production.

Presence of allergen-specific IgE on the cell surfaces of APCs is a unique feature of atopy. Importantly, the formation of high-affinity IgE receptor I (FcεRI)/IgE/allergen complexes on APC cell surfaces markedly facilitates allergen uptake and presentation. The clinical importance of this phenomenon is supported by the observation that FcεRI-positive Langerhans cells bearing IgE molecules are a prerequisite for skin-applied, aeroallergen provocation of eczematous lesions in patients with atopic dermatitis. The role of the low-affinity IgE receptor II (FcεRII, CD23) on monocytes-macrophages is less clear, although it appears that under certain conditions it can also facilitate antigen capture. Cross linking of FcεRII as well as FcεRI on monocytes-macrophages leads to the release of inflammatory mediators. There is a critical role for DCs in induction of oral tolerance; tolerogenic DCs are compartmentalized within the mucosa and present antigen through a mechanism designed to produce a Th1/Treg–suppressive response that ablates allergen-specific T cells.

Immunoglobulin E and its Receptors

The acute allergic response depends on IgE and its ability to bind selectively to the α chain of the high-affinity FcεRI or the low-affinity FcεRII (CD23). Cross linking of receptor-bound IgE molecules by allergen initiates a complex intracellular signaling cascade followed by the release of various mediators of allergic inflammation from mast cells and basophils. The FcεRI molecule is also found on the surface of antigen-presenting DCs (e.g., Langerhans cells), but differs from the structure found on mast cells/basophils in that the FcεRI molecule found on DCs lacks the β chain. CD23 is found on B cells, eosinophils, platelets, and dendritic cells. Cross linking and FcεRI aggregation on mast cells and basophils can also lead to anaphylaxis (Chapter 143). Differential expression of tyrosine kinases responsible for positive and negative regulation of mast cell/basophil degranulation are thought to be responsible for this aberrant allergic response.

The induction of IgE synthesis requires two major signals. The first signal (signal 1) initiates IL-4 or IL-13 activation of germline transcription at the ε immunoglobulin locus, which dictates isotype specificity. The second signal (signal 2) involves the engagement of CD40 on B cells by CD40 ligand expressed on T cells. This engagement results in activation of the recombination machinery, resulting in DNA switch recombination. Interactions between several co-stimulatory molecule pairs (CD28 and B7; lymphocyte function–associated antigen-1 [LFA-1] and intercellular adhesion molecule-1 [ICAM-1]; CD2 and CD58) can further amplify signal 1 and signal 2 to enhance IgE synthesis. Factors that inhibit IgE synthesis include Th1-type cytokines (IL-12, IFN-α, IFN-γ) and microbial DNA containing CpG (cytosine-phosphate-guanine) repeats.

Eosinophils

Allergic diseases are characterized by peripheral blood and tissue eosinophilia. Eosinophils participate in both innate and adaptive immune responses and, like mast cells, contain dense intracellular granules that are sources of inflammatory proteins. These granule proteins include major basic protein, eosinophil-derived neurotoxin, peroxidase, and cationic protein. Eosinophil granule proteins damage epithelial cells, induce airway hyperresponsiveness, and cause degranulation of basophils and mast cells. Major basic protein released from eosinophils can bind to an acidic moiety on the M2 muscarinic receptor and block its function, thereby leading to increased acetylcholine levels and the development of increased airway hyperreactivity. Eosinophils are also a rich source of prostaglandins and leukotrienes; in particular, cysteinyl leukotriene C4 contracts airway smooth muscle and increases vascular permeability. Other secretory products of eosinophils include cytokines (IL-4, IL-5, TNF-α), proteolytic enzymes, and reactive oxygen intermediates, all of which significantly enhance allergic tissue inflammation.

Several cytokines regulate the function of eosinophils in allergic disease. Eosinophils develop and mature in the bone marrow from myeloid precursor cells activated by IL-3, IL-5, and granulocyte-macrophage colony-stimulating factor (GM-CSF). Allergen exposure of allergic patients causes resident hematopoietic CD34 cells to express the IL-5 receptor. The IL5 receptor activation induces eosinophil maturation, causing eosinophils to synthesize granule proteins, prolonging their survival, potentiating degranulation of eosinophils, and stimulating release of eosinophils from the bone marrow. GM-CSF also enhances proliferation, cell survival, cytokine production, and degranulation of eosinophils. Certain chemokines, such as RANTES (regulated upon activation, normal T cell expressed and secreted), macrophage inflammatory protein-1α (MIP-1α), and eotaxins, are important for recruiting eosinophils into local allergic tissue inflammatory reactions. Eotaxins mobilize IL-5–dependent eosinophil colony–forming progenitor cells from the bone marrow. These progenitors are rapidly cleared from the blood and either return to the bone marrow or are recruited to inflamed tissue sites.

Mast Cells

Mast cells are derived from CD34 hematopoietic progenitor cells that arise in bone marrow. Upon entering the circulation, they travel to peripheral tissue, where they undergo tissue-specific maturation. Mast cell development and survival relies on interactions between the tyrosine kinase receptor c-kit expressed on the surface of mast cells and the fibroblast-derived c-kit ligand stem cell factor (SCF). Unlike mature basophils, mature mast cells do not typically circulate in the blood. They are, instead, widely distributed throughout connective tissues, where they often lie adjacent to blood vessels and beneath epithelial surfaces that are exposed to the external environment, such as the respiratory tract, gastrointestinal tract, and skin. So placed, mast cells are positioned anatomically to participate in allergic reactions. At least two subpopulations of human mast cells are recognized: mast cells with tryptase and mast cells with both tryptase and chymase. Mast cells with tryptase are the predominant type found in the lung and small intestinal mucosa, whereas mast cells with both tryptase and chymase are the predominant type found in skin, the gastrointestinal submucosa, and blood vessels.

Mast cells contain, or produce on appropriate stimulation, a diverse array of mediators that have different effects on allergic inflammation and organ function. They include preformed granule-associated mediators (histamine, serine proteases, proteoglycans) and membrane-derived lipid, cytokine, and chemokine mediators arising from de novo synthesis and release. The most important mast cell-derived lipid mediators are the cyclo-oxygenase and lipoxygenase metabolites of arachidonic acid, which have potent inflammatory activities. The major cyclo-oxygenase product of mast cells is PG D₂, and the major lipoxygenase products are the sulfidopeptide leukotrienes (LTs): LTC₄ and its peptidolytic derivatives LTD₄ and LTE₄. Mast cells also can produce cytokines that promote Th2-type responses (IL-4, IL-13, GM-CSF) and inflammation (TNF-α, IL-6) and regulate tissue remodeling (TGF, vascular endothelial cell growth factor). Immunologic activation of mast cells and basophils typically begins with cross linkage of IgE bound to the FcεRI with multivalent allergen. Mast cell surface FcεRI is increased by IL-4 and IgE. Surface levels of FcεRI decrease in subjects receiving treatment with anti-IgE antibody that lowers serum IgE, which is of potential therapeutic interest.