The Immunobiology and Treatment of Food Allergy.

IgE-mediated food allergy (IgE-FA) occurs due to a breakdown in immune tolerance that leads to a detrimental type 2 helper T cell (TH2) adaptive immune response. While the processes governing this loss of tolerance are incompletely understood, several host-related and environmental factors impacting the risk of IgE-FA development have been identified. Mounting evidence supports the role of an impaired epithelial barrier in the development of IgE-FA, with exposure of allergens through damaged skin and gut epithelium leading to the aberrant production of alarmins and activation of TH2-type allergic inflammation. The treatment of IgE-FA has historically been avoidance with acute management of allergic reactions, but advances in allergen-specific immunotherapy and the development of biologics and other novel therapeutics are rapidly changing the landscape of food allergy treatment. Here, we discuss the pathogenesis and immunobiology of IgE-FA in addition to its diagnosis, prognosis, and treatment.

The Microbiome and Food Allergy.

The gut-associated lymphoid tissue (GALT) faces a considerable challenge. It encounters antigens derived from an estimated 1014 commensal microbes and greater than 30 kg of food proteins yearly. It must distinguish these harmless antigens from potential pathogens and mount the appropriate host immune response. Local and systemic hyporesponsiveness to dietary antigens, classically referred to as oral tolerance, comprises a distinct complement of adaptive cellular and humoral immune responses. It is increasingly evident that a functional epithelial barrier engaged in intimate interplay with innate immune cells and the resident microbiota is critical to establishing and maintaining oral tolerance. Moreover, innate immune cells serve as a bridge between the microbiota, epithelium, and the adaptive immune system, parlaying tonic microbial stimulation into signals critical for mucosal homeostasis. Dysregulation of gut homeostasis and the subsequent disruption of tolerance therefore have clinically significant consequences for the development of food allergy.

Natural antibody repertoires: development and functional role in inhibiting allergic airway disease.

In this review we discuss the effects of microbial exposure on the B cell repertoire. Neonatal exposure to conserved bacterial carbohydrates and phospholipids permanently reprograms the natural antibody repertoire directed toward these antigens by clonal expansion, alterations in clonal dominance, and increased serum antibody levels. These epitopes are present not only in bacterial cell walls, but also in common environmental allergens. Neonatal immunization with bacterial polysaccharide vaccines results in attenuated allergic airway responses to fungi-, house dust mite-, and cockroach-associated allergens in mouse models. The similarities between mouse and human natural antibody repertoires suggest that reduced microbial exposure in children may have the opposite effect, providing a potential mechanistic explanation for the hygiene hypothesis. We propose that understanding the effects of childhood infections on the natural antibody repertoire and the mechanisms of antibody-mediated immunoregulation observed in allergy models will lead to the development of prevention/interventional strategies for treatment of allergic asthma.

Basophils add fuel to the flame of eczema itch.

Children and adults with atopic dermatitis suffer from intractable chronic itch and can also experience acute itch flare ups that significantly increase itch intensity. In this issue of Cell, Wang et al. demonstrate that a subset of basophils activates sensory neurons to drive allergen-evoked itch flare ups in atopic dermatitis.

Food allergy as a biological food quality control system.

Food is simultaneously a source of essential nutrients and a potential source of lethal toxins and pathogens. Consequently, multiple sensory mechanisms evolved to monitor the quality of food based on the presence and relative abundance of beneficial and harmful food substances. These include the olfactory, gustatory, and gut chemosensory systems. Here we argue that, in addition to these systems, allergic immunity plays a role in food quality control by mounting allergic defenses against food antigens associated with noxious substances. Exaggeration of these defenses can result in pathological food allergy.

A basophil-neuronal axis promotes itch.

Itch is an evolutionarily conserved sensation that facilitates expulsion of pathogens and noxious stimuli from the skin. However, in organ failure, cancer, and chronic inflammatory disorders such as atopic dermatitis (AD), itch becomes chronic, intractable, and debilitating. In addition to chronic itch, patients often experience intense acute itch exacerbations. Recent discoveries have unearthed the neuroimmune circuitry of itch, leading to the development of anti-itch treatments. However, mechanisms underlying acute itch exacerbations remain overlooked. Herein, we identify that a large proportion of patients with AD harbor allergen-specific immunoglobulin E (IgE) and exhibit a propensity for acute itch flares. In mice, while allergen-provoked acute itch is mediated by the mast cell-histamine axis in steady state, AD-associated inflammation renders this pathway dispensable. Instead, a previously unrecognized basophil-leukotriene (LT) axis emerges as critical for acute itch flares. By probing fundamental itch mechanisms, our study highlights a basophil-neuronal circuit that may underlie a variety of neuroimmune processes.

An αvß3 integrin checkpoint is critical for efficient TH2 cell cytokine polarization and potentiation of antigen-specific immunity.

Naive CD4+ T lymphocytes initially undergo antigen-specific activation to promote a broad-spectrum response before adopting bespoke cytokine expression profiles shaped by intercellular microenvironmental cues, resulting in pathogen-focused modular cytokine responses. Interleukin (IL)-4-induced Gata3 upregulation is important for the helper type 2 T cell (TH2 cell) polarization associated with anti-helminth immunity and misdirected allergic inflammation. Whether additional microenvironmental factors participate is unclear. Using whole mouse-genome CRISPR-Cas9 screens, we discovered a previously unappreciated role for αvß3 integrin in TH2 cell differentiation. Low-level αvß3 expression by naive CD4+ T cells contributed to pan-T cell activation by promoting T-T cell clustering and IL-2/CD25/STAT5 signaling. Subsequently, IL-4/Gata3-induced selective upregulation of αvß3 licensed intercellular αvß3-Thy1 interactions among TH2 cells, enhanced mammalian target of rapamycin (mTOR) signaling, supported differentiation and promoted IL-5/IL-13 production. In mice, αvß3 was required for efficient, allergen-driven, antigen-specific lung TH2 cell responses. Thus, αvß3-expressing TH2 cells form multicellular factories to propagate and amplify TH2 cell responses.

Physiological microbial exposure transiently inhibits mouse lung ILC2 responses to allergens.

Lung group 2 innate lymphoid cells (ILC2s) control the nature of immune responses to airway allergens. Some microbial products, including those that stimulate interferons, block ILC2 activation, but whether this occurs after natural infections or causes durable ILC2 inhibition is unclear. In the present study, we cohoused laboratory and pet store mice as a model of physiological microbial exposure. Laboratory mice cohoused for 2 weeks had impaired ILC2 responses and reduced lung eosinophilia to intranasal allergens, whereas these responses were restored in mice cohoused for ≥2 months. ILC2 inhibition at 2 weeks correlated with increased interferon receptor signaling, which waned by 2 months of cohousing. Reinduction of interferons in 2-month cohoused mice blocked ILC2 activation. These findings suggest that ILC2s respond dynamically to environmental cues and that microbial exposures do not control long-term desensitization of innate type 2 responses to allergens.

Allergen protease-activated stress granule assembly and gasdermin D fragmentation control interleukin-33 secretion.

Interleukin-33 (IL-33), an epithelial cell-derived cytokine that responds rapidly to environmental insult, has a critical role in initiating airway inflammatory diseases. However, the molecular mechanism underlying IL-33 secretion following allergen exposure is not clear. Here, we found that two cell events were fundamental for IL-33 secretion after exposure to allergens. First, stress granule assembly activated by allergens licensed the nuclear-cytoplasmic transport of IL-33, but not the secretion of IL-33. Second, a neo-form murine amino-terminal p40 fragment gasdermin D (Gsdmd), whose generation was independent of inflammatory caspase-1 and caspase-11, dominated cytosolic secretion of IL-33 by forming pores in the cell membrane. Either the blockade of stress granule assembly or the abolishment of p40 production through amino acid mutation of residues 309-313 (ELRQQ) could efficiently prevent the release of IL-33 in murine epithelial cells. Our findings indicated that targeting stress granule disassembly and Gsdmd fragmentation could reduce IL-33-dependent allergic airway inflammation.

Lung dendritic cells in respiratory viral infection and asthma: from protection to immunopathology.

Lung dendritic cells (DCs) bridge innate and adaptive immunity, and depending on context, they also induce a Th1, Th2, or Th17 response to optimally clear infectious threats. Conversely, lung DCs can also mount maladaptive Th2 immune responses to harmless allergens and, in this way, contribute to immunopathology. It is now clear that the various aspects of DC biology can be understood only if we take into account the functional specializations of different DC subsets that are present in the lung in homeostasis or are attracted to the lung as part of the inflammatory response to inhaled noxious stimuli. Lung DCs are heavily influenced by the nearby epithelial cells, and a model is emerging whereby direct communication between DCs and epithelial cells determines the outcome of the pulmonary immune response. Here, we have approached DC biology from the perspective of viral infection and allergy to illustrate these emerging concepts.

Environmental allergens trigger type 2 inflammation through ripoptosome activation.

Environmental allergens, including fungi, insects and mites, trigger type 2 immunity; however, the innate sensing mechanisms and initial signaling events remain unclear. Herein, we demonstrate that allergens trigger RIPK1-caspase 8 ripoptosome activation in epithelial cells. The active caspase 8 subsequently engages caspases 3 and 7, which directly mediate intracellular maturation and release of IL-33, a pro-atopy, innate immunity, alarmin cytokine. Mature IL-33 maintained functional interaction with the cognate ST2 receptor and elicited potent pro-atopy inflammatory activity in vitro and in vivo. Inhibiting caspase 8 pharmacologically and deleting murine Il33 and Casp8 each attenuated allergic inflammation in vivo. Clinical data substantiated ripoptosome activation and IL-33 maturation as likely contributors to human allergic inflammation. Our findings reveal an epithelial barrier, allergen-sensing mechanism that converges on the ripoptosome as an intracellular molecular signaling platform, triggering type 2 innate immune responses. These findings have significant implications for understanding and treating human allergic diseases.

Microbial metabolism of L-tyrosine protects against allergic airway inflammation.

The constituents of the gut microbiome are determined by the local habitat, which itself is shaped by immunological pressures, such as mucosal IgA. Using a mouse model of restricted antibody repertoire, we identified a role for antibody-microbe interactions in shaping a community of bacteria with an enhanced capacity to metabolize L-tyrosine. This model led to increased concentrations of p-cresol sulfate (PCS), which protected the host against allergic airway inflammation. PCS selectively reduced CCL20 production by airway epithelial cells due to an uncoupling of epidermal growth factor receptor (EGFR) and Toll-like receptor 4 (TLR4) signaling. Together, these data reveal a gut microbe-derived metabolite pathway that acts distally on the airway epithelium to reduce allergic airway responses, such as those underpinning asthma.

Homeostatic IL-13 in healthy skin directs dendritic cell differentiation to promote TH2 and inhibit TH17 cell polarization.

The signals driving the adaptation of type 2 dendritic cells (DC2s) to diverse peripheral environments remain mostly undefined. We show that differentiation of CD11blo migratory DC2s-a DC2 population unique to the dermis-required IL-13 signaling dependent on the transcription factors STAT6 and KLF4, whereas DC2s in lung and small intestine were STAT6-independent. Similarly, human DC2s in skin expressed an IL-4 and IL-13 gene signature that was not found in blood, spleen and lung DCs. In mice, IL-13 was secreted homeostatically by dermal innate lymphoid cells and was independent of microbiota, TSLP or IL-33. In the absence of IL-13 signaling, dermal DC2s were stable in number but remained CD11bhi and showed defective activation in response to allergens, with diminished ability to support the development of IL-4+GATA3+ helper T cells (TH), whereas antifungal IL-17+RORγt+ TH cells were increased. Therefore, homeostatic IL-13 fosters a noninflammatory skin environment that supports allergic sensitization.

How α-Helical Motifs Form Functionally Diverse Lipid-Binding Compartments.

Lipids are produced site-specifically in cells and then distributed nonrandomly among membranes via vesicular and nonvesicular trafficking mechanisms. The latter involves soluble amphitropic proteins extracting specific lipids from source membranes to function as molecular solubilizers that envelope their insoluble cargo before transporting it to destination sites. Lipid-binding and lipid transfer structural motifs range from multi-ß-strand barrels, to ß-sheet cups and baskets covered by α-helical lids, to multi-α-helical bundles and layers. Here, we focus on how α-helical proteins use amphipathic helical layering and bundling to form modular lipid-binding compartments and discuss the functional consequences. Preformed compartments generally rely on intramolecular disulfide bridging to maintain conformation (e.g., albumins, nonspecific lipid transfer proteins, saposins, nematode polyprotein allergens/antigens). Insights into nonpreformed hydrophobic compartments that expand and adapt to accommodate a lipid occupant are few and provided mostly by the three-layer, α-helical ligand-binding domain of nuclear receptors. The simple but elegant and nearly ubiquitous two-layer, α-helical glycolipid transfer protein (GLTP)-fold now further advances understanding.

A regulatory T cell Notch4-GDF15 axis licenses tissue inflammation in asthma.

Elucidating the mechanisms that sustain asthmatic inflammation is critical for precision therapies. We found that interleukin-6- and STAT3 transcription factor-dependent upregulation of Notch4 receptor on lung tissue regulatory T (Treg) cells is necessary for allergens and particulate matter pollutants to promote airway inflammation. Notch4 subverted Treg cells into the type 2 and type 17 helper (TH2 and TH17) effector T cells by Wnt and Hippo pathway-dependent mechanisms. Wnt activation induced growth and differentiation factor 15 expression in Treg cells, which activated group 2 innate lymphoid cells to provide a feed-forward mechanism for aggravated inflammation. Notch4, Wnt and Hippo were upregulated in circulating Treg cells of individuals with asthma as a function of disease severity, in association with reduced Treg cell-mediated suppression. Our studies thus identify Notch4-mediated immune tolerance subversion as a fundamental mechanism that licenses tissue inflammation in asthma.

Interleukin-33 activates regulatory T cells to suppress innate γδ T cell responses in the lung.

Foxp3+ regulatory T (Treg) cells expressing the interleukin (IL)-33 receptor ST2 mediate tissue repair in response to IL-33. Whether Treg cells also respond to the alarmin IL-33 to regulate specific aspects of the immune response is not known. Here we describe an unexpected function of ST2+ Treg cells in suppressing the innate immune response in the lung to environmental allergens without altering the adaptive immune response. Following allergen exposure, ST2+ Treg cells were activated by IL-33 to suppress IL-17-producing γδ T cells. ST2 signaling in Treg cells induced Ebi3, a component of the heterodimeric cytokine IL-35 that was required for Treg cell-mediated suppression of γδ T cells. This response resulted in fewer eosinophil-attracting chemokines and reduced eosinophil recruitment into the lung, which was beneficial to the host in reducing allergen-induced inflammation. Thus, we define a fundamental role for ST2+ Treg cells in the lung as a negative regulator of the early innate γδ T cell response to mucosal injury.

Serum amyloid A is a soluble pattern recognition receptor that drives type 2 immunity.

The molecular basis for the propensity of a small number of environmental proteins to provoke allergic responses is largely unknown. Herein, we report that mite group 13 allergens of the fatty acid-binding protein (FABP) family are sensed by an evolutionarily conserved acute-phase protein, serum amyloid A1 (SAA1), that promotes pulmonary type 2 immunity. Mechanistically, SAA1 interacted directly with allergenic mite FABPs (Der p 13 and Blo t 13). The interaction between mite FABPs and SAA1 activated the SAA1-binding receptor, formyl peptide receptor 2 (FPR2), which drove the epithelial release of the type-2-promoting cytokine interleukin (IL)-33 in a SAA1-dependent manner. Importantly, the SAA1-FPR2-IL-33 axis was upregulated in nasal epithelial cells from patients with chronic rhinosinusitis. These findings identify an unrecognized role for SAA1 as a soluble pattern recognition receptor for conserved FABPs found in common mite allergens that initiate type 2 immunity at mucosal surfaces.

From allergen genes to allergy vaccines.

IgE-mediated allergy is a hypersensitivity disease affecting more than 25% of the population. The structures of the most common allergens have been revealed through molecular cloning technology in the past two decades. On the basis of this knowledge of the sequences and three-dimensional structures of culprit allergens, investigators can now analyze the immune recognition of allergens and the mechanisms of allergic inflammation in allergic patients. Allergy vaccines have been constructed that are able to selectively target the aberrant immune responses in allergic patients via different pathways of the immune system. Here we review various types of allergy vaccines that have been developed based on allergen structures, results from their clinical application in allergic patients, and future strategies for allergen-specific immunotherapy and allergy prophylaxis.

House dust mites activate nociceptor-mast cell clusters to drive type 2 skin inflammation.

Allergic skin diseases, such as atopic dermatitis, are clinically characterized by severe itching and type 2 immunity-associated hypersensitivity to widely distributed allergens, including those derived from house dust mites (HDMs). Here we found that HDMs with cysteine protease activity directly activated peptidergic nociceptors, which are neuropeptide-producing nociceptive sensory neurons that express the ion channel TRPV1 and Tac1, the gene encoding the precursor for the neuropeptide substance P. Intravital imaging and genetic approaches indicated that HDM-activated nociceptors drive the development of allergic skin inflammation by inducing the degranulation of mast cells contiguous to such nociceptors, through the release of substance P and the activation of the cationic molecule receptor MRGPRB2 on mast cells. These data indicate that, after exposure to HDM allergens, activation of TRPV1+Tac1+ nociceptor-MRGPRB2+ mast cell sensory clusters represents a key early event in the development of allergic skin reactions.

Locally instructed CXCR4hi neutrophils trigger environment-driven allergic asthma through the release of neutrophil extracellular traps.

Low exposure to microbial products, respiratory viral infections and air pollution are major risk factors for allergic asthma, yet the mechanistic links between such conditions and host susceptibility to type 2 allergic disorders remain unclear. Through the use of single-cell RNA sequencing, we characterized lung neutrophils in mice exposed to a pro-allergic low dose of lipopolysaccharide (LPS) or a protective high dose of LPS before exposure to house dust mites. Unlike exposure to a high dose of LPS, exposure to a low dose of LPS instructed recruited neutrophils to upregulate their expression of the chemokine receptor CXCR4 and to release neutrophil extracellular traps. Low-dose LPS-induced neutrophils and neutrophil extracellular traps potentiated the uptake of house dust mites by CD11b+Ly-6C+ dendritic cells and type 2 allergic airway inflammation in response to house dust mites. Neutrophil extracellular traps derived from CXCR4hi neutrophils were also needed to mediate allergic asthma triggered by infection with influenza virus or exposure to ozone. Our study indicates that apparently unrelated environmental risk factors can shape recruited lung neutrophils to promote the initiation of allergic asthma.