The hygiene hypothesis: immunological mechanisms of airway tolerance The hygiene hypothesis: immunological mechanisms of airway tolerance Eline Haspeslagh1,2,3, Ines Heyndrickx1,3, Hamida Hammad1,3 and Bart N Lambrecht1,3,4 Available online at www.sciencedirect.com ScienceDirect The hygiene hypothesis was initially proposed as an explanation for the alarming rise in allergy prevalence in the last century. The immunological idea behind this hypothesis was a lack of infections associated with a Western lifestyle and a consequential reduction in type 1 immune responses. It is now understood that the development of tolerance to allergens depends on microbial colonization and immunostimulatory environmental signals during early-life or passed on by the mother. These environmental cues are sensed and integrated by barrier epithelial cells of the lungs and possibly skin, which in turn instruct dendritic cells to regulate or impede adaptive T cell responses. Recent reports also implicate immunoregulatory macrophages as powerful suppressors of allergy by the microbiome. We propose that loss of adequate microbial stimulation due to a Western lifestyle may result in hypersensitive barrier tissues and the observed rise in type 2 allergic disease. Addresses 1 Laboratory of Immunoregulation and Mucosal Immunology, VIB Center for Inflammation Research, Technologiepark 927, B-9052 Ghent (Zwijnaarde), Belgium 2 Department of Biomedical Molecular Biology, Ghent University, Technologiepark 927, B-9052 Ghent (Zwijnaarde), Belgium 3 Department of Internal Medicine, Ghent University, De Pintelaan 185 K12, B-9000 Ghent, Belgium 4 Department of Pulmonary Medicine, ErasmusMC, ’s-Gravendijkwal 230, 3015 CE Rotterdam, The Netherlands Corresponding authors: Hammad, Hamida (Hamida.Hammad@UGent. be), Lambrecht, Bart N (bart.lambrecht@ugent.be) Current Opinion in Immunology 2018, 54:102–108 This review comes from a themed issue on Allergy and hypersensitivity Edited by Onur Boyman, Alexander Eggel and Mario Noti https://doi.org/10.1016/j.coi.2018.06.007 0952-7915/ã 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creative- commons.org/licenses/by-nc-nd/4.0/). Introduction Allergic sensitization is characterized by the presence of allergen-specific immunoglobulin E (IgE) in serum. Exposure to allergens via inhalation, ingestion or contact with the skin can lead to diseases such as asthma, hay Current Opinion in Immunology 2018, 54:102–108 fever, eczema and, in some cases, to systemic anaphylaxis. During the last 150 years, allergies have emerged in a very rapid way and their prevalence is still on the rise. Nowa- days, more than 30% of children are allergic, up to 10% of children suffer from asthma and allergic rhinitis, and 5– 7% of children have developed food allergy. It is still not entirely clear why asthma prevalence is so high, but the rapid time frame of its origination and expansion suggests that environmental or behavioral changes in Western lifestyle are involved. A modern lifestyle is associated with dysbiosis An important evolution of the last 150 years is a successful decrease of infectious disease burden, due to the massive introduction of hygiene measures, antibiotics, and vac- cines. In 1989, Strachan observed that growing up in large families with more older siblings decreased the chances of developing hay fever or eczema [1]. He postulated that the recent increase in allergy incidence was a result of ‘declining family size, improvements in household ame- nities, and higher standards of personal cleanliness’, which had reduced ‘the opportunity for cross infection in young families’. The original ‘hygiene hypothesis’ was thus introduced. Since then, this hypothesis has been supported by numerous studies, especially in murine models, showing that exposure to bacteria, viruses, hel- minths or microbe-derived products could protect from allergy (reviewed in [2], [3 �� ]). However, it should be kept in mind that not all pathogens are protective; for instance, respiratory syncytial virus (RSV) or rhinovirus are associ- ated with a higher risk to develop wheeze and asthma up to adulthood [4]. Changes in lifestyle can also heavily influence the com- position and diversity of the microbiome at several muco- sal surfaces. These microbial communities have co- evolved with and within the human body for millions of years, and, consequently, the human immune system has been calibrated and fine-tuned so to maintain and shape symbiotic relationships with them (reviewed in [5]). Two theories, the ‘Old friends’ and the ‘Biodiversity’ hypotheses, have been proposed by Rook and by Haah- tela as a more accurate, or at least complementary, expla- nation for the recent allergy pandemic [6,7]. They stipu- late that the reason for the increased incidence in allergic disorders is a reduced exposure to such beneficial symbi- otic bacteria or parasites. Indeed, several studies have www.sciencedirect.com mailto:Hamida.Hammad@UGent.be mailto:Hamida.Hammad@UGent.be mailto:bart.lambrecht@ugent.be https://doi.org/10.1016/j.coi.2018.06.007 https://smart.servier.com/ https://smart.servier.com/ http://crossmark.crossref.org/dialog/?doi=10.1016/j.coi.2018.06.007&domain=pdf http://www.sciencedirect.com/science/journal/09527915 Hygiene hypothesis Haspeslagh et al. 103 reported that alterations in the composition of the skin, the nose or the gut microbiome are associated with eczema, asthma and food allergy [8–10]. These changes do not affect a single commensal, but rather reflect a reduced total microbial diversity [11], and they may be caused by several factors, including sibling order in the family [12], exposure to animals [13], and other early-life events [14]. The importance of a healthy microbiome in controlling allergies was further substantiated in mice, with germ-free mice being especially prone to develop overt allergic (airway) disease, a phenotype reverted by microbial recolonization [15,16]. However, other studies showed that germ-free mice are not universally more susceptible to house dust mite driven asthma, and that only selected strains of lung microbiota seem to suppress asthma [17]. During the last 30 years, the body of correla- tive epidemiological studies has expanded vastly, and is the subject of many excellent reviews. Here, we will zoom in on recent advances in the search for the under- lying immunological mechanisms explaining the observed effects. Microbes induce protective regulatory DCs and T cells Allergies are generally aberrant immune reactions to innocuous antigens, orchestrated by T helper 2 (Th2) cells and type 2 innate lymphoid cells (ILC2s). In the case of asthma, this type 2 cell activity leads to mucus hyper- secretion, goblet cell hyperplasia, smooth muscle cell hyperreactivity, and the infiltration and/or activation of eosinophils, mast cells and basophils, ultimately culmi- nating in breathing difficulties and airway remodeling [18]. Dendritic cells (DCs) are always found at the body’s barriers, and because they express a wide range of pattern recognition receptors (PRRs), they can sense the envi- ronment for the presence of danger signals [19]. Our group has shown that Th2 responses to house dust mite (HDM) allergens were induced by IRF4-dependent cDC2s in the lungs and in the skin [20,21 � ] (Figure 1). These cDC2s capture the HDM allergens in the airways and migrate to the draining lymph nodes, requiring ILC2- derived IL-13, where they present the allergens to naı̈ve T cells [22]. It is easy to imagine that environmental changes sensed at the level of the lungs, the skin but also of the gut will modify the context of allergen recognition by DCs, and either protect against or enhance allergic responses. Chronic Helicobacter pylori infection has been inversely linked to asthma in humans and can effectively protect mice from OVA-induced asthma [23,24]. In mice, H. pylori infection induced the accumulation of CD103+ cDCs in the lungs, which were required for the protec- tion, as was their IL-10 production [24]. In a recent study, semi-therapeutic H. pylori extract treatment also reduced airway allergy, shifted the CD11b+/CD103+ DC ratio in the lungs, and reduced the antigen processing by lung and www.sciencedirect.com lymph node DCs [25]. Other studies demonstrated pro- tective modulation of in vitro bone-marrow derived DC cultures (BMDCs). A synthetic TLR1/TLR2-agonist induced LPS-tolerance and IL-10 production in BMDCs, whereas the cowshed Lactococcus lactis instigated a Th1- polarizing program, both rendering the BMDCs unable to sensitize mice to OVA-allergen upon adoptive transfer [26,27 � ]. Trompette et al. recently found that feeding mice a fiber- rich diet changed the composition of the lung and gut microbiome, the latter metabolizing the fiber into circu- lating short-chain fatty acids (SCFA’s) [28]. The increased SCFA levels protected the mice from allergic lung inflammation. Mechanistically, the SCFA’s altered DC precursor generation in the bone marrow, and the DCs subsequently seeding the lungs had a higher phago- cytic capacity and were impaired in polarizing Th2 cells. Additional studies have supported the protective effect of dietary fiber supplementation on allergic asthma devel- opment in mice [29], and on wheeze in human infants when the fiber was given to the pregnant mother [30]. One mechanism by which the DCs in microbe-exposed animals can confer protection, is by inducing the genera- tion of regulatory T cells (Tregs). Microbial colonization in 2-week old mice was shown to be necessary for the transient upregulation of PD-L1 on lung CD11b + DCs, and the expansion of a specific pulmonary Treg subset [31]. PD-L1 blockade in neonates resulted in exaggerated responsiveness to HDM through adulthood, suggesting a crucial role for this microbial-induced DC–Treg axis for immunological tolerance. In another mouse model of H. pylori-mediated asthma protection, the Helicobacter infec- tion inhibited TLR-induced DC maturation and repro- grammed the DCs towards a FoxP3+ Treg-polarizing phenotype [32]. The bacterial component flagellin B, given semi-therapeutically together with allergen, could also inhibit murine allergic asthma symptoms in a DC� and CD25+ Treg-dependent manner [33 � ]. Although helminths are prototypical inducers of type 2 immunity, they have been correlated with reduced allergen skin prick test reactivity, and to some degree with asthma protection (reviewed in [34]). A general explanation for this non-intuitive association is that hel- minths induce a so-called ‘modified Th20 response, with immunoregulatory cells such as Tregs complementing the Th2-arm of immunity, and regulating the response to bystander antigens such as aeroallergens. Therefore, sev- eral groups have tried to find helminth-derived products with immunomodulatory properties that could be used to suppress Th2 immunity. For instance, an anti-inflamma- tory protein (-2; AIP-2) from the parasitic hookworm was identified to suppress murine airway allergy in a DC- dependent and Treg-dependent manner [35]. In another study, the helminth-derived immunomodulator Current Opinion in Immunology 2018, 54:102–108 104 Allergy and hypersensitivity Figure 1 Treg Th2 TLR4 IL-25 IL-33 GM-CSF CCL20 IL-1α Th0 IRF4+ CD11b+ OX40L Notch L IL-12 Allergens (HDM) Endotoxin Farming environment Microbial colonization RSV Second hand smoke Neonatal lungs H. polygyrus HES Microbial colonization H. pylori L. lactis SCFAs Flagellin B AIP-2 Th1 Th0 IL-1R IL-33 TLR4 TLR4 rM AvCystatin Herpes Current Opinion in Immunology Proposed model of airway tolerance. In the absence of immunoregulatory pathways, epithelial barrier cells readily respond to allergen binding on their pattern recognition receptors, among which TLR4, by the secretion of inflammatory mediators (IL-1a, IL-25, IL-33, GM-CSF, CCL20, and others). These mediators license antigen-bearing IRF4+ CD11b+ conventional dendritic cells (cDC2s) to polarize naı̈ve T cells to T helper 2 (Th2) cells in the lung-draining lymph nodes. Neonatal and germ-free mice are especially prone to develop such Th2 responses. Respiratory syncytial virus (RSV) infection and second hand cigarette smoke, two known asthma risk factors, increase IL-33 secretion and may thereby stimulate this pathway. Exposure to endotoxin, farm dust or microbial colonization blunts the epithelial response by increasing the expression of negative regulators. Epithelial IL-33 release is also inhibited by helminth-derived excreted and secreted products (HES). DCs devoid of epithelial activation signals do not induce T cell activation (Th0). Other protective factors impede T cell activity by influencing the maturation, antigen presentation, or phagocytic capacity of DCs. Some protective factors induce DCs that provoke regulatory T cell (Treg) activity or T helper 1 (Th1) activity. Regulatory macrophages (rM) can also induce Tregs, or block DC-mediated Th2-polarization. Abbreviations: HDM, house dust mite; TLR4, Toll like receptor 4; IL-1R, IL-1 receptor; H. pylori, Helicobacter pylori; L. lactis, Lactococcus lactis; SCFA, short chain fatty acids; AIP-2, anti-inflammatory protein 2. AvCystatin was demonstrated to induce regulatory macrophages that protected against experimental asthma upon adoptive transfer [36]. Regulatory alveolar macro- phages from bone marrow origin were recently also impli- cated in long-lasting protection conferred by a latent Current Opinion in Immunology 2018, 54:102–108 murine gammaherpesvirus infection, a model for Epstein- –Barr virus infection in mice [37 � ]. The regulatory macro- phages induced by the infection replaced the long-lived and self-replenishing alveolar macrophages that are gen- erated shortly after birth, and became long-lived as well. www.sciencedirect.com Hygiene hypothesis Haspeslagh et al. 105 This potentially explains the long-lasting effects of microbial stimuli in the lungs on allergy suppression. Allergic asthma is initiated by aberrant immune responses at barrier tissues To initiate an allergen-specific Th2 response, cDC2s need to be instructed by barrier epithelial cells (ECs) lining the airways. Barrier ECs are permanently exposed to environmental insults or innocuous signals and, like DCs, they are well-equipped to integrate these signals via a range of PRRs (reviewed in [38]). Activation of PRRs on ECs by allergens induces NF-kB activation and ROS production, resulting in the secretion of a wide range of inflammatory mediators, among which the cytokines IL- 33, IL-25 and TSLP. DCs react to these cytokines by OX40L and Notch ligand upregulation, and downregula- tion of IL-12 production, an activation state that favors Th2 polarization in the lung-draining lymph node [39,40]. Interestingly, the barrier tissue of the skin also constitutes a possible entry route for aeroallergens [21 � ]. Thus, barrier cells act very upstream in the inflammatory cas- cade of events leading to allergic sensitization (Figure 1). Our group has previously reported that the PRR toll-like receptor 4 (TLR4) on airway ECs was critically necessary to mount a Th2-mediated asthmatic response to HDM [41]. Strikingly, several HDM allergens have the intrinsic capacity to facilitate or amplify TLR4 signaling by bind- ing directly to proteins of the TLR4 signaling complex or to its ligands [42,43]. However, TLR4 is best known as the receptor for LPS, also termed endotoxin, a component of gram-negative bacteria. It is difficult to reconcile how a receptor specialized in bacterial sensing can contribute to Th2 immunity and allergy, especially given the fact that high endotoxin levels in children’s mattresses are protec- tive against atopic sensitization and asthma in humans and mice [44–46]. Another study also associated house dust endotoxin levels with a significantly reduced risk of allergic sensitization or eczema, specifically in children with a polymorphism in the CD14 gene [47]. In fact, a body of epidemiological studies have convincingly corre- lated a traditional farming environment, where endotoxin levels are high, with protection against hay fever, allergic sensitization, and asthma (reviewed in [48]). In a hallmark study, children growing up on agricultural Hutterite and Amish farms in the US were compared, and the latter were found to have six times less chance of developing atopy and asthma [49 �� ]. These two farming populations share a similar genetic ancestry and lifestyle. Farming practices, however, differ, and the Amish house dust contained almost 7 times more endotoxin than the house dust from Hutterite farms. Only the transfer of the Amish dust intranasally to mice inhibited subsequent experi- mental asthma development. We have recently confirmed that farm dust collected from Bavarian farms (in which farming practices resemble the Amish’s ones), and LPS, conferred protection against experimental asthma. This www.sciencedirect.com protection was mediated by an increased epithelial expression of TNFAIP3 (better known as A20), a nega- tive regulator of the NF-kB pathway, which blunted the epithelial cell response to HDM and downstream DC activity [50]. A similar tolerance to LPS mediated by A20 induction was demonstrated in intestinal ECs [51]. Inter- estingly, A20 expression was very low in neonatal rats, spontaneously increased shortly after birth coinciding with microbial colonization, and could be downregulated by treatment with antibiotics. It remains to be investi- gated if the gut or lung microbiota can similarly influence expression of A20 and other negative regulators in airway ECs. EC modulation has recently been demonstrated for Heligmosomoides polygyrus, a helminth often confirmed to protect against murine allergy [52]. Secreted and excreted products (HES) of this parasite inhibited IL-33 release by ECs and thereby suppressed Alternaria-induced airway allergy [3 �� ]). Mechanistically, the H. polygyrus alarmin release inhibitor (HpARI), a 26 kDa protein, binds to activated IL-33 and at the same time tethers IL-33 to the DNA of necrotic cells, thus inhibiting IL-33 action in a dual manner and inhibiting innate eosinophilic airway inflammation [53 �� ]. Other molecules secreted by the parasite are more related to TGFb and can induce a Foxp3+ Treg population with immunoregulatory poten- tial [54]. A dysregulated immune response at the skin can cause atopic dermatitis (AD), which in itself is a risk factor to accumulate more allergies later in life, among which asthma, a process known as ‘the atopic march’. In fact, AD and asthma share several risk factors. AD is strongly correlated with changes in the skin microbiome, the most well-known being pertinent Staphylococcus aureus coloni- zation of allergic skin. In a recent study, IL-17Ra�/� mice spontaneously developed AD with naturally occur- ring skin dysbiosis and a compromised skin barrier, and antibiotic treatment ameliorated skin inflammation [55]. A similar dysbiosis–AD axis has also been demonstrated in ADAM17�/� mice [56]. Topical treatment with non- pathogenic bacteria, on the other hand, can alleviate cutaneous inflammation in murine AD [57]. It has become clear in recent years that tonic sensing of skin commensals heavily shapes host DC and T cell functions [58–60]. It remains to be investigated what the relative importance is of passive barrier integrity and active sig- naling through keratinocyte PRRs, also poised to rapidly respond to innate immune ligands, in this microbe- immune cell cross-talk [61]. It will also be of great interest to study how environmental exposures influence the skin microbiome and/or the immune threshold of skin epithelium. One intriguing observation is that allergies tend to develop early in life. In the same time window, and even in utero, protective effects of environmental factors and the microbiome are also the strongest [62,63]. We Current Opinion in Immunology 2018, 54:102–108 106 Allergy and hypersensitivity recently demonstrated that the lung environment in neonatal mice is strongly type 2-skewed, with a gradual increase in IL-33 release by lung ECs, and with the spontaneous recruitment of several Th2-associated innate immune cells, peaking 2 weeks after birth [64 �� ]. This spontaneous wave of early type 2 immunity is likely to be caused by the mechanical stress induced by the breathing patterns [65], but also by the constant remodeling necessary to build up new lung structures. Interestingly, this period is prone to favor stronger Th2 sensitization to inhaled allergens [64 �� ], but also to favor lower immunity to bacteria [65]. Many environmental factors, like second hand smoking or RSV infection are known to facilitate Th2 sensitization in children. These triggers have in common that they induce high levels of IL-33 [66,67]. It is tempting to speculate that these risk factors act by prolonging or amplifying the epithelial cytokine response to allergens during early-life, and that combined early-life exposures thus define the final threshold for EC activation. Conclusion Effects of microbes on inducing Treg cells, Th1 cells and allergen cross-reactive antibody responses, are well- observed. In addition, we propose a model in which environmental and microbial stimuli are sensed and inte- grated by barrier tissues of the lung, the skin and the gut, resulting in a tonic DC activation status promoting either inflammatory or tolerogenic immunity. Together with direct effects on DCs and T cells, most protective stimuli thus seem to converge in the same central tolerogenic immune pathways. Fully understanding the fundamental immunological pathways underlying these protective triggers, their relative contribution, and how they interact, should hopefully allow us to pinpoint, modify or newly develop prophylactic or therapeutic therapies to cure asthma. Conflict of interest statement Nothing declared. Acknowledgements Research in the Lambrecht and Hammad lab is supported by an Advanced European Research Council (ERC) grant, an FWO Flanders Excellence of Science grant, and the World Without Asthma (AWWA) program of the Dutch Lung Foundation. Figure 1 includes derivative material from Servier Medical Art (https://smart.servier.com/) by Servier, available under a Creative Commons Attribution 3.0 Unported License (https:// creativecommons.org/licenses/by/3.0/). References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: � of special interest �� of outstanding interest 1. Strachan DP: Hay fever, hygiene, and household size. BMJ 1989, 299:1259-1260. Current Opinion in Immunology 2018, 54:102–108 2. Smits HH, Hiemstra PS, Prazeres da Costa C, Ege M, Edwards M, Garn H et al.: Microbes and asthma: opportunities for intervention. J Allergy Clin Immunol 2016, 137:690-697. 3. �� McSorley HJ, Blair NF, Smith KA, McKenzie AN, Maizels RM: Blockade of IL-33 release and suppression of type 2 innate lymphoid cell responses by helminth secreted products in airway allergy. Mucosal Immunol 2014, 7:1068-1078. Evidence of very early immunomodulation by a soluble parasite product. H. polygyrus excretory/secretory (HES) products were shown to block early Alternaria-induced IL-33 release by epithelial cells, and to strongly suppress the innate eosinophilia and ILC2 response. In an OVA asthma model using Alternaria as an adjuvant, HES also inhibited adaptive type 2 responses. The observation that parasites release products that block early epithelial IL-33 release, probably to inhibit the host’s anti-parasite immune responses, affirms the importance of this process in the devel- opment of type 2 immunity. 4. Feldman AS, He Y, Moore ML, Hershenson MB, Hartert TV: Toward primary prevention of asthma. Reviewing the evidence for early-life respiratory viral infections as modifiable risk factors to prevent childhood asthma. Am J Respir Crit Care Med 2015, 191:34-44. 5. Belkaid Y, Harrison OJ: Homeostatic Immunity and the Microbiota. 2017:562-576. 6. Rook GA: Hygiene hypothesis and autoimmune diseases. Clin Rev Allergy Immunol 2012, 42:5-15. 7. Hanski I, von Hertzen L, Fyhrquist N, Koskinen K, Torppa K, Laatikainen T et al.: Environmental biodiversity, human microbiota, and allergy are interrelated. Proc Natl Acad Sci U S A 2012, 109:8334-8339. 8. Abrahamsson TR, Jakobsson HE, Andersson AF, Bjorksten B, Engstrand L, Jenmalm MC: Low gut microbiota diversity in early infancy precedes asthma at school age. Clin Exp Allergy 2014, 44:842-850. 9. Clausen ML, Agner T, Lilje B, Edslev SM, Johannesen TB, Andersen PS: Association of disease severity with skin microbiome and filaggrin gene mutations in adult atopic dermatitis. JAMA Dermatol 2018, 154:293-300. 10. Savage JH, Lee-Sarwar KA, Sordillo J, Bunyavanich S, Zhou Y, O’Connor G et al.: A prospective microbiome-wide association study of food sensitization and food allergy in early childhood. Allergy: Eur J Allergy Clin Immunol 2018, 73:145-152. 11. Depner M, Ege MJ, Cox MJ, Dwyer S, Walker AW, Birzele LT et al.: Bacterial microbiota of the upper respiratory tract and childhood asthma. J Allergy Clin Immunol 2017, 139 826–834. e13. 12. Hasegawa K, Linnemann RW, Mansbach JM, Ajami NJ, Espinola JA, Fiechtner LG et al.: Household siblings and nasal and fecal microbiota in infants. Pediatr Int 2017, 59:473-481. 13. Tun HM, Konya T, Takaro TK, Brook JR, Chari R, Field CJ et al.: Exposure to household furry pets influences the gut microbiota of infant at 3–4 months following various birth scenarios. Microbiome 2017, 5:40. 14. Martin R, Makino H, Cetinyurek Yavuz A, Ben-Amor K, Roelofs M, Ishikawa E et al.: Early-life events, including mode of delivery and type of feeding, siblings and gender, shape the developing gut microbiota. PLoS ONE 2016, 11:e0158498. 15. Herbst T, Sichelstiel A, Schar C, Yadava K, Burki K, Cahenzli J et al.: Dysregulation of allergic airway inflammation in the absence of microbial colonization. Am J Respir Crit Care Med 2011, 184:198-205. 16. Atarashi K, Tanoue T, Oshima K, Suda W, Nagano Y, Nishikawa H et al.: Treg induction by a rationally selected mixture of Clostridia strains from the human microbiota. Nature 2013, 500:232-236. 17. Remot A, Descamps D, Noordine ML, Boukadiri A, Mathieu E, Robert V et al.: Bacteria isolated from lung modulate asthma susceptibility in mice. ISME J 2017, 11:1061-1074. 18. Lambrecht BN, Hammad H: The immunology of asthma. Nat Immunol 2015, 16:45-56. www.sciencedirect.com https://creativecommons.org/licenses/by/3.0/ https://creativecommons.org/licenses/by/3.0/ https://creativecommons.org/licenses/by/3.0/ http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0005 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0005 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0010 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0010 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0010 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0015 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0015 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0015 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0015 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0020 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0020 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0020 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0020 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0020 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0025 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0025 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0030 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0030 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0035 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0035 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0035 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0035 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0040 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0040 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0040 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0040 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0045 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0045 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0045 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0045 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0050 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0050 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0050 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0050 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0055 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0055 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0055 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0055 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0060 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0060 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0060 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0065 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0065 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0065 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0065 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0070 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0070 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0070 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0070 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0075 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0075 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0075 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0075 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0080 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0080 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0080 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0080 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0085 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0085 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0085 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0090 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0090 Hygiene hypothesis Haspeslagh et al. 107 19. Lambrecht BN: Dendritic cells and the regulation of the allergic immune response. Allergy 2005, 60:271-282. 20. Plantinga M, Guilliams M, Vanheerswynghels M, Deswarte K, Branco-Madeira F, Toussaint W et al.: Conventional and monocyte-derived CD11b(+) dendritic cells initiate and maintain T helper 2 cell-mediated immunity to house dust mite allergen. Immunity 2013, 38:322-335. 21. � Deckers J, Sichien D, Plantinga M, Van Moorleghem J, Vanheerswynghels M, Hoste E et al.: Epicutaneous sensitization to house dust mite allergen requires interferon regulatory factor 4-dependent dermal dendritic cells. J Allergy Clin Immunol 2017 http://dx.doi.org/10.1016/j.jaci.2016.12.970. This study reveals that unmanipulated skin is a possible entry route for HDM allergens, with IRF4-dependent skin cDC2s priming sensitization to subsequent airway allergen challenges. This implicates that perturbations or exposures at the level of the skin may be relevant for the ‘hygiene effect’ for asthma. 22. Halim TY, Steer CA, Matha L, Gold MJ, Martinez-Gonzalez I, McNagny KM et al.: Group 2 innate lymphoid cells are critical for the initiation of adaptive T helper 2 cell-mediated allergic lung inflammation. Immunity 2014, 40:425-435. 23. Amberbir A, Medhin G, Erku W, Alem A, Simms R, Robinson K et al.: Effects of Helicobacter pylori, geohelminth infection and selected commensal bacteria on the risk of allergic disease and sensitization in 3-year-old Ethiopian children. Clin Exp Allergy 2011, 41:1422-1430. 24. Engler DB, Reuter S, van Wijck Y, Urban S, Kyburz A, Maxeiner J et al.: Effective treatment of allergic airway inflammation with Helicobacter pylori immunomodulators requires BATF3- dependent dendritic cells and IL-10. Proc Natl Acad Sci U S A 2014, 111:11810-11815. 25. Van Wijck Y, De Kleijn S, John-Schuster G, Mertens TCJ, Hiemstra PS, Müller A et al.: Therapeutic application of an extract of therapeutic application of an extract of Helicobacter pylori ameliorates the development of allergic airway disease. J Immunol 2018, 200:1570-1579. 26. Stiehm M, Peters K, Wiesmuller KH, Bufe A, Peters M: A novel synthetic lipopeptide is allergy-protective by the induction of LPS-tolerance. Clin Exp Allergy 2013, 43:785-797. 27. � Stein K, Brand S, Jenckel A, Sigmund A, Chen ZJ, Kirschning CJ et al.: Endosomal recognition of Lactococcus lactis G121 and its RNA by dendritic cells is key to its allergy-protective effects. J Allergy Clin Immunol 2017, 139 667–678.e5. Study on direct protective DC modulation by the cowshedLactococcus lactis. Upon bacterial uptake, endosomal acidification, and TLR- mediated recognition of the released RNA, human moDCs or BMDCs secreted Th1 polarizing cytokines. In coculture, Lactococcus-pulsed DCs induced Th1 cells, and in vivo, they conferred protection against OVA- induced allergic asthma. 28. Trompette A, Gollwitzer ES, Yadava K, Sichelstiel AK, Sprenger N, Ngom-Bru C et al.: Gut microbiota metabolism of dietary fiber influences allergic airway disease and hematopoiesis. Nat Med 2014, 20:159-166. 29. Verheijden KA, Willemsen LE, Braber S, Leusink-Muis T, Jeurink PV, Garssen J et al.: The development of allergic inflammation in a murine house dust mite asthma model is suppressed by synbiotic mixtures of non-digestible oligosaccharides and Bifidobacterium breve M-16V. Eur J Nutr 2016, 55:1141-1151. 30. Thorburn AN, McKenzie CI, Shen S, Stanley D, Macia L, Mason LJ et al.: Evidence that asthma is a developmental origin disease influenced by maternal diet and bacterial metabolites. Nat Commun 2015, 6:7320. 31. Gollwitzer ES, Saglani S, Trompette A, Yadava K, Sherburn R, McCoy KD et al.: Lung microbiota promotes tolerance to allergens in neonates via PD-L1. Nat Med 2014, 20:642-647. 32. Oertli M, Sundquist M, Hitzler I, Engler DB, Arnold IC, Reuter S et al.: DC-derived IL-18 drives Treg differentiation, murine Helicobacter pylori-specific immune tolerance, and asthma protection. J Clin Invest 2012, 122:1082-1096. 33. � Shim JU, Lee SE, Hwang W, Lee C, Park JW, Sohn JH et al.: Flagellin suppresses experimental asthma by generating www.sciencedirect.com regulatory dendritic cells and T cells. J Allergy Clin Immunol 2016, 137:426-435. Use of microbe-derived flagellin B to suppress OVA-allergic or HDM- allergic asthma in a semi-therapeutic setting. The protection was depen- dent on TLR5 and on the generation of regulatory DCs, which could actively suppress asthma development upon adoptive transfer. Flagellin B treatment of PBMCs from HDM-allergic asthma patients rendered the DCs more prone to induce Tregs in an IL-10 dependent manner, making this microbe-derived product a possible candidate for cell-based asthma therapy. 34. Cruz AA, Cooper PJ, Figueiredo CA, Alcantara-Neves NM, Rodrigues LC, Barreto ML: Global issues in allergy and immunology: parasitic infections and allergy. J Allergy Clin Immunol 2017, 140:1217-1228. 35. Navarro S, Pickering DA, Ferreira IB, Jones L, Ryan S, Troy S et al.: Hookworm recombinant protein promotes regulatory T cell responses that suppress experimental asthma. Sci Transl Med 2016, 8 362ra. 36. Ziegler T, Rausch S, Steinfelder S, Klotz C, Hepworth MR, Kühl Aa et al.: A novel regulatory macrophage induced by a helminth molecule instructs IL-10 in CD4+ T cells and protects against mucosal inflammation. J Immunol 2015, 194:1555-1564. 37. � Machiels B, Dourcy M, Xiao X, Javaux J, Mesnil C, Sabatel C et al.: A gammaherpesvirus provides protection against allergic asthma by inducing the replacement of resident alveolar macrophages with regulatory monocytes. Nat Immunol 2017, 18:1310-1320. This mouse study demonstrates that infection with the murid herpesvirus MuHV-4 induces the replacement of resident alveolar macrophages (AMs) with monocytes from bone marrow origin, which persist long term in the lungs and are sufficient to block HDM-induced airway allergy. Preincubation of HDM-pulsed BMDCs with AMs from infected animals rendered the BMDCs less efficient in sensitizing acceptor mice to HDM- mediated asthma. 38. Hammad H, Lambrecht BN: Barrier epithelial cells and the control of type 2 immunity. Immunity 2015, 43:29-40. 39. Kuipers H, Heirman C, Hijdra D, Muskens F, Willart M, van Meirvenne S et al.: Dendritic cells retrovirally overexpressing IL-12 induce strong Th1 responses to inhaled antigen in the lung but fail to revert established Th2 sensitization. J Leukoc Biol 2004, 76:1028-1038. 40. Kuipers H, Lambrecht BN: The interplay of dendritic cells, Th2 cells and regulatory T cells in asthma. Curr Opin Immunol 2004, 16:702-708. 41. Hammad H, Chieppa M, Perros F, Willart MA, Germain RN, Lambrecht BN: House dust mite allergen induces asthma via Toll-like receptor 4 triggering of airway structural cells. Nat Med 2009, 15:410-416. 42. Trompette A, Divanovic S, Visintin A, Blanchard C, Hegde RS, Madan R et al.: Allergenicity resulting from functional mimicry of a Toll-like receptor complex protein. Nature 2009, 457:585- 588. 43. Choi HJ, Park SY, Cho JH, Park JW, Sohn JH, Kim YJ et al.: The TLR4-associated phospholipase D1 activation is crucial for der f 2-induced IL-13 production. Allergy Eur J Allergy Clin Immunol 2015, 70:1569-1579. 44. Weber J, Illi S, Nowak D, Schierl R, Holst O, von Mutius E et al.: Asthma and the hygiene hypothesis. Does cleanliness matter? Am J Respir Crit Care Med 2015, 191:522-529. 45. Braun-Fahrlander C, Riedler J, Herz U, Eder W, Waser M, Grize L et al.: Environmental exposure to endotoxin and its relation to asthma in school-age children. N Engl J Med 2002, 347:869- 877. 46. Kuipers H, Hijdra D, De Vries VC, Hammad H, Prins JB, Coyle AJ et al.: Lipopolysaccharide-induced suppression of airway Th2 responses does not require IL-12 production by dendritic cells. J Immunol 2003, 171:3645-3654. 47. Simpson A, John SL, Jury F, Niven R, Woodcock A, Ollier WE et al.: Endotoxin exposure, CD14, and allergic disease: an interaction between genes and the environment. Am J Respir Crit Care Med 2006, 174:386-392. Current Opinion in Immunology 2018, 54:102–108 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0095 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0095 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0100 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0100 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0100 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0100 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0100 http://dx.doi.org/10.1016/j.jaci.2016.12.970 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0110 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0110 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0110 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0110 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0115 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0115 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0115 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0115 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0115 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0120 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0120 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0120 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0120 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0120 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0125 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0125 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0125 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0125 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0125 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0130 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0130 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0130 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0135 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0135 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0135 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0135 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0140 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0140 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0140 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0140 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0145 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0145 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0145 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0145 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0145 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0145 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0150 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0150 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0150 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0150 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0155 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0155 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0155 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0160 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0160 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0160 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0160 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0165 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0165 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0165 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0165 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0170 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0170 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0170 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0170 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0175 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0175 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0175 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0175 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0180 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0180 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0180 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0180 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0180 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0185 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0185 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0185 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0185 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0185 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0190 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0190 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0195 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0195 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0195 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0195 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0195 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0200 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0200 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0200 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0205 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0205 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0205 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0205 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0210 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0210 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0210 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0210 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0215 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0215 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0215 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0215 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0220 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0220 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0220 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0225 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0225 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0225 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0225 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0230 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0230 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0230 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0230 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0235 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0235 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0235 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0235 108 Allergy and hypersensitivity 48. von Mutius E, Vercelli D: Farm living: effects on childhood asthma and allergy. Nat Rev Immunol 2010, 10:861-868. 49. �� Stein MM, Hrusch CL, Gozdz J, Igartua C, Pivniouk V, Murray SE et al.: Innate immunity and asthma risk in amish and hutterite farm children. N Engl J Med 2016, 375:411-421. This hallmark study compared Amish and Hutterite children, which share a similar genetic ancestry and lifestyle, but have different farming prac- tices. In the two groups of children, significant differences were found in the proportions, marker expression levels, and gene-expression profiles of peripheral-blood leukocytes. Importantly, Amish children had a decreased chance for allergy development, whereas median endotoxin content in their home dust samples was much higher. Only transfer of the Amish dust could inhibit HDM-induced asthma in a mouse model, and this was only the case in Myd88-sufficient and Trif-sufficient animals, consolidating the hypothesis that environmental innate immune signals shape asthma susceptibility. 50. Schuijs MJ, Willart MA, Vergote K, Gras D, Deswarte K, Ege MJ et al.: Farm dust and endotoxin protect against allergy through A20 induction in lung epithelial cells. Science 2015, 349:1106- 1110. 51. Wang J, Ouyang Y, Guner Y, Ford HR, Grishin AV: Ubiquitin- editing enzyme A20 promotes tolerance to lipopolysaccharide in enterocytes. J Immunol 2009, 183:1384-1392. 52. Kitagaki K, Businga TR, Racila D, Elliott DE, Weinstock JV, Kline JN: Intestinal helminths protect in a murine model of asthma. J Immunol 2006, 177:1628-1635. 53. �� Osbourn M, Soares DC, Vacca F, Cohen ES, Scott IC, Gregory WF et al.: HpARI Protein secreted by a helminth parasite suppresses interleukin-33. Immunity 2017, 47 739–751.e5. This paper provides a molecular mechanism of howH. polygyrus alarmin release inhibitor protein (HpARI) inhibits the biological activity of IL-33. HpARI binds to IL-33 and blocks its access to the IL-33R, while at the same time binding to DNA and inhibiting IL-33 release from necrotic cells as an alarmin. 54. Johnston CJC, Smyth DJ, Kodali RB, White MPJ, Harcus Y, Filbey KJ et al.: A structurally distinct TGF-beta mimic from an intestinal helminth parasite potently induces regulatory T cells. Nat Commun 2017, 8:1741. 55. Floudas A, Saunders SP, Moran T, Schwartz C, Hams E, Fitzgerald DC et al.: IL-17 receptor A maintains and protects the skin barrier to prevent allergic skin inflammation. J Immunol 2017, 199:707-717. 56. Kobayashi T, Glatz M, Horiuchi K, Kawasaki H, Akiyama H, Kaplan DH et al.: Dysbiosis and Staphyloccus aureus colonization drives inflammation in atopic dermatitis. Immunity 2015, 42:756-766. Current Opinion in Immunology 2018, 54:102–108 57. Volz T, Skabytska Y, Guenova E, Chen KM, Frick JS, Kirschning CJ et al.: Nonpathogenic bacteria alleviating atopic dermatitis inflammation induce IL-10-producing dendritic cells and regulatory Tr1 cells. J Investig Dermatol 2014, 134:96-104. 58. Naik S: Compartmentalized control of skin. Science 2012, 1115:1115-1120. 59. Naik S, Bouladoux N, Linehan JL, Han SJ, Harrison OJ, Wilhelm C et al.: Commensal–dendritic–cell interaction specifies a unique protective skin immune signature. Nature 2015, 520:104-108. 60. Scharschmidt TC, Vasquez KS, Truong HA, Gearty SV, Pauli ML, Nosbaum A et al.: A wave of regulatory T cells into neonatal skin mediates tolerance to commensal microbes. Immunity 2015, 43:1011-1021. 61. Takai T, Chen X, Xie Y, Vu AT, Le TA, Kinoshita H et al.: TSLP expression induced via Toll-like receptor pathways in human keratinocytes. Methods Enzymol 2014, 535:371-387. 62. Arrieta MC, Stiemsma LT, Dimitriu PA, Thorson L, Russell S, Yurist- Doutsch S et al.: Early infancy microbial and metabolic alterations affect risk of childhood asthma. Sci Transl Med 2015, 7:307ra. 63. Lambrecht BN, Hammad H: The immunology of the allergy epidemic and the hygiene hypothesis. Nat Immunol 2017, 18:1076-1083. 64. �� de Kleer IM, Kool M, de Bruijn MJ, Willart M, van Moorleghem J, Schuijs MJ et al.: Perinatal activation of the interleukin-33 pathway promotes type 2 Immunity in the developing lung. Immunity 2016, 45:1285-1298. Evidence for type 2 prone lung environment in neonatal mice and a decreased threshold in this period for innate immune responses to allergens. Developing lungs in the alveolarization phase spontaneously produced IL-33 and accumulated type 2 innate immune cells. Lung CD11b + cDCs in this period were scarce yet very efficient in presenting HDM allergen to T cells in draining lymph nodes and to promote Th2 polarization. 65. Saluzzo S, Gorki AD, Rana BM, Martins R, Scanlon S, Starkl P et al.: First-breath-induced type 2 pathways shape the lung immune environment. Cell Rep 2017, 18:1893-1905. 66. Lewis BW, Sultana R, Sharma R, Noël A, Langohr I, Patial S et al.: Early postnatal secondhand smoke exposure disrupts bacterial clearance and abolishes immune responses in muco-obstructive lung disease. J Immunol 2017, 199:1170- 1183. 67. Saravia J, You D, Shrestha B, Jaligama S, Siefker D, Lee GI et al.: Respiratory syncytial virus disease is mediated by age- variable IL-33. PLoS Pathog 2015, 11:e1005217. www.sciencedirect.com http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0240 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0240 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0245 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0245 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0245 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0250 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0250 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0250 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0250 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0255 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0255 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0255 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0260 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0260 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0260 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0265 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0265 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0265 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0270 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0270 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0270 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0270 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0275 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0275 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0275 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0275 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0280 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0280 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0280 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0280 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0285 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0285 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0285 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0285 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0290 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0290 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0295 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0295 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0295 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0300 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0300 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0300 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0300 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0305 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0305 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0305 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0310 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0310 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0310 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0310 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0315 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0315 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0315 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0320 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0320 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0320 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0320 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0325 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0325 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0325 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0330 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0330 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0330 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0330 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0330 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0335 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0335 http://refhub.elsevier.com/S0952-7915(18)30021-9/sbref0335 The hygiene hypothesis: immunological mechanisms of airway tolerance Introduction A modern lifestyle is associated with dysbiosis Microbes induce protective regulatory DCs and T cells Allergic asthma is initiated by aberrant immune responses at barrier tissues Conclusion Conflict of interest statement References and recommended reading Acknowledgements