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Rationale: Respiratory syncytial virus (RSV) bronchiolitis causes significant infant mortality. Bronchiolitis is characterized by airway epithelial cell (AEC) death; however, the mode of death remains unknown. Objectives: To determine whether necroptosis contributes to RSV b r onchiolitis pathogenesis via HMGB1 (high mobility group box 1) release. Methods: Nasopharyngeal samples were collected from children presenting to the hospital with acute respiratory infection. Primary human AECs and neonatal mice were inoculated with RSV and murine Pneumovirus, respectively. Necroptosis was determined via viability assays and immunohistochemistry for RIPK1 (receptor-interacting protein kinase-1), MLKL (mixed lineage kinase domain-like pseudokinase) protein, and caspase-3. Necroptosis was blocked using pharmacological inhibitors and RIPK1 kinase-dead knockin mice. Measurements and Main Results: HMGB1 levels were elevated in nasopharyngeal samples of children with acute RSV infection. RSV-induced epithelial cell death was associated with increased phosphorylated RIPK1 and phosphorylated MLKL but not active caspase-3 expression. Inhibition of RIPK1 or MLKL attenuated RSV-induced HMGBI translocation and release, and lowered viral load. MLKL inhibition increased active caspase-3 expression in a caspase-8/9-dependent manner. In susceptible mice, Pneumovirus infection upregulated RIPK1 and MLKL expression in the airway epithelium at 8 to 10 days after infection, coinciding with AEC sloughing, HMGB1 release, and neutrophilic inflammation. Genetic or pharmacological inhibition of RIPK1 or MLKL attenuated these pathologies, lowered viral load, and prevented type 2 inflammation and airway remodeling. Necroptosis inhibition in early life ameliorated asthma progression induced by viral or allergen challenge in later life. Conclusions: Pneumovirus infection induces AEC necroptosis. Inhibition of necroptosis may be a viable strategy to limit the severity of viral bronchiolitis and break its nexus with asthma.

Type-2 immunity elicits tissue repair and homeostasis, however dysregulated type-2 responses cause aberrant tissue remodelling, as observed in asthma. Severe respiratory viral infections in infancy predispose to later asthma, however, the processes that mediate tissue damage-induced type-2 inflammation and the origins of airway remodelling remain ill-defined. Here, using a preclinical mouse model of viral bronchiolitis, we find that increased epithelial and mesenchymal high-mobility group box 1 (HMGB1) expression is associated with increased numbers of IL-13-producing type-2 innate lymphoid cell (ILC2s) and the expansion of the airway smooth muscle (ASM) layer. Anti-HMGB1 ablated lung ILC2 numbers and ASM growth in vivo, and inhibited ILC2-mediated ASM cell proliferation in a co-culture model. Furthermore, we identified that HMGB1/RAGE (receptor for advanced glycation endproducts) signalling mediates an ILC2-intrinsic IL-13 auto-amplification loop. In summary, therapeutic targeting of the HMGB1/RAGE signalling axis may act as a novel asthma preventative by dampening ILC2-mediated type-2 inflammation and associated ASM remodelling., Author summary Asthma can start at any time in life, although most often begins in early childhood. Wheezy viral bronchiolitis is a major independent risk factor for subsequent asthma. However, key knowledge gaps exist in relation to the sequelae of severe viral bronchiolitis and the pathogenic processes that promote type-2 inflammation and airway wall remodelling, cardinal features of asthma. Our study addresses this gap by identifying high-mobility group box 1 as a pathogenic cytokine that contributes to group 2 innate lymphoid cell-induced airway smooth muscle growth.

Lynch et al. provide evidence of a causal relationship between RSV-bronchiolitis and asthma development and highlight a common but age-related Sema4a-mediated pathway by which pDCs and microbial colonization induce T reg cell expansion to confer protection against severe bronchiolitis and asthma., Respiratory syncytial virus–bronchiolitis is a major independent risk factor for subsequent asthma, but the causal mechanisms remain obscure. We identified that transient plasmacytoid dendritic cell (pDC) depletion during primary Pneumovirus infection alone predisposed to severe bronchiolitis in early life and subsequent asthma in later life after reinfection. pDC depletion ablated interferon production and increased viral load; however, the heightened immunopathology and susceptibility to subsequent asthma stemmed from a failure to expand functional neuropilin-1+ regulatory T (T reg) cells in the absence of pDC-derived semaphorin 4a (Sema4a). In adult mice, pDC depletion predisposed to severe bronchiolitis only after antibiotic treatment. Consistent with a protective role for the microbiome, treatment of pDC-depleted neonates with the microbial-derived metabolite propionate promoted Sema4a-dependent T reg cell expansion, ameliorating both diseases. In children with viral bronchiolitis, nasal propionate levels were decreased and correlated with an IL-6high/IL-10low microenvironment. We highlight a common but age-related Sema4a-mediated pathway by which pDCs and microbial colonization induce T reg cell expansion to protect against severe bronchiolitis and subsequent asthma.

Background A variant in the IL-6 receptor (IL-6R) gene increases asthma risk and is predicted to decrease IL-6 classic signaling and increase IL-6 trans-signaling. This suggests that inhibition of IL-6 trans-signaling, but not classic signaling, might suppress allergic airway inflammation. Objectives We sought to determine whether IL-6 signaling contributes to (1) acute experimental asthma induced by clinically relevant allergens and (2) variation in asthma clinical phenotypes in asthmatic patients. Methods Mice were sensitized to house dust mite (HDM) or cockroach at day 0, treated with IL-6R inhibitors at day 13, and challenged with the same allergen at days 14 to 17. End points were measured 3 hours after the final challenge. IL-6 and soluble IL-6 receptor (sIL-6R) expression in induced sputum of asthmatic patients was correlated with asthma clinical phenotypes. Results Both HDM and cockroach induced a type 2/type 17 cytokine profile and mixed granulocytic inflammation in the airways. Both allergens increased IL-6 expression in the airways, but only cockroach induced sIL-6R expression. Therefore HDM challenge promoted IL-6 classic signaling but not trans-signaling; in this model treatment with anti–IL-6R did not suppress airway inflammation. In contrast, cockroach-induced inflammation involved activation of IL-6 trans-signaling and production of IL-17A by γδ T cells. Anti–IL-6R, selective blockade of sIL-6R, or γδ T-cell deficiency significantly attenuated cockroach-induced inflammation. Asthmatic patients with high airway IL-6 and sIL-6R levels were enriched for the neutrophilic and mixed granulocytic subtypes. Conclusion Experimental asthma associated with both high IL-6 and high sIL-6R levels in the airways is attenuated by treatment with IL-6R inhibitors.

Respiratory syncytial virus (RSV)-bronchiolitis is a major cause of infant morbidity and mortality and a risk factor for subsequent asthma. We showed previously that toll-like receptor (TLR)7 in plasmacytoid dendritic cells (pDCs) is critical for protection against bronchiolitis and asthma in mice infected with pneumonia virus of mice (PVM), the mouse homolog of RSV. This lack of redundancy was unexpected as interferon-β promotor stimulator-1 (IPS-1) signalling, downstream of RIG-I-like receptor (RLR) and not TLR7 activation, contributes to host defence in hRSV-inoculated adult mice. To further clarify the role of IPS-1 signalling, we inoculated IPS-1−/− and WT mice with PVM in early-life, and again in later-life, to model the association between bronchiolitis and asthma. IPS-1 deficiency predisposed to severe PVM bronchiolitis, characterised by neutrophilic inflammation and necroptotic airway epithelial cell death, high mobility group box 1 (HMGB1) and IL-33 release, and downstream type-2 inflammation. Secondary infection induced an eosinophilic asthma-like pathophysiology in IPS-1−/− but not WT mice. Mechanistically, we identified that IPS-1 is necessary for pDC recruitment, IFN-α production and viral control. Our findings suggest that TLR7 and RLR signalling work collaboratively to optimally control the host response to pneumovirus infection thereby protecting against viral bronchiolitis and subsequent asthma.

The onset, progression and exacerbations of asthma are frequently associated with viral infections of the lower respiratory tract. An emerging paradigm suggests that this relationship may be underpinned by a defect in the host's antiviral response, typified by the impaired production of type I and type III interferons (IFNs). The failure to control viral burden probably causes damage to the lung architecture and contributes to an aberrant immune response, which together compromise lung function. Although a relatively rare cell type, the plasmacytoid dendritic cell dedicates much of its transcriptome to the synthesis of IFNs and is pre-armed with virus-sensing pattern recognition receptors. Thus, plasmacytoid dendritic cells are specialised to ensure early viral detection and the rapid induction of the antiviral state to block viral replication and spread. In addition, plasmacytoid dendritic cells can limit immunopathology, and promote peripheral tolerance to prevent allergic sensitisation to harmless antigens, possibly through the induction of regulatory T-cells. Thus, this enigmatic cell may lie at an important intersection, orchestrating the immediate phase of antiviral immunity to effect viral clearance while regulating tolerance. Here, we review the evidence to support the hypothesis that a primary defect in plasmacytoid dendritic function may underlie the development of asthma.

© 2017, eLife Sciences Publications Ltd. All rights reserved. Asthma is a chronic inflammatory disease. Although many patients with asthma develop type-2 dominated eosinophilic inflammation, a number of individuals develop paucigranulocytic asthma, which occurs in the absence of eosinophilia or neutrophilia. The aetiology of paucigranulocytic asthma is unknown. However, both respiratory syncytial virus (RSV) infection and mutations in the receptor for advanced glycation endproducts (RAGE) are risk factors for asthma development. Here, we show that RAGE deficiency impairs anti-viral immunity during an early-life infection with pneumonia virus of mice (PVM; a murine analogue of RSV). The elevated viral load was associated with the release of high mobility group box-1 (HMGB1) which triggered airway smooth muscle remodelling in early-life. Re-infection with PVM in later-life induced many of the cardinal features of asthma in the absence of eosinophilic or neutrophilic inflammation. Anti-HMGB1 mitigated both early-life viral disease and asthma-like features, highlighting HMGB1 as a possible novel therapeutic target.

Background Respiratory tract viruses are a major environmental risk factor for both the inception and exacerbations of asthma. Genetic defects in Toll-like receptor (TLR) 7–mediated signaling, impaired type I interferon responses, or both have been reported in asthmatic patients, although their contribution to the onset and exacerbation of asthma remains poorly understood. Objective We sought to determine whether Pneumovirus infection in the absence of TLR7 predisposes to bronchiolitis and the inception of asthma. Methods Wild-type and TLR7 -deficient ( TLR7 −/− ) mice were inoculated with the rodent-specific pathogen pneumonia virus of mice at 1 (primary), 7 (secondary), and 13 (tertiary) weeks of age, and pathologic features of bronchiolitis or asthma were assessed. In some experiments infected mice were exposed to low-dose cockroach antigen. Results TLR7 deficiency increased viral load in the airway epithelium, which became sloughed and necrotic, and promoted an IFN-α/β low , IL-12p70 low , IL-1β high , IL-25 high , and IL-33 high cytokine microenvironment that was associated with the recruitment of type 2 innate lymphoid cells/nuocytes and increased T H 2-type cytokine production. Viral challenge of TLR7 −/− mice induced all of the cardinal pathophysiologic features of asthma, including tissue eosinophilia, mast cell hyperplasia, IgE production, airway smooth muscle alterations, and airways hyperreactivity in a memory CD4 + T cell–dependent manner. Importantly, infections with pneumonia virus of mice promoted allergic sensitization to inhaled cockroach antigen in the absence but not the presence of TLR7. Conclusion TLR7 gene defects and Pneumovirus infection interact to establish an aberrant adaptive response that might underlie virus-induced asthma exacerbations in later life.