Allergin-1 on mast cells suppresses house dust mite-induced airway hyperresponsiveness in mice

Differential isoform expression of Allergin-1 during acute and chronic inflammation

INTRODUCTION

Neutrophils are crucial to antimicrobial defense, but excessive neutrophilic inflammation elicits immune pathology. Currently, no effective treatment exists to curb neutrophil activation. However, neutrophils express a variety of inhibitory receptors which may represent potential therapeutic targets to limit neutrophilic inflammation. Indeed, we previously showed that the inhibitory collagen receptor leukocyte-associated immunoglobulin-like receptor 1 (LAIR-1) regulates neutrophilic airway inflammation and inhibits neutrophil extracellular trap formation. The inhibitory receptor Allergin-1 is expressed by myeloid cells and B cells. Allergin-1 suppresses mast cell and basophil activation, but a potential regulatory role on neutrophils remains unexplored. We aimed to demonstrate the regulation of neutrophils by Allergin-1.

METHODS

We examine Allergin-1 isoform expression on human neutrophils during homeostatic (healthy donors) and chronic inflammatory (systemic lupus erythematosus patients) conditions in comparison to other circulating leukocytes by flow cytometry. To reveal a potential role for Allergin-1 in regulating neutrophilic inflammation, we experimentally infect wild-type (WT) and Allergin-1-deficient mice with a respiratory syncytial virus (RSV) and monitor disease severity and examine cellular airway infiltrate. Flow cytometry was used to confirm Allergin-1 expression by airway-infiltrated neutrophils in RSV infection-induced bronchiolitis patients.

RESULTS

Only the short 1 (S1) isoform, but not the long (L) or S2 isoform could be detected on blood leukocytes, with the exception of nonclassical monocytes, which exclusively express the S2 isoform. Allergin-1 expression levels did not vary significantly between healthy individuals and patients with the systemic inflammatory disease on any interrogated cell type. Airway-infiltrated neutrophils of pediatric RSV bronchiolitis patients were found to express Allergin-1S1. However, Allergin-1-deficient mice experimentally infected with RSV did not show exacerbated disease or increased neutrophil airway infiltration compared to WT littermates.

CONCLUSION

Allergin-1 isoform expression is unaffected by chronic inflammatory conditions. In stark contrast to fellow inhibitory receptor LAIR-1, Allergin-1 does not regulate neutrophilic inflammation in a mouse model of RSV bronchiolitis.

Immunoglobulin E-Dependent Activation of Immune Cells in Rhinovirus-Induced Asthma Exacerbation

Acute exacerbation is the major cause of asthma morbidity, mortality, and health-care costs. Respiratory viral infections, particularly rhinovirus (RV) infections, are associated with the majority of asthma exacerbations. The risk for bronchoconstriction with RV is associated with allergic sensitization and type 2 airway inflammation. The efficacy of the humanized anti-IgE monoclonal antibody omalizumab in treating asthma and reducing the frequency and severity of RV-induced asthma exacerbation is well-known. Despite these clinical data, mechanistic details of omalizumab's effects on RV-induced asthma exacerbation have not been well-defined for years due to the lack of appropriate animal models. In this Perspective, we discuss potential IgE-dependent roles of mast cells and dendritic cells in asthma exacerbations.

An inhibitory immunoreceptor, Allergin-1, suppresses FITC-induced type 2 contact hypersensitivity

Although allergic contact dermatitis (ACD) is the most common T cell-mediated inflammatory responses against an allergen in the skin, the pathogenesis of ACD remains incompletely understood. In the sensitization phase in ACD, hapten-bearing dermal dendritic cells (DCs) play a pivotal role in the transport of an antigen to the lymph nodes (LNs), where they present the antigen to naïve T cells. Here we report that Allergin-1, an inhibitory immunoreceptor containing immunoreceptor tyrosine-based inhibitory motif (ITIM) in the cytoplasmic region, is highly expressed on dermal DCs. Mice deficient in Allergin-1 exhibited exacerbated fluorescein isothiocyanate (FITC)-induced type 2 contact hypersensitivity (CHS) such as ear swelling and skin eosinophilia. Allergin-1-deficient mice also showed larger numbers of CD4⁺ T cells and FITC-bearing DCs and greater expressions of type 2 cytokines, including IL-5, IL-10 and IL-13, in the draining LNs than did wild type mice. In sharp contrast, Allergin-1-deficient mice showed comparable level of type 1 CHS induced by 2,4-dinitrofluorobenzene (DNFB). These results suggest that Allergin-1 on dermal DC inhibits type 2, but not type 1, immune responses in the sensitization phase of CHS.

Allergin-1 Immunoreceptor Suppresses House Dust Mite-Induced Allergic Airway Inflammation

House dust mite (HDM) allergens are leading causes of allergic asthma characterized by Th2 responses. The lung-resident CD11b⁺ dendritic cells (DCs) play a key role in Th2 cell development in HDM-induced allergic asthma. However, the regulatory mechanism of HDM-induced CD11b⁺ DC activation remains incompletely understood. In this study, we demonstrate that mice deficient in an inhibitory immunoreceptor, Allergin-1, showed exacerbated HDM-induced airway eosinophilia and serum IgE elevation. By using bone marrow-chimeric mice that were sensitized with adoptively transferred HDM-stimulated wild-type or Allergin-1-deficient CD11b⁺ bone marrow-derived cultured DCs (BMDCs), followed by challenge with HDM, we show that Allergin-1 on the BMDCs suppressed HDM-induced allergic airway inflammation. We also show that Allergin-1 suppressed HDM-induced PGE₂ production from CD11b⁺ BMDCs by inhibiting Syk tyrosine kinase activation through recruitment of SHP-1, subsequently leading to negative regulation of Th2 responses. These results suggest that Allergin-1 plays an important role in regulation of HDM-induced allergic airway inflammation.

Selective suppression of oral allergen-induced anaphylaxis by Allergin-1 on basophils in mice

Mast cells (MCs) play a critical role in oral allergen-induced anaphylaxis. However, the contribution of basophils to the anaphylaxis remains unclear. The inhibitory immunoreceptor Allergin-1 is highly expressed on MCs and basophils and inhibits FcεRI-mediated signaling in MCs. Here, we show that Allergin-1-deficient (Milr1−/−) mice developed more severe hypothermia, a higher mortality rate and a greater incidence of diarrhea than did wild-type (WT) mice in an oral ovalbumin (OVA)-induced food allergy model. MC-deficient Mas–TRECK mice, which had been reconstituted with either WT or Milr1−/− bone marrow-derived cultured MCs, did not develop hypothermia in this food allergy model. On the other hand, depletion of basophils by injection of anti-CD200R3 antibody rescued Milr1−/− mice from lethal hypothermia but not from diarrhea. In vitro analyses demonstrated that Allergin-1 inhibits IgE-dependent activation of both human and mouse basophils. Thus, Allergin-1 on basophils selectively suppresses oral allergen-induced anaphylaxis.