Microbial Regulation of Enteric Eosinophils and Its Impact on Tissue Remodeling and Th2 Immunity.

Eosinophils have emerged as multifaceted cells that contribute to tissue homeostasis. However, the impact of the microbiota on their frequency and function at mucosal sites remains unclear. Here, we investigated the role of the microbiota in the regulation of enteric eosinophils. We found that small intestinal (SI) eosinophilia was significantly greater in germ-free (GF) mice compared to specific pathogen free (SPF) controls. This was associated with changes in the production of enteric signals that regulate eosinophil attraction and survival, and was fully reversed by complex colonization. Additionally, SI eosinophils of GF mice exhibited more cytoplasmic protrusions and less granule content than SPF controls. Lastly, we generated a novel strain of eosinophil-deficient GF mice. These mice displayed intestinal fibrosis and were less prone to allergic sensitization as compared to GF controls. Overall, our study demonstrates that commensal microbes regulate intestinal eosinophil frequency and function, which impacts tissue repair and allergic sensitization to food antigens. These data support a critical interplay between the commensal microbiota and intestinal eosinophils in shaping homeostatic, innate, and adaptive immune processes in health and disease.

Lifelong memory responses perpetuate humoral TH2 immunity and anaphylaxis in food allergy.

BACKGROUND: A number of food allergies (eg, fish, shellfish, and nuts) are lifelong, without any disease-transforming therapies, and unclear in their underlying immunology. Clinical manifestations of food allergy are largely mediated by IgE. Although persistent IgE titers have been attributed conventionally to long-lived IgE+ plasma cells (PCs), this has not been directly and comprehensively tested. OBJECTIVE: We sought to evaluate mechanisms underlying persistent IgE and allergic responses to food allergens. METHODS: We used a model of peanut allergy and anaphylaxis, various knockout mice, adoptive transfer experiments, and in vitro assays to identify mechanisms underlying persistent IgE humoral immunity over almost the entire lifespan of the mouse (18-20 months). RESULTS: Contrary to conventional paradigms, our data show that clinically relevant lifelong IgE titers are not sustained by long-lived IgE+ PCs. Instead, lifelong reactivity is conferred by allergen-specific long-lived memory B cells that replenish the IgE+ PC compartment. B-cell reactivation requires allergen re-exposure and IL-4 production by CD4 T cells. We define the half-lives of antigen-specific germinal centers (23.3 days), IgE+ and IgG1+ PCs (60 and 234.4 days, respectively), and clinically relevant cell-bound IgE (67.3 days). CONCLUSIONS: These findings can explain lifelong food allergies observed in human subjects as the consequence of allergen exposures that recurrently activate memory B cells and identify these as a therapeutic target with disease-transforming potential.

Indigenous enteric eosinophils control DCs to initiate a primary Th2 immune response in vivo.

Eosinophils natively inhabit the small intestine, but a functional role for them there has remained elusive. Here, we show that eosinophil-deficient mice were protected from induction of Th2-mediated peanut food allergy and anaphylaxis, and Th2 priming was restored by reconstitution with il4(+/+) or il4(-/-) eosinophils. Eosinophils controlled CD103(+) dendritic cell (DC) activation and migration from the intestine to draining lymph nodes, events necessary for Th2 priming. Eosinophil activation in vitro and in vivo led to degranulation of eosinophil peroxidase, a granule protein whose enzymatic activity promoted DC activation in mice and humans in vitro, and intestinal and extraintestinal mouse DC activation and mobilization to lymph nodes in vivo. Further, eosinophil peroxidase enhanced responses to ovalbumin seen after immunization. Thus, eosinophils can be critical contributors to the intestinal immune system, and granule-mediated shaping of DC responses can promote both intestinal and extraintestinal adaptive immunity.