Resident intestinal eosinophils constitutively express antigen presentation markers and include two phenotypically distinct subsets of eosinophils.

Intestinal eosinophils are implicated in homeostatic and disease-associated processes, yet the phenotype of intestinal tissue-dwelling eosinophils is poorly defined and their roles in intestinal health or disease remain enigmatic. Here we probed the phenotype and localization of eosinophils constitutively homed to the small intestine of naive mice at baseline, and of antigen-sensitized mice following intestinal challenge. Eosinophils homed to the intestinal lamina propria of naive mice were phenotypically distinguished from autologous blood eosinophils, and constitutively expressed antigen-presenting cell markers, suggesting that intestinal eosinophils, unlike blood eosinophils, may be primed for antigen presentation. We further identified a previously unrecognized resident population of CD11chi eosinophils that are recovered with intraepithelial leucocytes, and that are phenotypically distinct from both lamina propria and blood eosinophils. To better visualize intestinal eosinophils in situ, we generated eosinophil reporter mice wherein green fluorescent protein expression is targeted to both granule-delimiting and plasma membranes. Analyses of deconvolved fluorescent z-section image stacks of intestinal tissue sections from eosinophil reporter mice revealed eosinophils within intestinal villi exhibited dendritic morphologies with cellular extensions that often contacted the basement membrane. Using an in vivo model of antigen acquisition in antigen-sensitized mice, we demonstrate that both lamina propria-associated and intraepithelium-associated eosinophils encounter, and are competent to acquire, lumen-derived antigen. Taken together these data provide new foundational insights into the organization and functional potential of intestinal tissue-dwelling eosinophils, including the recognition of different subsets of resident intestinal eosinophils, and constitutive expression of antigen-presenting cell markers.

Humoral Immunity Provides Resident Intestinal Eosinophils Access to Luminal Antigen via Eosinophil-Expressed Low-Affinity Fcγ Receptors.

Eosinophils are native to the healthy gastrointestinal tract and are associated with inflammatory diseases likely triggered by exposure to food allergens (e.g., food allergies and eosinophilic gastrointestinal disorders). In models of allergic respiratory diseases and in vitro studies, direct Ag engagement elicits eosinophil effector functions, including degranulation and Ag presentation. However, it was not known whether intestinal tissue eosinophils that are separated from luminal food Ags by a columnar epithelium might similarly engage food Ags. Using an intestinal ligated loop model in mice, in this study we determined that resident intestinal eosinophils acquire Ag from the lumen of Ag-sensitized but not naive mice in vivo. Ag acquisition was Ig-dependent; intestinal eosinophils were unable to acquire Ag in sensitized Ig-deficient mice, and passive immunization with immune serum or Ag-specific IgG was sufficient to enable intestinal eosinophils in otherwise naive mice to acquire Ag in vivo. Intestinal eosinophils expressed low-affinity IgG receptors, and the activating receptor FcγRIII was necessary for Ig-mediated acquisition of Ags by isolated intestinal eosinophils in vitro. Our combined data suggest that intestinal eosinophils acquire lumen-derived food Ags in sensitized mice via FcγRIII Ag focusing and that they may therefore participate in Ag-driven secondary immune responses to oral Ags.

A role for foxd3 and sox10 in the differentiation of gonadotropin-releasing hormone (GnRH) cells in the zebrafish Danio rerio.

Gonadotropin-releasing hormone (GnRH) is found in a wide range of vertebrate tissues, including the nervous system. In general, GnRH has two functions: endocrine, acting as a releasing hormone; and neuromodulatory, affecting neural activity in the peripheral and central nervous system. The best understood population of GnRH cells is that of the hypothalamus, which is essential for reproduction. Less well understood are the populations of GnRH cells found in the terminal nerve and midbrain, which appear to be neuromodulatory in function. The GnRH-containing cells of the midbrain are proposed to arise from the mesencephalic region of the neural tube. Previously, we showed that neuromodulatory GnRH cells of the terminal nerve arise from cranial neural crest. To test the hypothesis that neuromodulatory GnRH cells of the midbrain also arise from neural crest, we used gene knockdown experiments in zebrafish to disrupt neural crest development. We demonstrate that decrement of the function of foxd3 and/or sox10, two genes important for the development and specification of neural crest, resulted in a reduction and/or loss of GnRH cells of the midbrain, as well as a reduction in the number of terminal nerve GnRH cells. Therefore, our data support a neural crest origin for midbrain GnRH cells. Additionally, we demonstrate that knockdown of kallmann gene function resulted in the loss of endocrine GnRH cells of the hypothalamus, but not of neuromodulatory GnRH cells of the midbrain and terminal nerve, thus providing additional evidence for separate pathways controlling the development of neuromodulatory and endocrine GnRH cells.