Research Article

Pulmonary neuroendocrine cells amplify allergic asthma responses

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Science  08 Jun 2018:
Vol. 360, Issue 6393, eaan8546
DOI: 10.1126/science.aan8546

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Finding a role for PNECs in asthma

Pulmonary neuroendocrine cells (PNECs) are a rare cell type located in airway and alveolar epithelia and are often in contact with sensory nerve fibers. They have a wide phylogenic distribution and are found even in the relatively primitive lungs of amphibia and reptiles, suggesting a critical function. Sui et al. found that mice lacking PNECs have suppressed type 2 (allergic) immune responses. PNECs were observed in close proximity to group 2 innate lymphoid cells (ILC2s) around airway branch points. The PNECs enhanced ILC2 activity by secreting CGRP (calcitonin gene-related peptide). They also induced goblet-cell hyperplasia via the neurotransmitter GABA (γ-aminobutyric acid). Interestingly, human asthma patients were found to have increased PNEC numbers, suggesting a potential therapeutic target for the treatment of asthma.

Science, this issue p. eaan8546

Structured Abstract

INTRODUCTION

The lung, with its vast surface area, senses and responds to signals in inhaled air. Aberrant interactions between the lung and the environment underlie many diseases, including asthma. In vitro data show that pulmonary neuroendocrine cells (PNECs), a rare airway epithelial cell population, can act as chemosensors. Once stimulated in culture, they release dense core vesicles rich in neuropeptides, amines, and neurotransmitters. These bioactive molecules are capable of eliciting immune and physiological responses. A recent in vivo study by our group revealed that the proper development of PNECs into self-clustering units called neuroepithelial bodies is essential for restricting the number of immune cells in the naïve lung. However, whether PNECs can function in vivo to translate exogenous airway signals such as allergens into the cascade of downstream responses is unknown.

RATIONALE

To test the hypothesis that PNECs act as sensors in the lung, we generated mouse mutants that lack PNECs by inactivating Ascl1 in the airway epithelium—i.e., mutants that were depleted of PNECs starting at development. We exposed these mutants to either ovalbumin or house dust mites, following regimes of existing asthma models. We determined whether the mutants showed different asthmatic responses than controls. We elucidated the underlying mechanisms by identifying molecular effectors and cellular targets of PNECs. To complement the functional tests in mice, we investigated whether human asthma patients showed pathological changes in their PNECs.

RESULTS

Although normal at baseline, Ascl1-mutant mice exhibited severely reduced goblet cell hyperplasia and immune cell numbers compared with controls after allergen challenge. In investigating possible molecular effectors, we found that several PNEC products were decreased in mutants relative to controls after allergen challenge, including calcitonin gene-related peptide (CGRP) and γ-aminobutyric acid (GABA). In exploring possible cellular tar­gets, we found that innate lymphoid group 2 cells (ILC2s) were enriched at airway branch points, similar to PNECs. The PNEC product CGRP stimulated ILC2 production of interleukin-5 in culture. Conversely, inactivation of the CGRP recep­tor gene Calcrl in ILC2s led to dampened immune responses to allergens. In contrast to CGRP, GABA did not increase ILC2 cytokine secretion. Rather, inactivation of GABA biogenesis led to defective goblet cell hyperplasia after allergen challenge, suggesting that GABA is required for this response in the airway epithelium. The instillation of a mixture of CGRP and GABA in Ascl1 mutants restored both immune cell increases and goblet cell hyperplasia after allergen challenge, indicating that these products are the primary molecular effectors of PNECs in vivo. Consistent with these results from mice, we found increased PNEC numbers and cluster sizes in human asthma patients, which may underlie the heightened response to allergens in these individuals.

CONCLUSION

Our results demonstrate that PNECs, despite being a rare population of cells in the airway, are critical for amplifying the airway allergen signal into mucosal type 2 responses. Specifically, PNECs act through their product GABA to stimulate airway epithelial mucus production. In parallel, PNECs act through another product, CGRP, to stimulate ILC2 production of cytokines, which in turn recruit downstream immune cells. PNECs and ILC2s form neuroimmunological modules at the airway branch points, which are also the sites where airway particles are enriched. Our findings indicate that the PNEC-ILC2 axis functions to sense inhaled inputs, such as allergens, and amplify them into lung outputs, such as the allergic asthma response.

PNECs are preferentially localized at branch points.

A mouse airway stained by antibody against CGRP, to label PNECs (magenta) and antibody against SCGB1A1 to label club cells (green) (200× magnification). PNECs often cluster into neuroepithelial bodies and are preferentially localized at branch points.

Abstract

Pulmonary neuroendocrine cells (PNECs) are rare airway epithelial cells whose function is poorly understood. Here we show that Ascl1-mutant mice that have no PNECs exhibit severely blunted mucosal type 2 response in models of allergic asthma. PNECs reside in close proximity to group 2 innate lymphoid cells (ILC2s) near airway branch points. PNECs act through calcitonin gene-related peptide (CGRP) to stimulate ILC2s and elicit downstream immune responses. In addition, PNECs act through the neurotransmitter γ-aminobutyric acid (GABA) to induce goblet cell hyperplasia. The instillation of a mixture of CGRP and GABA in Ascl1-mutant airways restores both immune and goblet cell responses. In accordance, lungs from human asthmatics show increased PNECs. These findings demonstrate that the PNEC-ILC2 neuroimmunological modules function at airway branch points to amplify allergic asthma responses.

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