Mucosal Immunol. 2016 Nov;9(6):1466-1476.

Mast cell-derived neurotrophin 4 mediates allergen-induced airway hyperinnervation in early life

Kruti R. Patel, Linh Aven, Fengzhi Shao, Nandini Krishnamoorthy, Melody G. Duvall, Bruce D. Levy, and Xingbin Ai

The Pulmonary Center, Department of Medicine, Boston University School of Medicine, Boston, Massachusetts, USA. Division of Critical Care Medicine, Department of Anesthesia, Perioperative and Pain Medicine, Boston Children’s Hospital, Boston, Massachusetts, USA. Division of Pulmonary and Critical Care Medicine, Brigham & Women’s Hospital, Harvard Medical School, 75 Francis Street, Boston, Massachusetts, USA.



Asthma often progresses from early episodes of insults. How early life events connect to long-term airway dysfunction remains poorly understood. We demonstrated previously that increased neurotrophin 4 (NT4) levels following early life allergen exposure cause persistent changes in airway smooth muscle (ASM) innervation and airway hyper-reactivity (AHR) in mice. Herein, we identify pulmonary mast cells as a key source of aberrant NT4 expression following early insults. NT4 is selectively expressed by ASM and mast cells in mice, nonhuman primates and humans. We show in mice that mast cell-derived NT4 is dispensable for ASM innervation during development. However, upon insults, mast cells expand in number and degranulate to release NT4 and thus become the major source of NT4 under pathological condition. Adoptive transfer of wild type mast cells, but not NT4 -/- mast cells restores ASM hyperinnervation and AHR in KitW-sh/W-sh mice following early life insults. Notably, an infant nonhuman primate model of asthma also exhibits ASM hyperinnervation associated with the expansion and degranulation of mast cells. Together, these findings identify an essential role of mast cells in mediating ASM hyperinnervation following early life insults by producing NT4. This role may be evolutionarily conserved in linking early insults to long-term airway dysfunction.



Childhood asthma is linked with exposure to pollutants, cigarette smoke, allergens or viral respiratory infection (1),(2),(3) . All of these factors may differentially determine the degree of atopy and wheeze later in life (4). Data from longitudinal birth cohort studies show that children who presented with recurrent wheeze in pre-school years continued to have reduced lung function even as adults with or without asthma symptoms (5),(6). This indicates that insults to a developing lung may cause permanent alterations in the lung thereby lowering lung function. Although childhood asthma is inflammation driven, structural changes occurring in the postnatal developing lung may contribute to the disease progression independent of inflammation. To target the inflammation there are several oral and inhaled corticosteroid treatment options available for asthmatics which help relieve the symptoms but are not effective in altering the progression of the disease. Thus, it is crucial to understand the underlying mechanisms involved in the pathophysiology of neonatal asthma for effective therapeutics.


The lung continues to develop post-birth and is considered fully mature in early adulthood. During embryogenesis, the formation of ASM is closely associated with outgrowth of a combination of nerves guided by the neuronal growth factors called neurotrophins. Neurotrophins form a chemical gradient thereby providing signals required for the survival, development and functions of the neurons (7). Based on our neonatal mouse model of ovalbumin (OVA) and cockroach allergen exposure, we discovered that early-life insults have persistent dysregulation in ASM tone due to aberrant innervation changes. However, the effects of increasing innervation are limited to the early postnatal period. Therefore, it is important to understand how these specific innervation changes are brought about in childhood and how are they associated with a sustained decline in lung function into adulthood.

In this study, we identify a critical role of mast cells in NT4 overproduction, ASM hyperinnervation, and AHR following early life insult in mice. This role is only evident and essential under pathological conditions. The proximity of mucosal mast cells and nerves is conserved through species (figure 1 and 2).



Figure 1 and 2: (1) Double staining for mast cells (red) and nerves (green) in mouse lungs at P21 using a tryptase antibody and TuJ1 antibody. Scale bar, 50 mm. (2) Toluidine blue staining for mast cells in 6 month old primate lungs.


Together, building upon our findings in both mice and nonhuman primates, we propose a model for ASM hyperinnervation following early life insults. In our model, NT4 from ASM serves as an essential trophic factor for innervating nerves that express the NT4 receptor TrkB, thereby establishing ASM innervation during normal development (Figure 3). Following allergen exposure, NT4 expression by ASM is unchanged. However, mast cells increase in number and degranulate to release NT4, thereby becoming a key source of aberrant NT4 expression that in turn causes ASM hyperinnervation and AHR (Figure 3). Without mast cells, such as in KitW-sh/W-sh mice, the only cellular source of NT4 in the lung is ASM. As a result, allergen exposure has no effect on ASM innervation and fails to elicit AHR (Figure 3). Notably, NT4 expression by ASM and pulmonary mast cells is conserved between mice, nonhuman primates and humans. In addition, the expansion of the mast cell pool and degranulation similarly occur in rodents, nonhuman primates and humans in response to a variety of risk factors for asthma. Therefore, mast cells may play a conserved role in ASM hyperinnervation in the infant nonhuman primate model of asthma and thus may contribute to pathogenesis of asthma in human.



Figure 3 :A model of pulmonary mast cells as a key source of elevated NT4 for early-life allergen-induced neuroplasticity. Allergen exposure increases the number of mast cells and triggers degranulation to release NT4, thereby increasing NT4-dependent ASM innervation. This in turn leads to AHR. Without mast cells in the lung, early-life allergen-induced neuroplasticity no longer happens. As a result, there is a lack of AHR in KitW-sh/W-sh mice after early-life allergen exposure.


Combining our findings from previous and current studies, mast cell degranulation and NT4 release serve as upstream events that ultimately trigger long-lasting changes in ASM innervation and function following early life insults. These findings suggest that blockade of mast cell degranulation may be a preventative strategy for young children at high risk of asthma. This study shows a unique mechanism of crosstalk between mast cells, deregulated cholinergic nerves and ASM in early-life. Our findings suggest that blockade of mast cell degranulation may be a preventative strategy for young children at high risk of asthma.



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