Introduction
The normal sinonasal epithelium comprises of ciliated cell, goblet cell,
club cell and basal cell1.
Tight
junctions (TJs), which is located at the most apical part of
intercellular junction between these epithelial cells, serves as a
physical barrier of nasal airway epithelium to protect it from the
external environment2. The main components of TJs are
occludin, claudins, junctional adhesion molecules and scaffold protein
zonula occludens (ZO). These TJ proteins are the key structural proteins
which maintain epithelial polarity, regulate pericellular permeability
and participate in epithelial cell proliferation, differentiation and
migration3,4. These functions play an important role
in distinct tissue compartmentalization and homeostasis of nasal
epithelium. Disruption of TJs may cause reduction in epithelial cohesion
and integrity which may lead to a variety of pathological conditions. It
has been demonstrated that TJ proteins are involved in the
pathophysiology of chronic airway inflammatory disorders including
chronic rhinosinusitis with nasal polyps (CRSwNP), allergic rhinitis and
asthma5-7.
IL-13 is the key regulator in type-2 mediated inflammation in CRSwNP.
IL-13 induces goblet cell hyperplasia, loss of cilia, inducible nitric
oxide syntheses production and fibrosis to accelerate inflammation and
promote remodeling8,9. The elevated levels of
pro-inflammatory cytokines contributing to diseases pathophysiology and
barrier dysfunctions have been implicated in a variety of
tissues9-11. Studies have shown that
IL-13
impaired barrier function by reducing expression of occludin, ZO-1 and
β-catenin in primary bronchial epithelial cells of asthmatic
patients12 and also disrupted intestinal barrier by
upregulation of claudin-213. Additionally, it was
found that with IL-4/IL-13 and IL-5 stimulation, E-cadherin and ZO-1
were downregulated in cultured epithelial cells of patients with
allergic rhinitis14. Hence, these studies further
support the hypothesis that change in expression and localization of TJs
accounts for epithelial barrier dysfunctions in chronic inflammatory
diseases.
In CRSwNP, persistent and prolonged mucosal inflammation is well
characterized and is closely related to elevated type-2 cytokines.
However, the role of these cytokines on impairing the nasal epithelial
barrier is still unknown. On the other hand, human nasal epithelial
cells (hNECs), being the primary entry point of most inhaled pathogens,
are the key players in regulating inflammatory responses and are an
important source of pro-inflammatory cytokines15.
Among respiratory viruses, rhinovirus (RV) is most commonly associated
with exacerbation of chronic airway disease16,17. It
was found that RV infection damaged TJs integrity in airway epithelial
cells of asthmatic patients by reducing ZO-1, occludin and claudin-1
protein expression7. In addition, while it is reported
that RV caused transient barrier disruption in a model of normal
air-liquid interface (ALI) differentiated airway epithelium, RV
infection at initial stage of differentiation in an injury model
prolonged barrier dysfunction by decreasing transepithelial electrical
resistance (TEER) and occludin level18. Furthermore,
recent study has shown that RV infection increased production of
pro-inflammatory mediators and contributed to the exaggerated
inflammatory response in in vitro cancer cell
line19. While RV is reported highly prevalent in
chronic rhinosinusitis patients20, the underlying
mechanism of the association of airway disease with chronic inflammation
and virus comorbidity is still poorly understood.
Our previously established in vitro IL-13-matured hNECs model
using IL-13 stimulation closely mimicked the physiological condition and
epithelium responses of in vivo nasal mucosa in
CRSwNP21. Using this model, we investigate the direct
effect of IL-13 on hNECs and TJ proteins expression in pseudostratified
layers to analyze the nasal epithelial barrier functions.
Moreover, to better define the
effects of respiratory viruses on TJs of inflammatory airway model, we
have also analyzed the nasal epithelial barrier integrity, remodeling
and immune responses of IL-13-treated hNECs
against RV acute infection.