CRS can be understood as a dysfunctional host-environment interaction at the nasal and sinus mucosa.1011 Bacterial and viral infections, fungal extracts, and protease allergens play potential roles in CRS as external stimuli.12 For the evidence of bacterial roles, the superantigen hypothesis has been strongly suggested. Colonized Staphylococcus aureus secretes enterotoxins that augment local T helper 2 (Th2) inflammation and may play a role in the pathogenesis of NP as a disease modifier rather than an initiator.13 Another hypothesis of bacterial roles is that bacterial biofilms may be involved in CRS pathogenesis. Biofilms are organized structures comprised of bacteria protectively encased within extracellular matrix (ECM), which is associated with local inflammatory markers and disease severity.14 Although many studies have attempted to find evidence, the role of fungi in CRS has drawn lots of controversies in the last decade.15 There is still relatively sparse evidence that viral infection is a chronic source of CRS or involved in the initiation and development of sinonasal allergic inflammation. However, ds-RNA, a toll-like receptor 3 (TLR3) ligand, is a potent inducer of thymic stromal lymphopoietin (TSLP)16 and a potential changer of innate lymphoid cell (ILC) phenotypes,17 which may be associated with chronicity of airway disease. Moreover, viral infection has been clearly implicated in the exacerbation of allergic airway diseases, such as asthma, in a number of studies.18

The epithelial barrier is the first line of defense; its breakdown can play a significant role in allowing external stimuli to enter nasal tissue and provoke immune responses.19 Functional and mechanical defects have been reported in NP. The protease-activated receptor (PAR) contributes to the production of cytokines and chemokines from the epithelium in response to external stimuli, such as bacteria, fungi, and allergens.1220 Epithelial barrier destroyed by protease activities enables allergens to pass physical epithelial barriers, culminating in allergen sensitization.21 It also signals epithelial cells to secrete innate cytokines and then facilitates the induction of eosinophilic inflammation. Epithelial-derived innate cytokines, such as interleukin (IL)-25, IL-33, and TSLP, may also participate in the evolution of NP.22 IL-33 is secreted by immune cells, such as macrophages and dendritic cells as well as epithelial cells.23 Full-length IL-33 is extracellularly released when epithelial cells undergo necrosis and necroptosis via tissue damage caused by external stimuli. Biologically active full-length IL-33 plays a role in mucosal inflammation recruiting neutrophils via chemokines, including chemokine (C-X-C motif) ligand (CXCL)-1 and CXCL-2.242526 Gordon et al.27 have suggested a potential role of IL-33 in eosinophilic inflammation by demonstrating a splice variant of IL-33 missing exons 3 and 4, which localizes to the cytoplasm of epithelial cells, is actively released and strongly related to Th2 inflammation, whereas full length is not. Several studies have sought to investigate the expression and role of IL-33 in CRS. There have been conflicting results of the expression of IL-33 in CRS. It has been reported that IL-33 mRNA is highly expressed in nasal mucosa but was not elevated in NP or other inflamed areas of the sinuses in CRSwNP.282930 A significant up-regulation of ST2 expression has been demonstrated in ethmoid mucosa from CRSwNP, but the concentration of IL-33 protein is not significantly different between nasal polyp and control tissue.31 Kim et al.23 have recently demonstrated that IL-33 is up-regulated in other CRS tissues compared to eosinophilic NP and correlates with Th1/Th17 cytokines. IL-33 may contribute to the induction of different types of inflammation under various microenvironments.

IL-17E, also known as IL-25, is released by Th2 cells, mast cells, eosinophils as well as epithelial cells. It is produced and stored in the cytoplasm of epithelial cells as a result of external stimuli, including allergen proteases.32 IL-25 transcript levels have been reported to increase in CRS tissues, including NP and to correlate with disease severity and blood eosinophilia,3033 whereas an earlier study has reported that IL-25 and GATA-binding protein 3 (GATA-3) transcripts were decreased in NP vs control tissues.29 Additionally, polyp-derived IL-17RB (+) Th2 cells were identified in NP, which co-expressed ST2 and enhanced IL-5 and IL-13 production in response to IL-25 and IL-33.34 Protein levels of IL-25 are up-regulated in non-eosinophilic NP as well as eosinophilic NP.2235 Of note, the fact that IL-25, known as a cytokine involved in diverse Th2-mediated diseases, also correlated with inflammatory mediators involved in Th1 and Th17 responses in Asian subjects suggests that it may play diverse roles in polypogenesis besides promoting Th2 inflammation.3336 Blockade of IL-25 reduced the burden of NP in a mouse model of NP and represented a potential novel therapeutic target.36

TSLP is well known to be induced in airway epithelial cells by viruses, TLR3 agonists, protease, and pro-inflammatory cytokines.16373839 IL-1β and tumor necrosis factor (TNF)-α regulate TSLP transcript expression in an nuclear factor-kappa B (NF-κB)-dependent manner.39 Several researchers have demonstrated that TSLP mRNA is overexpressed in eosinophilic NP and associated with Th2 inflammation.404142 TSLP induces the differentiation of naïve T cells into effector Th2 cells via enhancement of OX40 ligand (OX40L)-OX40 axis on the interaction between dendritic cells and CD4 T cells.43 TSLP protein is post-translationally modified by the endogenous protease. The cleaved TSLP shows higher activity, producing IL-5 when stimulated with IL-1β, than the full-length form.40 Of interest, Kim et al.44 authors recently demonstrated that TSLP production was induced by periostin in epithelial cells under the Th2 high inflammatory condition like eosinophilic NP. Until now, TSLP has been consistently reported to play a pathological role in eosinophilic NP unlike IL-25 and IL-33.

Epithelial-derived cytokines, such as IL-25, IL-33, and TSLP, exert effects on type 2 ILCs (ILC2s).45 ILCs are lymphocyte-like cells, but lack markers of mature lymphocytes, and do not express allergen-specific T cell receptors. ILC2s are regarded as innate counterparts of Th2 cells because both share the same functional module by their mutual production of signature cytokines, such as IL-5 and IL-13.46 For example, GATA-3 is a key transcriptional factor that plays parallel roles in the development and function of both Th2 cells and ILC2s.47 Moreover, the signal transducer and activator of transcription (STAT)-6 is also an important factor for Th2 polarization and plays a role in the post-developmental role in ILC2s, though it is not required for the development of ILC2s.46 Interestingly, IL-33- and IL-25-activated ILC2s can induce eosinophilic airway inflammation accompanied by airway hyperresponsiveness even in recombination-activating gene (Rag) knockout mice, which means ILC2s have function independent of acquired immunity.4849 ILC2s are abundant and also have a close relationship with higher tissue and blood eosinophilia in NP, clinically related to worsening nasal symptom scores and asthma comorbidity.5051 Bal et al.52 have reported that there is spatial co-localization between ILC2s and eosinophils in NP, and that co-culture of eosinophils and ILC2s augmented the activation of eosinophils and prolonged their survival, and in return pre-activated eosinophils enhanced IL-5 production of ILC2s in an IL-4 dependent manner. Of note, ILC2s have functional plasticity responsive to environmental cues, including viral infection. Mouse ILC2s in the lung undergo T-bet-mediated plasticity in response to infection, including influenza virus, respiratory syncytial virus, Haemophilus influenza, and Staphylococcal aureus.17 Human ILC2s can be converted into ILC1s by IL-12 and reversed by IL-4,52 or into interferon (IFN)-γ/IL-13 dual-producing ILC1s in response to both IL-1β and IL-12.53 T cells that are able to produce both IFN-γ and IL-13 induce enhanced airway hyperresponsiveness compared to conventional Th2 cells.54 Thus, ILC2 plasticity may contribute to disease heterogeneity which might lead to recalcitrance and exacerbation of inflammatory diseases.

Three major subsets of CD4+ T effectors, classified as Th1, Th2, and Th17, function in host defense in response to various types of pathogens and are involved in different types of tissue injury in immunologic diseases. Regulatory T cells (Tregs) modulate T cell response inducing the termination of inflammation. Definite characteristics of differentiated subsets of T effector cells are cytokines they produce and transcriptional factors they express. Signature cytokines produced by the major CD4+ T cell subsets are IFN-γ for Th1 cells; IL-4, IL-5, and IL-13 for Th2 cells; IL-17 and IL-22 for Th17 cells, and IL-10 and transforming growth factor-beta (TGF-β) for Treg. Involved transcriptional factors are T-bet for Th1, GATA-3 for Th2, retinoid-related orphan receptor C (RORC) for Th17, and FOXP3 for Treg. CRS was initially classified according to the presence of NP which reflects predominantly eosinophil-infiltrating Th2 inflammation at the molecular levels in Western countries.55 However, Asian populations, including Chinese population, showed multiple inflammatory cell infiltrates, including neutrophils and T cells classified as Th1/Th17 cells.56 Furthermore, mixed Th17/Th2 inflammatory patterns were also demonstrated throughout a single nasal polyp tissue, and eosinophilic NP also encompass neutrophils and their biological markers such as IL-8 and myeloperoxidase.2 Therefore, NP begin to be understood from the dichotomization of Th1 or Th2 to a multi-dimensionally evolved concept. A European multicenter case-control study group (GA²LEN Sinusitis Cohort group) recently conducted the clustering of CRS subjects by T cell subset-based cytokines and explored whether these cytokines adequately reflect phenotype, such as the presence of NP or asthma comorbidities.57 This study analyzed tissue cytokines from 173 CRS subjects and 89 controls in a phenotype-free manner and classified 10 clusters: 4 clusters with low or undetectable IL-5, immunoglobulin E (IgE), and eosinophilic cationic protein (ECP); and 6 clusters having moderate to high concentrations of these markers. In detail, cluster 1 has minimal inflammatory markers, cluster 2 associated with Th22, cluster 3 representing Th1, and cluster 4 showing mixed Th17/Th22/Th1. The latter 6 clusters with elevated IL-5 were classified into 2 groups: clusters 8-10 showed the highest IL-5 with enhanced IgE against Staphylococcal enterotoxins plus neutrophilic markers plus Th17/Th22 cytokines implying sophisticated pathomechanism, whereas clusters 5-7 showed mainly IL-5 dominant CRS. This clustering roughly reflects phenotypes: clusters 1-3 represent CRS without NP (CRSsNP) and low asthma comorbidity, but the proportion of NP and asthma comorbidities seems to increase from cluster 4 to cluster 10. Furthermore, it was reported to be a distinct regional difference in endotypes.2 CRSwNP tissues from patients of European countries, such as Benelux and Berlin, had a high proportion of IL-5 (62%-82%); those from Adelaide, Australia showed IL-5 in 46% and mixed IL-5 plus IL-17 or IFN-r in 27%. Most of the Western NP were Th2-biased. However, there was a remarkable difference even in Chinese NP. Beijing NP showed mixed IL-5/IL-17 portion of 42%; NP in Chengdu, a Chinese city, mainly had a negative cytokine pattern of 57%. These results imply that CRS is not a dichotomous disease but has multi-dimensional continuum worldwide. To make this concept clearer and more understandable, longitudinal cohort studies would be required rather than cross-sectional, case-control studies conducted in the past.

CRS is characterized by edematous remodeling patterns, basement membrane thickening, and goblet cell hyperplasia.5859 TGF-β is not only a chemoattractant for fibroblasts, enhancing their proliferation and collagen deposition, but also an inducer for Treg. High TGF-β is characteristic for CRSsNP, but low TGF-β was a signature finding in patients with CRSwNP.5860 Low TGF-β is also associated with a defect of Treg, namely, failing to maintain homeostasis, additionally, failure of deposition of collagens and other ECMs. High matrix metallopeptidases (MMPs), which are usually observed in NP, support ECM degradation.61 Besides the roles of TGF-β in the remodeling process in NP, the dysregulated coagulation system can be engaged in this process. Coagulation and fibrinolysis cascades are the biological processes involved in fibrin deposition and dissolution.62 The first study to suggest local activation of the coagulation system in sinus tissue showed that the concentration of thrombin and thrombin-antithrombin complex were increased in CRSwNP, which in turn participated in the remodeling process via a vascular endothelial growth factor pathway.63 Since that study, well-conducted reports have documented that the down-regulation of tissue plasminogen activator (t-PA) and activated coagulation factor XIIIa are involved in fibrin deposition and enhancement of fibrin crosslinking, culminating in edematous remodeling patterns in NP.636465 Of interest, the concentration of t-PA in CRS tissues shows a negative correlation with the concentration of ECP, a marker of eosinophils, and t-PA is mainly stained in epithelial cells on immunohistochemistry and down-regulated in epithelial cells by Th2 cytokines, such as IL-4 and IL-13.64 Furthermore, M2 macrophages, alternatively activated by Th2 cytokines,66 produce coagulation factor XIIIa which acts enzymatically and contribute to the formation of a tight tetrameric complex (FXIIIA2B2), a cross-linking process of fibrin.65 In summary, Th2-induced coagulopathy causes excessive fibrin deposition to induce edematous remodeling patterns in NP. The pathogenesis of NP described above is depicted in Figure.