Underlying immunological mechanisms
Pathology of CRSwNP has been well studied and tissue remodeling, epithelial dysfunction, activation of innate and adaptive inflammatory responses and fibrin deposition seem to be common features in CRSwNP45,46. CRSwNP is frequently divided into two key endotypes, eosinophilic CRSwNP (ECRSwNP) and non-eosinophilic CRSwNP (NECRSwNP) based on the presence and absence of large numbers of eosinophils in nasal polyp (NP) tissue. Analysis of gene expression in CRS tissues ultimately shifted focus to molecules produced by T lymphocytes of types 1, 2 and 3 (also known as 17) that produce primary cytokines that drive the inflammatory patterns observed in tissues (referred to here as T1, T2 and T3 and Tun, signifying “untypeable”).Figure 1 shows microarray data from Chicago using samples from CRS patients illustrating the stark contrast between, and the strong similarity among, patients in the distinct endotypes, and Figure 2 shows a summary/overview of the driving cytokine, source cells and effector cells in the tissues. ECRSwNP is characterized by presence of type 2 (T2) cytokines (IL-4, IL-5 and IL-13) and accumulation of type 2 immune cells including mast cells, basophils, CD4 T helper 2 (Th2) cells, group 2 innate lymphoid cells (ILC2), B cells, M2 macrophages and dendritic cells in addition to eosinophils. For this reason, ECRSwNP is also called T2 CRSwNP 45-55. NECRSwNP can also be subdivided based on presence of inflammatory cytokines, including the type 1 (T1) endotype based on IFN-γ signaling and type 3 (T3) endotype based on IL-17A signaling; infiltrated cell types include neutrophils, lymphocytes and plasma cells 52,56-59. NECRSwNP is highly heterogeneous and the frequency of each endotype varies geographically 56-58. Transcriptomic approaches help to identify immunological mechanisms in NP 60-63. Of note, two groups presented distinct gene expression profiles in NP between ECRSwNP and NECRSwNP in Asia and results also showed several similarities 56,63. Both studies showed that eosinophil- and T2-associated genes including CLC , CCL23 ,CCL26 , SIGLEC8 , PRSS33 and ALOX15 were upregulated in ECRSwNP, confirming that ECRSwNP is associated with the T2 endotype 56,63. In contrast, NECRSwNP showed up-regulation of IFN-γ-induced genes (CXCL9 , CXCL10 andCXCL11 ), IL-17A-induced genes (serum amyloid A, CXCL6 and CHI3L1) and neutrophil chemokines (IL-8, CXCL1 and CXCL6), suggesting that NECRSwNP in Asia may display mainly a mixed T1 and T3 endotype with neutrophilia 56,63. Indeed, Wang and colleagues reported that the T1 and T3 mixed (IFN-γ+, IL-17A+) endotype is the most common endotype in NECRSwNP in Beijing 56. Although ECRSwNP and NECRSwNP showed distinct immunological mechanisms, both result in NP formation, suggesting that some phenotypic features, such as formation of polyps, are not reliable indicators of the transcriptomic pattern or molecular endotype. A summary of gene expression in the primary endotypes and mixed endotypes is found in Figure 3 .
Transcriptome analysis may have value in identifying genes that are associated with NP formation by extracting shared dysregulated genes in NPs from both ECRSwNP and NECRSwNP. Epithelial-to-mesenchymal transition (EMT) associated genes including HIF1A are elevated in both ECRSwNP and NECRSwNP, suggesting that EMT may be a key event in NP formation46,56,64. Although transcriptome analysis identifies similarities between ECRSwNP and NECRSwNP, the upstream pathway for the affected molecules may be different in each endotype. For example, reduced fibrinolysis associated with down-regulation of tissue plasminogen activator (tPA) is a common feature of NPs in T2wNP (ECRSwNP) and T1wNP (NECRSwNP) 32,34,46.
In contrast to CRSwNP, studies in CRSsNP have been complicated by the use of variable sinonasal biopsy sites which have inherent tissue-specific molecular differences that obscure the underlying heterogeneity of inflammation 57,58,62,65. By the exclusive use of ethmoid sinus mucosa for microarray, we recently identified gene expression profiles in three inflammatory endotypes; T1sNP, T2sNP and T3sNP, and predicted molecular mechanisms and biomarkers for each endotype 55. The gene signatures suggested that T1sNP is associated with T cells (Th1 cells and CD8+ cytotoxic T cells), NK cells and antigen presenting cells (APC); T2sNP is associated with eosinophils, mast cells, basophils, Th2 cells, ILC2 and APC that are also found in T2wNP; and T3sNP is associated with neutrophils, Th17 cells, B cells and APC(see Figure 2) 55. We further found that T1 (CXCL9 and CXCL10), T2 (eosinophilic proteins and CCL26) and T3 (CSF3) endotypic biomarkers can distinguish tissue endotypes in nasal lavage fluids from patients with CRSsNP 55.