Frontline Science: Nasal epithelial GM-CSF contributes to TLR5-mediated modulation of airway dendritic cells and subsequent IgA response

Abstract

Flagellin, as a TLR5 agonist, is an established mucosal adjuvant for enhancing mucosal IgA responses by i.n. immunization. Nasal epithelial cells (NECs) are the first sentinel cells to be exposed to antigen and adjuvant in i.n. immunization, and it is suggested that they play an important role in the mucosal adjuvant activity of flagellin. However, the molecular mechanism leading to modulation and the response by flagellin-activated NECs remain unknown. We aimed to identify the soluble mediator(s) from flagellin-activated NECs that modulate the functions of airway dendritic cells (DCs) and enhance subsequent IgA response. In vitro studies showed that compared with the TLR4 agonist LPS, flagellin directly triggered slight up-regulation of CD80 on airway DCs but was insufficient to affect CD86 expression and DC-mediated IgA response. With the use of an in vitro system for culturing mouse primary NECs (mNECs), we demonstrated that flagellin-activated mNECs could functionally modulate airway DCs, which manifested as significant up-regulation of CD80/CD86 and enhancement of IgA production. The functional modulation of airway DCs was dependent on TLR5 activation of mNECs rather than direct TLR5 activation of airway DCs. With the use of cytokine array and antibody-blocking assays, we further identified that GM-CSF, a cytokine secreted from TLR5-activated mNECs, contributes to the activation of mNECs to airway DCs and subsequent IgA enhancement. In vivo blocking experiments confirmed that GM-CSF is an important factor in recombinant flagellin derived from (FliC)-induced airway DC activation and antigen-specific IgA enhancement. Our data directly demonstrate that nasal epithelial GM-CSF contributes to TLR5-mediated modulation of airway DCs and a subsequent IgA response.

Vascular Adhesion Protein 1 Mediates Gut Microbial Flagellin-Induced Inflammation, Leukocyte Infiltration, and Hepatic Steatosis

“…Thus, it may be that only the FLG-induced increase in IL1β is truly VAP-1 mediated. Nevertheless, in WT mice the stimulation of GM-CSF and MCP-1 by FLG is in agreement with previous studies [24,25]. In addition, the increase in MCP-1 and circulating neutrophils, monocytes and lymphocytes-recruiting GM-CSF can explain the increased infiltration of CD45-positive leukocytes into the VAT of FLG-treated mice that seemed to be VAP-1-mediated.…”

Flagellin: a unique microbe-associated molecular pattern and a multi-faceted immunomodulator

“…11 We developed a series of recombinant flagellins based on different combinations between its dual activities for mucosal adjuvant, which can enhance the antigen-specific IgA antibody response by priming nasal epithelial cells to modulate airway DCs and B cells. 12 Using a fusion strategy, we developed an anti-caries mucosal vaccine by either direct fusion or replacement of the highly variable region of flagellin with caries vaccine antigen, which can promote a protective salivary IgA antibody response and inhibit dental caries prophylactically and therapeutically in rat caries model (unpublished data). 13,14 As a unique microbe-associated molecular pattern, does flagellin possess the ability to activate some other unknown pathway in addition to the TLR5 and NAIPs/NLRC4 inflammasome pathway?…”

Editorial: The nose knows

“…I n this issue, Cao et al [1] explore the tripartite interplay among the local nasal epithelial tissue, resident DCs, and adaptive immune cells. Their results reveal a succession of interactions that culminate in the characteristic IgA production associated with intranasal challenge (schematized in Fig.…”

“…It is within this context that Cao et al [1] elected to probe the mechanisms through which flagellin fosters IgA in response to intranasal immunization. In their first set of experiments, they show that intranasal introduction of FLiC leads to local DC activation within 4 h, as evidenced by CD80 and CD86 upregulation.…”

“…Although they were identified more than a century ago in blood, sputum, and lung lavage samples, they remain a controversial cell type for their role in immunity, particularly in lung immune responses. Several studies using congenitally eosinophil-deficient strains of mice, PHIL, DdblGATA, and iPHIL, have demonstrated a prominent role for pulmonary eosinophils in regulating immune responses in allergic asthma as well as in pulmonary fibrosis, although some reports have cast these findings into doubt [1][2][3]. Possible reasons for the discrepancy in these reports may be that the three eosinophil-deficient strains of mice so far developed rely on the use of cytocidal reagents (diphtheria toxin) or gene targeting of key transcription factors (i.e., GATA-1).…”