The host defence peptide cathelicidin (LL-37 in humans, mCRAMP in mice) is released from neutrophils by de-granulation, NETosis and necrotic death; it has potent anti-pathogen activity as well as being a broad immunomodulator. Here we report that cathelicidin is a powerful Th17 potentiator which enhances aryl hydrocarbon receptor (AHR) and RORγt expression, in a TGF-β1-dependent manner. In the presence of TGF-β1, cathelicidin enhanced SMAD2/3 and STAT3 phosphorylation, and profoundly suppressed IL-2 and T-bet, directing T cells away from Th1 and into a Th17 phenotype. Strikingly, Th17, but not Th1, cells were protected from apoptosis by cathelicidin. We show that cathelicidin is released by neutrophils in mouse lymph nodes and that cathelicidin-deficient mice display suppressed Th17 responses during inflammation, but not at steady state. We propose that the neutrophil cathelicidin is required for maximal Th17 differentiation, and that this is one method by which early neutrophilia directs subsequent adaptive immune responses.

Figure 1 IL22 induces an ER stress/unfolded protein response transcriptional module in colonic epithelial cells. (A) Heat map demonstrating pathway specific transcript expression in murine colonoids treated with (+IL22, n=3) or without (control, n=3) recombinant IL22. Mouse gene 2.0 ST array platform (affymetrix). (B) GSEA evaluating enrichment of ER stress response transcriptional module in IL22 treated colonoids. A core set of colonic epithelial-specific ER stress genes was defined by analysing significantly differentially expressed (p<0.05 and absolute value of the log2 fold change >±2) transcripts in colonoids treated with tunicamycin (n=3) or medium alone (n=3). (C) Expression of ER stress response transcripts in IL22 treated WT and Il22ra1 −/− colonoids (RNA-seq dataset ERR247358-ERR247389, Pham et al, 2014). 18 (D) Enrichment analysis for ER stress-related functional annotation groups (GO biological processes) in IL22-treated colonoids from dataset ERR247358-ERR247389. (E) Microarray analysis of core ER stress response transcripts in colonoids treated with tunicamycin (n=3), tunicamycin+IL22 (n=3) or untreated (control, n=3). (F) Real-time PCR quantification of ER stress transcripts in colonoids treated with IL22 (n=11), IL17A (n=6) and IL22+IL17A (n=6) and unexposed controls. *P<0.01. (G) Immunoblot and densitometry quantification (H) detecting GRP78 protein expression in colonoids treated with different cytokines. *P<0.026, one tailed t test. ER, endoplasmic reticulum; GO, Gene Ontology; GSEA, Gene Set Enrichment Analysis; IL22, interleukin-22.on July 6, 2020 by guest. Protected by copyright.

Neutrophils are the most abundant leukocytes in peripheral blood and respond rapidly to danger, infiltrating tissues within minutes of infectious or sterile injury. Neutrophils were long thought of as simple killers, but now we recognise them as responsive cells able to adapt to inflammation and orchestrate subsequent events with some sophistication. Here, we discuss how these rapid responders release mediators which influence later adaptive T cell immunity through influences on DC priming and directly on the T cells themselves. We consider how the release of granule contents by neutrophils—through NETosis or degranulation—is one way in which the innate immune system directs the phenotype of the adaptive immune response.