Here we identified a population of bone marrow neutrophils that constitutively express RORγt and which can produce and respond to IL-17A (IL-17). IL-6, IL-23 and RORγt, but not T cells or NK cells, are required for IL-17 production in neutrophils. IL-6 and IL-23 induced IL-17RC and Dectin-2 expression in neutrophils, and expression of IL-17RC was augmented by Aspergillus and Dectin-2 activation. Autocrine IL-17A–IL-17 receptor activity induced production of reactive oxygen species (ROS), and increased fungal killing in vitro and in a model of Aspergillus keratitis. Human neutrophils also expressed RORγt, and induced IL-17A, IL-17RC and Dectin-2 expression following IL-6 and IL-23 stimulation. These findings identify a population of human and murine neutrophils that exhibit autocrine IL-17 activity, and which likely contribute to the etiology of microbial and inflammatory diseases.

P. aeruginosa is a leading cause of blinding corneal ulcers worldwide. To determine the role of type III secretion in the pathogenesis of P. aeruginosa keratitis, corneas of C57BL/6 mice were infected with P. aeruginosa strain PAO1 or PAK, which express ExoS, ExoT and ExoY, but not ExoU. PAO1 and PAK infected corneas developed severe disease with pronounced opacification and rapid bacterial growth. In contrast, corneas infected with ΔpscD or ΔpscJ mutants that cannot assemble a Type III secretion system, or with mutants lacking the translocator proteins, do not develop clinical disease, and are rapidly killed by infiltrating neutrophils. Further, survival of PAO1 and PAK strains in the cornea and development of corneal disease was impaired in ΔexoS, ΔexoT and ΔexoST mutants of both strains, but not in a ΔexoY mutant. ΔexoST mutants were also rapidly killed in neutrophils in vitro and were impaired in their ability to promote neutrophil apoptosis in vivo compared with PAO1. Point mutations in the ADP ribosyltransferase (ADPR) regions of ExoS or ExoT also impaired pro-apoptotic activity in infected neutrophils, and exoST(ADPR-) mutants replicated the ΔexoST phenotype in vitro and in vivo, whereas mutations in rho-GAP showed the same phenotype as PAO1. Together, these findings demonstrate that the pathogenesis of P. aeruginosa keratitis in ExoS and ExoT producing strains is almost entirely due to their ADPR activities, which subvert the host response by targeting the anti-bacterial activity of infiltrating neutrophils.

SUMMARYFungal infection stimulates the canonical C-type lectin receptors (CLRs) signaling pathway via Syk activation. Here we show that SHP-2 plays a crucial role in mediating CLRs-induced Syk activation. Genetic ablation of Shp-2 (Ptpn11) in dendritic cells (DCs) and macrophages impaired Syk-mediated signaling and abrogated pro-inflammatory gene expression following fungal stimulation. Mechanistically, SHP-2 operates as a scaffold facilitating the recruitment of Syk to dectin-1 or FcRγ, through its N-SH2 domain and a previously unrecognized C-terminal ITAM motif. We demonstrate that DC-derived SHP-2 is crucial for the induction of IL-1β, IL-6 and IL-23, and anti-fungal TH17 cell responses to control Candida albicans infection. Together, these data reveal a mechanism by which SHP-2 mediates Syk activation in response to fungal infections

Fusarium and Aspergillus species of mould are major causes of corneal infections in the USA and worldwide, resulting in severe visual impairment and blindness. As there is evidence for T cell responses to these pathogenic fungi in infected individuals, we examined the role of IL-17A (IL-17) and IFN-γ in murine models of fungal keratitis. We found that C57BL/6 mice given intratracheal or subcutaneous immunization of conidia prior to corneal infection exhibited enhanced fungal killing and lower corneal opacity compared with unimmunized mice. Protective immunity was associated with temporal recruitment of IL-17 producing neutrophils, Th17 and Th1 cells, and was dependent on production of IL-17 but not IFN-γ. Protection was also impaired in neutrophil depleted and in Rag2−/− mice. Together, the results of these studies identify an essential role for IL-17 producing neutrophils and Th17 cells in regulating the growth of fungal hyphae and the severity of corneal disease.

Diabetic retinopathy is a prevailing diabetes complication, and one of the leading causes of blindness worldwide. IL-17A is a cytokine involved in the onset of diabetic complications. In the current study, we examined the role of IL-17A in the development of retinal inflammation and long-term vascular pathology in diabetic mice. We found IL-17A expressing T cells and neutrophils in the retinal vasculature. Further, the IL-17A receptor was expressed on Muller glia, retinal endothelial cells, and photoreceptors. Finally, diabetes-mediated retinal inflammation, oxidative stress, and vascular leakage were all significantly lower in IL-17A −/− mice. These are all clinically meaningful abnormalities that characterize the onset of diabetic retinopathy.

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IL-6 and IL-23 (IL-6/23) induce IL-17A (IL-17) production by a subpopulation of murine and human neutrophils, resulting in autocrine IL-17 activation, enhanced production of reactive oxygen species, and increased fungal killing. As IL-6 and IL-23 receptors trigger JAK1, -3/STAT3 and JAK2/STAT3 phosphorylation, respectively, we examined the role of this pathway in a murine model of fungal keratitis and also examined neutrophil elastase and gelatinase (matrix metalloproteinase 9) activity by IL-6/23-stimulated human neutrophils in vitro. We found that STAT3 phosphorylation of neutrophils in Aspergillus fumigatus-infected corne as was inhibited by the JAK/STAT inhibitor Ruxolitinib, resulting in impaired fungal killing and decreased matrix metalloproteinase 9 activity. In vitro, we showed that fungal killing by IL-6/23-stimulated human peripheral blood neutrophils was impaired by JAK/STAT inhibitors Ruxolitinib and Stattic, and by the retinoic acid receptor-related orphan receptor γt inhibitor SR1001. This was also associated with decreased reactive oxygen species, IL-17A production, and retinoic acid receptor-related orphan receptor γt translocation to the nucleus. We also demonstrate that IL-6/23-activated neutrophils exhibit increased elastase and gelatinase (matrix metalloproteinase 9) activity, which is inhibited by Ruxolitinib and Stattic but not by SR1001. Taken together, these observations indicate that the regulation of activity of IL-17-producing neutrophils by JAK/STAT inhibitors impairs reactive oxygen species production and fungal killing activity but also blocks elastase and gelatinase activity that can cause tissue damage.

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