Interleukin (IL) 25 (IL-17E), a distinct member of the IL-17 cytokine family, plays important roles in evoking T helper type 2 (Th2) cell–mediated inflammation that features the infiltrations of eosinophils and Th2 memory cells. However, the cellular sources, target cells, and underlying mechanisms remain elusive in humans. We demonstrate that human Th2 memory cells expressing distinctive levels of IL-25 receptor (R) are one of the responding cell types. IL-25 promotes cell expansion and Th2 cytokine production when Th2 central memory cells are stimulated with thymic stromal lymphopoietin (TSLP)–activated dendritic cells (DCs), homeostatic cytokines, or T cell receptor for antigen triggering. The enhanced functions of Th2 memory cells induced by IL-25 are associated with sustained expression of GATA-3, c-MAF, and JunB in an IL-4–independent manner. Although keratinocytes, mast cells, eosinophils, and basophils express IL-25 transcripts, activated eosinophils and basophils from normal and atopic subjects were found to secrete bioactive IL-25 protein, which augments the functions of Th2 memory cells. Elevated expression of IL-25 and IL-25R transcripts was observed in asthmatic lung tissues and atopic dermatitis skin lesions, linking their possible roles with exacerbated allergic disorders. Our results provide a plausible explanation that IL-25 produced by innate effector eosinophils and basophils may augment the allergic inflammation by enhancing the maintenance and functions of adaptive Th2 memory cells.

IL-17-producing CD4

Previous studies suggest that thymus produces a homogenous population of natural regulatory T (Treg) cells that express a transcriptional factor FOXP3 and control autoimmunity through a cell-contact-dependent mechanism. We found two subsets of FOXP3+ natural Treg cells defined by the expression of the costimulatory molecule ICOS in the human thymus and periphery. Whereas the ICOS+FOXP3+ Treg cells used interleukin-10 to suppress dendritic cell function and transforming growth factor (TGF)-beta to suppress T cell function, the ICOS-FOXP3+ Treg cells used TGF-beta only. The survival and proliferation of the two subsets of Treg cells were differentially regulated by signaling through ICOS or CD28, respectively. We suggest that the selection of natural Treg cells in thymus is coupled with Treg cell differentiation into two subsets imprinted with different cytokine expression potentials and use both cell-contact-dependent and independent mechanisms for immunosuppression in periphery.

Toll-like receptor 9 (TLR9) senses microbial DNA and triggers type I IFN responses in plasmacytoid dendritic cells (pDCs). Previous studies suggest the presence of myeloid differentiation primary response gene 88 (MyD88)-dependent DNA sensors other than TLR9 in pDCs. Using MS, we investigated C-phosphate-G (CpG)-binding proteins from human pDCs, pDC-cell lines, and interferon regulatory factor 7 (IRF7)-expressing B-cell lines. CpG-A selectively bound the aspartate-glutamate-any amino acid-aspartate/histidine (DExD/H)-box helicase 36 (DHX36), whereas CpG-B selectively bound DExD/H-box helicase 9 (DHX9). Although the aspartateglutamate-alanine-histidine box motif (DEAH) domain of DHX36 was essential for CpG-A binding, the domain of unknown function 1605 (DUF1605 domain) of DHX9 was required for CpG-B binding. DHX36 is associated with IFN-α production and IRF7 nuclear translocation in response to CpG-A, but DHX9 is important for TNF-α and IL-6 production and NF-κB activation in response to CpG-B. Knocking down DHX9 or DHX36 significantly reduced the cytokine responses of pDCs to a DNA virus but had no effect on the cytokine responses to an RNA virus. We further showed that both DHX9 and DHX36 are localized within the cytosol and are directly bound to the Toll-interleukin receptor domain of MyD88 via their helicase-associated domain 2 and DUF domains. This study demonstrates that DHX9/DHX36 represent the MyD88-dependent DNA sensors in the cytosol of pDCs and suggests a much broader role for DHX helicases in viral sensing.cytosolic sensor | innate immunity

Plasmacytoid dendritic cells (pDCs) produce copious type I interferon (IFN) upon sensing nucleic acids through Toll-like receptor (TLR) 7 and TLR9. Uncontrolled pDC activation and IFN production are implicated in lymphopenia and autoimmune diseases; therefore, a mechanism controlling pDC IFN production is essential. Human pDCs specifically express an orphan receptor, immunoglobulin-like transcript 7 (ILT7). Here, we discovered an ILT7 ligand expressed by human cell lines and identified it as bone marrow stromal cell antigen 2 (BST2; CD317). BST2 directly binds to purified ILT7 protein, initiates signaling via the ILT7–FcεRIγ complex, and strongly inhibits production of IFN and proinflammatory cytokines by pDCs. Readily induced by IFN and other proinflammatory cytokines, BST2 may modulate the human pDC’s IFN responses through ILT7 in a negative feedback fashion.

Epithelial ovarian cancer (EOC) is the fifth most common cause of cancer death among women. Despite its immunogenicity, effective antitumor responses are limited, due, in part, to the presence of forkhead box protein 3–positive (Foxp3+) T regulatory (Treg) cells in the tumor microenvironment. However, the mechanisms that regulate the accumulation and the suppressive function of these Foxp3+ Treg cells are poorly understood. Here, we found that the majority of Foxp3+ Treg cells accumulating in the tumor microenvironment of EOCs belong to the subset of Foxp3+ Treg cells expressing inducible costimulator (ICOS). The expansion and the suppressive function of these cells were strictly dependent on ICOS-L costimulation provided by tumor plasmacytoid dendritic cells (pDC). Accordingly, ICOS+Foxp3+Treg cells were found to localize in close vicinity of tumor pDCs, and their number directly correlated with the numbers of pDCs in the tumors. Furthermore, pDCs and ICOS+ Foxp3+Treg cells were found to be strong predictors for disease progression in patients with ovarian cancer, with ICOS+Treg cell subset being a stronger predictor than total Foxp3+Treg cells. These findings suggest an essential role for pDCs and ICOS-L in immunosuppression mediated by ICOS+ Foxp3+ Treg cells, leading to tumor progression in ovarian cancer.

T cell homeostasis is a self-regulating process for maintaining the size of the peripheral T cell pool. Although dendritic cells (DCs) seem to be important in T cell homeostasis, the molecular regulation of DC-mediated T cell homeostasis is unknown. We show that human DCs activated by thymic stromal lymphopoietin (TSLP) induced a robust expansion of autologous CD4(+) T cell populations, which depended on self peptide-major histocompatibility complex. The proliferating T cells adopted and maintained a central memory polyclonal phenotype and could differentiate into T helper type 1 or type 2 effector cells. These results, together with findings of TSLP expression in epithelial cells of mucosal lymphoid tissues and thymus, indicate that TSLP is involved in DC-mediated CD4(+) T cell homeostasis.

Summary The NLRP3 inflammasome plays a major role in innate immune responses by activating caspase-1, resulting in secretion of interleukin (IL)-18 and IL-1β. Although cytosolic double-stranded RNA (dsRNA) and bacterial RNA are known to activate the NLRP3 inflammasome, the upstream sensor is unknown. We investigated the potential function of DExD/H-box RNA helicase family members (previously shown to sense cytosolic DNA and RNA to induce type 1 interferon responses) in RNA-induced NLRP3 inflammasome activation. Among the helicase family members tested, we found that targeting of DHX33 expression by short hairpin RNA efficiently blocked the activation of caspase-1 and secretion of IL-18/IL-1β in human macrophages that were activated by cytosolic poly I:C, reoviral RNA or bacterial RNA. DHX33 bound dsRNA via the helicase C domain. DHX33 interacted with NLRP3 and formed the inflammasome complex following stimulation with RNA. We, therefore, identified DHX33 as a cytosolic RNA sensor that activates the NLRP3 inflammasome.