Dynamic Hippo pathway activity underlies mesenchymal differentiation during lung alveolar morphogenesis

Alveologenesis, the final stage in lung development, substantially remodels the distal lung, expanding the alveolar surface area for efficient gas exchange. Secondary crest myofibroblasts (SCMF) exist transiently in the neonatal distal lung and are crucial for alveologenesis. However, the pathways that regulate SCMF function, proliferation and temporal identity remain poorly understood. To address this, we purified SCMFs from reporter mice, performed bulk RNA-seq and found dynamic changes in Hippo-signaling components during alveologenesis. We deleted the Hippo effectors Yap/Taz from Acta2-expressing cells at the onset of alveologenesis, causing a significant arrest in alveolar development. Using single cell RNA-seq, we identified a distinct cluster of cells in mutant lungs with altered expression of marker genes associated with proximal mesenchymal cell types, airway smooth muscle and alveolar duct myofibroblasts. In vitro studies confirmed that Yap/Taz regulates myofibroblast-associated gene signature and contractility. Together, our findings show that Yap/Taz is essential for maintaining functional myofibroblast identity during postnatal alveologenesis.

Dysregulated alveolar epithelial cell progenitor function and identity in Hermansky-Pudlak syndrome pulmonary fibrosis

Hermansky-Pudlak syndrome (HPS) is a genetic disorder associated with pulmonary fibrosis in specific subtypes, including HPS-1 and HPS-2. Single mutant HPS1 and HPS2 mice display increased fibrotic sensitivity while double mutant HPS1/2 mice exhibit spontaneous fibrosis with aging, which has been attributed to HPS mutations in alveolar epithelial type II (AT2) cells. Unifying mechanisms of AT2 cell dysfunction in genetic and sporadic fibrotic lung diseases remain unknown. Incorporating AT2 cell lineage tracing in HPS mice, we observed a progressive decline in AT2 cell numbers with aging and aberrant differentiation with increased AT2-derived alveolar epithelial type I cells. HPS AT2 cell proliferation was impaired ex vivo and in vivo , suggesting an intrinsic progenitor defect. Transcriptomic analysis of HPS AT2 cells revealed elevated expression of genes associated with aberrant differentiation and cellular senescence. Through lineage tracing and organoid modeling, we demonstrated that HPS AT2 cells were primed to persist in a Krt8 + reprogrammed transitional state, mediated by p53 activity. These findings suggest that pulmonary fibrosis in HPS may be driven by AT2 cell progenitor dysfunction in the setting of p53-mediated senescence, highlighting a novel potential therapeutic target in HPS and suggesting unifying mechanisms underlying HPS and other forms of pulmonary fibrosis.

Circadian regulation of lung repair and regeneration

Optimal lung repair and regeneration are essential for recovery from viral infections, including influenza A virus (IAV). We have previously demonstrated that acute inflammation and mortality induced by IAV is under circadian control. However, it is not known whether the influence of the circadian clock persists beyond the acute outcomes. Here, we utilize the UK Biobank to demonstrate an association between poor circadian rhythms and morbidity from lower respiratory tract infections, including the need for hospitalization and mortality after discharge; this persists even after adjusting for common confounding factors. Furthermore, we use a combination of lung organoid assays, single-cell RNA sequencing, and IAV infection in different models of clock disruption to investigate the role of the circadian clock in lung repair and regeneration. We show that lung organoids have a functional circadian clock and the disruption of this clock impairs regenerative capacity. Finally, we find that the circadian clock acts through distinct pathways in mediating lung regeneration - in tracheal cells via the Wnt/β-catenin pathway and through IL-1β in alveolar epithelial cells. We speculate that adding a circadian dimension to the critical process of lung repair and regeneration will lead to novel therapies and improve outcomes.

Engineering and Modeling the Lung Mesenchyme

The structure of the mammalian lung controls the flow of air through the airways and into the distal alveolar region where gas exchange occurs. Specialized cells in the lung mesenchyme produce the extracellular matrix (ECM) and growth factors required for lung structure. Historically, characterizing the mesenchymal cell subtypes was challenging due to their ambiguous morphology, overlapping expression of protein markers, and limited cell-surface molecules needed for isolation. The recent development of single-cell RNA sequencing (scRNA-seq) complemented with genetic mouse models demonstrated that the lung mesenchyme comprises transcriptionally and functionally heterogeneous cell-types. Bioengineering approaches that model tissue structure clarify the function and regulation of mesenchymal cell types. These experimental approaches demonstrate the unique abilities of fibroblasts in mechanosignaling, mechanical force generation, ECM production, and tissue regeneration. This chapter will review the cell biology of the lung mesenchyme and experimental approaches to study their function.

CXCL12 defines lung endothelial heterogeneity and promotes distal vascular growth

There is a growing amount of data uncovering the cellular diversity of the pulmonary circulation and mechanisms governing vascular repair after injury. However, the molecular and cellular mechanisms contributing to the morphogenesis and growth of the pulmonary vasculature during embryonic development are less clear. Importantly, deficits in vascular development lead to significant pediatric lung diseases, indicating a need to uncover fetal programs promoting vascular growth. To address this, we used a transgenic mouse reporter for expression of Cxcl12, an arterial endothelial hallmark gene, and performed single-cell RNA sequencing on isolated Cxcl12-DsRed+ endothelium to assess cellular heterogeneity within pulmonary endothelium. Combining cell annotation with gene ontology and histological analysis allowed us to segregate the developing artery endothelium into functionally and spatially distinct subpopulations. Expression of Cxcl12 is highest in the distal arterial endothelial subpopulation, a compartment enriched in genes for vascular development. Accordingly, disruption of CXCL12 signaling led to, not only abnormal branching, but also distal vascular hypoplasia. These data provide evidence for arterial endothelial functional heterogeneity and reveal conserved signaling mechanisms essential for pulmonary vascular development.

A census of the lung: CellCards from LungMAP

The human lung plays vital roles in respiration, host defense, and basic physiology. Recent technological advancements such as single-cell RNA sequencing and genetic lineage tracing have revealed novel cell types and enriched functional properties of existing cell types in lung. The time has come to take a new census. Initiated by members of the NHLBI-funded LungMAP Consortium and aided by experts in the lung biology community, we synthesized current data into a comprehensive and practical cellular census of the lung. Identities of cell types in the normal lung are captured in individual cell cards with delineation of function, markers, developmental lineages, heterogeneity, regenerative potential, disease links, and key experimental tools. This publication will serve as the starting point of a live, up-to-date guide for lung research at https://www.lungmap.net/cell-cards/. We hope that Lung CellCards will promote the community-wide effort to establish, maintain, and restore respiratory health.

Genomic, epigenomic, and biophysical cues controlling the emergence of the lung alveolus

The lung alveolus is the functional unit of the respiratory system required for gas exchange. During the transition to air breathing at birth, biophysical forces are thought to shape the emerging tissue niche. However, the intercellular signaling that drives these processes remains poorly understood. Applying a multimodal approach, we identified alveolar type 1 (AT1) epithelial cells as a distinct signaling hub. Lineage tracing demonstrates that AT1 progenitors align with receptive, force-exerting myofibroblasts in a spatial and temporal manner. Through single-cell chromatin accessibility and pathway expression (SCAPE) analysis, we demonstrate that AT1-restricted ligands are required for myofibroblasts and alveolar formation. These studies show that the alignment of cell fates, mediated by biophysical and AT1-derived paracrine signals, drives the extensive tissue remodeling required for postnatal respiration.

STAT3-BDNF-TrkB signalling promotes alveolar epithelial regeneration after lung injury

Alveolar epithelial regeneration is essential for recovery from devastating lung diseases. This process occurs when type II alveolar pneumocytes (AT2 cells) proliferate and transdifferentiate into type I alveolar pneumocytes (AT1 cells). We used genome-wide analysis of chromatin accessibility and gene expression following acute lung injury to elucidate repair mechanisms. AT2 chromatin accessibility changed substantially following injury to reveal STAT3 binding motifs adjacent to genes that regulate essential regenerative pathways. Single-cell transcriptome analysis identified brain-derived neurotrophic factor (Bdnf) as a STAT3 target gene with newly accessible chromatin in a unique population of regenerating AT2 cells. Furthermore, the BDNF receptor tropomyosin receptor kinase B (TrkB) was enriched on mesenchymal alveolar niche cells (MANCs). Loss or blockade of AT2-specific Stat3, Bdnf or mesenchyme-specific TrkB compromised repair and reduced Fgf7 expression by niche cells. A TrkB agonist improved outcomes in vivo following lung injury. These data highlight the biological and therapeutic importance of the STAT3-BDNF-TrkB axis in orchestrating alveolar epithelial regeneration.

Defining the role of pulmonary endothelial cell heterogeneity in the response to acute lung injury

Pulmonary endothelial cells (ECs) are an essential component of the gas exchange machinery of the lung alveolus. Despite this, the extent and function of lung EC heterogeneity remains incompletely understood. Using single-cell analytics, we identify multiple EC populations in the mouse lung, including macrovascular endothelium (maEC), microvascular endothelium (miECs), and a new population we have termed Car4-high ECs. Car4-high ECs express a unique gene signature, and ligand-receptor analysis indicates they are primed to receive reparative signals from alveolar type I cells. After acute lung injury, they are preferentially localized in regenerating regions of the alveolus. Influenza infection reveals the emergence of a population of highly proliferative ECs that likely arise from multiple miEC populations and contribute to alveolar revascularization after injury. These studies map EC heterogeneity in the adult lung and characterize the response of novel EC subpopulations required for tissue regeneration after acute lung injury.

Dnmt1 is required for proximal-distal patterning of the lung endoderm and for restraining alveolar type 2 cell fate

Lung endoderm development occurs through a series of finely coordinated transcriptional processes that are regulated by epigenetic mechanisms. However, the role of DNA methylation in regulating lung endoderm development remains poorly understood. We demonstrate that DNA methyltransferase 1 (Dnmt1) is required for early branching morphogenesis of the lungs and for restraining epithelial fate specification. Loss of Dnmt1 leads to an early branching defect, a loss of epithelial polarity and proximal endodermal cell differentiation, and an expansion of the distal endoderm compartment. Dnmt1 deficiency also disrupts epithelial-mesenchymal crosstalk and leads to precocious distal endodermal cell differentiation with premature expression of alveolar type 2 cell restricted genes. These data reveal an important requirement for Dnmt1 mediated DNA methylation in early lung development to promote proper branching morphogenesis, maintain proximal endodermal cell fate, and suppress premature activation of the distal epithelial fate.

Regeneration of the lung alveolus by an evolutionarily conserved epithelial progenitor

Functional tissue regeneration is required for the restoration of normal organ homeostasis after severe injury. Some organs, such as the intestine, harbour active stem cells throughout homeostasis and regeneration; more quiescent organs, such as the lung, often contain facultative progenitor cells that are recruited after injury to participate in regeneration. Here we show that a Wnt-responsive alveolar epithelial progenitor (AEP) lineage within the alveolar type 2 cell population acts as a major facultative progenitor cell in the distal lung. AEPs are a stable lineage during alveolar homeostasis but expand rapidly to regenerate a large proportion of the alveolar epithelium after acute lung injury. AEPs exhibit a distinct transcriptome, epigenome and functional phenotype and respond specifically to Wnt and Fgf signalling. In contrast to other proposed lung progenitor cells, human AEPs can be directly isolated by expression of the conserved cell surface marker TM4SF1, and act as functional human alveolar epithelial progenitor cells in 3D organoids. Our results identify the AEP lineage as an evolutionarily conserved alveolar progenitor that represents a new target for human lung regeneration strategies.

IL-17A-Induced PLET1 Expression Contributes to Tissue Repair and Colon Tumorigenesis

This study identifies a novel mechanism linking IL-17A with colon tissue repair and tumor development. Abrogation of IL-17A signaling in mice attenuated tissue repair of dextran sulfate sodium (DSS)-induced damage in colon epithelium and markedly reduced tumor development in an azoxymethane/DSS model of colitis-associated cancer. A novel IL-17A target gene, PLET1 (a progenitor cell marker involved in wound healing), was highly induced in DSS-treated colon tissues and tumors in an IL-17RC-dependent manner. PLET1 expression was induced in LGR5⁺ colon epithelial cells after DSS treatment. LGR5⁺PLET1⁺ marks a highly proliferative cell population with enhanced expression of IL-17A target genes. PLET1 deficiency impaired tissue repair of DSS-induced damage in colon epithelium and reduced tumor formation in an azoxymethane/DSS model of colitis-associated cancer. Our results suggest that IL-17A-induced PLET1 expression contributes to tissue repair and colon tumorigenesis.

T cell-intrinsic ASC critically promotes T(H)17-mediated experimental autoimmune encephalomyelitis

Interleukin 1β (IL-1β) is critical for the in vivo survival, expansion and effector function of IL-17-producing helper T (T(H)17) cells during autoimmune responses, including experimental autoimmune encephalomyelitis (EAE). However, the spatiotemporal role and cellular source of IL-1β during EAE pathogenesis are poorly defined. In the present study, we uncovered a T cell-intrinsic inflammasome that drives IL-1β production during T(H)17-mediated EAE pathogenesis. Activation of T cell antigen receptors induced expression of pro-IL-1β, whereas ATP stimulation triggered T cell production of IL-1β via ASC-NLRP3-dependent caspase-8 activation. IL-1R was detected on T(H)17 cells but not on type 1 helper T (T(H)1) cells, and ATP-treated T(H)17 cells showed enhanced survival compared with ATP-treated T(H)1 cells, suggesting autocrine action of T(H)17-derived IL-1β. Together these data reveal a critical role for IL-1β produced by a T(H)17 cell-intrinsic ASC-NLRP3-caspase-8 inflammasome during inflammation of the central nervous system.

Human Colon Tumors Express a Dominant-Negative Form of SIGIRR That Promotes Inflammation and Colitis-Associated Colon Cancer in Mice

BACKGROUND & AIMS

Single immunoglobulin and toll-interleukin 1 receptor (SIGIRR), a negative regulator of the Toll-like and interleukin-1 receptor (IL-1R) signaling pathways, controls intestinal inflammation and suppresses colon tumorigenesis in mice. However, the importance of SIGIRR in human colorectal cancer development has not been determined. We investigated the role of SIGIRR in development of human colorectal cancer.

METHODS

We performed RNA sequence analyses of pairs of colon tumor and nontumor tissues, each collected from 68 patients. Immunoblot and immunofluorescence analyses were used to determine levels of SIGIRR protein in primary human colonic epithelial cells, tumor tissues, and colon cancer cell lines. We expressed SIGIRR and mutant forms of the protein in Vaco cell lines. We created and analyzed mice that expressed full-length (control) or a mutant form of Sigirr (encoding SIGIRR(N86/102S), which is not glycosylated) specifically in the intestinal epithelium. Some mice were given azoxymethane (AOM) and dextran sulfate sodium to induce colitis-associated cancer. Intestinal tissues were collected and analyzed by immunohistochemical and gene expression profile analyses.

RESULTS

RNA sequence analyses revealed increased expression of a SIGIRR mRNA isoform, SIGIRR(ΔE8), in colorectal cancer tissues compared to paired nontumor tissues. SIGIRR(ΔE8) is not modified by complex glycans and is therefore retained in the cytoplasm-it cannot localize to the cell membrane or reduce IL1R signaling. SIGIRR(ΔE8) interacts with and has a dominant-negative effect on SIGIRR, reducing its glycosylation, localization to the cell surface, and function. Most SIGIRR detected in human colon cancer tissues was cytoplasmic, whereas in nontumor tissues it was found at the cell membrane. Mice that expressed SIGIRR(N86/102S) developed more inflammation and formed larger tumors after administration of azoxymethane and dextran sulfate sodium than control mice; colon tissues from these mutant mice expressed higher levels of the inflammatory cytokines IL-17A and IL-6 had activation of the transcription factors STAT3 and NFκB. SIGIRR(N86/102S) expressed in colons of mice did not localize to the epithelial cell surface.

CONCLUSION

Levels of SIGIRR are lower in human colorectal tumors, compared with nontumor tissues; tumors contain the dominant-negative isoform SIGIRR(ΔE8). This mutant protein blocks localization of full-length SIGIRR to the surface of colon epithelial cells and its ability to downregulate IL1R signaling. Expression of SIGIRR(N86/102S) in the colonic epithelium of mice increases expression of inflammatory cytokines and formation and size of colitis-associated tumors.

A novel IL-17 signaling pathway controlling keratinocyte proliferation and tumorigenesis via the TRAF4-ERK5 axis

Wu et al. report a novel IL-17–mediated cascade via the IL-17R–TRAF4–ERK5 axis that directly stimulates keratinocyte proliferation and skin tumor formation in mice.

Although IL-17 is emerging as an important cytokine in cancer promotion and progression, the underlining molecular mechanism remains unclear. Previous studies suggest that IL-17 (IL-17A) sustains a chronic inflammatory microenvironment that favors tumor formation. Here we report a novel IL-17–mediated cascade via the IL-17R–Act1–TRAF4–MEKK3–ERK5 positive circuit that directly stimulates keratinocyte proliferation and tumor formation. Although this axis dictates the expression of target genes Steap4 (a metalloreductase for cell metabolism and proliferation) and p63 (a transcription factor for epidermal stem cell proliferation), Steap4 is required for the IL-17–induced sustained expansion of p63⁺ basal cells in the epidermis. P63 (a positive transcription factor for the Traf4 promoter) induces TRAF4 expression in keratinocytes. Thus, IL-17–induced Steap4-p63 expression forms a positive feedback loop through p63-mediated TRAF4 expression, driving IL-17–dependent sustained activation of the TRAF4–ERK5 axis for keratinocyte proliferation and tumor formation.

A novel IL-25 signaling pathway through STAT5

IL-25 is a member of the IL-17 family of cytokines that promotes Th2 cell-mediated inflammatory responses. IL-25 signals through a heterodimeric receptor (IL-25R) composed of IL-17RA and IL-17RB, which recruits the adaptor molecule Act1 for downstream signaling. Although the role of IL-25 in potentiating type 2 inflammation is well characterized by its ability to activate the epithelium as well as T cells, the components of its signaling cascade remain largely unknown. In this study, we found that IL-25 can directly activate STAT5 independently of Act1. Furthermore, conditional STAT5 deletion in T cells or epithelial cells led to a defective IL-25-initiated Th2 polarization as well as defective IL-25 enhancement of Th2 responses. Finally, we found that STAT5 is recruited to the IL-25R in a ligand-dependent manner through unique tyrosine residues on IL-17RB. Together, these findings reveal a novel Act1-independent IL-25 signaling pathway through STAT5 activation.

TRAF4-SMURF2-mediated DAZAP2 degradation is critical for IL-25 signaling and allergic airway inflammation

IL-25 promotes type 2 immunity by inducing the expression of Th2-associated cytokines. Although it is known that the IL-25R (IL-17RB) recruits the adaptor protein ACT1, the IL-25R signaling mechanism remains poorly understood. While screening for IL-25R components, we found that IL-25 responses were impaired in Traf4 (-/-) cells. Administering IL-25 to Traf4 (-/-) mice resulted in blunted airway eosinophilia and Th2 cytokine production. Notably, IL-25R recruitment of TRAF4 was required for the ACT1/IL-25R interaction. Mechanistically, TRAF4 recruited the E3-ligase SMURF2, to degrade the IL-25R-inhibitory molecule DAZAP2. Silencing Dazap2 increased ACT1/IL-25R interaction and IL-25 responsiveness. Moreover, a tyrosine within the IL-25R elicited DAZAP2 interference. This study indicates that TRAF4-SMURF2-mediated DAZAP2 degradation is a crucial initiating event for the IL-25 response.

IL-32 promotes angiogenesis

IL-32 is a multifaceted cytokine with a role in infections, autoimmune diseases, and cancer, and it exerts diverse functions, including aggravation of inflammation and inhibition of virus propagation. We previously identified IL-32 as a critical regulator of endothelial cell (EC) functions, and we now reveal that IL-32 also possesses angiogenic properties. The hyperproliferative ECs of human pulmonary arterial hypertension and glioblastoma multiforme exhibited a markedly increased abundance of IL-32, and, significantly, the cytokine colocalized with integrin αVβ3. Vascular endothelial growth factor (VEGF) receptor blockade, which resulted in EC hyperproliferation, increased IL-32 three-fold. Small interfering RNA-mediated silencing of IL-32 negated the 58% proliferation of ECs that occurred within 24 h in scrambled-transfected controls. Reduction of IL-32 neither affected apoptosis (insignificant changes in Bak-1, Bcl-2, Bcl-xL, lactate dehydrogenase, annexin V, and propidium iodide) nor VEGF or TGF-β levels, but siIL-32-transfected adult and neonatal ECs produced up to 61% less NO, IL-8, and matrix metalloproteinase-9, and up to 3-fold more activin A and endostatin. In coculture-based angiogenesis assays, IL-32γ dose-dependently increased tube formation up to 3-fold; an αVβ3 inhibitor prevented this activity and reduced IL-32γ-induced IL-8 by 85%. In matrigel plugs loaded with IL-32γ, VEGF, or vehicle and injected into live mice, we observed the anticipated VEGF-induced increase in neocapillarization (8-fold versus vehicle), but unexpectedly, IL-32γ was equally angiogenic. A second signal such as IFN-γ was required to render cells responsive to exogenous IL-32γ; importantly, this was confirmed using a completely synthetic preparation of IL-32γ. In summary, we add angiogenic properties that are mediated by integrin αVβ3 but VEGF-independent to the portfolio of IL-32, implicating a role for this versatile cytokine in pulmonary arterial hypertension and neoplastic diseases.

Cutting edge: TNF receptor-associated factor 4 restricts IL-17-mediated pathology and signaling processes

The effector T cell subset, Th17, plays a significant role in the pathogenesis of multiple sclerosis and of other autoimmune diseases. The signature cytokine, IL-17, engages the IL-17R and recruits the E3-ligase NF-κB activator 1 (Act1) upon stimulation. In this study, we examined the role of TNFR-associated factor (TRAF)4 in IL-17 signaling and Th17-mediated autoimmune encephalomyelitis. Primary cells from TRAF4-deficient mice displayed markedly enhanced IL-17-activated signaling pathways and induction of chemokine mRNA. Adoptive transfer of MOG35-55 specific wild-type Th17 cells into TRAF4-deficient recipient mice induced an earlier onset of disease. Mechanistically, we found that TRAF4 and TRAF6 used the same TRAF binding sites on Act1, allowing the competition of TRAF4 with TRAF6 for the interaction with Act1. Taken together, the results of this study reveal the necessity of a unique role of TRAF4 in restricting the effects of IL-17 signaling and Th17-mediated disease.

Protection from RNA and DNA viruses by IL-32

Several studies have documented a proinflammatory role for IL-32, which induces IL-1α, IL-1β, IL-6, TNF, and chemokines via NF-κB, p38MAPK, and AP-1. However, IL-32 also participates in the responses to infection with viruses such as HIV-1 and influenza. In this study, we explored these antiviral properties of IL-32. Vital staining assays demonstrated that low concentrations (5-10 ng/ml) of rIL-32γ protected epithelial WISH cells from vesicular stomatitis virus-induced cell death. By lactate dehydrogenase assays, treatment with IL-32γ resulted in a 3- to 4-fold decrease in viral load. Specific silencing of IL-32 revealed that the antiviral responses triggered by the synthetic analogs of ssRNA viruses (polyuridine) and dsRNA viruses (polyinosinic-polycytidylic acid) were significantly weaker (2- to 3-fold more virus) in WISH cells in the absence of IL-32. Importantly, we discovered that the polyinosinic-polycytidylic acid-induced increase in production of IFN-α in human PBMC was nearly completely abolished when IL-32 was silenced. Moreover, we observed that IL-32 antagonizes the DNA virus HSV-2 in epithelial Vero cells as well as in human umbilical cord endothelial cells, as production of HSV-2 increased 8-fold upon silencing of IL-32 (p < 0.001). Mechanistically, we found that IL-32 used the PKR-eIF-2α as well as the MxA antiviral pathways. Unexpectedly, a considerable part of the antiviral properties of IL-32 was not dependent on IFNs; specific blockade of IFN activity reduced the antiviral properties of IL-32 only moderately. In conclusion, these data suggest a central role for IL-32 in the immune response to RNA and DNA viruses, which may be exploitable for clinical use in the future.

Increased cytokine production in interleukin-18 receptor alpha-deficient cells is associated with dysregulation of suppressors of cytokine signaling

Since interleukin (IL)-18 is a proinflammatory cytokine, mice lacking IL-18 or its ligand-binding receptor (IL-18R) should exhibit decreased cytokine and chemokine production. Indeed, production of IL-1alpha, IL-6, and MIP-1alpha was reduced in IL-18 knock-out (ko) mouse embryonic fibroblast (MEF)-like cells. Unexpectedly, we observed a paradoxical 10-fold increase in IL-1beta-induced IL-6 production in MEF cells from mice deficient in the IL-18R alpha-chain (IL-18Ralpha) compared with wild type MEF. Similar increases were observed for IL-1alpha, MIP-1alpha, and prostaglandin E2. Likewise, coincubation with a specific IL-18Ralpha-blocking antibody augmented IL-1beta-induced cytokines in wild type and IL-18 ko MEF. Stable lines of IL-18Ralpha-depleted human A549 cells were generated using shRNA, resulting in an increase of IL-1beta-induced IL-1alpha, IL-6, and IL-8 compared to scrambled small hairpin RNA. In addition, we silenced IL-18Ralpha with small interfering RNA in primary human blood cells and observed up to 4-fold increases in the secretion of lipopolysaccharide- and IL-12/IL-18-induced IL-1beta, IL-6, interferon-gamma, and CD40L. Mechanistically, despite increases in Stat1 and IL-6, induction of SOCS1 and -3 (suppressor of cytokine signaling 1 and 3) was markedly reduced in the absence of IL-18Ralpha. Consistent with these observations, activation of the p38alpha/beta and ERK1/2 MAPKs and of protein kinase B/Akt increased in IL-18Ralpha ko MEF, whereas the negative feedback kinase MSK2 was more active in IL-18 ko cells. These data reveal a role for SOCS1 and -3 in the seemingly paradoxical hyperresponsive state in cells deficient in IL-18Ralpha, supporting the concept that IL-18Ralpha participates in both pro- and anti-inflammatory responses and that an endogenous ligand engages IL-18Ralpha to deliver an inhibitory signal.

Endogenous IL-32 controls cytokine and HIV-1 production

IL-32, a proinflammatory cytokine that activates the p38MAPK and NF-kappaB pathways, induces other cytokines, for example, IL-1beta, IL-6, and TNF-alpha. This study investigated the role of endogenous IL-32 in HIV-1 infection by reducing IL-32 with small interfering (si)RNA in freshly infected PBMC and in the latently infected U1 macrophage cell line. When PBMC were pretreated with siRNA to IL-32 (siIL-32), IL-6, IFN-gamma, and TNF-alpha were reduced by 57, 51, and 36%, respectively, compared with scrambled siRNA. Cotransfection of NF-kappaB and AP-1 reporter constructs with siIL-32 decreased DNA binding of these transcription factors by 42 and 46%, respectively. Cytokine protein array analysis revealed that the inhibitory activity of siIL-32 primarily targeted Th1 and proinflammatory cytokines and chemokines, e.g., MIP-1alpha/beta. Unexpectedly, HIV-1 production (as measured by p24) increased 4-fold in these same PBMC when endogenous IL-32 was reduced. Because IFN-gamma was lower in siIL-32-treated PBMC, we blocked IFN-gamma bioactivity, which enhanced the augmentation of p24 by siIL-32. Furthermore, siIL-32 reduced the natural ligands of the HIV-1 coreceptors CCR5 (MIP-1alpha/beta and RANTES) and CXCR4 (SDF-1). Inhibition of endogenous IL-32 in U1 macrophages also increased HIV-1. When rhIL-32gamma was added to these cells, p24 levels fell by 72%; however, in the same cultures IFN-alpha increased 4-fold. Blockade of IFN-alpha/beta bioactivity in IL-32gamma-stimulated U1 cells revealed that IFN-alpha conveys the anti-HIV-1 effect of rhIL-32gamma. In summary, depletion of endogenous IL-32 reduced the levels of Th1 and proinflammatory cytokines but paradoxically increased p24, proposing IL-32 as a natural inhibitor of HIV-1.