Microglia C-lectin/selectin' neurons to eat.

ß-glucosylceramide (ß-GlcCer) accumulates in Gaucher disease, but how ß-GlcCer, a Mincle ligand, causes characteristic neuroinflammation and neuronopathy is poorly understood. In this issue of Immunity, Shimizu et al. reveal that Mincle-dependent activation of microglia led to phagocytosis of neurons and neurologic symptoms.

Dectin-1 limits autoimmune neuroinflammation and promotes myeloid cell-astrocyte crosstalk via Card9-independent expression of Oncostatin M.

Pathologic roles of innate immunity in neurologic disorders are well described, but their beneficial aspects are less understood. Dectin-1, a C-type lectin receptor (CLR), is largely known to induce inflammation. Here, we report that Dectin-1 limited experimental autoimmune encephalomyelitis (EAE), while its downstream signaling molecule, Card9, promoted the disease. Myeloid cells mediated the pro-resolution function of Dectin-1 in EAE with enhanced gene expression of the neuroprotective molecule, Oncostatin M (Osm), through a Card9-independent pathway, mediated by the transcription factor NFAT. Furthermore, we find that the Osm receptor (OsmR) functioned specifically in astrocytes to reduce EAE severity. Notably, Dectin-1 did not respond to heat-killed Mycobacteria, an adjuvant to induce EAE. Instead, endogenous Dectin-1 ligands, including galectin-9, in the central nervous system (CNS) were involved to limit EAE. Our study reveals a mechanism of beneficial myeloid cell-astrocyte crosstalk regulated by a Dectin-1 pathway and identifies potential therapeutic targets for autoimmune neuroinflammation.

Skewing of the population balance of lymphoid and myeloid cells by secreted and intracellular osteopontin.

The balance of myeloid populations and lymphoid populations must be well controlled. Here we found that osteopontin (OPN) skewed this balance during pathogenic conditions such as infection and autoimmunity. Notably, two isoforms of OPN exerted distinct effects in shifting this balance through cell-type-specific regulation of apoptosis. Intracellular OPN (iOPN) diminished the population size of myeloid progenitor cells and myeloid cells, and secreted OPN (sOPN) increase the population size of lymphoid cells. The total effect of OPN on skewing the leukocyte population balance was observed as host sensitivity to early systemic infection with Candida albicans and T cell-mediated colitis. Our study suggests previously unknown detrimental roles for two OPN isoforms in causing the imbalance of leukocyte populations.

Are Neuropeptide-Reactive T Cells behind Narcolepsy?

Researchers have previously hypothesized autoimmune origins for narcolepsy on the basis of its strong genetic association with an MHC class II allele. In a recent issue of Nature, Latorre et al. (2018) discovered that narcolepsy patients had autoreactive T cells specific to the neuronal antigen hypocretin, providing more evidence of the potential immune origin of the disease.

Host immune responses in the central nervous system during fungal infections.

Fungal infections in the central nervous system (CNS) cause high morbidity and mortality. The frequency of CNS mycosis has increased over the last two decades as more individuals go through immunocompromised conditions for various reasons. Nevertheless, options for clinical interventions for CNS mycoses are still limited. Thus, there is an urgent need to understand the host-pathogen interaction mechanisms in CNS mycoses for developing novel treatments. Although the CNS has been regarded as an immune-privileged site, recent studies demonstrate the critical involvement of immune responses elicited by CNS-resident and CNS-infiltrated cells during fungal infections. In this review, we discuss mechanisms of fungal invasion in the CNS, fungal pathogen detection by CNS-resident cells (microglia, astrocytes, oligodendrocytes, neurons), roles of CNS-infiltrated leukocytes, and host immune responses. We consider that understanding host immune responses in the CNS is crucial for endeavors to develop treatments for CNS mycosis.

Osteopontin (OPN)/SPP1: from its biochemistry to biological functions in the innate immune system and the central nervous system (CNS).

Osteopontin (OPN) is a multifunctional protein, initially identified in osteosarcoma cells with its role of mediating osteoblast adhesion. Later studies revealed that OPN is associated with many inflammatory conditions caused by infections, allergic responses, autoimmunity and tissue damage. Many cell types in the peripheral immune system express OPN with various functions, which could be beneficial or detrimental. Also, more recent studies demonstrated that OPN is highly expressed in the central nervous system (CNS), particularly in microglia during CNS diseases and development. However, understanding of mechanisms underlying OPN's functions in the CNS is still limited. In this review, we focus on peripheral myeloid cells and CNS-resident cells to discuss the expression and functions of OPN.

Emerging roles of Dectin-1 in noninfectious settings and in the CNS.

Dectin-1 is a C-type lectin receptor (CLR) expressed on the surface of various mammalian myeloid cells. Dectin-1 recognizes ß-glucans and elicits antifungal proinflammatory immune responses. Recent studies have begun to examine the biology of Dectin-1 in previously less explored settings, such as homeostasis, sterile inflammation, and in the central nervous system. Indeed, in certain contexts, Dectin-1 is now known to promote tolerance, and anti-inflammatory and neuroprotective responses. In this review, we provide an overview of the current understanding of the roles of Dectin-1 in immunology beyond the context of fungal infections, mainly focusing on in vivo neuroimmunology studies, which could reveal new therapeutic approaches to modify innate immune responses in neurologic disorders.

Targeting Inflammasomes to Treat Neurological Diseases.

Inflammasomes are multimeric protein complexes that can sense a plethora of microbe- and damage-associated molecular signals. They play important roles in innate immunity and are key regulators of inflammation in health and disease. Inflammasome-mediated processing and secretion of proinflammatory cytokines such as interleukin (IL) 1ß and IL-18 and induction of pyroptosis, a proinflammatory form of cell death, have been associated with the development and progression of common immune-mediated and degenerative central nervous system (CNS) diseases such as Alzheimer disease, multiple sclerosis, brain injury, stroke, epilepsy, Parkinson disease, and amyotrophic lateral sclerosis. A growing number of pharmacological compounds inhibiting inflammasome activation and signaling show therapeutic efficacy in preclinical models of the aforementioned disease conditions. Here, we illustrate regulatory mechanisms of inflammasome activation during CNS homeostasis and tissue injury. We highlight the evidence for inflammasome activation as a mechanistic underpinning in a wide range of CNS diseases and critically discuss the promise and potential limitations of therapeutic strategies that aim to inhibit the inflammasome components in neurological disorders. ANN NEUROL 2021;90:177-188.

The Ubiquitin-Modifying Enzyme A20 Terminates C-Type Lectin Receptor Signals and Is a Suppressor of Host Defense against Systemic Fungal Infection.

C-type lectin receptors (CLRs) play key roles in antifungal defense. CLR-induced NF-κB is central to CLR functions in immunity, and thus, molecules that control the amplitude of CLR-induced NF-κB could profoundly influence host defense against fungal pathogens. However, little is known about the mechanisms that negatively regulate CLR-induced NF-κB, and molecules which act on the CLR family broadly and which directly regulate acute CLR-signaling cascades remain unidentified. Here, we identify the ubiquitin-editing enzyme A20 as a negative regulator of acute NF-κB activation downstream of multiple CLR pathways. Absence of A20 suppression results in exaggerated CLR responses in cells which are A20 deficient and also cells which are A20 haplosufficient, including multiple primary immune cells. Loss of a single allele of A20 results in enhanced defense against systemic Candida albicans infection and prolonged host survival. Thus, A20 restricts CLR-induced innate immune responses in vivo and is a suppressor of host defense against systemic fungal infection.

Yes, research matters.

My father was diagnosed with stomach cancer recently. Luckily, it was still at an early stage, and endoscopic surgery successfully took care of it. My father was fortunate; since people with stomach cancer do not show clear symptoms in the early stages, the disease is often not diagnosed until it becomes advanced. In his case, the diagnosis started from a suggestion by his doctor to check whether he had a gastric infection with Helicobacter pylori, a bacterial species found in the digestive tract. In Japan, where he lives, a majority of gastric cancer patients (more than 99%) have been infected with H. pylori [1], and the causative role of this bacterial species in promoting gastric cancer is very well established. Now, scientific understanding connecting gastric cancer to H. pylori is saving the lives of many people, including my father. Thinking about this recent personal experience, I wonder if the connection between bacteria and cancer might have been considered a crazy idea decades ago. Research makes it possible to connect seemingly unrelated matters. My laboratory works on seemingly unrelated research topics, such as fungal infections and autoimmunity. However, my question is the same whatever the topic: How do leukocytes elicit and regulate inflammation when they detect infections or endogenous signals? In fact, host receptors detecting pathogens can induce autoimmunity, and autoimmunity alters host sensitivity to pathogens due to the imbalance in the immune system. We are beginning to gain some insight into this question, as revealed by some of our recent studies. For example, the NLR family, pyrin domain containing 3 (NLRP3) inflammasome, which is known to sense a wide variety of pathogens, can also change the course of experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS). In particular, our study suggested that disease treatment approaches need to be changed based on the activation status of the NLRP3 inflammasome [2]. Another recent study from our laboratory demonstrated that a protein, termed osteopontin (OPN), skews the balance of population sizes between myeloid cells (i.e., innate immunity) and lymphoid cells (i.e., adaptive immunity) during infections and other biological insults [3]. An intracellular isoform of OPN (iOPN) negatively regulates emergency myelopoiesis. Thus, OPN attenuates host resistance by limiting neutrophil supply at the early stage of systemic Candida infection. In contrast, a secreted OPN (sOPN) isoform positively regulates the expansion of T lymphocytes and ends up triggering autoimmune colitis. I am an immunologist but obtained my PhD in mycology. Nevertheless, it took some time for me to appreciate that research enables us to connect the dots placed far apart. This is a truly exciting time to connect seemingly unrelated biological phenomena, because scientists are exponentially increasing our understanding of nature. This is particularly true in innate immunity, which is not only the central alarming system in host-microbe interactions but also relates to almost any human disease we can imagine. However, we are facing a dark age for science and research, in which certain interests wrongfully discredit some research fields. There are things that can be achieved only by research. I am always ready to tell anyone, "Yes, research matters!".

Metabolic Alterations Contribute to Enhanced Inflammatory Cytokine Production in Irgm1-deficient Macrophages.

The immunity-related GTPases (IRGs) are a family of proteins that are induced by interferon (IFN)-γ and play pivotal roles in immune and inflammatory responses. IRGs ostensibly function as dynamin-like proteins that bind to intracellular membranes and promote remodeling and trafficking of those membranes. Prior studies have shown that loss of Irgm1 in mice leads to increased lethality to bacterial infections as well as enhanced inflammation to non-infectious stimuli; however, the mechanisms underlying these phenotypes are unclear. In the studies reported here, we found that uninfected Irgm1-deficient mice displayed high levels of serum cytokines typifying profound autoinflammation. Similar increases in cytokine production were also seen in cultured, IFN-γ-primed macrophages that lacked Irgm1. A series of metabolic studies indicated that the enhanced cytokine production was associated with marked metabolic changes in the Irgm1-deficient macrophages, including increased glycolysis and an accumulation of long chain acylcarnitines. Cells were exposed to the glycolytic inhibitor, 2-deoxyglucose, or fatty acid synthase inhibitors to perturb the metabolic alterations, which resulted in dampening of the excessive cytokine production. These results suggest that Irgm1 deficiency drives metabolic dysfunction in macrophages in a manner that is cell-autonomous and independent of infectious triggers. This may be a significant contributor to excessive inflammation seen in Irgm1-deficient mice in different contexts.

Osteopontin has a protective role in prostate tumor development in mice.

Osteopontin (OPN) is a protein, generally considered to play a pro-tumorigenic role, whereas several reports have demonstrated the anti-tumorigenic function of OPN during tumor development. These opposing anti- and pro-tumorigenic functions are not fully understood. Here, we report that host-derived OPN plays an anti-tumorigenic role in the transgenic adenocarcinoma of the mouse prostate (TRAMP) model and a TRAMP tumor transplant model. Tumor suppression mediated by OPN in Rag2-/- mice suggests that OPN is dispensable in the adaptive immune response. We found that host-derived OPN enhanced infiltration of natural killer (NK) cells into TRAMP tumors. The requirement of OPN in NK cell migration towards TRAMP cells was confirmed by an ex vivo cell migration assay. In contrast to TRAMP cells, in vivo B16 tumor development was not inhibited by OPN, and B16 tumors did not show OPN-mediated cell recruitment. It is possible that low levels of chemokine expression by B16 cells do not allow OPN to enhance immune cell recruitment. In addition to demonstrating the anti-tumorigenic role of OPN in TRAMP tumor development, this study also suggests that the contribution of OPN to tumor development depends on the type of tumor as well as the source and isoform of OPN.

Leptin directly promotes T-cell glycolytic metabolism to drive effector T-cell differentiation in a mouse model of autoimmunity.

Upon activation, T cells require energy for growth, proliferation, and function. Effector T (Teff) cells, such as Th1 and Th17 cells, utilize high levels of glycolytic metabolism to fuel proliferation and function. In contrast, Treg cells require oxidative metabolism to fuel suppressive function. It remains unknown how Teff/Treg-cell metabolism is altered when nutrients are limited and leptin levels are low. We therefore examined the role of malnutrition and associated hypoleptinemia on Teff versus Treg cells. We found that both malnutrition-associated hypoleptinemia and T cell-specific leptin receptor knockout suppressed Teff-cell number, function, and glucose metabolism, but did not alter Treg-cell metabolism or suppressive function. Using the autoimmune mouse model EAE, we confirmed that fasting-induced hypoleptinemia altered Teff-cell, but not Treg-cell, glucose metabolism, and function in vivo, leading to decreased disease severity. To explore potential mechanisms, we examined HIF-1α, a key regulator of Th17 differentiation and Teff-cell glucose metabolism, and found HIF-1α expression was decreased in T cell-specific leptin receptor knockout Th17 cells, and in Teff cells from fasted EAE mice, but was unchanged in Treg cells. Altogether, these data demonstrate a selective, cell-intrinsic requirement for leptin to upregulate glucose metabolism and maintain function in Teff, but not Treg cells.

Lung inflammation stalls Th17-cell migration en route to the central nervous system during the development of experimental autoimmune encephalomyelitis.

Recruiting pathogenic T cells to the central nervous system (CNS) is a critical step during the development of experimental autoimmune encephalomyelitis (EAE). Here, we report that the absence of autophagy and microtubule-associated protein 1A/1B-light chain 3-associated phagocytosis significantly delayed the onset of EAE in Atg7 conditional knockout (Atg7 CKO) mice in myeloid cells. T-helper cell-cell priming appeared to be normal in the Atg7 CKO mice, but the mice showed significant accumulation of Th17 cells in the lung. The data suggested that the stalling of Th17 cells in the lung en route to the CNS caused the delay. The lung of Atg7 CKO mice, in which we previously demonstrated spontaneous mild inflammation, showed high expression of CCL20, a chemokine that attracts Th17 cells. We have also shown that LPS intranasal instillation delayed EAE onset, suggesting that pulmonary inflammation has an impact on EAE development. Based on our data, therapeutic immunomodulation targeted to the lung, rather than systemically, might be a possible future option to treat multiple sclerosis.

Engagement of the type I interferon receptor on dendritic cells inhibits T helper 17 cell development: role of intracellular osteopontin.

Mechanisms that prevent inappropriate or excessive interleukin-17-producing T helper (Th17) cell responses after microbial infection may be necessary to avoid autoimmunity. Here, we define a pathway initiated by engagement of type I IFN receptor (IFNAR) expressed by dendritic cells (DC) that culminated in suppression of Th17 cell differentiation. IFNAR-dependent inhibition of an intracellular translational isoform of Osteopontin, termed Opn-i, derepressed interleukin-27 (IL-27) secretion and prevented efficient Th17 responses. Moreover, Opn-i expression in DC and microglia regulated the type and intensity of experimental autoimmune encephalomyelitis (EAE). Mice containing DC deficient in Opn-i produced excessive amounts of IL-27 and developed a delayed disease characterized by an enhanced Th1 response compared with the dominant Th17 response of Opn-sufficient mice. Definition of the IFNAR-Opn-i axis that controls Th17 development provides insight into regulation of Th cell sublineage development and the molecular basis of type I interferon therapy for MS and other autoimmune diseases.

Cutting edge: Role of osteopontin and integrin αv in T cell-mediated anti-inflammatory responses in endotoxemia.

The immune system is equipped with mechanisms that downregulate hyperinflammation to avoid collateral damage. We demonstrated recently that unprimed T cells downregulate macrophage TNF production through direct interaction with macrophages in the spleen during LPS endotoxemia. How T cell migration toward macrophages occurs upon LPS injection is still not clear. In this study, we demonstrate that secreted osteopontin (sOPN) plays a role in the T cell migration to initiate the suppression of hyperinflammation during endotoxemia. Osteopontin levels in splenic macrophages were upregulated 2 h after LPS treatment, whereas T cell migration toward macrophages was observed 3 h after treatment. Neutralization of sOPN and blockade of its receptor, integrin αv, significantly inhibited CD4(+) T cell migration and increased susceptibility to endotoxemia. Our study demonstrates that the sOPN/integrin αv axis, which induces T cell chemotaxis toward macrophages, is critical for suppressing hyperinflammation during the first 3 h of endotoxemia.

The lung is protected from spontaneous inflammation by autophagy in myeloid cells.

The lung is constantly exposed to the outer environment; thus, it must maintain a state of immune ignorance or tolerance not to overrespond to harmless environmental stimuli. How cells in the lung control immune responses under nonpathogenic condition is not fully understood. In this study, we found that autophagy plays a critical role in the lung-specific immune regulation that prevents spontaneous inflammation. Autophagy in pulmonary myeloid cells plays a role in maintaining low burdens of environmental microbes in the lung, as well as in lowering mitochondrial reactive oxygen species production and preventing overresponse to TLR4 ligands in alveolar macrophages. Based on these mechanisms, we also found that intranasal instillation of antibiotics or an inhibitor of reactive oxygen species was efficient in preventing spontaneous pulmonary inflammation. Thus, autophagy in myeloid cells, particularly alveolar macrophages, is critical for inhibiting spontaneous pulmonary inflammation, and pulmonary inflammation caused by dysfunctional autophagy is pharmacologically prevented.

T cells down-regulate macrophage TNF production by IRAK1-mediated IL-10 expression and control innate hyperinflammation.

Endotoxemia is caused by excessive inflammation, but the immune system has various mechanisms to avoid collateral organ damage in endotoxemia. A handful of reports have shown that innate immune responses are suppressed by the adaptive immune system. However, the molecular mechanism by which adaptive immune cells suppress innate inflammatory responses is not clear. Here, we report that T cells are shown to interact with macrophages at the early stage of enodotoxemia and to prolong survival of mice through controlling TNF and IL-10 levels by macrophage CD40 stimulation. The cross-talk between CD40 and toll-like receptor (TLR4) signaling first mediates IL-1 receptor-associated kinase 1 (IRAK1) nuclear translocation and its binding to the IL-10 gene promoter in macrophages, without interfering with the NFκB pathway. IL-10 is then detected by macrophages in an autocrine fashion to destabilize Tnfa mRNA. To induce IRAK1-mediated IL-10 expression, signals from both CD40 and TLR4 are essential. CD40 signaling induces IRAK1 sumoylation in the presence of TNF receptor-associated factor 2 (TRAF2) and intracellular isoform of osteopontin (iOPN) whereas TLR4 signaling provides IFN regulatory factor 5 (IRF5) as a chaperone for sumoylated IRAK1 nuclear translocation. Interaction of T cells with macrophages was observed in the spleen in vivo after endotoxemia induction with LPS injection. Our study demonstrates a mechanistic basis for the immunosuppressive role of macrophage CD40 in LPS endotoxemia.

Calcineurin orchestrates dimorphic transitions, antifungal drug responses and host-pathogen interactions of the pathogenic mucoralean fungus Mucor circinelloides.

Calcineurin plays essential roles in virulence and growth of pathogenic fungi and is a target of the natural products FK506 and Cyclosporine A. In the pathogenic mucoralean fungus Mucor circinelloides, calcineurin mutation or inhibition confers a yeast-locked phenotype indicating that calcineurin governs the dimorphic transition. Genetic analysis in this study reveals that two calcineurin A catalytic subunits (out of three) are functionally diverged. Homology modeling illustrates modes of resistance resulting from amino substitutions in the interface between each calcineurin subunit and the inhibitory drugs. In addition, we show how the dimorphic transition orchestrated by calcineurin programs different outcomes during host-pathogen interactions. For example, when macrophages phagocytose Mucor yeast, subsequent phagosomal maturation occurs, indicating host cells respond appropriately to control the pathogen. On the other hand, upon phagocytosis of spores, macrophages fail to form mature phagosomes. Cytokine production from immune cells differs following exposure to yeast versus spores (which germinate into hyphae). Thus, the morphogenic transition can be targeted as an efficient treatment option against Mucor infection. In addition, genetic analysis (including gene disruption and mutational studies) further strengthens the understanding of calcineurin and provides a foundation to develop antifungal agents targeting calcineurin to deploy against Mucor and other pathogenic fungi.

Mnk1 and 2 are dispensable for T cell development and activation but important for the pathogenesis of experimental autoimmune encephalomyelitis.

T cell development and activation are usually accompanied by expansion and production of numerous proteins that require active translation. The eukaryotic translation initiation factor 4E (eIF4E) binds to the 5' cap structure of mRNA and is critical for cap-dependent translational initiation. It has been hypothesized that MAPK-interacting kinase 1 and 2 (Mnk1/2) promote cap-dependent translation by phosphorylating eIF4E at serine 209 (S209). Pharmacologic studies using inhibitors have suggested that Mnk1/2 have important roles in T cells. However, genetic evidence supporting such conclusions is lacking. Moreover, the signaling pathways that regulate Mnk1/2 in T cells remain unclear. We demonstrate that TCR engagement activates Mnk1/2 in primary T cells. Such activation is dependent on Ras-Erk1/2 signaling and is inhibited by diacylglycerol kinases α and ζ. Mnk1/2 double deficiency in mice abolishes TCR-induced eIF4E S209 phosphorylation, indicating their absolute requirement for eIF4E S209 phosphorylation. However, Mnk1/2 double deficiency does not affect the development of conventional αß T cells, regulatory T cells, or NKT cells. Furthermore, T cell activation, in vivo primary and memory CD8 T cell responses to microbial infection, and NKT cell cytokine production were not obviously altered by Mnk1/2 deficiency. Although Mnk1/2 deficiency causes decreased IL-17 and IFN-γ production by CD4 T cells following immunization of mice with myelin oligodendrocyte glycoprotein peptide in complete Freund's adjuvant, correlating with milder experimental autoimmune encephalomyelitis scores, it does not affect Th cell differentiation in vitro. Together, these data suggest that Mnk1/2 has a minimal role in T cell development and activation but may regulate non-T cell lineages to control Th1 and Th17 differentiation in vivo.