Brush cells fine-tune neurogenic inflammation in the airways

Airway epithelial cells, once considered a simple barrier layer, are now recognized as providing an active site for antigen sensing and immune response initiation. Most mucosal sites contain chemosensory epithelial cells, rare and specialized cells gaining recognition for their unique functions in sensing and directing the immune response symphony. In this issue of the JCI, Hollenhorst, Nandigama, et al. demonstrated that tracheal chemosensory brush cells detected bitter-tasting substances, including quorum-sensing molecules (QSMs) generated by pathogenic Pseudomonas aeruginosa. The authors used various techniques, including genetic deletion of brush cells, genetic manipulation of brush cell signaling, deletion of sensory neurons, in vivo imaging, and infection models with P. aeruginosa, to show that QSMs increased vascular permeability and innate immune cell influx into the trachea. These findings link the recognition of bacterial QSMs to the innate immune response in the airways, with translational implications for airway inflammation and infectious pathology.     

Evolutionarily conserved recognition and innate immunity to fungal pathogens by the scavenger receptors SCARF1 and CD36

Receptors involved in innate immunity to fungal pathogens have not been fully elucidated. We show that the Caenorhabditis elegans receptors CED-1 and C03F11.3, and their mammalian orthologues, the scavenger receptors SCARF1 and CD36, mediate host defense against two prototypic fungal pathogens, Cryptococcus neoformans and Candida albicans. CED-1 and C03F11.1 mediated antimicrobial peptide production and were necessary for nematode survival after C. neoformans infection. SCARF1 and CD36 mediated cytokine production and were required for macrophage binding to C. neoformans, and control of the infection in mice. Binding of these pathogens to SCARF1 and CD36 was β-glucan dependent. Thus, CED-1/SCARF1 and C03F11.3/CD36 are β-glucan binding receptors and define an evolutionarily conserved pathway for the innate sensing of fungal pathogens.Yeasts such as Cryptococcus neoformans and Candida albicans are an emerging group of infectious pathogens in patients with impaired T cell–mediated immunity, such as those with AIDS, solid organ transplant recipients, and hospitalized patients (1, 2). Although much work has been devoted in the past decade to uncovering the role of pattern recognition receptors in the innate response to bacteria and viruses, our understanding of how the innate immune system senses fungal pathogens is less clear (3). The macrophage, a central component of the innate immune system, is essential to the effective immune response to pathogenic yeast. Macrophages have direct antimicrobial activity against these organisms; they promote antigen presentation, polysaccharide sequestration, and cytokine and chemokine production (4–6). There is also evidence that persistent infection is associated with the intracellular residence of yeast cells in macrophages. Furthermore, infected circulating macrophages can transfer these pathogens and cause dissemination of these infections hematogenously (7).Recently, the C-type lectin-like receptor Dectin-1 was shown to be a macrophage receptor for β-glucans and to bind several fungi (8–12). β-glucan is a major carbohydrate found in the fungal cell wall. Dectin-1 mediates both Toll-like receptor (TLR)–dependent and TLR-independent responses to fungi in vitro. However, the role of Dectin-1 in the host response to fungal pathogens in vivo is less clear (13–15), suggesting that additional receptors contribute to the innate immune response to fungal pathogens.Scavenger receptors constitute a diverse family of pattern recognition receptors that recognize both endogenous and pathogen-derived ligands. These receptors are expressed on cells patrolling potential portals of pathogen entry, including macrophages, dendritic cells, microglia, and endothelial cells, and are believed to be involved in the pathogenesis of chronic inflammatory conditions such as atherosclerosis and Alzheimer''s disease, and in the host response to some bacterial pathogens (16–21). To date, a role for scavenger receptors in the recognition of fungal and yeast pathogens has not been described.Using an shRNA screen in mouse macrophages, we found that two evolutionarily conserved members of the scavenger receptor family, SCARF1 and CD36, and their Caenorhabditis elegans orthologues, CED-1 and C03F11.3, are required for the induction of protective immune responses in worms and mice to pathogenic yeast. We show that these receptors recognize β-glucans and have an essential function in antifungal immunity and host defense by mediating yeast binding and subsequent macrophage activation.     

STALing B cell responses with CD22

Suppressing unwanted humoral immune responses without compromising the host’s ability to respond to foreign pathogens is a primary goal for therapies aimed at ameliorating harmful autoantibody production. Global suppression of the immune system via lymphocyte depletion and/or immunosuppressive drugs can have off-target effects, a limitation to conventional therapies. In this issue of the JCI, Macauley and colleagues utilize a novel platform to inhibit antigen-specific antibody production that preserves the immune system’s ability to respond to unrelated antigens. B cells are critical for providing protection from disease because the Abs they produce offer an important first line of defense from invading pathogens. However, B cells can also contribute to harmful immune responses especially when their Ab production is directed toward self antigens (Ags), as is the case in many autoimmune diseases. Finding ways to manipulate B cell responses — either enhancing them for vaccine development or inhibiting them in patients with autoimmune and allergic diseases — has been a long-standing goal for vaccinologists and clinicians alike. One promising strategy that has emerged for immune modulation is the targeted delivery of Ag to specific immune cell subsets (1). This approach uses mAbs or ligands specific for cell surface receptors to deliver linked Ags directly to immune cells upon injection in vivo. In this issue of the JCI, Macauley and colleagues introduce a novel Ag-targeting approach to inhibit B cell responses that combines both Ag specificity and negative regulation of B cell receptor (BCR) signaling (2).     

TREM-2 promotes macrophage survival and lung disease after respiratory viral infection

Viral infections and type 2 immune responses are thought to be critical for the development of chronic respiratory disease, but the link between these events needs to be better defined. Here, we study a mouse model in which infection with a mouse parainfluenza virus known as Sendai virus (SeV) leads to long-term activation of innate immune cells that drive IL-13–dependent lung disease. We find that chronic postviral disease (signified by formation of excess airway mucus and accumulation of M2-differentiating lung macrophages) requires macrophage expression of triggering receptor expressed on myeloid cells-2 (TREM-2). Analysis of mechanism shows that viral replication increases lung macrophage levels of intracellular and cell surface TREM-2, and this action prevents macrophage apoptosis that would otherwise occur during the acute illness (5–12 d after inoculation). However, the largest increases in TREM-2 levels are found as the soluble form (sTREM-2) long after clearance of infection (49 d after inoculation). At this time, IL-13 and the adapter protein DAP12 promote TREM-2 cleavage to sTREM-2 that is unexpectedly active in preventing macrophage apoptosis. The results thereby define an unprecedented mechanism for a feed-forward expansion of lung macrophages (with IL-13 production and consequent M2 differentiation) that further explains how acute infection leads to chronic inflammatory disease.A critical step toward improved diagnosis and treatment of chronic inflammatory diseases depends on defining the immune mechanisms for the persistent accumulation of activated immune cells in the target tissue. In the case of the lung, clinical evidence suggests that acute infection with a respiratory virus might lead to chronic lung diseases such as asthma and COPD (Holtzman, 2012). To determine precisely how acute infection causes chronic lung disease, we developed a high-fidelity mouse model of this process. In this model, mouse parainfluenza virus (also known as Sendai virus, SeV) is substituted for the related human pathogen to achieve more efficient viral replication and thereby produce the severe acute illness and subsequent chronic respiratory disease that is typical of the pathology found in humans (Walter et al., 2002). Using this model system, we determined that postviral lung disease depends on airway progenitor epithelial cell (APEC) production of IL-33 to drive invariant NK T cells (iNKT cells) and lung macrophages toward IL-13 production (Kim et al., 2008; Byers et al., 2013). The result is IL-13–dependent inflammation (signified by type 2 activation and accumulation of lung macrophages) and airway mucus production (signified by MUC5AC mucin gene expression). This innate epithelial to immune cell loop also appears relevant to human disease because increased numbers of IL-33–expressing APECs are found in association with an IL-13 gene expression signature (including increased MUC5AC mRNA and protein) in the lungs of humans with severe chronic obstructive pulmonary disease (COPD; Kim et al., 2008; Agapov et al., 2009; Alevy et al., 2012; Byers et al., 2013).In our previous work, we recognized that the APEC population was capable of self-renewal and inducible release of IL-33 to sustain ongoing activation of the innate immune system (Holtzman et al., 2014). However, the existing data did not explain the selective activation of the lung macrophage population and the special dominance of type 2 (M2) macrophages as a downstream part of the disease process. In the present study, we therefore aimed to better understand how the lung macrophage component of this disease process is triggered by acute infection and then is manifest for months. We reasoned that triggering receptor expressed on myeloid cells 2 (TREM-2) might contribute to this process because M2 polarization is associated with TREM-2 expression in isolated macrophages (Turnbull et al., 2006). In pursuing this possibility, we found that the soluble form of TREM-2 (sTREM-2) was linked to the development of chronic postviral lung disease and was active in promoting macrophage survival. The data stand in contrast to the conventional view that cleavage of cell surface TREM-2 to sTREM-2 results in an inactive end product. The results thereby provide for a previously unrecognized control over macrophage survival and a consequent type 2 immune response that can serve both as a pathogenic mechanism and as a therapeutic target and accompanying biomarker for chronic inflammatory disease.     

Respiratory influenza virus infection induces intestinal immune injury via microbiota-mediated Th17 cell–dependent inflammation

Influenza in humans is often accompanied by gastroenteritis-like symptoms such as diarrhea, but the underlying mechanism is not yet understood. We explored the occurrence of gastroenteritis-like symptoms using a mouse model of respiratory influenza infection. We found that respiratory influenza infection caused intestinal injury when lung injury occurred, which was not due to direct intestinal viral infection. Influenza infection altered the intestinal microbiota composition, which was mediated by IFN-γ produced by lung-derived CCR9⁺CD4⁺ T cells recruited into the small intestine. Th17 cells markedly increased in the small intestine after PR8 infection, and neutralizing IL-17A reduced intestinal injury. Moreover, antibiotic depletion of intestinal microbiota reduced IL-17A production and attenuated influenza-caused intestinal injury. Further study showed that the alteration of intestinal microbiota significantly stimulated IL-15 production from intestinal epithelial cells, which subsequently promoted Th17 cell polarization in the small intestine in situ. Thus, our findings provide new insights into an undescribed mechanism by which respiratory influenza infection causes intestinal disease.Influenza is an infectious respiratory disease affecting many bird and mammal species (Laver and Webster, 1979; Reid et al., 1999). Clinically, the most common symptoms include cough, fever, headache, and weakness (Monto et al., 2000). These symptoms are often accompanied by gastroenteritis-like symptoms in many influenza patients, such as abdominal pain, nausea, vomiting, and diarrhea, especially in young children (Baden et al., 2009; Shinde et al., 2009; Dilantika et al., 2010). However, the immune mechanisms underlying these clinical manifestations in the intestine during a lung-tropic viral influenza infection remain unclear.The intestinal tracts in humans and other animals are inhabited by hundreds of diverse species of commensal bacteria, which are essential in shaping intestinal immune responses during both health and disease (Hooper and Gordon, 2001; Chervonsky, 2009). Distinct components of commensal bacteria were associated with special status of the immune system. Although most commensal bacteria are beneficial (Ichinohe et al., 2011), a few can be potentially harmful in some conditions; for example, some commensal bacteria have been suggested to influence susceptibility to inflammatory bowel disease (IBD; Garrett et al., 2007; Mazmanian et al., 2008). Thus, when conditions in the host are unfavorable, such as during infection, the resulting changes within the intestinal tract environment may promote growth of the harmful bacteria that induce intestinal disease.It is well known that the respiratory and intestinal tracts are both mucosal tissues. Over 30 years ago, John Bienenstock hypothesized that the immune cells and structures contained in mucosal tissues were universally connected within the whole body. This “common mucosal immune system” concept speculated that the mucosal immune system was itself an “organ” in which the mucosal immune cells distributed throughout the body could interplay between or among different mucosal tissues or organs (McDermott and Bienenstock, 1979; McDermott et al., 1980). Although this term was coined three decades ago, appreciation of its importance is only just beginning. Much was learned from the numerous studies conducted on the mucosal immune system during this time, which mainly focused on understanding its individual components (Holmgren and Czerkinsky, 2005; Sato and Kiyono, 2012). Although a few studies have suggested that the mucosal immune system is a system-wide organ (Gallichan et al., 2001; Sobko et al., 2010), some questions still need to be clarified. For example, how do the different components affect each other, and how is cross talk achieved among the various mucosal sites (Gill et al., 2010)?In this study, we found that lymphocytes derived from the respiratory mucosa specifically migrated into the intestinal mucosa during respiratory influenza infection by the CCL25–CCR9 chemokine axis and destroyed the intestinal microbiota homeostasis in the small intestine, finally leading to intestinal immune injury. Our findings may provide new insights into not only the mechanisms underlying intestinal immune injury induced by influenza infection of the lung but also the interplay of immune cells between or among different mucosal sites.     

Antigen persistence and the control of local T cell memory by migrant respiratory dendritic cells after acute virus infection

Acute viral infections induce robust adaptive immune responses resulting in virus clearance. Recent evidence suggests that there may be depots of viral antigen that persist in draining lymph nodes (DLNs) after virus clearance and could, therefore, affect the adaptive immune response and memory T cell formation. The nature of these residual antigen depots, the mechanism of antigen persistence, and the impact of the persistent antigen on memory T cells remain ill defined. Using a mouse model of influenza virus infection of the respiratory tract, we identified respiratory dendritic cells (RDCs) as essential for both sampling and presenting residual viral antigen. RDCs in the previously infected lung capture residual viral antigen deposited in an irradiation-resistant cell type. RDCs then transport the viral antigen to the LNs draining the site of infection, where they present the antigen to T cells. Lastly, we document preferential localization of memory T cells to the DLNs after virus clearance as a consequence of presentation of residual viral antigen by the migrant RDC.The induction of the adaptive immune response to an infectious agent consists of a defined series of events prompted by invasion of the body by the organism, followed by replication of the organism and the subsequent initiation of an innate and adaptive immune response. For organisms that enter at a body surface and confine replication primarily to that site, for example, infection of the respiratory tract by seasonal type A influenza, innate responses are triggered by infection of cells at the body surfaces and activation of sentinel innate immune cells in the surrounding tissues. Induction of the adaptive immune response typically requires the uptake/infection by the organism or uptake of its constituents by tissue-resident professional APCs, most notably immature DCs (Banchereau and Steinman, 1998; Lambrecht et al., 2001; de Heer et al., 2005). These professional APCs transport the organism or its products to secondary lymphoid organs, usually LNs draining the site of infection (draining LN [DLN]) where adaptive immune (T and B cells) responses are initiated. In the case of experimental mouse influenza infection, at least two distinct subsets of respiratory DCs (RDCs) have been implicated as critical APCs in the induction of primary T cell responses to this virus (GeurtsvanKessel et al., 2008; Kim and Braciale, 2009).The outcome of the interaction between an invading microorganism and the immune system is either clearance of the agent or the development of chronic persistent infection. Viruses such as HIV, HCV, and, in the mouse model, LCMV, can produce chronic persistent infection with high titers of the virus because of the ability of these viruses to subvert or suppress the host immune response (Ahmed et al., 1984; Bevan and Braciale, 1995; Letvin and Walker, 2003; Wherry et al., 2003; Rehermann and Nascimbeni, 2005). Persistence of viral antigen at high levels in these infections may contribute to the dysregulation of the adaptive immune response observed in these infections (Shin and Wherry, 2007). In contrast, acute infection with many viruses results in rapid virus clearance (even with an initial high level of virus replication) and, presumably, elimination of virus-infected cells by the action of the adaptive and innate immune response.Recently, however, evidence has emerged to suggest that after acute viral infection viral antigen can be detectable for an extended period, i.e., weeks to months, after clearance of infectious virus. This phenomenon is not only observed with viruses capable of establishing persistent infection in vitro, for example, respiratory syncytial virus (RSV; P’ringle et al., 1978; Schwarze et al., 2004) and Sendai virus (Mori et al., 1995), but has also been observed for classically lytic viruses, such as type A influenza (Jelley-Gibbs et al., 2005; Zammit et al., 2006) and VSV (Turner et al., 2007), which are not believed to produce chronic persistent infection of cells. In these instances, viral antigen was detected using the proliferative response of adoptively transferred TCR transgenic (Tg) T cells into previously infected animals as a sensitive measure of antigen persistence. Although the nature and the mechanism of antigen persistence in these instances was not defined, several studies provided evidence that the presentation of this residual viral antigen to T cells may influence the quality of the memory T cell response to infection (Jelley-Gibbs et al., 2005, 2007; Zammit et al., 2006; Turner et al., 2007; Woodland and Kohlmeier, 2009).In several studies reporting persistence of viral antigen after acute infection and subsequent virus clearance, the reservoir of antigen (as detected by adoptive transfer of virus-specific T cells) was localized within secondary lymphoid organs, most notably the LN draining the site of infection (Jelley-Gibbs et al., 2005, 2007; Zammit et al., 2006). In this paper, we describe a series of experiments to investigate the underlying mechanism of the persistence of antigen presentation after respiratory tract infection with type A influenza virus. We found that a reservoir of viral antigen (i.e., RNA and protein) is present for an extended period after an acute influenza virus infection at the site of infection (i.e., the lungs) and localized to both nonhematopoietic (CD45⁻) and hematopoietic (CD45⁺) cell types within residual mild lung inflammatory foci. We further demonstrate that the residual viral antigen deposited in the previously infected lung is captured and transported by RDC to the regional LN, where the migrant RDC function in presenting the residual antigen to T cells. In addition, we provide evidence that lung follicular DCs (FDCs) may play an important role in regulating the response of RDC. Finally, our results suggest that as a consequence of presentation of residual viral antigen by the migrant RDC to T cells in the LNs draining the site of infection, there is preferential localization of memory T cells to the DLN after virus clearance. The implications of these observations are discussed.     

T2R38 taste receptor polymorphisms underlie susceptibility to upper respiratory infection

Innate and adaptive defense mechanisms protect the respiratory system from attack by microbes. Here, we present evidence that the bitter taste receptor T2R38 regulates the mucosal innate defense of the human upper airway. Utilizing immunofluorescent and live cell imaging techniques in polarized primary human sinonasal cells, we demonstrate that T2R38 is expressed in human upper respiratory epithelium and is activated in response to acyl-homoserine lactone quorum-sensing molecules secreted by Pseudomonas aeruginosa and other gram-negative bacteria. Receptor activation regulates calcium-dependent NO production, resulting in stimulation of mucociliary clearance and direct antibacterial effects. Moreover, common polymorphisms of the TAS2R38 gene were linked to significant differences in the ability of upper respiratory cells to clear and kill bacteria. Lastly, TAS2R38 genotype correlated with human sinonasal gram-negative bacterial infection. These data suggest that T2R38 is an upper airway sentinel in innate defense and that genetic variation contributes to individual differences in susceptibility to respiratory infection.     

Respiratory virus–induced EGFR activation suppresses IRF1-dependent interferon λ and antiviral defense in airway epithelium

Viruses suppress host responses to increase infection, and understanding these mechanisms has provided insights into cellular signaling and led to novel therapies. Many viruses (e.g., Influenza virus, Rhinovirus [RV], Cytomegalovirus, Epstein-Barr virus, and Hepatitis C virus) activate epithelial epidermal growth factor receptor (EGFR), a tyrosine kinase receptor, but the role of EGFR in viral pathogenesis is not clear. Interferon (IFN) signaling is a critical innate antiviral host response and recent experiments have implicated IFN-λ, a type III IFN, as the most significant IFN for mucosal antiviral immune responses. Despite the importance of IFN-λ in epithelial antiviral responses, the role and mechanisms of epithelial IFN-λ signaling have not been fully elucidated. We report that respiratory virus-induced EGFR activation suppresses endogenous airway epithelial antiviral signaling. We found that Influenza virus– and RV-induced EGFR activation suppressed IFN regulatory factor (IRF) 1–induced IFN-λ production and increased viral infection. In addition, inhibition of EGFR during viral infection augmented IRF1 and IFN-λ, which resulted in decreased viral titers in vitro and in vivo. These findings describe a novel mechanism that viruses use to suppress endogenous antiviral defenses, and provide potential targets for future therapies.Respiratory viral infections, which cause pneumonia and exacerbations of chronic lung diseases, are responsible for significant morbidity and mortality. Despite substantial disease burden, there are limited therapies for treating virus-induced pulmonary disease. Viruses induce inflammation, which impairs host responses. Upon infection of airway epithelial cells (AECs), the primary cell type for respiratory viral infection, viruses induce epithelial production of IL-8 (Choi and Jacoby, 1992; Subauste et al., 1995). Our research, and that of other investigators, has shown that virus-induced AEC IL-8 production requires epidermal growth factor receptor (EGFR) activation (Monick et al., 2005; Koff et al., 2008; Liu et al., 2008). Therefore, we investigated the effect of virus-induced EGFR activation on airway epithelial antiviral responses.EGFR (ErbB1/HER1), a tyrosine kinase receptor present in epithelial cells, is activated in a ligand-dependent manner (Shao et al., 2003). In AECs, EGFR activation involves an integrated signaling pathway that includes NADPH oxidase (Nox) activation of a metalloproteinase (MP), which cleaves an EGFR pro-ligand that is released to bind to, and to activate EGFR (Shao and Nadel, 2005; Burgel and Nadel, 2008). Recently, viruses have been shown to activate EGFR via this signaling pathway in AECs (Koff et al., 2008; Zhu et al., 2009; Barbier et al., 2012).IFN signaling is a critical innate antiviral host response. Recent experiments have suggested that IFN-λ, a recently discovered type III IFN, is the most significant IFN in AECs (Khaitov et al., 2009; Mordstein et al., 2010). Studies suggest that IFN-λ is the primary IFN that regulates mucosal responses to viral infection, whereas type I IFNs (e.g., IFN-α and -β) are essential for clearance of systemic infection (Jewell et al., 2010; Mordstein et al., 2010). Despite the importance of IFN-λ in epithelial antiviral responses, the kinetics of airway epithelial IFN-λ production has not been fully elucidated. For example, IFN regulatory factors (IRFs), critical for type I and II IFN signaling (Tamura et al., 2008), have not been analyzed in epithelial IFN-λ production. In addition, the potential for EGFR signaling to suppress IFN-λ has not been explored.Influenza A virus (IAV) and Rhinovirus (RV) are ssRNA viruses that are significant pathogens that cause viral pneumonia and induce exacerbations of asthma and chronic obstructive pulmonary disease (Johnston, 2005). Recently, both viruses were shown to activate EGFR via Nox and MP-induced release of EGFR ligand (Liu et al., 2008; Zhu et al., 2009; Barbier et al., 2012). Both IAV and RV stimulate epithelial IFN-λ production, and IFN-λ was implicated in effective clearance of these viruses (Contoli et al., 2006; Jewell et al., 2010). Although the role of IRF in epithelial IFN-λ production has not been explored, RV was found to activate IRF1, IRF3, and IRF7 in AECs (Wang et al., 2009b; Zaheer and Proud, 2010).Here, we examined the interaction between virus-induced EGFR signaling and IFN-λ production in AECs. IAV and RV activated EGFR, and EGFR activation suppressed IRF1-induced IFN-λ production and increased viral infection. In addition, inhibition of EGFR during viral infection augmented IRF1 and IFN-λ production, which resulted in decreased viral titers in vitro and in vivo.     

Defense mechanism against respiratory infection

Most of acute respiratory diseases are caused by infection with various viruses, bacteria and other microorganisms; a mixed infection with these pathogens often results in exacerbation of the disease. In addition to nonspecific protective mechanisms that constantly function in the respiratory tracts, innate and adaptive immunities play important roles in the protection of these pathogens. Secretory IgA and/or cytotoxic T cells in the mucus may be the most effective protection machineries of the adaptive immune systems. Recently, it has been proven that Toll-like receptors, each of which recognizes a conserved structure of pathogens, are the key molecules of the innate immune systems; this recognition step is considered a prerequisite for the adaptive immunity to function in the respiratory tracts.     

Resolution of immune activation defines nonpathogenic SIV infection

Natural nonhuman primate hosts of SIV do not succumb to AIDS despite significant viral replication, a phenomenon attributed to reduced levels of chronic and deleterious “immune activation.” Two studies in this issue of the JCI, by Bosinger et al. and Jacquelin et al., now show that SIV induces vigorous immune activation and upregulation of IFN-stimulated genes in both natural and susceptible hosts, but strikingly, the responses resolve only in the former (see the related articles, beginning on pages 3556 and 3544, respectively). Thus, natural hosts for SIV actively engage mechanisms to abort sustained immune activation and its associated harmful effects. Chronic, generalized immune activation is believed to advance the pathogenesis of progressive HIV and SIV infection (1, 2). The phenomenon, observed in humans and in susceptible strains of monkeys (e.g., rhesus macaques [RMs]), is associated with ineffective control of virus replication; accelerated apoptosis and turnover of T and B lymphocytes; increased levels of T cells expressing markers of activation and proliferation (e.g., CD38, antigen identified by monoclonal antibody Ki-67 [MKI67]); and elevated serum levels of proinflammatory cytokines including type I IFN (reviewed in ref. 2). Of significance, the level of immune activation in the early phase of HIV-1 infection is prognostic of disease outcome. In contrast, SIV infection in natural hosts (e.g., African green monkeys [AGMs] and sooty mangabeys [SMs]) is nonpathogenic despite persistent viral replication, possibly because they do not develop features of chronic immune activation (3). Although the etiology of immune activation is multifactorial, it is generally thought that chronic high levels of type I IFN and proinflammatory cytokines are central to its maintenance. Type I IFN, in particular, besides upregulating IFN-stimulated genes (ISGs) and inducing an antiviral state, contributes to generalized activation of lymphocytes and NK cells, exhaustion and apoptosis of T cells, defects in thymopoiesis, maturation of immature DCs, and aberrant hematopoietic stem cell proliferation (reviewed in ref. 4). Diminished production of IFN-α by plasmacytoid DCs (pDCs) due to mutations in the transactivation domain of IFN regulatory factor 7 (IRF7) has been postulated to account for the nonpathogenicity of infection in natural hosts (5). However, natural hosts mount anti-SIV T cell responses not dissimilar to RMs (6), maintain normal CD4⁺ T cell homeostasis in blood, and have evidence of strong type I IFN responses that subsequently resolve (7). Thus, it remains unresolved whether the lack of sustained immune activation is due to a general attenuated response to infection, or to induction of regulatory mechanisms that suppress immune responses generated during the acute infection.     

DAMPs ramp up drug toxicity

The clinical syndrome of acetaminophen-induced liver injury represents the combined result of drug toxicity and a potent innate immune response that follows drug-induced cell death. In this issue of the JCI, Imaeda and colleagues report that DNA released from dying hepatocytes is a key stimulus of innate immune activation in the acetaminophen-treated mouse liver (see the related article beginning on page 305). They present evidence indicating that hepatocyte DNA promotes immune activation by acting as a danger-associated molecular pattern (DAMP) that stimulates cytokine production in neighboring sinusoidal endothelial cells via Tlr9 and the Nalp3 inflammasome. The analgesic acetaminophen is widely known for its potential to cause severe and sometimes lethal liver injury. When ingested in large amounts, acetaminophen overwhelms the normal metabolic pathways of glucuronidation and sulfation and undergoes oxidation to form the highly reactive intermediate N-acetyl-p-benzoquinone-imine (NAPQI). NAPQI is not harmful if it combines rapidly with glutathione; however, when hepatic glutathione stores are depleted, NAPQI escapes detoxification, resulting in liver cell death (1). An important but underappreciated aspect of acetaminophen toxicity is that direct, drug-induced harm accounts for only part of the overall syndrome of acetaminophen-induced liver injury. The reason for this is that the initial wave of drug-induced hepatocellular destruction is followed by a robust innate immune response, in which invading inflammatory cells release toxic oxidants and cause a second wave of destruction. The collateral damage inflicted by inflammatory cells can be so severe as to double the degree of tissue injury caused by acetaminophen alone (2).     

Streptococcus pneumoniae colonization associates with impaired adaptive immune responses against SARS-CoV-2

BackgroundAlthough recent epidemiological data suggest that pneumococci may contribute to the risk of SARS-CoV-2 disease, cases of coinfection with Streptococcus pneumoniae in patients with coronavirus disease 2019 (COVID-19) during hospitalization have been reported infrequently. This apparent contradiction may be explained by interactions of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and pneumococci in the upper airway, resulting in the escape of SARS-CoV-2 from protective host immune responses.MethodsHere, we investigated the relationship of these 2 respiratory pathogens in 2 distinct cohorts of health care workers with asymptomatic or mildly symptomatic SARS-CoV-2 infection identified by systematic screening and patients with moderate to severe disease who presented to the hospital. We assessed the effect of coinfection on host antibody, cellular, and inflammatory responses to the virus.ResultsIn both cohorts, pneumococcal colonization was associated with diminished antiviral immune responses, which primarily affected mucosal IgA levels among individuals with mild or asymptomatic infection and cellular memory responses in infected patients.ConclusionOur findings suggest that S. pneumoniae impair host immunity to SARS-CoV-2 and raise the question of whether pneumococcal carriage also enables immune escape of other respiratory viruses and facilitates reinfection.Trial registrationISRCTN89159899 (FASTER study) and ClinicalTrials.gov {"type":"clinical-trial","attrs":{"text":"NCT03502291","term_id":"NCT03502291"}}NCT03502291 (LAIV study).     

Type 3 innate lymphoid cells maintain intestinal epithelial stem cells after tissue damage

Disruption of the intestinal epithelial barrier allows bacterial translocation and predisposes to destructive inflammation. To ensure proper barrier composition, crypt-residing stem cells continuously proliferate and replenish all intestinal epithelial cells within days. As a consequence of this high mitotic activity, mucosal surfaces are frequently targeted by anticancer therapies, leading to dose-limiting side effects. The cellular mechanisms that control tissue protection and mucosal healing in response to intestinal damage remain poorly understood. Type 3 innate lymphoid cells (ILC3s) are regulators of homeostasis and tissue responses to infection at mucosal surfaces. We now demonstrate that ILC3s are required for epithelial activation and proliferation in response to small intestinal tissue damage induced by the chemotherapeutic agent methotrexate. Multiple subsets of ILC3s are activated after intestinal tissue damage, and in the absence of ILC3s, epithelial activation is lost, correlating with increased pathology and severe damage to the intestinal crypts. Using ILC3-deficient Lgr5 reporter mice, we show that maintenance of intestinal stem cells after damage is severely impaired in the absence of ILC3s or the ILC3 signature cytokine IL-22. These data unveil a novel function of ILC3s in limiting tissue damage by preserving tissue-specific stem cells.The intestinal epithelium combines efficient uptake of nutrients and water while providing a physical barrier between the intestinal microbiota and the body (Peterson and Artis, 2014). Damage sustained by intestinal epithelial cells (IECs) needs to be swiftly and efficiently repaired to prevent inappropriate immune responses to commensal bacteria. Intestinal damage is an early event in the development of both graft-versus-host disease (Reddy and Ferrara, 2003) and alimentary mucositis (Sonis, 2004) and a driver of bacterial translocation and T cell activation in inflammatory bowel disease (Salim and Söderholm, 2011).A major pathway involved in the intestinal epithelial response to damage is the activation of Stat3, which is expressed along the crypt–villus axis of the intestinal epithelium (Grivennikov et al., 2009; Heneghan et al., 2013). Phosphorylated Stat3 translocates to the nucleus and activates genes involved in proliferation, survival, and mucosal defense (Bollrath et al., 2009; Pickert et al., 2009; Ernst et al., 2014). Mutations in STAT3 have been identified as susceptibility factors for inflammatory bowel disease (Bollrath et al., 2009; Anderson et al., 2011; Demaria et al., 2012), and in mice, upon DSS-induced colitis, epithelial Stat3 is required for mucosal wound healing (Pickert et al., 2009).Intestinal regeneration depends on the continuous differentiation of epithelial cells from crypt-residing intestinal stem cells (ISCs; Potten et al., 1978; Günther et al., 2013; Ritsma et al., 2014). Even though multiple intestinal progenitor cells have been described, the best-characterized populations are the Lgr5-expressing cells that reside at the crypt bottom, interspersed with Paneth cells. These stem cells have the ability to give rise to all IECs ex vivo (Sato et al., 2009). Similar to its role in differentiated epithelial cells, Stat3 activation is also an important pathway for survival of intestinal epithelial stem cells (Matthews et al., 2011).Type 3 innate lymphoid cells (ILC3s) are innate immune cells that reside in the lamina propria of both the small and large intestines and are involved in tissue homeostasis, early defense against enteric pathogens, and containment of microbiota (Spits and Cupedo, 2012; Artis and Spits, 2015). In the intestines, multiple ILC3 subsets exist, two of which can be distinguished by mutual exclusive expression of the natural cytotoxicity receptor NKp46 and the chemokine receptor CCR6 (Sawa et al., 2010; Reynders et al., 2011). Most Nkp46⁺ ILC3s are found dispersed throughout the lamina propria, a localization that depends on the expression of CXCR6 (Satoh-Takayama et al., 2014). In contrast, the majority of CCR6⁺ ILC3s are located in close proximity to the intestinal crypts in anatomically defined sites known as cryptopatches (Kanamori et al., 1996). Recent findings indicated that under inflammatory conditions, such as experimental graft-versus-host disease, ILC3s can interact with the epithelial stem cells in the crypts, protecting them from T cell–mediated killing (Hanash et al., 2012).The well-known ability of ILC3s to condition the local microenvironment, the close proximity of ILC3s to intestinal crypts, and the ability of ILC3s to communicate with epithelial stem cells led us to hypothesize that ILC3s are involved in directing intestinal epithelial responses to tissue damage. Using the methotrexate (MTX) model of small intestinal damage, we now show that ILC3s are activated immediately after MTX administration, leading to a rapid activation of epithelial Stat3 and maintenance of ISCs. Our data reveal a novel function for ILC3s as organizers of the intestinal epithelial response to tissue damage through activation of epithelial cells and maintenance of ISCs and suggest that ILC3s might in future be therapeutically harnessed to prevent stem cell loss during chemotherapy.     

Antimicrobial peptides from Bombyx mori: a splendid immune defense response in silkworms

Bombyx mori L., a primary producer of silk, is the main tool in the sericulture industry and provides the means of livelihood to a large number of people. Silk cocoon crop losses due to bacterial infection pose a major threat to the sericulture industry. Bombyx mori L., a silkworm of the mulberry type, has a sophisticated inherent innate immune mechanism to combat such invasive pathogens. Among all the components in this defense system, antimicrobial peptides (AMPs) are notable due to their specificity towards the invading pathogens without harming the normal host cells. Bombyx mori L. so far has had AMPs identified that belong to six different families, namely cecropin, defensin, moricin, gloverin, attacin and lebocin, which are produced by the Toll and immune deficiency (IMD) pathways. Their diverse modes of action depend on microbial pathogens and are still under investigation. This review examines the recent progress in understanding the immune defense mechanism of Bombyx mori based on AMPs.

AMPs produced by B. mori induced by microbial challenge in the fat body.     

Ketolides in the treatment of community-acquired respiratory tract infections: A review

Background:

The increasing prevalence of resistance to established antibiotics among key respiratory bacterial pathogens highlights a need for new antibacterial agents for the treatment of community-acquired respiratory tract infections (RTIs). Ketolides are a new class of antibiotics specifically developed for the treatment of RTIs.

Objective:

The aim of this review was to present the current status of treatment of RTIs with ketolides, focusing on telithromycin—the first ketolide to be approved by the US Food and Drug Administration for clinical use.

Methods:

To gather evidence on the current status of ketolides, a literature search was conducted using MEDLINE (years: 1990-2005; key terms: ketolides, telithromycin, and HMR3647).

Results:

Telithromycin shows strong in vitro activity against the major respiratorypathogens, including strains resistant to other antibiotics, as well as the atypical respiratory pathogens. The pharmacokinetic properties of telithromycin are compatible with once-daily dosing. Clinical trials have demonstrated that telithromycin 800 mg QD for 5 to 10 days is effective in the treatment of acute bacterial sinusitis, acute bacterial exacerbations of chronic bronchitis, and mild to moderate community-acquired pneumonia. Overall, telithromycin is well tolerated by patients. Drug-drug interactions are similar to those reported for macrolides.

Conclusion:

Evidence to date indicates that telithromycin is an effective andwell-tolerated empiric treatment for community-acquired RTIs.     

BCG vaccination in health care providers and the protection against COVID-19

A number of coronavirus disease 2019 (COVID-19) vaccine candidates have shown promising results, but substantial uncertainty remains regarding their effectiveness and global rollout. Boosting innate immunity with bacillus Calmette Guérin (BCG) or other live attenuated vaccines may also play a role in the fight against the COVID-19 pandemic. BCG has long been known for its nonspecific beneficial effects that are most likely explained by epigenetic and metabolic reprogramming of innate immune cells, termed trained immunity. In this issue of the JCI, Rivas et al. add to these arguments by showing that BCG-vaccinated health care providers from a Los Angeles health care organization had lower rates of COVID-19 diagnoses and seropositivity compared with unvaccinated individuals. Prospective clinical trials are thus warranted to explore the effects of BCG vaccination in COVID-19. We posit that beyond COVID-19, vaccines such as BCG that elicit trained immunity may mitigate the impact of emerging pathogens in future pandemics.     

Immunité de la muqueuse respiratoire : physiologie et implications en réanimation

Respiratory mucosa is the first-line defense against external threat, notably from microbial origin. Indeed, it is a physical barrier against pathogens invasion but it also has major immune functions. Epithelial cells have innate immunity receptors (pathogen recognition receptors), which allow them to detect molecular patterns presented by microorganisms. Activation of these receptors leads to an inflammatory response of variable intensity. Epithelial cells also have the ability to modulate and lessen the inflammatory response triggered. Regulation of inflammatory response of the respiratory mucosa is highly complex and implies numerous cellular and molecular effectors, but two cytokines, Interleukin (IL)-17 and IL-22, are noteworthy. Thus, respiratory epithelium acts as both a sensor and an effector of the immune response, and this double function can be deregulated under various pathologic conditions. Dysfunction of epithelial response can be induced by respiratory supportive treatments, notably mechanical ventilation. However, alterations of epithelial functions are principally described during inflammatory process, either acute (e.g., acute respiratory distress syndrome) or chronic (e.g., chronic obstructive pulmonary disease). Respiratory epithelium is a key player in the physical and functional restoration of respiratory mucosa and so developing the therapeutic strategies to protect epithelial integrity might help in preventing pathological amplification of local inflammation and/or limiting pathogens dissemination.     

Cryptosporidiosis: host immune responses and the prospects for effective immunotherapies

Cryptosporidium spp. that develop in intestinal epithelial cells are responsible for the diarrhoeal disease cryptosporidiosis, which is common in humans of all ages and in neonatal livestock. Following infection, parasite reproduction increases for a number of days before it is blunted and then impeded by innate and adaptive immune responses. Immunocompromised hosts often cannot establish strong immunity and develop chronic infections that can lead to death. Few drugs consistently inhibit parasite reproduction in the host, and chemotherapy might be ineffective in immunodeficient hosts. Future options for prevention or treatment of cryptosporidiosis might include vaccines or recombinant immunological molecules, but this will probably require a better understanding of both the mucosal immune system and intestinal immune responses to the parasite.