Bioorthogonal Metabolic Labeling of the Virulence Factor Phenolic Glycolipid in Mycobacteria

Surface lipids on pathogenic mycobacteria modulate infection outcomes by regulating host immune responses. Phenolic glycolipid (PGL) is a host-modulating surface lipid that varies among clinical Mycobacterium tuberculosis strains. PGL is also found in Mycobacterium marinum, where it promotes infection of zebrafish through effects on the innate immune system. Given the important role this lipid plays in the host-pathogen relationship, tools for profiling its abundance, spatial distribution, and dynamics are needed. Here, we report a strategy for imaging PGL in live mycobacteria using bioorthogonal metabolic labeling. We functionalized the PGL precursor p-hydroxybenzoic acid (pHB) with an azide group (3-azido pHB). When fed to mycobacteria, 3-azido pHB was incorporated into the cell surface, which could then be visualized via the bioorthogonal conjugation of a fluorescent probe. We confirmed that 3-azido pHB incorporates into PGL using mass spectrometry methods and demonstrated selectivity for PGL-producing M. marinum and M. tuberculosis strains. Finally, we applied this metabolic labeling strategy to study the dynamics of PGL within the mycobacterial membrane. This new tool enables visualization of PGL that may facilitate studies of mycobacterial pathogenesis.

CDKL5 regulates p62-mediated selective autophagy and confers protection against neurotropic viruses

Virophagy, the selective autophagosomal engulfment and lysosomal degradation of viral components, is crucial for neuronal cell survival and antiviral immunity. However, the mechanisms leading to viral antigen recognition and capture by autophagic machinery remain poorly understood. Here, we identified cyclin-dependent kinase-like 5 (CDKL5), known to function in neurodevelopment, as an essential regulator of virophagy. Loss-of-function mutations in CDKL5 are associated with a severe neurodevelopmental encephalopathy. We found that deletion of CDKL5 or expression of a clinically relevant pathogenic mutant of CDKL5 reduced virophagy of Sindbis virus (SINV), a neurotropic RNA virus, and increased intracellular accumulation of SINV capsid protein aggregates and cellular cytotoxicity. Cdkl5-knockout mice displayed increased viral antigen accumulation and neuronal cell death after SINV infection and enhanced lethality after infection with several neurotropic viruses. Mechanistic studies demonstrated that CDKL5 directly binds the canonical selective autophagy receptor p62 and phosphorylates p62 at T269/S272 to promote its interaction with viral capsid aggregates. We found that CDKL5-mediated phosphorylation of p62 facilitated the formation of large p62 inclusion bodies that captured viral capsids to initiate capsid targeting to autophagic machinery. Overall, these findings identify a cell-autonomous innate immune mechanism for autophagy activation to clear intracellular toxic viral protein aggregates during infection.

Pathogenicity and virulence of Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) is the causative agent of tuberculosis, an infectious disease with one of the highest morbidity and mortality rates worldwide. Leveraging its highly evolved repertoire of non-protein and protein virulence factors, Mtb invades through the airway, subverts host immunity, establishes its survival niche, and ultimately escapes in the setting of active disease to initiate another round of infection in a naive host. In this review, we will provide a concise synopsis of the infectious life cycle of Mtb and its clinical and epidemiologic significance. We will also take stock of its virulence factors and pathogenic mechanisms that modulate host immunity and facilitate its spread. Developing a greater understanding of the interface between Mtb virulence factors and host defences will enable progress toward improved vaccines and therapeutics to prevent and treat tuberculosis.

BECN1F121A mutation increases autophagic flux in aged mice and improves aging phenotypes in an organ-dependent manner

Macroautophagy/autophagy is necessary for lifespan extension in multiple model organisms and autophagy dysfunction impacts age-related phenotypes and diseases. Introduction of an F121A mutation into the essential autophagy protein BECN1 constitutively increases basal autophagy in young mice and reduces cardiac and renal age-related changes in longer lived Becn1F121A mutant mice. However, both autophagic and lysosomal activities decline with age. Thus, whether autophagic flux is maintained during aging and whether it is enhanced in Becn1F121A mice is unknown. Here, we demonstrate that old wild-type mice maintained functional autophagic flux in heart, kidney and skeletal muscle but not liver, and old Becn1F121A mice had increased autophagic flux in those same organs compared to wild type. In parallel, Becn1F121A mice were not protected against age-associated hepatic phenotypes but demonstrated reduced skeletal muscle fiber atrophy. These findings identify an organ-specific role for the ability of autophagy to impact organ aging phenotypes.

Infectious and Inflammatory Pathways to Cough

Coughing is a dynamic physiological process resulting from input of vagal sensory neurons innervating the airways and perceived airway irritation. Although cough serves to protect and clear the airways, it can also be exploited by respiratory pathogens to facilitate disease transmission. Microbial components or infection-induced inflammatory mediators can directly interact with sensory nerve receptors to induce a cough response. Analysis of cough-generated aerosols and transmission studies have further demonstrated how infectious disease is spread through coughing. This review summarizes the neurophysiology of cough, cough induction by respiratory pathogens and inflammation, and cough-mediated disease transmission.

Bag it, tag it: ubiquitin ligases and host resistance to Mycobacterium tuberculosis

Infection with Mycobacterium tuberculosis (Mtb), the etiological agent of tuberculosis (TB), remains a significant global epidemic. Host resistance to Mtb depends on both adaptive and innate immunity mechanisms, including development of antigen-specific CD4 and CD8 T cells, production of inflammatory cytokines, bacterial phagocytosis and destruction within phagolysosomes, host cell apoptosis, and autophagy. A key regulatory mechanism in innate immunity is the attachment of the small protein ubiquitin to protein and lipid targets by the enzymatic activity of ubiquitin ligases. Here, we summarize the latest advances on the role of ubiquitination and ubiquitin ligases in host immunity against Mtb, with a focus on innate immunity signaling, inflammation, and antimicrobial autophagy. Understanding how ubiquitin ligases mediate immunity to Mtb, and the specific substrates of distinct ubiquitin ligases in the context of Mtb infection, could facilitate development of new host-directed antimicrobials.

Identification of scavenger receptor B1 as the airway microfold cell receptor for Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) can enter the body through multiple routes, including via specialized transcytotic cells called microfold cells (M cell). However, the mechanistic basis for M cell entry remains undefined. Here, we show that M cell transcytosis depends on the Mtb Type VII secretion machine and its major virulence factor EsxA. We identify scavenger receptor B1 (SR-B1) as an EsxA receptor on airway M cells. SR-B1 is required for Mtb binding to and translocation across M cells in mouse and human tissue. Together, our data demonstrate a previously undescribed role for Mtb EsxA in mucosal invasion and identify SR-B1 as the airway M cell receptor for Mtb.

Mycobacterium tuberculosis Sulfolipid-1 Activates Nociceptive Neurons and Induces Cough

Pulmonary tuberculosis, a disease caused by Mycobacterium tuberculosis (Mtb), manifests with a persistent cough as both a primary symptom and mechanism of transmission. The cough reflex can be triggered by nociceptive neurons innervating the lungs, and some bacteria produce neuron-targeting molecules. However, how pulmonary Mtb infection causes cough remains undefined, and whether Mtb produces a neuron-activating, cough-inducing molecule is unknown. Here, we show that an Mtb organic extract activates nociceptive neurons in vitro and identify the Mtb glycolipid sulfolipid-1 (SL-1) as the nociceptive molecule. Mtb organic extracts from mutants lacking SL-1 synthesis cannot activate neurons in vitro or induce cough in a guinea pig model. Finally, Mtb-infected guinea pigs cough in a manner dependent on SL-1 synthesis. Thus, we demonstrate a heretofore unknown molecular mechanism for cough induction by a virulent human pathogen via its production of a complex lipid.

Screening Mycobacterium tuberculosis Secreted Proteins Identifies Mpt64 as a Eukaryotic Membrane-Binding Bacterial Effector

Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis, is one of the most successful human pathogens. One reason for its success is that Mtb can reside within host macrophages, a cell type that normally functions to phagocytose and destroy infectious bacteria. However, Mtb is able to evade macrophage defenses in order to survive for prolonged periods of time. Many intracellular pathogens secrete virulence factors targeting host membranes and organelles to remodel their intracellular environmental niche. We hypothesized that Mtb secreted proteins that target host membranes are vital for Mtb to adapt to and manipulate the host environment for survival. Thus, we characterized 200 secreted proteins from Mtb for their ability to associate with eukaryotic membranes using a unique temperature-sensitive yeast screen and to manipulate host trafficking pathways using a modified inducible secretion screen. We identified five Mtb secreted proteins that both associated with eukaryotic membranes and altered the host secretory pathway. One of these secreted proteins, Mpt64, localized to the endoplasmic reticulum during Mtb infection of murine and human macrophages and impaired the unfolded protein response in macrophages. These data highlight the importance of secreted proteins in Mtb pathogenesis and provide a basis for further investigation into their molecular mechanisms.IMPORTANCE Advances have been made to identify secreted proteins of Mycobacterium tuberculosis during animal infections. These data, combined with transposon screens identifying genes important for M. tuberculosis virulence, have generated a vast resource of potential M. tuberculosis virulence proteins. However, the function of many of these proteins in M. tuberculosis pathogenesis remains elusive. We have integrated three cell biological screens to characterize nearly 200 M. tuberculosis secreted proteins for eukaryotic membrane binding, host subcellular localization, and interactions with host vesicular trafficking. In addition, we observed the localization of one secreted protein, Mpt64, to the endoplasmic reticulum (ER) during M. tuberculosis infection of macrophages. Interestingly, although Mpt64 is exported by the Sec pathway, its delivery into host cells was dependent upon the action of the type VII secretion system. Finally, we observed that Mpt64 impairs the ER-mediated unfolded protein response in macrophages.

165. Mycobacterium tuberculosis Produces Molecules That Trigger Nociceptive Neurons to Activate Cough

Background

Methods

Results

Conclusion

Disclosures

A baculovirus-conjugated mimotope vaccine targeting Mycobacterium tuberculosis lipoarabinomannan

Mycobacterium tuberculosis, the causative agent of tuberculosis, is a major cause of morbidity and mortality worldwide. However, an effective vaccine for M. tuberculosis is lacking. We panned a phage display library using monoclonal antibodies against M. tuberculosis liporabinomannan (LAM), an important component of the M. tuberculosis cell wall, and identified two peptide sequences, HSFKWLDSPRLR or SGVYKVAYDWQH, with high antibody affinity after multiple rounds of panning. Only the HSFKWLDSPRLR peptide induced an anti-LAM response when conjugated to either keyhole limpet hemocyanin (KLH) or to the baculovirus Autographa californica multicapsid nucleopolyherovirus (AcMNPV) when introduced into mice by injection or via intranasal inoculation, respectively. Vaccination with AcMNPV conjugated HSFKWLDSPRLR peptide delayed mortality in a mouse model of tuberculosis. Thus, we report a proof of principle M. tuberculosis vaccination strategy combining an anti-LAM mimotope with a baculovirus delivery system.

Role of M Cells in Human Mucosal Immunity to Mycobacterium tuberculosis

Background

Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB), is a bacterial pathogen that infects roughly one-third of the worldÕs population and causes 1–2 million deaths per year. The current paradigm is that phagocytosis of Mtb by patrolling alveolar macrophages initiates Mtb infection. While this model can account for pulmonary TB, it does not adequately explain the occurrence of extrapulmonary forms of TB that manifest in the absence of obvious lung involvement, such as tuberculous cervical lymphadenitis, also known as scrofula. We hypothesized that specialized epithelial cells called microfold cells (M cells) may be an alternate portal of entry for Mtb. Previously we demonstrated that Mtb is able to transcytose across an epithelial barrier in an M cell dependent manner and that M cell mediated transcytosis is vital for Mtb pathogenesis in a mouse model of tuberculosis.

Methods

We used an in vitro M-cell mediated translocation assay and a Mtb mutant lacking a key virulence factor, ESAT6. We used biochemistry and genetics to identify a novel receptor for ESAT6. We also developed a novel explanted human adenoid Mtb infection model to study mucosal immunity.

Results

We now demonstrate that the Mtb virulence factor ESAT6 is necessary and sufficient to mediate binding and transcytosis by M cells in vitro and in vivo, and that uptake of Mtb by M cells requires a unique cell surface ESAT6 receptor. We developed a novel explanted human adenoid model of M cell biology and demonstrate rapid Mtb transcytosis by primary human tissue within 60–120 minutes. Using flow cytometry we find that Mtb is first ingested by M cells and then after transcytosis, by tissue resident antigen-presenting cells. Explanted adenoids from 10 independent donors display a wide range of Mtb uptake.

Conclusion

We conclude that Mtb ESAT6 is necessary for Mtb uptake by M-cells and that binding and transcytosis require a host receptor. Because explanted adenoids display a wide range of Mtb uptake, M cell mediated transcytosis may confer differential susceptibility to scrofula and disseminated disease. These findings are significant as M cells could potentially serve as the basis for novel therapeutic targets against primary Mtb infection.

Disclosures

All authors: No reported disclosures.

The Ubiquitin Ligase Smurf1 Functions in Selective Autophagy of Mycobacterium tuberculosis and Anti-tuberculous Host Defense

During antibacterial autophagy, ubiquitination of intracellular bacteria recruits proteins that mediate bacterial delivery to the lysosome for degradation. Smurf1 is an E3 ubiquitin ligase whose role in selective bacterial autophagy is unknown. We show that Smurf1 facilitates selective autophagy of the human pathogen Mycobacterium tuberculosis (Mtb). Smurf1-/- macrophages are defective in recruiting polyubiquitin, the proteasome, the ubiquitin-binding autophagy adaptor NBR1, the autophagy protein LC3, and the lysosomal marker LAMP1 to Mtb-associated structures and are more permissive for Mtb growth. This function of Smurf1 requires both its ubiquitin-ligase and C2 phospholipid-binding domains, and involves K48- rather than K63-linked ubiquitination. Chronically infected Smurf1-/- mice have increased bacterial load, increased lung inflammation, and accelerated mortality. SMURF1 controls Mtb replication in human macrophages and associates with bacteria in lungs of patients with pulmonary tuberculosis. Thus, Smurf1 is required for selective autophagy of Mtb and host defense against tuberculosis infection.

Microfold Cells Actively Translocate Mycobacterium tuberculosis to Initiate Infection

The prevailing paradigm is that tuberculosis infection is initiated when patrolling alveolar macrophages and dendritic cells within the terminal alveolus ingest inhaled Mycobacterium tuberculosis (Mtb). However, definitive data for this model are lacking. Among the epithelial cells of the upper airway, a specialized epithelial cell known as a microfold cell (M cell) overlies various components of mucosa-associated lymphatic tissue. Here, using multiple mouse models, we show that Mtb invades via M cells to initiate infection. Intranasal Mtb infection in mice lacking M cells either genetically or by antibody depletion resulted in reduced invasion and dissemination to draining lymph nodes. M cell-depleted mice infected via aerosol also had delayed dissemination to lymph nodes and reduced mortality. Translocation of Mtb across two M cell transwell models was rapid and transcellular. Thus, M cell translocation is a vital entry mechanism that contributes to the pathogenesis of Mtb.

Heme Oxygenase-1 Regulates Inflammation and Mycobacterial Survival in Human Macrophages during Mycobacterium tuberculosis Infection

Mycobacterium tuberculosis, the causative agent of tuberculosis, is responsible for 1.5 million deaths annually. We previously showed that M. tuberculosis infection in mice induces expression of the CO-producing enzyme heme oxygenase (HO1) and that CO is sensed by M. tuberculosis to initiate a dormancy program. Further, mice deficient in HO1 succumb to M. tuberculosis infection more readily than do wild-type mice. Although mouse macrophages control intracellular M. tuberculosis infection through several mechanisms, such as NO synthase, the respiratory burst, acidification, and autophagy, how human macrophages control M. tuberculosis infection remains less well understood. In this article, we show that M. tuberculosis induces and colocalizes with HO1 in both mouse and human tuberculosis lesions in vivo, and that M. tuberculosis induces and colocalizes with HO1 during primary human macrophage infection in vitro. Surprisingly, we find that chemical inhibition of HO1 both reduces inflammatory cytokine production by human macrophages and restricts intracellular growth of mycobacteria. Thus, induction of HO1 by M. tuberculosis infection may be a mycobacterial virulence mechanism to enhance inflammation and bacterial growth.

Sensing of Mycobacterium tuberculosis and consequences to both host and bacillus

Mycobacterium tuberculosis (Mtb), the primary causative agent of human tuberculosis, has killed more people than any other bacterial pathogen in human history and remains one of the most important transmissible diseases worldwide. Because of the long-standing interaction of Mtb with humans, it is no surprise that human mucosal and innate immune cells have evolved multiple mechanisms to detect Mtb during initial contact. To that end, the cell surface of human cells is decorated with numerous pattern recognition receptors for a variety of mycobacterial ligands. Furthermore, once Mtb is ingested into professional phagocytes, other host molecules are engaged to report on the presence of an intracellular pathogen. In this review, we discuss the role of specific mycobacterial products in modulating the host's ability to detect Mtb. In addition, we describe the specific host receptors that mediate the detection of mycobacterial infection and the role of individual receptors in mycobacterial pathogenesis in humans and model organisms.

Cyclic GMP-AMP Synthase Is an Innate Immune DNA Sensor for Mycobacterium tuberculosis

Activation of the DNA-dependent cytosolic surveillance pathway in response to Mycobacterium tuberculosis infection stimulates ubiquitin-dependent autophagy and inflammatory cytokine production, and plays an important role in host defense against M. tuberculosis. However, the identity of the host sensor for M. tuberculosis DNA is unknown. Here we show that M. tuberculosis activated cyclic guanosine monophosphate-adenosine monophosphate (cGAMP) synthase (cGAS) in macrophages to produce cGAMP, a second messenger that activates the adaptor protein stimulator of interferon genes (STING) to induce type I interferons and other cytokines. cGAS localized with M. tuberculosis in mouse and human cells and in human tuberculosis lesions. Knockdown or knockout of cGAS in human or mouse macrophages blocked cytokine production and induction of autophagy. Mice deficient in cGAS were more susceptible to lethality caused by infection with M. tuberculosis. These results demonstrate that cGAS is a vital innate immune sensor of M. tuberculosis infection.

Effect of carbon monoxide on Mycobacterium tuberculosis pathogenesis

The intracellular pathogen Mycobacterium tuberculosis (Mtb) is exposed to multiple host antimicrobial pathways, including toxic gases such as superoxide, nitric oxide and carbon monoxide (CO). To survive, mycobacteria evolved mechanisms to resist the toxic environment, and in this review we focus on a relatively new field, namely, the role of macrophage heme oxygenase and its enzymatic product CO in Mtb pathogenesis. In particular, we focus on (i) the induction of heme oxygenase during Mtb infection and its relevance to Mtb pathogenesis, (ii) the ability of mycobacteria to catabolize CO, (iii) the transcriptional reprogramming of Mtb by exposure to CO, (iv) the general antimicrobial properties of CO and (v) new genetic evidence characterizing the ability of Mtb to resist CO toxicity. Developing a complete molecular and genetic understanding of the pathogenesis of Mtb is essential to its eventual eradication.

Mycobacterium tuberculosis activates the DNA-dependent cytosolic surveillance pathway within macrophages

Cytosolic bacterial pathogens activate the cytosolic surveillance pathway (CSP) and induce innate immune responses, but how the host detects vacuolar pathogens like Mycobacterium tuberculosis is poorly understood. We show that M. tuberculosis also initiates the CSP upon macrophage infection via limited perforation of the phagosome membrane mediated by the ESX-1 secretion system. Although the bacterium remains within the phagosome, this permeabilization results in phagosomal and cytoplasmic mixing and allows extracellular mycobacterial DNA to access host cytosolic receptors, thus blurring the distinction between "vacuolar" and "cytosolic" pathogens. Activation of cytosolic receptors induces signaling through the Sting/Tbk1/Irf3 axis, resulting in IFN-β production. Surprisingly, Irf3(-/-) mice, which cannot respond to cytosolic DNA, are resistant to long-term M. tuberculosis infection, suggesting that the CSP promotes M. tuberculosis infection. Thus, cytosolic sensing of mycobacterial DNA plays a key role in M. tuberculosis pathogenesis and likely contributes to the high type I IFN signature in tuberculosis.

Mycobacterium tuberculosis MycP1 protease plays a dual role in regulation of ESX-1 secretion and virulence

Mycobacterium tuberculosis uses the ESX-1 secretion system to deliver virulence proteins during infection of host cells. Here we report a mechanism of posttranscriptional control of ESX-1 mediated by MycP1, a M. tuberculosis serine protease. We show that MycP1 is required for ESX-1 secretion but that, unexpectedly, genetic inactivation of MycP1 protease activity increases secretion of ESX-1 substrates. We demonstrate that EspB, an ESX-1 substrate required for secretion, is a target of MycP1 in vitro and in vivo. During macrophage infection, an inactive MycP1 protease mutant causes hyperactivation of ESX-1-stimulated innate signaling pathways. MycP1 is required for growth in mice during acute infection, while loss of its protease activity leads to attenuated virulence during chronic infection. As the key ESX-1 substrates ESAT-6 and CFP-10 are highly immunogenic, fine-tuning of their secretion by MycP1 may balance virulence and immune detection and be essential for successful maintenance of long-term M. tuberculosis infection.

To catch a killer. What can mycobacterial models teach us about Mycobacterium tuberculosis pathogenesis?

Mycobacterium tuberculosis is the causative agent of the global tuberculosis epidemic. To combat this successful human pathogen we need a better understanding of the basic biology of mycobacterial pathogenesis. The use of mycobacterial model systems has the potential to greatly facilitate our understanding of how M. tuberculosis causes disease. Recently, studies using mycobacterial models, including M. bovis BCG, M. marinum, and M. smegmatis have significantly contributed to understanding M. tuberculosis. Specifically, there have been advances in genetic manipulation of M. tuberculosis using inducible promoters and recombineering that alleviate technical limitations in working with mycobacteria. Model systems have helped elucidate how secretion systems function at both the molecular level and during virulence. Mycobacterial models have also led to interesting hypotheses about how M. tuberculosis mediates latent infection and host response. While there is utility in using model systems to understand tuberculosis, each of these models represent distinct mycobacterial species with unique environmental adaptations. Directly comparing findings in model mycobacteria to those in M. tuberculosis will illuminate the similarities and differences between these species and increase our understanding of why M. tuberculosis is such a potent human pathogen.

Mycobacterium tuberculosis senses host-derived carbon monoxide during macrophage infection

Mycobacterium tuberculosis (MTB) expresses a set of genes known as the dormancy regulon in vivo. These genes are expressed in vitro in response to nitric oxide (NO) or hypoxia, conditions used to model MTB persistence in latent infection. Although NO, a macrophage product that inhibits respiration, and hypoxia are likely triggers in vivo, additional cues could activate the dormancy regulon during infection. Here, we show that MTB infection stimulates expression of heme oxygenase (HO-1) by macrophages and that the gaseous product of this enzyme, carbon monoxide (CO), activates expression of the dormancy regulon. Deletion of macrophage HO-1 reduced expression of the dormancy regulon. Furthermore, we show that the MTB DosS/DosT/DosR two-component sensory relay system is required for the response to CO. Together, these findings demonstrate that MTB senses CO during macrophage infection. CO may represent a general cue used by pathogens to sense and adapt to the host environment.

A glutamate-alanine-leucine (EAL) domain protein of Salmonella controls bacterial survival in mice, antioxidant defence and killing of macrophages: role of cyclic diGMP

Signature-tagged transposon mutagenesis of Salmonella with differential recovery from wild-type and immunodeficient mice revealed that the gene here named cdgR[for c-diguanylate (c-diGMP) regulator] is required for the bacterium to resist host phagocyte oxidase in vivo. CdgR consists solely of a glutamate-alanine-leucine (EAL) domain, a predicted cyclic diGMP (c-diGMP) phosphodiesterase. Disruption of cdgR decreased bacterial resistance to hydrogen peroxide and accelerated bacterial killing of macrophages. An ultrasensitive assay revealed c-diGMP in wild-type Salmonella with increased levels in the CdgR-deficient mutant. Thus, besides its known role in regulating cellulose synthesis and biofilm formation, bacterial c-diGMP also regulates host-pathogen interactions involving antioxidant defence and cytotoxicity.

Reactive oxygen and nitrogen intermediates in the relationship between mammalian hosts and microbial pathogens

This review summarizes recent evidence from knock-out mice on the role of reactive oxygen intermediates and reactive nitrogen intermediates (RNI) in mammalian immunity. Reflections on redundancy in immunity help explain an apparent paradox: the phagocyte oxidase and inducible nitric oxide synthase are each nonredundant, and yet also mutually redundant, in host defense. In combination, the contribution of these two enzymes appears to be greater than previously appreciated. The remainder of this review focuses on a relatively new field, the basis of microbial resistance to RNI. Experimental tuberculosis provides an important example of an extended, dynamic balance between host and pathogen in which RNI play a major role. In diseases such as tuberculosis, a molecular understanding of host-pathogen interactions requires characterization of the defenses used by microbes against RNI, analogous to our understanding of defenses against reactive oxygen intermediates. Genetic and biochemical approaches have identified candidates for RNI-resistance genes in Mycobacterium tuberculosis and other pathogens.

Secretory leukocyte protease inhibitor, an inhibitor of neutrophil activation, is elevated in serum in human sepsis and experimental endotoxemia

OBJECTIVES

To document changes in serum secretory leukocyte protease inhibitor (SLPI) in human sepsis and in experimental endotoxemia in vivo. To compare changes in serum SLPI in human sepsis with changes in interleukin (IL)-6, IL-10, and tumor necrosis factor (TNF)-alpha. To determine whether or not changes in SLPI correlate with the severity of multiple organ dysfunction syndrome as measured by the maximal multiple organ dysfunction score. Finally, because neutrophils have been implicated in tissue injury associated with organ dysfunction, to determine whether recombinant human SLPI blocks activation of isolated human neutrophils.

DESIGN

Case-control study and ex-vivo cellular assay.

SETTING

Surgical intensive care unit and clinical research center of university hospitals; laboratory of a medical school.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

There was a significant dose-dependent elevation (50.2+/-4.0 ng/mL, p = .01) in plasma SLPI 12 hrs after administration of lipopolysaccharide to seven healthy adults (36.4+/-2.3 ng/mL). Further, serum concentrations of SLPI (132+/-15 ng/mL) were elevated in septic surgical patients compared with healthy controls (43+/-2 ng/mL, p < .01) and nonseptic surgical controls (69+/-10 ng/mL, p = .01). Serum SLPI concentrations correlated (r2 = .71, p < .01) better with organ dysfunction as measured by maximal multiple organ dysfunction score than did serum IL-6 (r2 = .49, p < .01), IL-10 (r2 = .05, p = .22), or TNF-alpha (r2 = .02, p = .44). We found that recombinant human SLPI in vitro inhibits TNF-alpha-induced hydrogen peroxide production by human neutrophils (ID50 = 1-2 microg/mL).

CONCLUSIONS

Serum SLPI is elevated in human sepsis and experimental endotoxemia. Maximal concentrations of serum SLPI correlate significantly with maximal multiple organ dysfunction scores in patients with sepsis. Secretory leukocyte protease inhibitor may function to limit ongoing neutrophil-mediated tissue injury associated with organ dysfunction.

Reactive nitrogen intermediates and the pathogenesis of Salmonella and mycobacteria

Over the past decade, reactive nitrogen intermediates joined reactive oxygen intermediates as a biochemically parallel and functionally non-redundant pathway for mammalian host resistance to many microbial pathogens. The past year has brought a new appreciation that these two pathways are partially redundant, such that each can compensate in part for the absence of the other. In combination, their importance to defense of the murine host is greater than previously appreciated. In addition to direct microbicidal actions, reactive nitrogen intermediates have immunoregulatory effects relevant to the control of infection. Genes have been characterized in Mycobacterium tuberculosis and Salmonella typhimurium that may regulate the ability of pathogens to resist reactive nitrogen and oxygen intermediates produced by activated macrophages.

Lethality of endotoxin in mice genetically deficient in the respiratory burst oxidase, inducible nitric oxide synthase, or both

Two classes of oxidants are thought to play a critical role in tissue damage in septic shock: reactive oxygen intermediates (ROI) and reactive nitrogen intermediates (RNI). Particular importance has been ascribed to peroxynitrite, a product arising from the reaction of nitric oxide with superoxide. A major source of ROI is the respiratory burst oxidase of neutrophils, eosinophils, monocytes, and macrophages. A major source of RNI is inducible nitric oxide synthase (iNOS), an enzyme expressed in leukocytes, hepatocytes, vascular smooth muscle cells, endothelium, and cardiac myocytes during inflammation. In previous studies using various mouse models of endotoxic shock, genetic deficiency of iNOS as a sole intervention did not consistently alter survival. Here, using Salmonella typhimurium endotoxic bacterial lipopolysaccharide (LPS) as a sole challenge, genetic deficiency of iNOS was associated with no protection or a reduction in survival, depending on the dose of LPS. Further, no protection from lethality was observed when LPS was injected into mice genetically deficient in the 91 kDa subunit of the respiratory burst oxidase (gp91phox) nor in mice genetically deficient in both gp91phox and iNOS (gp91phox-/-/NOS2-/- mice). For the latter experiments, mice were challenged either with S. typhimurium LPS alone or with inactivated bacille Calmette-Guerin (BCG) followed by Escherichia coli LPS. Deficiency of gp91phox impaired the inflammatory response to inactivated Propionobacterium acnes, rendering survival studies following priming with P. acnes difficult to interpret. Thus, in two models of endotoxic shock, major reductions in the ability to form nitric oxide or superoxide, alone or in combination, failed to improve survival.

Phenotype of mice and macrophages deficient in both phagocyte oxidase and inducible nitric oxide synthase

The two genetically established antimicrobial mechanisms of macrophages are production of reactive oxygen intermediates by phagocyte oxidase (phox) and reactive nitrogen intermediates by inducible nitric oxide synthase (NOS2). Mice doubly deficient in both enzymes (gp91(phox-/-)/NOS2(-/-)) formed massive abscesses containing commensal organisms, mostly enteric bacteria, even when reared under specific pathogen-free conditions with antibiotics. Neither parental strain showed such infections. Thus, phox and NOS2 appear to compensate for each other's deficiency in providing resistance to indigenous bacteria, and no other pathway does so fully. Macrophages from gp91(phox-/-)/NOS2(-/-) mice could not kill virulent Listeria. Their killing of S. typhimurium, E. coli, and attenuated Listeria was markedly diminished but demonstrable, establishing the existence of a mechanism of macrophage antibacterial activity independent of phox and NOS2.

Periplasmic superoxide dismutase protects Salmonella from products of phagocyte NADPH-oxidase and nitric oxide synthase

Superoxide dismutase (SOD) catalyzes the conversion of superoxide radical to hydrogen peroxide. Periplasmic localization of bacterial Cu,Zn-SOD has suggested a role of this enzyme in defense against extracellular phagocyte-derived reactive oxygen species. Sequence analysis of regions flanking the Salmonella typhimurium sodC gene encoding Cu,Zn-SOD demonstrates significant homology to lambda phage proteins, reflecting possible bacteriophage-mediated horizontal gene transfer of this determinant among pathogenic bacteria. Salmonella deficient in Cu,Zn-SOD has reduced survival in macrophages and attenuated virulence in mice, which can be restored by abrogation of either the phagocyte respiratory burst or inducible nitric oxide synthase. Moreover, a sodC mutant is extremely susceptible to the combination of superoxide and nitric oxide. These observations suggest that SOD protects periplasmic or inner membrane targets by diverting superoxide and limiting peroxynitrite formation, and they demonstrate the ability of the respiratory burst and nitric oxide synthase to synergistically kill microbial pathogens in vivo.

A novel antioxidant gene from Mycobacterium tuberculosis

Among the major antimicrobial products of macrophages are reactive intermediates of the oxidation of nitrogen (RNI) and the reduction of oxygen (ROI). Selection of recombinants in acidified nitrite led to the cloning of a novel gene, noxR1, from a pathogenic clinical isolate of Mycobacterium tuberculosis. Expression of noxR1 conferred upon Escherichia coli and Mycobacterium smegmatis enhanced ability to resist RNI and ROI, whether the bacteria were exposed to exogenous compounds in medium or to endogenous products in macrophages. These studies provide the first identification of an RNI resistance mechanism in mycobacteria, point to a new mechanism for resistance to ROI, and raise the possibility that inhibition of the noxR1 pathway might enhance the ability of macrophages to control tuberculosis.

Evaluation of bacterial survival and phagocyte function with a fluorescence-based microplate assay

To compare antibacterial function in macrophages from mice deficient in the respiratory burst oxidase or inducible nitric oxide synthase, we developed a fluorescence-based microplate assay of bacterial survival. As bacteria grow, they convert a formulation of resazurin termed AlamarBlue from its nonfluorescent oxidized state to its fluorescent reduced state. The time required to achieve a given fluorescence is inversely proportional to the number of viable bacteria present when the dye is added. This relationship allows a precise, accurate assessment of bacterial numbers with greater sensitivity and throughput and at less cost than conventional assays. The assay facilitated quantification of the killing of Escherichia coli by S-nitrosoglutathione and hydrogen peroxide and of Salmonella typhimurium by human neutrophils and mouse macrophages. Mouse macrophages lacking the 91-kDa subunit of the respiratory burst oxidase were deficient in their ability to kill S. typhimurium, while those lacking inducible nitric oxide synthase were unimpaired.