Cyclopropane Fatty Acids Are Important for Salmonella enterica Serovar Typhimurium Virulence.

A variety of eubacteria, plants, and protozoa can modify membrane lipids by cyclopropanation, which is reported to modulate membrane permeability and fluidity. The ability to cyclopropanate membrane lipids has been associated with resistance to oxidative stress in Mycobacterium tuberculosis, organic solvent stress in Escherichia coli, and acid stress in E. coli and Salmonella. In bacteria, the cfa gene encoding cyclopropane fatty acid (CFA) synthase is induced during the stationary phase of growth. In the present study, we constructed a cfa mutant of Salmonella enterica serovar Typhimurium 14028s (S. Typhimurium) and determined the contribution of CFA-modified lipids to stress resistance and virulence in mice. Cyclopropane fatty acid content was quantified in wild-type and cfa mutant S. Typhimurium. CFA levels in the cfa mutant were greatly reduced compared to CFA levels in the wild type, indicating that CFA synthase is the major enzyme responsible for cyclopropane modification of lipids in Salmonella. S. Typhimurium cfa mutants were more sensitive to extreme acid pH, the protonophore CCCP, and hydrogen peroxide compared to the wild type. In addition, cfa mutants exhibited reduced viability in murine macrophages and could be rescued by the addition of the NADPH phagocyte oxidase inhibitor diphenyleneiodonium (DPI) chloride. S. Typhimurium lacking cfa was also attenuated for virulence in mice. These observations indicate that CFA modification of lipids makes an important contribution to Salmonella virulence.

Keeping the host alive - lessons from obligate intracellular bacterial pathogens.

Mammals have evolved sophisticated host cell death signaling pathways as an important immune mechanism to recognize and eliminate cell intruders before they establish their replicative niche. However, intracellular bacterial pathogens that have co-evolved with their host have developed a multitude of tactics to counteract this defense strategy to facilitate their survival and replication. This requires manipulation of pro-death and pro-survival host signaling pathways during infection. Obligate intracellular bacterial pathogens are organisms that absolutely require an eukaryotic host to survive and replicate, and therefore they have developed virulence factors to prevent diverse forms of host cell death and conserve their replicative niche. This review encapsulates our current understanding of these host-pathogen interactions by exploring the most relevant findings of Anaplasma spp., Chlamydia spp., Rickettsia spp. and Coxiella burnetii modulating host cell death pathways. A detailed comprehension of the molecular mechanisms through which these obligate intracellular pathogens manipulate regulated host cell death will not only increase the current understanding of these difficult-to-study pathogens but also provide insights into new tools to study regulated cell death and the development of new therapeutic approaches to control infection.

Mass production of human airway-like neutrophils via transmigration in an organotypic model of human airways.

Neutrophils are difficult to study, particularly in tissues, due to their short half-life and propensity for activation. We describe an organotypic airway model that uses patient airway fluid to enable the transmigration of blood neutrophils to acquire an airway-like phenotype in order to better understand their contribution to airway diseases. In particular, we showcase how conditioned neutrophils modulate their bacteria-killing abilities. For complete details on the use and execution of this protocol, please refer to Margaroli et al. (2021).

Differential responses of chicken monocyte-derived dendritic cells infected with Salmonella Gallinarum and Salmonella Typhimurium.

Salmonella enterica serovar Gallinarum is a host-restricted bacterial pathogen that causes a serious systemic disease exclusively in birds of all ages. Salmonella enterica serovar Typhimurium is a host-generalist serovar. Dendritic cells (DCs) are key antigen-presenting cells that play an important part in Salmonella host-restriction. We evaluated the differential response of chicken blood monocyte-derived dendritic cells (chMoDCs) exposed to S. Gallinarum or S. Typhimurium. S. Typhimurium was found to be more invasive while S. Gallinarum was more cytotoxic at the early phase of infection and later showed higher resistance against chMoDCs killing. S. Typhimurium promoted relatively higher upregulation of costimulatory and other immune function genes on chMoDCs in comparison to S. Gallinarum during early phase of infection (6 h) as analyzed by real-time PCR. Both Salmonella serovars strongly upregulated the proinflammatory transcripts, however, quantum was relatively narrower with S. Gallinarum. S. Typhimurium-infected chMoDCs promoted relatively higher proliferation of naïve T-cells in comparison to S. Gallinarum as assessed by mixed lymphocyte reaction. Our findings indicated that host restriction of S. Gallinarum to chicken is linked with its profound ability to interfere the DCs function. Present findings provide a valuable roadmap for future work aimed at improved vaccine strategies against this pathogen.

The Frequency and Effect of Granulocytic Myeloid-Derived Suppressor Cells on Mycobacterial Survival in Patients With Tuberculosis: A Preliminary Report.

Introduction: Protective host responses in those exposed to or infected with tuberculosis (TB) is thought to require a delicate balance between pro-inflammatory and regulatory immune responses. Myeloid-derived suppressor cells (MDSCs), regulatory cells that dampen T-cell function, have been described in cancer and other infectious diseases but there are limited data on their role in TB. Methods: Peripheral blood was obtained from patients with active pulmonary TB and participants with presumed latent TB infection (LTBI) from Cape Town, South Africa. MDSC frequency was ascertained by flow cytometry. Purified MDSCs were used to assess (i) their suppressive effect on T-cell proliferation using a Ki67 flow cytometric assay and (ii) their effect on mycobacterial containment by co-culturing with H37Rv-infected monocyte-derived macrophages and autologous pre-primed effector T-cells with or without MDSCs. Mycobacterial containment was measured by plating colony forming units (CFU). Results: MDSCs (CD15+HLA-DR-CD33+) had significantly higher median frequencies (IQR) in patients with active TB (n=10) versus LTBI (n= 10) [8.2% (6.8-10.7) versus 42.2% (27-56) respectively; p=0.001]. Compared to MDSC-depleted peripheral blood mononuclear and effector T cell populations, dilutions of purified MDSCs isolated from active TB patients suppressed T-cell proliferation by up to 72% (n=6; p=0.03) and significantly subverted effector T-cell-mediated containment of H37Rv in monocyte-derived macrophages (n=7; 0.6% versus 8.5%; p=0.02). Conclusion: Collectively, these data suggest that circulating MDSCs are induced during active TB disease and can functionally suppress T-cell proliferation and subvert mycobacterial containment. These data may inform the design of vaccines and immunotherapeutic interventions against TB but further studies are required to understand the mechanisms underpinning the effects of MDSCs.

Nontuberculous Mycobacteria, Macrophages, and Host Innate Immune Response.

Although nontuberculous mycobacteria (NTM) are considered opportunistic infections, incidence and prevalence of NTM infection are increasing worldwide becoming a major public health threat. Innate immunity plays an essential role in mediating the initial host response against these intracellular bacteria. Specifically, macrophages phagocytose and eliminate NTM and act as antigen-presenting cells, which trigger downstream activation of cellular and humoral adaptive immune responses. Identification of macrophage receptors, mycobacterial ligands, phagosome maturation, autophagy/necrosis, and escape mechanisms are important components of this immunity network. The role of the macrophage in mycobacterial disease has mainly been studied in tuberculosis (TB), but limited information exists on its role in NTM. In this review, we focus on NTM immunity, the role of macrophages, and host interaction in NTM infection.

Staphylococcus aureus Peptide Methionine Sulfoxide Reductases Protect from Human Whole-Blood Killing.

The generation of oxidative stress is a host strategy used to control Staphylococcus aureus infections. Sulfur-containing amino acids, cysteine and methionine, are particularly susceptible to oxidation because of the inherent reactivity of sulfur. Due to the constant threat of protein oxidation, many systems evolved to protect S. aureus from protein oxidation or to repair protein oxidation after it occurs. The S. aureus peptide methionine sulfoxide reductase (Msr) system reduces methionine sulfoxide to methionine. Staphylococci have four Msr enzymes, which all perform this reaction. Deleting all four msr genes in USA300 LAC (Δmsr) sensitizes S. aureus to hypochlorous acid (HOCl) killing; however, the Δmsr strain does not exhibit increased sensitivity to H2O2 stress or superoxide anion stress generated by paraquat or pyocyanin. Consistent with increased susceptibility to HOCl killing, the Δmsr strain is slower to recover following coculture with both murine and human neutrophils than USA300 wild type. The Δmsr strain is attenuated for dissemination to the spleen following murine intraperitoneal infection and exhibits reduced bacterial burdens in a murine skin infection model. Notably, no differences in bacterial burdens were observed in any organ following murine intravenous infection. Consistent with these observations, USA300 wild-type and Δmsr strains have similar survival phenotypes when incubated with murine whole blood. However, the Δmsr strain is killed more efficiently by human whole blood. These findings indicate that species-specific immune cell composition of the blood may influence the importance of Msr enzymes during S. aureus infection of the human host.

Gefitinib Results in Robust Host-Directed Immunity Against Salmonella Infection Through Proteo-Metabolomic Reprogramming.

The global rise of antibiotic-resistant strains of Salmonella has necessitated the development of alternative therapeutic strategies. Recent studies have shown that targeting host factors may provide an alternative approach for the treatment of intracellular pathogens. Host-directed therapy (HDT) modulates host cellular factors that are essential to support the replication of the intracellular pathogens. In the current study, we identified Gefitinib as a potential host directed therapeutic drug against Salmonella. Further, using the proteome analysis of Salmonella-infected macrophages, we identified EGFR, a host factor, promoting intracellular survival of Salmonella via mTOR-HIF-1α axis. Blocking of EGFR, mTOR or HIF-1α inhibits the intracellular survival of Salmonella within the macrophages and in mice. Global proteo-metabolomics profiling indicated the upregulation of host factors predominantly associated with ATP turn over, glycolysis, urea cycle, which ultimately promote the activation of EGFR-HIF1α signaling upon infection. Importantly, inhibition of EGFR and HIF1α restored both proteomics and metabolomics changes caused by Salmonella infection. Taken together, this study identifies Gefitinib as a host directed drug that holds potential translational values against Salmonella infection and might be useful for the treatment of other intracellular infections.

A CD4+CD161+ T-Cell Subset Present in Unexposed Humans, Not Tb Patients, Are Fast Acting Cells That Inhibit the Growth of Intracellular Mycobacteria Involving CD161 Pathway, Perforin, and IFN-γ/Autophagy.

It remains undefined whether a subset of CD4+ T cells can function as fast-acting cells to control Mycobacterium tuberculosis (Mtb) infection. Here we show that the primary CD4+CD161+ T-cell subset, not CD4+CD161-, in unexposed healthy humans fast acted as unconventional T cells capable of inhibiting intracellular Mtb and BCG growth upon exposure to infected autologous and allogeneic macrophages or lung epithelial A549 cells. Such inhibition coincided with the ability of primary CD4+CD161+ T cells to rapidly express/secrete anti-TB cytokines including IFN-γ, TNF-α, IL-17, and perforin upon exposure to Mtb. Mechanistically, blockades of CD161 pathway, perforin or IFN-γ by blocking mAbs abrogated the ability of CD4+CD161+ T cells to inhibit intracellular mycobacterial growth. Pre-treatment of infected macrophages with inhibitors of autophagy also blocked the CD4+CD161+ T cell-mediated growth inhibition of mycobacteria. Furthermore, adoptive transfer of human CD4+CD161+ T cells conferred protective immunity against mycobacterial infection in SCID mice. Surprisingly, CD4+CD161+ T cells in TB patients exhibited a loss or reduction of their capabilities to produce perforin/IFN-γ and to inhibit intracellular growth of mycobacteria in infected macrophages. These immune dysfunctions were consistent with PD1/Tim3 up-regulation on CD4+CD161+ T cells in active tuberculosis patients, and the blockade of PD1/Tim3 on this subset cells enhanced the inhibition of intracellular mycobacteria survival. Thus, these findings suggest that a fast-acting primary CD4+CD161+T-cell subset in unexposed humans employs the CD161 pathway, perforin, and IFN-γ/autophagy to inhibit the growth of intracellular mycobacteria, thereby distinguishing them from the slow adaptive responses of conventional CD4+ T cells. The presence of fast-acting CD4+CD161+ T-cell that inhibit mycobacterial growth in unexposed humans but not TB patients also implicates the role of these cells in protective immunity against initial Mtb infection.

Sialic Acid Protects Nontypeable Haemophilus influenzae from Natural IgM and Promotes Survival in Murine Respiratory Tract.

Nontypeable Haemophilus influenzae (NTHi), a common inhabitant of the human nasopharynx and upper airways, causes opportunistic respiratory tract infections that are frequently recurring and chronic. NTHi utilizes sialic acid from the host to evade antibacterial defenses and persist in mucosal tissues; however, the role of sialic acid scavenged by NTHi during infection is not fully understood. We previously showed that sialylation protects specific epitopes on NTHi lipooligosaccharide (LOS) targeted by bactericidal IgM in normal human serum. Here, we evaluated the importance of immune evasion mediated by LOS sialylation in the mouse respiratory tract using wild-type H. influenzae and an isogenic siaB mutant incapable of sialylating the LOS. Sialylation protected common NTHi glycan structures recognized by human and murine IgM and protected NTHi from complement-mediated killing directed by IgM against these structures. Protection from IgM binding by sialylated LOS correlated with decreased survival of the siaB mutant versus the wild type in the murine lung. Complement depletion with cobra venom factor increased survival of the siaB mutant in the nasopharynx but not in the lungs, suggesting differing roles of sialylation at these sites. Prior infection increased IgM against H. influenzae but not against sialic acid-protected epitopes, consistent with sialic acid-mediated immune evasion during infection. These results provide mechanistic insight into an NTHi evasive strategy against an immune defense conserved across host species, highlighting the potential of the mouse model for development of anti-infective strategies targeting LOS antigens of NTHi.

Contribution of Complement System pathways to the killing of Leptospira spp.

The Complement System (CS) plays an important role in the immune response against leptospirosis and can be activated by the Alternative and Lectin Pathways (Innate Immunity) and by the Classical Pathway (Acquired Immunity). Here we analyzed a broad range of nonpathogenic and pathogenic Leptospira strains considering their interaction with each CS pathway. We determined bacterial survival rate and CS protein deposition in the presence of purified proteins, specific component depleted sera and NHS treated with the chelating agents EDTA (inhibits all three activation pathways) or EGTA (inhibits the Classical and Lectin Pathways). We suggest that the Lectin and the Alternative Pathways have an important role to eliminate saprophytic leptospires since i) approximately 50% survival of both saprophytic strains was observed in the presence of MBL-deficient serum; ii) approximately 50% survival of Leptospira biflexa Patoc I was observed in the presence of NHS - EGTA and iii) C1q-depleted serum caused significant bacterial lysis. In all serovars investigated the deposition of C5-C9 proteins on saprophytic Leptospira strains was more pronounced when compared to pathogenic species confirming previous studies in the literature. No difference on C3 deposition was observed between nonpathogenic and pathogenic strains. In conclusion, Leptospira strains interact to different degrees with CS proteins, especially those necessary to form MAC, indicating that some strains and specific ligands could favor the binding of certain CS proteins.

Bacterial Defense Systems against the Neutrophilic Oxidant Hypochlorous Acid.

Neutrophils kill invading microbes and therefore represent the first line of defense of the innate immune response. Activated neutrophils assemble NADPH oxidase to convert substantial amounts of molecular oxygen into superoxide, which, after dismutation into peroxide, serves as the substrate for the generation of the potent antimicrobial hypochlorous acid (HOCl) in the phagosomal space. In this minireview, we explore the most recent insights into physiological consequences of HOCl stress. Not surprisingly, Gram-negative bacteria have evolved diverse posttranslational defense mechanisms to protect their proteins, the main targets of HOCl, from HOCl-mediated damage. We discuss the idea that oxidation of conserved cysteine residues and partial unfolding of its structure convert the heat shock protein Hsp33 into a highly active chaperone holdase that binds unfolded proteins and prevents their aggregation. We examine two novel members of the Escherichia coli chaperone holdase family, RidA and CnoX, whose thiol-independent activation mechanism differs from that of Hsp33 and requires N-chlorination of positively charged amino acids during HOCl exposure. Furthermore, we summarize the latest findings with respect to another bacterial defense strategy employed in response to HOCl stress, which involves the accumulation of the universally conserved biopolymer inorganic polyphosphate. We then discuss sophisticated adaptive strategies that bacteria have developed to enhance their survival during HOCl stress. Understanding bacterial defense and survival strategies against one of the most powerful neutrophilic oxidants may provide novel insights into treatment options that potentially compromise the ability of pathogens to resist HOCl stress and therefore may increase the efficacy of the innate immune response.

Beyond bacterial killing: NADPH oxidase 2 is an immunomodulator.

NADPH oxidase 2 is a superoxide-generating enzymatic complex based on the catalytic subunit gp91phox that is also known as Nox2. Initially identified in neutrophils, NADPH oxidase 2 was long considered responsible only for the killing of phagocytized microorganisms. However, advances in knowledge about redox signalling and the discovery of Nox2 expression in different cell types, including macrophages, endothelial cells (ECs), dendritic cells (DCs), B and T lymphocytes, have changed this paradigm. For instance, Nox2 expressed in macrophages and neutrophils limits the transcription of cytokines and toll-like receptors (TLRs) induced by lipopolysaccharide (LPS), whereas DC Nox2 facilitates antigen cross-presentation to T cells. More recently, our group observed that Nox2 inhibits the suppressive ability of regulatory T cells (Tregs) by limiting NF-κB and FoxP3 activation. In this review, we discuss non-canonical microbicidal functions and redox-signalling-associated roles of Nox2 in different cell types, emphasizing its roles in the innate and adaptive immune system.

Characterising viable virus from air exhaled by H1N1 influenza-infected ferrets reveals the importance of haemagglutinin stability for airborne infectivity.

The transmissibility and pandemic potential of influenza viruses depends on their ability to efficiently replicate and be released from an infected host, retain viability as they pass through the environment, and then initiate infection in the next host. There is a significant gap in knowledge about viral properties that enable survival of influenza viruses between hosts, due to a lack of experimental methods to reliably isolate viable virus from the air. Using a novel technique, we isolate and characterise infectious virus from droplets emitted by 2009 pandemic H1N1-infected ferrets. We demonstrate that infectious virus is predominantly released early after infection. A virus containing a mutation destabilising the haemagglutinin (HA) surface protein displayed reduced survival in air. Infectious virus recovered from droplets exhaled by ferrets inoculated with this virus contained mutations that conferred restabilisation of HA, indicating the importance of influenza HA stability for between-host survival. Using this unique approach can improve knowledge about the determinants and mechanisms of influenza transmissibility and ultimately could be applied to studies of airborne virus exhaled from infected people.

Two-Component Signal Transduction Systems in the Human Pathogen Streptococcus agalactiae.

Streptococcus agalactiae (group B Streptococcus [GBS]) is an important cause of invasive infection in newborns, maternal women, and older individuals with underlying chronic illnesses. GBS has many mechanisms to adapt and survive in its host, and these mechanisms are often controlled via two-component signal transduction systems. In GBS, more than 20 distinct two-component systems (TCSs) have been classified to date, consisting of canonical TCSs as well as orphan and atypical sensors and regulators. These signal transducing systems are necessary for metabolic regulation, resistance to antibiotics and antimicrobials, pathogenesis, and adhesion to the mucosal surfaces to colonize the host. This minireview discusses the structures of these TCSs in GBS as well as how selected systems regulate essential cellular processes such as survival and colonization. GBS contains almost double the number of TCSs compared to the closely related Streptococcus pyogenes and Streptococcus pneumoniae, and while research on GBS TCSs has been increasing in recent years, no comprehensive reviews of these TCSs exist, making this review especially relevant.

Effect of trimethylamine N-oxide on inflammation and the gut microbiota in Helicobacter pylori-infected mice.

Diet is one of the factors contributing to symptom of Helicobacter pylori (H. pylori) infection. Trimethylamine N-oxide (TMAO), a diet-related microbial metabolite, is associated with inflammatory and metabolic diseases. The aim of this study is to investigate the effects of TMAO intake on inflammation and gut microbiota composition in H. pylori-infected mice via 16S rRNA sequencing and biochemical analyses. The in vitro experiments showed that TMAO not only increased the expression of growth- and metabolism-associated genes and the urease activity of H. pylori, but increased the production of virulence factors. Moreover, TMAO intake increased the production of inflammatory markers and reduced the richness and diversity of the gut microbiota in H. pylori-infected mice. Further analysis showed that TMAO increased the relative abundance of Escherichia_Shigella in H. pylori-infected mice, which had positive correlation with the levels of LPS, CRP, and CXCL1. Collectively, our results suggest that TMAO may aggravate H. pylori-induced inflammation by increasing the viability and virulence of H. pylori and may aggravate inflammation in association with the gut microbiota in H. pylori-infected mice. This study may provide a novel insight into the mechanism for the effect of diet-derived metabolites such as TMAO on H. pylori-induced disease development.

The tst gene associated Staphylococcus aureus pathogenicity island facilitates its pathogenesis by promoting the secretion of inflammatory cytokines and inducing immune suppression.

Staphylococcus aureus (S. aureus) is an important pathogen causing various limited or systemic infections. Methicillin resistant S. aureus (MRSA) in particular presents a major clinical and public health problem. Toxic shock syndrome toxin-1 (TSST-1) encoded by the gene tst is an important virulence factor of tst positive S. aureus, leading to multi-organ malfunction. However, the mechanism of TSST-1 in pathogenesis is only partly clear. In this study, we investigated the prevalence of the tst gene in clinical isolates of S. aureus. Then, animal experiments were performed to further evaluate the influence of the presence of the tst gene associated Staphylococcus aureus Pathogenicity Island (SaPI) on body weight, serum cytokine concentrations and the bacterial load in different organs. In addition, macrophages were used to analyze the secretion of cytokines in vitro and bacterial survival in the cytoplasm. Finally, pathological analysis was carried out to evaluate organ tissue impairment. The results demonstrated that the prevalence of tst gene was approximately 17.8% of the bacterial strains examined. BALB/c mice infected with tst gene associated SaPI positive isolates exhibited a severe loss of body weight and a high bacterial load in the liver, heart, kidney and spleen. Pathological analysis demonstrated that tissue impairment was more severe after infection with tst gene associated SaPI positive isolates. Moreover, the secretion of IL-6, IL-2 and IL17A by macrophages infected with tst gene associated SaPI positive isolates clearly increased. Notably, IL-6 secretion in BALB/c mice infected with tst gene associated SaPI positive isolates was higher than that in BALB/c mice infected with negative ones. Together, these results indicated that the tst gene associated SaPI may play a critical role in the pathological process of infection via a direct and persistent toxic function, and by promoting the secretion of inflammatory cytokines that indirectly induce immune suppression.

Type I-F CRISPR-Cas resistance against virulent phages results in abortive infection and provides population-level immunity.

Type I CRISPR-Cas systems are abundant and widespread adaptive immune systems in bacteria and can greatly enhance bacterial survival in the face of phage infection. Upon phage infection, some CRISPR-Cas immune responses result in bacterial dormancy or slowed growth, which suggests the outcomes for infected cells may vary between systems. Here we demonstrate that type I CRISPR immunity of Pectobacterium atrosepticum leads to suppression of two unrelated virulent phages, ɸTE and ɸM1. Immunity results in an abortive infection response, where infected cells do not survive, but viral propagation is severely decreased, resulting in population protection due to the reduced phage epidemic. Our findings challenge the view of CRISPR-Cas as a system that protects the individual cell and supports growing evidence of abortive infection by some types of CRISPR-Cas systems.

ClpC affects the intracellular survival capacity of Staphylococcus aureus in non-professional phagocytic cells.

Invasion and persistence of bacteria within host cells requires that they adapt to life in an intracellular environment. This adaptation induces bacterial stress through events such as phagocytosis and enhanced nutrient-restriction. During stress, bacteria synthesize a family of proteins known as heat shock proteins (HSPs) to facilitate adaptation and survival. Previously, we determined the Staphylococcus aureus HSP ClpC temporally alters bacterial metabolism and persistence. This led us to hypothesize that ClpC might alter intracellular survival. Inactivation of clpC in S. aureus strain DSM20231 significantly enhanced long-term intracellular survival in human epithelial (HaCaT) and endothelial (EA.hy926) cell lines, without markedly affecting adhesion or invasion. This phenotype was similar across a genetically diverse collection of S. aureus isolates, and was influenced by the toxin/antitoxin encoding locus mazEF. Importantly, MazEF alters mRNA synthesis and/or stability of S. aureus virulence determinants, indicating ClpC may act through the mRNA modulatory activity of MazEF. Transcriptional analyses of total RNAs isolated from intracellular DSM20231 and isogenic clpC mutant cells identified alterations in transcription of α-toxin (hla), protein A (spa), and RNAIII, consistent with the hypothesis that ClpC negatively affects the intracellular survival of S. aureus in non-professional phagocytic cells, via modulation of MazEF and Agr.

Nigericin Promotes NLRP3-Independent Bacterial Killing in Macrophages.

Altered microbiota has been associated with a number of diseases, including inflammatory bowel diseases, diabetes, and cancer. This dysregulation is thought to relate the host inflammatory response to enteric pathogens. Macrophages play a key role in host response to microbes and are involved in bacterial killing and clearance. This process is partially mediated through the potassium efflux-dependent, cytosolic, PYCARD-containing inflammasome protein complex. Surprisingly, we discovered an alternative mechanism for bacterial killing, independent of the NLRP3 inflammasome/PYCARD. Using the NLRP3 inflammasome-deficient Raw 264.7 and PYCARD-deficient J77 macrophages, which both lack PYCARD, we found that the potassium efflux activator nigericin enhances bacterial killing. Macrophage response to nigericin was examined by RT gene profiling and subsequent qPCR, which demonstrated altered expression of a series of genes involved in the IL-18 bacterial killing pathway. Based on our results we propose a model of bacterial killing, unrelated to NLRP3 inflammasome activation in macrophage cells. Improving understanding of the molecular pathways driving bacterial clearance within macrophage cells will aid in the development of novel immune-targeted therapeutics in a number of diseases.