Innate Immune Interactions between Bacillus anthracis and Host Neutrophils

The subversion of toll-like receptor signaling by bacterial and viral proteases during the development of infectious diseases

Toll-like receptors (TLRs) are pattern recognition receptors (PRRs) that respond to pathogen-associated molecular patterns (PAMPs). The recognition of specific microbial ligands by TLRs triggers an innate immune response and also promotes adaptive immunity, which is necessary for the efficient elimination of invading pathogens. Successful pathogens have therefore evolved strategies to subvert and/or manipulate TLR signaling. Both the impairment and uncontrolled activation of TLR signaling can harm the host, causing tissue destruction and allowing pathogens to proliferate, thus favoring disease progression. In this context, microbial proteases are key virulence factors that modify components of the TLR signaling pathway. In this review, we discuss the role of bacterial and viral proteases in the manipulation of TLR signaling, highlighting the importance of these enzymes during the development of infectious diseases.

Active genetics comes alive: Exploring the broad applications of CRISPR-based selfish genetic elements (or gene-drives): Exploring the broad applications of CRISPR-based selfish genetic elements (or gene-drives)

Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-based "active genetic" elements developed in 2015 bypassed the fundamental rules of traditional genetics. Inherited in a super-Mendelian fashion, such selfish genetic entities offered a variety of potential applications including: gene-drives to disseminate gene cassettes carrying desired traits throughout insect populations to control disease vectors or pest species, allelic drives biasing inheritance of preferred allelic variants, neutralizing genetic elements to delete and replace or to halt the spread of gene-drives, split-drives with the core constituent Cas9 endonuclease and guide RNA (gRNA) components inserted at separate genomic locations to accelerate assembly of complex arrays of genetic traits or to gain genetic entry into novel organisms (vertebrates, plants, bacteria), and interhomolog based copying systems in somatic cells to develop tools for treating inherited or infectious diseases. Here, we summarize the substantial advances that have been made on all of these fronts and look forward to the next phase of this rapidly expanding and impactful field.

Bacillus licheniformis and Bacillus subtilis, Probiotics That Induce the Formation of Macrophage Extracellular Traps

Probiotics are considered living microorganisms that help preserve the health of the host who uses them. Bacillus are a genus of Gram-positive bacteria used as probiotics for animal and human consumption. They are currently distributed in various commercial forms. Two of the species used as probiotics are B. licheniformis and B. subtilis. Macrophages are central cells in the immune response, being fundamental in the elimination of microbial pathogens, for which they use various mechanisms, including the formation of extracellular traps (METs). There have been very few studies carried out on the participation of macrophages in response to the interaction of probiotics of the genus Bacillus with the host. In this work, we used macrophages from the J774A mouse cell line.1, and we found that they are susceptible to infection by the two Bacillus species. However, both species were eliminated as the infection progressed. Using confocal microscopy, we identified the formation of METs from the first hours of infection, which were characterized by the presence of myeloperoxidase (MPO) and citrullinated histone (Hit3Cit). Quantitative data on extracellular DNA release were also obtained; release was observed starting in the first hour of infection. The induction of METs by B. licheniformis caused a significant decrease in the colony-forming units (CFU) of Staphylococcus aureus. The induction of METS by bacteria of the Bacillus genus is a mechanism that participates in controlling the probiotic and potentially pathogenic bacteria such as S. aureus. The induction of METs to control pathogens may be a novel mechanism that could explain the beneficial effects of probiotics of the genus Bacillus.

Complement C5 inhibition protects against hemolytic anemia and acute kidney injury in anthrax peptidoglycan-induced sepsis in baboons

Late-stage anthrax infections are characterized by dysregulated immune responses and hematogenous spread of Bacillus anthracis, leading to extreme bacteremia, sepsis, multiple organ failure, and, ultimately, death. Despite the bacterium being nonhemolytic, some fulminant anthrax patients develop a secondary atypical hemolytic uremic syndrome (aHUS) through unknown mechanisms. We recapitulated the pathology in baboons challenged with cell wall peptidoglycan (PGN), a polymeric, pathogen-associated molecular pattern responsible for the hemostatic dysregulation in anthrax sepsis. Similar to aHUS anthrax patients, PGN induces an initial hematocrit elevation followed by progressive hemolytic anemia and associated renal failure. Etiologically, PGN induces erythrolysis through direct excessive activation of all three complement pathways. Blunting terminal complement activation with a C5 neutralizing peptide prevented the progressive deposition of membrane attack complexes on red blood cells (RBC) and subsequent intravascular hemolysis, heme cytotoxicity, and acute kidney injury. Importantly, C5 neutralization did not prevent immune recognition of PGN and shifted the systemic inflammatory responses, consistent with improved survival in sepsis. Whereas PGN-induced hemostatic dysregulation was unchanged, C5 inhibition augmented fibrinolysis and improved the thromboischemic resolution. Overall, our study identifies PGN-driven complement activation as the pathologic mechanism underlying hemolytic anemia in anthrax and likely other gram-positive infections in which PGN is abundantly represented. Neutralization of terminal complement reactions reduces the hemolytic uremic pathology induced by PGN and could alleviate heme cytotoxicity and its associated kidney failure in gram-positive infections.

A Stochastic Intracellular Model of Anthrax Infection With Spore Germination Heterogeneity

We present a stochastic mathematical model of the intracellular infection dynamics of Bacillus anthracis in macrophages. Following inhalation of B. anthracis spores, these are ingested by alveolar phagocytes. Ingested spores then begin to germinate and divide intracellularly. This can lead to the eventual death of the host cell and the extracellular release of bacterial progeny. Some macrophages successfully eliminate the intracellular bacteria and will recover. Here, a stochastic birth-and-death process with catastrophe is proposed, which includes the mechanism of spore germination and maturation of B. anthracis. The resulting model is used to explore the potential for heterogeneity in the spore germination rate, with the consideration of two extreme cases for the rate distribution: continuous Gaussian and discrete Bernoulli. We make use of approximate Bayesian computation to calibrate our model using experimental measurements from in vitro infection of murine peritoneal macrophages with spores of the Sterne 34F2 strain of B. anthracis. The calibrated stochastic model allows us to compute the probability of rupture, mean time to rupture, and rupture size distribution, of a macrophage that has been infected with one spore. We also obtain the mean spore and bacterial loads over time for a population of cells, each assumed to be initially infected with a single spore. Our results support the existence of significant heterogeneity in the germination rate, with a subset of spores expected to germinate much later than the majority. Furthermore, in agreement with experimental evidence, our results suggest that most of the spores taken up by macrophages are likely to be eliminated by the host cell, but a few germinated spores may survive phagocytosis and lead to the death of the infected cell. Finally, we discuss how this stochastic modelling approach, together with dose-response data, allows us to quantify and predict individual infection risk following exposure.

Structural insights of macromolecules involved in bacteria-induced apoptosis in the pathogenesis of human diseases

Numbers of pathogenic bacteria can induce apoptosis in human host cells and modulate the cellular pathways responsible for inducing or inhibiting apoptosis. These pathogens are significantly recognized by host proteins and provoke the multitude of several signaling pathways and alter the cellular apoptotic stimuli. This process leads the bacterial entry into the mammalian cells and evokes a variety of responses like phagocytosis, release of mitochondrial cytochrome c, secretion of bacterial effectors, release of both apoptotic and inflammatory cytokines, and the triggering of apoptosis. Several mechanisms are involved in bacteria-induced apoptosis including, initiation of the endogenous death machinery, pore-forming proteins, and secretion of superantigens. Either small molecules or proteins may act as a binding partner responsible for forming the protein complexes and regulate enzymatic activity via protein-protein interactions. The bacteria induce apoptosis, attack the human cell and gain control over various types of cells and tissue. Since these processes are intricate in the defense mechanisms of host organisms against pathogenic bacteria and play an important function in host-pathogen interactions. In this chapter, we focus on the various bacterial-induced apoptosis mechanisms in host cells and discuss the important proteins and bacterial effectors that trigger the host cell apoptosis. The structural characterization of bacterial effector proteins and their interaction with human host cells are also considered.

Convergent evolution of diverse Bacillus anthracis outbreak strains toward altered surface oligosaccharides that modulate anthrax pathogenesis

Bacillus anthracis, a spore-forming gram-positive bacterium, causes anthrax. The external surface of the exosporium is coated with glycosylated proteins. The sugar additions are capped with the unique monosaccharide anthrose. The West African Group (WAG) B. anthracis have mutations rendering them anthrose deficient. Through genome sequencing, we identified 2 different large chromosomal deletions within the anthrose biosynthetic operon of B. anthracis strains from Chile and Poland. In silico analysis identified an anthrose-deficient strain in the anthrax outbreak among European heroin users. Anthrose-deficient strains are no longer restricted to West Africa so the role of anthrose in physiology and pathogenesis was investigated in B. anthracis Sterne. Loss of anthrose delayed spore germination and enhanced sporulation. Spores without anthrose were phagocytized at higher rates than spores with anthrose, indicating that anthrose may serve an antiphagocytic function on the spore surface. The anthrose mutant had half the LD50 and decreased time to death (TTD) of wild type and complement B. anthracis Sterne in the A/J mouse model. Following infection, anthrose mutant bacteria were more abundant in the spleen, indicating enhanced dissemination of Sterne anthrose mutant. At low sample sizes in the A/J mouse model, the mortality of ΔantC-infected mice challenged by intranasal or subcutaneous routes was 20% greater than wild type. Competitive index (CI) studies indicated that spores without anthrose disseminated to organs more extensively than a complemented mutant. Death process modeling using mouse mortality dynamics suggested that larger sample sizes would lead to significantly higher deaths in anthrose-negative infected animals. The model was tested by infecting Galleria mellonella with spores and confirmed the anthrose mutant was significantly more lethal. Vaccination studies in the A/J mouse model showed that the human vaccine protected against high-dose challenges of the nonencapsulated Sterne-based anthrose mutant. This work begins to identify the physiologic and pathogenic consequences of convergent anthrose mutations in B. anthracis.

Experimental infection of hepatitis E virus induces pancreatic necroptosis in miniature pigs

Infection by hepatitis E virus (HEV) via the oral route causes acute hepatitis. Extra-hepatic manifestations of HEV infection may stem from various causes; however, its distribution in organs such as the liver, as well as the mechanisms underlying HEV-induced cell injury, remain unclear. The objective of this study was to determine the chronological distribution of HEV in various tissues of HEV-challenged miniature pigs and to investigate the mechanisms underlying HEV-induced cell death in the pancreas and liver. Virological and serological analyses were performed on blood and faecal samples. Histopathology of the liver and extra-hepatic tissues was analysed. Cell death pathways and immune cell characterisation in inflammatory lesions were analysed using immunohistochemistry. The liver and pancreas displayed inflammation and cellular injury, and a large amount of HEV was observed in the lesions. The liver was infiltrated by T and natural killer cells. HEV was identified in all organs except the heart, and was associated with immune cells. Although the liver and the pancreas strongly expressed TNF-α and TRAIL, TUNEL assay results were negative. RIP3 and pMLKL were expressed in the pancreas. RIP3, but not pMLKL, was expressed in the liver. Pancreatitis induced in HEV-infected miniature pigs is associated with necroptosis.

C3 Opsonization of Anthrax Bacterium and Peptidoglycan Supports Recognition and Activation of Neutrophils

Neutrophils are the most abundant innate cell population and a key immune player against invading pathogens. Neutrophils can kill both bacterium and spores of Bacillus anthracis, the causative anthrax pathogen. Unlike interactions with professional phagocytes, the molecular recognition of anthrax by neutrophils is largely unknown. In this study, we investigated the role of complement C3 deposition on anthrax particles for neutrophil recognition of bacterium and/or its cell wall peptidoglycan, an abundant pathogen-associated molecular pattern that supports anthrax sepsis. C3 opsonization and recognition by complement receptors accounted for 70-80% of the affinity interactions between neutrophils and anthrax particles at subphysiologic temperatures. In contrast, C3 supported up to 50% of the anthrax particle ingestion under thermophysiologic conditions. Opsonin-dependent low affinity interactions and, to a lower extent, opsonin-independent mechanisms, provide alternative entry routes. Similarly, C3 supported 58% of peptidoglycan-induced degranulation and, to a lower extent, 23% of bacterium-induced degranulation. Interestingly, an opsonin independent mechanism mediated by complement C5, likely through C5a anaphylatoxin, primes azurophilic granules in response to anthrax particles. Overall, we show that C3 deposition supports anthrax recognition by neutrophils but is dispensable for pathogen ingestion and neutrophil degranulation, highlighting immune recognition redundancies that minimize the risk of pathogen evasion.

Structural Integrity of the Alveolar-Capillary Barrier in Cynomolgus Monkeys Challenged with Fully Virulent and Toxin-Deficient Strains of Bacillus anthracis

Inhalational anthrax, a disease caused by inhaling Bacillus anthracis spores, leads to respiratory distress, vascular leakage, high-level bacteremia, and often death within days. Anthrax lethal toxin and edema toxin, which are composed of protective antigen (PA) plus either lethal factor (LF) or edema factor (EF), respectively, play an important yet incompletely defined role in the pulmonary pathophysiology. To better understand their contribution, we examined the structural integrity of the alveolar-capillary barrier in archival formalin-fixed lungs of cynomolgus monkeys challenged with the fully virulent B. anthracis Ames wild-type strain or the isogenic toxin-deficient mutants ΔEF, ΔLF, and ΔPA. Pulmonary spore challenge with the wild-type strain caused high mortality, intra-alveolar hemorrhages, extensive alveolar septal sequestration of bacteria and neutrophils, diffuse destabilization of epithelial and endothelial junctions, increased markers of coagulation and complement activation (including tissue factor and C5a), and multifocal intra-alveolar fibrin deposition. ΔEF challenge was lethal and showed similar alveolar-capillary alterations; however, intra-alveolar hemorrhages, bacterial deposition, and markers of coagulation or complement were absent or markedly lower. In contrast, ΔLF or ΔPA challenges were nonlethal and showed no signs of alveolar bacterial deposition or alveolar-capillary changes. These findings provide evidence that lethal toxin plays a determinative role in bacterial dissemination and alveolar-capillary barrier dysfunction, and edema toxin may significantly exacerbate pulmonary pathologies in a systemic infection.

Bacillus anthracis Poly-γ-D-Glutamate Capsule Inhibits Opsonic Phagocytosis by Impeding Complement Activation

Bacillus anthracis poly-γ-D-glutamic acid (PGA) capsule is an essential virulent factor that helps the bacterial pathogen to escape host immunity. Like other encapsulated bacterial species, the B. anthracis capsule may also inhibit complement-mediated clearance and ensure bacterial survival in the host. Previous reports suggest that B. anthracis spore proteins inhibit complement activation. However, the mechanism through which the B. anthracis capsule imparts a survival advantage to the active bacteria has not been demonstrated till date. Thus, to evaluate the role of the PGA capsule in evading host immunity, we have undertaken the present head-to-head comparative study of the phagocytosis and complement activation of non-encapsulated and encapsulated B. anthracis strains. The encapsulated virulent strain exhibited resistance toward complement-dependent and complement-independent bacterial phagocytosis by human macrophages. The non-encapsulated Sterne strain was highly susceptible to phagocytosis by THP-1 macrophages, after incubation with normal human serum (NHS), heat-inactivated serum, and serum-free media, thus indicating that the capsule inhibited both complement-dependent and complement-independent opsonic phagocytosis. An increased binding of C3b and its subsequent activation to C3c and C3dg, which functionally act as potent opsonins, were observed with the non-encapsulated Sterne strain compared with the encapsulated strain. Other known mediators of complement fixation, IgG, C-reactive protein (CRP), and serum amyloid P component (SAP), also bound more prominently with the non-encapsulated Sterne strain. Studies with complement pathway-specific, component-deficient serum demonstrated that the classical pathway was primarily involved in mediating C3b binding on the non-encapsulated bacteria. Both strains equally bound the complement regulatory proteins C4BP and factor H. Importantly, we demonstrated that the negative charge of the PGA capsule was responsible for the differential binding of the complement proteins between the non-encapsulated and encapsulated strains. At lower pH closer to the isoelectric point of PGA, the neutralization of the negative charge was associated with an increased binding of C3b and IgG with the encapsulated B. anthracis strain. Overall, our data have demonstrated that the B. anthracis capsule inhibits complement fixation and opsonization resulting in reduced phagocytosis by macrophages, thus allowing the bacterial pathogen to evade host immunity.

Chlorpyrifos Suppresses Neutrophil Extracellular Traps in Carp by Promoting Necroptosis and Inhibiting Respiratory Burst Caused by the PKC/MAPK Pathway

Neutrophil extracellular traps (NETs) are reticular structures formed by myeloperoxidase (MPO), histones, and neutrophil elastase (NE) that are released from neutrophils in response to pathogenic stimuli. Chlorpyrifos (CPF) is wildly used as an organophosphorus pesticide that causes a range of toxicological and environmental problems. Exposure to CPF can increase the production of neutrophils in carps, and this increase can be considered a biomarker of water pollution. To explore a relationship between NETs and CPF and its mechanism of influence, we treated neutrophils from the blood of carp with 1 μg/mL phorbol 12-myristate 13-acetate (PMA), 0.325 mg/L CPF, or 20 μM necrostatin-1 (Nec-1). The production of MPO and NETs was reduced in the CPF+PMA group compared with that in the PMA group. CPF can cause an increase in reactive oxygen species (ROS), while inhibiting respiratory burst caused by PMA stimulation. We found that the expression levels of protein-coupled receptor 84 (gpr84), dystroglycan (DAG), proto-oncogene serine/threonine kinase (RAF), protein kinase C (PKC), and mitogen-activated protein kinase 3 (MAPK3) in the CPF+PMA group were lower than those in the PMA group, indicating that the PKC-MAPK pathway was suppressed. The expression levels of cylindromatosis (CYLD), mixed lineage kinase domain-like pseudokinase (MLKL), receptor-interacting serine-threonine kinase 1 (RIP1), and receptor-interacting serine-threonine kinase 3 (RIP3) were increased, and the expression levels of caspase 8 were reduced by CPF, indicating that CPF may cause necroptosis. The addition of Nec-1 restored the number of NETs in the CPF+PMA group. The results indicate that CPF reduced the production of NETs by inhibiting respiratory burst and increasing necroptosis. The results contribute to the understanding of the immunotoxicological mechanism of CPF and provide a reference for comparative medical studies.