Protease-dependent mechanisms of complement evasion by bacterial pathogens

The complement system as a key modulator of the oral microbiome in health and disease

In this review, we address the interplay between the complement system and host microbiomes in health and disease, focussing on oral bacteria known to contribute to homeostasis or to promote dysbiosis associated with dental caries and periodontal diseases. Host proteins modulating complement activities in the oral environment and expression profiles of complement proteins in oral tissues were described. In addition, we highlight a sub-set of bacterial proteins involved in complement evasion and/or dysregulation previously characterized in pathogenic species (or strains), but further conserved among prototypical commensal species of the oral microbiome. Potential roles of these proteins in host-microbiome homeostasis and in the emergence of commensal strain lineages with increased virulence were also addressed. Finally, we provide examples of how commensal bacteria might exploit the complement system in competitive or cooperative interactions within the complex microbial communities of oral biofilms. These issues highlight the need for studies investigating the effects of the complement system on bacterial behaviour and competitiveness during their complex interactions within oral and extra-oral host sites.

A guide to complement biology, pathology and therapeutic opportunity

Complement has long been considered a key innate immune effector system that mediates host defence and tissue homeostasis. Yet, growing evidence has illuminated a broader involvement of complement in fundamental biological processes extending far beyond its traditional realm in innate immunity. Complement engages in intricate crosstalk with multiple pattern-recognition and signalling pathways both in the extracellular and intracellular space. Besides modulating host-pathogen interactions, this crosstalk guides early developmental processes and distinct cell trajectories, shaping tissue immunometabolic and regenerative programmes in different physiological systems. This Review provides a guide to the system-wide functions of complement. It highlights illustrative paradigm shifts that have reshaped our understanding of complement pathobiology, drawing examples from evolution, development of the central nervous system, tissue regeneration and cancer immunity. Despite its tight spatiotemporal regulation, complement activation can be derailed, fuelling inflammatory tissue pathology. The pervasive contribution of complement to disease pathophysiology has inspired a resurgence of complement therapeutics with major clinical developments, some of which have challenged long-held dogmas. We thus highlight major therapeutic concepts and milestones in clinical complement intervention.

Multiplexed DNA and Protease Detection with Orthogonal Energy Transfer on a Single Quantum Dot Scaffolded Biosensor

Almost all pathogens, whether viral or bacterial, utilize key proteolytic steps in their pathogenesis. The ability to detect a pathogen's genomic material along with its proteolytic activity represents one approach to identifying the pathogen and providing initial evidence of its viability. Here, we report on a prototype biosensor design assembled around a single semiconductor quantum dot (QD) scaffold that is capable of detecting both nucleic acid sequences and proteolytic activity by using orthogonal energy transfer (ET) processes. The sensor consists of a central QD assembled via peptidyl-PNA linkers with multiple DNA sequences that encode complements to genomic sequences originating from the Ebola, Influenza, and COVID-19 viruses, which we use as surrogate targets. These are hybridized to complement strands labeled with a terbium (Tb) chelate, AlexaFluor647 (AF647), and Cy5.5 dyes, giving rise to two potential FRET cascades: the first includes Tb → QD → AF647 → Cy5.5 (→ = ET step), which is detected in a time-gated modality, and QD → AF647 → Cy5.5, which is detected from direct excitation. The labeled DNA-displaying QD construct is then further assembled with a RuII-modified peptide, which quenches QD photoluminescence by charge transfer and is recognized by a protease to yield the full biosensor. Each of the labeled DNAs and peptides can be ratiometrically assembled to the QD in a controllable manner to tune each of the ET pathways. Addition of a given target DNA displaces its labeled complement on the QD, disrupting that FRET channel, while protease addition disrupts charge transfer quenching of the central QD scaffold and boosts its photoluminescence and FRET relay capabilities. Along with characterizing the ET pathways and verifying biosensing in both individual and multiplexed formats, we also demonstrate the ability of this construct to function in molecular logic and perform Boolean operations; this highlights the construct's ability to discriminate and transduce signals between different inputs or pathogens. The potential application space for such a sensor device is discussed.

Bacterial Proteases as Potentially Exploitable Modulators of SARS-CoV-2 Infection: Logic from the Literature, Informatics, and Inspiration from the Dog

(1) Background: The COVID-19 pandemic left many intriguing mysteries. Retrospective vulnerability trends tie as strongly to odd demographics as to exposure profiles, genetics, health, or prior medical history. This article documents the importance of nasal microbiome profiles in distinguishing infection rate trends among differentially affected subgroups. (2) Hypothesis: From a detailed literature survey, microbiome profiling experiments, bioinformatics, and molecular simulations, we propose that specific commensal bacterial species in the Pseudomonadales genus confer protection against SARS-CoV-2 infections by expressing proteases that may interfere with the proteolytic priming of the Spike protein. (3) Evidence: Various reports have found elevated Moraxella fractions in the nasal microbiomes of subpopulations with higher resistance to COVID-19 (e.g., adolescents, COVID-19-resistant children, people with strong dietary diversity, and omnivorous canines) and less abundant ones in vulnerable subsets (the elderly, people with narrower diets, carnivorous cats and foxes), along with bioinformatic evidence that Moraxella bacteria express proteases with notable homology to human TMPRSS2. Simulations suggest that these proteases may proteolyze the SARS-CoV-2 spike protein in a manner that interferes with TMPRSS2 priming.

Potential probiotic lactobacilli strains isolated from artisanal Mexican Cocido cheese: evidence-based biosafety and probiotic action-related traits on in vitro tests

The biosafety of four potentially probiotic lactobacilli strains, isolated from artisanal Mexican Cocido cheese, was assessed through in vitro tests aimed to determine (1) the antibiotic susceptibility profile by broth microdilution, (2) the transferability of antibiotic resistance determinants by filter-mating, and (3) the phenotypic and genotypic stability during serial batch sub-culture (100-day period) by evaluating physiological and probiotic features and RAPD-PCR fingerprinting. Lactobacilli strains exhibited multidrug-resistance; however, resistance determinants were not transferred in the filter-mating assay. Significant (p < 0.05) differences were observed in bacterial morphology and some functional and technological properties when strains were serially sub-cultured over 50 generations (G50), compared to the initial cultures (G0). Conversely, the strains did not show mucinolytic and hemolytic activities either at G0 or after 100 generations (G100). Genetic polymorphism and genomic template instability on selected strains were detected, which suggest possible evolutionary arrangements that may occur when these bacteria are largely cultured. Our findings suggest that the assessed strains did not raise in vitro biosafety concerns; however, complementary studies are still needed to establish the safe potential applications in humans and animals.

Computational docking investigation of phytocompounds from bergamot essential oil against Serratia marcescens protease and FabI: Alternative pharmacological strategy

The rapid development of multi-drug resistant (MDR) pathogens adds urgency to search for novel and safe drugs having promising action on new and re-emerging infectious pathogens. Serratia marcescens is an MDR pathogen that causes several-healthcare associated infections. Curbing bacterial virulence, rather than inhibiting its growth, is a promising strategy to diminish the pathogenesis of infectious bacteria, reduce the development of antimicrobial resistance, and boost the host immune power to eradicate infections. Bergamot essential oil (BEO) is a remarkable source of promising therapeutics against pathogens. Therefore, the present investigation aimed to analyze the major phytocompounds from BEO against S. marcescens virulent proteins using in silico studies. The analysis of BEO phytocompounds was achieved by Gas chromatography-mass spectrometry (GC-MS) method. The molecular docking was carried out using the SP and XP docking protocol of the Glide program. The drug-likeness and pharmacokinetics properties (ADMET properties) were analyzed with SwissADME and pkCSM server. The results revealed that the major compounds present in BEO are Linalool (8.17%), D-Limonene (21.26%), and Linalyl acetate (26.91%). Molecular docking analysis revealed that these compounds docked strongly within the binding cavities of Serratia protease and FabI model which in turn curb the pathogenesis of this bacteria. Linalool interacted with the Serratia protease and FabI with a binding energy of - 3.130 kcal/mol and - 3.939 kcal/mol, respectively. Based on the pharmacokinetics findings all lead BEO phytocompounds appear to be promising drug candidates. Overall, these results represent a significant step in the development of plant-based compounds as a promising inhibitor of the virulent proteins of the MDR S. marcescens.

Omptin Proteases of Enterobacterales Show Conserved Regulation by the PhoPQ Two-Component System but Exhibit Divergent Protection from Antimicrobial Host Peptides and Complement

Bacteria that colonize eukaryotic surfaces interact with numerous antimicrobial host-produced molecules, including host defense peptides, complement, and antibodies. Bacteria have evolved numerous strategies to both detect and resist these molecules, and in the Enterobacterales order of bacteria these include alterations of the cell surface lipopolysaccharide structure and/or charge and the production of proteases that can degrade these antimicrobial molecules. Here, we show that omptin family proteases from Escherichia coli and Citrobacter rodentium are regulated by the PhoPQ system. Omptin protease activity is induced by growth in low Mg²⁺, and deletion of PhoP dramatically reduces omptin protease activity, transcriptional regulation, and protein levels. We identify conserved PhoP-binding sites in the promoters of the E. coli omptin genes ompT, ompP, and arlC as well as in croP of Citrobacter rodentium and show that mutation of the putative PhoP-binding site in the ompT promoter abrogates PhoP-dependent expression. Finally, we show that although regulation by PhoPQ is conserved, each of the omptin proteins has differential activity toward host defense peptides, complement components, and resistance to human serum, suggesting that each omptin confers unique survival advantages against specific host antimicrobial factors.

Hyperglycemia in Diabetic Skin Infections Promotes Staphylococcus aureus Virulence Factor Aureolysin: Visualization by Molecular Imaging

Bacterial skin infections are common in diabetic patients, with Staphylococcus aureus (S. aureus) being the most commonly isolated, causing comorbidities such as increased mortality and long-term hospitalization. While precise mechanisms remain to be determined, hyperglycemia represents an important pathogenetic factor responsible for the increased risk of S. aureus infection. Herein, we constructed a series of ratiometric fluorescent molecular probes for aureolysin (Aur), a major virulence factor in S. aureus. Using probe 1, we were able to determine specific Aur activity in both cells and tissues. We also observed that elevated glucose levels led to 2-fold higher Aur expression in S. aureus cultures. In a diabetic mouse model, we used molecular imaging to demonstrate that hyperglycemia tripled S. aureus Aur virulence compared to nondiabetic mice, resulting in more severe infections.

Secreted Autotransporter Toxin (Sat) Mediates Innate Immune System Evasion

Several strategies are used by Escherichia coli to evade the host innate immune system in the blood, such as the cleavage of complement system proteins by secreted proteases. Members of the Serine Proteases Autotransporters of Enterobacteriaceae (SPATE) family have been described as presenting proteolytic effects against complement proteins. Among the SPATE-encoding genes sat (secreted autotransporter toxin) has been detected in high frequencies among strains of E. coli isolated from bacteremia. Sat has been characterized for its cytotoxic action, but the possible immunomodulatory effects of Sat have not been investigated. Therefore, this study aimed to evaluate the proteolytic effects of Sat on complement proteins and the role in pathogenesis of BSI caused by extraintestinal E. coli (ExPEC). E. coli EC071 was selected as a Sat-producing ExPEC strain. Whole-genome sequencing showed that sat sequences of EC071 and uropathogenic E. coli CFT073 present 99% identity. EC071 was shown to be resistant to the bactericidal activity of normal human serum (NHS). Purified native Sat was used in proteolytic assays with proteins of the complement system and, except for C1q, all tested substrates were cleaved by Sat in a dose and time-dependent manner. Moreover, E. coli DH5α survived in NHS pre-incubated with Sat. EC071-derivative strains harboring sat knockout and in trans complementations producing either active or non-active Sat were tested in a murine sepsis model. Lethality was reduced by 50% when mice were inoculated with the sat mutant strain. The complemented strain producing active Sat partially restored the effect caused by the wild-type strain. The results presented in this study show that Sat presents immunomodulatory effects by cleaving several proteins of the three complement system pathways. Therefore, Sat plays an important role in the establishment of bloodstream infections and sepsis.

An Introduction to Bacterial Biofilms and Their Proteases, and Their Roles in Host Infection and Immune Evasion

Bacterial biofilms represent multicellular communities embedded in a matrix of extracellular polymeric substances, conveying increased resistance against environmental stress factors but also antibiotics. They are shaped by secreted enzymes such as proteases, which can aid pathogenicity by degrading host proteins of the connective tissue or the immune system. Importantly, both secreted proteases and the capability of biofilm formation are considered key virulence factors. In this review, we focus on the basic aspects of proteolysis and protein secretion, and highlight various secreted bacterial proteases involved in biofilm establishment and dispersal, and how they aid bacteria in immune evasion by degrading immunoglobulins and components of the complement system. Thus, secreted proteases represent not only prominent antimicrobial targets but also enzymes that can be used for dedicated applications in biotechnology and biomedicine, including their use as laundry detergents, in mass spectrometry for the glycoprofiling of antibodies, and the desensitization of donor organs intended for positive crossmatch patients.

Phylogenetic analysis of the bacterial Pro-Pro-endopeptidase domain reveals a diverse family including secreted and membrane anchored proteins

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Bacterial Pro-Pro-endopeptidase (PPEP) is the latest member of the metalloendopeptidase class (E.C. 3.4.24.89).

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PPEP homologs are found in two firmicutes orders, clostridiales and bacillales spread over 9 genera and more than 130 species.

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Some PPEP homologs have acquired additional anchor domains that bind noncovalently to various elements of the bacterial peptidoglycan cell wall.

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Prototype family members, PPEP-1 and PPEP-2, target bacterial surface adhesion proteins, but homologs could target other extracellular proteins.

Pro-Pro-endopeptidases (PPEP, EC 3.4.24.89) are secreted, zinc metalloproteases that have the unusual capacity to cleave a peptide bond between two prolines, a bond that is generally less sensitive to proteolytic cleavage. Two well studied members of the family are PPEP-1 and PPEP-2, produced by Clostridioides difficile, a human pathogen, and Paenibacillus alvei, a bee secondary invader, respectively. Both proteases seem to be involved in mediating bacterial adhesion by cleaving cell surface anchor proteins on the bacterium itself.

By using basic alignment and phylogenetic profiling analysis, this work shows that the complete family of proteins that contain a PPEP domain includes proteins from more than 130 species spread over 9 genera. These analyses also suggest that the PPEP domain spread through horizontal gene transfer events between species within the Firmicutes’ classes Bacilli and Clostridia. Bacterial species containing PPEP homologs are found in diverse habitats, varying from human pathogens and gut microbiota to free-living bacteria, which were isolated from various environments, including extreme conditions such as hot springs, desert soil and salt lakes.

The phylogenetic tree reveals the relationships between family members and suggests that smaller subgroups could share cleavage specificity, substrates and functional similarity. Except for PPEP-1 and PPEP-2, no cleavage specificity, specific physiological target, or function has been assigned for any of the other PPEP-family members. Some PPEP proteins have acquired additional domains that recognize and bind noncovalently to various elements of the bacterial peptidoglycan cell-wall, anchoring these PPEPs. Secreted or anchored to the cell-wall surface PPEP proteins seem to perform various functions.

To React or Not to React: The Dilemma of Fish Immune Systems Facing Myxozoan Infections

Myxozoans are microscopic, metazoan, obligate parasites, belonging to the phylum Cnidaria. In contrast to the free-living lifestyle of most members of this taxon, myxozoans have complex life cycles alternating between vertebrate and invertebrate hosts. Vertebrate hosts are primarily fish, although they are also reported from amphibians, reptiles, trematodes, mollusks, birds and mammals. Invertebrate hosts include annelids and bryozoans. Most myxozoans are not overtly pathogenic to fish hosts, but some are responsible for severe economic losses in fisheries and aquaculture. In both scenarios, the interaction between the parasite and the host immune system is key to explain such different outcomes of this relationship. Innate immune responses contribute to the resistance of certain fish strains and species, and the absence or low levels of some innate and regulatory factors explain the high pathogenicity of some infections. In many cases, immune evasion explains the absence of a host response and allows the parasite to proliferate covertly during the first stages of the infection. In some infections, the lack of an appropriate regulatory response results in an excessive inflammatory response, causing immunopathological consequences that are worse than inflicted by the parasite itself. This review will update the available information about the immune responses against Myxozoa, with special focus on T and B lymphocyte and immunoglobulin responses, how these immune effectors are modulated by different biotic and abiotic factors, and on the mechanisms of immune evasion targeting specific immune effectors. The current and future design of control strategies for myxozoan diseases is based on understanding this myxozoan-fish interaction, and immune-based strategies such as improvement of innate and specific factors through diets and additives, host genetic selection, passive immunization and vaccination, are starting to be considered.

The Role of Bacterial Proteases in Microbe and Host-microbe Interactions

BACKGROUND

Secreted proteases are an important class of factors used by bacterial to modulate their extracellular environment through the cleavage of peptides and proteins. These proteases can range from broad, general proteolytic activity to high degrees of substrate specificity. They are often involved in interactions between bacteria and other species, even across kingdoms, allowing bacteria to survive and compete within their niche. As a result, many bacterial proteases are of clinical importance. The immune system is a common target for these enzymes, and bacteria have evolved ways to use these proteases to alter immune responses for their benefit. In addition to the wide variety of human proteins that can be targeted by bacterial proteases, bacteria also use these secreted factors to disrupt competing microbes, ranging from outright antimicrobial activity to disrupting processes like biofilm formation.

OBJECTIVE

In this review, we address how bacterial proteases modulate host mechanisms of protection from infection and injury, including immune factors and cell barriers. We also discuss the contributions of bacterial proteases to microbe-microbe interactions, including antimicrobial and anti-biofilm dynamics.

CONCLUSION

Bacterial secreted proteases represent an incredibly diverse group of factors that bacteria use to shape and thrive in their microenvironment. Due to the range of activities and targets of these proteases, some have been noted for having potential as therapeutics. The vast array of bacterial proteases and their targets remains an expanding field of research, and this field has many important implications for human health.

Surface loading of nanoparticles on engineered or natural erythrocytes for prolonged circulation time: strategies and applications

Nano drug-delivery systems (DDS) may significantly improve efficiency and reduce toxicity of loaded drugs, but a few nano-DDS are highly successful in clinical use. Unprotected nanoparticles in blood flow are often quickly cleared, which could limit their circulation time and drug delivery efficiency. Elongating their blood circulation time may improve their delivery efficiency or grant them new therapeutic possibilities. Erythrocytes are abundant endogenous cells in blood and are continuously renewed, with a long life span of 100-120 days. Hence, loading nanoparticles on the surface of erythrocytes to protect the nanoparticles could be highly effective for enhancing their in vivo circulation time. One of the key questions here is how to properly attach nanoparticles on erythrocytes for different purposes and different types of nanoparticles to achieve ideal results. In this review, we describe various methods to attach nanoparticles and drugs to the erythrocyte surface, and discuss the key factors that influence the stability and circulation properties of the erythrocytes-based delivery system in vivo. These data show that using erythrocytes as a host for nanoparticles possesses great potential for further development.

A deep-sea pathogenic Bacillus subtilis isolate employs different strategies to escape the killing of teleost and murine complements

Bacillus subtilis subsp. subtilis G7 was isolated from a deep-sea hydrothermal vent and is pathogenic to pathogenic to fish (Japanese flounder) and mice. G7 is able to survive in host sera and phagocytes. In this study, we investigated the underlying mechanism of G7 serum resistance. We found that (i) the remaining complement activity was very low in G7-incubated flounder serum but high in G7-incubated mouse serum; (ii) cleaved C3 and C5 components were detected on flounder serum-incubated G7 but not on mouse serum-incubated G7; (iii) abundant uncleaved C5 was localized in G7-incubated mouse, but not flounder, serum; (iv) G7-incubated flounder, but not mouse, serum exhibited strong chemotactic activity; (v) pre-treatment with low-dose lysozyme abolished the serum resistance of G7. Hence, G7 activates flounder complement but is protected from complement-mediated destruction by its cell wall structure, while G7 prevents the activation of mouse complement. These results indicate that G7 employs different mechanisms to avoid the complement killing of different hosts.

Molecular Basis for Bordetella pertussis Interference with Complement, Coagulation, Fibrinolytic, and Contact Activation Systems: the Cryo-EM Structure of the Vag8-C1 Inhibitor Complex

Complement, contact activation, coagulation, and fibrinolysis are serum protein cascades that need strict regulation to maintain human health. Serum glycoprotein, a C1 inhibitor (C1-INH), is a key regulator (inhibitor) of serine proteases of all the above-mentioned pathways. Recently, an autotransporter protein, virulence-associated gene 8 (Vag8), produced by the whooping cough pathogen, Bordetella pertussis, was shown to bind to C1-INH and interfere with its function. Here, we present the structure of the Vag8-C1-INH complex determined using cryo-electron microscopy at a 3.6-Å resolution. The structure shows a unique mechanism of C1-INH inhibition not employed by other pathogens, where Vag8 sequesters the reactive center loop of C1-INH, preventing its interaction with the target proteases.IMPORTANCE The structure of a 10-kDa protein complex is one of the smallest to be determined using cryo-electron microscopy at high resolution. The structure reveals that C1-INH is sequestered in an inactivated state by burial of the reactive center loop in Vag8. By so doing, the bacterium is able to simultaneously perturb the many pathways regulated by C1-INH. Virulence mechanisms such as the one described here assume more importance given the emerging evidence about dysregulation of contact activation, coagulation, and fibrinolysis leading to COVID-19 pneumonia.

LPS modifications and AvrA activity of Salmonella enterica serovar Typhimurium are required to prevent Perforin-2 expression by infected fibroblasts and intestinal epithelial cells

Cellular Perforin-2 (MPEG1) is a pore-forming MACPF family protein that plays a critical role in the defense against bacterial pathogens. Macrophages, neutrophils, and several other cell types that are part of the front line of innate defenses constitutively express high levels of Perforin-2; whereas, most other cell types must be induced to express Perforin-2 by interferons (α, β and γ) and/or PAMPs such as LPS. In this study, we demonstrate that many bacterial pathogens can limit the expression of Perforin-2 in cells normally inducible for Perforin-2 expression, while ordinarily commensal or non-pathogenic bacteria triggered high levels of Perforin-2 expression in these same cell types. The mechanisms by which pathogens suppress Perforin-2 expression was explored further using Salmonella enterica serovar Typhimurium and cultured MEFs as well as intestinal epithelial cell lines. These studies identified multiple factors required to minimize the expression of Perforin-2 in cell types inducible for Perforin-2 expression. These included the PmrAB and PhoPQ two-component systems, select LPS modification enzymes and the Type III secretion effector protein AvrA.

Inherently and Conditionally Essential Protein Catabolism Genes of Porphyromonas gingivalis

Proteases are critical virulence determinants of Porphyromonas gingivalis, an emerging Alzheimer's disease, cancer, and arthritis pathogen and established agent of periodontitis. Transposon sequencing has been employed to define the core essential genome of this bacterium and genes conditionally essential in multiple environments - abscess formation; epithelial colonization; and cigarette smoke toxin exposure; as well as to elucidate genes required for iron acquisition and a functional type 9 secretion system. Validated and predicted protein catabolism genes identified include a combination of established virulence factors and a larger set of seemingly more mundane proteolytic genes. The functions and relevance of genes that share essentiality in multiple disease-relevant conditions are examined. These common stress-related genes may represent particularly attractive therapeutic targets for the control of P. gingivalis infections.

An Irreversible Inhibitor to Probe the Role of Streptococcus pyogenes Cysteine Protease SpeB in Evasion of Host Complement Defenses

Members of the CA class of cysteine proteases have multifaceted roles in physiology and virulence for many bacteria. Streptococcal pyrogenic exotoxin B (SpeB) is secreted by Streptococcus pyogenes and implicated in the pathogenesis of the bacterium through degradation of key human immune effector proteins. Here, we developed and characterized a clickable inhibitor, 2S-alkyne, based on X-ray crystallographic analysis and structure-activity relationships. Our SpeB probe showed irreversible enzyme inhibition in biochemical assays and labeled endogenous SpeB in cultured S. pyogenes supernatants. Importantly, application of 2S-alkyne decreased S. pyogenes survival in the presence of human neutrophils and supports the role of SpeB-mediated proteolysis as a mechanism to limit complement-mediated host defense. We posit that our SpeB inhibitor will be a useful chemical tool to regulate, label, and quantitate secreted cysteine proteases with SpeB-like activity in complex biological samples and a lead candidate for new therapeutics designed to sensitize S. pyogenes to host immune clearance.

CD89/CD35 Expression Ratio in Salivary Neutrophil as an Early Detection Marker for Severe Early Childhood Caries

Objectives  To analyze CD35/CD89 expression ratio on the surface of neutrophils as an early detection marker for S-ECC.

Materials and Methods  Saliva was collected from 4- to 6-year-old kindergarten students. Salivary neutrophils were obtained by instructing the subjects to rinse their mouth with 1 mL of sterile 1.5% NaCl for 30 seconds before expectorating it into a sterile glass. The expression of CFSE ⁺ CD35 ⁺ and CFSE ⁺ CD89 ⁺ was measured and analyzed using flow cytometry.

Results The expression of CFSE ⁺ CD89 ⁺ in the caries-free group (2.46 ± 0.39) was significantly lower than that in the S-ECC group (3.41 ± 1.11), with a p -value of 0.0001, while the expression of CFSE ⁺ CD35 ⁺ in the caries-free group was (2.35 ± 0.56) compared with (1.54 ± 0.35) ( p = 0.0001) in the S-ECC group.

Conclusions  The expression ratio of CFSE ⁺ CD89 ⁺ and CFSE ⁺ CD35 ⁺ constitutes a marker for S-ECC.

Salivary Total Protease Activity Based on a Broad-Spectrum Fluorescence Resonance Energy Transfer Approach to Monitor Induction and Resolution of Gingival Inflammation

OBJECTIVE

Salivary total protease and chitinase activities were measured by a broad-spectrum fluorescence resonance energy transfer approach as predictors of induction and resolution of gingival inflammation in healthy individuals by applying an experimental human gingivitis model.

METHODS

Dental biofilm accumulated (21 days, Induction Phase) by omitting oral hygiene practices followed by a 2-week Resolution Phase to restore gingival health in an experimental gingivitis study. Plaque accumulation, as assessed by the Turesky Modification of the Quigley-Hein Plaque Index (TQHPI), and gingival inflammation, assessed using the Modified Gingival Index (MGI), scores were recorded and unstimulated saliva was collected weekly. Saliva was analysed for total protein, albumin, total protease activity and chitinase activity (n = 18).

RESULTS

The TQHPI and MGI scores, as well as total protease activity, increased until day 21. After re-establishment of oral hygiene, gingival inflammation levels returned to values similar to baseline (day 0). Levels of protease activity decreased significantly, but not to baseline values. Furthermore, 'fast' responders, who responded immediately to plaque, exhibited significantly higher proteolytic activity throughout the experimental course than 'slow' responders, who showed a lagged inflammatory response.

CONCLUSION

The results indicate that differential inflammatory responses encompass inherent variations in total salivary proteolytic activities, which could be further utilised in contemporary diagnostic, prognostic and treatment modalities for periodontal diseases.

Role of the Complement Pathway in Inflammatory Skin Diseases: A Focus on Hidradenitis Suppurativa

Although the role of immune dysregulation in hidradenitis suppurativa (HS) has yet to be elucidated, recent studies identified several complement abnormalities in patients with HS. The complement system serves a critical role in the modulation of immune response and regulation of cutaneous commensal bacteria. Complement is implicated in several inflammatory skin diseases including systemic lupus erythematosus, angioedema, pemphigus, bullous pemphigoid, and HS. A model of HS pathogenesis is proposed, integrating the role of commensal bacteria, cutaneous immune responses, and complement dysregulation. The role of complement in disease pathogenesis has led to the development of novel anticomplement agents and clinical trials investigating the efficacy of such treatments in HS.

Aeromonas hydrophila suppresses complement pathways via degradation of complement C3 in bony fish by metalloprotease

Aeromonas hydrophila is a pathogen that causes high mortality in the grass carp. The complement system, as a frontline defence of innate immunity, plays an important role in the immune response against pathogens. However, the immunity evasion mechanism of A. hydrophila against the complement system of grass carp remains unclear. In this study, we described an additional mechanism used by A. hydrophila GD18 to evade the complement system and survive in grass carp serum. First, A. hydrophila evaded the bactericidal activity of grass carp serum. Second, the haemolytic activity assays showed that A. hydrophila obviously suppressed the alternative pathway, which depended on preventing the formation or disabling the function of the membrane-attack complex (MAC). Further research indicated that A. hydrophila targeted complement C3, the central component of the three complement pathways, and degraded it in the grass carp serum, leading to the inhibition of the complement pathways, which resulted in the serum-resistance of A. hydrophila. Furthermore, cleavage analyses showed that extracellular proteases (ECPases) of A. hydrophila efficiently cleaved purified C3 as well as C3 in grass carp serum. Finally, protease inhibitor studies and mass spectrum analysis identified the secreted metalloprotease elastase (AhE), which was present in large amounts in crude ECPases, as the central molecule responsible for C3 cleavage. Compared to wild strain GD18, the AhE knockout, Δahe was dramatically reduced in the ability of serum resistance. Our findings suggested that A. hydrophila escaped serum-killing by suppressing the complement pathways via the degradation of complement C3 in bony fish, which was related to secreted metalloproteases.

Comparative analysis of homologous aminopeptidase PepN from pathogenic and non-pathogenic mycobacteria reveals divergent traits

Mycobacterium tuberculosis (Mtb) secretes proteases and peptidases to subjugate its host. Out of its sixty plus proteases, atleast three are reported to reach host macrophages. In this study, we show that Mtb also delivers a lysyl alanine aminopeptidase, PepN (Rv2467) into host macrophage cytosol. Our comparative in silico analysis shows PepNMtb highly conserved across all pathogenic mycobacteria. Non-pathogenic mycobacteria including M. smegmatis (Msm) also encode pepN. PepN protein levels in both Mtb (pathogenic) and Msm (non-pathogenic) remain uniform across all in vitro growth phases. Despite such tight maintenance of PepNs' steady state levels, upon supplementation, Mtb alone allows accumulation of any excessive PepN. In contrast, Msm does not. It not only proteolyzes, but also secretes out the excessive PepN, be it native or foreign. Interestingly, while PepNMtb is required for modulating virulence in vivo, PepNMsm is essential for Msm growth in vitro. Despite such essentiality difference, both PepNMtb and PepNMsm harbor almost identical N-terminal M1-type peptidase domains that significantly align in their amino acid sequences and overlap in their secondary structures. Their C-terminal ERAP1_C-like domains however align much more moderately. Our in vitro macrophage-based infection experiments with MtbΔpepN-expressing pepNMsm reveals PepNMsm also retaining the ability to reach host cytosol. Lastly, but notably, we determined the PepNMtb and PepNMsm interactomes and found them to barely coincide. While PepNMtb chiefly interacts with Mtb's secreted proteins, PepNMsm primarily coimmunoprecipitates with Msm's housekeeping proteins. Thus, despite high sequence homology and several common properties, our comparative analytical study reveals host-centric traits of pathogenic and bacterial-centric traits of non-pathogenic PepNs.

Complement-dependent outer membrane perturbation sensitizes Gram-negative bacteria to Gram-positive specific antibiotics

Gram-negative bacteria are refractory to the action of many antibiotics due to their impermeable outer membrane. An important player of the immune system is the complement system, a protein network in serum that directly kills Gram-negative bacteria through pore-formation by the Membrane Attack Complexes (MAC). We here show that the MAC rapidly perforates the outer membrane but that inner membrane damage, which is essential for killing, is relatively slow. Importantly, we demonstrate that MAC-induced outer membrane damage sensitizes Gram-negative bacteria to otherwise ineffective, Gram-positive-specific, antimicrobials. Synergy between serum and nisin was observed for 22 out of 53 tested Gram-negative clinical isolates and for multi-drug resistant (MDR) blood isolates. The in vivo relevance of this process is further highlighted by the fact that blood sensitizes a MDR K. pneumoniae strain to vancomycin. Altogether, these data imply that antibiotics that are considered ineffective to treat infections with Gram-negatives may have different functional outcomes in patients, due to the presence of the complement system.

Complement Activation as a Helping Hand for Inflammophilic Pathogens and Cancer

The complement system, an evolutionarily ancient component of innate immunity, is capable of protecting hosts from invading pathogens, either directly, by lysis of target cells, or indirectly, by mobilization of host immune mechanisms. However, this potentially cytotoxic cascade must be tightly regulated, since improperly controlled complement can damage healthy cells and tissues. The practical importance of this axis is highlighted when impairment of complement regulators or bacterial mechanisms of complement evasion result in pathogenic conditions. Recognition of complement as a "double-edged sword" is widely acknowledged, but another, currently underappreciated aspect of complement function has emerged as an important player in homeostatic balance-the dual outcome of complement-mediated inflammation. In most cases, the proinflammatory properties of complement are beneficial to the host. However, certain pathogens have developed the ability to utilize local inflammation as a source of nutrients and as a way to establish a niche for further colonization. Such a strategy can be illustrated in the example of periodontitis. Interestingly, certain tumors also seem to benefit from complement activation products, which promote a proangiogenic and immunosuppressive microenvironment.

Complement and Bacterial Infections: From Molecular Mechanisms to Therapeutic Applications

Complement is a complex protein network of plasma, and an integral part of the innate immune system. Complement activation results in the rapid clearance of bacteria by immune cells, and direct bacterial killing via large pore-forming complexes. Here we review important recent discoveries in the complement field, focusing on interactions relevant for the defense against bacteria. Understanding the molecular interplay between complement and bacteria is of great importance for future therapies for infectious and inflammatory diseases. Antibodies that support complement-dependent bacterial killing are of interest for the development of alternative therapies to treat infections with antibiotic-resistant bacteria. Furthermore, a variety of novel therapeutic complement inhibitors have been developed to prevent unwanted complement activation in autoimmune inflammatory diseases. A better understanding of how such inhibitors may increase the risk of bacterial infections is essential if such therapies are to be successful.

Pathogenic Leptospira: Advances in understanding the molecular pathogenesis and virulence

Leptospirosis is a common zoonotic disease has emerged as a major public health problem, with developing countries bearing disproportionate burdens. Although the diverse range of clinical manifestations of the leptospirosis in humans is widely documented, the mechanisms through which the pathogen causes disease remain undetermined. In addition, leptospirosis is a much-neglected life-threatening disease although it is one of the most important zoonoses occurring in a diverse range of epidemiological distribution. Recent advances in molecular profiling of pathogenic species of the genus Leptospira have improved our understanding of the evolutionary factors that determine virulence and mechanisms that the bacteria employ to survive. However, a major impediment to the formulation of intervention strategies has been the limited understanding of the disease determinants. Consequently, the association of the biological mechanisms to the pathogenesis of Leptospira, as well as the functions of numerous essential virulence factors still remain implicit. This review examines recent advances in genetic screening technologies, the underlying microbiological processes, the virulence factors and associated molecular mechanisms driving pathogenesis of Leptospira species.

Acquisition of C1 inhibitor by Bordetella pertussis virulence associated gene 8 results in C2 and C4 consumption away from the bacterial surface

Whooping cough, or pertussis, is a contagious disease of the respiratory tract that is re-emerging worldwide despite high vaccination coverage. The causative agent of this disease is the Gram-negative Bordetella pertussis. Knowledge on complement evasion strategies of this pathogen is limited. However, this is of great importance for future vaccine development as it has become apparent that a novel pertussis vaccine is needed. Here, we unravel the effect of Virulence associated gene 8 (Vag8) of B. pertussis on the human complement system at the molecular level. We show that both recombinant and endogenously secreted Vag8 inhibit complement deposition on the bacterial surface at the level of C4b. We reveal that Vag8 binding to human C1-inhibitor (C1-inh) interferes with the binding of C1-inh to C1s, C1r and MASP-2, resulting in the release of active proteases that subsequently cleave C2 and C4 away from the bacterial surface. We demonstrate that the depletion of these complement components in the bacterial surrounding and subsequent decreased deposition on B. pertussis leads to less complement-mediated bacterial killing. Vag8 is the first protein described that specifically prevents C1s, C1r and MASP-2 binding to C1-inh and thereby mediates complement consumption away from the bacterial surface. Unravelling the mechanism of this unique complement evasion strategy of B. pertussis is one of the first steps towards understanding the interactions between the first line of defense complement and B. pertussis.

Despite wide-spread vaccination, whooping cough caused by the Gram-negative bacterium Bordetella pertussis remains a public health problem and has been re-emerging in the past decades. To this end, new vaccination strategies are being explored including the use of complement evasion molecules as vaccine candidates. Autotransporter Virulence associated gene 8 (Vag8) was previously shown to be involved in complement evasion. However, the molecular mechanism of this immune evasion was not understood. Considering knowledge on molecular mechanisms is crucial for further studies regarding vaccine development, we investigated the underlying mechanism of Vag8 induced complement evasion of B. pertussis. We show that both recombinant Vag8 as well as endogenously secreted Vag8 inhibits complement activation via the classical and lectin complement pathway at the level of C4 and C2. We identified a novel bacterial complement evasion strategy initiated by the binding of Vag8 to C1-inhibitor. This binding interferes with the interactions between C1-inhibitor and the proteases C1s, C1r and MASP-2 resulting in the release of active proteases that cleave C4 and C2 away from the bacterial surface. This environmental consumption of C4 and C2 leads to decreased complement deposition on the bacterial surface and hence inhibits complement-mediated killing of B. pertussis.

Porphyromonas gingivalis disturbs host-commensal homeostasis by changing complement function

Porphyromonas gingivalis is a Gram-negative anaerobic rod that has been proposed as an orchestrator of complement-dependent dysbiotic inflammation. This notion was suggested from its capacities to manipulate the complement–Toll-like receptor crosstalk in ways that promote dysbiosis and periodontal disease in animal models. Specifically, while at low colonization levels, P. gingivalis interferes with innate immunity and leads to changes in the counts and composition of the oral commensal microbiota. The resulting dysbiotic microbial community causes disruption of host–microbial homeostasis, leading to inflammatory bone loss. These findings suggested that P. gingivalis can be considered as a keystone pathogen. The concept of keystone pathogens is one where their effects have community-wide significance and are disproportionate of their abundance. The present review summarizes the relevant literature and discusses whether the results from the animal models can be extrapolated to man.

Novel insights into the expression pattern of anaphylatoxin receptors in mice and men

The anaphylatoxins (AT) C3a and C5a play important roles as mediators of inflammation. Further, they regulate and control multiple innate and adaptive immune responses through binding and activation of their cognate G protein-coupled receptors, i.e. C3a receptor (C3aR), C5a receptor 1 (C5aR1) and C5a receptor 2 (C5aR2), although the latter lacks important sequence motifs for G protein-coupling. Based on their pleiotropic functions, they contribute not only to tissue homeostasis but drive, perpetuate and resolve immune responses in many inflammatory diseases including infections, malignancies, autoimmune as well as allergic diseases. During the past few years, transcriptome expression data provided detailed insights into AT receptor tissue mRNA expression. In contrast, our understanding of cellular AT receptor expression in human and mouse tissues under steady and inflammatory conditions is still sketchy. Ligand binding studies, flow cytometric and immunohistochemical analyses convincingly demonstrated tissue-specific C5aR1 expression in various cells of myeloid origin. However, a detailed map for C3aR or C5aR2 expression in human or mouse tissue cells is still lacking. Also, reports about AT expression in lymphoid cells is still controversial. To understand the multiple roles of the ATs in the innate and adaptive immune networks, a detailed understanding of their receptor expression in health and disease is required. Recent findings obtained with novel GFP or tdTomato AT-receptor knock-in mice provide detailed insights into their expression pattern in tissue immune and stroma cells. Here, we will provide an update about our current knowledge of AT receptor expression pattern in humans and mice.

Mirolysin, a LysargiNase from Tannerella forsythia, proteolytically inactivates the human cathelicidin, LL-37

Tannerella forsythia is a periodontal pathogen expressing six secretory proteolytic enzymes with a unique multidomain structure referred to as KLIKK proteases. Two of these proteases, karilysin and mirolysin, were previously shown to protect the bacterium against complement-mediated bactericidal activity. The latter metalloprotease, however, was not characterized at the protein level. Therefore, we purified recombinant mirolysin and subjected it to detailed biochemical characterization. Mirolysin was obtained as a 66 kDa zymogen, which autoproteolytically processed itself into a 31 kDa active form via truncations at both the N- and C-termini. Further autodegradation was prevented by calcium. Substrate specificity was determined by the S1' subsite of the substrate-binding pocket, which shows strong preference for Arg and Lys at the carbonyl side of a scissile peptide bond (P1' residue). The protease cleaved an array of host proteins, including human fibronectin, fibrinogen, complement proteins C3, C4, and C5, and the antimicrobial peptide, LL-37. Degradation of LL-37 abolished not only the bactericidal activity of the peptide, but also its ability to bind lipopolysaccharide (LPS), thus quenching the endotoxin proinflammatory activity. Taken together, these results indicate that, through cleavage of LL-37 and complement proteins, mirolysin might be involved in evasion of the host immune response.

Let's Tie the Knot: Marriage of Complement and Adaptive Immunity in Pathogen Evasion, for Better or Worse

The complement system is typically regarded as an effector arm of innate immunity, leading to recognition and killing of microbial invaders in body fluids. Consequently, pathogens have engaged in an arms race, evolving molecules that can interfere with proper complement responses. However, complement is no longer viewed as an isolated system, and links with other immune mechanisms are continually being discovered. Complement forms an important bridge between innate and adaptive immunity. While its roles in innate immunity are well-documented, its function in adaptive immunity is less characterized. Therefore, it is no surprise that the field of pathogenic complement evasion has focused on blockade of innate effector functions, while potential inhibition of adaptive immune responses (via complement) has been overlooked to a certain extent. In this review, we highlight past and recent developments on the involvement of complement in the adaptive immune response. We discuss the mechanisms by which complement aids in lymphocyte stimulation and regulation, as well as in antigen presentation. In addition, we discuss microbial complement evasion strategies, and highlight specific examples in the context of adaptive immune responses. These emerging ties between complement and adaptive immunity provide a catalyst for future discovery in not only the field of adaptive immune evasion but in elucidating new roles of complement.

Complement Evasion by Pathogenic Leptospira

Leptospirosis is a neglected infectious disease caused by spirochetes from the genus Leptospira. Pathogenic microorganisms, notably those which reach the blood circulation such as Leptospira, have evolved multiple strategies to escape the host complement system, which is important for innate and acquired immunity. Leptospira avoid complement-mediated killing through: (i) recruitment of host complement regulators; (ii) acquisition of host proteases that cleave complement proteins on the bacterial surface; and, (iii) secretion of proteases that inactivate complement proteins in the Leptospira surroundings. The recruitment of host soluble complement regulatory proteins includes the acquisition of Factor H (FH) and FH-like-1 (alternative pathway), C4b-binding protein (C4BP) (classical and lectin pathways), and vitronectin (Vn) (terminal pathway). Once bound to the leptospiral surface, FH and C4BP retain cofactor activity of Factor I in the cleavage of C3b and C4b, respectively. Vn acquisition by leptospires may result in terminal pathway inhibition by blocking C9 polymerization. The second evasion mechanism lies in plasminogen (PLG) binding to the leptospiral surface. In the presence of host activators, PLG is converted to enzymatically active plasmin, which is able to degrade C3b, C4b, and C5 at the surface of the pathogen. A third strategy used by leptospires to escape from complement system is the active secretion of proteases. Pathogenic, but not saprophytic leptospires, are able to secrete metalloproteases that cleave C3 (central complement molecule), Factor B (alternative pathway), and C4 and C2 (classical and lectin pathways). The purpose of this review is to fully explore these complement evasion mechanisms, which act together to favor Leptospira survival and multiplication in the host.

Hijacking Complement Regulatory Proteins for Bacterial Immune Evasion

The human complement system plays an important role in the defense against invading pathogens, inflammation and homeostasis. Invading microbes, such as bacteria, directly activate the complement system resulting in the formation of chemoattractants and in effective labeling of the bacteria for phagocytosis. In addition, formation of the membrane attack complex is responsible for direct killing of Gram-negative bacteria. In turn, bacteria have evolved several ways to evade complement activation on their surface in order to be able to colonize and invade the human host. One important mechanism of bacterial escape is attraction of complement regulatory proteins to the microbial surface. These molecules are present in the human body for tight regulation of the complement system to prevent damage to host self-surfaces. Therefore, recruitment of complement regulatory proteins to the bacterial surface results in decreased complement activation on the microbial surface which favors bacterial survival. This review will discuss recent advances in understanding the binding of complement regulatory proteins to the bacterial surface at the molecular level. This includes, new insights that have become available concerning specific conserved motives on complement regulatory proteins that are favorable for microbial binding. Finally, complement evasion molecules are of high importance for vaccine development due to their dominant role in bacterial survival, high immunogenicity and homology as well as their presence on the bacterial surface. Here, the use of complement evasion molecules for vaccine development will be discussed.

Trypsin differentially modulates the surface expression and function of channel catfish leukocyte immune-type receptors

Channel catfish (Ictalurus punctatus) leukocyte immune-type receptors (IpLITRs) are immunoregulatory proteins that control innate immune cellular responses. Previously, we demonstrated that two representative IpLITR forms, IpLITR 2.6b and IpLITR 1.1b, engage distinct components of the phagocytic machinery resulting in unique target capture and engulfment phenotypes. IpLITR-induced phagocytic mechanisms were also differentially susceptible to temperature and pharmacological inhibitors of canonical signaling mediators. In the present study, we examined the sensitivity of IpLITR-mediated phagocytosis to the endogenous serine-protease trypsin, a well-known mediator of immunoregulatory receptor functions. Trypsin selectively reduced IpLITR 1.1b cell surface expression and phagocytic activity in a dose-dependent manner. We also observed a significant alteration of the IpLITR 1.1b phagocytic phenotype post-trypsin exposure; whereas, the IpLITR 2.6b-mediated target engulfment phenotype was unchanged. Recovery experiments suggested that trypsin-induced inhibition of IpLITR 1.1b-dependent phagocytosis was reversible and that the re-establishment of phagocytic function was associated with a recovery of receptor surface expression. Cell-surface biotinylation and immunoprecipitation studies demonstrated that IpLITR 1.1b normally exists as a mature (∼70 kDa) protein on the cell surface. However, trypsin treatment reduced expression of the mature receptor and processed IpLITR 1.1b into an ∼60 kDa form. The trypsin-generated and putative immature IpLITR 1.1b form was not present on the cell surface; suggesting that the cleaved receptor may have been internalized, post-processing, by regulated endocytosis. Taken together, these results reveal a unique role for trypsin as a selective modulator of IpLITR-mediated phagocytosis and highlight a conserved role for serine proteases as potent immunomodulatory factors.

Calcium-induced Tetramerization and Zinc Chelation Shield Human Calprotectin from Degradation by Host and Bacterial Extracellular Proteases

Calprotectin (CP, S100A8/S100A9 oligomer, MRP-8/14 oligomer, calgranulins A and B) is a protein component of the innate immune system that contributes to the metal-withholding response by sequestering bioavailable transition metal ions at sites of infection. Human CP employs Ca(II) ions to modulate its quaternary structure, transition metal binding properties, and antimicrobial activity. In this work, we report the discovery that Ca(II)-induced self-association of human CP to afford heterotetramers protects the protein scaffold from degradation by host serine proteases. We present the design and characterization of two new human CP-Ser variants, S100A8(C42S)(I60E)/S100A9(C3S) and S100A8(C42S)(I60K)/S100A9(C3S), that exhibit defective tetramerization properties. Analytical size exclusion chromatography and analytical ultracentrifugation reveal that both variants, hereafter I60E and I60K, persist as heterodimers in the presence of Ca(II) only, and form heterotetramers in the presence of Mn(II) only and both Ca(II) and Mn(II). Coordination to Fe(II) also causes I60E and I60K to form heterotetramers in both the absence and presence of Ca(II). The Ca(II)-bound I60E and I60K heterodimers are readily degraded by trypsin, chymotrypsin and human neutrophil elastase, whereas the Ca(II)-bound CP-Ser heterotetramers and the Ca(II)- and Mn(II)-bound I60E and I60K heterotetramers are resistant to degradation by these host proteases. The staphylococcal extracellular protease GluC cuts the S100A8 subunit of CP-Ser at the C-terminal end of residue 89 to afford a ΔSKHE variant. The GluC cleavage site is in close proximity to the His3Asp metal-binding site, which coordinates Zn(II) with high affinity, and Zn(II) chelation protects the S100A8 subunit from GluC cleavage. Taken together, these results provide new insight into how Ca(II) ions and transition metals modulate the chemistry and biology of CP, and indicate that coordination to divalent cations transforms human CP into a protease-resistant form and enables innate immune function in the hostile conditions of an infection site.

What Makes a Bacterial Species Pathogenic?:Comparative Genomic Analysis of the Genus Leptospira

Leptospirosis, caused by spirochetes of the genus Leptospira, is a globally widespread, neglected and emerging zoonotic disease. While whole genome analysis of individual pathogenic, intermediately pathogenic and saprophytic Leptospira species has been reported, comprehensive cross-species genomic comparison of all known species of infectious and non-infectious Leptospira, with the goal of identifying genes related to pathogenesis and mammalian host adaptation, remains a key gap in the field. Infectious Leptospira, comprised of pathogenic and intermediately pathogenic Leptospira, evolutionarily diverged from non-infectious, saprophytic Leptospira, as demonstrated by the following computational biology analyses: 1) the definitive taxonomy and evolutionary relatedness among all known Leptospira species; 2) genomically-predicted metabolic reconstructions that indicate novel adaptation of infectious Leptospira to mammals, including sialic acid biosynthesis, pathogen-specific porphyrin metabolism and the first-time demonstration of cobalamin (B12) autotrophy as a bacterial virulence factor; 3) CRISPR/Cas systems demonstrated only to be present in pathogenic Leptospira, suggesting a potential mechanism for this clade's refractoriness to gene targeting; 4) finding Leptospira pathogen-specific specialized protein secretion systems; 5) novel virulence-related genes/gene families such as the Virulence Modifying (VM) (PF07598 paralogs) proteins and pathogen-specific adhesins; 6) discovery of novel, pathogen-specific protein modification and secretion mechanisms including unique lipoprotein signal peptide motifs, Sec-independent twin arginine protein secretion motifs, and the absence of certain canonical signal recognition particle proteins from all Leptospira; and 7) and demonstration of infectious Leptospira-specific signal-responsive gene expression, motility and chemotaxis systems. By identifying large scale changes in infectious (pathogenic and intermediately pathogenic) vs. non-infectious Leptospira, this work provides new insights into the evolution of a genus of bacterial pathogens. This work will be a comprehensive roadmap for understanding leptospirosis pathogenesis. More generally, it provides new insights into mechanisms by which bacterial pathogens adapt to mammalian hosts.

Gingipains: Critical Factors in the Development of Aspiration Pneumonia Caused by Porphyromonas gingivalis

Aspiration pneumonia is a life-threatening infectious disease often caused by oral anaerobic and periodontal pathogens such as Porphyromonas gingivalis. This organism produces proteolytic enzymes, known as gingipains, which manipulate innate immune responses and promote chronic inflammation. Here, we challenged mice with P. gingivalis W83 and examined the role of gingipains in bronchopneumonia, lung abscess formation, and inflammatory responses. Although gingipains were not required for P. gingivalis colonization and survival in the lungs, they were essential for manifestation of clinical symptoms and infection-related mortality. Pathologies caused by wild-type (WT) P. gingivalis W83, including hemorrhage, necrosis, and neutrophil infiltration, were absent from lungs infected with gingipain-null isogenic strains or WT bacteria preincubated with gingipain-specific inhibitors. Damage to lung tissue correlated with systemic inflammatory responses, as manifested by elevated levels of TNF, IL-6, IL-17, and C-reactive protein. These effects were unequivocally dependent on gingipain activity. Gingipain activity was also implicated in the observed increase in IL-17 in lung tissues. Furthermore, gingipains increased platelet counts in the blood and activated platelets in the lungs. Arginine-specific gingipains made a greater contribution to P. gingivalis-related morbidity and mortality than lysine-specific gingipains. Thus, inhibition of gingipain may be a useful adjunct treatment for P. gingivalis-mediated aspiration pneumonia.

Calcium Regulates the Activity and Structural Stability of Tpr, a Bacterial Calpain-like Peptidase

Porphyromonas gingivalis is a peptide-fermenting asaccharolytic periodontal pathogen. Its genome contains several genes encoding cysteine peptidases other than gingipains. One of these genes (PG1055) encodes a protein called Tpr (thiol protease) that has sequence similarity to cysteine peptidases of the papain and calpain families. In this study we biochemically characterize Tpr. We found that the 55-kDa Tpr inactive zymogen proteolytically processes itself into active forms of 48, 37, and 33 kDa via sequential truncations at the N terminus. These processed molecular forms of Tpr are associated with the bacterial outer membrane where they are likely responsible for the generation of metabolic peptides required for survival of the pathogen. Both autoprocessing and activity were dependent on calcium concentrations >1 mm, consistent with the protein's activity within the intestinal and inflammatory milieus. Calcium also stabilized the Tpr structure and rendered the protein fully resistant to proteolytic degradation by gingipains. Together, our findings suggest that Tpr is an example of a bacterial calpain, a calcium-responsive peptidase that may generate substrates required for the peptide-fermenting metabolism of P. gingivalis. Aside from nutrient generation, Tpr may also be involved in evasion of host immune response through degradation of the antimicrobial peptide LL-37 and complement proteins C3, C4, and C5. Taken together, these results indicate that Tpr likely represents an important pathogenesis factor for P. gingivalis.

A Metalloproteinase Mirolysin of Tannerella forsythia Inhibits All Pathways of the Complement System

Recent reports focusing on virulence factors of periodontal pathogens implicated proteinases as major determinants of remarkable pathogenicity of these species, with special emphasis on their capacity to modulate complement activity. In particular, bacteria-mediated cleavage of C5 and subsequent release of C5a seems to be an important phenomenon in the manipulation of the local inflammatory response in periodontitis. In this study, we present mirolysin, a novel metalloproteinase secreted by Tannerella forsythia, a well-recognized pathogen strongly associated with periodontitis. Mirolysin exhibited a strong effect on all complement pathways. It inhibited the classical and lectin complement pathways due to efficient degradation of mannose-binding lectin, ficolin-2, ficolin-3, and C4, whereas inhibition of the alternative pathway was caused by degradation of C5. This specificity toward complement largely resembled the activity of a previously characterized metalloproteinase of T. forsythia, karilysin. Interestingly, mirolysin released the biologically active C5a peptide in human plasma and induced migration of neutrophils. Importantly, we demonstrated that combination of mirolysin with karilysin, as well as a cysteine proteinase of another periodontal pathogen, Prevotella intermedia, resulted in a strong synergistic effect on complement. Furthermore, mutant strains of T. forsythia, devoid of either mirolysin or karilysin, showed diminished survival in human serum, providing further evidence for the synergistic inactivation of complement by these metalloproteinases. Taken together, our findings on interactions of mirolysin with complement significantly add to the understanding of immune evasion strategies of T. forsythia and expand the knowledge on molecular mechanisms driving pathogenic events in the infected periodontium.

Edwardsiella tarda evades serum killing by preventing complement activation via the alternative pathway

Edwardsiella tarda is a Gram-negative bacterium with a broad host range that includes a wide variety of farmed fish as well as humans. E. tarda has long been known to be able to survive in host serum, but the relevant mechanism is unclear. In this study, we investigated the fundamental question, i.e. whether E. tarda activated serum complement or not. We found that (i) when incubated with flounder serum, E. tarda exhibited a high survival rate (87.6%), which was slightly but significantly reduced in the presence of Mg(2+); (ii) E. tarda-incubated serum possessed strong hemolytic activity and bactericidal activity, (iii) compared to the serum incubated with a complement-sensitive laboratory Escherichia coli strain, E. tarda-incubated serum exhibited much less chemotactic activity, (iv) in contrast to the serum incubated with live E. tarda, the serum incubated with heat-inactivated E. tarda exhibited no apparent hemolytic capacity. Taken together, these results indicate for the first time that E. tarda circumvents serum attack by preventing, to a large extent, complement activation via the alternative pathway, and that heat-labile surface structures likely play an essential role in the complement evasion of E. tarda.

Peptidyl arginine deiminase from Porphyromonas gingivalis abolishes anaphylatoxin C5a activity

Evasion of killing by the complement system, a crucial part of innate immunity, is a key evolutionary strategy of many human pathogens. A major etiological agent of chronic periodontitis, the Gram-negative bacterium Porphyromonas gingivalis, produces a vast arsenal of virulence factors that compromise human defense mechanisms. One of these is peptidylarginine deiminase (PPAD), an enzyme unique to P. gingivalis among bacteria, which converts Arg residues in polypeptide chains into citrulline. Here, we report that PPAD citrullination of a critical C-terminal arginine of the anaphylatoxin C5a disabled the protein function. Treatment of C5a with PPAD in vitro resulted in decreased chemotaxis of human neutrophils and diminished calcium signaling in monocytic cell line U937 transfected with the C5a receptor (C5aR) and loaded with a fluorescent intracellular calcium probe: Fura-2 AM. Moreover, a low degree of citrullination of internal arginine residues by PPAD was also detected using mass spectrometry. Further, after treatment of C5 with outer membrane vesicles naturally shed by P. gingivalis, we observed generation of C5a totally citrullinated at the C-terminal Arg-74 residue (Arg74Cit). In stark contrast, only native C5a was detected after treatment with PPAD-null outer membrane vesicles. Our study suggests reduced antibacterial and proinflammatory capacity of citrullinated C5a, achieved via lower level of chemotactic potential of the modified molecule, and weaker cell activation. In the context of previous studies, which showed crosstalk between C5aR and Toll-like receptors, as well as enhanced arthritis development in mice infected with PPAD-expressing P. gingivalis, our findings support a crucial role of PPAD in the virulence of P. gingivalis.

A pathogenic trace of Tannerella forsythia - shedding of soluble fully active tumor necrosis factor α from the macrophage surface by karilysin

Tannerella forsythia is implicated as a pathogen causing chronic and aggressive periodontitis. However, its virulence factors, including numerous putative proteases, are mostly uncharacterized. Karilysin is a newly described matrix metalloprotease-like enzyme of T. forsythia. Since pathogen-derived proteases may affect the host defense system via modulation of the cytokine network, the aim of this study was to determine the influence of karilysin on tumor necrosis factor-α (TNF-α). The results showed that karilysin cleaved the membrane form of TNF-α on the surface of macrophages, and that this led to an increased concentration of soluble TNF-α in the conditioned medium. Importantly, despite partial degradation of soluble TNF-α by karilysin, the released cytokine retained its biological activity, inducing apoptosis and stimulating autocrine pathway of pro-inflammatory gene expression. Notably, the observed effect required proteolytic activity by karilysin, since a catalytically inactive mutant of the enzyme did not affect TNF-α secretion. The shedding was independent of the activity of ADAM17, a major endogenous TNF-α converting enzyme. Karilysin-dependent TNF-α release from the cell surface is likely to occur in vivo because human plasma, the main constituent of gingival crevicular fluid, only slightly affected the sheddase activity of karilysin. Taken together, these results indicate that karilysin modulates the host immune response through regulation of TNF-α secretion, and should therefore be considered as a new virulence factor of T. forsythia.

The link between periodontal disease and rheumatoid arthritis: an updated review

Porphyromonas gingivalis is a leading pathogen in chronic periodontitis, a disease process involving progressive destruction of the tissues that support the teeth. Recently, the organism has been reported to produce a unique bacterial enzyme, P. gingivalis peptidyl-arginine deiminase (PPAD), which has the ability to convert arginine residues in proteins to citrulline. Protein citrullination alters protein structure and function; hence, PPAD may be involved in deregulation of the host’s signalling network and immune evasion. Further, accumulating evidence suggests a role for autoimmunity against citrullinated proteins in the development of rheumatoid arthritis (RA). As inflammatory conditions in the lungs of cigarette smokers contribute to the breakdown of immune tolerance to citrullinated epitopes, chronic exposure to citrullinated proteins at periodontitis sites may also predispose susceptible individuals to the development of autoantibodies and the initiation of RA. In this review, we discuss evidence that PPAD may represent a mechanistic link between periodontitis and RA, diseases that are known to be significantly associated at the epidemiological level.

Immune evasion by pathogenic Leptospira strains: the secretion of proteases that directly cleave complement proteins

Leptospirosis is an infectious disease of public health importance. To successfully colonize the host, pathogens have evolved multiple strategies to escape the complement system. Here we demonstrate that the culture supernatant of pathogenic but not saprophytic Leptospira inhibit the three complement pathways. We showed that the proteolytic activity in the supernatants of pathogenic strains targets the central complement molecule C3 and specific proteins from each pathway, such as factor B, C2, and C4b. The proteases cleaved α and β chains of C3 and work in synergy with host regulators to inactivate C3b. Proteolytic activity was inhibited by 1,10-phenanthroline, suggesting the participation of metalloproteases. A recombinant leptospiral metalloprotease from the thermolysin family cleaved C3 in serum and could be one of the proteases responsible for the supernatant activity. We conclude that pathogenic leptospiral proteases can deactivate immune effector molecules and represent potential targets to the development of new therapies in leptospirosis.