Regulation of Apoptosis by Gram-Positive Bacteria: Mechanistic Diversity and Consequences for Immunity

Superantigens, a Paradox of the Immune Response

Staphylococcal enterotoxins are a wide family of bacterial exotoxins with the capacity to activate as much as 20% of the host T cells, which is why they were called superantigens. Superantigens (SAgs) can cause multiple diseases in humans and cattle, ranging from mild to life-threatening infections. Almost all S. aureus isolates encode at least one of these toxins, though there is no complete knowledge about how their production is triggered. One of the main problems with the available evidence for these toxins is that most studies have been conducted with a few superantigens; however, the resulting characteristics are attributed to the whole group. Although these toxins share homology and a two-domain structure organization, the similarity ratio varies from 20 to 89% among different SAgs, implying wide heterogeneity. Furthermore, every attempt to structurally classify these proteins has failed to answer differential biological functionalities. Taking these concerns into account, it might not be appropriate to extrapolate all the information that is currently available to every staphylococcal SAg. Here, we aimed to gather the available information about all staphylococcal SAgs, considering their functions and pathogenicity, their ability to interact with the immune system as well as their capacity to be used as immunotherapeutic agents, resembling the two faces of Dr. Jekyll and Mr. Hyde.

Evaluation of Enterococcus faecalis, Lactobacillus acidophilus, and Lactobacillus plantarum in Biopsy Samples of Colorectal Cancer and Polyp Patients Compared to Healthy People

Background: Colorectal cancer (CRC) is one of the leading causes of death in both men and women worldwide. According to different studies, infectious agents or microbiota dysbiosis can play a role in CRC progression. Objective: This study aimed to evaluate the prevalence of Enterococcus faecalis, Lactobacillus acidophilus, and Lactobacillus plantarum in people with polyps or CRC compared to healthy individuals. Methods: In this study, 60 biopsy samples were collected from three groups, including patients with CRC, polyps, and healthy people. The genomic DNA was extracted from the collected samples and amplified by polymerase chain reaction (PCR) to detect E. faecalis, L. acidophilus, and L. plantarum. In the next step, quantitative Real-Time PCR was used to evaluate the copy number of the bacteria in the studied groups. Results: There was no statistically significant difference between the studied groups regarding age and gender (P > 0.05). The mean number of E. faecalis was higher in patients with CRC than in patients with polyps and healthy individuals (P < 0.05). Also, the mean numbers of L. acidophilus and L. plantarum were higher in healthy individuals than in patients with polyps and CRC (P < 0.05). Conclusions: Our findings indicate that L. acidophilus and L. plantarum in people with a family history of CRC and patients with polyps may effectively prevent or reduce CRC progression.

Impact of smokeless tobacco-associated bacteriome in oral carcinogenesis

Smokeless tobacco products possess a complex community of microorganisms. The microbial community ferment compounds present in the smokeless tobacco products and convert them into carcinogens like tobacco-associated nitrosamines. However, the potential of smokeless tobacco products associated bacteriome to manipulate systemic inflammation and other signaling pathways involved in the etiology of oral cancer will be a risk factor for oral cancer. Further, damage to oral epithelial cells causes a leaky oral layer that leads to increased infiltration of bacterial components like lipopolysaccharide, flagellin, and toxins, etc. The consumption of smokeless tobacco products can cause damage to the oral layer and dysbiosis of oral microbiota. Hence, the enrichment of harmful microbes due to dysbiosis in the oral cavity can produce high levels of bacterial metabolites and provoke inflammation as well as carcinogenesis. Understanding the complex and dynamic interrelation between the smokeless tobacco-linked bacteriome and host oral microbiome may help to unravel the mechanism of oral carcinogenesis stimulated by smokeless tobacco products. This review provides an insight into smokeless tobacco product-associated bacteriome and their potential in the progression of oral cancer. In the future, this will guide in the evolution of prevention and treatment strategies against smokeless tobacco products-induced oral cancer. Besides, it will assist the government organizations for better management and cessation policy building for the worldwide problem of smokeless tobacco addiction.

Mitochondrion-Dependent Apoptosis Is Essential for Rickettsia parkeri Infection and Replication in Vector Cells

Apoptosis is an innate immune response induced by infection in eukaryotes that contributes significantly to protection from pathogens. However, little is known about the role of apoptosis in the interactions of arthropod vectors with the rickettsiae that they transmit. Rickettsia spp. are vector-borne obligately intracellular bacteria and display different degrees of virulence in their eukaryotic hosts. In this study, we found that infection with Rickettsia parkeri (Rp) activated the apoptosis pathway in an Amblyomma americanum tick cell line (AAE2), as evidenced by the loss of phospholipid membrane asymmetry and DNA fragmentations. Additionally, infection with Rp also led to apoptosis activation in cell lines of different tick species. Interestingly, suppressing apoptosis decreased Rp infection and replication, while the activation of apoptosis increased Rp accumulation at the early stage of infection. Moreover, mitochondrion-dependent apoptosis was essential for Rp infection and replication in vector cells, and apoptosis induction required intracellular rickettsia replication. We further showed that Rp utilizes two different survival strategies to modulate apoptosis in the arthropod vectors and mammalian host cells. There was no direct correlation between apoptosis activation in vector cells and rickettsial pathogenicity. These novel findings indicate a possible mechanism whereby apoptosis facilitates infection and replication of a Rickettsia sp. in an arthropod vector. These results contribute to our understanding of how the vector's responses to pathogen infection affect pathogen replication and therefore transmission.

IMPORTANCE Rickettsioses, infections caused by the genus Rickettsia, are among the oldest known infectious diseases. Ticks are essential arthropod vectors for rickettsiae, and knowledge about the interactions between ticks, their hosts, and pathogens is fundamental for identifying drivers of tick-borne rickettsioses. Despite the rapid development in apoptosis research with rickettsiae, little is known regarding the role of apoptosis in the interactions between Rickettsia spp., vertebrate hosts, and arthropod vectors. Here, we demonstrated that mitochondrion-dependent apoptosis is essential for rickettsial infection and replication in vector cells and that apoptosis induction requires intracellular rickettsial replication. However, rickettsial pathogenicity is not linked with apoptosis activation in tick cells. Our findings improve understanding of the apoptosis mechanism in arthropods exploited by rickettsiae and also the potential to discover specific targets for new vaccines and drugs to prevent or treat rickettsial infections.

Selective Host Cell Death by Staphylococcus aureus: A Strategy for Bacterial Persistence

Host cell death programs are fundamental processes that shape cellular homeostasis, embryonic development, and tissue regeneration. Death signaling and downstream host cell responses are not only critical to guide mammalian development, they often act as terminal responses to invading pathogens. Here, we briefly review and contrast how invading pathogens and specifically Staphylococcus aureus manipulate apoptotic, necroptotic, and pyroptotic cell death modes to establish infection. Rather than invading host cells, S. aureus subverts these cells to produce diffusible molecules that cause death of neighboring hematopoietic cells and thus shapes an immune environment conducive to persistence. The exploitation of cell death pathways by S. aureus is yet another virulence strategy that must be juxtaposed to mechanisms of immune evasion, autophagy escape, and tolerance to intracellular killing, and brings us closer to the true portrait of this pathogen for the design of effective therapeutics and intervention strategies.

Effect of the Streptococcus agalactiae Virulence Regulator CovR on the Pathogenesis of Urinary Tract Infection

Background

Streptococcus agalactiae can cause urinary tract infection (UTI). The role of the S. agalactiae global virulence regulator, CovR, in UTI pathogenesis is unknown.

Methods

We used murine and human bladder uroepithelial cell models of UTI and S. agalactiae mutants in covR and related factors, including β-hemolysin/cytolysin (β-h/c), surface-anchored adhesin HvgA, and capsule to study the role of CovR in UTI.

Results

We found that covR-deficient serotype III S. agalactiae 874391 was significantly attenuated for colonization in mice and adhesion to uroepithelial cells. Mice infected with covR-deficient S. agalactiae produced less proinflammatory cytokines than those infected with wild-type 874391. Acute cytotoxicity in uroepithelial cells triggered by covR-deficient but not wild-type 874391 was associated with significant caspase 3 activation. Mechanistically, covR mutation significantly altered the expression of several genes in S. agalactiae 874391 that encode key virulence factors, including β-h/c and HvgA, but not capsule. Subsequent mutational analyses revealed that HvgA and capsule, but not the β-h/c, exerted significant effects on colonization of the murine urinary tract in vivo.

Conclusions

S. agalactiae CovR promotes bladder infection and inflammation, as well as adhesion to and viability of uroepithelial cells. The pathogenesis of S. agalactiae UTI is complex, multifactorial, and influenced by virulence effects of CovR, HvgA, and capsule.

Evidencing the Role of Erythrocytic Apoptosis in Malarial Anemia

In the last decade it has become clear that, similarly to nucleated cells, enucleated red blood cells (RBCs) are susceptible to programmed apoptotic cell death. Erythrocytic apoptosis seems to play a role in physiological clearance of aged RBCs, but it may also be implicated in anemia of different etiological sources including drug therapy and infectious diseases. In malaria, severe anemia is a common complication leading to death of children and pregnant women living in malaria-endemic regions of Africa. The pathogenesis of malarial anemia is multifactorial and involves both ineffective production of RBCs by the bone marrow and premature elimination of non-parasitized RBCs, phenomena potentially associated with apoptosis. In the present overview, we discuss evidences associating erythrocytic apoptosis with the pathogenesis of severe malarial anemia, as well as with regulation of parasite clearance in malaria. Efforts to understand the role of erythrocytic apoptosis in malarial anemia can help to identify potential targets for therapeutic intervention based on apoptotic pathways and consequently, mitigate the harmful impact of malaria in global public health.

Crystal Structures of Group B Streptococcus Glyceraldehyde-3-Phosphate Dehydrogenase: Apo-Form, Binary and Ternary Complexes

Glyceraldehyde 3-phosphate dehydrogenase or GAPDH is an evolutionarily conserved glycolytic enzyme. It catalyzes the two step oxidative phosphorylation of D-glyceraldehyde 3-phosphate into 1,3-bisphosphoglycerate using inorganic phosphate and NAD+ as cofactor. GAPDH of Group B Streptococcus is a major virulence factor and a potential vaccine candidate. Moreover, since GAPDH activity is essential for bacterial growth it may serve as a possible drug target. Crystal structures of Group B Streptococcus GAPDH in the apo-form, two different binary complexes and the ternary complex are described here. The two binary complexes contained NAD+ bound to 2 (mixed-holo) or 4 (holo) subunits of the tetrameric protein. The structure of the mixed-holo complex reveals the effects of NAD+ binding on the conformation of the protein. In the ternary complex, the phosphate group of the substrate was bound to the new Pi site in all four subunits. Comparison with the structure of human GAPDH showed several differences near the adenosyl binding pocket in Group B Streptococcus GAPDH. The structures also reveal at least three surface-exposed areas that differ in amino acid sequence compared to the corresponding areas of human GAPDH.

Staphylococcus epidermidis Biofilm-Released Cells Induce a Prompt and More Marked In vivo Inflammatory-Type Response than Planktonic or Biofilm Cells

Staphylococcus epidermidis biofilm formation on indwelling medical devices is frequently associated with the development of chronic infections. Nevertheless, it has been suggested that cells released from these biofilms may induce severe acute infections with bacteraemia as one of its major associated clinical manifestations. However, how biofilm-released cells interact with the host remains unclear. Here, using a murine model of hematogenously disseminated infection, we characterized the interaction of cells released from S. epidermidis biofilms with the immune system. Gene expression analysis of mouse splenocytes suggested that biofilm-released cells might be particularly effective at activating inflammatory and antigen presenting cells and inducing cellular apoptosis. Furthermore, biofilm-released cells induced a higher production of pro-inflammatory cytokines, in contrast to mice infected with planktonic cells, even though these had a similar bacterial load in livers and spleens. Overall, these results not only provide insights into the understanding of the role of biofilm-released cells in S. epidermidis biofilm-related infections and pathogenesis, but may also help explain the relapsing character of these infections.

Positive Feedback Cycle of TNFα Promotes Staphylococcal Enterotoxin B-Induced THP-1 Cell Apoptosis

Staphylococcal enterotoxin B (SEB) has been demonstrated to be of importance in Staphylococcus aureus related diseases, such as atopic dermatitis (AD). Dysregulated apoptosis in AD is remarkable, and SEB can induce apoptosis of various cell types. However, the mechanisms by which SEB induces apoptosis and influences disease processes remain unclear. In this study, the recombinant SEB-induced THP-1 monocyte apoptosis was demonstrated in the absence of preliminary cell activation in a time- and dose-dependent manner. SEB could up-regulate the expression of tumor necrosis factor alpha (TNFα) in THP-1 cells and induce apoptosis via an extrinsic pathway. TNFα could in turn increase the expression of HLA-DRa, the SEB receptor on the cell surface. As a result, a positive feedback cycle of TNFα was established. TNFα expression and SEB-induced apoptosis were decreased by knocking down the expression of either HLA-DRa or TNFR1. Therefore, the feedback cycle of TNFα is crucial for SEB functions. This work provides insights into the mechanisms of SEB-induced monocyte apoptosis and emphasizes the major role of TNFα in future related studies.

Induction of Central Host Signaling Kinases during Pneumococcal Infection of Human THP-1 Cells

Streptococcus pneumoniae is a widespread colonizer of the mucosal epithelia of the upper respiratory tract of human. However, pneumococci are also responsible for numerous local as well as severe systemic infections, especially in children under the age of five and the elderly. Under certain conditions, pneumococci are able to conquer the epithelial barrier, which can lead to a dissemination of the bacteria into underlying tissues and the bloodstream. Here, specialized macrophages represent an essential part of the innate immune system against bacterial intruders. Recognition of the bacteria through different receptors on the surface of macrophages leads thereby to an uptake and elimination of bacteria. Accompanied cytokine release triggers the migration of leukocytes from peripheral blood to the site of infection, where monocytes differentiate into mature macrophages. The rearrangement of the actin cytoskeleton during phagocytosis, resulting in the engulfment of bacteria, is thereby tightly regulated by receptor-mediated phosphorylation cascades of different protein kinases. The molecular cellular processes including the modulation of central protein kinases are only partially solved. In this study, the human monocytic THP-1 cell line was used as a model system to examine the activation of Fcγ and complement receptor-independent signal cascades during infection with S. pneumoniae. Pneumococci cultured either in chemically defined or complex medium showed no significant differences in pneumococcal phagocytosis by phorbol 12-myristate 13-acetate (PMA) differentiated THP-1 cells. Double immuno-fluorescence microscopy and antibiotic protection assays demonstrated a time-dependent uptake and killing of S. pneumoniae 35A inside of macrophages. Infections of THP-1 cells in the presence of specific pharmacological inhibitors revealed a crucial role of actin polymerization and importance of the phosphoinositide 3-kinase (PI3K) and Protein kinase B (Akt) as well during bacterial uptake. The participation of essential host cell signaling kinases in pneumococcal phagocytosis was deciphered for the kinase Akt, ERK1/2, and p38 and phosphoimmunoblots showed an increased phosphorylation and thus activation upon infection with pneumococci. Taken together, this study deciphers host cell kinases in innate immune cells that are induced upon infection with pneumococci and interfere with bacterial clearance after phagocytosis.

Exploiting death: apoptotic immunity in microbial pathogenesis

Innate immunity typically is responsible for initial host responses against infections. Independently, nucleated cells that die normally as part of the physiological process of homeostasis in mammals (including humans) suppress immunity. Specifically, the physiological process of cell death (apoptosis) generates cells that are recognized specifically by viable cells of all types and elicit a profound transient suppression of host immunity (termed 'innate apoptotic immunity' (IAI)). IAI appears to be important normally for the maintenance of self-tolerance and for the resolution of inflammation. In addition, pathogens are able to take advantage of IAI through a variety of distinct mechanisms, to enable their proliferation within the host and enhance pathogenicity. For example, the protist pathogen Leishmania amazonensis, at its infective stage, mimics apoptotic cells by expressing apoptotic-like protein determinants on the cell surface, triggering immunosuppression directly. In contrast, the pathogenic bacterium Listeria monocytogenes triggers cell death in host lymphocytes, relying on those apoptotic cells to suppress host immune control and facilitate bacterial expansion. Finally, although the inhibition of apoptotic cell death is a common attribute of many viruses which facilitates their extended replication, it is clear that adenoviruses also reprogram the non-apoptotic dead cells that arise subsequently to manifest apoptotic-like immunosuppressive properties. These three instances represent diverse strategies used by microbial pathogens to exploit IAI, focusing attention on the potency of this facet of host immune control. Further examination of these cases will be revealing both of varied mechanisms of pathogenesis and the processes involved in IAI control.

The role of macrophage 1 antigen in polymicrobial sepsis

PURPOSE

Macrophage 1 antigen (Mac-1, CD11bCD18) is a leukocyte adhesion molecule that is involved in many functions including leukocyte recruitment, phagocytosis, and neutrophil apoptosis. The previous report of mild polymicrobial, abdominal sepsis showed that the administration of anti-CD11b-blocking antibody administration attenuated lung injury without any survival benefit. Here we tested the impact of Mac-1 deficiency in severe polymicrobial abdominal sepsis model.

METHODS

Polymicrobial sepsis was studied using cecal ligation and puncture model in wild-type (WT) or Mac-1-deficient (CD11b knockout [KO]) mice, and their outcomes were examined. Bacterial tissue load and the recruitment of neutrophils to the abdominal cavity were assessed. In vitro bacterial killing assay was performed. Serum cytokine levels were measured using multiarray. Apoptosis of spleen tissues was assessed using Western blot analysis and immunohistochemistry (cleaved caspase 3 and TUNEL staining). In addition, in vitro apoptosis assay was performed using primary splenocytes from both WT and KO mice. The recruitment of neutrophils to lung was assessed by measuring myeloperoxidase activity.

RESULTS

Macrophage 1 antigen deficiency significantly decreased survival (survival percentage WT 43.5% vs. KO 13.0%; P = 0.0038) with higher bacterial load in blood and more severe systemic inflammation. Knockout mice demonstrated higher apoptosis both in vivo and in vitro. The recruitment of neutrophils to lung was not different between WT and KO mice.

CONCLUSIONS

Macrophage 1 antigen deficiency was associated with poorer outcomes, more bacterial load, systemic inflammation, and splenic apoptosis. However, Mac-1 deficiency did not attenuate neutrophil recruitment to lung.

Structure of Streptococcus agalactiae glyceraldehyde-3-phosphate dehydrogenase holoenzyme reveals a novel surface

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a conserved cytosolic enzyme, which plays a key role in glycolysis. GAPDH catalyzes the oxidative phosphorylation of D-glyceraldehyde 3-phosphate using NAD or NADP as a cofactor. In addition, GAPDH localized on the surface of some bacteria is thought to be involved in macromolecular interactions and bacterial pathogenesis. GAPDH on the surface of group B streptococcus (GBS) enhances bacterial virulence and is a potential vaccine candidate. Here, the crystal structure of GBS GAPDH from Streptococcus agalactiae in complex with NAD is reported at 2.46 Å resolution. Although the overall structure of GBS GAPDH is very similar to those of other GAPDHs, the crystal structure reveals a significant difference in the area spanning residues 294-307, which appears to be more acidic. The amino-acid sequence of this region of GBS GAPDH is also distinct compared with other GAPDHs. This region therefore may be of interest as an immunogen for vaccine development.

Enterococcus faecalis infection activates phosphatidylinositol 3-kinase signaling to block apoptotic cell death in macrophages

Apoptosis is an intrinsic immune defense mechanism in the host response to microbial infection. Not surprisingly, many pathogens have evolved various strategies to manipulate this important pathway to benefit their own survival and dissemination in the host during infection. To our knowledge, no attempts have been made to explore the host cell survival signals modulated by the bacterium Enterococcus faecalis. Here, we show for the first time that during early stages of infection, internalized enterococci can prevent host cell (RAW264.7 cells, primary macrophages, and mouse embryonic fibroblasts [MEFs]) apoptosis induced by a wide spectrum of proapoptotic stimuli. Activation of caspase 3 and cleavage of the caspase 3 substrate poly(ADP-ribose) polymerase were inhibited in E. faecalis-infected cells, indicating that E. faecalis protects macrophages from apoptosis by inhibiting caspase 3 activation. This antiapoptotic activity in E. faecalis-infected cells was dependent on the activation of the phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway, which resulted in the increased expression of the antiapoptotic factor Bcl-2 and decreased expression of the proapoptotic factor Bax. Further analysis revealed that active E. faecalis physiology was important for inhibition of host cell apoptosis, and this feature seemed to be a strain-independent trait among E. faecalis isolates. Employing a mouse peritonitis model, we also determined that cells collected from the peritoneal lavage fluid of E. faecalis-infected mice showed reduced levels of apoptosis compared to cells from uninfected mice. These results show early modulation of apoptosis during infection and have important implications for enterococcal pathogenesis.

The role of coagulation/fibrinolysis during Streptococcus pyogenes infection

The hemostatic system comprises platelet aggregation, coagulation and fibrinolysis and is a host defense mechanism that protects the integrity of the vascular system after tissue injury. During bacterial infections, the coagulation system cooperates with the inflammatory system to eliminate the invading pathogens. However, pathogenic bacteria have frequently evolved mechanisms to exploit the hemostatic system components for their own benefit. Streptococcus pyogenes, also known as Group A Streptococcus, provides a remarkable example of the extraordinary capacity of pathogens to exploit the host hemostatic system to support microbial survival and dissemination. The coagulation cascade comprises the contact system (also known as the intrinsic pathway) and the tissue factor pathway (also known as the extrinsic pathway), both leading to fibrin formation. During the early phase of S. pyogenes infection, the activation of the contact system eventually leads to bacterial entrapment within a fibrin clot, where S. pyogenes is immobilized and killed. However, entrapped S. pyogenes can circumvent the antimicrobial effect of the clot by sequestering host plasminogen on the bacterial cell surface that, after conversion into its active proteolytic form, plasmin, degrades the fibrin network and facilitates the liberation of S. pyogenes from the clot. Furthermore, the surface-localized fibrinolytic activity also cleaves a variety of extracellular matrix proteins, thereby enabling S. pyogenes to migrate across barriers and disseminate within the host. This review summarizes the knowledge gained during the last two decades on the role of coagulation/fibrinolysis in host defense against S. pyogenes as well as the strategies developed by this pathogen to evade and exploit these host mechanisms for its own benefit.

Infection-induced IL-10 and JAK-STAT: A review of the molecular circuitry controlling immune hyperactivity in response to pathogenic microbes

Generation of effective immune responses against pathogenic microbes depends on a fine balance between pro- and anti-inflammatory responses. Interleukin-10 (IL-10) is essential in regulating this balance and has garnered renewed interest recently as a modulator of the response to infection at the JAK-STAT signaling axis of host responses. Here, we examine how IL-10 functions as the “master regulator” of immune responses through JAK-STAT, and provide a perspective from recent insights on bacterial, protozoan, and viral infection model systems. Pattern recognition and subsequent molecular events that drive activation of IL-10-associated JAK-STAT circuitry are reviewed and the implications for microbial pathogenesis are discussed.

Listr1 locus regulates innate immunity against Listeria monocytogenes infection in the mouse liver possibly through Cxcl11 polymorphism

Inbred stains of mice display differential susceptibility to infection with the common foodborne pathogen Listeria monocytogenes (Lm). Previously, Listr1 and Listr2, two genetic loci that control differential sensitivity to Lm infection between BALB/cByJ and C57BL/6ByJ mice, were identified. To analyze the role of Listr1 in innate immune responses, we employed congenic mice (C.B6By-Listr1/Rag2 (-/-) ) bearing the C57BL/6ByJ-derived Listr1 locus on a BALB/c-Rag2 (-/-) background. Consistent with the results of a previous genetic analysis, the congenic mice showed increased susceptibility to Lm infection. The bacterial burden in the liver between the congenic and control lines was significantly different (P < 0.05) from 24 h postinfection with Lm. Analysis of genes within the Listr1 locus identified a frameshift mutation in the Cxcl11 gene of the C57BL/6 strain that prevents production of the mature chemokine CXCL11. No differences in inflammatory cell infiltration or cells expressing CXCR3 and CXCR7 which are the receptors of CXCL11 occurred because of CXCL11 deficiency in the congenic mice spleens. However, these mice lacked a distinct population of CD14(+) positive resident mononuclear cells that express intermediate levels of CXCR3 and CXCR7 in the liver. There were fewer microabscesses in the liver of CXCL11-deficient mice during the early stage of infection, which is consistent with their decreased ability to resist Lm. Our results, when taken together, show that the Listr1 locus plays an important role in early control of Lm infection in the mouse liver and that Cxcl11 is a candidate gene for disease severity within this locus.

Infection and cellular defense dynamics in a novel 17β-estradiol murine model of chronic human group B streptococcus genital tract colonization reveal a role for hemolysin in persistence and neutrophil accumulation

Genital tract carriage of group B streptococcus (GBS) is prevalent among adult women; however, the dynamics of chronic GBS genital tract carriage, including how GBS persists in this immunologically active host niche long term, are not well defined. To our knowledge, in this study, we report the first animal model of chronic GBS genital tract colonization using female mice synchronized into estrus by delivery of 17β-estradiol prior to intravaginal challenge with wild-type GBS 874391. Cervicovaginal swabs, which were used to measure bacterial persistence, showed that GBS colonized the vaginal mucosa of mice at high numbers (10(6)-10(7) CFU/swab) for at least 90 d. Cellular and histological analyses showed that chronic GBS colonization of the murine genital tract caused significant lymphocyte and PMN cell infiltrates, which were localized to the vaginal mucosal surface. Long-term colonization was independent of regular hormone cycling. Immunological analyses of 23 soluble proteins related to chemotaxis and inflammation showed that the host response to GBS in the genital tract comprised markers of innate immune activation including cytokines such as GM-CSF and TNF-α. A nonhemolytic isogenic mutant of GBS 874391, Δcyle9, was impaired for colonization and was associated with amplified local PMN responses. Induction of DNA neutrophil extracellular traps, which was observed in GBS-infected human PMNs in vitro in a hemolysin-dependent manner, appeared to be part of this response. Overall, this study defines key infection dynamics in a novel murine model of chronic GBS genital tract colonization and establishes previously unknown cellular and soluble defense responses to GBS in the female genital tract.

Intestinal epithelial cell apoptosis and loss of barrier function in the setting of altered microbiota with enteral nutrient deprivation

Total parenteral nutrition (TPN), a commonly used treatment for patients who cannot receive enteral nutrition, is associated with significant septic complications due in part to a loss of epithelial barrier function (EBF). While the underlying mechanisms of TPN-related epithelial changes are poorly understood, a mouse model of TPN-dependence has helped identify several contributing factors. Enteral deprivation leads to a shift in intestinal microbiota to predominantly Gram-negative Proteobacteria. This is associated with an increase in expression of proinflammatory cytokines within the mucosa, including interferon-γ and tumor necrosis factor-α. A concomitant loss of epithelial growth factors leads to a decrease in epithelial cell proliferation and increased apoptosis. The resulting loss of epithelial tight junction proteins contributes to EBF dysfunction. These mechanisms identify potential strategies of protecting against TPN-related complications, such as modification of luminal bacteria, blockade of proinflammatory cytokines, or growth factor replacement.

Apoptosis-associated uncoupling of bone formation and resorption in osteomyelitis

The mechanisms underlying the destruction of bone tissue in osteomyelitis are only now being elucidated. While some of the tissue damage associated with osteomyelitis likely results from the direct actions of bacteria and infiltrating leukocytes, perhaps exacerbated by bacterial manipulation of leukocyte survival pathways, infection-induced bone loss predominantly results from an uncoupling of the activities of osteoblasts and osteoclasts. Bacteria or their products can directly increase osteoclast formation and activity, and the inflammatory milieu at sites of infection can further promote bone resorption. In addition, osteoclast activity is critically regulated by osteoblasts that can respond to bacterial pathogens and foster both inflammation and osteoclastogenesis. Importantly, bone loss during osteomyelitis is also brought about by a decline in new bone deposition due to decreased bone matrix synthesis and by increased rates of osteoblast apoptosis. Extracellular bacterial components may be sufficient to reduce osteoblast viability, but the causative agents of osteomyelitis are also capable of inducing continuous apoptosis of these cells by activating intrinsic and extrinsic cell death pathways to further uncouple bone formation and resorption. Interestingly, bacterial internalization appears to be required for maximal osteoblast apoptosis, and cytosolic inflammasome activation may act in concert with autocrine/paracrine death receptor-ligand signaling to induce cell death. The manipulation of apoptotic pathways in infected bone cells could be an attractive new means to limit inflammatory damage in osteomyelitis. However, the mechanism that is the most important in bacterium-induced bone loss has not yet been identified. Furthermore, it remains to be determined whether the host would be best served by preventing osteoblast cell death or by promoting apoptosis in infected cells.

The early phase transcriptome of bovine monocyte-derived macrophages infected with Staphylococcus aureus in vitro

Background

In the mammary gland, local recruitment and action of macrophages is a key immunological defence mechanism against infection. Macrophages are members of the innate immune system, serve as the first line of the defence against invading pathogens and are critical effectors and regulators of inflammation. We have examined the early phase response of bovine macrophages to infection with live Staphylococcus aureus. Genome-wide transcript profiling of blood monocyte-derived macrophages from six Norwegian Red heifers infected with live S. aureus for 2 and 6 hours in vitro was performed.

Results

About 420 of the 17 000 genes on the ARK-Genomics bovine cDNA array were differentially regulated at 6 hours post infection. Approximately 70% of the responding genes had a known identity (Entrez Gene ID) and were used in the identification of overrepresented pathways and biological functions in the dataset.

Analysis of a subset of differentially regulated genes (List eQG) obtained by comparison with data from genome-wide association mapping in Norwegian Red cattle identified anti-inflammatory cytokines interleukin 4 and interleukin 13 as putative expression quantitative trait loci, suggesting that S. aureus infection triggers alternative activation of macrophages. Moreover, several classical activation pathways were found, mainly cellular immune response and cytokine signaling pathways, i.e. triggering receptor expressed on myeloid cells 1 (TREM1) and nucleotide-binding and oligomerization domain-like receptor (NLR) pathways. Tumor necrosis factor receptor superfamily member 5 (CD40 ligand) was identified as an upstream regulator which points toward CD40 likely acting as a co-stimulatory receptor during Toll-like receptor 2(TLR2)-mediated inflammatory response of bovine macrophages to S. aureus infection. Furthermore, peptidoglycan was identified as an upstream regulator in the List eQG, which indicates that this bacterial cell-wall component might be pivotal in macrophage intracellular bacterial recognition during early inflammation.

Conclusions

Here we have shown that in vitro infection of bovine macrophages with live S. aureus induced both alternative and classical activation pathways. Alternative activation of macrophages may be a mechanism contributing to intracellular persistence of S. aureus in the course of inflammation such as during mastitis in dairy cattle.

Modulation of host immune defenses by Aeromonas and Yersinia species: convergence on toxins secreted by various secretion systems

Like other pathogenic bacteria, Yersinia and Aeromonas species have been continuously co-evolving with their respective hosts. Although the former is a bonafide human pathogen, the latter has gained notararity as an emerging disease-causing agent. In response to immune cell challenges, bacterial pathogens have developed diverse mechanism(s) enabling their survival, and, at times, dominance over various host immune defense systems. The bacterial type three secretion system (T3SS) is evolutionarily derived from flagellar subunits and serves as a vehicle by which microbes can directly inject/translocate anti-host factors/effector proteins into targeted host immune cells. A large number of Gram-negative bacterial pathogens possess a T3SS empowering them to disrupt host cell signaling, actin cytoskeleton re-arrangements, and even to induce host-cell apoptotic and pyroptotic pathways. All pathogenic yersiniae and most Aeromonas species possess a T3SS, but they also possess T2- and T6-secreted toxins/effector proteins. This review will focus on the mechanisms by which the T3SS effectors Yersinia outer membrane protein J (YopJ) and an Aeromonas hydrophila AexU protein, isolated from the diarrheal isolate SSU, mollify host immune system defenses. Additionally, the mechanisms that are associated with host cell apoptosis/pyroptosis by Aeromonas T2SS secreted Act, a cytotoxic enterotoxin, and Hemolysin co-regulated protein (Hcp), an A. hydrophila T6SS effector, will also be discussed.

Role of apoptosis in disease

Since the initial description of apoptosis, a number of different forms of cell death have been described. In this review we will focus on classic caspase-dependent apoptosis and its variations that contribute to diseases. Over fifty years of research have clarified molecular mechanisms involved in apoptotic signaling as well and shown that alterations of these pathways lead to human diseases. Indeed both reduced and increased apoptosis can result in pathology. More recently these findings have led to the development of therapeutic approaches based on regulation of apoptosis, some of which are in clinical trials or have entered medical practice.

Genome-wide mapping of cystitis due to Streptococcus agalactiae and Escherichia coli in mice identifies a unique bladder transcriptome that signifies pathogen-specific antimicrobial defense against urinary tract infection

The most common causes of urinary tract infections (UTIs) are Gram-negative pathogens such as Escherichia coli; however, Gram-positive organisms, including Streptococcus agalactiae, or group B streptococcus (GBS), also cause UTI. In GBS infection, UTI progresses to cystitis once the bacteria colonize the bladder, but the host responses triggered in the bladder immediately following infection are largely unknown. Here, we used genome-wide expression profiling to map the bladder transcriptome of GBS UTI in mice infected transurethrally with uropathogenic GBS that was cultured from a 35-year-old women with cystitis. RNA from bladders was applied to Affymetrix Gene-1.0ST microarrays; quantitative reverse transcriptase PCR (qRT-PCR) was used to analyze selected gene responses identified in array data sets. A surprisingly small significant-gene list of 172 genes was identified at 24 h; this compared to 2,507 genes identified in a side-by-side comparison with uropathogenic E. coli (UPEC). No genes exhibited significantly altered expression at 2 h in GBS-infected mice according to arrays despite high bladder bacterial loads at this early time point. The absence of a marked early host response to GBS juxtaposed with broad-based bladder responses activated by UPEC at 2 h. Bioinformatics analyses, including integrative system-level network mapping, revealed multiple activated biological pathways in the GBS bladder transcriptome that regulate leukocyte activation, inflammation, apoptosis, and cytokine-chemokine biosynthesis. These findings define a novel, minimalistic type of bladder host response triggered by GBS UTI, which comprises collective antimicrobial pathways that differ dramatically from those activated by UPEC. Overall, this study emphasizes the unique nature of bladder immune activation mechanisms triggered by distinct uropathogens.

Combining quantitative trait loci and heterogeneous microarray data analyses reveals putative candidate pathways affecting mastitis in cattle

Mastitis is a frequent disease and considerable problem for the global dairy industry. Identification of solutions leading to the development of new control strategies is therefore of high importance. In this study, we have integrated genomic data from genome-wide association mapping in cattle with transcriptomic data from microarray studies of several mastitis pathogens and host species in vitro and in vivo. To identify significant candidate pathways directly and indirectly involved in the immune response to mastitis, ingenuity pathway analysis (ipa) and database for annotation, visualization and integrated discovery bioinformatic (david) were applied. Several candidate pathways were found. Of great interest are IL-17 and IL-8 signalling pathways, responsible for the recruitment and migration of inflammatory cells into tissue during inflammation and infection. These results may emphasize further functional studies for identification of factors contributing to resistance to mastitis pathogens in cattle.

Recent insights into microbial triggers of interleukin-10 production in the host and the impact on infectious disease pathogenesis

Since its initial description as a Th2-cytokine antagonistic to interferon-alpha and granulocyte-macrophage colony-stimulating factor, many studies have shown various anti-inflammatory actions of interleukin-10 (IL-10), and its role in infection as a key regulator of innate immunity. Studies have shown that IL-10 induced in response to microorganisms and their products plays a central role in shaping pathogenesis. IL-10 appears to function as both sword and shield in the response to varied groups of microorganisms in its capacity to mediate protective immunity against some organisms but increase susceptibility to other infections. The nature of IL-10 as a pleiotropic modulator of host responses to microorganisms is explained, in part, by its potent and varied effects on different immune effector cells which influence antimicrobial activity. A new understanding of how microorganisms trigger IL-10 responses is emerging, along with recent discoveries of how IL-10 produced during disease might be harnessed for better protective or therapeutic strategies. In this review, we summarize studies from the past 5 years that have reported the induction of IL-10 by different classes of pathogenic microorganisms, including protozoa, nematodes, fungi, viruses and bacteria and discuss the impact of this induction on the persistence and/or clearance of microorganisms in the host.

Epithelial cell coculture models for studying infectious diseases: benefits and limitations

Countless in vitro cell culture models based on the use of epithelial cell types of single lineages have been characterized and have provided insight into the mechanisms of infection for various microbial pathogens. Diverse culture models based on disease-relevant mucosal epithelial cell types derived from gastrointestinal, genitourinary, and pulmonary organ systems have delineated many key host-pathogen interactions that underlie viral, parasitic, and bacterial disease pathogenesis. An alternative to single lineage epithelial cell monoculture, which offers more flexibility and can overcome some of the limitations of epithelial cell culture models based on only single cell types, is coculture of epithelial cells with other host cell types. Various coculture models have been described, which incorporate epithelial cell types in culture combination with a wide range of other cell types including neutrophils, eosinophils, monocytes, and lymphocytes. This paper will summarize current models of epithelial cell coculture and will discuss the benefits and limitations of epithelial cell coculture for studying host-pathogen dynamics in infectious diseases.

Phylogenetic lineage and pilus protein Spb1/SAN1518 affect opsonin-independent phagocytosis and intracellular survival of Group B Streptococcus

Opsonin-independent phagocytosis of Group B Streptococcus (GBS) is important in defense against neonatal GBS infections. A recent study indicated a role for GBS pilus in macrophage phagocytosis (Maisey et al Faseb J 22 2008 1715-24). We studied 163 isolates from different phylogenetic backgrounds and those possessing or lacking the gene encoding the pilus backbone protein, Spb1 (SAN1518, PI-2b) and spb1-deficient mutants of wild-type (WT) serotype III-3 GBS 874391 in non-opsonic phagocytosis assays using J774A.1 macrophages. Numbers of GBS phagocytosed differed up to 23-fold depending on phylogenetic background; isolates possessing spb1 were phagocytosed more than isolates lacking spb1. Comparing WT GBS and isogenic spb1-deficient mutants showed WT was phagocytosed better compared to mutants; Spb1 also enhanced intracellular survival as mutants were killed more efficiently. Complementation of mutants restored phagocytosis and resistance to killing in J774A.1 macrophages. Spb1 antiserum revealed surface expression in WT GBS and spatial distribution relative to capsular polysaccharide. spb1 did not affect macrophage nitric oxide and TNF-alpha responses; differences in phagocytosis did not correlate with N-acetyl d-glucosamine (from GBS cell-wall) according to enzyme-linked lectin-sorbent assay. Together, these findings support a role for phylogenetic lineage and Spb1 in opsonin-independent phagocytosis and intracellular survival of GBS in J774A.1 macrophages.

Haemolysin II is a Bacillus cereus virulence factor that induces apoptosis of macrophages

Bacillus cereus is a Gram-positive spore-forming bacterium causing food poisoning and serious opportunistic infections. These infections are characterized by bacterial accumulation despite the recruitment of phagocytic cells. The precise mechanisms and the bacterial factors allowing B. cereus to circumvent host immune responses remain to be elucidated. We have previously shown that B. cereus induces macrophage cell death by an unknown mechanism. Here we identified the toxic component from the B. cereus supernatant. We report that Haemolysin II (HlyII) provokes macrophage cell death by apoptosis through its pore-forming activity. The HlyII-induced apoptotic pathway is caspase 3 and 8 dependent, thus most likely mediated by the death receptor pathway. Using insects and mice as in vivo models, we show that deletion of hlyII strongly reduces virulence. In addition, we show that after infection of Bombyx mori larvae, the immune cells are apoptotic, demonstrating that HlyII induces apoptosis of phagocytic cells in vivo. Altogether, our results clearly unravel HlyII as a novel virulence protein that induces apoptosis in phagocytic cells in vitro and in vivo.

Effect of platelet additive solution on bacterial dynamics and their influence on platelet quality in stored platelet concentrates

BACKGROUND

Platelet additive solutions (PASs) are an alternative to plasma for the storage of platelet concentrates (PCs). However, little is known about the effect of PAS on the growth dynamics of contaminant bacteria. Conversely, there have been no studies on the influence of bacteria on platelet (PLT) quality indicators when suspended in PAS.

STUDY DESIGN AND METHODS

Eight buffy coats were pooled, split, and processed into PCs suspended in either plasma or PAS (SSP+, MacoPharma). PCs were inoculated with 10 and 100 colony-forming units (CFUs)/bag of either Serratia liquefaciens or Staphylococcus epidermidis. Bacterial growth was measured over 5 days by colony counts and bacterial biofilm formation was assayed by scanning electron microscopy and crystal violet staining. Concurrently, PLT markers were measured by an assay panel and flow cytometry.

RESULTS

S. liquefaciens exhibited an apparent slower doubling time in plasma-suspended PCs (plasma-PCs). Biofilm formation by S. liquefaciens and S. epidermidis was significantly greater in PCs stored in plasma than in PAS. Although S. liquefaciens altered several PLT quality markers by Days 3 to 4 postinoculation in both PAS- and plasma-PCs, S. epidermidis contamination did not produce measurable PLT changes.

CONCLUSIONS

S. liquefaciens can be detected more quickly in PAS-suspended PCs (PAS-PCs) than in plasma-PCs by colony counting. Furthermore, reduced biofilm formation by S. liquefaciens and S. epidermidis during storage in PAS-PCs increases bacteria availability for sampling detection. Culture-based detection remains the earliest indicator of bacterial presence in PAS-PCs, while changes of PLT quality can herald S. liquefaciens contamination when in excess of 10(8) CFUs/mL.

Streptococcus adherence and colonization

Streptococci readily colonize mucosal tissues in the nasopharynx; the respiratory, gastrointestinal, and genitourinary tracts; and the skin. Each ecological niche presents a series of challenges to successful colonization with which streptococci have to contend. Some species exist in equilibrium with their host, neither stimulating nor submitting to immune defenses mounted against them. Most are either opportunistic or true pathogens responsible for diseases such as pharyngitis, tooth decay, necrotizing fasciitis, infective endocarditis, and meningitis. Part of the success of streptococci as colonizers is attributable to the spectrum of proteins expressed on their surfaces. Adhesins enable interactions with salivary, serum, and extracellular matrix components; host cells; and other microbes. This is the essential first step to colonization, the development of complex communities, and possible invasion of host tissues. The majority of streptococcal adhesins are anchored to the cell wall via a C-terminal LPxTz motif. Other proteins may be surface anchored through N-terminal lipid modifications, while the mechanism of cell wall associations for others remains unclear. Collectively, these surface-bound proteins provide Streptococcus species with a "coat of many colors," enabling multiple intimate contacts and interplays between the bacterial cell and the host. In vitro and in vivo studies have demonstrated direct roles for many streptococcal adhesins as colonization or virulence factors, making them attractive targets for therapeutic and preventive strategies against streptococcal infections. There is, therefore, much focus on applying increasingly advanced molecular techniques to determine the precise structures and functions of these proteins, and their regulatory pathways, so that more targeted approaches can be developed.

Granzymes drive a rapid listeriolysin O-induced T cell apoptosis

The Listeria monocytogenes protein listeriolysin O (LLO) is a pore-forming protein essential for virulence. Although the major role for LLO is to allow L. monocytogenes entry into the cytosol, it also induces apoptosis of activated lymphocytes, an obligatory cellular response that modulates the infection. Induction of apoptosis by LLO proceeds through a fast, caspase-dependent pathway and a slow, caspase-independent pathway. Polyclonal T cell lines were generated from either normal mice or mice deficient in granzyme and perforin proteins, and then treated with apoptogenic doses of LLO. In this study we show that apoptosis of lymphocytes induced by LLO was characterized by activation of caspases as quickly as 30 min that was dependent on the expression of granzymes. In the absence of granzymes, all parameters of apoptosis such as caspase activation, phosphatidylserine exposure, mitochondrial depolarization, and DNA fragmentation were dramatically reduced in magnitude. Removal of perforin inhibited the apoptotic effect of LLO on cells by approximately 50%. Neutralization of intracellular acidification using chloroquine inhibited the rapid apoptotic death. In agreement with these findings granzyme-deficient mice harbored lower bacterial titers and decrease splenic pathology compared with normal mice following L. monocytogenes infection. Thus, LLO exploits apoptotic enzymes of the adaptive immune response to eliminate immune cells and increase its virulence.

A case of hereditary protein S deficiency presenting with cerebral sinus venous thrombosis and deep vein thrombosis at high altitude

A 35-year-old healthy male with no history of any past medical illness developed severe headache, vomiting and drowsiness while at high altitude (4,572 m) in the eastern Himalayan ranges. He was evacuated to a tertiary-care hospital where he was diagnosed to have cerebral sinus venous thrombosis (CSVT) on magnetic resonance imaging, with deep vein thrombosis (DVT) of his right popliteo-femoral vein on color Doppler study. Investigation for thrombophilia revealed protein S (PS) deficiency in this patient. Family screening revealed low levels of PS in two elder brothers. One brother had a history of 'stroke in young' at the age of 20 years with the other being asymptomatic. This established the hereditary nature of PS deficiency. We are not aware of any previously published report on hereditary PS deficiency combined with CSVT and DVT occurring at high altitude. However, 1 case of protein C deficiency with CSVT has been reported previously.