The modulation of host cell apoptosis by intracellular bacterial pathogens

Functional analysis of OmpA and its contribution to pathogenesis of Edwardsiella tarda

Outer membrane protein A (OmpA), a major component of outer membrane proteins in gram-negative bacteria, is considered to be an important virulence factor in various pathogenic bacteria, but its underlying mechanisms involved in pathogenic process of Edwardsiella tarda has not yet been fully elucidated. E. tarda is an important facultative intracellular pathogen with a broad host range. This bacterium could survive and replicate in macrophages as an escape mechanism from the host defense. To address the functions of OmpA and its potential roles in the pathogenesis of E. tarda, ΔompA mutant strain and ΔompA-C complementary strain were constructed by the allelic exchange method in this study. Here, we demonstrate that the abilities of motility, biofilm formation and adherence to RAW264.7 cells of ΔompA were significantly impaired, although there was no difference in growth between wild-type (WT) strain and ΔompA. Moreover, inactivation of ompA rendered E. tarda more sensitive to oxidative, heat shock and osmotic stress, which simulate the in vivo conditions that E. tarda encounters within the intramacrophage environment. Consist with this observation, ΔompA was also found to be markedly attenuated for growth within macrophages. In addition, compared with the WT strain, ΔompA activated macrophages to release more inflammatory mediators, including tumor necrosis factor alpha (TNF-α), reactive oxygen species (ROS) and nitric oxide (NO). However, flow cytometry analysis revealed that ΔompA induced less apoptosis of RAW264.7 cells as compared with WT strain, characterized by decreased Annexin V binding and the activation of caspase-3. Overall, our findings suggest an importance of OmpA to E. tarda and provide the first comprehensive insight into its functions and potential roles in the pathogenesis of E. tarda, including its effect on interaction with macrophages.

Intestinal epithelial cells of Japanese flounder (Paralichthys olivaceus) as an in vitro model for studying intestine immune function based on transcriptome analysis

Japanese flounder (Paralichthys olivaceus) is an economically crucial marine species, but diseases like hemorrhagic septicemia caused by Edwardsiella tarda have resulted in significant economic losses. E. tarda infects various hosts, and its pathogenicity in fish is not fully understood. Lipopolysaccharides (LPS) are components of the outer membrane of Gram-negative bacteria and are representative of typical PAMP molecules that cause activation of the immune system. The PoIEC cell line is a newly established intestinal epithelial cell line from P. olivaceus. In order to investigate whether it can be used as an in vitro model for studying the pathogenesis of E. tarda and LPS stimulation, we conducted RNA-seq experiments for the PoIECs model of E. tarda infection and LPS stimulation. In this study, transcriptome sequencing was carried out in the PoIEC cell line after treatment with LPS and E. tarda. A total of 62.52G of high-quality data from transcriptome sequencing results were obtained in nine libraries, of which an average of 87.96% data could be aligned to the P. olivaceus genome. Data analysis showed that 283 and 414 differentially expressed genes (DEGs) in the LPS versus Control (LPS-vs-Con) and E. tarda versus Control groups (Et-vs-Con), respectively, of which 60 DEGs were shared in two comparation groups. The GO terms were predominantly enriched in the extracellular space, inflammatory response, and cytokine activity in the LPS-vs-Con group, whereas GO terms were predominantly enriched in nucleus and positive regulation of transcription by RNA polymerase II in the Et-vs-Con group. KEGG analysis revealed that three immune-related pathways were co-enriched in both comparison groups, including the Toll-like receptor signaling pathway, C-type lectin receptor signaling pathway, and Cytokine-cytokine receptor interaction. Five genes were randomly screened to confirm the validity and accuracy of the transcriptome data. These results suggest that PoIEC cell line can be an ideal in vitro model for studies of marine fish gut immunity and pathogenesis of Edwardsiellosis.

Mitochondria in health, disease, and aging

Mitochondria are well known as organelles responsible for the maintenance of cellular bioenergetics through the production of ATP. Although oxidative phosphorylation may be their most important function, mitochondria are also integral for the synthesis of metabolic precursors, calcium regulation, the production of reactive oxygen species, immune signaling, and apoptosis. Considering the breadth of their responsibilities, mitochondria are fundamental for cellular metabolism and homeostasis. Appreciating this significance, translational medicine has begun to investigate how mitochondrial dysfunction can represent a harbinger of disease. In this review, we provide a detailed overview of mitochondrial metabolism, cellular bioenergetics, mitochondrial dynamics, autophagy, mitochondrial damage-associated molecular patterns, mitochondria-mediated cell death pathways, and how mitochondrial dysfunction at any of these levels is associated with disease pathogenesis. Mitochondria-dependent pathways may thereby represent an attractive therapeutic target for ameliorating human disease.

Seasonality in Synergism with Multi-Pathogen Presence Leads to Mass Mortalities of the Highly Endangered Pinna nobilis in Greek Coastlines: A Pathophysiological Approach

Mortalities of Pinna nobilis populations set at risk the survival of the species from many Mediterranean coastline habitats. In many cases, both Haplosporidium pinnae and Mycobacterium spp. are implicated in mass mortalities of P. nobilis populations, leading the species into extinction. In the context of the importance of these pathogens' role in P. nobilis mortalities, the present study investigated two Greek populations of the species hosting different microbial loads (one only H. pinnae and the second both pathogens) by the means of pathophysiological markers. More specifically, the populations from Kalloni Gulf (Lesvos Island) and from Maliakos Gulf (Fthiotis), seasonally sampled, were chosen based on the host pathogens in order to investigate physiological and immunological biomarkers to assess those pathogens' roles. In order to determine if the haplosporidian parasite possesses a major role in the mortalities or if both pathogens are involved in these phenomena, a variety of biomarkers, including apoptosis, autophagy, inflammation and heat shock response were applied. The results indicated a decreased physiological performance of individuals hosting both pathogens in comparison with those hosting only H. pinnae. Our findings provide evidence for the synergistic role of those pathogens in the mortality events, which is also enhanced by the influence of seasonality.

Gene expression and involvement of signaling pathways during host-pathogen interplay in Orientia tsutsugamushi infection

Scrub typhus is a neglected tropical disease that affects one-third of the world’s population. The disease is caused by Orientia tsutsugamushi (OT), an obligate intracellular Gram-negative bacterium. OT efficiently escapes from the endosomal pathway after entering the host cell and replicates inside cytosol. OT infection promotes cellular autophagy, the autonomous defense mechanism unlike other bacteria. This study has discussed the bacterial invasion process through the extracellular matrix and the immune response activated by the bacterium within the hosts. Furthermore, we have emphasized the importance of extracellular matrix and their cross-talk with the immune cells, such as, macrophages, neutrophils, and dendritic cells followed by their inflammatory response. We have also put an insight into the host factors associated with signaling pathways during scrub typhus disease with a special focus on the OT-induced stress response, autophagy, apoptosis, and innate immunity. Multiple cytokines and chemokines play a significant role in activating different immune-related signaling pathways. Due to the presence of high antigenic diversity among strains, the signaling pathways during the host–pathogen interplay of OT with its host is very complicated. Thus, it hinders to mitigate the severity of the pandemic occurred by the respective pathogen. Our investigation will provide a useful guide to better understand the virulence and physiology of this intracellular pathogen which will lead towards a better therapeutic diagnosis and vaccine development.

Histopathological, Immunohistochemical and Biochemical Studies of Murine Hepatosplenic Tissues Affected by Chronic Toxoplasmosis

Toxoplasmosis is a serious health problem in humans and animals resulting from obligatory intracellular invasion of reticuloendothelial tissue by Toxoplasma gondii. The profound pathologic effect of toxoplasmosis is confined to nervous tissue, but many other organs, including the liver and spleen, are insulted. Many molecules like caspase-3, CD3, and CD138 are implicated in the tissue immune response in a trial to alleviate hazardous toxoplasmosis impact. This study aimed to investigate the effect of chronic toxoplasmosis on the liver and spleen tissues of mice using biochemical and histopathological techniques and to detect the activity and level of expression of caspase-3, CD3, and CD138 in these tissues using immunohistochemical labeling. Compared with normal control, altered normal histological features accompanied by inflammatory reaction were recorded in hepatosplenic reticuloendothelial tissues in chronically infected mice. The biochemical profile of the liver has been changed in the form of increased liver enzymes, and oxidative stress has been evidenced by elevated nitric oxide (NO) concentration in liver homogenate. The levels of caspase3, CD3, and CD138 were markedly expressed in the liver and spleen of infected mice. Our findings revealed the persistent effect of latent toxoplasmosis on the host's histological architecture, metabolic, and immunological profile, creating a continued challenging host-parasite relationship.

Vibrio splendidus flagellin C binds tropomodulin to induce p38 MAPK-mediated p53-dependent coelomocyte apoptosis in Echinodermata

As a typical pathogen-associated molecular pattern, bacterial flagellin can bind Toll-like receptor 5 and the intracellular NAIP5 receptor component of the NLRC4 inflammasome to induce immune responses in mammals. However, these flagellin receptors are generally poorly understood in lower animal species. In this study, we found that the isolated flagellum of Vibrio splendidus AJ01 destroyed the integrity of the tissue structure of coelomocytes and promoted apoptosis in the sea cucumber Apostichopus japonicus. To further investigate the molecular mechanism, the novel intracellular LRR domain-containing protein tropomodulin (AjTmod) was identified as a protein that interacts with flagellin C (FliC) with a dissociation constant (K_d) of 0.0086 ± 0.33 μM by microscale thermophoresis assay. We show that knockdown of AjTmod also depressed FliC-induced apoptosis of coelomocytes. Further functional analysis with different inhibitor treatments revealed that the interaction between AjTmod and FliC could specifically activate p38 MAPK, but not JNK or ERK MAP kinases. We demonstrate that the transcription factor p38 is then translocated into the nucleus, where it mediates the expression of p53 to induce coelomocyte apoptosis. Our findings provide the first evidence that intracellular AjTmod serves as a novel receptor of FliC and mediates p53-dependent coelomocyte apoptosis by activating the p38 MAPK signaling pathway in Echinodermata.

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.

Targeting Autophagy as a Strategy for Developing New Vaccines and Host-Directed Therapeutics Against Mycobacteria

Mycobacterial disease is an immense burden worldwide. This disease group includes tuberculosis, leprosy (Hansen's disease), Buruli Ulcer, and non-tuberculous mycobacterial (NTM) disease. The burden of NTM disease, both pulmonary and ulcerative, is drastically escalating globally, especially in developed countries such as America and Australia. Mycobacteria's ability to inhibit or evade the host immune system has contributed significantly to its continued prevalence. Pre-clinical studies have highlighted promising candidates that enhance endogenous pathways and/or limit destructive host responses. Autophagy is a cell-autonomous host defense mechanism by which intracytoplasmic cargos can be delivered and then destroyed in lysosomes. Previous studies have reported that autophagy-activating agents, small molecules, and autophagy-activating vaccines may be beneficial in restricting intracellular mycobacterial infection, even with multidrug-resistant strains. This review will examine how mycobacteria evade autophagy and discusses how autophagy could be exploited to design novel TB treatment strategies, such as host-directed therapeutics and vaccines, against Mycobacterium tuberculosis and NTMs.

Differential expression of intestinal genes in necrotic enteritis challenged broiler chickens with 2 different Clostridium perfringens strains

The primary cause of necrotic enteritis (NE) disease in chickens is the NetB-positive Clostridium perfringens bacterium. Many factors are known to affect the severity of NE in the challenge models of broiler chickens, and one of these factors is the virulence of C. perfringens strain. This study was conducted to evaluate the effect of 2 pathogenic C. perfringens strains in a NE challenge model on gut health and mRNA expression of genes encoding apoptosis, tight junction, immunity, and nutrient transporters in broilers. Day-old Ross-308 male broilers (n = 468) were allocated in a 2 × 3 factorial arrangement of treatments with in-feed antibiotics (no or yes) and challenge (Non, C. perfringens strain NE18, and C. perfringens strain NE36) as the factors. The birds in the challenged groups were inoculated with Eimeria species on day 9 and with a fresh suspension of C. perfringens NE18 or NE36 on day 14 and 15. Sample collection was performed on 2 birds of each pen on day 16. Necrotic enteritis challenge, impaired feed conversion ratio during day 0 to 16 compared with the control group where the effect of the NE36 challenge was more severe than that with NE18 (P < 0.001). The mRNA expression of mucin-2, immunoglobulin-G, occludin (P < 0.001), and tight junction protein-1 (P < 0.05) genes were downregulated in both challenged groups compared with the nonchallenged counterparts. Antibiotic supplementation, on the other hand, increased weight gain, and feed intake in all challenged birds (P < 0.01), but upregulated mucin-5ac and alanine, serine, cysteine, and threonine transporter-1 (P < 0.05) only in the NE18 challenged birds. The challenge with NE36 significantly upregulated caspase-8 and claudin-1 (P < 0.001), but downregulated glucose transporter-2 (P < 0.001) compared with the NE18 challenge. These results suggest that NE challenge is detrimental to the performance of broilers through compromised intestinal health, and different C. perfringens strains can affect the severity of the disease through modulating the expression of intestinal genes encoding proteins responsible for apoptosis, gut integrity, immunity, mucus production, and nutrient transporters.

Computational Proteome-Wide Study for the Prediction of Escherichia coli Protein Targeting in Host Cell Organelles and Their Implication in Development of Colon Cancer

Enterohemorrhagic Escherichia coli infection is associated with gastrointestinal disorders, including diarrhea and colorectal cancer. Although evidences have established the involvement of E. coli in the growth of colon cancer, the molecular mechanisms of carcinogenesis of cancer growth and development are not well understood. We analyzed E. coli protein targeting in host cell organelles and the implication in colon cancer using in silico approaches. Our results indicated that many E. coli proteins targeted the endoplasmic reticulum (ER), ER membranes, Golgi apparatus, Golgi apparatus membranes, peroxisomes, nucleus, nuclear membrane, mitochondria, and mitochondrial membrane of host cells. These targeted proteins in ER, Golgi apparatus, peroxisomes, nucleus, and mitochondria may alter the normal functioning of various pathways including DNA repair, apoptosis, replication, transcription, and protein folding in E. coli-infected host cells. The results of the current in silico study provide insights into E. coli pathogenesis and may aid in designing new preventive and therapeutic strategies.

Sodium Butyrate Abrogates the Growth and Pathogenesis of Mycobacterium bovis via Regulation of Cathelicidin (LL37) Expression and NF-κB Signaling

Mycobacterium bovis is the causative agent of bovine tuberculosis, has been identified a serious threat to human population. It has been found that sodium butyrate (NaB), the inhibitor of histone deacetylase, can promote the expression of cathelicidin (LL37) and help the body to resist a variety of injuries. In the current study, we investigate the therapeutic effect of NaB on the regulation of host defense mechanism against M. bovis infection. We found an increased expression of LL37 in M. bovis infected THP-1 cells after NaB treatment. In contrast, NaB treatment significantly down-regulated the expression of Class I HDAC in THP-1 cells infected with M. bovis. Additionally, NaB reduced the expression of phosphorylated P65 (p-P65) and p-IκBα, indicating the inhibition of nuclear factor-κB (NF-κB) signaling. Furthermore, we found that NaB treatment reduced the production of inflammatory cytokines (IL-1β, TNF-α, and IL-10) and a key anti-apoptotic marker protein Bcl-2 in THP-1 cell infected with M. bovis. Notably, mice showed high resistance to M. bovis infection after NaB treatment. The reduction of viable M. bovis bacilli indicates that NaB-induced inhibition of M. bovis infection mediated by upregulation of LL37 and inhibition of NF-κB signaling pathway. These observations illustrate that NaB mediate protective immune responses against M. bovis infection. Overall, these results suggest that NaB can be exploited as a therapeutic strategy for the control of M. bovis in animals and human beings.

Roles of EvpP in Edwardsiella piscicida-Macrophage Interactions

Edwardsiella piscicida is found to be an important facultative intracellular pathogen with a broad host range. These organisms can replicate and survive within host macrophages to escape from the subversion of the immune defense. E. piscicida-macrophage interaction is very important in determining the outcome of edwardsiellasis. As an effector protein of E. piscicida T6SS, EvpP has been determined to be a very important virulence factor for E. piscicida, although its precise role in E. piscicida-macrophage interactions is not yet clear. In this study, the roles of EvpP in E. piscicida-macrophage interactions were characterized. Here, we constructed the deletion mutants of evpP (ΔevpP) and complementation (ΔevpP-C) by the allelic exchange method. Compared to wild type strain (WT), ΔevpP was found to be attenuated for growth within macrophages. In line with this observation, we found its survival capacity was lower than WT under oxidative and acid stress in vitro, which simulate conditions encountered in host macrophages. Attenuation of ΔevpP also correlated with enhanced activation of macrophages, as reflected by augmented NO production in ΔevpP-treated macrophages. Moreover, compared to WT, ΔevpP induced markedly increased apoptosis of macrophages, characterized by increased Annexin V binding and the activation of cleaved caspase-3. These findings provided strong evidence that EvpP is involved in the process of E. piscicida-macrophage interactions and is required for its survival and replication in macrophages. Thus, we propose that EvpP might be an important factor that controlling the fate of E. piscicida inside macrophages. To further exploring the underlying mechanism of EvpP action, the cDNA library was constructed from E. piscicida-infected macrophages and a yeast two-hybrid screen was performed to search for cellular proteins interacting with EvpP. Ribosomal protein S5 (RPS5) was identified as a target of EvpP. Furthermore, the interaction was validated with co-immunoprecipitation assay. This result implies that the observed effect of EvpP on macrophages might be related to RPS5-mediated regulation, contributing to a better understanding of the mechanisms of EvpP involved in E. piscicida-macrophage interactions.

Effects of Vitamin D2 (Ergocalciferol) and D3 (Cholecalciferol) on Atlantic Salmon (Salmo salar) Primary Macrophage Immune Response to Aeromonas salmonicida subsp. salmonicida Infection

Vitamin D₂ (ergocalciferol) and vitamin D₃ (cholecalciferol) are fat-soluble secosteroid hormones obtained from plant and animal sources, respectively. Fish incorporates vitamin D₂ and D₃ through the diet. In mammals, vitamin D forms are involved in mineral metabolism, cell growth, tissue differentiation, and antibacterial immune response. Vitamin D is an essential nutrient in aquafeeds for finfish. However, the influence of vitamin D on fish cell immunity has not yet been explored. Here, we examined the effects of vitamin D₂ and vitamin D₃ on Salmo salar primary macrophage immune response to A. salmonicida subspecies salmonicida infection under in vitro conditions. We determined that high concentrations of vitamin D₂ (100,000 ng/ml) and D₃ (10,000 ng/ml) affect the growth of A. salmonicida and decrease the viability of S. salar primary macrophages. In addition, we determined that primary macrophages pre-treated with a biologically relevant concentration of vitamin D₃ for 24 h showed a decrease of A. salmonicida infection. In contrast, vitamin D₂ did not influence the antibacterial activity of the S. salar macrophages infected with A. salmonicida. Vitamin D₂ and D₃ did not influence the expression of canonical genes related to innate immune response. On the other hand, we found that A. salmonicida up-regulated the expression of several canonical genes and suppressed the expression of leukocyte-derived chemotaxin 2 (lect-2) gene, involved in neutrophil recruitment. Primary macrophages pre-treated for 24 h with vitamin D₃ counteracted this immune suppression and up-regulated the transcription of lect-2. Our results suggest that vitamin D₃ affects A. salmonicida attachment to the S. salar primary macrophages, and as a consequence, the A. salmonicida invasion decreased. Moreover, our study shows that the positive effects of vitamin D₃ on fish cell immunity seem to be related to the lect-2 innate immunity mechanisms. We did not identify positive effects of vitamin D₂ on fish cell immunity. In conclusion, we determined that the inactive form of vitamin D₃, cholecalciferol, induced anti-bacterial innate immunity pathways in Atlantic salmon primary macrophages, suggesting that its utilization as a component of a healthy aquafeed diet in Atlantic salmon could enhance the immune response against A. salmonicida.

Macrophages Infected by a Pathogen and a Non-pathogen Spotted Fever Group Rickettsia Reveal Differential Reprogramming Signatures Early in Infection

Despite their high degree of genomic similarity, different spotted fever group (SFG) Rickettsia are often associated with very different clinical presentations. For example, Rickettsia conorii causes Mediterranean spotted fever, a life-threatening disease for humans, whereas Rickettsia montanensis is associated with limited or no pathogenicity to humans. However, the molecular basis responsible for the different pathogenicity attributes are still not understood. Although killing microbes is a critical function of macrophages, the ability to survive and/or proliferate within phagocytic cells seems to be a phenotypic feature of several intracellular pathogens. We have previously shown that R. conorii and R. montanensis exhibit different intracellular fates within macrophage-like cells. By evaluating early macrophage responses upon insult with each of these rickettsial species, herein we demonstrate that infection with R. conorii results in a profound reprogramming of host gene expression profiles. Transcriptional programs generated upon infection with this pathogenic bacteria point toward a sophisticated ability to evade innate immune signals, by modulating the expression of several anti-inflammatory molecules. Moreover, R. conorii induce the expression of several pro-survival genes, which may result in the ability to prolong host cell survival, thus protecting its replicative niche. Remarkably, R. conorii-infection promoted a robust modulation of different transcription factors, suggesting that an early manipulation of the host gene expression machinery may be key to R. conorii proliferation in THP-1 macrophages. This work provides new insights into the early molecular processes hijacked by a pathogenic SFG Rickettsia to establish a replicative niche in macrophages, opening several avenues of research in host-rickettsiae interactions.

Identification of Chlamydia pneumoniae candidate genes that interact with human apoptotic factor caspase-9

Chlamydia pneumoniae is an obligate intracellular pathogen responsible for respiratory diseases, including pneumonia and bronchitis, and is highly involved in chronic diseases, including atherosclerosis, asthma, and Alzheimer's disease. We previously showed that the host apoptotic factor caspase-9 played a crucial role for chlamydial multiplication and host apoptosis inhibition by chlamydial infection. To identify chlamydial genes interacting with human caspase-9, yeast two-hybrid screening was performed and 5 chlamydial genes, including Cpj0838 and pmpG were isolated from the C. pneumoniae genomic library. Pull-down experiments showed that caspase-9 physically bound to the Cpj0838 product and chlamydial cells, which contain PmpG proteins. This study could provide a clue to understanding host-Chlamydia interactions, especially the apoptosis repression by Chlamydia infection.

An Update on Host-Pathogen Interplay and Modulation of Immune Responses during Orientia tsutsugamushi Infection

The obligate intracellular bacterium Orientia tsutsugamushi is the causative agent of scrub typhus in humans, a serious mite-borne disease present in a widespread area of endemicity, which affects an estimated 1 million people every year. This disease may exhibit a broad range of presentations, ranging from asymptomatic to fatal conditions, with the latter being due to disseminated endothelial infection and organ injury. Unique characteristics of the biology and host-pathogen interactions of O. tsutsugamushi, including the high antigenic diversity among strains and the highly variable, short-lived memory responses developed by the host, underlie difficulties faced in the pursuit of an effective vaccine, which is an imperative need. Other factors that have hindered scientific progress relative to the infectious mechanisms of and the immune response triggered by this bacterium in vertebrate hosts include the limited number of mechanistic studies performed on animal models and the lack of genetic tools currently available for this pathogen. However, recent advances in animal model development are promising to improve our understanding of host-pathogen interactions. Here, we comprehensively discuss the recent advances in and future perspectives on host-pathogen interactions and the modulation of immune responses related to this reemerging disease, highlighting the role of animal models.

Cellular Exit Strategies of Intracellular Bacteria

The coevolution of intracellular bacteria with their eukaryotic hosts has presented these pathogens with numerous challenges for their evolutionary progress and survival. Chief among these is the ability to exit from host cells, an event that is fundamentally linked to pathogen dissemination and transmission. Recent years have witnessed a major expansion of research in this area, and this chapter summarizes our current understanding of the spectrum of exit strategies that are exploited by intracellular pathogens. Clear themes regarding the mechanisms of microbial exit have emerged and are most easily conceptualized as (i) lysis of the host cell, (ii) nonlytic exit of free bacteria, and (iii) release of microorganisms into membrane-encased compartments. The adaptation of particular exit strategies is closely linked with additional themes in microbial pathogenesis, including host cell death, manipulation of host signaling pathways, and coincident activation of proinflammatory responses. This chapter will explore the molecular determinants used by intracellular pathogens to promote host cell escape and the infectious advantages each exit pathway may confer, and it will provide an evolutionary framework for the adaptation of these mechanisms.

Differential expression of virulence genes in Legionella pneumophila growing in Acanthamoeba and human monocytes

Legionella pneumophila, the causative agent of Legionnaires’ disease, is widely distributed throughout natural and artificial water systems and can replicate in macrophages and amoebae. Amoebae are the natural hosts of L. pneumophila, whereas macrophages are incidentally infected. The life cycle of L. pneumophila comprises a replicative phase within the Legionella-containing vacuole (LCV) and a transmissive phase during which bacterial cells become motile and are released via killing of the host. Although the host death mechanisms induced by L. pneumophila have been studied, the expression patterns of related L. pneumophila genes have not been reported. The present study compared the expression patterns of host cell death-associated genes in L. pneumophila grown in the human monocytic cell line THP-1 and Acanthamoeba castellanii. Notably, when L. pneumophila was grown in THP-1, expression of the gene flaA, which is involved in the induction of pyroptosis, was downregulated during the course of infection. In contrast, sdhA associated indirectly with host death, was upregulated. Expression of the genes vipD and sidF, which are involved in the induction and suppression of apoptosis, changed by less than 2-fold. Notably, a lower percentage of pyroptotic cells was observed among infected THP-1 cells relative to uninfected cells, and the latter exhibited stronger expression of caspase-1. A different pattern was observed when L. pneumophila was grown in A. castellanii: flaA and vipD were activated, whereas sdhA and sidF were downregulated during the later stage of replication. The percentage of non-viable (annexin-V⁺ PI⁺ or annexin-V⁺PI⁻) A. castellanii organisms increased with Legionella infection, and the expression of metacaspase-1, which is involved in encystation was up-regulated at late infection time. In summary, L. pneumophila can multiply intracellularly in both amoebae and macrophages to induce cell death and secondary infection, and this characteristic is essential for its survival in water and the lungs. The gene expression profiles observed in this study indicated the increased cytotoxicity of L. pneumophila in A. castellanii, suggesting an increased adaptation of Legionella to this host.

Lactoferricin Peptides Increase Macrophages' Capacity To Kill Mycobacterium avium

The genus Mycobacterium comprises several pathogenic species, including M. tuberculosis, M. leprae, M. avium, etc. Infections caused by these bacteria are particularly difficult to treat due to their intrinsic impermeability, low growth rate, and intracellular localization. Antimicrobial peptides are increasingly acknowledged as potential treatment tools, as they have a high spectrum of activity, low tendency to induce bacterial resistance, and immunomodulatory properties. In this study, we show that peptides derived from bovine lactoferricin (LFcin) improve the antimicrobial activity of ethambutol against Mycobacterium avium growing inside macrophages. Moreover, the d-enantiomer of a short version of lactoferricin containing amino acids 17 to 30 (d-LFcin17–30) causes intramacrophagic death of M. avium by increasing the formation of lysosomes and autophagosomes. This work opens the way to the use of lactoferricin-derived peptides to treat infections caused by mycobacteria and highlights important modulatory effects of d-FLcin17–30 on macrophages, which may be useful under other conditions in which macrophage activation is needed.

Mycobacterial infections cause a significant burden of disease and death worldwide. Their treatment is long, toxic, costly, and increasingly prone to failure due to bacterial resistance to currently available antibiotics. New therapeutic options are thus clearly needed. Antimicrobial peptides represent an important source of new antimicrobial molecules, both for their direct activity and for their immunomodulatory potential. We have previously reported that a short version of the bovine antimicrobial peptide lactoferricin with amino acids 17 to 30 (LFcin17–30), along with its variants obtained by specific amino acid substitutions, killed Mycobacterium avium in broth culture. In the present work, those peptides were tested against M. avium living inside its natural host cell, the macrophage. We found that the peptides increased the antimicrobial action of the conventional antibiotic ethambutol inside macrophages. Moreover, the d-enantiomer of the lactoferricin peptide (d-LFcin17–30) was more stable and induced significant killing of intracellular mycobacteria by itself. Interestingly, d-LFcin17–30 did not localize to M. avium-harboring phagosomes but induced the production of proinflammatory cytokines and increased the formation of lysosomes and autophagosome-like vesicles. These results lead us to conclude that d-LFcin17–30 primes macrophages for intracellular microbial digestion through phagosomal maturation and/or autophagy, culminating in mycobacterial killing.

IMPORTANCE The genus Mycobacterium comprises several pathogenic species, including M. tuberculosis, M. leprae, M. avium, etc. Infections caused by these bacteria are particularly difficult to treat due to their intrinsic impermeability, low growth rate, and intracellular localization. Antimicrobial peptides are increasingly acknowledged as potential treatment tools, as they have a high spectrum of activity, low tendency to induce bacterial resistance, and immunomodulatory properties. In this study, we show that peptides derived from bovine lactoferricin (LFcin) improve the antimicrobial activity of ethambutol against Mycobacterium avium growing inside macrophages. Moreover, the d-enantiomer of a short version of lactoferricin containing amino acids 17 to 30 (d-LFcin17–30) causes intramacrophagic death of M. avium by increasing the formation of lysosomes and autophagosomes. This work opens the way to the use of lactoferricin-derived peptides to treat infections caused by mycobacteria and highlights important modulatory effects of d-FLcin17–30 on macrophages, which may be useful under other conditions in which macrophage activation is needed.

Targeting calpains: A novel immunomodulatory approach for microbial infections

Calpains are a family of Ca²⁺ dependent cytosolic non-lysosomal proteases with well conserved cysteine-rich domains for enzymatic activity. Due to their functional dependency on Ca²⁺ concentrations, they are involved in various cellular processes that are regulated by intracellular ca²⁺ concentration (i.e. embryo development, cell development and migration, maintenance of cellular architecture and structure etc.). Calpains are widely studied proteases in mammalian (i.e. mouse and human) physiology and pathophysiology due to their ubiquitous presence. For example, these proteases have been found to be involved in various inflammatory disorders such as neurodegeneration, cancer, brain and myocardial ischemia and infarction, cataract and muscular dystrophies etc. Besides their role in these sterile inflammatory conditions, calpains have also been shown to regulate a wide range of infectious diseases (i.e. sepsis, tuberculosis, gonorrhoea and bacillary dysentery etc.). One of these regulatory mechanisms mediated by calpains (i.e. calpain 1 and 2) during microbial infections involves the regulation of innate immune response, inflammation and cell death. Thus, the major emphasis of this review is to highlight the importance of calpains in the pathogenesis of various microbial (i.e. bacterial, fungal and viral) diseases and the use of calpain modulators as potential immunomodulators in microbial infections.

Swine IRF3/IRF7 attenuates inflammatory responses through TLR4 signaling pathway

To explore the role of IRF3/IRF7 during inflammatory responses, we investigated the effects of swine IRF3/IRF7 on TLR4 signaling pathway and inflammatory factors expression in porcine kidney epithelial PK15 cell lines. We successfully constructed eukaryotic vectors PB-IRF3 and PB-IRF7, transfected these vectors into PK15 cells and observed GFP under a fluorescence microscope. In addition, RT-PCR was also used to detect transfection efficiency. We found that IRF3/IRF7 was efficiently overexpressed in PK15 cells. Moreover, we evaluated the effects of IRF3/IRF7 on the TLR4 signaling pathway and inflammatory factors by RT-PCR. Transfected cells were treated with lipopolysaccharide (LPS) alone, or in combination with a TBK1 inhibitor (LiCl). We revealed that IRF3/IRF7 enhanced IFNα production, and decreased IL-6 mRNA expression. Blocking the TBK1 pathway, inhibited the changes in IFNα, but not IL-6 mRNA. This illustrated that IRF3/IRF7 enhanced IFNα production through TLR4/TBK1 signaling pathway and played an anti-inflammatory role, while IRF3/IRF7 decreased IL-6 expression independent of the TBK1 pathway. Trends in MyD88, TRAF6, TBK1 and NFκB mRNA variation were similar in all treatments. LPS increased MyD88, TRAF6, TBK1 and NFκB mRNA abundance in PBR3/PBR7 and PBv cells, while LiCl blocked the LPS-mediated effects. The levels of these four factors in PBR3/PBR7 cells were higher than those in PBv. These results demonstrated that IRF3/IRF7 regulated the inflammatory response through the TLR4 signaling pathway. Overexpression of swine IRF3/IRF7 in PK15 cells induced type I interferons production, and attenuated inflammatory responses through TLR4 signaling pathway.

Heterogeneous Family of Cyclomodulins: Smart Weapons That Allow Bacteria to Hijack the Eukaryotic Cell Cycle and Promote Infections

Some bacterial pathogens modulate signaling pathways of eukaryotic cells in order to subvert the host response for their own benefit, leading to successful colonization and invasion. Pathogenic bacteria produce multiple compounds that generate favorable conditions to their survival and growth during infection in eukaryotic hosts. Many bacterial toxins can alter the cell cycle progression of host cells, impairing essential cellular functions and impeding host cell division. This review summarizes current knowledge regarding cyclomodulins, a heterogeneous family of bacterial effectors that induce eukaryotic cell cycle alterations. We discuss the mechanisms of actions of cyclomodulins according to their biochemical properties, providing examples of various cyclomodulins such as cycle inhibiting factor, γ-glutamyltranspeptidase, cytolethal distending toxins, shiga toxin, subtilase toxin, anthrax toxin, cholera toxin, adenylate cyclase toxins, vacuolating cytotoxin, cytotoxic necrotizing factor, Panton-Valentine leukocidin, phenol soluble modulins, and mycolactone. Special attention is paid to the benefit provided by cyclomodulins to bacteria during colonization of the host.

In vitro model to estimate Edwardsiella tarda-macrophage interactions using RAW264.7 cells

Edwardsiella tarda has been recognized as an important facultative intracellular pathogen of fish with capability of survival and replication within macrophages. E. tarda-macrophage interactions play a very important role in the defense mechanism of fish against infection. The mechanisms that E. tarda use to infect and persist inside macrophages are not well characterized. To gain insight concerning this process, RAW264.7 cells was used to investigate the interactions between E. tarda and macrophages. Using an in vitro model involving RAW264.7 cells, internalization assay demonstrated that MOIs of 10:1 and 100:1 could result in a satisfactory infection rate after a 2 h infection period. Consistent with the performance in fish macrophages, E. tarda could survive, replicate and induce iNOS-mediated NO production in RAW264.7 cells. Light and electron microscopy confirmed the internalization and replication of E. tarda in RAW264.7 cells, showing once inside macrophages, numberous bacteria may be destroyed within phagolysosomes and those that successfully subvert phagocyte defenses are capable of extensively replicating within the vacuolar-like compartment in macrophages. In addition, E. tarda-induced apoptosis was observed in RAW264.7 cells in a dose-and time-dependent manner, characterized by increased Annexin V binding and the activation of caspase-3. The results described here indicate that RAW264.7 cells could model the behavior of fish macrophages in response to E. tarda in many ways and may serve as a cell model for study on interactions between E. tarda and macrophages. The successful establishment of E. tarda-invaded RAW264.7 cells model may contribute to providing a basis for more detailed understanding of E. tarda pathogenesis.

OMP31 of Brucella melitensis 16M impairs the apoptosis of macrophages triggered by TNF-α

Outer membrane proteins (OMPs) of microorganisms play important roles in directly interacting with host cells. Brucella species inhibit the apoptosis of host cells to benefit their own intracellular survival and replication. However, the association between OMP31 of Brucella and host cell apoptosis, and the underlying mechanism are unclear. In this study, an OMP31 gene deletion mutant based on B. melitensis 16M was constructed. Following the infection of RAW264.7 cells with B. melitensis 16M or the mutant strain, colony formation, apoptosis, tumor necrosis factor (TNF)-α levels and the levels of key downstream factors of the apoptosis pathways triggered by TNF-α, namely caspase-3, -8 and -9, cytochrome c, B-cell lymphoma 2 (Bcl-2) and Bcl-2-associated X protein (Bax) were detected. The mutant strain was shown to have the same phenotype as the parent strain using traditional microbiological tests. However, the mutant strain had impaired intracellular survival, with higher levels of apoptosis and TNF-α expression in infected RAW164.7 macrophages than the parent strain. The downstream factors of apoptosis triggered by TNF-α, including increased caspase-8, -3 and -9, cytochrome c and Bax, and decreased Bcl-2, indicated that the classical and mitochondrial cell death pathways were involved. It may be concluded that OMP31 from Brucella inhibited apoptosis and benefitted the intracellular survival of this microorganism. Furthermore, TNF-α may have served as a switch triggering classical death and mitochondrial cell death pathways.

Caspase-3 serves as an intracellular immune receptor specific for lipopolysaccharide in oyster Crassostrea gigas

Apoptosis is a form of programmed cell death process controlled by a family of cysteine proteases called caspases, which plays a crucial role in the immune system homeostasis. The apoptosis and the detailed regulation mechanism have been well studied in vertebrate, but the information in lower animals, especially invertebrates, is still very limited. In the present study, Caspase-3 in the Pacific oyster Crassostrea gigas (designated CgCaspase-3) was enriched by lipopolysaccharide (LPS) affinity chromatography and further identified by MALDI-TOF/TOF-mass spectrometry. The binding activity of CgCaspase-3 to LPS was confirmed by enzyme-linked immunosorbent assay, and surface plasmon resonance analysis revealed its high binding specificity and moderate binding affinity (KD = 1.08 × 10(-6) M) to LPS. The recombinant CgCaspase-3 exhibited high proteolytic activity to substrate Ac-DEVD-pNA and relatively weak activity to substrate Ac-DMQD-pNA and Ac-VDQQD-pNA. The binding of CgCaspase-3 to LPS significantly inhibited its proteolytic activity toward AC-DEVD-pNA in vitro. The over-expression of CgCaspase-3 leaded to the phosphatidylserine exposure on the external plasma membrane and the cleavage of poly (ADP-ribose) polymerase, which reduced cell viability, and finally induced cell apoptosis. But the cell apoptosis mediated by CgCaspase-3 in vivo was significantly inhibited by the treatment of LPS. These results collectively indicated that CgCaspase-3 could serve as an intracellular LPS receptor, and the interaction of LPS with CgCaspase-3 specifically inhibited the cell apoptosis induced by CgCaspase-3.

An epistatic effect of apaf-1 and caspase-9 on chlamydial infection

Chlamydia is an obligate intracellular bacterial pathogen that replicates solely within a membrane-bound vacuole termed an inclusion. Chlamydia seems to perturb multiple cellular processes of the host, such as, rearrangement of the membrane trafficking system for its intracellular multiplication, and inhibition of host cell apoptosis for persistent infection. In an attempt to clarify host factor involvement in apoptosis regulation, we found that inhibition of Caspase-9 restricted, while Apaf-1 promoted, Chlamydia pneumoniae infection in HEp-2, HeLa, and mouse epithelial fibroblast (MEF) cells. These opposition contributions to the chlamydial infection were confirmed using caspase-9^−/− and apaf-1^−/− MEFs. Similar phenomena also appeared in the case of infection with Chlamydia trachomatis. Interestingly, caspase-9 in apaf-1^−/− MEFs was activated by chlamydial infection but during the infection caspase-3 was not activated. That is, caspase-9 was activated without support for multiplication and activation by Apaf-1, and the activated caspase-9 may be physically disconnected from the caspase cascade. This may be partially explained by the observation of caspase-9 accumulation within chlamydial inclusions. The sequestration of caspase-9 by chlamydia seems to result in apoptosis repression, which is crucial for the chlamydial development cycle. Because Apaf-1 shares domains with intracellular innate immune receptor NOD1, it may play a key role in the strategy to regulate chlamydial infection.

Role of eosinophils and apoptosis in PDIMs/PGLs deficient mycobacterium elimination in adult zebrafish

The cell wall lipids phthiocerol dimycocerosates (PDIMs) and its structurally-related compound, phenolic glycolipids (PGLs) are major virulence factors of mycobacterium, as shown by the reduced growth of PDIMs/PGLs deficient mutants in various animal models. PDIMs/PGLs play active roles in modulating host immune responses. However, the cellular and molecular mechanisms of how PDIMs/PGLs deficient mutant was eliminated in vivo are still elusive. Our aim was to investigate what host immune responses have effect on mycobacterium elimination in vivo. Using microarray, we find PDIMs/PGLs modulate divergent host responses, including chemotaxis and focal adhesion's downstream pathway and apoptosis. We examine these two host responses by Diff-Quik stain, coupled with transmission electron microscopy and TUNEL stain respectively. The ultrastructure observation showed that eosinophils appeared in WT-infected zebrafish at day 1, however eosinophils arrived was delayed to day 7 in PDIMs/PGLs-deficient mutant-infected animals. More intriguingly, apoptosis was markedly increased in PDIMs/PGLs-mutant infected zebrafish at day 1 after infection, compared to WT-infected fishes at this time. However, apoptosis trend was fully reversed by day 7, with increased apoptosis were detected in WT-infected zebrafish compared with the PDIMs/PGLs-deficient mutant, especially more apoptosis within the granuloma. This study shows that the anti-apoptotic effects of PDIMs/PGLs and the recruitment of eosinophils in tissue during the early infection in zebrafish might promote bacterium growth in vivo.

Leishmania mexicana amastigotes inhibit p38 and JNK and activate PI3K/AKT: role in the inhibition of apoptosis of dendritic cells

Leishmania mexicana is the causal agent of cutaneous leishmaniasis in Mexico. Dendritic cells (DC) are one of the host cells of Leishmania parasites. Intracellular microorganisms inhibit host cell apoptosis as a strategy to ensure their survival in infected cells. We have previously shown that Leishmania mexicana promastigotes and amastigotes inhibit camptothecin-induced apoptosis of monocyte-derived dendritic cells (moDC), but the mechanisms underlying the inhibition of apoptosis of DC by Leishmania have not been established. MAP kinases and PI3K participate in the process of apoptosis and are modulated by different species of Leishmania. As shown in this study, the infection of moDC with L. mexicana amastigotes diminished significantly the phosphorylation of the MAP kinases p38 and JNK. The inhibition of both kinases diminished significantly DNA fragmentation in moDC stimulated with camptothecin. On the other hand, L. mexicana amastigotes were able to activate the anti-apoptotic pathways PI3K and AKT. Our results indicate that L. mexicana amastigotes have the capacity to diminish MAP kinases activation and activate PI3K and AKT, which is probably one of the strategies employed by L. mexicana amastigotes to inhibit apoptosis in the infected moDC.

Leishmania mexicana promastigotes down regulate JNK and p-38 MAPK activation: Role in the inhibition of camptothecin-induced apoptosis of monocyte-derived dendritic cells

Dendritic cells (DC) are one of the principal host cells of the obligate intracellular parasite Leishmania. Inhibition of host cell apoptosis is a strategy employed by multiple pathogens to ensure their survival in the infected cell. We have previously shown that the infection of monocyte-derived dendritic cells (moDC) with Leishmania mexicana inhibits campthotecin-induced apoptosis. Nevertheless, the mechanisms involved in the inhibition of apoptosis of dendritic cells by Leishmania have not been established. Mitogen-activated protein kinases (MAPK) are key participants in the process of apoptosis and different species of Leishmania have been shown to regulate these kinases. In the present study, we analyzed the effect of L. mexicana promastigotes in the activation of JNK and p38 MAP kinase and their participation in the inhibition of apoptosis. The infection of moDC with L. mexicana promastigotes diminished significantly the phosphorylation of the MAP kinases JNK and p38. The inhibition of both kinases diminished DNA fragmentation, but in a major extent was the reduction of DNA fragmentation when JNK was inhibited. The capacity of L. mexicana promastigotes to diminish MAP kinases activation is probably one of the strategies employed to delay apoptosis induction in the infected moDC and may have implications for Leishmania pathogenesis by favoring the invasion of its host and the persistence of the parasite in the infected cells.

Subversion of Cell-Autonomous Host Defense by Chlamydia Infection

Obligate intracellular bacteria entirely depend on the metabolites of their host cell for survival and generation of progeny. Due to their lifestyle inside a eukaryotic cell and the lack of any extracellular niche, they have to perfectly adapt to compartmentalized intracellular environment of the host cell and counteract the numerous defense strategies intrinsically present in all eukaryotic cells. This so-called cell-autonomous defense is present in all cell types encountering Chlamydia infection and is in addition closely linked to the cellular innate immune defense of the mammalian host. Cell type and chlamydial species-restricted mechanisms point a long-term evolutionary adaptation that builds the basis of the currently observed host and cell-type tropism among different Chlamydia species. This review will summarize the current knowledge on the strategies pathogenic Chlamydia species have developed to subvert and overcome the multiple mechanisms by which eukaryotic cells defend themselves against intracellular pathogens.

Exploiting the ubiquitin and phosphoinositide pathways by the Legionella pneumophila effector, SidC

Intracellular bacterial pathogens use secreted effector proteins to alter host cellular processes, with the goal of subverting host defenses and allowing the infection to progress. One such pathogen, Legionella pneumophila, secretes ~300 proteins into its host to alter a number of pathways including intracellular trafficking, phosphoinositide metabolism, and cell signaling. The Legionella effector SidC was previously found to bind to PI(4)P and was responsible for the enrichment of ER proteins and ubiquitinated species on the Legionella-containing vacuoles. Through our recent work, we have discovered that SidC contains a unique N-terminal E3 ubiquitin ligase domain and a C-terminal novel PI(4)P-binding domain. Our results demonstrate that SidC serves to link two distinct cellular pathways, ubiquitin and phosphoinositide. However, how the ubiquitin ligase activity regulates host membrane trafficking events remains to be investigated.

Influence of the Paracoccidioides brasiliensis 14-3-3 and gp43 proteins on the induction of apoptosis in A549 epithelial cells

The fungal strain Paracoccidioides brasiliensis remains viable inside of epithelial cells and can induce apoptosis in this population. However, until now, the molecules that participate in this process remained unknown. Thus, this study evaluated the contribution of two P. brasiliensis molecules, the 14-3-3 and glycoprotein of 43 kDa proteins, which had been previously described as extracellular matrix adhesins and apoptosis inductors in human pneumocytes. Accordingly, epithelial cells were treated with these molecules for different periods of time and the expression of the apoptosis regulating-proteins Bak, Bax, Bcl-2, p53 and caspases were evaluated by terminal deoxynucleotidyl transferase dUTP nick end labelling, flow cytometry and real-time polymerase chain reaction analysis. Our results demonstrated that treatment with these molecules induces apoptosis signalling in pulmonary epithelial cells, showing the same pattern of programmed cell-death as that observed during infection with P. brasiliensis. Thus, we could conclude that P. brasiliensis uses these molecules as virulence factors that participate not only in the fungal adhesion process to host cells, but also in other important cellular mechanisms such as apoptosis.

Protective effect of melittin against inflammation and apoptosis on Propionibacterium acnes-induced human THP-1 monocytic cell

Melittin is a cationic, hemolytic peptide that is the main toxic component in the venom of the honey bee (Apis mellifera). It has been used in treatment of various chronic inflammatory diseases. However, the cellular mechanism and the anti-apoptotic effect of melittin in Propionibactierium acnes (P. acnes)-induced THP-1 cells have not been explored. In the present study, we investigated the anti-inflammatory and anti-apoptotic mechanism by examining the effect of melittin on P. acnes-induced THP-1 monocytic cells. THP-1 monocytic cells were stimulated by heat-killed P. acnes in the presence of melittin. The expression levels of pro-inflammatory cytokines, NF-κB signaling, caspase family, and PARP signaling were measured by ELISA or Western blot analysis. The number of apoptotic cells and changes of cell morphology were examined using fluorescence microscopy and flow cytometry. Heat-killed P. acnes increased the secretion of pro-inflammatory cytokines and cleavage of caspase-3 and -8 in heat-killed P. acnes-induced THP-1 cells. However, treatment with melittin inhibited the pro-inflammatory cytokines and cleavage of the caspase-3 and -8. Moreover, the cleaved PARP appeared after 8h of heat-killed P. acnes treatment and its cleavage was reduced by melittin treatment. These results demonstrate that 1.0×10(7) CFU/ml of heat-killed P. acnes induces THP-1 cell apoptosis and secretion of inflammatory cytokines. Also, administration of melittin significantly decreases the expression of various inflammatory cytokines in heat-killed P. acnes-treated THP-1 monocytic cells. In particular, melittin exerts anti-apoptotic effects against 1.0×10(7) CFU/ml of heat-killed P. acnes injury to THP-1 cells.

Microorganisms and Biotic Interactions

Most ecosystems are populated by a large number of diversified microorganisms, which interact with one another and form complex interaction networks. In addition, some of these microorganisms may colonize the surface or internal parts of plants and animals, thereby providing an additional level of interaction complexity. These microbial relations range from intraspecific to interspecific interactions, and from simple short-term interactions to intricate long-term ones. They have played a key role in the formation of plant and animal kingdoms, often resulting in coevolution; they control the size, activity level, and diversity patterns of microbial communities. Therefore, they modulate trophic networks and biogeochemical cycles, regulate ecosystem productivity, and determine the ecology and health of plant and animal partners. A better understanding of these interactions is needed to develop microbe-based ecological engineering strategies for environmental sustainability and conservation, to improve environment-friendly approaches for feed and food production, and to address health challenges posed by infectious diseases. The main types of biotic interactions are presented: interactions between microorganisms, interactions between microorganisms and plants, and interactions between microorganisms and animals.

Helicobacter pylori sensitizes TNF-related apoptosis-inducing ligand (TRAIL)-mediated apoptosis in human gastric epithelial cells through regulation of FLIP

Helicobacter pylori (H. pylori) infection is associated with chronic gastritis, peptic ulcer and gastric cancer. Apoptosis induced by microbial infections is implicated in the pathogenesis of H. pylori infection. Here we show that human gastric epithelial cells sensitized to H. pylori confer susceptibility to TRAIL-mediated apoptosis via modulation of death receptor signaling. Human gastric epithelial cells are intrinsically resistant to TRAIL-mediated apoptosis. The induction of TRAIL sensitivity by H. pylori is dependent on the activation of caspase-8 and its downstream pathway. H. pylori induces caspase-8 activation via enhanced assembly of the TRAIL death-inducing signaling complex (DISC) through downregulation of cellular FLICE-inhibitory protein (FLIP). Overexpression of FLIP abolished the H. pylori-induced TRAIL sensitivity in human gastric epithelial cells. Our study thus demonstrates that H. pylori induces sensitivity to TRAIL apoptosis by regulation of FLIP and assembly of DISC, which initiates caspase activation, resulting in the breakdown of resistance to apoptosis, and provides insight into the pathogenesis of gastric damage in Helicobacter infection. Modulation of host apoptosis signaling by bacterial interaction adds a new dimension to the pathogenesis of Helicobacter.

Brucella dissociation is essential for macrophage egress and bacterial dissemination

It has long been observed that smooth Brucella can dissociate into rough mutants that are cytotoxic to macrophages. However, the in vivo biological significance and/or mechanistic details of Brucella dissociation and cytotoxicity remain incomplete. In the current report, a plaque assay was developed using Brucella strains exhibiting varying degrees of cytotoxicity. Infected monolayers were observed daily using phase contrast microscopy for plaque formation while Brucella uptake and replication were monitored using an immunofluorescence assay (IFA). Visible plaques were detected at 4-5 days post infection (p.i.) with cytotoxic Brucella 16MΔmanBA at an MOI of 0.1. IFA staining demonstrated that the plaques consisted of macrophages with replicating Brucella. Visible plaques were not detected in monolayers infected with non-cytotoxic 16MΔmanBAΔvirB2 at an MOI of 0.1. However, IFA staining did reveal small groups of macrophages (foci) with replicating Brucella in the monolayers infected with 16MΔmanBAΔvirB2. The size of the foci observed in macrophage monolayers infected with rough Brucella correlated directly with cytotoxicity measured in liquid culture, suggesting that cytotoxicity was essential for Brucella egress and dissemination. In monolayers infected with 16M, small and large foci were observed. Double antibody staining revealed spontaneous rough mutants within the large, but not the small foci in 16M infected monolayers. Furthermore, plaque formation was observed in the large foci derived from 16M infections. Finally, the addition of gentamicin to the culture medium inhibited plaque formation, suggesting that cell-to-cell spread occurred only following release of the organisms from the cells. Taken together, these results demonstrate that Brucella-induced cytotoxicity is critical for Brucella egress and dissemination.

Leishmania donovani prevents oxidative burst-mediated apoptosis of host macrophages through selective induction of suppressors of cytokine signaling (SOCS) proteins

One of the mechanisms for establishment of infection employed by intra-macrophage pathogen-like Leishmania is inhibition of oxidative burst-mediated macrophage apoptosis to protect their niche for survival and replication. We tried to elucidate the underlying mechanism for this by using H2O2 for induction of apoptosis. Leishmania donovani-infected macrophages were much more resistant to H2O2-mediated apoptosis compared with control. Although infected cells were capable of comparable reactive oxygen species production, there was less activation of the downstream cascade consisting of caspase-3 and -7 and cleaved poly(ADP)-ribose polymerase. Suppressors of cytokine signaling (SOCS) 1 and 3 proteins and reactive oxygen species scavenging enzyme thioredoxin, known to be involved in stabilization of protein-tyrosine phosphatases, were found to be induced during infection. Induction of SOCS proteins may be mediated by Egr1, and silencing of Socs1 and -3 either alone or in combination resulted in reduced thioredoxin levels, enhanced activation of caspases, and increased apoptosis of infected macrophages. The induction of protein-tyrosine phosphatases, thioredoxin, SOCS, and Egr1 in L. donovani-infected macrophages was found to be unaffected by H2O2 treatment. SOCS knocked down cells also displayed decreased parasite survival thus marking reduction in disease progression. Taken together, these results suggest that L. donovani may exploit SOCS for subverting macrophage apoptotic machinery toward establishing its replicative niche inside the host.

RNA-seq profiles of immune related genes in the staghorn coral Acropora cervicornis infected with white band disease

Coral diseases are among the most serious threats to coral reefs worldwide, yet most coral diseases remain poorly understood. How the coral host responds to pathogen infection is an area where very little is known. Here we used next-generation RNA-sequencing (RNA-seq) to produce a transcriptome-wide profile of the immune response of the Staghorn coral Acropora cervicornis to White Band Disease (WBD) by comparing infected versus healthy (asymptomatic) coral tissues. The transcriptome of A. cervicornis was assembled de novo from A-tail selected Illumina mRNA-seq data from whole coral tissues, and parsed bioinformatically into coral and non-coral transcripts using existing Acropora genomes in order to identify putative coral transcripts. Differentially expressed transcripts were identified in the coral and non-coral datasets to identify genes that were up- and down-regulated due to disease infection. RNA-seq analyses indicate that infected corals exhibited significant changes in gene expression across 4% (1,805 out of 47,748 transcripts) of the coral transcriptome. The primary response to infection included transcripts involved in macrophage-mediated pathogen recognition and ROS production, two hallmarks of phagocytosis, as well as key mediators of apoptosis and calcium homeostasis. The strong up-regulation of the enzyme allene oxide synthase-lipoxygenase suggests a key role of the allene oxide pathway in coral immunity. Interestingly, none of the three primary innate immune pathways - Toll-like receptors (TLR), Complement, and prophenoloxydase pathways, were strongly associated with the response of A. cervicornis to infection. Five-hundred and fifty differentially expressed non-coral transcripts were classified as metazoan (n = 84), algal or plant (n = 52), fungi (n = 24) and protozoans (n = 13). None of the 52 putative Symbiodinium or algal transcript had any clear immune functions indicating that the immune response is driven by the coral host, and not its algal symbionts.

A Correlation between Phagocytosis and Apoptosis in THP-1 Cells Infected with Prevalent Strains ofMycobacterium tuberculosis

The innate ability of infected macrophages to undergo programmed cell death (apoptosis) and curtail the infection is crucial for the host defense. Although phagocytosis and intracellular killing mechanisms leading to apoptosis in macrophages are highly effective in eliminating the infecting tuberculous bacilli, some Mycobacterium tuberculosis(Mtb) strains have evolved strategies to inhibit this microbicidal function and make use of macrophage for its successful and prolonged survival. Two clinical strains of Mtb (S7 and S10) found to be prevalent and primitive, based on molecular epidemiological studies, were used to study the magnitude in induction of apoptosis in THP-1 cells at various time points of infection and to correlate it with phagocytosis. The percentage of phagocytosis did not show any strain-specific association with differentiated THP-1 cells. But in the phagocytic index, the clinical strains showed a low dose of infection in the 1-10 bacilli category thereby exerting less burden on the cells. The induction of apoptosis was strain dependent. The THP-1 cells infected with H37Ra and S10 showed an increase in apoptosis at all time points while the S7 strain induced minimum apoptosis. A negative correlation between apoptosis and phagocytic index was observed in the 1-10 category and a positive correlation in the > 20 category of the phagocytic index. This novel observation indicates that the magnitude of THP-1 cell apoptosis is a function of the number of internalized mycobacteria. These results indicated a differential mode of infection by clinical strains and their adaptation to different survival strategies that may lead to immune suppression and pathogenesis of the disease.

Prison break: pathogens' strategies to egress from host cells

A wide spectrum of pathogenic bacteria and protozoa has adapted to an intracellular life-style, which presents several advantages, including accessibility to host cell metabolites and protection from the host immune system. Intracellular pathogens have developed strategies to enter and exit their host cells while optimizing survival and replication, progression through the life cycle, and transmission. Over the last decades, research has focused primarily on entry, while the exit process has suffered from neglect. However, pathogen exit is of fundamental importance because of its intimate association with dissemination, transmission, and inflammation. Hence, to fully understand virulence mechanisms of intracellular pathogens at cellular and systemic levels, it is essential to consider exit mechanisms to be a key step in infection. Exit from the host cell was initially viewed as a passive process, driven mainly by physical stress as a consequence of the explosive replication of the pathogen. It is now recognized as a complex, strategic process termed "egress," which is just as well orchestrated and temporally defined as entry into the host and relies on a dynamic interplay between host and pathogen factors. This review compares egress strategies of bacteria, pathogenic yeast, and kinetoplastid and apicomplexan parasites. Emphasis is given to recent advances in the biology of egress in mycobacteria and apicomplexans.

Porphyromonas gingivalis modulates Pseudomonas aeruginosa-induced apoptosis of respiratory epithelial cells through the STAT3 signaling pathway

Pseudomonas aeruginosa is an important opportunistic bacterial pathogen, causing infections of respiratory and other organ systems in immunocompromised hosts that may invade and proliferate in mucosal epithelial cells to induce apoptosis. Previous studies suggest that oral bacteria, especially gram-negative periodontal pathogens, may enhance P. aeruginosa invasion into respiratory epithelial cells to augment tissue destruction. In this study, we investigated the effect of the periodontopathogen Porphyromonas gingivalis on P. aeruginosa-induced epithelial cell apoptosis. P. gingivalis invasion transiently inhibited P. aeruginosa-induced apoptosis in respiratory epithelial cells via the signal transducer and activator of transcription 3 (STAT3) signaling pathway. The activated STAT3 up-regulated the downstream anti-apoptotic moleculars survivin and B-cell leukemia-2 (bcl-2). This process was accompanied by down-regulation of pro-apoptosis molecular Bcl-2-associated death promoter (bad) and caspase-3 activity inhibition. In addition, the activation of the STAT3 pathway was affected by P. gingivalis in a dose-dependent manner. Finally, co-invasion of P. aeruginosa and P. gingivalis led to greater cell death compared with P. aeruginosa challenge alone. These results suggest that regulation of P. aeruginosa-induced apoptosis by P. gingivalis contributes to the pathogenesis of respiratory disease. Interference with this process may provide a potential therapeutic strategy for the treatment and prevention of respiratory disease.

Evaluation of nanoparticles as endocytic tracers in cellular microbiology

The study of pathogen interactions with eukaryotic host cells requires the introduction of fluorescent probes to visualize processes such as endocytosis, intracellular transport or host cell manipulation by the pathogen. Here, three types of fluorescent nanoparticles (NPs), i.e. Rhodamine-labeled polymethacrylate (PMA) NPs, silica NPs and gold NPs, were employed to label the host cellular endolysosomal system and monitor manipulations by the pathogen Salmonella enterica. Using live cell imaging, we investigated the performance of NPs in cellular uptake, labeling of endocytic vesicles and lysosomes, as well as interaction with the pathogen. We show that fluorescent gold and silica, but not PMA NPs appropriately label host cell structures and efficiently track rearrangements of the host endosomal system by the activities of intracellular Salmonella. Silica NPs slightly aggregated and located in Salmonella-induced compartments as isolated dots, while gold NPs distributed uniformly inside such structures. Both silica and gold NPs exhibited no adverse impact on either host cells or pathogens, and are versatile tools for infection biology.

Involvement of apoptosis in host-parasite interactions in the zebra mussel

The question of whether cell death by apoptosis plays a biological function during infection is key to understanding host-parasite interactions. We investigated the involvement of apoptosis in several host-parasite systems, using zebra mussels Dreissena polymorpha as test organisms and their micro- and macroparasites. As a stress response associated with parasitism, heat shock proteins (Hsp) can be induced. In this protein family, Hsp70 are known to be apoptosis inhibitors. Mussels were diagnosed for their respective infections by standard histological methods; apoptosis was detected using the TUNEL methods on paraffin sections and Hsp70 by immunohistochemistry on cryosections. Circulating hemocytes were the main cells observed in apoptosis whereas infected tissues displayed no or few apoptotic cells. Parasitism by intracellular bacteria Rickettsiales-like and the trematode Bucephalus polymorphus were associated with the inhibition of apoptosis whereas ciliates Ophryoglena spp. or the trematode Phyllodistomum folium did not involve significant differences in apoptosis. Even if some parasites were able to modulate apoptosis in zebra mussels, we did not see evidence of any involvement of Hsp70 on this mechanism.