PLoS Pathogens: New Articles

Rac1 Is Required for Pathogenicity and Chm1-Dependent Conidiogenesis in Rice Fungal Pathogen Magnaporthe grisea

2008-11-14T08:00:00Z

by Jisheng Chen, Wu Zheng, Shiqin Zheng, Dongmei Zhang, Weijian Sang, Xiao Chen, Guangpu Li, Guodong Lu, Zonghua Wang

Author Summary

The fungus Magnaporthe grisea (M. grisea) is an important pathogen in plants and has a great impact on agriculture. Its infection of rice causes one of the most destructive diseases, the rice blast disease, around the world. M. grisea starts infection by producing conidia, which generate infectious structures and determine disease epidemics. However, the mechanism of conidial production is not well-understood. In this study, we have employed genetic and molecular techniques to silence the function of certain genes in M. grisea and found that the Rac1 gene is required for conidial production. Importantly, we have identified the mechanism for the Rac1 requirement in conidial production, which involves the interaction between Rac1 and its downstream effector Chm1. Furthermore, our study shows that the Rac1/Chm1-mediated conidiation is necessary but not sufficient for the pathogenicity of M. grisea in plants. Additional Rac1 effectors such as the Nox gene products are necessary for M. grisea to cause disease symptoms in rice and barley. Our study provides new insights into the mechanism of conidiation and pathogenicity of M. grisea during its infection in plants.

Cryoelectron Tomography of HIV-1 Envelope Spikes: Further Evidence for Tripod-Like Legs

2008-11-14T08:00:00Z

by Ping Zhu, Hanspeter Winkler, Elena Chertova, Kenneth A. Taylor, Kenneth H. Roux

Author Summary

The envelope (Env) spikes on the surface of HIV-1 and SIV virions facilitate target cell tropism, binding, and entry, and serve as the sole targets of humoral (antibody-mediated) immunity. X-ray crystallography has previously revealed the atomic structures of key core domains and peptides of the gp120 and gp41 Env spike subunits, but the manner by which these components are arranged in the Env spike is still speculative. Cryoelectron tomography (cryoET) affords a view of the entire Env spike in the context of the intact virion. We have previously published a cryoET model of the SIV Env spike which showed a unique tripod-like leg configuration for the solvent-exposed (external) gp41 stalk region. This model is consistent with, and helps explain, many of the unique biophysical and immunological features of this region. Subsequently another group using similar technology and virions reported a spike model displaying a compact gp41 stalk inconsistent with our splayed-leg spike model. In this report, we apply enhanced analytical cryoET procedures to show that HIV-1 also displays the tripod-like leg configuration, and shows considerable gp41 leg flexibility/heteromorphology. These results have implications for the design of effective vaccines targeting this region and may provide new insights into Env spike function.

Alteration of Blood–Brain Barrier Integrity by Retroviral Infection

2008-11-14T08:00:00Z

by Philippe V. Afonso, Simona Ozden, Marie-Christine Cumont, Danielle Seilhean, Luis Cartier, Payam Rezaie, Sarah Mason, Sophie Lambert, Michel Huerre, Antoine Gessain, Pierre-Olivier Couraud, Claudine Pique, Pierre-Emmanuel Ceccaldi, Ignacio A. Romero

Author Summary

The blood–brain barrier (BBB) forms the interface between the blood and the central nervous system (CNS). BBB disruption is considered to be a key event in the pathogenesis of retroviral-associated neurological diseases. The present paper deals with the susceptibility of the endothelial cells (i.e., one of the main cellular components of BBB) to retroviral infection, and with the impact of infection in BBB function. This study focuses on the Human T-Lymphotropic Virus (HTLV-1), which infects 20 million people worldwide, and is the etiological agent of a neurodegenerative disease called HTLV-1 Associated Myelopathy/Tropical Spastic Paraparesis (HAM/TSP). We first demonstrated that the cerebral endothelial cells express the receptors for the retrovirus in vitro, and on spinal cord autopsy sections from non-infected and HAM/TSP patients. We found on these latter that vascular-like structures were infected and confirmed in vitro that the endothelial cells could be productively infected by HTLV-1. We demonstrated that such an infection impairs BBB properties in vitro, as well as tight junctions, that are cell adhesion structures. This study is the first to demonstrate the impact of HTLV-1 infection on human BBB integrity; such a susceptibility has to be considered in the design of future therapeutics strategies.

Resistance of Bovine Spongiform Encephalopathy (BSE) Prions to Inactivation

2008-11-14T08:00:00Z

by Kurt Giles, David V. Glidden, Robyn Beckwith, Rose Seoanes, David Peretz, Stephen J. DeArmond, Stanley B. Prusiner

Author Summary

“Mad cow” disease, formally known as bovine spongiform encephalopathy (BSE), belongs to a family of diseases affecting humans and a number of commercially important animal species. These diseases are not spread by bacteria or viruses, but by infectious proteins, termed “prions.” Prions are known to be very difficult to inactivate, but little is known about the relative difficulty of inactivation for prions from different species. Here, we studied the inactivation of BSE prions and compared it to the inactivation of prions from humans, mice, and hamsters. We used highly sensitive, genetically engineered mouse models to detect low levels of infectivity. We then quantified the levels of inactivation for a range of treatments and calculated differences between prions from different species. We found that naturally occurring BSE prions can be up to 1 million times more difficult to inactivate than the most commonly used hamster prions. BSE prions were also 1,000 times more difficult to inactivate than a mouse prion that was thought to be a surrogate for BSE prions. This study demonstrates that prion inactivation procedures need to be validated directly against the prion strains for which they are intended to be used.

The Host Cell Sulfonation Pathway Contributes to Retroviral Infection at a Step Coincident with Provirus Establishment

2008-11-14T08:00:00Z

by James W. Bruce, Paul Ahlquist, John A. T. Young

Author Summary

A genetic screen was used to identify host cell functions important for the replication of retroviruses, including human immunodeficiency viruses. These studies have uncovered a heretofore unexpected role for the cellular sulfonation pathway in an intracellular step of retroviral replication. Through the addition of sulfate groups, this pathway is responsible for modifying and regulating different types of cellular factors including proteins, lipids, carbohydrates and hormones. The role of this pathway was further confirmed by using specific chemical inhibitors. The sulfonation requirement was mapped to a step during viral DNA integration into the host genome that has a subsequent effect upon the level of expression of viral genes. These studies have uncovered a new regulatory mechanism of retroviral replication and suggest that components of the host cell sulfonation pathway might represent attractive targets for antiviral development.

Key Role of Splenic Myeloid DCs in the IFN-αβ Response to Adenoviruses In Vivo

2008-11-14T08:00:00Z

by György Fejer, Lisa Drechsel, Jan Liese, Ulrike Schleicher, Zsolt Ruzsics, Nicola Imelli, Urs F. Greber, Simone Keck, Bernd Hildenbrand, Anne Krug, Christian Bogdan, Marina A. Freudenberg

Author Summary

Adenoviruses (Ads) are important pathogens and promising vectors for gene therapy applications. In the course of adenoviral infections innate immune responses are activated, which can be beneficial for the antiviral host defense but also detrimental if activated in a deregulated manner. Type I IFNs are crucial for the innate immune control of various viral infections in the mammalian host. So far, the early, systemic release of IFN-αβ during viral infections has been attributed to specialized immune cells, the plasmacytoid dendritic cells. Here, in a mouse infection model, we show that wild type Ads, as well as adenoviral vectors, elicit rapid IFN-αβ production almost exclusively in another cell population, the splenic myeloid dendritic cells. This IFN-αβ storm depends on viral escape from endosomes to the cytosol and the requirements of the response are suggestive of a novel viral induction pathway. Furthermore, we show that virus induced IFN-αβ is the key mediator of Ad-induced hypersensitivity to the cytokine-inducing and toxic activity of lipopolysaccharide, a common constituent of Gram-negative bacteria. Since these bacteria comprise several commensals and pathogens, enhanced susceptibility to lipopolysaccharide may contribute to toxic reactions observed during adenoviral gene therapy and to the clinical symptoms of adenoviral diseases.

Foamy Macrophages from Tuberculous Patients' Granulomas Constitute a Nutrient-Rich Reservoir for M. tuberculosis Persistence

2008-11-11T08:00:00Z

by Pascale Peyron, Julien Vaubourgeix, Yannick Poquet, Florence Levillain, Catherine Botanch, Fabienne Bardou, Mamadou Daffé, Jean-François Emile, Bruno Marchou, Pere-Joan Cardona, Chantal de Chastellier, Frédéric Altare

Author Summary

Mycobacterium tuberculosis, the causative agent of tuberculosis, is responsible for dramatic health problems globally. It is estimated that this pathogen infects one-third of the human population and causes three million deaths annually. Most individuals remain asymptomatic for several years before developing an active disease. In such individuals, the bacilli are not cleared but rather persist in a dormant state. Major goals of TB research are to (i) understand how the bacilli remain alive for years within infected individuals, and (ii) find how to prevent their reactivation and hence clinical disease. During dormancy, most of the bacilli are confined to granulomas that consist of well-defined aggregates of different host immune cells. Granulomas prevent spreading of bacilli. In this study, we analyzed the role of a particular cell population found within granulomas, the “foamy macrophages”. These cells are filled with droplets of lipids, a well-known nutrient for persistent bacilli. We found that within these cells, the bacilli do not replicate, but remain alive and seem to internalize host lipids. The foamy macrophages might thus constitute a reservoir for persisting bacilli within their human host, and could provide a relevant model for screening of new antimicrobials against non-replicating persistent mycobacteria.

Influenza A Virus Inhibits Type I IFN Signaling via NF-κB-Dependent Induction of SOCS-3 Expression

2008-11-07T08:00:00Z

by Eva-K. Pauli, Mirco Schmolke, Thorsten Wolff, Dorothee Viemann, Johannes Roth, Johannes G. Bode, Stephan Ludwig

Author Summary

The type I interferon (IFN) system is one of the most powerful innate defenses against viral pathogens. Most RNA viruses are sensitive to the action of type I IFN. Therefore, these pathogens have evolved strategies to evade this response. For example, influenza viruses express a viral protein, the non-structural protein 1 (NS1), that suppresses production of IFNβ by lowering cellular sensitivity to viral nucleic acid as a pathogen pattern. Here we present data indicating that influenza A viruses are not only capable of suppressing production of the IFNβ gene but also inhibit action of this antiviral cytokine on cells. This occurs by viral induction of a cellular protein, the suppressor of cytokine signaling (SOCS)-3, a potent endogenous inhibitor of IFN signaling. This is a novel mechanism by which influenza viruses inhibit the antiviral response of the host and paves the path to efficient virus replication. This may be especially relevant for influenza viruses that induce high cytokine responses (cytokine burst), such as highly pathogenic avian influenza viruses of the H5N1 subtype. Induction of SOCS-3 expression would allow efficient replication despite high IFN and cytokine levels.

Broadening of Neutralization Activity to Directly Block a Dominant Antibody-Driven SARS-Coronavirus Evolution Pathway

2008-11-07T08:00:00Z

by Jianhua Sui, Daniel R. Aird, Azaibi Tamin, Akikazu Murakami, Meiying Yan, Anuradha Yammanuru, Huaiqi Jing, Biao Kan, Xin Liu, Quan Zhu, Qing-an Yuan, Gregory P. Adams, William J. Bellini, Jianguo Xu, Larry J. Anderson, Wayne A. Marasco

Author Summary

The SARS-CoV caused a worldwide epidemic of SARS in 2002/03 and was responsible for this zoonotic infectious disease. The role of neutralizing antibody (nAb) mediated immune pressure in the evolution of SARS-CoV during the 2002/03 outbreak and a second 2003/04 zoonotic transmission is unknown. Here we demonstrate nAb responses elicited during natural infection clearly have strain-specific components which could have been the driving force for virus evolution in spike protein during intra-species transmission. In vitro immune pressure using 2002/03 strain-specific nAb 80R recapitulate a dominant escape mutation that was present in all 2003/04 animal and human viruses. We investigated how to generate a single broad nAb (BnAb) with activity against various natural viral variants of the 2002/03 and 2003/04 outbreaks as well as nAb escape mutants. Remarkably, amino acid changes in an activation-induced cytidine deaminase (AID) “hot spot” of somatic hypermutation and localized to a single VL CDR were successful in generating BnAbs. These results provide an effective strategy for generating BnAbs that should be generally useful for improving immune based anti-viral therapies as well as providing a foundation to directly manipulate virus evolution by blocking escape pathways.

Urea-Mediated Cross-Presentation of Soluble Epstein-Barr Virus BZLF1 Protein

2008-11-07T08:00:00Z

by Sascha Barabas, Regina Gary, Tanja Bauer, Juha Lindner, Petra Lindner, Birgit Weinberger, Wolfgang Jilg, Hans Wolf, Ludwig Deml

Author Summary

CD8⁺ T lymphocytes (CTL) play a key role in the immunological control of persistent intracellular pathogens and tumors. Thus, the development of improved technologies for the monitoring and expansion of protein-specific CTL represents a major challenge in clinical immunology. CTL specifically target infected cells through the recognition of peptides displayed by surface exposed HLA class I molecules. In most cell types, HLA class I–associated peptides are generally derived from cytosolic proteins. In contrast, delivery of soluble exogenous proteins to the endogenous HLA class I processing pathway is scarce. Here we exemplified with the Epstein-Barr virus immediate early protein BZLF1 a novel and simple urea-based strategy to deliver soluble proteins to the class I processing pathway of antigen-presenting cells by cross-presentation. We showed that urea formulated BZLF1 but not urea-free BZLF1 reveals a strong capacity to reactivate CD8⁺ T cells in blood cells of EBV-positive donors. Accordingly, dendritic cells, monocytes, but also B cells are able to cross-present BZLF1-derived epitopes to CTL. This technology could improve the development of T cell diagnostics for microbial diseases and may facilitate a novel strategy for the expansion of protein-specific CTL for therapeutic application.

A Sterol-Regulatory Element Binding Protein Is Required for Cell Polarity, Hypoxia Adaptation, Azole Drug Resistance, and Virulence in Aspergillus fumigatus

2008-11-07T08:00:00Z

by Sven D. Willger, Srisombat Puttikamonkul, Kwang-Hyung Kim, James B. Burritt, Nora Grahl, Laurel J. Metzler, Robert Barbuch, Martin Bard, Christopher B. Lawrence, Robert A. Cramer

Author Summary

The incidence of potentially lethal infections caused by normally benign molds has increased tremendously over the last two decades. One disease in particular, invasive pulmonary aspergillosis (IPA), caused by the common mold Aspergillus fumigatus, has become the leading cause of death due to invasive mycoses. Currently, we have a limited understanding of how this opportunistic pathogen causes disease in immunocompromised patients. In this study, we discover a previously unexplored mechanism required by this mold to cause disease, hypoxia (low oxygen) adaptation. We report that hypoxia adaptation in A. fumigatus is mediated in part by a highly conserved transcription factor, SrbA, a protein in the sterol regulatory element binding protein family. A null mutant of SrbA was unable to grow in hypoxia, displayed increased susceptibility to the azole class of antifungal drugs, and was avirulent in two distinct murine models of IPA. Importantly, we report the discovery of a novel function of SrbA in molds related to maintenance of cell polarity. The finding that SrbA regulates resistance to the azole class of antifungal drugs presents an opportunity to uncover new mechanisms of antifungal drug resistance in A. fumigatus.

Kinome-Wide RNAi Screen Implicates at Least 5 Host Hepatocyte Kinases in Plasmodium Sporozoite Infection

2008-11-07T08:00:00Z

by Miguel Prudêncio, Cristina D. Rodrigues, Michael Hannus, Cécilie Martin, Eliana Real, Lígia A. Gonçalves, Céline Carret, Robert Dorkin, Ingo Röhl, Kerstin Jahn-Hoffmann, Adrian J. F. Luty, Robert Sauerwein, Christophe J. Echeverri, Maria M. Mota

Author Summary

During a mammalian malaria infection, Plasmodium sporozoites injected by an infected mosquito travel to the liver where they invade hepatocytes and multiply into thousands of new parasites. These newly formed merozoites are then released into the bloodstream where they infect red blood cells and cause the symptoms of the disease. Although asymptomatic, the liver stage of malaria is an obligatory step in the parasite's lifecycle and constitutes an appealing target for prophylatic intervention. The marked tropism of sporozoites for hepatocytes suggests the latter may provide the parasite with a molecular environment that it can exploit to its own benefit. The identification of host factors that influence hepatic infection can thus provide clues for potential anti-malarial strategies. To this end, we carried out an RNA interference screen of the entire human kinome and associated signaling molecules and assessed the effect of knockdown of their expression in the infection of a human hepatoma cell line by Plasmodium. This strategy identified at least 5 kinases whose down-regulation leads to a marked decrease in infection. Further characterisation of one of these proteins, PKCζ, confirmed that it plays a role in infection by influencing the parasite's invasion of the host liver cells.

PLoS Pathogens Issue Image | Vol. 4(10) October 2008

2008-10-31T07:00:00Z

Young boy ploughing a rice paddy field in Thailand.

Burkholderia pseudomallei, the causative agent of melioidosis, is often found in wet soils, and farmers in endemic areas can be exposed due to lack of appropriate foot protection (see Sim et al., doi:10.1371/journal.ppat.1000178).

Image Credit: Vanaporn Wuthiekanun, Mahidol University, Thailand

Waterfowl—The Missing Link in Epidemic and Pandemic Cholera Dissemination?

2008-10-31T07:00:00Z

by Malka Halpern, Yigal Senderovich, Ido Izhaki

Role of Cellular Heparan Sulfate Proteoglycans in Infection of Human Adenovirus Serotype 3 and 35

2008-10-31T07:00:00Z

by Sebastian Tuve, Hongjie Wang, Jeffrey D. Jacobs, Roma C. Yumul, David F. Smith, André Lieber

Author Summary

In this study, we attempted to identify binding receptors that are used by the two human adenovirus (Ad) serotypes 3 and 35. Ad3 uses yet-unknown receptors and is one of the most common Ads causing epidemic conjunctivitis, and respiratory and gastrointestinal diseases. Ad35 uses the complement receptor CD46 as an attachment receptor and mainly causes infections of the kidney and urinary tract. We utilized novel high-throughput techniques in combination with the recombinant viral proteins (fiber knobs), which mediate the initial interaction of Ads with host cells. We found that both serotypes interacted with cellular heparan sulfate proteoglycans (HSPGs). In subsequent assays, we show that HSPGs were not major receptors, but acted as low-affinity co-receptors for both Ad3 and Ad35. Ad3 and Ad35 used different viral proteins in order to interact with HSPGs. Both serotypes, however, used the same regions within HSPGs that show high levels of sulfation and are important for binding of extracellular located physiologic ligands. In summary, we show that Ad3 and Ad35 evolved to “highjack” yet another class of cellular surface molecules that are essential for the function of the target host cells and are ubiquitously expressed. This provides new insights into the emerging picture of the infection mechanism of Ad3 and Ad35.

The Antimicrobial Peptide Histatin-5 Causes a Spatially Restricted Disruption on the Candida albicans Surface, Allowing Rapid Entry of the Peptide into the Cytoplasm

2008-10-31T07:00:00Z

by A. Brian Mochon, Haoping Liu

Author Summary

In most healthy individuals, the yeast Candida albicans is found within the oral cavity as part of the normal microflora. Though under immunocompromising conditions, this benign microbe can become an opportunistic pathogen causing oral candidiasis (i.e. thrush). Although antifungal drugs are typically efficacious, the paucity of drugs and their increasing usage has led to rising drug resistance. Thus, researchers have begun to look at alternative therapeutics, such as the candidacidal salivary peptide histatin-5. To date, little is known about the initial binding and subsequent internalization that facilitates Hst-5's killing activity. Thus, our study attempted to determine how Hst-5 is internalized into C. albicans. It was thought that Hst-5 is transported into the cytoplasm without disruption of the plasma membrane. However, our study finds that Hst-5, under physiological concentrations, disrupts the plasma membrane and is rapidly translocated into the cytoplasm, leading to cell death. Interestingly, the internalization of Hst-5 is initiated from a single spatially restricted site on the plasma membrane rather than multiple breaches on the cell surface. This novel mechanism of membrane disruption provides new insights into how Hst-5 and other antimicrobial peptides may be acting on pathogenic microorganisms.

Repetitive N-WASP–Binding Elements of the Enterohemorrhagic Escherichia coli Effector EspF_U Synergistically Activate Actin Assembly

2008-10-31T07:00:00Z

by Kenneth G. Campellone, Hui-Chun Cheng, Douglas Robbins, Anosha D. Siripala, Emma J. McGhie, Richard D. Hayward, Matthew D. Welch, Michael K. Rosen, Vassilis Koronakis, John M. Leong

Author Summary

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 is a food-borne pathogen that causes diarrhea and life-threatening systemic illnesses. EHEC colonizes the intestine by adhering tightly to host cells and injecting bacterial molecules that trigger the formation of a “pedestal” below bound bacteria. These pedestals are generated by reorganizing the actin cytoskeleton into densely packed filaments beneath the plasma membrane. Pedestal formation is therefore not only important for EHEC disease, it provides a means to study how mammalian cells control their shape. We show here that two EHEC proteins, Tir and EspF_U, are sufficient to trigger pedestal formation. Tir localizes to the mammalian plasma membrane, and its central function is to promote clustering of EspF_U. EspF_U contains multiple repeat sequences that stimulate actin polymerization by binding N-WASP, a host protein that initiates actin assembly. Although a single repeat of EspF_U can generate pedestals, multi-repeat variants promote actin assembly cooperatively. One explanation for this synergy is that tandem repeats can potently trigger the formation of a complex of mammalian proteins that modulate the actin cytoskeleton. These findings define the minimal set of EHEC effectors required for pedestal formation and the elements within those effectors that confer their ability to alter cell shape.

Direct Identification of the Meloidogyne incognita Secretome Reveals Proteins with Host Cell Reprogramming Potential

2008-10-31T07:00:00Z

by Stéphane Bellafiore, Zhouxin Shen, Marie-Noelle Rosso, Pierre Abad, Patrick Shih, Steven P. Briggs

Author Summary

Parasitic nematodes are microscopic worms that cause major diseases of plants, animals, and humans. Infection is associated with secretion of proteins by the parasite; these proteins suppress the immune system and cause other changes to host cells that are required for infection. Identification of secreted proteins has been difficult because they are released only in trace amounts. We have developed very sensitive methods that enabled the discovery of 486 proteins secreted by the root knot nematode, Meloidogyne incognita; prior to this, only a handful of secreted proteins were known. Several secreted proteins appear to mimic normal plant proteins, and they may participate in the process by which the nematode hijacks the plant cell for its own purposes. Meloidogyne species infect many crops, including corn, soybean, cotton, rice, tomato, carrots, alfalfa, and tobacco. The discovery of these secreted proteins could lead to new methods for protecting these important crops from nematode damage. We observed that the secretome of the human pathogen, Brugia malayi, overlaps that of M. incognita, suggesting a common parasitic behavior between pathogens of plants and animals.

Interferon-β Pretreatment of Conventional and Plasmacytoid Human Dendritic Cells Enhances Their Activation by Influenza Virus

2008-10-31T07:00:00Z

by Hannah Phipps-Yonas, Jeremy Seto, Stuart C. Sealfon, Thomas M. Moran, Ana Fernandez-Sesma

Author Summary

Influenza infection leads to a serious respiratory infection of the lung epithelium. Lying directly below the epithelial cells are immune system sentinels known as dendritic cells. These cells interact with the virus and carry parts of the virus to draining lymph nodes to activate killer T cells. In order to effectively carry out this function, DCs must perceive the presence of a virus using receptors specially adapted for this function. However, when DCs are mixed with influenza virus in the laboratory, no activation occurs because the virus produces a protein called NS1 that blocks the receptors. Yet, patients infected with influenza virus develop a strong adaptive response that leads to recovery from infection. This observation suggests that additional factors must be present that contribute to the activation of the DCs. The most likely contributor is type I interferon, a ubiquitous protein released from many cells upon exposure to virus. In this study, we mixed influenza virus with DCs in the presence of type I interferon and found that this greatly enhanced their activation. Treatment with interferon allowed the DC to bypass the block in activation mediated by the influenza NS1 protein. Our data suggest that the production of type I interferon within an infected patient may endow the DCs with the ability to fully respond to influenza virus.

Recruitment of the Complete hTREX Complex Is Required for Kaposi's Sarcoma–Associated Herpesvirus Intronless mRNA Nuclear Export and Virus Replication

2008-10-31T07:00:00Z

by James R. Boyne, Kevin J. Colgan, Adrian Whitehouse

Author Summary

Following gene expression in the nucleus, newly transcribed messenger RNA (mRNA) is exported to the cytoplasm, where it is translated into protein. In mammals the vast majority of mRNAs contain introns that must be removed by the spliceosome prior to nuclear export. In addition to excising introns, splicing is also essential for the recruitment of a several protein complexes to mRNA, one example being the human transcription/export complex, which is required for mRNA export. Herpesviruses, such as Kaposi's sarcoma–associated herpesvirus, replicate by hijacking components of the host cells biological machinery, including those proteins necessary for mRNA export. An intriguing caveat in herpesvirology is that herpesviruses, such as Kaposi's sarcoma–associated herpesvirus, produce some mRNAs that lack introns and do not undergo splicing. How then are these intronless mRNAs exported to the cytoplasm? The answer lies in a virus protein called ORF57 that is able to bind to the intronless mRNA and then export them to the cytoplasm. ORF57 achieves this function by mimicking splicing and recruiting the human transcription/export complex to the intronless viral mRNA, thus facilitating its export into the cytoplasm.

Proteomic Profiling of Plasmodium Sporozoite Maturation Identifies New Proteins Essential for Parasite Development and Infectivity

2008-10-31T07:00:00Z

by Edwin Lasonder, Chris J. Janse, Geert-Jan van Gemert, Gunnar R. Mair, Adriaan M. W. Vermunt, Bruno G. Douradinha, Vera van Noort, Martijn A. Huynen, Adrian J. F. Luty, Hans Kroeze, Shahid M. Khan, Robert W. Sauerwein, Andrew P. Waters, Matthias Mann, Hendrik G. Stunnenberg

Author Summary

Human malaria is caused by Plasmodium falciparum, a unicellular protozoan parasite that is transmitted by Anopheles mosquitoes. An infectious mosquito injects saliva containing sporozoite forms of the parasite and these then migrate from the skin to the liver, where they establish an infection. Many intervention strategies are currently focused on preventing the establishment of infection by sporozoites. Clearly, an understanding of the biology of the sporozoite is essential for developing new intervention strategies. Sporozoites are produced within the oocyst, located on the outside wall of the mosquito midgut, and migrate after release from the oocysts to the salivary glands where they are stored as mature infectious forms. Comparison of the proteomes of sporozoites derived from either the oocyst or from the salivary gland reveals remarkable differences in the protein content of these stages despite their similar morphology. The changes in protein content reflect the very specific preparations the sporozoites make in order to establish an infection of the liver. Analysis of the function of several previously uncharacterized, conserved proteins revealed proteins essential for sporozoite development at distinct points of their maturation.

Horizontal versus Familial Transmission of Helicobacter pylori

2008-10-24T07:00:00Z

by Sandra Schwarz, Giovanna Morelli, Barica Kusecek, Andrea Manica, Francois Balloux, Robert J. Owen, David Y. Graham, Schalk van der Merwe, Mark Achtman, Sebastian Suerbaum

Author Summary

More than half of the world population is infected with Helicobacter pylori. It was widely believed that the primary mode of transmission is intra-familial, but the chains of infection are poorly understood. We have applied multilocus sequence analysis to H. pylori from two large multi-generation families in rural South Africa. Observations were compared with H. pylori from small, nuclear families in urban areas of the United States, United Kingdom, Colombia, and Korea, as well as with a large global collection from unrelated individuals. Intra-familial transmission of H. pylori was common in urban families but less common in the rural South African families. Instead, the South African families were infected with widely diverse strains, and multiple infections were common. We argue that the apparent predominance of intra-familial transmission in urban societies is a result of good socioeconomic conditions. In high-prevalence areas, opportunities for horizontal transmission are higher, which can result in greater diversity of H. pylori within a family. The patterns of frequent horizontal spread in rural South African families may be representative of large parts of the developing world. This interpretation is supported by the global sample which yielded ample evidence for horizontal inter-familial transmission in many areas of the world.

Regulated Expression of an Essential Allosteric Activator of Polyamine Biosynthesis in African Trypanosomes

2008-10-24T07:00:00Z

by Erin K. Willert, Margaret A. Phillips

Author Summary

Human African trypanosomiasis (HAT) is an important vector-borne pathogen. The World Health Organization estimates that more than 50 million people are at risk for the disease, which occurs focally, in remote regions, and periodically reaches epidemic levels. Untreated HAT is always fatal, and the available drugs compromise toxicity and emerging resistance. The only safe treatment for late-stage disease is an inhibitor of an essential metabolic pathway that is involved in the synthesis of small organic cations termed polyamines. In this paper, we use genetic approaches to demonstrate how the parasite regulates this essential metabolic pathway. By modulating the protein levels of a trypanosome-specific activator of polyamine biosynthesis, the parasite has developed a mechanism to regulate pathway output. We also demonstrate that this pathway activator is essential to parasite growth. Our data strengthen the genetic and chemical validation of a key enzyme in this pathway as a drug target in the parasite, and they provide new insight into parasite-specific approaches that could be used to design novel drugs against this deadly disease.

Discerning the Complexity of Community Interactions Using a Drosophila Model of Polymicrobial Infections

2008-10-24T07:00:00Z

by Christopher D. Sibley, Kangmin Duan, Carrie Fischer, Michael D. Parkins, Douglas G. Storey, Harvey R. Rabin, Michael G. Surette

Author Summary

Bacterial infections often involve more than one species. The lung disease of cystic fibrosis (CF) patients provides examples of polymicrobial infections whereby diverse and dynamic microbial communities are a characteristic of CF airways. The significance of microbe–microbe interactions and the interplay of the communities with the host have not been thoroughly investigated. We describe a novel Drosophila model to discern the biological interactions between microbes within microbial communities, as well as the interactions between the communities and the innate immune system. Using fly survival as a readout of relevant interactions, we show that mixed infections may additively or synergistically enhance the pathogenicity of a microbial community. The polymicrobial infection model was used to differentiate the bacterial flora in CF sputum, revealing that a large proportion of the organisms in CF airways has the ability to influence the outcome of an infection when in combination with the principal CF pathogen Pseudomonas aeruginosa. We show that during the synergistic-type mixed infections, P. aeruginosa virulence gene expression is altered within live Drosophila compared to mono-species infections. The immune response to microbial communities takes many forms and can include synergistic activation of antimicrobial peptide gene expression. We postulate that the biological interactions exposed using this model may contribute to the transition from chronic stable infections to acute pulmonary exacerbation infections in CF.

Immunity to HIV-1 Is Influenced by Continued Natural Exposure to Exogenous Virus

2008-10-24T07:00:00Z

by Christian B. Willberg, J. Jeff McConnell, Emily M. Eriksson, Larry A. Bragg, Vanessa A. York, Teri J. Liegler, Fredrick M. Hecht, Robert M. Grant, Douglas F. Nixon

Author Summary

Serosorting, the practice of seeking to engage in unprotected sexual activities only with partners who are of the same HIV-1 status, is a growing trend. Unprotected sexual intercourse between two HIV-1 infected individuals can lead to consequences such as HIV-1 super-infection. However, continued exposure to HIV-1 may also have an important influence on the immune response. Here, we explored this influence in a cohort of HIV-1 infected individuals who were in long-term partnerships with other HIV-1 infected individuals. We found that individuals, who regularly engaged in unprotected receptive sexual intercourse with an HIV-1 infected viremic partner, displayed higher T cell responses to HIV proteins compared to those who were not regularly exposed to a viremic partner. None of the individuals within this study showed evidence of systemic super-infection. Exposure had limited impact on general activation or poly-functionality. These results are clearly of importance for HIV-1 infected individuals who chose to engage in unprotected sexual activity with other HIV-1 infected individuals. These data also reveal a more general mechanism that occurs in infectious diseases: immune responses to chronic viruses are influenced not only by the virus within the host, but also by exposure to the virus from without.

Prevention of Cytotoxic T Cell Escape Using a Heteroclitic Subdominant Viral T Cell Determinant

2008-10-24T07:00:00Z

by Noah S. Butler, Alex Theodossis, Andrew I. Webb, Roza Nastovska, Sri Harsha Ramarathinam, Michelle A. Dunstone, Jamie Rossjohn, Anthony W. Purcell, Stanley Perlman

Author Summary

Enhancing the immune responses to pathogens is a chief goal of vaccine development. Here, we describe the development of an engineered CD8+ T cell epitope that elicits an immune response to the native epitope that is more potent than the one that occurs during the natural infection. We showed that this “improved” (heteroclitic) epitope protects against clinical disease and against cytotoxic T cell escape that frequently occurs in the immunodominant epitope expressed by the virus. We also performed structural analyses and showed that enhanced immunogenicity was associated with changes in the conformations of both the peptide and the region of the MHC class I molecule that is in close association with the peptide. These studies provide a model for designing T cell epitopes with enhanced immunogenicity that will be useful in vaccine development, with particular emphasis on diseases, such as HIV and hepatitis C, in which epitope mutation and escape is common.

Transmission of Vibrio cholerae Is Antagonized by Lytic Phage and Entry into the Aquatic Environment

2008-10-24T07:00:00Z

by Eric J. Nelson, Ashrafuzzaman Chowdhury, James Flynn, Stefan Schild, Lori Bourassa, Yue Shao, Regina C. LaRocque, Stephen B. Calderwood, Firdausi Qadri, Andrew Camilli

Author Summary

The biological factors that control the transmission of water-borne pathogens like Vibrio cholerae during outbreaks are ill defined. In this study, a molecular analysis of the active but non-culturable (ABNC) state of V. cholerae provides insights into the physiology of environmental adaptation. The ABNC state, lytic phage, and hyperinfectivity were concurrently followed as V. cholerae passaged from cholera patients to an aquatic reservoir. The relevance to transmission of each factor was weighed against the others. As the bacteria transitioned from the patient to pond water, there was a rapid decay into the ABNC state and a rise of lytic phage that compounded to block transmission in a mouse model. These two factors give reason for V. cholerae to make a quick transit through the environment and onto the next human host. Thus, in over-crowded locations with failed water infrastructure, the opportunity for fast transmission coupled with the increased infectivity and culturability of recently shed V. cholerae creates a charged setting for explosive cholera outbreaks.

Host Cell Egress and Invasion Induce Marked Relocations of Glycolytic Enzymes in Toxoplasma gondii Tachyzoites

2008-10-24T07:00:00Z

by Sebastien Pomel, Flora C. Y. Luk, Con J. M. Beckers

Author Summary

Protozoan parasites of humans are important causes of disease throughout the world. Amongst these, the apicomplexan parasites are an especially important group of pathogens, as they include Plasmodium species, the causative agents of malaria, and Toxoplasma gondii, an important cause of disease in people with AIDS. These parasites divide their stay in the infected human into two discrete stages, the motile extracellular stage that is responsible for parasite spread throughout its host, and the intracellular stage that is devoted entirely to replication. Here we demonstrate that Toxoplasma relocates its main source of energy when it moves between the intracellular and extracellular environment. It appears that this allows the parasite to optimize energy delivery to those processes that are critical for extracellular survival and invasion into new host cells and those required for subsequent intracellular growth and replication.

PLoS Pathogens: New Articles

Rac1 Is Required for Pathogenicity and Chm1-Dependent Conidiogenesis in Rice Fungal Pathogen Magnaporthe grisea

Cryoelectron Tomography of HIV-1 Envelope Spikes: Further Evidence for Tripod-Like Legs

Alteration of Blood–Brain Barrier Integrity by Retroviral Infection

Resistance of Bovine Spongiform Encephalopathy (BSE) Prions to Inactivation

The Host Cell Sulfonation Pathway Contributes to Retroviral Infection at a Step Coincident with Provirus Establishment

Key Role of Splenic Myeloid DCs in the IFN-αβ Response to Adenoviruses In Vivo

Foamy Macrophages from Tuberculous Patients' Granulomas Constitute a Nutrient-Rich Reservoir for M. tuberculosis Persistence

Influenza A Virus Inhibits Type I IFN Signaling via NF-κB-Dependent Induction of SOCS-3 Expression

Broadening of Neutralization Activity to Directly Block a Dominant Antibody-Driven SARS-Coronavirus Evolution Pathway

Urea-Mediated Cross-Presentation of Soluble Epstein-Barr Virus BZLF1 Protein

A Sterol-Regulatory Element Binding Protein Is Required for Cell Polarity, Hypoxia Adaptation, Azole Drug Resistance, and Virulence in Aspergillus fumigatus

Kinome-Wide RNAi Screen Implicates at Least 5 Host Hepatocyte Kinases in Plasmodium Sporozoite Infection

PLoS Pathogens Issue Image | Vol. 4(10) October 2008

Waterfowl—The Missing Link in Epidemic and Pandemic Cholera Dissemination?

Role of Cellular Heparan Sulfate Proteoglycans in Infection of Human Adenovirus Serotype 3 and 35

The Antimicrobial Peptide Histatin-5 Causes a Spatially Restricted Disruption on the Candida albicans Surface, Allowing Rapid Entry of the Peptide into the Cytoplasm

Repetitive N-WASP–Binding Elements of the Enterohemorrhagic Escherichia coli Effector EspFU Synergistically Activate Actin Assembly

Direct Identification of the Meloidogyne incognita Secretome Reveals Proteins with Host Cell Reprogramming Potential

Interferon-β Pretreatment of Conventional and Plasmacytoid Human Dendritic Cells Enhances Their Activation by Influenza Virus

Recruitment of the Complete hTREX Complex Is Required for Kaposi's Sarcoma–Associated Herpesvirus Intronless mRNA Nuclear Export and Virus Replication

Proteomic Profiling of Plasmodium Sporozoite Maturation Identifies New Proteins Essential for Parasite Development and Infectivity

Horizontal versus Familial Transmission of Helicobacter pylori

Regulated Expression of an Essential Allosteric Activator of Polyamine Biosynthesis in African Trypanosomes

Discerning the Complexity of Community Interactions Using a Drosophila Model of Polymicrobial Infections

Immunity to HIV-1 Is Influenced by Continued Natural Exposure to Exogenous Virus

Prevention of Cytotoxic T Cell Escape Using a Heteroclitic Subdominant Viral T Cell Determinant

Transmission of Vibrio cholerae Is Antagonized by Lytic Phage and Entry into the Aquatic Environment

Host Cell Egress and Invasion Induce Marked Relocations of Glycolytic Enzymes in Toxoplasma gondii Tachyzoites

Repetitive N-WASP–Binding Elements of the Enterohemorrhagic Escherichia coli Effector EspF_U Synergistically Activate Actin Assembly