Advances in experimental methods for the elucidation of Pseudomonas syringae effector function with a focus on AvrPtoB

Pseudomonas syringae infects a wide range of plant species through the use of a type III secretion system. The effector proteins injected into the plant cell through this molecular syringe serve as promoters of disease by subverting the plant immune response to the benefit of the bacteria in the intercellular space. The targets and activities of a subset of effectors have been elucidated recently. In this article, we focus on the experimental approaches that have proved most successful in probing the molecular basis of effectors, ranging from loss-of-function to gain-of-function analyses utilizing several techniques for effector delivery into plants. In particular, we highlight how these diverse approaches have been applied to the study of one effector--AvrPtoB--a multifunctional protein with the ability to suppress both effector-triggered immunity and pathogen (or microbe)-associated molecular pattern-triggered immunity. Taken together, advances in this field illustrate the need for multiple experimental approaches when elucidating the function of a single effector.

Roadmap for future research on plant pathogen effectors

Bacterial and eukaryotic plant pathogens deliver effector proteins into plant cells to promote pathogenesis. Bacterial pathogens containing type III protein secretion systems are known to inject many of these effectors into plant cells. More recently, oomycete pathogens have been shown to possess a large family of effectors containing the RXLR motif, and many effectors are also being discovered in fungal pathogens. Although effector activities are largely unknown, at least a subset suppress plant immunity. A plethora of new plant pathogen genomes that will soon be available thanks to next-generation sequencing technologies will allow the identification of many more effectors. This article summarizes the key approaches used to identify plant pathogen effectors, many of which will continue to be useful for future effector discovery. Thus, it can be viewed as a 'roadmap' for effector and effector target identification. Because effectors can be used as tools to elucidate components of innate immunity, advances in our understanding of effectors and their targets should lead to improvements in agriculture.

Fungal effector proteins: past, present and future

The pioneering research of Harold Flor on flax and the flax rust fungus culminated in his gene-for-gene hypothesis. It took nearly 50 years before the first fungal avirulence (Avr) gene in support of his hypothesis was cloned. Initially, fungal Avr genes were identified by reverse genetics and map-based cloning from model organisms, but, currently, the availability of many sequenced fungal genomes allows their cloning from additional fungi by a combination of comparative and functional genomics. It is believed that most Avr genes encode effectors that facilitate virulence by suppressing pathogen-associated molecular pattern-triggered immunity and induce effector-triggered immunity in plants containing cognate resistance proteins. In resistant plants, effectors are directly or indirectly recognized by cognate resistance proteins that reside either on the plasma membrane or inside the plant cell. Indirect recognition of an effector (also known as the guard model) implies that the virulence target of an effector in the host (the guardee) is guarded by the resistance protein (the guard) that senses manipulation of the guardee, leading to activation of effector-triggered immunity. In this article, we review the literature on fungal effectors and some pathogen-associated molecular patterns, including those of some fungi for which no gene-for-gene relationship has been established.

Ten things to know about oomycete effectors

Long considered intractable organisms by fungal genetic research standards, the oomycetes have recently moved to the centre stage of research on plant-microbe interactions. Recent work on oomycete effector evolution, trafficking and function has led to major conceptual advances in the science of plant pathology. In this review, we provide a historical perspective on oomycete genetic research and summarize the state of the art in effector biology of plant pathogenic oomycetes by describing what we consider to be the 10 most important concepts about oomycete effectors.

The evolution of Pseudomonas syringae host specificity and type III effector repertoires

The discovery 45 years ago that many Pseudomonas syringae pathovars elicit the hypersensitive response in plant species other than their hosts fostered the use of these bacteria as experimental models. However, the basis for host specificity and the corresponding resistance of nonhosts remain unclear. Pseudomonas syringae is now known to inject into the host cytoplasm, via the type III secretion system, effector proteins that suppress basal innate immunity, but may be recognized by cognate resistance (R) proteins in a second level of defence. The identification and manipulation of complete repertoires of type III effectors have revealed the highly polymorphic nature of effector repertoires and their potential to limit the host range. However, the maintenance of compatible effector repertoires may be driven by adaptations to life in a given plant species involving many factors. Tools are now available to test several hypotheses for the nature and evolution of P. syringae host specificity and nonhost resistance.

Feline leukemia virus immunity induced by whole inactivated virus vaccination

A fraction of cats exposed to feline leukemia virus (FeLV) effectively contain virus and resist persistent antigenemia/viremia. Using real-time PCR (qPCR) to quantitate circulating viral DNA levels, previously we detected persistent FeLV DNA in blood cells of non-antigenemic cats considered to have resisted FeLV challenge. In addition, previously we used RNA qPCR to quantitate circulating viral RNA levels and determined that the vast majority of viral DNA is transcriptionally active, even in the absence of antigenemia. A single comparison of all USDA-licensed commercially available FeLV vaccines using these modern sensitive methods has not been reported. To determine whether FeLV vaccination would prevent nucleic acid persistence, we assayed circulating viral DNA, RNA, antigen, infectious virus, and virus neutralizing (VN) antibody in vaccinated and unvaccinated cats challenged with infectious FeLV. We identified challenged vaccinates with undetectable antigenemia and viremia concomitant with persistent FeLV DNA and/or RNA. Moreover, these studies demonstrated that two whole inactivated virus (WIV) adjuvanted FeLV vaccines (Fort Dodge Animal Health's Fel-O-Vax Lv-K) and Schering-Plough Animal Health's FEVAXYN FeLV) provided effective protection against FeLV challenge. In nearly every recipient of these vaccines, neither viral DNA, RNA, antigen, nor infectious virus could be detected in blood after FeLV challenge. Interestingly, this effective viral containment occurred despite a weak to undetectable VN antibody response. The above findings reinforce the precept of FeLV infection as a unique model of effective retroviral immunity elicited by WIV vaccination, and as such holds valuable insights into retroviral immunoprevention and therapy.

Pathogenesis, treatment, and prevention of pneumococcal pneumonia

Pneumococcus remains the most common cause of community-acquired pneumonia worldwide. Streptococcus pneumoniae is well adapted to people, and is a frequent inhabitant of the upper airways in healthy hosts. This seemingly innocuous state of colonisation is a dynamic and competitive process in which the pathogen attempts to engage the host, proliferate, and invade the lower airways. The host in turn continuously deploys an array of innate and acquired cellular and humoral defences to prevent pneumococci from breaching tissue barriers. Discoveries into essential molecular mechanisms used by pneumococci to evade host-sensing systems that are designed to contain the pathogen provide new insights into potential treatment options. Versatility of the genome of pneumococci and the bacteria's polygenic virulence capabilities show that a multifaceted approach with many vaccine antigens, antibiotic combinations, and immunoadjuvant therapies will be needed to control this microbe.

[The efficiency of the bactericidal action of serum raised by complement and lysozyme against bacteria which avoid the immunological response of higher organisms]

This paper presents some processes of the antibacterial effect of serum, which mainly results from the activities of complement (C) and lysozyme (muramidase, LZ). The C system consists ofa group of serum proteins and tissue fluids which are activated in a particular order. Complement,operating together with lysozyme, constitutes the main protection from microorganisms entering the body. Pathogenic microorganisms are able to avoid natural protective mechanisms by, among others, molecular mimicry, binding complement control proteins, or secreting proteolytic enzymes.The effectiveness of the cytolytic action of C proteins and LZ also depends on the surface structures of the microorganisms. Imbalance between the activation and deactivation of inflammatory reactions in the presence of pathogens can lead to various pathological states, such as autoimmunological diseases.

Th17 cytokines and host-pathogen interactions at the mucosa: dichotomies of help and harm

The mucosal surfaces are often the first site of interaction between pathogenic microorganisms and the host. Activation of the mucosal immune response has the important function of containing an infection and preventing dissemination of pathogens to systemic sites (barrier function). Numerous lines of evidence suggest that the barrier function is orchestrated by a subset of cytokines (interleukin (IL-)17 and IL-22), which belong to the Th17 family. IL-17 and IL-22 induce expression of antimicrobial peptides and neutrophil chemoattractants at mucosal sites, and thus play an important role in controlling mucosal infections. However, there is increasing evidence that mucosal pathogens achieve greater colonization during inflammation because they are resistant to a subset of these antimicrobial responses. In this review we compare the antimicrobial responses elicited by Th17 cytokines during mucosal infections with four different pathogens: Klebsiella pneumoniae, Citrobacter rodentium, Candida albicans and Salmonella typhimurium. We will then discuss which responses may constitute the mucosal barrier, thus providing a benefit to the host, and which ones may promote the colonization of pathogens, thereby providing a benefit to the microbes.

Coexistence conditions for strains of influenza with immune cross-reaction

The accumulation of cross-immunity in the host population is an important factor driving the antigenic evolution of viruses such as influenza A. Mathematical models have shown that the strength of temporary non-specific cross-immunity and the basic reproductive number are both key determinants for evolutionary branching of the antigenic phenotype. Here we develop deterministic and stochastic versions of one such model. We examine how the time of emergence or introduction of a novel strain affects co-existence with existing strains and hence the initial establishment of a new evolutionary branch. We also clarify the roles of cross-immunity and the basic reproductive number in this process. We show that the basic reproductive number is important because it affects the frequency of infection, which influences the long term immune profile of the host population. The time at which a new strain appears relative to the epidemic peak of an existing strain is important because it determines the environment the emergent mutant experiences in terms of the short term immune profile of the host population. Strains are more likely to coexist, and hence to establish a new clade in the viral phylogeny, when there is a significant time overlap between their epidemics. It follows that the majority of antigenic drift in influenza is expected to occur in the earlier part of each transmission season and this is likely to be a key surveillance period for detecting emerging antigenic novelty.

Torquetenovirus DNA drives proinflammatory cytokines production and secretion by immune cells via toll-like receptor 9

Active infection with torquetenovirus (TTV) has been associated with an increased severity of diseases in which inflammation plays a particularly important pathogenetic role. Here, we report that cloned DNA of a genogroup 4 TTV (ViPiSAL) is an activator of proinflammatory cytokine production by murine spleen cells and that the effect is mediated via toll-like receptor (TLR)9. The same DNA also increased the levels of proinflammatory cytokines induced by two well-characterized TLR9 stimulants. Finally, in silico analyses of the genomes of ViPiSAL and other TTVs revealed marked differences in the representation of CpG motifs known to be most effective at activating immune cells via TLR9. These findings demonstrate for the first time that at least one TTV isolate has the potential to stimulate and co-stimulate inflammatory responses.

Suppression of cell-mediated immunity following recognition of phagosome-confined bacteria

Listeria monocytogenes is a facultative intracellular pathogen capable of inducing a robust cell-mediated immune response to sub-lethal infection. The capacity of L. monocytogenes to escape from the phagosome and enter the host cell cytosol is paramount for the induction of long-lived CD8 T cell–mediated protective immunity. Here, we show that the impaired T cell response to L. monocytogenes confined within a phagosome is not merely a consequence of inefficient antigen presentation, but is the result of direct suppression of the adaptive response. This suppression limited not only the adaptive response to vacuole-confined L. monocytogenes, but negated the response to bacteria within the cytosol. Co-infection with phagosome-confined and cytosolic L. monocytogenes prevented the generation of acquired immunity and limited expansion of antigen-specific T cells relative to the cytosolic L. monocytogenes strain alone. Bacteria confined to a phagosome suppressed the production of pro-inflammatory cytokines and led to the rapid MyD88-dependent production of IL-10. Blockade of the IL-10 receptor or the absence of MyD88 during primary infection restored protective immunity. Our studies demonstrate that the presence of microbes within a phagosome can directly impact the innate and adaptive immune response by antagonizing the signaling pathways necessary for inflammation and the generation of protective CD8 T cells.

Little is understood about how the immune system distinguishes between pathogenic and non-pathogenic microbes. Limiting or preventing infections by intracellular pathogens requires the activation of innate immunity and the consequent generation of effector and memory T cells, which recognize and kill infected cells. Investigators are currently testing attenuated versions of pathogenic microbes as vaccines in an attempt to generate pathogen-specific T cells without causing disease. Unfortunately, attenuated microbes often fail to elicit long-lived protective immunity. We hypothesized that attenuated bacterial vaccines do not immunize because they fail to activate a stimulatory arm of host innate immune receptors. However, we found that these attenuated bacterial vaccines are not simply prevented from activating immunity, but rather generate a negative signal that inhibits the desired immune response. These studies may explain why the addition of an adjuvant to ineffective vaccines does not necessarily improve immunogenicity. Furthermore, these studies provide a framework for the development of attenuated vaccines that do not inhibit the desired immune responses.

O-antigen delays lipopolysaccharide recognition and impairs antibacterial host defense in murine intestinal epithelial cells

Although Toll-like receptor (TLR) 4 signals from the cell surface of myeloid cells, it is restricted to an intracellular compartment and requires ligand internalization in intestinal epithelial cells (IECs). Yet, the functional consequence of cell-type specific receptor localization and uptake-dependent lipopolysaccharide (LPS) recognition is unknown. Here, we demonstrate a strikingly delayed activation of IECs but not macrophages by wildtype Salmonella enterica subsp. enterica sv. (S.) Typhimurium as compared to isogenic O-antigen deficient mutants. Delayed epithelial activation is associated with impaired LPS internalization and retarded TLR4-mediated immune recognition. The O-antigen-mediated evasion from early epithelial innate immune activation significantly enhances intraepithelial bacterial survival in vitro and in vivo following oral challenge. These data identify O-antigen expression as an innate immune evasion mechanism during apical intestinal epithelial invasion and illustrate the importance of early innate immune recognition for efficient host defense against invading Salmonella.

The mammalian host recognizes infection by the detection of particular microbial structures. Recognition of these structures leads to activation of host defense effector mechanisms that in turn combat infection. A very potent activating microbial structure is lipopolysaccharide, a cell wall component released by many bacteria such as Salmonella, one of the most frequent causative agents of foodborne infection of the gut. We previously showed that cells lining the gut surface require uptake of bacterial lipopolysaccharide for its detection. The functional consequence of lipopolysaccharide uptake, however, was unknown. Here, we demonstrate that the uptake of lipopolysaccharide released by Salmonella is impaired by its extensive sugar modification. Impaired lipopolysaccharide uptake prevents early activation of host defense mechanisms and thereby allows Salmonella to better survive and proliferate within the host's intestinal cells. Thus, this lipopolysaccharide modification represents a mechanism by which Salmonella impairs recognition by the mammalian host to more efficiently cause infection of the intestinal mucosa.

CD8(+) T cells restrict Yersinia pseudotuberculosis infection: bypass of anti-phagocytosis by targeting antigen-presenting cells

All Yersinia species target and bind to phagocytic cells, but uptake and destruction of bacteria are prevented by injection of anti-phagocytic Yop proteins into the host cell. Here we provide evidence that CD8⁺ T cells, which canonically eliminate intracellular pathogens, are important for restricting Yersinia, even though bacteria are primarily found in an extracellular locale during the course of disease. In a model of infection with attenuated Y. pseudotuberculosis, mice deficient for CD8⁺ T cells were more susceptible to infection than immunocompetent mice. Although exposure to attenuated Y. pseudotuberculosis generated T_H1-type antibody responses and conferred protection against challenge with fully virulent bacteria, depletion of CD8⁺ T cells during challenge severely compromised protective immunity. Strikingly, mice lacking the T cell effector molecule perforin also succumbed to Y. pseudotuberculosis infection. Given that the function of perforin is to kill antigen-presenting cells, we reasoned that cell death marks bacteria-associated host cells for internalization by neighboring phagocytes, thus allowing ingestion and clearance of the attached bacteria. Supportive of this model, cytolytic T cell killing of Y. pseudotuberculosis–associated host cells results in engulfment by neighboring phagocytes of both bacteria and target cells, bypassing anti-phagocytosis. Our findings are consistent with a novel function for cell-mediated immune responses protecting against extracellular pathogens like Yersinia: perforin and CD8⁺ T cells are critical for hosts to overcome the anti-phagocytic action of Yops.

Pathogenic Yersinia are bacteria that cause diverse diseases such as gastroenteritis and plague. Yersinia binds to specialized immune cells called macrophages, which attempt to engulf and destroy the bacteria. The bacteria resist destruction by injecting proteins called Yops into macrophages, which stops the engulfment process. Yersinia thus survives as attached but extracellular bacteria to cause disease. Yersinia disease can be prevented by immunization. In this study, we identified one mechanism of protective immunity—that host cells called CD8⁺ T lymphocytes are important to restrict Yersinia infection. This observation is unusual because CD8⁺ T cells generally protect against intracellular pathogens: T cells destroy the host cell harboring the pathogen, thus preventing the pathogen's replication. We present data consistent with the model that CD8⁺ T cells can also restrict extracellular bacteria by showing that T cells target host cells with extracellularly attached Yersinia, thus allowing the host cells and associated bacteria to be engulfed and removed by neighboring macrophages.

Differential modulation of beta-defensin gene expression by Salmonella Enteritidis in intestinal epithelial cells from resistant and susceptible chicken inbred lines

beta-Defensins are important components of innate immunity in mucosal tissue, a major entry site for several pathogens. These small cationic peptides possess antimicrobial activity against various microorganisms including Salmonella. Two chicken inbred lines, 6 and 15I, diverge phenotypically with respect to levels of Salmonella Enteritidis intestinal carriage and to level of gene expression of two beta-defensins, AvBD1 and AvBD2. The cellular source of these two defensins in the intestinal tissue has not previously been explored. Therefore embryonic intestinal cells were isolated from both chicken lines. Primary intestinal cell cultures expressed epithelial specific markers (villin and E-cadherin) and differentially expressed two beta-defensin genes AvBD1 and AvBD2 according to chicken line. Furthermore, S. Enteritidis interfered with AvBD2 expression only in the cells from the susceptible line 15I. Our embryonic cell culture model demonstrated that intestinal epithelium express beta-defensin antimicrobial peptides that may play a role in immunoprotection against Salmonella Enteritidis.

[Salmonella enterica and cytokine system of the host]

Contemporary data on mechanisms of immune defense during salmonellosis as well as on ability of Salmonella to resist these mechanisms are presented in the review. Main emphasis is made on reaction of cytokine system of innate and adaptive immunity described in experimental and clinical studies.

Infection, immunity and the neuroendocrine response

The Central Nervous (CNS) and Immune Systems (IS) are the two major adaptive systems which respond rapidly to numerous challenges that are able to compromise health. The defensive response strictly linking innate to acquired immunity, works continuously to limit pathogen invasion and damage. The efficiency of the innate response is crucial for survival and for an optimum priming of acquired immunity. During infection, the immune response is modulated by an integrated neuro-immune network which potentiates innate immunity, controls potential harmful effects and also addresses metabolic and nutritional modifications supporting immune function. In the last decade much knowledge has been gained on the molecular signals that orchestrate this integrated adaptive response, with focus on the systemic mediators which have a crucial role in driving and controlling an efficient protective response. These mediators are also able to signal alterations and control pathway dysfunctions which may be involved in the persistence and/or overexpression of inflammation that may lead to tissue damage and to a negative metabolic impact, causing retarded growth. This review aims to describe some important signalling pathways which drive bidirectional communication between the Immune and Nervous Systems during infection. Particular emphasis is placed on pro-inflammatory cytokines, immunomodulator hormones such as Glucocorticoids (GCs), Growth hormone (GH), Insulin-like Growth Factor-1 (IGF-1), and Leptin, as well as nutritional factors such as Zinc (Zn). Finally, the review includes up-to-date information on this neuroimmune cross-talk in domestic animals. Data in domestic animal species are still limited, but there are several exciting areas of research, like the potential interaction pathways between mediators (i.e. cytokine-HPA regulation, IL-6-GCS-Zn, cytokines-GH/IGF-1, IL-6-GH-Leptin and thymus activity) that are or could be promising topics of future research in veterinary medicine.

Analysis of genome expression in the response of Oryza granulata to Xanthomonas oryzae pv oryzae

In order to understand the mechanism of the strong resistance of Oryza granulata to Xanthomonas oryzae pv oryzae (Xoo), cDNA microarrays containing 2,436 cDNA clones of Oryza granulata derived from Suppression subtractive library and cDNA library were constructed and genome expression patterns after inoculating Xoo were investigated. Three hundred and 83 clones were up-regulated, 836 clones were down-regulated after pathogen infection. Approximately 800 clones were sequenced and BLAST search were carried out. There are no homologous sequences for 35 clones of them. The functions of the homologous sequences for most clones are unknown. The known functions of the homologous sequences involved in photosynthesis, respiration, material transport, signal transduction, pathogenesis-related proteins, transcription factors, the active oxygen scavenging system and so on. The putative functions of them in responding to Xoo were discussed.

Linking development to defense: auxin in plant-pathogen interactions

Although the plant growth hormone auxin has long been recognized as a regulator of plant defense, the molecular mechanisms involved are still largely unknown. Recent studies reviewed here reveal new insights into the role of auxin in plant defense. Similar to the signaling pathways of the defense-associated plant hormones salicylic acid (SA) and jasmonic acid (JA), auxin signaling differentially affects resistance to separate pathogen groups. Recent evidence suggests that the auxin and SA pathways act in a mutually antagonistic manner during plant defense, whereas auxin and JA signaling share many commonalities. Auxin also affects disease outcomes indirectly through effects on development. Here, we discuss the multiple ways in which auxin regulation of plant growth and development might be intimately linked to plant defense.

Epigenetic silencing of host cell defense genes enhances intracellular survival of the rickettsial pathogen Anaplasma phagocytophilum

Intracellular bacteria have evolved mechanisms that promote survival within hostile host environments, often resulting in functional dysregulation and disease. Using the Anaplasma phagocytophilum–infected granulocyte model, we establish a link between host chromatin modifications, defense gene transcription and intracellular bacterial infection. Infection of THP-1 cells with A. phagocytophilum led to silencing of host defense gene expression. Histone deacetylase 1 (HDAC1) expression, activity and binding to the defense gene promoters significantly increased during infection, which resulted in decreased histone H3 acetylation in infected cells. HDAC1 overexpression enhanced infection, whereas pharmacologic and siRNA HDAC1 inhibition significantly decreased bacterial load. HDAC2 does not seem to be involved, since HDAC2 silencing by siRNA had no effect on A. phagocytophilum intracellular propagation. These data indicate that HDAC up-regulation and epigenetic silencing of host cell defense genes is required for A. phagocytophilum infection. Bacterial epigenetic regulation of host cell gene transcription could be a general mechanism that enhances intracellular pathogen survival while altering cell function and promoting disease.

Although the main function of defense cells is to eliminate invading infections, some intracellular bacterial pathogens manage to turn defense cells into suitable hosts for bacterial propagation. In doing so, intracellular pathogens dysregulate host cell function and cause disease. With genomic and metabolic resources thousands of times more limited than the host's, intracellular bacteria have evolved very efficient mechanisms to globally subvert the host defense. Here, we define a mechanism by which the intracellular pathogen Anaplasma phagocytophilum globally inhibits host cell defenses by affecting mechanisms of epigenetic control of defense gene expression. Silencing or inhibition of the host protein HDAC1 has a negative effect on intracellular bacterial replication, whereas HDAC1 overexpression leads to defense gene silencing and facilitates intracellular bacterial survival. This study not only provides new insight into a mechanism of host cell subversion, but also identifies a potential target for future development of novel therapeutic intervention strategies.

Structure and function of murine cytomegalovirus MHC-I-like molecules: how the virus turned the host defense to its advantage

The mouse cytomegalovirus (CMV), a beta-herpesvirus, exploits its large (~230 kb) double-stranded DNA genome for both essential and non-essential functions. Among the non-essential functions are those that offer the virus a selective advantage in eluding both the innate and adaptive immune responses of the host. Several non-essential genes of MCMV are thought to encode MHC-I-like genes and to function as immunoevasins. To understand further the evolution and function of these viral MHC-I (MHC-Iv) molecules, X-ray structures of several of them have been determined, not only confirming the overall MHC-I-like structure, but also elucidating features unique to this family. Future efforts promise to clarify the nature of the molecular ligands of these molecules, their evolution in the context of the adapting immune response of the murine host, and by analogy the evolution of the host response to human CMV as well.

[Cooperation between picornaviral proteins for the overcoming of cellular protective mechanisms]

Picornaviruses can overcome cellular mechanisms of antiviral defense by inhibiting cellular RNA and protein synthesis, nuclear-cytoplasmic transport and secretory pathways. The participation of proteases 2A and 3C and membrane proteins 2B and 3A in these processes is described. Four steps of the viral inhibition of cellular gene expression and the reciprocal supplement between these steps are discussed.

Location of pathogenic bacteria during persistent infections: insights from an analysis using game theory

Bacterial persistent infections are responsible for a significant amount of the human morbidity and mortality. Unlike acute bacterial infections, it is very difficult to treat persistent bacterial infections (e.g. tuberculosis). Knowledge about the location of pathogenic bacteria during persistent infection will help to treat such conditions by designing novel drugs which can reach such locations. In this study, events of bacterial persistent infections were analyzed using game theory. A game was defined where the pathogen and the host are the two players with a conflict of interest. Criteria for the establishment of Nash equilibrium were calculated for this game. This theoretical model, which is very simple and heuristic, predicts that during persistent infections pathogenic bacteria stay in both intracellular and extracellular compartments of the host. The result of this study implies that a bacterium should be able to survive in both intracellular and extracellular compartments of the host in order to cause persistent infections. This explains why persistent infections are more often caused by intracellular pathogens like Mycobacterium and Salmonella. Moreover, this prediction is in consistence with the results of previous experimental studies.

Hepatitis C virus (HCV) employs multiple strategies to subvert the host innate antiviral response

Hepatitis C virus (HCV) is a serious global health problem which accounts for approximately 40% of chronic liver diseases worldwide. HCV frequently establishes a persistent infection, although it is recognized and targeted by innate immunity as well as cellular and humoral immune mechanisms. This suggests that HCV has developed powerful strategies to escape elimination by innate and adaptive immunity. HCV-induced liver injury is thought to be mainly immune-mediated rather than due to direct cytopathic effects of the virus. Hence, therapeutic strategies should target those mechanisms favoring viral persistence since unspecific enhancement of host antiviral immunity may theoretically also promote liver injury. The present review summarizes our current understanding of how the hepatitis C virus interferes with the innate antiviral host-response to establish persistent infection.

Chromosomal location and inheritance of stem rust resistance transferred from Hordeum bulbosum into cultivated barley (H. vulgare)

Stem rust, caused by Puccinia graminis f. sp. tritici, is an important disease on barley (Hordeum vulgare). Host resistance has effectively controlled stem rust, primarily through use of gene Rpg1. However, virulence to Rpg1 is present in North America, and a new race (TTKSK, or Ug99) from eastern Africa threatens barley production. A search for novel resistance was previously conducted, and an interspecific barley breeding line (212Y1) with introgressed chromatin from H. bulbosum was identified as carrying resistance to races MCCF and QCCJ. This study evaluated the inheritance of resistance in 212Y1 using populations from crosses to Morex (Rpg1 donor) and Q21861 (rpg4 donor) and the pathogen races MCCF (avirulent on Rpg1 and rpg4) and QCCJ (virulent on Rpg1 and avirulent on rpg4), and determined the chromosomal position of the introgression using genomic in situ hybridization (GISH) and chromosome-specific polymerase chain reaction (PCR)-based markers. Progeny from the 212Y1/Q21861 F(2) population segregated for resistant and susceptible plants, indicating different gene loci. Genetic analyses of Morex/212Y1 F(3) families fit a 7 homozygous resistant (HR):8 segregating:1 homozygous susceptible (HS) family segregation ratio to race MCCF, indicating that two genes controlled resistance. Plants in segregating families were in 3R:1S (Rpg1), 13R:3S (Rpg1+212Y1), and 1R:3S (212Y1) ratios. Genetic analyses of the same F(3) families fit a 1HR:2 segregating:1HS family segregation ratio to race QCCJ, indicating monogenic inheritance. Plants in segregating families were in a 1R: 3S ratio, indicating recessive inheritance in 212Y1. The introgression from H. bulbosum into H. vulgare was positioned on chromosome 6HS based on GISH and the PCR-based markers. No known stem rust resistance gene has previously been mapped to that region. Thus, it is proposed to name this novel gene from H. bulbosum as rpg6.

Characterization of resistance mechanisms to powdery mildew (Erysiphe betae) in beet (Beta vulgaris)

Beet powdery mildew incited by Erysiphe betae is a serious foliar fungal disease of worldwide distribution causing losses of up to 30%. In the present work, we searched for resistance in a germplasm collection of 184 genotypes of Beta vulgaris including fodder (51 genotypes), garden (60 genotypes), leaf (51 genotypes), and sugar (22 genotypes) beet types. Resistant genotypes were identified in the four beet types under study. In addition, mechanisms underlying resistance were dissected through histological studies. These revealed different resistance mechanisms acting at different fungal developmental stages, i.e., penetration resistance, early and late cell death, or posthaustorial resistance. Most genotypes were able to hamper fungal development at several stages. The later are interesting for breeding aiming to resistance durability. Furthermore, characterization of defense mechanisms will be useful for further cellular and molecular studies to unravel the bases of resistance in this species.

The role of NF-kappaB factor REL2 in the Aedes aegypti immune response

Mosquitoes transmit numerous diseases that continue to be an enormous burden on public health worldwide. Transgenic mosquitoes impervious to vector-borne pathogens, in concert with vector control and drug and vaccine development, comprise an arsenal of means anticipated to defeat mosquito-spread diseases in the future. Mosquito transgenesis allows tissue-specific manipulation of their major immune pathways and enhances the ability to study mosquito-pathogen interactions. Here, we report the generation of two independent transgenic strains of Aedes aegypti overexpressing the NF-?B transcriptional factor REL2, a homologue of Drosophila Relish, which is shown to be under the control of the vitellogenin promoter in the mosquito fat body after a blood meal. We show that this REL2 overexpression in the fat body results in transcriptional activation of Defensins A, C, and D, and Cecropins A and N, as well as translation and secretion of Defensin A protein into the hemolymph. We also demonstrate that induction of REL2 results in the increased resistance of the mosquito to tested Gram-negative and Gram-positive bacteria. Importantly, induction of transgenic REL2 leads to the significant decrease in susceptibility of A. aegypti to Plasmodium gallinaceum infection. Consistently, RNAi knockdown of REL2 in wild-type mosquitoes results in a delay in Defensin A and Cecropin A expression in response to infection and in increased susceptibility to both bacteria and P. gallinaceum. Moreover, our transgenic assays demonstrate that the N-terminus of the mosquito REL2, which includes the His/Gln-rich and serine-rich regions, plays a role in its transactivation properties.

MAVS-mediated apoptosis and its inhibition by viral proteins

Background

Host responses to viral infection include both immune activation and programmed cell death. The mitochondrial antiviral signaling adaptor, MAVS (IPS-1, VISA or Cardif) is critical for host defenses to viral infection by inducing type-1 interferons (IFN-I), however its role in virus-induced apoptotic responses has not been elucidated.

Principal Findings

We show that MAVS causes apoptosis independent of its function in initiating IFN-I production. MAVS-induced cell death requires mitochondrial localization, is caspase dependent, and displays hallmarks of apoptosis. Furthermore, MAVS^−/− fibroblasts are resistant to Sendai virus-induced apoptosis. A functional screen identifies the hepatitis C virus NS3/4A and the Severe Acute Respiratory Syndrome coronavirus (SARS-CoV) nonstructural protein (NSP15) as inhibitors of MAVS-induced apoptosis, possibly as a method of immune evasion.

Significance

This study describes a novel role for MAVS in controlling viral infections through the induction of apoptosis, and identifies viral proteins which inhibit this host response.

Bovine natural killer cells

Natural killer (NK) cells have received much attention due to their cytotoxic abilities, often with a focus on their implications for cancer and transplantation. But despite their name, NK cells are also potent producers of cytokines like interferon-gamma. Recent discoveries of their interplay with dendritic cells and T-cells have shown that NK cells participate significantly in the onset and shaping of adaptive cellular immune responses, and increasingly these cells have become associated with protection from viral, bacterial and parasitic infections. Furthermore, they are substantially present in the placenta, apparently participating in the establishment of normal pregnancy. Consequently, NK cells have entered arenas of particular relevance in veterinary immunology. Limited data still exist on these cells in domestic animal species, much due to the lack of specific markers. However, bovine NK cells can be identified as NKp46 (CD335) expressing, CD3(-) lymphocytes. Recent studies have indicated a role for NK cells in important infectious diseases of cattle, and identified important bovine NK receptor families, including multiple KIRs and a single Ly49. In this review we will briefly summarize the current understanding of general NK cell biology, and then present the knowledge obtained thus far in the bovine species.

Porcine Toll-like receptors: the front line of pathogen monitoring and possible implications for disease resistance

Toll-like receptors (TLRs) are the most famous pattern-recognition receptors (PRRs); they monitor pathogen-associated molecular patterns and play a critical role in activation of the immune system against infection. TLR gene mutations may affect the gene products in terms of their ligand-binding ability or their signal transduction ability after ligand binding; such changes have a great influence on pathogen monitoring and disease resistance. Thirteen mammalian TLRs have been identified, and genes corresponding to all 10 TLR genes identified in humans have been fully cloned in pigs. Porcine TLR gene coding sequences possess a large number of nonsynonymous single nucleotide polymorphisms (SNPs). They are concentrated in ectodomains, and may increase the variability of pathogen recognition in pig populations. We summarize the current knowledge of TLR molecules in mammals and livestock (particularly pigs) and speculate on the relationship between SNPs in porcine TLRs and their application to vaccine design and disease-resistance breeding.

Porcine host defense peptides: expanding repertoire and functions

Host defense peptides (HDPs) are a large group of innate immune effectors that are also termed antimicrobial peptides. Because of the rapid progress that has been made in completing several animal genomes, many HDPs have been systemically defined using bioinformatic analysis and partially characterized using reverse genomic approaches. In pigs, about 30 HDPs have been identified and partially characterized relative to structure and function. Antimicrobial activity of porcine HDPs has been extensively evaluated against a broad spectrum of microorganisms in vitro and evaluated for their protective role in vivo. Increasing evidence indicates that HDPs are functionally differentiated during posttranslational and postsecretory processing, and that the structural units for antimicrobial and immunoregulatory functions are separate. These findings suggest promising new avenues for therapeutic drug design based on HDPs, including porcine HDPs. This review summarizes and discusses advances in porcine HDPs research during the last decade with an emphasis on the rapidly expanding profiles and biological functions.

N-Acylhomoserine lactone-mediated quorum sensing: a twist in the tail and a blow for host immunity

Communication through quorum sensing (QS) enables bacterial populations to coordinate their behavior. Recent work on N-acylhomoserine lactone-mediated QS has revealed that some soil bacteria exploit host-derived substrates to generate an alternative N-substituted homoserine lactone. New light has also been shed on the mechanism by which N-(3-oxo-dodecanoyl)-L-homoserine lactone modulates host inflammatory signaling pathways to promote bacterial survival.

Antibody-based strategy to identify Candida albicans genes expressed during infections

Investigators have long used antibody-based screening strategies to identify Candida albicans immunogenic proteins and the genes that encode them during infections. With the recent availability of the C. albicans genome sequence and the development of genomic and proteomic technologies, it is now possible to efficiently conduct large-scale screening in standard research labs. C. albicans proteins and genes identified with a variety of screening methods have been implicated as important determinants of candidal virulence and exploited as vaccine and therapeutic targets. In this chapter, we describe methods used in our lab, in which sera recovered from patients with candidiasis are used to screen a C. albicans genomic DNA expression library. Immunoreactive colonies are detected by reaction with anti-human immunoglobulin, and the corresponding open reading frames are identified using the genome sequence database. The methods are also suitable for use with cDNA expression libraries, and they are complementary to proteomic screening strategies described elsewhere in this volume.

Does intra-individual major histocompatibility complex diversity keep a golden mean?

An adaptive immune response is usually initiated only if a major histocompatibility complex (MHC) molecule presents pathogen-derived peptides to T-cells. Every MHC molecule can present only peptides that match its peptide-binding groove. Thus, it seems advantageous for an individual to express many different MHC molecules to be able to resist many different pathogens. However, although MHC genes are the most polymorphic genes of vertebrates, each individual has only a very small subset of the diversity at the population level. This is an evolutionary paradox. We provide an overview of the current data on infection studies and mate-choice experiments and conclude that overall evidence suggests that intermediate intra-individual MHC diversity is optimal. Selective forces that may set an upper limit to intra-individual MHC diversity are discussed. An updated mathematical model based on recent findings on T-cell selection can predict the natural range of intra-individual MHC diversity. Thus, the aim of our review is to evaluate whether the number of MHC alleles usually present in individuals may be optimal to balance the advantages of presenting an increased range of peptides versus the disadvantages of an increased loss of T-cells.

In vitro systems for studying the interaction of fungal pathogens with primary cells from the mammalian innate immune system

The incidence of invasive fungal diseases has increased over the past decades, particularly in relation with the increase of immunocompromised patient cohorts (e.g., HIV-infected patients, transplant recipients, immunosuppressed patients with cancer). Opportunistic fungal pathogens such as Candida spp. are most often associated with serious systemic infections. Currently available antifungal drugs are rather unspecific, often with severe side effects. In some cases, their prophylactic use has favored emergence of resistant fungal strains. Major antifungal drugs target the biosynthesis of lipid components of the fungal plasma membrane or the assembly of the cell wall. For a more specific and efficient treatment and prevention of fungal infection, new therapeutic strategies are needed, including strengthening or stimulation of the residual host immune response. Achieving such a goal requires a better understanding of factors important for the defense and the survival of the host combating Candida spp. Where possible, primary cultures of mammalian immune cells of the innate immune system constitute a better suited model than transformed cell lines to study host-pathogen response and virulence. Hence, in vitro primary cell culture systems are a good strategy for a first screening of mutant strains of Candida spp. to identify virulence traits with regard to host cell response and pathogen invasion.

RNA recognition and signal transduction by RIG-I-like receptors

Viral infection is detected by cellular sensor molecules as foreign nucleic acids and initiates innate antiviral responses, including the activation of proinflammatory cytokines and type I interferon (IFN). Recent identification of cytoplasmic viral sensors, such as retinoic acid-inducible gene-I-like receptors (RLRs), highlights their significance in the induction of antiviral innate immunity. Moreover, it is intriguing to understand how they can discriminate endogenous RNA from foreign viral RNA and initiate signaling cascades leading to the induction of type I IFNs. This review focuses on the current understanding of the molecular machinery underlying RNA recognition and subsequent signal transduction by RLRs.

Pathogen recognition by innate immunity and its signaling

Mammalian immune response can be divided into innate and acquired immunity. Furthermore, much evidence has demonstrated that activation of innate immunity is a prerequisite to induction of acquired immunity. This paradigm shift has changed our thinking on the pathogenesis and treatment of infections, immune diseases, allergy, and cancers.

DAP10- and DAP12-associated receptors in innate immunity

The DAP10 and DAP12 signaling subunits are highly conserved in evolution and associate with a large family of receptors in hematopoietic cells, including dendritic cells, plasmacytoid dendritic cells, neutrophils, basophils, eosinophils, mast cells, monocytes, macrophages, natural killer cells, and some B and T cells. Some receptors are able to associate with either DAP10 or DAP12, which contribute unique intracellular signaling functions. Studies of humans and mice deficient in these signaling subunits have provided surprising insights into the physiological functions of DAP10 and DAP12, demonstrating that they can either activate or inhibit immune responses. DAP10- and DAP12-associated receptors have been shown to recognize both host-encoded ligands and ligands encoded by microbial pathogens, indicating that they play an important role in innate immune responses.

Approaching the RNA ligand for RIG-I?

Innate and antigen-specific antiviral immunity are triggered by immunorecognition of viral nucleic acids. The helicase retinoic acid-inducible gene I (RIG-I) (also known as DDX58) is the key sensor of negative strand RNA viruses in the cytosol of cells. RNA containing a triphosphate at the 5'-end was shown to activate RIG-I, but the exact structure of RNA supporting 5'-triphosphate recognition, the requirement of a 5'-triphosphate group, as well as the existence of RNA structures detected by RIG-I in the absence of 5'-triphosphate remain controversial. Here, we revisit the literature on RIG-I and RIG-I ligands. The literature proposes at least six different RIG-I ligands: (i) single strand with a 5'-triphosphate, (ii) double-stranded RNA with a 5'-triphosphate, (iii) 5'-triphosphate single-stranded RNA with A- and U-rich 3'-sequences, (iv) double-stranded RNA of intermediate length (>300 and <2000 bp) without 5'-triphosphate, (v) blunt-end short double-stranded RNA (23-30 bp) without 5'-triphosphate, and (vi) short double-stranded RNA (23-30 bp) with 5'-monophosphate. RIG-I thus seems promiscuous for a variety of different RNA molecules, very similar to the Toll-like receptors, of which 10 family members are sufficient for the safe detection of the microbial cosmos. In the light of these outstanding publications, it seems an unlikely possibility that there is a fundamental shortcoming in the design of all studies. Looking closely, the only issue that comes to mind is the in vitro transcription technique used by all investigators without confirming the identity of RNA products. This technique, together with the different biological systems used, the lack of dose responses and of proper comparison of different published ligands and controls leave us with more questions than answers as to what the exact RIG-I ligand is, if in fact it exists.

Neuroimmunomodulation during infectious diseases: mechanisms, causes and consequences for the host

In this special issue of Neuroimmunomodulation, the reader will find reviewed some of the hottest topics in the field of neuroendocrine-immune interactions, with emphasis on infectious diseases. For instance, the role that immunoendocrine alterations have during human tuberculosis, a chronic infectious disease, is discussed, and it is concluded that these immunoendocrine interactions may play a detrimental role during the disease, in terms of the development of protective immune responses, control of tissue damage and metabolic disorders. Also, the role that neuroimmunomodulation plays in parasitic diseases is presented, and emphasizes the importance of the host-parasite neuroimmunoendocrine network during helminth infection. Moreover, the influence of beta-androstenes on resistance to viral and bacterial infections is discussed, as well as the impact of infectious diseases upon neuroendocrine circuits. Also, the reader will find contributions to the study of sex hormones and modulation of immunity against leishmaniasis, as well as recent work on the immune mechanisms associated with resistance or susceptibility to parasitic diseases during pregnancy. We hope that our readers will find the first ever special issue devoted to neuroimmunomodulation and infectious diseases fascinating and enticing.

Innate immune recognition in tuberculosis infection

In this review, an overview of the host's innate immune response against Mycobacterium tuberculosis will be provided. In particular, M. tuberculosis interaction with Toll-like receptors (TLRs), lung surfactant proteins and the antimicrobial mechanisms in the macrophage will be discussed along with their importance in shaping adaptive immunity to tuberculosis infection.

Toll gates to periodontal host modulation and vaccine therapy

Toll-like receptors (TLRs) are central mediators of innate antimicrobial and inflammatory responses and play instructive roles in the development of the adaptive immune response. Thus when stimulated by certain agonists, TLRs serve as adjuvant receptors that link innate and adaptive immunity. However, when excessively activated or inadequately controlled during an infection, TLRs may contribute to immunopathology associated with inflammatory diseases, such as periodontitis. Moreover, certain microbial pathogens appear to exploit aspects of TLR signalling in ways that enhance their adaptive fitness. The diverse and important roles played by TLRs suggest that therapeutic manipulation of TLR signalling may have implications in the control of infection, attenuation of inflammation, and the development of vaccine adjuvants for the treatment of periodontitis. Successful application of TLR-based therapeutic modalities in periodontitis would require highly selective and precisely targeted intervention. This would in turn necessitate precise characterization of TLR signalling pathways in response to periodontal pathogens, as well as development of effective and specific agonists or antagonists of TLR function and signalling. This review summarizes the current status of TLR biology as it relates to periodontitis, and evaluates the potential of TLR-based approaches for host-modulation therapy in this oral disease.

Innate immunity to virus infection

The innate immune system is essential for the initial detection of invading viruses and subsequent activation of adaptive immunity. Three classes of receptors, designated retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs), Toll-like receptors (TLRs), and nucleotide oligomerization domain (NOD)-like receptors (NLRs), sense viral components, such as double-stranded RNA (dsRNA), single-stranded RNA, and DNA. RLRs and TLRs play essential roles in the production of type I interferons (IFNs) and proinflammatory cytokines in cell type-specific manners. While the RLRs play essential roles in the recognition of RNA viruses in various cells, plasmacytoid dendritic cells utilize TLRs for detecting virus invasion. NLRs play a role in the production of mature interleukin-1 beta to dsRNA stimulation. Activation of innate immune cells is critical for mounting adaptive immune responses. In this review, we discuss recent advances in our understanding of the mechanisms of viral RNA recognition by these different types of receptors and its relation to acquired immune responses.