Signalling C‐Type lectin receptors,microbial recognition and immunity

Signalling C‐type lectin receptors (CLRs) are crucial in shaping the immune response to fungal pathogens, but comparably little is known about the role of these receptors in bacterial, viral and parasitic infections. CLRs have many diverse functions depending on the signalling motifs in their cytoplasmic domains, and can induce endocytic, phagocytic, antimicrobial, pro‐inflammatory or anti‐inflammatory responses which are either protective or not during an infection. Understanding the role of CLRs in shaping anti‐microbial immunity offers great potential for the future development of therapeutics for disease intervention. In this review we will focus on the recognition of bacterial, viral and parasitic pathogens by CLRs, and how these receptors influence the outcome of infection. We will also provide a brief update on the role of CLRs in antifungal immunity.     

CpG oligodeoxynucleotides stimulate protective innate immunity against pulmonary Klebsiella infection

Bacterial pneumonia is a leading cause of mortality in the United States. Innate immune responses, including type-1 cytokine production, are critical to the effective clearance of bacterial pathogens from the lung. Synthetic oligodeoxynucleotides (ODN) containing unmethylated CpG dinucleotide motifs (CpG ODN), which mimic the effects of bacterial DNA, have been shown to enhance type-1 cytokine responses during infection due to intracellular pathogens, resulting in enhanced microbial clearance. The role of CpG ODN in modulating protective innate immunity against extracellular pathogens is unknown. Using a murine model of Gram-negative pneumonia, we found that CpG ODN administration stimulated protective immunity against Klebsiella pneumoniae. Specifically, intratracheal (i.t.) administration of CpG ODN (30 microg) 48 h before i.t. K. pneumoniae challenge resulted in increased survival, compared with animals pretreated with control ODN or saline. Pretreatment with CpG ODN resulted in enhanced bacterial clearance in lung and blood, and higher numbers of pulmonary neutrophils, NKT cells, gammadelta-T cells, and activated NK1.1+ cells and gammadelta-T lymphocytes during infection. Furthermore, pretreatment with CpG ODN enhanced the production of TNF-alpha, and type-1 cytokines, including IL-12, IFN-gamma, and the IFN-gamma-dependent ELR- CXC chemokines IFN-gamma-inducible protein-10 and monokine induced by IFN-gamma in response to Klebsiella challenge, compared with control mice. These findings indicate that i.t. administration of CpG ODN can stimulate multiple components of innate immunity in the lung, and may form the basis for novel therapies directed at enhancing protective immune responses to severe bacterial infections of the lung.     

Protozoan encounters with Toll-like receptor signalling pathways: implications for host parasitism

Toll-like receptors (TLRs) have emerged as a major receptor family involved in non-self recognition. They have a vital role in triggering innate immunity and orchestrate the acquired immune response during bacterial and viral infection. However, the role of TLRs during infection with protozoan pathogens is less clear. Nevertheless, our understanding of how these parasitic microorganisms engage the host TLR signalling system has now entered a phase of rapid expansion. This Review describes recent insights into how parasitic protozoans are sensed by TLR molecules, and how the TLR system itself can be targeted by these microbial pathogens for their own survival.     

Role of direct effects of IFN-gamma on T cells in the regulation of CD8 T cell homeostasis

It is well recognized that IFN-gamma plays a critical role in the control of CD8 T cell expansion and contraction during immune responses to several intracellular pathogens. However, our understanding of the mechanisms underlying the regulation of T cell fate by IFN-gamma is sorely incomplete. Specifically, it is unclear whether regulation of CD8 T cell homeostasis occurs by a T cell intrinsic IFN-gamma pathway. In this study, we have determined the role of the direct effects of IFN-gamma on T cells in regulating the expansion, contraction, and memory phases of the polyclonal CD8 T cell response to an acute viral infection. Using two complementary approaches we demonstrate that the direct effects of IFN-gamma suppress IL-7R expression on Ag-specific effector CD8 T cells, but clonal expansion or deletion of activated CD8 T cells in vivo can occur in the apparent absence of IFN-gammaR signaling in T cells. These findings have clarified fundamental features of control of T cell homeostasis by IFN-gamma in the context of CD8 T cell memory and protective immunity.     

Activation of virus-specific CD8+ T cells by lipopolysaccharide-induced IL-12 and IL-18

Virus-specific T cells represent a hallmark of Ag-specific, adaptive immunity. However, some T cells also demonstrate innate functions, including non-Ag-specific IFN-gamma production in response to microbial products such as LPS or exposure to IL-12 and/or IL-18. In these studies we examined LPS-induced cytokine responses of CD8(+) T cells directly ex vivo. Following acute viral infection, 70-80% of virus-specific T cells will produce IFN-gamma after exposure to LPS-induced cytokines, and neutralization experiments indicate that this is mediated almost entirely through production of IL-12 and IL-18. Different combinations of these cytokines revealed that IL-12 decreases the threshold of T cell activation by IL-18, presenting a new perspective on IL-12/IL-18 synergy. Moreover, memory T cells demonstrate high IL-18R expression and respond effectively to the combination of IL-12 and IL-18, but cannot respond to IL-18 alone, even at high cytokine concentrations. This demonstrates that the synergy between IL-12 and IL-18 in triggering IFN-gamma production by memory T cells is not simply due to up-regulation of the surface receptor for IL-18, as shown previously with naive T cells. Together, these studies indicate how virus-specific T cells are able to bridge the gap between innate and adaptive immunity during unrelated microbial infections, while attempting to protect the host from cytokine-induced immunopathology and endotoxic shock.     

CRISPR-Induced Distributed Immunity in Microbial Populations

In bacteria and archaea, viruses are the primary infectious agents, acting as virulent, often deadly pathogens. A form of adaptive immune defense known as CRISPR-Cas enables microbial cells to acquire immunity to viral pathogens by recognizing specific sequences encoded in viral genomes. The unique biology of this system results in evolutionary dynamics of host and viral diversity that cannot be fully explained by the traditional models used to describe microbe-virus coevolutionary dynamics. Here, we show how the CRISPR-mediated adaptive immune response of hosts to invading viruses facilitates the emergence of an evolutionary mode we call distributed immunity - the coexistence of multiple, equally-fit immune alleles among individuals in a microbial population. We use an eco-evolutionary modeling framework to quantify distributed immunity and demonstrate how it emerges and fluctuates in multi-strain communities of hosts and viruses as a consequence of CRISPR-induced coevolution under conditions of low viral mutation and high relative numbers of viral protospacers. We demonstrate that distributed immunity promotes sustained diversity and stability in host communities and decreased viral population density that can lead to viral extinction. We analyze sequence diversity of experimentally coevolving populations of Streptococcus thermophilus and their viruses where CRISPR-Cas is active, and find the rapid emergence of distributed immunity in the host population, demonstrating the importance of this emergent phenomenon in evolving microbial communities.     

Plasmodesmata: the battleground against intruders

Plasmodesmata are intercellular channels that establish a symplastic communication pathway between neighboring cells in plants. Owing to this role, opportunistic microbial pathogens have evolved to exploit plasmodesmata as gateways to spread infection from cell to cell within the plant. However, although these pathogens have acquired the capacity to breach the plasmodesmal trafficking pathway, plants are unlikely to relinquish control over a structure essential for their survival so easily. In this review, we examine evidence that suggests plasmodesmata play an active role in plant immunity against viral, fungal and bacterial pathogens. We discuss how these pathogens differ in their lifestyles and infection modes, and present the defense strategies that plants have adopted to prevent the intercellular spread of an infection.     

Neutrophils, dendritic cells and Toxoplasma

Toxoplasma gondii rapidly elicits strong Type 1 cytokine-based immunity. The necessity for this response is well illustrated by the example of IFN-gamma and IL-12 gene knockout mice that rapidly succumb to the effects of acute infection. The parasite itself is skilled at sparking complex interactions in the innate immune system that lead to protective immunity. Neutrophils are one of the first cell types to arrive at the site of infection, and the cells release several proinflammatory cytokines and chemokines in response to Toxoplasma. Dendritic cells are an important source of IL-12 during infection with T. gondii and other microbial pathogens, and they are also specialized for high-level antigen presentation to T lymphocytes. Tachyzoites express at least two types of molecules that trigger innate immune cell cytokine production. One of these involves Toll-like receptor/MyD88 pathways common to many microbial pathogens. The second pathway is less conventional and involves molecular mimicry between a parasite cyclophilin and host CC chemokine receptor 5-binding ligands. Neutrophils, dendritic cells and Toxoplasma work together to elicit the immune response required for host survival. Cytokine and chemokine cross-talk between parasite-triggered neutrophils and dendritic cells results in recruitment, maturation and activation of the latter. Neutrophil-empowered dendritic cells possess properties expected of highly potent antigen presenting cells that drive T helper 1 generation.     

Inhibition of pulmonary antibacterial defense by interferon-gamma during recovery from influenza infection

Secondary bacterial infection often occurs after pulmonary virus infection and is a common cause of severe disease in humans, yet the mechanisms responsible for this viral-bacterial synergy in the lung are only poorly understood. We now report that pulmonary interferon-gamma (IFN-gamma) produced during T cell responses to influenza infection in mice inhibits initial bacterial clearance from the lung by alveolar macrophages. This suppression of phagocytosis correlates with lung IFN-gamma abundance, but not viral burden, and leads to enhanced susceptibility to secondary pneumococcal infection, which can be prevented by IFN-gamma neutralization after influenza infection. Direct inoculation of IFN-gamma can mimic influenza infection and downregulate the expression of the class A scavenger receptor MARCO on alveolar macrophages. Thus, IFN-gamma, although probably facilitating induction of specific anti-influenza adaptive immunity, suppresses innate protection against extracellular bacterial pathogens in the lung.     

Autophagy controls plant basal immunity in a pathogenic lifestyle-dependent manner

Plant genomes harbor autophagy-related (ATG) genes that encode major components of the eukaryotic autophagic machinery. Autophagy in plants has been functionally linked to senescence, oxidative stress adaptation and the nutrient starvation response. In addition, plant autophagy has been assigned negative ('anti-death') and positive ('pro-death') regulatory functions in controlling cell death programs that establish sufficient immunity to microbial infection. The role of autophagy in plant disease and basal immunity to microbial infection has, however, not been studied in detail. We have employed a series of autophagy-deficient genotypes of the genetic model plant Arabidopsis thaliana in various infection systems. Genotypes lacking ATG5, ATG10 or ATG18a develop spreading necrosis and enhanced disease susceptibility upon infection with toxin-producing pathogens preferring a necrotrophic lifestyle. These findings suggest that autophagy positively controls the containment of host tissue integrity upon infections by host-destructive microbes. In contrast, autophagy-deficient genotypes exhibit markedly increased immunity to infections by biotrophic pathogens through altered homeostasis of the plant hormone salicylic acid, thus suggesting an additional negative regulatory role of autophagy in plant basal immunity. In sum, our findings suggest that the role of plant autophagy in immunity cannot be generalized, and depends critically on the lifestyle and infection strategy of invading microbes.     

Tonsilar NK cells restrict B cell transformation by the Epstein-Barr virus via IFN-gamma

Cells of the innate immune system act in synergy to provide a first line of defense against pathogens. Here we describe that dendritic cells (DCs), matured with viral products or mimics thereof, including Epstein-Barr virus (EBV), activated natural killer (NK) cells more efficiently than other mature DC preparations. CD56(bright)CD16(-) NK cells, which are enriched in human secondary lymphoid tissues, responded primarily to this DC activation. DCs elicited 50-fold stronger interferon-gamma (IFN-gamma) secretion from tonsilar NK cells than from peripheral blood NK cells, reaching levels that inhibited B cell transformation by EBV. In fact, 100- to 1,000-fold less tonsilar than peripheral blood NK cells were required to achieve the same protection in vitro, indicating that innate immune control of EBV by NK cells is most efficient at this primary site of EBV infection. The high IFN-gamma concentrations, produced by tonsilar NK cells, delayed latent EBV antigen expression, resulting in decreased B cell proliferation during the first week after EBV infection in vitro. These results suggest that NK cell activation by DCs can limit primary EBV infection in tonsils until adaptive immunity establishes immune control of this persistent and oncogenic human pathogen.     

Cutting edge: conventional CD8 alpha+ dendritic cells are generally involved in priming CTL immunity to viruses

Dendritic cells (DCs) play a central role in initiating immune responses. Despite this, there is little understanding how different DC subsets contribute to immunity to different pathogens. CD8alpha(+) DC have been shown to prime immunity to HSV. Whether this very limited capacity of a single DC subset priming CTL immunity is restricted to HSV infection or is a more general property of anti-viral immunity was examined. Here, we show that the CD8alpha(+) DCs are the principal DC subset that initiates CTL immunity to s.c. infection by influenza virus, HSV, and vaccinia virus. This same subset also dominated immunity after i.v. infection with all three viruses, suggesting a similar involvement in other routes of infection. These data highlight the general role played by CD8alpha(+) DCs in CTL priming to viral infection and raises the possibility that this DC subset is specialized for viral immunity.     

Viral evasion of autophagy

Autophagy is an evolutionarily ancient pathway for survival during different forms of cellular stress, including infection with viruses and other intracellular pathogens. Autophagy may protect against viral infection through degradation of viral components (xenophagy), by promoting the survival or death of infected cells, through delivery of Toll-like receptor (TLR) ligands to endosomes to activate innate immunity, or by feeding antigens to MHC class II compartments to activate adaptive immunity. Given this integral role of autophagy in innate and adaptive antiviral immunity, selective pressure likely promoted the emergence of escape mechanisms by pathogenic viruses. This review will briefly summarize the current understanding of autophagy as an antiviral pathway, and then discuss strategies that viruses may utilize to evade this host defense mechanism.     

Viral evasion of autophagy

Autophagy is an evolutionarily ancient pathway for survival during different forms of cellular stress, including infection with viruses and other intracellular pathogens. Autophagy may protect against viral infection through degradation of viral components (xenophagy), by promoting the survival or death of infected cells, through delivery of Toll-like receptor (TLR) ligands to endosomes to activate innate immunity, or by feeding antigens to MHC class II compartments to activate adaptive immunity. Given this integral role of autophagy in innate and adaptive antiviral immunity, selective pressure likely promoted the emergence of escape mechanisms by pathogenic viruses. This review will briefly summarize the current understanding of autophagy as an antiviral pathway, and then discuss strategies that viruses may utilize to evade this host defense mechanism.     

CXCR2 deficiency confers impaired neutrophil recruitment and increased susceptibility during Toxoplasma gondii infection.

Neutrophil migration to the site of infection is a critical early step in host immunity to microbial pathogens, in which chemokines and their receptors play an important role. In this work, mice deficient in expression of the chemokine receptor CXCR2 were infected with Toxoplasma gondii and the outcome was monitored. Gene-deleted animals displayed completely defective neutrophil recruitment, which was apparent at 4 h and sustained for at least 36 h. Kit(W)/Kit(W-v) animals also displayed defective polymorphonuclear leukocyte migration, suggesting mast cells as one source of chemokines driving the response. Tachyzoite infection and replication were accelerated in CXCR2(-/-) animals, resulting in establishment of higher cyst numbers in the brain relative to wild-type controls. Furthermore, serum and spleen cell IFN-gamma levels in infected, gene-deleted mice were reduced 60-75% relative to infected normal animals, and spleen cell TNF-alpha was likewise reduced by approximately 50%. These results highlight an important role for CXCR2 in neutrophil migration, which may be important for early control of infection and induction of immunity during Toxoplasma infection.     

Anti-immunology: evasion of the host immune system by bacterial and viral pathogens

Multicellular organisms possess very sophisticated defense mechanisms that are designed to effectively counter the continual microbial insult of the environment within the vertebrate host. However, successful microbial pathogens have in turn evolved complex and efficient methods to overcome innate and adaptive immune mechanisms, which can result in disease or chronic infections. Although the various virulence strategies used by viral and bacterial pathogens are numerous, there are several general mechanisms that are used to subvert and exploit immune systems that are shared between these diverse microbial pathogens. The success of each pathogen is directly dependant on its ability to mount an effective anti-immune response within the infected host, which can ultimately result in acute disease, chronic infection, or pathogen clearance. In this review, we highlight and compare some of the many molecular mechanisms that bacterial and viral pathogens use to evade host immune defenses.     

Autophagy controls plant basal immunity in a pathogenic lifestyle-dependent manner

Plant genomes harbor autophagy-related (ATG) genes that encode major components of the eukaryotic autophagic machinery. Autophagy in plants has been functionally linked to senescence, oxidative stress adaptation and the nutrient starvation response. In addition, plant autophagy has been assigned negative (‘anti-death’) and positive (‘pro-death’) regulatory functions in controlling cell death programs that establish sufficient immunity to microbial infection. The role of autophagy in plant disease and basal immunity to microbial infection has, however, not been studied in detail. We have employed a series of autophagy-deficient genotypes of the genetic model plant Arabidopsis thaliana in various infection systems. Genotypes lacking ATG5, ATG10 or ATG18a develop spreading necrosis and enhanced disease susceptibility upon infection with toxin-producing pathogens preferring a necrotrophic lifestyle. These findings suggest that autophagy positively controls the containment of host tissue integrity upon infections by host-destructive microbes. In contrast, autophagy-deficient genotypes exhibit markedly increased immunity to infections by biotrophic pathogens through altered homeostasis of the plant hormone salicylic acid, thus suggesting an additional negative regulatory role of autophagy in plant basal immunity. In sum, our findings suggest that the role of plant autophagy in immunity cannot be generalized, and depends critically on the lifestyle and infection strategy of invading microbes.     

Protein vaccines induce uncommitted IL-2-secreting human and mouse CD4 T cells, whereas infections induce more IFN-gamma-secreting cells

Mouse and human CD4 T cells primed during an immune response may differentiate into effector phenotypes such as Th1 (secreting IFN-gamma) or Th2 (secreting IL-4) that mediate effective immunity against different classes of pathogen. However, primed CD4 T cells can also remain uncommitted, secreting IL-2 and chemokines, but not IFN-gamma or IL-4. We now show that human CD4 T cells primed by protein vaccines mostly secreted IL-2, but not IFN-gamma, whereas in the same individuals most CD4 T cells initially primed by infection with live pathogens secreted IFN-gamma. We further demonstrate that many tetanus-specific IL-2+IFN-gamma- cells are uncommitted and that a single IL-2+IFN-gamma- cell can differentiate into Th1 or Th2 phenotypes following in vitro stimulation under appropriate polarizing conditions. In contrast, influenza-specific IL-2+IFN-gamma- CD4 cells maintained a Th1-like phenotype even under Th2-polarizing conditions. Similarly, adoptively transferred OTII transgenic mouse T cells secreted mainly IL-2 after priming with OVA in alum, but were biased toward IFN-gamma secretion when primed with the same OVA peptide presented as a pathogen Ag during live infection. Thus, protein subunit vaccines may prime a unique subset of differentiated, but uncommitted CD4 T cells that lack some of the functional properties of committed effectors induced by infection. This has implications for the design of more effective vaccines against pathogens requiring strong CD4 effector T cell responses.     

Osteopontin is not required for the development of Th1 responses and viral immunity

Osteopontin (OPN) has been defined as a key cytokine promoting the release of IL-12 and hence inducing the development of protective cell-mediated immunity to viruses and intracellular pathogens. To further characterize the role of OPN in antiviral immunity, OPN-deficient (OPN-/-) mice were analyzed after infection with influenza virus and vaccinia virus. Surprisingly, we found that viral clearance, lung inflammation, and recruitment of effector T cells to the lung were unaffected in OPN-/- mice after influenza infection. Furthermore, effector status of T cells was normal as demonstrated by normal IFN-gamma production and CTL lytic activity. Moreover, activation and Th1 differentiation of naive TCR transgenic CD4+ T cells by dendritic cells and cognate Ag was normal in the absence of OPN in vitro. Contrary to a previous report, we found that OPN-/- mice mounted a normal immune response to Listeria monocytogenes. In conclusion, OPN is dispensable for antiviral immune responses against influenza virus and vaccinia virus.