Bacillus anthracis: a multi-faceted role for anthrax lethal toxin in thwarting host immune defenses

Lethal factor (LF), along with its receptor-binding partner protective antigen (PA), forms lethal toxin (LT), a critical virulence factor for Bacillus anthracis. LF is a Zn(2+) protease that cleaves specific mitogen activated protein kinase kinases (MAPKKs), inactivating signal transduction intermediates required for normal immune function. Initial research emphasized the role of LT in attenuating pro-inflammatory responses by macrophages, the primary targets of infection. More recent studies have revealed that LT affects a broad range of immune cells. In addition to direct effects on macrophages and neutrophils, LT suppresses the costimulatory functions of dendritic cells, thereby impeding essential cross-talk between innate and adaptive immune responses. Moreover, LT acts directly on T and B lymphocytes, blocking antigen receptor-dependent proliferation, cytokine production and Ig production. In this manner, LT mounts a broad-based attack on host immunity, thus providing B. anthracis with multiple mechanisms for avoiding protective host responses.     

Functional inactivation of immature dendritic cells by the intracellular parasite Toxoplasma gondii

Despite its noted ability to induce strong cellular immunity, and its known susceptibility to IFN-gamma-dependent immune effector mechanisms, the protozoan Toxoplasma gondii is a highly successful parasite, able to replicate, disseminate, and either kill the host or, more commonly, establish resistant encysted life forms before the emergence of protective immune responses. We sought to understand how the parasite gains the advantage. Using transgenic clonal parasite lines engineered to express fluorescent markers in combination with dendritic cells (DC) grown from the bone marrow of wild-type mice or transgenic mice expressing fluorescent protein-tagged MHC class II molecules, we used flow cytometry and fluorescence microscopy to analyze the responses of infected DC to both invasion by the parasite and subsequent DC maturation signals. We found that T. gondii preferentially invades immature dendritic cells but fails to activate them in the process, and renders them resistant to subsequent activation by TLR ligands or the immune-system-intrinsic maturation signal CD40L. The functional consequences of T. gondii-mediated suppression of DC activation are manifested in a relative inability of infected immature DC to activate naive CD4(+) Th lymphocytes, or to secrete cytokines, such IL-12 and TNF-alpha, that play important roles in innate and/or adaptive immunity. The findings reveal that T. gondii suppresses the ability of immature DC to participate in innate immunity and to induce adaptive immune responses. The ability of T. gondii to temporarily evade recognition could provide a selective advantage that permits dissemination and establishment before adaptive immune response initiation.     

Ebolavirus VP35 suppresses IFN production from conventional but not plasmacytoid dendritic cells

Ebolaviruses naturally infect a wide variety of cells including macrophages and dendritic cells (DCs), and the resulting cytokine and interferon-α/β (IFN) responses of infected cells are thought to influence viral pathogenesis. The VP35 protein impairs RIG-I-like receptor-dependent signaling to inhibit IFN production, and this function has been suggested to promote the ineffective host immune response characteristic of ebolavirus infection. To assess the impact of VP35 on innate immunity in biologically relevant primary cells, we used a recombinant Newcastle disease virus encoding VP35 (NDV/VP35) to infect macrophages and conventional DCs, which primarily respond to RNA virus infection via RIG-I-like pathways. VP35 suppressed not only IFN but also tumor necrosis factor (TNF)-α secretion, which are normally produced from these cells upon NDV infection. Additionally, in cells susceptible to the activity of VP35, IRF7 activation is impaired. In contrast, NDV/VP35 infection of plasmacytoid DCs, which activate IRF7 and produce IFN through TLR-dependent signaling, leads to robust IFN production. When plasmacytoid DCs deficient for TLR signaling were infected, NDV/VP35 was able to inhibit IFN production. Consistent with this, VP35 was less able to inhibit TLR-dependent versus RIG-I-dependent signaling in vitro. These data demonstrate that ebolavirus VP35 suppresses both IFN and cytokine production in multiple primary human cell types. However, cells that utilize the TLR pathway can circumvent this inhibition, suggesting that the presence of multiple viral sensors enables the host to overcome viral immune evasion mechanisms.     

Innate immunity to <Emphasis Type="Italic">Legionella</Emphasis> and toll-like receptors — <Emphasis Type="Italic">review</Emphasis>

Legionella is a parasite of eukaryotic cells, able to survive and replicate in a wide range of protozoan hosts. It can also infect humans as an opportunistic pathogen, primarily by interaction with alveolar macrophages. These bacteria can cause life-threatening pneumonia, especially in immunocompromised individuals. However, most infections triggered by Legionella are cleared by an efficient host immune system. The protective immune responses against Legionella are complex and multifaceted, involving many components of the immune system. Recognition of such components as LPS, flagellum, and peptidoglycan of L. pneumophila by the TLRs, which orchestrates the innate immune responses to Legionella, lays an important role in activation of monocytes and alveolar macrophages and, thus, in inhibition of intracellular proliferation of bacteria. MyD88-dependent signaling pathways are important for host protection against Legionella.     

The Yin and Yang of regulatory T cells in infectious diseases and avenues to target them

CD4⁺CD25⁺FoxP3⁺ regulatory T cells (Tregs) are key players for maintaining immune tolerance and for reducing the inflammation‐mediated tissue damage following infection. However, Tregs also suppress protective immune responses to pathogens (including virus, bacteria, parasites, and fungi) and vaccines and enhance pathogen persistence by inhibiting the activation and functions of both innate and adaptive immune cells such as dendritic cells, macrophages, and T and B lymphocytes and by promoting immunosuppressive environment. Therefore, equilibrium in the Treg number and function is important to ensure pathogen clearance and protection from infection‐associated immunopathologies. Recent advances in understanding of Treg influence on the outcome of infection opened new avenues to target them. Various small molecules, pharmacological inhibitors, monoclonal antibodies that target Tregs provided proof of concept in experimental models. The field also benefits from advances in other subjects, particularly oncology and autoimmunity, where Treg‐targeted therapies are exploited in the clinic to a greater extent. The future research should aim at translating this preclinical success to human application.     

Signal transduction in T helper cells: CD4 coreceptors exert complex regulatory effects on T cell activation and function

The immune system provides a highly sophisticated surveillance mechanism to detect diverse antigens and to protect the host organism from invading pathogens and altered cells (e.g., virus-infected and tumor cells). Adaptive immune responses depend on the recognition of antigen by specific antigen receptors that are expressed on the surface of T and B lymphocytes. Helper T cells provide regulatory functions and direct the adaptive immune system to respond appropriately to a particular antigen (i.e., cytotoxic T cell responses against viral infections and tumor cells, humoral responses against extracellular bacteria and parasitic worms). Helper T cells express CD4 coreceptors, which recognize conserved domains on proteins expressed by the class II major histocompatibility complex, the same proteins that present antigen to the T cell receptor. Recent progress in T cell biology has identified multiple regulatory functions of CD4 during thymocyte development and antigen stimulation of mature T helper cells. Signaling pathways induced by engagement of CD4 independently of T cell receptor signaling mediate these regulatory functions. In this review, we discuss the regulation of T cell signaling and emphasize the functional consequences of proper and improper CD4 coreceptor signaling.     

Opa proteins and CEACAMs: pathways of immune engagement for pathogenic Neisseria

Neisseria meningitidis and Neisseria gonorrhoeae are globally important pathogens, which in part owe their success to their ability to successfully evade human immune responses over long periods. The phase-variable opacity-associated (Opa) adhesin proteins are a major surface component of these organisms, and are responsible for bacterial adherence and entry into host cells and interactions with the immune system. Most immune interactions are mediated via binding to members of the carcinoembryonic antigen cell adhesion molecule (CEACAM) family. These Opa variants are able to bind to different receptors of the CEACAM family on epithelial cells, neutrophils, and T and B lymphocytes, influencing the innate and adaptive immune responses. Increased epithelial cell adhesion creates the potential for prolonged infection, invasion and dissemination. Furthermore, Opa proteins may inhibit T-lymphocyte activation and proliferation, B-cell antibody production, and innate inflammatory responses by infected epithelia, in addition to conferring increased resistance to antibody-dependent, complement-mediated killing. While vaccines containing Opa proteins could induce adhesion-blocking and bactericidal antibodies, the consequence of CEACAM binding by a candidate Opa-containing vaccine requires further investigation. This review summarizes current knowledge of the immunological consequences of the interaction between meningococcal and gonococcal Opa proteins and human CEACAMs, considering the implications for pathogenesis and vaccine development.     

Regulatory T cells: friend or foe in immunity to infection?

Homeostasis in the immune system depends on a balance between the responses that control infection and tumour growth and the reciprocal responses that prevent inflammation and autoimmune diseases. It is now recognized that regulatory T cells have a crucial role in suppressing immune responses to self-antigens and in preventing autoimmune diseases. Evidence is also emerging that regulatory T cells control immune responses to bacteria, viruses, parasites and fungi. This article explores the possibility that regulatory T cells can be both beneficial to the host, through limiting the immunopathology associated with anti-pathogen immune responses, and beneficial to the pathogen, through subversion of the protective immune responses of the host.     

Bordetella adenylate cyclase toxin: a swift saboteur of host defense

Bordetella that infect mammals produce a multifunctional repeat in toxin (RTX) adenylate cyclase toxin known as CyaA, an excellent example of bacterial sophistication in subverting host defense. Recent reports show that interaction of CyaA with tracheal epithelial cells aids adhesion of Bordetella to ciliated mucosa and induces production of the pro-inflammatory cytokine interleukin, IL-6. Myeloid phagocytes, attracted to the site of infection are the target of freshly secreted CyaA that binds to the alpha(M)beta2 integrin (CD11b/CD18), penetrates cells and promptly suppresses their bactericidal functions by converting cellular ATP to cAMP. Such uncontrolled cAMP signaling can also drive CD11b-expressing immature dendritic cells into a semi-mature state, possibly hijacking them to shape the local adaptive immune response towards tolerance of the pathogen.     

Antitumor immunity and cellular cancer therapies

The identification of tumor specific antigens has provided important advance in tumor immunology. It is now established that specific cytotoxic T lymphocytes (CTL) and natural killer cells infiltrate tumor tissues and are effector cells able to control tumor growth. However, such a natural antitumor immunity has limited effects in cancer patients. Failure of host defenses against tumor is consecutive to several mechanisms which are becoming targets to design new immunotherapeutic approaches. CTL are critical components of the immune response to human tumors and induction of strong CTL responses is the goal of most current vaccine strategies. Effectiveness of cytokine therapy, cancer vaccines and injection of cells improving cellular immunity have been established in tumor grafted murine models. Clinical trials are underway. To day, interest is particularly focused on cell therapy: injected cells are either "ready to use" effector cells (lymphocytes) or antigen presenting cells able to induce a protective immune reaction in vivo (dendritic cells). The challenge ahead lie in the careful optimization of the most promising strategies in clinical situation.     

Dendritic cells as sensors of environmental perturbations

Dendritic cells were discovered 25 years ago as professional antigen presenting cells bridging together innate and adaptive immunity. Recently additional functions of dendritic cells have been uncovered indicating a relevant role of dendritic cells in immune system regulation. Indeed, they are the professional sensors of the immune system that can detect perturbations caused by non-self infectious as well as self non-infectious signals in most tissues. Dendritic cells discriminate both antigen amounts and antigen persistence through their receptor repertoire via the integration of different signaling pathways. The environment plays an essential role in conditioning the effector functions of dendritic cells leading either to the activation or suppression of adaptive immunity.     

Association of Neisseria gonorrhoeae OpaCEA with Dendritic Cells Suppresses Their Ability to Elicit an HIV-1-Specific T Cell Memory Response

Infection with Neisseria gonorrhoeae (N. gonorrhoeae) can trigger an intense local inflammatory response at the site of infection, yet there is little specific immune response or development of immune memory. Gonococcal surface epitopes are known to undergo antigenic variation; however, this is unlikely to explain the weak immune response to infection since individuals can be re-infected by the same serotype. Previous studies have demonstrated that the colony opacity-associated (Opa) proteins on the N. gonorrhoeae surface can bind human carcinoembryonic antigen-related cellular adhesion molecule 1 (CEACAM1) on CD4⁺ T cells to suppress T cell activation and proliferation. Interesting in this regard, N. gonorrhoeae infection is associated with impaired HIV-1 (human immunodeficiency virus type 1)-specific cytotoxic T-lymphocyte (CTL) responses and with transient increases in plasma viremia in HIV-1-infected patients, suggesting that N. gonorrhoeae may also subvert immune responses to co-pathogens. Since dendritic cells (DCs) are professional antigen presenting cells (APCs) that play a key role in the induction of an adaptive immune response, we investigated the effects of N. gonorrhoeae Opa proteins on human DC activation and function. While morphological changes reminiscent of DC maturation were evident upon N. gonorrhoeae infection, we observed a marked downregulation of DC maturation marker CD83 when the gonococci expressing CEACAM1-specific Opa_CEA, but not other Opa variants. Consistent with a gonococcal-induced defect in maturation, Opa_CEA binding to CEACAM1 reduced the DCs’ capacity to stimulate an allogeneic T cell proliferative response. Moreover, Opa_CEA-expressing N. gonorrhoeae showed the potential to impair DC-dependent development of specific adaptive immunity, since infection with Opa_CEA-positive gonococci suppressed the ability of DCs to stimulate HIV-1-specific memory CTL responses. These results reveal a novel mechanism to explain why infection of N. gonorrhoeae fails to trigger an effective specific immune response or develop immune memory, and may affect the potent synergy between gonorrhea and HIV-1 infection.     

Induction of enhanced immunity to intestinal nematodes using IL-9-producing dendritic cells

Dendritic cells can be considered natural adjuvants and are able to act as cellular vaccines to protect against disease. Adoptive transfer of Ag-pulsed bone marrow-derived dendritic cells (BMDCs) enhanced expulsion of the intestinal nematode, Trichinella spiralis, from the small intestine. IL 9 is a critical cytokine in protective immunity to intestinal nematode infection and is believed to enhance Th2 immune responses. Deriving dendritic cells from an IL-9 transgenic (IL-9t) mouse has enabled a detailed investigation of the importance of IL-9 during Ag presentation. Indeed, IL-9t dendritic cells significantly enhanced T cell proliferation and Th2 responses and, after adoptive transfer, enhanced parasite-specific IgG1 and intestinal mastocytosis in vivo, leading to accelerated expulsion of adult worms from the intestine. Overall, this paper demonstrates that dendritic cell vaccination can be used to successfully protect the host against intestinal nematode infection and suggests that IL-9 can act as a potent type 2 adjuvant during Ag presentation and the early stages of Th2 activation.     

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.     

IL-1R-associated kinase 4 is required for lipopolysaccharide-induced activation of APC

The bacterial product LPS is a critical stimulus for the host immune system in the response against the corresponding bacterial infection. LPS provides an activation stimulus for macrophages and a maturation signal for dendritic cells to set up innate and adaptive immune responses, respectively. The signaling cascade of myeloid differentiation factor 88-->IL-1R-associated kinase (IRAK)-->TNFR-associated factor 6 has been implicated in mediating LPS signaling. In this report, we studied the function of IRAK-4 in various LPS-induced signals. We found that IRAK-4-deficient cells were severely impaired in producing some IFN-regulated genes as well as inflammatory cytokines in response to LPS. Among the critical downstream signaling pathways induced by LPS, NF-kappaB activation but not IFN regulatory factor 3 or STAT1 activation was defective in cells lacking IRAK-4. IRAK-4 was also required for the proper maturation of dendritic cells by LPS stimulation, particularly in terms of cytokine production and the ability to stimulate Th cell differentiation. Our results demonstrate that IRAK-4 is critical for the LPS-induced activations of APCs.     

Toll-like receptors: linking innate and adaptive immunity

Detection of and response to microbial infections by the immune system depends largely on a family of pattern-recognition receptors called Toll-like receptors (TLRs). These receptors recognize conserved molecular products derived from various classes of pathogens, including Gram-positive and -negative bacteria, DNA and RNA viruses, fungi and protozoa. Recognition of ligands by TLRs leads to a series of signaling events resulting in induction of acute responses necessary to kill the pathogen. TLRs are also responsible for the induction of dendritic cell maturation, which is responsible and necessary for initiation of adaptive immune responses. Although TLRs control induction of adaptive immunity, it is not clear at this point how responses are appropriately tailored by individual TLRs to the advantage of the host.