Macrophages acquire neutrophil granules for antimicrobial activity against intracellular pathogens

A key target of many intracellular pathogens is the macrophage. Although macrophages can generate antimicrobial activity, neutrophils have been shown to have a key role in host defense, presumably by their preformed granules containing antimicrobial agents. Yet the mechanism by which neutrophils can mediate antimicrobial activity against intracellular pathogens such as Mycobacterium tuberculosis has been a long-standing enigma. We demonstrate that apoptotic neutrophils and purified granules inhibit the growth of extracellular mycobacteria. Phagocytosis of apoptotic neutrophils by macrophages results in decreased viability of intracellular M. tuberculosis. Concomitant with uptake of apoptotic neutrophils, granule contents traffic to early endosomes, and colocalize with mycobacteria. Uptake of purified granules alone decreased growth of intracellular mycobacteria. Therefore, the transfer of antimicrobial peptides from neutrophils to macrophages provides a cooperative defense strategy between innate immune cells against intracellular pathogens and may complement other pathways that involve delivery of antimicrobial peptides to macrophages.     

Why intracellular parasitism need not be a degrading experience for Mycobacterium.

The success of mycobacteria as pathogens hinges on their ability to infect and persist within the macrophages of their host. However, activation of host macrophages by cytokines from a productive cellular immune response can stimulate the cells to kill their resident pathogens. This suggests that the interaction between host cell and microbe is in delicate balance, which can be tipped in favour of either organism. Biochemical analysis of mycobacterial vacuoles has shown them to be integral to the host cell''s recycling endosomal system. As such they show limited acidification and hydrolytic activity despite possession of known lysosomal constituents such as cathepsins D, B and L, and LAMP 1. Even in established infections, they remain dynamic compartments accessible to several plasmalemma-derived constituents. Once the macrophage has been activated by IFN-gamma and TNF-alpha the vacuoles coalesce and acidify. This marks a distinct alteration in vacuole physiology and leads to stasis and death of the mycobacteria. Mycobacteria have developed several strategies to avoid this outcome. Most notably, live bacilli-induce sustained release of IL-6 from infected macrophages. IL-6 blocks the ability of both polyclonal primary T cells and T-cell hybridomas to respond to appropriate stimuli. Such an activity could render the centres of infection foci, such as granulomas, anergic and thus avoid release of macrophage-activating cytokines. This paper discusses both the mechanisms by which mycobacteria try to ensure their success as intracellular pathogens and the relevance of these strategies to the overall understanding of mycobacterial diseases.     

Immunity to vacuolar pathogens: What can we learn from Legionella?

Intracellular pathogens can manipulate host cellular pathways to create specialized organelles. These pathogen-modified vacuoles permit the survival and replication of bacterial and protozoan microorganisms inside of the host cell. By establishing an atypical organelle, intracellular pathogens present unique challenges to the host immune system. To understand pathogenesis, it is important to not only investigate how these organisms create unique subcellular compartments, but to also determine how mammalian immune systems have evolved to detect and respond to pathogens sequestered in specialized vacuoles. Recent studies have identified genes in the respiratory pathogen Legionella pneumophila that are essential for establishing a unique endoplasmic reticulum-derived organelle inside of mammalian macrophages, making this pathogen an attractive model system for investigations on host immune responses that are specific for bacteria that establish vacuoles disconnected from the endocytic pathway. This review will focus on the host immune response to Legionella and highlight areas of Legionella research that should help elucidate host strategies to combat infections by intracellular pathogens.     

Immune Subversion by Mycobacterium tuberculosis through CCR5 Mediated Signaling: Involvement of IL-10

Tuberculosis is characterized by severe immunosuppression of the host macrophages, resulting in the loss of the host protective immune responses. During Mycobacterium tuberculosis infection, the pathogen modulates C-C Chemokine Receptor 5 (CCR5) to enhance IL-10 production, indicating the possible involvement of CCR5 in regulation of the host immune response. Here, we found that Mycobacterium infection significantly increased CCR5 expression in macrophages there by facilitating the activation of its downstream signaling. These events culminated in up-regulation of the immunosuppressive cytokine IL-10 production, which was further associated with the down-regulation of macrophage MHC-II expression along with the up-regulation of CCR5 expression via engagement of STAT-3 in a positive feedback loop. Treatment of macrophages with CCR5 specific siRNA abrogated the IL-10 production and restored MHCII expression. While, in vivo CCR5 silencing was also effective for the restoration of host immune responses against tuberculosis. This study demonstrated that CCR5 played a very critical role for the immune subversion mechanism employed by the pathogen.     

Breach of IL-12 monopoly in the initiation of type 1 immunity to intracellular infections: IL-12 is not required.

IL-12 is believed to play an important role in type 1 T-cell differentiation and type 1 cytokine IFN-gamma release by T- and NK-cells and macrophages in host defense against intracellular infections by bacteria, parasites, fungi and viruses. However, recent studies by us and others have provided unequivocal evidence that while IL- 12 is critically required for the development of type 1 immunity to the majority of intracellular bacterial, parasitic and fungal infections, it is not required for anti-viral type 1 immune responses. These findings have provoked our re-thinking about the role of IL-12 in type 1 immunity and the search for additional cytokines capable of initiating anti-viral type 1 immunity. We hypothesize that there exist multiple cytokines including IL-12 which play a redundant role in the initiation of type 1 immunity against viral infection. These cytokines are likely released from not only antigen-presenting macrophages/dendritic cells but many other cell types, which suits the mode of viral infection. The existence of multiple factors capable of driving type 1 immunity endows the host with additional safeguards to cope with prevalent viral foes.     

The multiple facets of HIV attachment to dendritic cell lectins

Entry of enveloped viruses into host cells depends on the interactions of viral surface proteins with cell surface receptors. Many enveloped viruses maximize the efficiency of receptor engagement by first binding to attachment‐promoting factors, which concentrate virions on target cells and thus increase the likelihood of subsequent receptor engagement. Cellular lectins can recognize glycans on viral surface proteins and mediate viral uptake into immune cells for subsequent antigen presentation. Paradoxically, many viral and non‐viral pathogens target lectins to attach to immune cells and to subvert cellular functions to promote their spread. Thus, it has been proposed that attachment of HIV to the dendritic cell lectin DC‐SIGN enables the virus to hijack cellular transport processes to ensure its transmission to adjacent T cells. However, recent studies show that the consequences of viral capture by immune cell lectins can be diverse, and can entail negative and positive regulation of viral spread. Here, we will describe key concepts proposed for the role of lectins in HIV attachment to host cells, and we will discuss recent findings in this rapidly evolving area of research.     

Masters of manipulation: Viral modulation of the immunological synapse

In order to thrive, viruses have evolved to manipulate host cell machinery for their own benefit. One major obstacle faced by pathogens is the immunological synapse. To enable efficient replication and latency in immune cells, viruses have developed a range of strategies to manipulate cellular processes involved in immunological synapse formation to evade immune detection and control T‐cell activation. In vitro, viruses such as human immunodeficiency virus 1 and human T‐lymphotropic virus type 1 utilise structures known as virological synapses to aid transmission of viral particles from cell to cell in a process termed trans‐infection. The formation of the virological synapse provides a gateway for virus to be transferred between cells avoiding the extracellular space, preventing antibody neutralisation or recognition by complement. This review looks at how viruses are able to subvert intracellular signalling to modulate immune function to their advantage and explores the role synapse formation has in viral persistence and cell‐to‐cell transmission.     

Possible mechanisms of acquisition of herpesvirus virokines

The genomes of certain types of human and primate herpesviruses contain functional homologs of important host cytokines (IL-6, IL-17, and IL-10), or so-called virokines. Virokines can interact with immune cell receptors, transmit a signal to them, and thus switch the type of immune response that facilitates viral infection development. In this work, we have summarized possible ways of virokine origin and proposed an evolutionary scenario of virokine acquisition with involvement of retroviral coinfection of the host. This scenario is probably valid for vIL-6 of HHV-8 and MRV-5 viruses, vIL-17 of HVS virus, and vIL-10 of HHV-4, Bonobo-HV, RhLCV, and BaLCV viruses. The ability to acquire cytokine genes allows herpesviruses to implement unique strategies of avoiding the immune response and provides them an evolutionary advantage: more than 90% of the host population can be chronically infected with different herpesviruses. It is possible that the biological success of herpesviruses can be partially due to their cooperation with another group of viruses. This hypothesis emphasizes the importance of studies on the reciprocal influence of pathogens on their coinfection, as well as their impact on the host organism.     

Chemokines and chemokine receptors in infectious diseases

Today, 10 years after the discovery of IL-8, chemokines (chemotactic cytokines) are seen as the stimuli that largely control leucocyte migration. Chemokines are low molecular weight chemoattractant cytokines secreted by a variety of cells, including leucocytes, epithelial cells, endothelial cells, fibroblasts and numerous other cell types. They are produced in response to exogenous stimuli, such as viruses and bacterial LPS, and endogenous stimuli, such as IL-1, TNF and IFN. These factors mediate chemotaxis and leucocyte activation. They also regulate leucocyte extravasation from the blood and/or lymph vessel luminal surface to the tissue space, the site of inflammation. There is no doubt that chemokines and chemokine receptors are critical for defence against infectious pathogens. It is also clear that these pathogens have evolved to accommodate the workings of the host immune system. Survival of these infectious agents appears dependent upon strategies that can evade, suppress, counteract or otherwise confound the constellation of host responses to invading pathogens. In this regard, the chemokines and their receptors are a major target. Reviewed in the present paper are several examples in which microbial pathogens have usurped the mammalian chemokine system to subvert the host immune response.     

Targeting of the Hydrophobic Metabolome by Pathogens

The hydrophobic molecules of the metabolome – also named the lipidome – constitute a major part of the entire metabolome. Novel technologies show the existence of a staggering number of individual lipid species, the biological functions of which are, with the exception of only a few lipid species, unknown. Much can be learned from pathogens that have evolved to take advantage of the complexity of the lipidome to escape the immune system of the host organism and to allow their survival and replication. Different types of pathogens target different lipids as shown in interaction maps, allowing visualization of differences between different types of pathogens. Bacterial and viral pathogens target predominantly structural and signaling lipids to alter the cellular phenotype of the host cell. Fungal and parasitic pathogens have complex lipidomes themselves and target predominantly the release of polyunsaturated fatty acids from the host cell lipidome, resulting in the generation of eicosanoids by either the host cell or the pathogen. Thus, whereas viruses and bacteria induce predominantly alterations in lipid metabolites at the host cell level, eukaryotic pathogens focus on interference with lipid metabolites affecting systemic inflammatory reactions that are part of the immune system. A better understanding of the interplay between host–pathogen interactions will not only help elucidate the fundamental role of lipid species in cellular physiology, but will also aid in the generation of novel therapeutic drugs.      

Autophagy in viral replication and pathogenesis

Autophagy is a catabolic process that is important for the removal of damaged organelles and long-lived proteins for the maintenance of cellular homeostasis. It can also serve as innate immunity to remove intracellular microbial pathogens. A growing list of viruses has been shown to affect this cellular pathway. Some viruses suppress this pathway for their survival, while others enhance or exploit this pathway to benefit their replication. The effect of viruses on autophagy may also sensitize cells to death or enhance cell survival and play a critical role in viral pathogenesis. In this article, we review the relationships between different viruses and autophagy and discuss how these relationships may affect viruses and their host cells.     

Bacterial entry into cells: a role for the endocytic machinery

Increasing evidence indicates that pathogens have evolved highly efficient strategies to induce their internalization within host cells. Viruses and bacteria express and expose on their surface, molecules that mimic endogenous ligands to cell receptors, thereby inducing specific intracellular signalling cascades. More recently it has become clear that, as most viruses, bacteria can enter cells via the clathrin-mediated pathway, indicating a key role for endocytosis in pathogens entry into cells. Here we review the pathways followed by Listeria monocytogenes to enter into non-phagocytic cells, as a model for the subversion of cellular functions to induce pathogens internalization.     

Modulation of inflammasome pathways by bacterial and viral pathogens

Inflammasomes are emerging as key regulators of the host response against microbial pathogens. These cytosolic multiprotein complexes recruit and activate the cysteine protease caspase-1 when microbes invade sterile tissues or elicit cellular damage. Inflammasome-activated caspase-1 induces inflammation by cleaving the proinflammatory cytokines IL-1β and IL-18 into their biologically active forms and by releasing the alarmin HMGB1 into the extracellular milieu. Additionally, inflammasomes counter bacterial replication and clear infected immune cells through an inflammatory cell death program termed pyroptosis. As a countermeasure, bacterial and viral pathogens evolved virulence factors to antagonize inflammasome pathways. In this review, we discuss recent progress on how inflammasomes contribute to host defense against bacterial and viral pathogens, and we review how viruses and bacteria modulate inflammasome function to their benefit.     

Infection of human macrophages and dendritic cells with Mycobacterium tuberculosis induces a differential cytokine gene expression that modulates T cell response

Macrophages and dendritic cells (DC) play an essential role in the initiation and maintenance of immune response to pathogens. To analyze early interactions between Mycobacterium tuberculosis (Mtb) and immune cells, human peripheral blood monocyte-derived macrophages (MDM) and monocyte-derived dendritic cells (MDDC) were infected with Mtb. Both cells were found to internalize the mycobacteria, resulting in the activation of MDM and maturation of MDDC as reflected by enhanced expression of several surface Ags. After Mtb infection, the proinflammatory cytokines TNF-alpha, IL-1, and IL-6 were secreted mainly by MDM. As regards the production of IFN-gamma-inducing cytokines, IL-12 and IFN-alpha, was seen almost exclusively from infected MDDC, while IL-18 was secreted preferentially by macrophages. Moreover, Mtb-infected MDM also produce the immunosuppressive cytokine IL-10. Because IL-10 is a potent inhibitor of IL-12 synthesis from activated human mononuclear cells, we assessed the inhibitory potential of this cytokine using soluble IL-10R. Neutralization of IL-10 restored IL-12 secretion from Mtb-infected MDM. In line with these findings, supernatants from Mtb-infected MDDC induced IFN-gamma production by T cells and enhanced IL-18R expression, whereas supernatants from MDM failed to do that. Neutralization of IFN-alpha, IL-12, and IL-18 activity in Mtb-infected MDDC supernatants by specific Abs suggested that IL-12 and, to a lesser extent, IFN-alpha and IL-18 play a significant role in enhancing IFN-gamma synthesis by T cells. During Mtb infection, macrophages and DC may have different roles: macrophages secrete proinflammatory cytokines and induce granulomatous inflammatory response, whereas DC are primarily involved in inducing antimycobacterial T cell immune response.     

Immune Response Against Sporothrix schenckii in TLR-4-Deficient Mice

For many fungal diseases, macrophages are the major cell population implicated in host protection, primarily by their ability to eliminate the invading fungal pathogen through phagocytosis. In sporotrichosis, this remains true, because of macrophages’ ability to recognize Sporothrix schenckii through specific receptors for some of the fungus’ cellular surface constituents. Further confirmation for macrophages’ pivotal role in fungal diseases came with the identification of toll-like receptors, and the subsequent numerous associations found between TLR-4 deficiency and host susceptibility to diverse fungal pathogens. Involvement of TLR-4 in immune response against sporotrichosis has been conducted to investigate how TLR-4 signaling could affect inflammatory response development through evaluation of H2O2 production and IL-1β, IL-6 and TGF-β release during the course of S. schenckii infection on TLR-4-deficient mice. The results showed that macrophages are largely dependent on TLR-4 for inflammatory activation and that in the absence of TLR-4 signaling, increased TGF-β release may be one of the contributing factors for the abrogated inflammatory activation of peritoneal exudate cells during mice sporotrichosis.     

A critical role for SOCS3 in innate resistance to Toxoplasma gondii

The innate and adaptive immune responses that confer resistance to the intracellular pathogen Toxoplasma gondii critically depend on IL-12 production, which drives interferon-γ (IFN-γ) expression. Certain cytokines can activate STAT3 and limit IL-12 production to prevent infection-associated immune pathology, but T.?gondii also directly activates STAT3 to evade host immunity. We show that suppressor of cytokine signaling molecule 3 (SOCS3), a target of STAT3 that limits signaling by the pleiotropic cytokine IL-6, is upregulated in response to?infection but is dispensable for the immune-inhibitory effects of T.?gondii. Unexpectedly, mice with targeted deletion of SOCS3 in macrophages and neutrophils have reduced IL-12 responses and succumb to toxoplasmosis. Anti-IL-6 administration or IL-12 treatment blocked disease susceptibility, suggesting that in the absence of SOCS3, macrophages are hypersensitive to the anti-inflammatory properties of IL-6. Thus, SOCS3 has a critical role in suppressing IL-6 signals and promoting immune responses to control T.?gondii infection.     

Conventional dendritic cells mount a type I IFN response against Candida spp. requiring novel phagosomal TLR7-mediated IFN-β signaling

Human fungal pathogens such as the dimorphic Candida albicans or the yeast-like Candida glabrata can cause systemic candidiasis of high mortality in immunocompromised individuals. Innate immune cells such as dendritic cells and macrophages establish the first line of defense against microbial pathogens and largely determine the outcome of infections. Among other cytokines, they produce type I IFNs (IFNs-I), which are important modulators of the host immune response. Whereas an IFN-I response is a hallmark immune response to bacteria and viruses, a function in fungal pathogenesis has remained unknown. In this study, we demonstrate a novel mechanism mediating a strong IFN-β response in mouse conventional dendritic cells challenged by Candida spp., subsequently orchestrating IFN-α/β receptor 1-dependent intracellular STAT1 activation and IFN regulatory factor (IRF) 7 expression. Interestingly, the initial IFN-β release bypasses the TLR 4 and TLR2, the TLR adaptor Toll/IL-1R domain-containing adapter-inducing IFN-β and the β-glucan/phagocytic receptors dectin-1 and CD11b. Notably, Candida-induced IFN-β release is strongly impaired by Src and Syk family kinase inhibitors and strictly requires completion of phagocytosis as well as phagosomal maturation. Strikingly, TLR7, MyD88, and IRF1 are essential for IFN-β signaling. Furthermore, in a mouse model of disseminated candidiasis we show that IFN-I signaling promotes persistence of C. glabrata in the host. Our data uncover for the first time a pivotal role for endosomal TLR7 signaling in fungal pathogen recognition and highlight the importance of IFNs-I in modulating the host immune response to C. glabrata.