The 19-kDa Mycobacterium tuberculosis protein induces macrophage apoptosis through Toll-like receptor-2

The mycobacterial 38-kilodalton glycolipoprotein antigen activates the mitogen-activated protein kinase pathway and release of proinflammatory cytokines through Toll-like receptors 2 and 4 in human monocytes

Although the 38-kDa glycolipoprotein of Mycobacterium tuberculosis H37Rv is known to evoke prominent cellular and humoral immune responses in human tuberculosis (TB), little information is known about intracellular regulatory mechanisms involved in 38-kDa antigen (Ag)-induced host responses. In this study, we found that purified 38-kDa glycolipoprotein activates mitogen-activated protein kinases (MAPKs; extracellular signal-regulated kinase 1/2 [ERK1/2] and p38) and induces tumor necrosis factor alpha (TNF-alpha) and interleukin 6 (IL-6) in human monocytes. When the 38-kDa Ag was applied to monocytes from TB patients and healthy controls, the activation of ERK1/2 and p38 MAPK and the subsequent cytokine secretion were greater in the monocytes from the active pulmonary TB patients than in monocytes from the healthy controls. Additionally, neutralizing antibodies for Toll-like receptor 2 (TLR2) or TLR4 significantly reduced the ERK1/2 and p38 activation induced by the 38-kDa protein when the antibodies were applied to HEK293 cells overexpressing TLR2 or TLR4 as well as human primary monocytes. Furthermore, the inhibition of TLR2 significantly, and that of TLR4 partially, decreased the 38-kDa Ag-induced secretion of TNF-alpha and IL-6 in human monocytes. The intact protein moieties of the 38-kDa protein were responsible for biologic activities by this Ag. These data collectively demonstrate that the 38-kDa glycolipoprotein, acting through both TLR2 and TLR4, induces the activation of the ERK1/2 and p38 MAPK pathways, which in turn play an essential role in TNF-alpha and IL-6 expression during mycobacterial infection.

Macrophage apoptosis in response to high intracellular burden of Mycobacterium tuberculosis is mediated by a novel caspase-independent pathway

We previously reported that macrophage exposure to attenuated strains of pathogenic mycobacteria at multiplicities of infection (MOI) < or = 10 triggers TNF-alpha-mediated apoptosis which reduces the viability of intracellular bacilli. Virulent strains were found to suppress macrophage apoptosis, and it was proposed that apoptosis is an innate defense against intracellular Mycobacterium tuberculosis analogous to apoptosis of virus-infected cells. The potential similarity of host cell responses to intracellular infection with mycobacteria and viruses suggests that M. tuberculosis might lyse infected macrophage when that niche is no longer needed. To investigate this question, we challenged murine macrophages with high intracellular bacillary loads. A sharp increase in cytolysis within 24 h was observed at MOI > or = 25. The primary death mode was apoptosis, based on nuclear morphology and phosphatidyl serine exposure, although the apoptotic cells progressed rapidly to necrosis. Apoptosis at high MOI differs markedly from low MOI apoptosis: it is potently induced by virulent M. tuberculosis, it is TNF-alpha-independent, and it does not reduce mycobacterial viability. Caspase inhibitors failed to prevent high MOI apoptosis, and macrophages deficient in caspase-3, MyD88, or TLR4 were equally susceptible as wild type. Apoptosis was reduced in the presence of cathepsin inhibitors, suggesting the involvement of lysosomal proteases in this novel death response. We conclude that the presence of high numbers of intracellular M. tuberculosis bacilli triggers a macrophage cell death pathway that could promote extracellular spread of infection and contribute to the formation of necrotic lesions in tuberculosis.

TLR3 Can Directly Trigger Apoptosis in Human Cancer Cells

TLRs function as molecular sensors to detect pathogen-derived products and trigger protective responses ranging from secretion of cytokines that increase the resistance of infected cells and chemokines that recruit immune cells to cell death that limits microbe spreading. Viral dsRNA participate in virus-infected cell apoptosis, but the signaling pathway involved remains unclear. In this study we show that synthetic dsRNA induces apoptosis of human breast cancer cells in a TLR3-dependent manner, which involves the molecular adaptor Toll/IL-1R domain-containing adapter inducing IFN-beta and type I IFN autocrine signaling, but occurs independently of the dsRNA-activated kinase. Moreover, detailed molecular analysis of dsRNA-induced cell death established the proapoptotic role of IL-1R-associated kinase-4 and NF-kappaB downstream of TLR3 as well as the activation of the extrinsic caspases. The direct proapoptotic activity of endogenous human TLR3 expressed by cancerous cells reveals a novel aspect of the multiple-faced TLR biology, which may open new clinical prospects for using TLR3 agonists as cytotoxic agents in selected cancers.

Secretory phospholipase A2 induces apoptosis through TNF-α and cytochrome c-mediated caspase cascade in murine macrophage RAW 264.7 cells

Phospholipase A2 (PLA2) is an esterase that cleaves the sn-2 ester bond in glycerophospholipids, thereby releasing free fatty acids and lysophospholipids. In addition to the apoptotic activity of cytosolic PLA2 and Ca2+-independent PLA2, recent studies showed that secretory PLA2 (sPLA2) also play a role in apoptosis. However, the details of molecular mechanism have not been fully elucidated. Our data demonstrated that group IB PLA (IB PLA2)-exposed murine macrophage 264.7 cells showed characteristic features of apoptosis such as morphological changes, DNA laddering, staining positive for propidium iodide (PI) as well as Annexin V and activation of caspases and subsequent cleavage of poly (ADP-ribose) polymerase (PARP) in dose- and time-dependent manner. Moreover, IB PLA2 was found to elicit tumor necrosis factor (TNF)-alpha production and release of cytochrome c, suggesting that IB PLA2 exerts its apoptotic activity via the induction of TNF-alpha production and cytochrome c release, which results in triggering the activation of caspase cascade and PARP cleavage.

Apoptotic vesicles crossprime CD8 T cells and protect against tuberculosis

CD8 T lymphocytes are important effectors in protective immunity against Mycobacterium tuberculosis. We recently characterized the detour pathway of CD8 T cell activation in tuberculosis mediated by apoptotic vesicles from infected cells that transport mycobacterial antigens to dendritic cells (DCs). Here we demonstrate that apoptotic vesicles from mycobacteria-infected macrophages stimulate CD8 T cells in vivo. Homing of DCs to draining lymph nodes was critically required for effective crosspriming. Subsequent fate of vesicle-associated antigens in recipient DCs was characterized by endosomal mechanisms predominating over proteasomal processing. In addition, vesicle processing depended on the presence of saposins to disintegrate apoptotic membranes. Apoptotic vesicles displayed potent adjuvant activity by stimulating through Toll-like receptors (TLR). Ultimately, vaccination with vesicles from infected cells induced protection against M. tuberculosis infection. Taken together, we propose the detour pathway to represent a genuine immunological mechanism mediating crosspriming of CD8 T cells in vivo and protection against tuberculosis.

Abrupt Expression of TLR4 in TLR4-Deficient Macrophages Imposes a Selective Disadvantage: Genetic Evidence for TLR4-Dependent Responses to Endogenous, Nonmicrobial Stimuli

TLR4 is crucial for macrophage responses to LPS. It is less clear whether TLR4 may also transduce signals from host factors, and if so, with what consequences. Immortalized bone marrow-derived macrophage cell lines, termed T4Cr and T4ko, were established from TLR4null strains, C57BL/10ScNCr and TLR4 knockout mice, respectively. Multiple transfections and selections were conducted to stably introduce TLR4 into these cell lines. Among 196 individual clones isolated, 48 expressed TLR4 on the cell surface but did not respond to LPS due to a deletion in the MyD88 gene. The remaining clones integrated TLR4 DNA into the genome but expressed neither detectable TLR4 mRNA nor TLR4 protein. To test the possibility that TLR4null cells lack modulating factors to protect against a harmful effect of TLR4, 15 stably transfected clones were generated in the presence of conditioned media from wild-type macrophages. Some of these cells expressed a small amount of TLR4 and regained responsiveness to LPS. Because no microbial ligands were available to the cell lines during their generation, signaling via endogenous ligands is likely to have occurred in TLR4-expressing, signal-competent macrophages and imposed a proliferative or other selective disadvantage. These studies support the existence of constitutive signaling via TLR4 during in vitro culture of macrophages without microbial products, and help account for the lack of reports of restoration of TLR4 expression in normally TLR4-expressing types of cells in vitro whose TLR4 genes are deleted or disrupted.

Heterozygous Arg753Gln polymorphism of human TLR-2 impairs immune activation by Borrelia burgdorferi and protects from late stage Lyme disease

Lyme disease (LD) is caused by Borrelia burgdorferi and displays different stages, including localized, early disseminated, and persistent infection, all of which are associated with profound inflammatory reactions in the host. Induction of proinflammatory cytokines by B. burgdorferi is mainly mediated by outer surface proteins interacting with TLR-2/TLR-1 heterodimers. In this study, we show that TNF-alpha induction by Borrelia lysate was impaired in heterozygous TLR-2 knockout mice, while reactivity to lipoteichoic acid, another TLR-2 ligand signaling via TLR-2/TLR-6 heterodimers, was unaffected. Blood from individuals heterozygous for the TLR-2 polymorphism Arg753Gln was tested for cytokine release upon stimulation with Borrelia lysate, and induction of TNF-alpha and IFN-gamma was significantly lower as compared with individuals not exhibiting this variation. Overexpression of TLR-2 carrying the Arg753Gln polymorphism in HEK 293 cells led to a significantly stronger impairment of activation by TLR-2/TLR-1 ligands as compared with TLR-2/TLR-6 ligands. To study whether heterozygosity for the Arg753Gln variant of TLR-2 influenced susceptibility for LD, we analyzed 155 patients for this polymorphism. The Arg753Gln variant occurs at a significantly lower frequency in LD patients as compared with matched controls (5.8 vs 13.5%, odds ratio 0.393, 95% confidence interval 0.17-0.89, p = 0.033), with an even more pronounced difference when late stage disease was observed (2.3 vs 12.5%, odds ratio 0.163, 95% confidence interval 0.04-0.76, p = 0.018). These data suggest that Arg753Gln may protect from the development of late stage LD due to a reduced signaling via TLR-2/TLR-1.

Effect of deletion or overexpression of the 19-kilodalton lipoprotein Rv3763 on the innate response to Mycobacterium tuberculosis

The 19-kDa lipoprotein of Mycobacterium tuberculosis is an important target of the innate immune response. To investigate the immune biology of this antigen in the context of the whole bacillus, we derived a recombinant M. tuberculosis H37Rv that lacked the 19-kDa-lipoprotein gene (Delta19) and complemented this strain by reintroduction of the 19-kDa-lipoprotein gene on a multicopy vector to produce Delta19::pSMT181. The Delta19 strain multiplied less well than Delta19::pSMT181 in human monocyte-derived macrophages (MDM) (P = 0.039). Surface expression of major histocompatibility complex class II molecules was reduced in phagocytes infected with M. tuberculosis; this effect was not seen in cells infected with Delta19. Delta19 induced lower interleukin 1beta (IL-1beta) secretion from monocytes and MDM. Overexpression of the 19-kDa protein increased IL-1beta, IL-12p40, and tumor necrosis factor alpha secretion irrespective of phagocyte maturity. These data support reports that the 19-kDa lipoprotein has pleiotropic effects on the interaction of M. tuberculosis with phagocytes. However, this analysis indicates that in the context of the whole bacillus, the 19-kDa lipoprotein is only one of a number of molecules that mediate the innate response to M. tuberculosis.

Overexpression of TLR2 and TLR4 susceptibility to serum deprivation-induced apoptosis in CHO cells

We examined the effect of overexpression of TLR2 and TLR4 on apoptosis. TLR2 and TLR4 transfected CHO cells were subjected to serum deprivation for 0, 24, and 48 h. CHO cells served as control. The survival was 80.4% and 66.8% in CHO cells, 73.8% and 47.6% in TLR2/CHO, and 70.5% and 53.0% in TLR4/CHO, respectively. Flow cytometry examination suggested that apoptotic cells were 7.17% and 32.91% in control CHO cells, 29.0% and 64.6% in TLR2/CHO, and 41.4% and 64.6% in TLR4/CHO, respectively. The levels of FasL and caspase-8 activity in TLR2/CHO and TLR4/CHO cells were significantly higher than that of CHO cells. Transfection of dominant negative FADD into TLR2/CHO and TLR4/CHO cells significantly reduced apoptosis. Our results suggest that overexpression of TLR2 and TLR4 in CHO cells sensitizes the cells to serum deprivation-induced apoptosis and that the mechanisms are involved in the death receptor-mediated signaling pathway.

Mycobacterial manipulation of the host cell

Phagosome biogenesis, the process by which macrophages neutralize ingested pathogens and initiate antigen presentation, has entered the field of cellular mycobacteriology research largely owing to the discovery 30 years ago that phagosomes harboring mycobacteria are refractory to fusion with lysosomes. In the past decade, the use of molecular genetics and biology in different model systems to study phagosome biogenesis have made significant advances in understanding subtle mechanisms by which mycobacteria inhibit the maturation of its phagosome. Thus, we are beginning to appreciate the extent to which these pathogens are able to interfere with innate immune responses and manipulate defense mechanisms to enhance their survival within the human host cell. Here, we summarize current knowledge about phagosome maturation arrest in infected macrophages and the subsequent attenuation of the macrophage-initiated adaptive anti-mycobacterial immune defenses.

Toll-like receptors and chronic lung disease

TLRs (Toll-like receptors) comprise a family of proteins whose function is principally to facilitate the detection of, and response to, pathogens. Protozoa, helminths, viruses, bacteria and fungi can all activate TLR signalling, and these signals have important roles in the activation of host defence. TLRs may also respond to products of tissue damage, providing them with roles in infective and sterile inflammation. Their role as detectors of pathogens and pathogen-associated molecules provides molecular mechanisms to underpin the observations leading to the hygiene hypothesis. Targeting of TLR signalling has implications in the control of infection, vaccine design, desensitization to allergens and down-regulation of inflammation. This review will explore TLR history, molecular signalling and the potential roles of TLRs in chronic lung disease.

Toll-like receptors and their role in the trophoblast

The aim of this review is to discuss the role of Toll-like receptors at the maternal-fetal interface and the capacity of trophoblast cells to initiate innate immune responses towards infection. The maternal-fetal interface represents an immunologically unique site that must promote tolerance to the allogenic fetus, whilst maintaining host defense against a diverse array of possible pathogens. Clinical studies have shown a strong association between certain complications of pregnancy and intrauterine infections. Therefore, innate immune responses against microorganisms at the maternal-fetal interface may have a significant impact on the success of a pregnancy. There is growing evidence that trophoblast cells are able to recognize and respond to pathogens through the expression of Toll-like receptors, an important part of innate immunity. This review will discuss the role of Toll-like receptors at the maternal-fetal interface, the potential for trophoblast cells to function as components of the innate immune system and the impact TLR-mediated trophoblast responses may have on pregnancy outcome.

The role of toll-like receptors in the pathogenesis and treatment of dermatological disease

Toll-like receptors (TLR) are crucial players in the innate immune response to microbial invaders. These receptors are expressed on immune cells, such as monocytes, macrophages, dendritic cells, and granulocytes. Importantly, TLR are not only expressed by peripheral blood cells, but their expression has been demonstrated in airway epithelium and skin, important sites of host-pathogen interaction. Host cells expressing TLR are capable of recognizing conserved pathogen-associated molecular patterns, such as lipopolysaccharide and CpG DNA, and their activation triggers signaling pathways that result in the expression of immune response genes and cytokine production. As TLR are instrumental in both launching innate immune responses and influencing adaptive immunity, regulation of TLR expression at sites of disease such as in leprosy, acne, and psoriasis may be important in the pathophysiology of these diseases. Furthermore, since TLR are vital players in infectious and inflammatory diseases, they have been identified as potential therapeutic targets. Indeed, synthetic TLR agonists such as imiquimod have already established utility in treating viral pathogens and skin cancers. In the future, it seems possible there may also be drugs capable of blocking TLR activation and thus TLR-dependent inflammatory responses, providing new treatment options for inflammatory diseases.

The 19-kDa antigen of Mycobacterium tuberculosis is a major adhesin that binds the mannose receptor of THP-1 monocytic cells and promotes phagocytosis of mycobacteria

Identification of mycobacterial adhesins is needed to understand better the pathogenesis of tuberculosis and to develop new strategies to fight this infection. In this work, THP-1 monocytic cells were incubated with Mycobacterium tuberculosis culture filtrate proteins labelled with biotin and a dominant 19-kDa adhesin was found. This adhesin was characterized as the glycosylated and acylated 19-kDa antigen (Rv 3763). These findings were confirmed in assays with culture filtrate proteins and cell-wall fractions from a recombinant Mycobacterium smegmatis strain that overexpresses the 19-kDa antigen. Further, fluorescent microspheres coated with recombinant culture filtrate proteins adhere to cells in higher numbers than microspheres coated with native M. smegmatis proteins. The binding of the 19-kDa antigen to cells was inhibited with mannose receptor competitor sugars, Ca(2+) chelators and with a monoclonal antibody to the human mannose receptor. Phagocytosis assays showed high-level binding of bacilli to THP-1 cells that was inhibited with alpha-methyl-mannoside, mannan, EDTA and mAbs to the mannose receptor and to the 19-kDa M. tuberculosis antigen. Immunoprecipitation, cell-surface ELISA and immunostaining confirmed the expression of the mannose receptor by THP-1 cells. In conclusion, here we show that the macrophage mannose receptor, considered a pathogen pattern recognition receptor, may interact with mannose residues of mycobacterial glycoproteins that could promote the phagocytosis of mycobacteria.

The role of endothelial cell apoptosis in inflammatory and immune diseases

The integrity of the endothelial lining of the vasculature is essential for vascular homeostasis and normal organ function. Endothelial injury or dysfunction has been implicated in the pathogenesis of diverse vascular diseases. Studies in vitro have demonstrated that a wide variety of stimuli can induce programmed cell death (apoptosis) of endothelial cells, and have suggested that apoptosis could be an important mechanism of vascular injury, resulting in vascular leak, inflammation, and coagulation. In this review, we focus on the potential role of endothelial apoptosis in the initiation and progression of inflammatory and immune disorders, reviewing human diseases and in vivo models in which endothelial cell apoptosis has been demonstrated. Although endothelial cell apoptosis is observed in many inflammatory and immune disorders, we find that there is, as yet, only limited experimental evidence demonstrating that it is critical to the pathogenesis of disease.

No life without death—apoptosis as prerequisite for T cell activation

The orchestrated death of infected cells is key to our understanding of CD8 T cell activation against pathogens. Most intracellular bacteria including Mycobacterium tuberculosis, the etiologic agent of tuberculosis, remain enclosed in phagosomes of infected macrophages. CD8 T cells play a critical role in defense of infection and recognize antigens originating from the cytosol presented by MHC-I molecules. Since mycobacteria do not gain access to the cytosolic MHC-I presentation pathway, the fundamental question as to how CD8 T cells encounter mycobacterial antigens remains to be solved. In this review, we focus on solutions for this enigma and describe the detour pathway of T cell activation. Mycobacteria induce cell death of infected macrophages which thereby leave a last message by releasing apoptotic vesicles. Subsequently, these antigen-containing entities are engulfed by dendritic cells which process the mycobacterial cargo for efficient antigen presentation and CD8 T cell activation. Since the dying infected cell is the origin of a protective T cell response destined to preserve life and individuality, the detour pathway represents an altruistic principle at a cellular level which corresponds to the macroscopic world where death is the precondition to perpetuate the living.

TLR4, but not TLR2, signals autoregulatory apoptosis of cultured microglia: a critical role of IFN-beta as a decision maker

TLRs mediate diverse signaling after recognition of evolutionary conserved pathogen-associated molecular patterns such as LPS and lipopeptides. Both TLR2 and TLR4 are known to trigger a protective immune response as well as cellular apoptosis. In this study, we present evidence that TLR4, but not TLR2, mediates an autoregulatory apoptosis of activated microglia. Brain microglia underwent apoptosis upon stimulation with TLR4 ligand (LPS), but not TLR2 ligands (Pam(3)Cys-Ser-Lys(4), peptidoglycan, and lipoteichoic acid). Based on studies using TLR2-deficient or TLR4 mutant mice and TLR dominant-negative mutants, we also demonstrated that TLR4, but not TLR2, is necessary for microglial apoptosis. The critical difference between TLR2 and TLR4 signalings in microglia was IFN regulatory factor-3 (IRF-3) activation, followed by IFN-beta expression: while TLR4 agonist induced the activation of IRF-3/IFN-beta pathway, TLR2 did not. Nevertheless, both TLR2 and TLR4 agonists strongly induced NF-kappaB activation and NO production in microglia. Neutralizing Ab against IFN-beta attenuated TLR4-mediated microglial apoptosis. IFN-beta alone, however, did not induce a significant cell death. Meanwhile, TLR2 activation induced microglial apoptosis with help of IFN-beta, indicating that IFN-beta production following IRF-3 activation determines the apoptogenic action of TLR signaling. TLR4-mediated microglial apoptosis was mediated by MyD88 and Toll/IL-1R domain-containing adaptor-inducing IFN-beta, and was associated with caspase-11 and -3 activation rather than Fas-associated death domain protein/caspase-8 pathway. Taken together, TLR4 appears to signal a microglial apoptosis via autocrine/paracrine IFN-beta production, which may act as an apoptotic sensitizer.

Immunogenic membrane-associated proteins of Mycobacterium tuberculosis revealed by proteomics

Membrane-associated proteins ofMycobacterium tuberculosisoffer a challenge, as well as an opportunity, in the quest for better therapeutic and prophylactic interventions against tuberculosis. The authors have previously reported that extraction with the detergent Triton X-114 (TX-114) is a useful step in proteomic analysis of mycobacterial cell membranes, and detergent-soluble membrane proteins of mycobacteria are potent stimulators of human T cells. In this study 1-D and 2-D gel electrophoresis-based protocols were used for the analysis of proteins in the TX-114 extract ofM. tuberculosismembranes. Peptide mass mapping (using MALDI-TOF-MS, matrix assisted laser desorption/ionization time of flight mass spectrometry) of 116 samples led to the identification of 105 proteins, 9 of which were new to theM. tuberculosisproteome. Functional orthologues of 73 of these proteins were also present inMycobacterium leprae, suggesting their relative importance. Bioinformatics predicted that as many as 73 % of the proteins had a hydrophobic disposition. 1-D gel electrophoresis revealed more hydrophobic/transmembrane and basic proteins than 2-D gel electrophoresis. Identified proteins fell into the following major categories: protein synthesis, cell wall biogenesis/architecture and conserved hypotheticals/unknowns. To identify immunodominant proteins of the detergent phase (DP), 14 low-molecular-mass fractions prepared by continuous-elution gel electrophoresis were subjected to T cell activation assays using blood samples from BCG-vaccinated healthy donors from a tuberculosis endemic area. Analysis of the responses (cell proliferation and IFN-γproduction) showed that the immunodominance of certain DP fractions was most probably due to ribosomal proteins, which is consistent with both their specificity for mycobacteria and their abundance. Other membrane-associated proteins, including transmembrane proteins/lipoproteins and ESAT-6, did not appear to contribute significantly to the observed T cell responses.

Colon cancer cell-derived high mobility group 1/amphoterin induces growth inhibition and apoptosis in macrophages

High mobility group (HMGB)1/amphoterin is a multifunctional cytokine involved in invasion and metastasis of cancer and in inflammation. To investigate HMGB1/amphoterin effects on macrophages, U937 human monocytic leukemia cells and rat peritoneal and human alveolar macrophages were examined. U937 cells expressed low levels of an HMGB1/amphoterin receptor, receptor for advanced glycation end-products (RAGE), whereas RAGE production was induced in differentiated phorbol 12-myristate 13-acetate (PMA)-U937 cells. Treatment with cultured medium of HMGB1/amphoterin-secreting WiDr human colon cancer cells showed growth inhibition of both U937 and PMA-U937 cells and apoptosis in PMA-U937 cells. The number of PMA-U937 cells was markedly decreased by co-culture with WiDr cells exposed to HMGB1/amphoterin sense S-oligodeoxynucleotide (ODN) in spheroids or monolayers. In contrast, PMA-U937 cells co-cultured with WiDr cells exposed to HMGB1/amphoterin anti-sense S-ODN were preserved in number. PMA-U937 cells exposed to RAGE anti-sense S-ODN were insensitive to WiDr-cultured medium. Recombinant human HMGB1/amphoterin induced growth inhibition in thioglycollate-induced rat peritoneal macrophages, PMA-U937 cells, and human alveolar macrophages, an effect that was abrogated by absorption with anti-HMGB1 antibody. Phosphorylation of JNK and Rac1 was induced in PMA-U937 cells treated with HMGB1/amphoterin. These results suggest that HMGB1/amphoterin induces growth inhibition and apoptosis in macrophages through RAGE intracellular signaling pathway.

Cross-Talk between CD14 and Complement Receptor 3 Promotes Phagocytosis of Mycobacteria: Regulation by Phosphatidylinositol 3-Kinase and Cytohesin-1

The glycosylphosphatidyl anchored molecule CD14 to the monocyte membrane plays a prominent role in innate immunity, and the paradigms for CD14 selective signaling are beginning to be elucidated. In this study, transfected human monocytic cell line THP-1 and Chinese hamster ovary (CHO) fibroblastic cells were used to examine phagocytosis of Mycobacterium bovis bacillus Calmette-Guerin (BCG). Flow cytometry was combined with molecular and biochemical approaches to demonstrate a dual mechanism for BCG internalization involving either CD14 alone or a CD14-regulated complement receptor (CR)3-dependent pathway. Phagocytosis by CD14-positive THP-1 cells was attenuated by phosphatidylinositol-3 inhibitors LY294002 and wortmannin and experiments using transfected CHO cells showed substantial accumulation of phosphatidylinositol-3,4,5-trisphosphate at the BCG attachment site in CHO cells expressing CD14 and TLR2 suggesting that bacteria bind to CD14 and use TLR2 to initiate a PI3K signaling pathway. Additional experiments using blocking Abs showed that anti-TLR2 Abs inhibit phagocytosis of BCG by THP-1 cells. Furthermore, knockdown of cytohesin-1, a PI3K-regulated adaptor molecule for beta(2) integrin activation, specifically abrogated CD14-regulated CR3 ingestion of BCG consistent with the observation of physical association between CR3 and cytohesin-1 in cells stimulated with mycobacterial surface components. These findings reveal that mycobacteria promote their uptake through a process of "inside-out" signaling involving CD14, TLR2, PI3K, and cytohesin-1. This converts low avidity CR3 into an active receptor leading to increased bacterial internalization.

Divergent trophoblast responses to bacterial products mediated by TLRs

Intrauterine infections have been associated with pregnancy complications that are also linked with increased trophoblast apoptosis. TLRs are key components of the innate immune system which recognize conserved sequences on the surface of pathogens and trigger effector cell functions. We hypothesize that intrauterine infections may cause the excessive trophoblast cell apoptosis observed in abnormal pregnancies and that TLR may provide a mechanism of pathogenesis. In this study we describe the expression and function of TLR-2 and TLR-4 in first trimester trophoblast cells. Although ligation of TLR4 induced cytokine production by trophoblast cells, TLR-2 activation induced apoptosis. TLR-2 mediated apoptosis was dependent upon the Fas-associated death domain, the inactivation of the X-linked inhibitor of apoptosis, and the activation of caspases 8, 9, and 3. These results suggest that certain intrauterine infections may directly induce trophoblast cell death through TLR-2. Our findings provide a novel mechanism of pathogenesis for certain pregnancy complications in which there is engagement of the innate immune system.

Lipoproteins ofMycobacterium tuberculosis: an abundant and functionally diverse class of cell envelope components

Mycobacterium tuberculosis remains the predominant bacterial scourge of mankind. Understanding of its biology and pathogenicity has been greatly advanced by the determination of whole genome sequences for this organism. Bacterial lipoproteins are a functionally diverse class of membrane-anchored proteins. The signal peptides of these proteins direct their export and post-translational lipid modification. These signal peptides are amenable to bioinformatic analysis, allowing the lipoproteins encoded in whole genomes to be catalogued. This review applies bioinformatic methods to the identification and functional characterisation of the lipoproteins encoded in the M. tuberculosis genomes. Ninety nine putative lipoproteins were identified and so this family of proteins represents ca. 2.5% of the M. tuberculosis predicted proteome. Thus, lipoproteins represent an important class of cell envelope proteins that may contribute to the virulence of this major pathogen.

Toll-like receptor pathways in the immune responses to mycobacteria

The control of Mycobacterium tuberculosis infection depends on recognition of the pathogen and the activation of both the innate and adaptive immune responses. Toll-like receptors (TLR) were shown to play a critical role in the recognition of several pathogens. Mycobacterial antigens recognise distinct TLR resulting in rapid activation of cells of the innate immune system. Recent evidence from in vitro and in vivo investigations, summarised in this review demonstrates TLR-dependent activation of innate immune response, while the induction of adaptive immunity to mycobacteria may be TLR independent.

Signaling of Apoptosis through TLRs Critically Involves Toll/IL-1 Receptor Domain-Containing Adapter Inducing IFN-β, but Not MyD88, in Bacteria-Infected Murine Macrophages

TLRs are important sensors of the innate immune system that serve to identify conserved microbial components to mount a protective immune response. They furthermore control the survival of the challenged cell by governing the induction of pro- and antiapoptotic signaling pathways. Pathogenic Yersinia spp. uncouple the balance of life and death signals in infected macrophages, which compels the macrophage to undergo apoptosis. The initiation of apoptosis by Yersinia infection specifically involves TLR4 signaling, although Yersinia can activate TLR2 and TLR4. In this study we characterized the roles of downstream TLR adapter proteins in the induction of TLR-responsive apoptosis. Experiments using murine macrophages defective for MyD88 or Toll/IL-1R domain-containing adapter inducing IFN-beta (TRIF) revealed that deficiency of TRIF, but not of MyD88, provides protection against Yersinia-mediated cell death. Similarly, apoptosis provoked by treatment of macrophages with the TLR4 agonist LPS in the presence of a proteasome inhibitor was inhibited in TRIF-defective, but not in MyD88-negative, cells. The transfection of macrophages with TRIF furthermore potently promoted macrophage apoptosis, a process that involved activation of a Fas-associated death domain- and caspase-8-dependent apoptotic pathway. These data indicate a crucial function of TRIF as proapoptotic signal transducer in bacteria-infected murine macrophages, an activity that is not prominent for MyD88. The ability to elicit TRIF-dependent apoptosis was not restricted to TLR4 activation, but was also demonstrated for TLR3 agonists. Together, these results argue for a specific proapoptotic activity of TRIF as part of the host innate immune response to bacterial or viral infection.

IL-6 and IL-10 Induction from Dendritic Cells in Response to Mycobacterium tuberculosis Is Predominantly Dependent on TLR2-Mediated Recognition

The initial TLR-mediated interaction between Mycobacterium tuberculosis and dendritic cells is critical, since the cytokine production that ensues can greatly influence the class of adaptive immunity that is generated to the pathogen. In this study, we therefore determined the dependency on TLR2 and TLR4 for M. tuberculosis-induced cytokine production by murine dendritic cells. A key new finding of this study is that production of IL-6 and IL-10 from dendritic cells in response to M. tuberculosis is principally dependent on TLR2. The study also indicates that M. tuberculosis can induce IL-12 production in the absence of either TLR2 or TLR4, suggesting redundancy or possibly involvement of other receptors in IL-12 production. In addition, the data also reveal that lack of TLR2 or TLR4 does not impact on dendritic cell maturation or on their ability to influence the polarity of differentiating naive T cells. Collectively, data presented here provide a mechanistic insight for the contribution of TLR2 and TLR4 to tuberculosis disease progression and offer strategies for regulating IL-6 and IL-10 production in dendritic cell-based vaccine strategies.

Prolonged Toll-like receptor stimulation leads to down-regulation of IRAK-4 protein

Interleukin-1 receptor-associated kinase (IRAK)-4 is a key mediator in the Toll-like receptor (TLR) signaling. We found that stimulation of TLR2, TLR4, or TLR9, but not TLR3, caused a decrease in IRAK-4 protein without affecting its mRNA level in a mouse macrophage cell line, RAW 264. The decrease in IRAK-4 was accompanied by the appearance of a smaller molecular weight protein (32 kD), which was recognized by an anti-IRAK-4 antibody raised against the C-terminal region. The decrease in IRAK-4 and the appearance of the 32-kD protein occurred with slower kinetics than the activation of IRAK-1 and were suppressed by inhibitors of the proteasome, inducible inhibitor of kappaBalpha phosphorylation or protein synthesis, but not by caspase inhibitors. These results indicate that prolonged stimulation of TLR2, TLR4, or TLR9 causes a down-regulation of IRAK-4 protein, which may be mediated through cleavage of IRAK-4 by a protease induced by the activation of nuclear factor-kappaB.

Apoptosis paves the detour path for CD8 T cell activation against intracellular bacteria

Intracellular bacteria such as Mycobacterium tuberculosis primarily infect macrophages. Within these host cells, the pathogens are confined to phagosomes and their antigens are secluded from the classical MHC I presentation pathway. Moreover, macrophages fail to express certain antigen presenting molecules like CD1 proteins. As a result of this intracellular lifestyle, the pathways for the induction of MHC I- and CD1-restricted CD8 T cells by such microorganisms remain elusive. Based on recent findings in tuberculosis and salmonellosis, we propose a new detour pathway for CD8 T cell activation against intracellular bacteria through apoptotic blebs from infected macrophages. Pathogen-derived antigens including proteins and lipids are delivered from infected cells to non-infected dendritic cells. Subsequently, these professional antigen presenting cells display microbial antigens through MHC I and CD1 to T cells. Thus, cross-priming mediated by apoptotic vesicles is not just a matter of antigen distribution, but an intrinsic immunological function due to the nature of phagosomally located intracellular bacteria. We consider infection-induced apoptosis the conditio sine qua non for antigen-specific CD8 T cell activation by phagosome-enclosed pathogens. This important new function of cell death in antibacterial immunity requires consideration for rational vaccine design.

Combined DNA Vaccines Formulated in DDA Enhance Protective Immunity against Tuberculosis

This study evaluated the adjuvant Dimethyldioctyldecyl Ammonium Bromide (DDA) effect on the protective immunity induced by a combination of plasmids containing genes encoding antigens Ag85B, MPT-83, and ESAT-6 from Mycobacterium tuberculosis. The combined DNA vaccines in DDA resulted in significant increases in both specific IgG and splenic T-cell-derived Th1-type cytokine gamma interferon (IFN-gamma) production in response to the three purified antigens when compared to that of combined DNA vaccines in saline. Vaccines in DDA increased the protective efficacy of mice challenged with M. tuberculosis H37Rv as measured by reduced relative CFU counts in their lungs. Mice immunized with the combined DNA vaccines were shown to limit the growth of tubercle bacilli both in lungs and in spleens. Histopathological analyses showed that vaccinated mice had substantially improved postinfection lung pathology relative to the controls. We suggest that our combination of antigens together with DDA formulation may provide a new insight into tuberculosis prevention.

Mitogenicity of the recombinant mycobacterial 27-kilodalton lipoprotein is not connected to its antiprotective effect

We reported previously that even though immunization with the recombinant mycobacterial 27-kDa lipoprotein (r27) induced a Th1-type response in mice, the vaccinated mice became more susceptible to challenge with Mycobacterium tuberculosis. In this study we show that r27 stimulates naive splenocytes to proliferate. Acylation of r27 was crucial for this effect, since a nonacylated mutant of r27, termed r27DeltaSP, failed to stimulate splenocytes either in vitro or in vivo. Depletion experiments indicated that only B cells were proliferating in a T-cell-independent manner. We also found that r27 is recognized by TLR2, which is involved in mitogenic stimulation. Interestingly, r27 but not r27DeltaSP induced high gamma interferon levels in splenocyte supernatants, whereas no significant interleukin-2 levels were detected. Since B-cell polyclonal activation might aggravate pathogen infection, we asked whether the antiprotective effect of the r27 lipoprotein is associated with its mitogenicity. We showed that, as in the case of r27, immunization of mice with the nonmitogenic r27DeltaSP lipoprotein resulted in increased M. tuberculosis multiplication. We conclude that the antiprotective effect of the r27 lipoprotein must be linked to properties of the polypeptide portion of the lipoprotein rather than to its lipid moiety and its mitogenicity.

Tuberculosis: a problem with persistence

Mycobacterium tuberculosis is one of most successful pathogens of mankind, infecting one-third of the global population and claiming two million lives every year. The ability of the bacteria to persist in the form of a long-term asymptomatic infection, referred to as latent tuberculosis, is central to the biology of the disease. The persistence of bacteria in superficially normal tissue was recognized soon after the discovery of the tubercle bacillus, and much of our knowledge about persistent populations of M. tuberculosis dates back to the first half of the last century. Recent advances in microbial genetics and host immunity provide an opportunity for renewed investigation of this persistent threat to human health.

To be, or not to be: NF-kappaB is the answer--role of Rel/NF-kappaB in the regulation of apoptosis

During their lifetime, cells encounter many life or death situations that challenge their very own existence. Their survival depends on the interplay within a complex yet precisely orchestrated network of proteins. The Rel/NF-kappaB signaling pathway and the transcription factors that it activates have emerged as critical regulators of the apoptotic response. These proteins are best known for the key roles that they play in normal immune and inflammatory responses, but they are also implicated in the control of cell proliferation, differentiation, apoptosis and oncogenesis. In recent years, there has been remarkable progress in understanding the pathways that activate the Rel/NF-kappaB factors and their role in the cell's decision to either fight or surrender to apoptotic challenge. Whereas NF-kappaB is most commonly involved in suppressing apoptosis by transactivating the expression of antiapoptotic genes, it can promote programmed cell death in response to certain death-inducing signals and in certain cell types. This review surveys our current understanding of the role of NF-kappaB in the apoptotic response and focuses on many developments since this topic was last reviewed in Oncogene 4 years ago. These recent findings shed new light on the activity of NF-kappaB as a critical regulator of apoptosis in the immune, hepatic, epidermal and nervous systems, on the mechanisms through which it operates and on its role in tissue development, homoeostasis and cancer.

A dominant role of Toll-like receptor 4 in the signaling of apoptosis in bacteria-faced macrophages

Conserved bacterial components potently activate host immune cells through transmembrane Toll-like receptors (TLRs), which trigger a protective immune response but also may signal apoptosis. In this study, we investigated the roles of TLR2 and TLR4 as inducers of apoptosis in Yersinia enterocolitica-infected macrophages. Yersiniae suppress activation of the antiapoptotic NF-kappaB signaling pathway in host cells by inhibiting inhibitory kappaB kinase-beta. This leads to macrophage apoptosis under infection conditions. Experiments with mouse macrophages deficient for TLR2, TLR4, or both receptors showed that, although yersiniae could activate signaling through both TLR2 and TLR4, loss of TLR4 solely diminished Yersinia-induced apoptosis. This suggests implication of TLR4, but not of TLR2, as a proapoptotic signal transducer in Yersinia-conferred cell death. In the same manner, agonist-specific activation of TLR4 efficiently mediated macrophage apoptosis in the presence of the proteasome inhibitor MG-132, an effect that was less pronounced for activation through TLR2. Furthermore, the extended stimulation of overexpressed TLR4 elicited cellular death in epithelial cells. A dominant-negative mutant of Fas-associated death domain protein could suppress TLR4-mediated cell death, which indicates that TLR4 may signal apoptosis through a Fas-associated death domain protein-dependent pathway. Together, these data show that TLR4 could act as a potent inducer of apoptosis in macrophages that encounter a bacterial pathogen.

CD1-Dependent Regulation of Chronic Central Nervous System Inflammation in Experimental Autoimmune Encephalomyelitis

The existence of T cells restricted for the MHC I-like molecule CD1 is well established, but the function of these cells is still obscure; one implication is that CD1-dependent T cells regulate autoimmunity. In this study, we investigate their role in experimental autoimmune encephalomyelitis (EAE), an animal model for multiple sclerosis, using CD1-deficient mice on a C57BL/6 background. We show that CD1-/- mice develop a clinically more severe and chronic EAE compared with CD1+/+ C57BL/6 mice, which was histopathologically confirmed with increased demyelination and CNS infiltration in CD1-/- mice. Autoantigen rechallenge in vitro revealed similar T cell proliferation in CD+/+ and CD1-/- mice but an amplified cytokine response in CD1-/- mice as measured by both the Th1 cytokine IFN-gamma and the Th2 cytokine IL-4. Investigation of cytokine production at the site of inflammation showed a CNS influx of TGF-beta1-producing cells early in the disease in CD1+/+ mice, which was absent in the CD1-/- mice. Passive transfer of EAE using an autoreactive T cell line induced equivalent disease in both groups, which suggested additional requirements for activation of the CD1-dependent regulatory pathway(s). When immunized with CFA before T cell transfer, the CD1-/- mice again developed an augmented EAE compared with CD1+/+ mice. We suggest that CD1 exerts its function during CFA-mediated activation, regulating development of EAE both through enhancing TGF-beta1 production and through limiting autoreactive T cell activation, but not necessarily via effects on the Th1/Th2 balance.

Two distinct mechanisms for induction of dendritic cell apoptosis in response to intact Streptococcus pneumoniae

Apoptotic dendritic cells (DCs) are ineffective at inducing immunity. Thus, parameters that regulate DC viability during a primary infection will help to determine the outcome of the subsequent immune response. In this regard, pathogens have developed strategies to promote DC apoptosis to counterbalance the nascent primary immune response. We demonstrate, using cultured bone marrow-derived DCs, that Streptococcus pneumoniae can induce DC apoptosis through two distinct mechanisms: 1) a rapid, caspase-independent mechanism of apoptosis induction, critically dependent on bacterial expression of pneumolysin, and 2) a delayed-onset, caspase-dependent mechanism of apoptosis induction associated with terminal DC maturation. Delayed-onset apoptosis does not require bacterial internalization, but rather is triggered by the interaction of bacterial subcapsular components and bone marrow-derived DC (likely Toll-like) receptors acting in a myeloid differentiation factor 88-dependent manner. In this regard, heavy polysaccharide encapsulation interferes with both DC maturation and apoptosis induction. In contrast, neither CD95/CD95 ligand interactions nor TNF-alpha appear to play a role in the delayed onset of apoptosis. These data are the first to define two mechanistically distinct pathways of DC apoptosis induction in response to an extracellular bacterium that likely have important consequences for the establishment of antibacterial immunity.

In situ expression of CD40, CD40L (CD154), IL-12, TNF-alpha, IFN-gamma and TGF-beta1 in murine lungs during slowly progressive primary tuberculosis

The distribution and expression of CD40, its ligand CD40L (154) and related cytokines interleukin-12 (IL-12), tumour necrosis factor-alpha (TNF-alpha), interferon-gamma (IFN-gamma) and transforming growth factor-beta1 (TGF-beta1) were studied in the lungs of B6D2F1 hybrid mice during slowly progressive primary tuberculosis (TB) by immunohistochemistry. CD40 and CD40L are implicated in cell-mediated immunity (CMI) causing activation or apoptosis of infected cells. The phenomenon of apoptosis is associated with Mycobacterium tuberculosis survival. In this study, using frozen lung sections (n = 33), our results showed increased CD40, IL-12 and TGF-beta1 expression in macrophages with progression of disease. High percentages of mycobacterial antigens (M.Ags), CD40L and IFN-gamma expression were maintained throughout infection, and TNF-alpha-expressing cells were decreased. In lymphocytes, the percentage of IFN-gamma-positive cells was increased, but CD40L and IL-12 were maintained with the progression of disease. M.Ags, CD40 and CD40L were expressed in the same areas of the lesions. We conclude that changes in the expression of CD40-CD40L and cytokines associated with M. tuberculosis infection favour the hypothesis that M. tuberculosis causes resistance of host cells to apoptosis causing perpetuation of infection.

Immune evasion by Mycobacterium tuberculosis: living with the enemy

Mycobacterium tuberculosis is successful as a pathogen because of its ability to persist in an immunocompetent host. This bacterium lives within the macrophage, a cell whose function is the elimination of microbes. Recent advances have improved our understanding of how M. tuberculosis evades two major antimicrobial mechanisms of macrophages: phagolysosome fusion and the production of toxic reactive nitrogen intermediates. M. tuberculosis also modulates antigen presentation to prevent the detection of infected macrophages by CD4(+) T cells.