Signal transduction pathways activated by CpG-DNA

TLR Stimulation during T-cell Activation Lowers PD-1 Expression on CD8+ T Cells

Expression of T-cell checkpoint receptors can compromise antitumor immunity. Blockade of these receptors, notably PD-1 and LAG-3, which become expressed during T-cell activation with vaccination, can improve antitumor immunity. We evaluated whether T-cell checkpoint expression could be separated from T-cell activation in the context of innate immune stimulation with TLR agonists. We found that ligands for TLR1/2, TLR7, and TLR9 led to a decrease in expression of PD-1 on antigen-activated CD8+ T cells. These effects were mediated by IL12 released by professional antigen-presenting cells. In two separate tumor models, treatment with antitumor vaccines combined with TLR1/2 or TLR7 ligands induced antigen-specific CD8+ T cells with lower PD-1 expression and improved antitumor immunity. These findings highlight the role of innate immune activation during effector T-cell development and suggest that at least one mechanism by which specific TLR agonists can be strategically used as vaccine adjuvants is by modulating the expression of PD-1 during CD8+ T-cell activation. Cancer Immunol Res; 6(11); 1364-74. ©2018 AACR.

Role of p38 Mitogen-Activated Protein Kinase (MAPK) for Vacuole Formation in Lipopolysaccharide (LPS)-Stimulated Macrophages

The role of p38 mitogen-activated protein kinase (MAPK) on vacuole formation in lipopolysaccharide (LPS)-stimulated RAW 264.7 cells was examined. LPS definitely induced the formation of vacuoles in RAW 264.7 cells and SB202190 as a p38 specific inhibitor also induced slight vacuole formation. The simultaneous treatment with LPS and SB202190 induced many more vacuoles in RAW 264.7 cells than the treatment with LPS or SB202190 alone, and the vacuoles were extraordinarily large in size. On the other hand, an inactive inhibitor of p38 MAPK did not augment LPS-induced vacuole formation. Further, the inhibitors of other MAPKs and nuclear factor (NF)-kappaB pathways did not affect it. The extraordinarily large vacuoles in RAW 264.7 cells treated with LPS and SB202190 were possibly formed via fusion of small vacuoles. However, SB202190 did not augment vacuole formation in CpG DNA or interferon (IFN)-gamma-stimulated RAW 264.7 cells. The role of p38 MAPK in the vacuole formation in LPS-stimulated macrophages is discussed.

Enterocyte-specific A20 deficiency sensitizes to tumor necrosis factor-induced toxicity and experimental colitis

A20 is a nuclear factor kappaB (NF-kappaB) target gene that encodes a ubiquitin-editing enzyme that is essential for the termination of NF-kappaB activation after tumor necrosis factor (TNF) or microbial product stimulation and for the prevention of TNF-induced apoptosis. Mice lacking A20 succumb to inflammation in several organs, including the intestine, and A20 mutations have been associated with Crohn's disease. However, ablation of NF-kappaB activity, specifically in intestinal epithelial cells (IECs), promotes intestinal inflammation. As A20 deficiency sensitizes cells to TNF-induced apoptosis yet also promotes NF-kappaB activity, it is not clear if A20 deficiency in IECs would exacerbate or ameliorate intestinal inflammation. We generated mice lacking A20 specifically in IECs. These mice did not show spontaneous intestinal inflammation but exhibited increased susceptibility to experimental colitis, and their IECs were hypersensitive to TNF-induced apoptosis. The resulting TNF-driven breakdown of the intestinal barrier permitted commensal bacterial infiltration and led to systemic inflammation. These studies define A20 as a major antiapoptotic protein in the intestinal epithelium and further indicate that defects in A20 might contribute to inflammatory bowel disease in humans.

Cytokine association with bacterial DNA in serum of patients with inflammatory bowel disease

BACKGROUND

The pathogenesis of inflammatory bowel disease (IBD) involves the interaction between genetic susceptibility, mucosal immunity, and intestinal bacteria. Bacterial translocation is a common event in these patients and plays an important role in the perpetuation of chronic intestinal inflammation. Blood microbiological cultures, however, are frequently negative. The aim was to evaluate the presence of bacterial DNA (bactDNA) and the associated cytokine response in patients with IBD.

METHODS

Fifteen healthy donors, 29 patients with ulcerative colitis (UC), and 33 patients with Crohn's disease (CD) were studied. The presence of bactDNA was pursued by PCR followed by nucleotide sequencing analysis. Microbiological cultures were carried out among all controls and patients. Cytokine serum levels were measured by enzyme-linked immunosorbent assay (ELISA).

RESULTS

BactDNA was detected in 14 out of 33 patients with CD (42.4%) and in 15 out of 29 patients with UC (51.7%). BactDNA translocation was present in 7 out of 21 (33%) and in 10 out of 15 (34%) patients with CD and UC in remission, respectively. None of healthy controls showed bactDNA in serum. A statistically significant increase in all Th1-derived cytokines in CD but not in UC patients with the presence of bactDNA was observed in comparison with patients without bactDNA and controls.

CONCLUSIONS

BactDNA is present in IBD patients, irrespective of their disease activity. This fact is associated with a marked Th1-driven immune reaction in CD patients, even in those in remission. Whether bactDNA is inducing or is favored by an increased inflammatory scenario in these patients remains under discussion.

Toll-like receptor 9-dependent macrophage activation by Entamoeba histolytica DNA

Activation of the innate immune system by bacterial DNA and DNA of other invertebrates represents a pathogen recognition mechanism. In this study we investigated macrophage responses to DNA from the intestinal protozoan parasite Entamoeba histolytica. E. histolytica genomic DNA was purified from log-phase trophozoites and tested with the mouse macrophage cell line RAW 264.7. RAW cells treated with E. histolytica DNA demonstrated an increase in levels of tumor necrosis factor alpha (TNF-alpha) mRNA and protein production. TNF-alpha production was blocked by pretreatment with chloroquine or monensin. In fact, an NF-kappaB luciferase reporter assay in HEK cells transfected with human TLR9 demonstrated that E. histolytica DNA signaled through Toll-like receptor 9 (TLR9) in a manner similar to that seen with CpG-ODN. Immunofluorescence assays confirmed NF-kappaB activation in RAW cells, as seen by nuclear translocation of the p65 subunit. Western blot analysis demonstrated mitogen-activated protein kinase activation by E. histolytica DNA. E. histolytica DNA effects were abolished in MYD88-/- mouse-derived macrophages. In the context of disease, immunization with E. histolytica DNA protected gerbils from an E. histolytica challenge infection. Taken together, these results demonstrate that E. histolytica DNA is recognized by TLR9 to activate macrophages and may provide an innate defense mechanism characterized by the induction of the inflammatory mediator TNF-alpha.

Translocation of bacterial DNA from Gram-positive microorganisms is associated with a species-specific inflammatory response in serum and ascitic fluid of patients with cirrhosis

Translocation of bacterial-DNA in patients with cirrhosis and ascites triggers an innate immune response. Identification of characteristics to which this response is sensitive is relevant from a clinical standpoint. The aim of this study has been to determine if the proinflammatory immune response established in vivo in cirrhotic patients with ascites as a consequence of bacterial-DNA translocation is related to the identified bacterial species and their frequency of cytosine-guanosine content in serum and ascitic fluid. Patients with advanced cirrhosis and ascites were included in the study and distributed into groups I and II according to the absence or presence of bacterial-DNA translocation, respectively. Serum and ascitic fluid levels of proinflammatory cytokines after normalization of bacterial-DNA concentration and the activated form of nuclear factor-kappa B in ascitic fluid pellets were measured by enzyme-linked immunosorbent assay techniques. Translocation of bacterial-DNA with higher cytosine-guanosine content induced the highest cytokine response, which was higher than that in patients without bacterial-DNA translocation. The activated form of nuclear factor-kappa B in ascitic fluid pellets of patients with bacterial-DNA translocation was greater in patients with higher bacterial-DNA cytosine-guanosine content, whereas the amount of total nuclear factor-kappa B remained unaltered. Bacterial-DNA translocation induces a marked immune reaction in vivo in patients with advanced cirrhosis and ascites which is related, among other factors, to the bacterial-DNA cytosine-guanosine content. Therefore, the host's immune response to bacterial-DNA translocation constitutes a species-specific phenomenon.

A newly established murine immature dendritic cell line can be differentiated into a mature state, but exerts tolerogenic function upon maturation in the presence of glucocorticoid

The phenotype and function of murine dendritic cells (DCs) are primarily studied using bone-marrow–derived DCs (BM-DCs), but may be hampered by the heterogenous phenotype of BM-DCs due to their differential state of maturation. Here we characterize a newly established murine DC line (SP37A3) of myeloid origin. During maintainance in the presence of granulocyte-macrophage colony-stimulating factor (GM-CSF) and M-CSF, SP37A3 cells resemble immature DCs characterized by low expression of major histocompatibility complex (MHC) II and costimulatory molecules and low T-cell stimulatory capacity. Upon stimulation, SP37A3 cells acquire a mature phenotype and activate naive T cells as potently as BM-DCs. Similar to BM-DCs, SP37A3 cells activated in the presence of dexamethasone-induced regulatory T cells, which were anergic upon restimulation and suppressed proliferation of naive T cells. This tolerogenic state was reflected by lower expression levels of costimulatory molecules and proinflammatory cytokines compared with mature cells, as well as up-regulated expression of FcγRIIB and interleukin-1RA (IL-1RA). SP37A3 cells were responsive to dexamethasone even when applied at later time points during activation, suggesting functional plasticity. Thus, DC line SP37A3 represents a suitable model to study functions of immature and mature as well as tolerogenic myeloid DCs, circumventing restrictions associated with the use of primary DCs and BM-DCs.

The antimalarial artemisinin synergizes with antibiotics to protect against lethal live Escherichia coli challenge by decreasing proinflammatory cytokine release

In the present study artemisinin (ART) was found to have potent anti-inflammatory effects in animal models of sepsis induced by CpG-containing oligodeoxy-nucleotides (CpG ODN), lipopolysaccharide (LPS), heat-killed Escherichia coli 35218 or live E. coli. Furthermore, we found that ART protected mice from a lethal challenge by CpG ODN, LPS, or heat-killed E. coli in a dose-dependent manner and that the protection was related to a reduction in serum tumor necrosis factor alpha (TNF-alpha). More significantly, the administration of ART together with ampicillin or unasyn (a complex of ampicillin and sulbactam) decreased mortality from 100 to 66.7% or 33.3%, respectively, in mice subjected to a lethal live E. coli challenge. Together with the observation that ART alone does not inhibit bacterial growth, this result suggests that ART protection is achieved as a result of its anti-inflammatory activity rather than an antimicrobial effect. In RAW264.7 cells, pretreatment with ART potently inhibited TNF-alpha and interleukin-6 release induced by CpG ODN, LPS, or heat-killed E. coli in a dose- and time-dependent manner. Experiments utilizing affinity sensor technology revealed no direct binding of ART with CpG ODN or LPS. Flow cytometry further showed that ART did not alter binding of CpG ODN to cell surfaces or the internalization of CpG ODN. In addition, upregulated levels of TLR9 and TLR4 mRNA were not attenuated by ART treatment. ART treatment did, however, block the NF-kappaB activation induced by CpG ODN, LPS, or heat-killed E. coli. These findings provide compelling evidence that ART may be an important potential drug for sepsis treatment.

[Cancer immunotherapy with CpG-ODN]

Bacterial DNA and synthetic oligodeoxynucleotides containing CpG motifs (CpG-ODN) are the ligands for the Toll-like receptor 9 (TLR9), which is expressed by B-lymphocytes and a subset of dendritic cells. CpG-ODN are strong activators of both innate and specific immunity, and drive the immune response towards the Th1 phenotype. Given the promising results obtained in several experimental models of allergies or infections, CpG-ODN are now entering clinical trials for these diseases. In cancer, promising approaches combined CpG-ODN with tumor antigens, monoclonal antibodies or dendritic cells. When no relevant tumor antigen is known, CpG-ODN can be used alone to activate locally the innate immunity and trigger a tumor-specific immune response, overcoming the need for the identification of a tumoral antigen. Preclinical models have shown impressive results and several clinical trials are on-going worldwide in melanoma, lymphoma, renal carcinoma, breast cancer and glioblastoma.

Chloroquine protects mice from challenge with CpG ODN and LPS by decreasing proinflammatory cytokine release

Bacterial DNA (bDNA) and lipopolysaccharide (LPS) are potent activators of immune cells such as monocytes and macrophages, which contribute to systemic inflammatory response syndrome (SIRS) and sepsis. To date, no effective anti-sepsis drugs have been developed for clinical use. Chloroquine (CQ), a diprotic weak base traditionally used for treating malaria, was recently shown to decrease cytokine release from macrophages induced by LPS and CpG oligonucleotide (CpG ODN). In the present study, Escherichia coli DNA (EC DNA), CpG ODN and LPS were used to induce SIRS/sepsis in animal models. We found that 30 mg/kg of CQ could protect mice from lethal challenge by CpG ODN and LPS, and 25 mg/kg of CQ could decrease serum TNF-alpha and IL-6 in rats injected with sublethal doses of CpG ODN and LPS. In addition, treatment of murine macrophage ANA-1 cells with 2 mM CQ potently inhibited the release of TNF-alpha, IL-6 and IL-12 induced by CpG ODN and LPS. In human peripheral blood mononuclear cells (hPBMC), 100-200 microM CQ almost completely abrogated release of both TNF-alpha and IL-6 induced by CpG ODN and LPS, whereas IL-6 release induced by EC DNA was not significantly affected by 50 microM CQ. Furthermore, CQ reduced the expression of TLR9 and TLR4 mRNA and the activation of NFkappaB and AP-1 stimulated by CpG ODN and LPS in ANA-1 cells. Flow cytometry and confocal microscopy revealed that CQ increased the accumulation of CpG ODN within ANA-1 cells without influence on its uptake, suggesting that the delayed degradation of CpG ODN was associated with the reduction of proinflammatory cytokine release from the cells. Our results demonstrated that CQ-mediated protection of lethal challenge by CpG ODN and LPS was associated with the reduction of proinflammatory cytokine release.

TOLL-like receptors linking innate and adaptive immune response

Invading pathogens are controlled by the innate and adaptive arms of the immune system. Adaptive immunity, which is mediated by B and T lymphocytes, recognises pathogens by rearranged high affinity receptors. However, the establishment of adaptive immunity is often not rapid enough to eradicate microorganisms as it involves cell proliferation, gene activation and protein synthesis. More rapid defense mechanisms are provided by innate immunity, which recognises invading pathogens by germ-line-encoded pattern recognition receptors (PRR). Recent evidence shows that this recognition can mainly be attributed to the family of TOLL-like receptors (TLR). Binding of pathogen-associated molecular patterns (PAMP) to TLR induces the production of reactive oxygen and nitrogen intermediates (ROI and RNI), pro-inflammatory cytokines, and up-regulates expression of co-stimulatory molecules, subsequently initiating the adaptive immunity. In this review, we will summarize the discovery and the critical roles of the TLR family in host defense, briefly allude to signaling mechanisms mediating the response to TLR ligands, and will provide an update on current knowledge regarding the ligand specificity of these receptors and their role in immunity of domestic animals, particularly cattle.

Differential involvement of IFN-beta in Toll-like receptor-stimulated dendritic cell activation

Toll-like receptor (TLR) can activate dendritic cells (DC) through common signaling pathways requiring a cytoplasmic adapter, MyD88. However, the signaling is differentially regulated among TLR family members. TLR4 can activate MyD88-deficient bone marrow-derived DC (BMDC), and lead to induction of IFN-inducible genes and up-regulation of co-stimulatory molecules such as CD40, implying that the MyD88-independent signaling pathway functions downstream of TLR4. Because these effects can also be induced by type I IFN, we have analyzed whether type I IFN is involved in TLR4-induced responses. In response to lipopolysaccharide (LPS), IFN-beta gene expression was augmented in both wild-type and MyD88-deficient BMDC. Expression of all IFN-inducible genes except immune-responsive gene 1 (IRG1) was abolished and CD40 up-regulation was decreased in LPS-stimulated BMDC lacking either IFN-alpha/beta receptor (IFN-alpha/betaR) or signal transducer and activator of transcription 1 (STAT-1). Similar to the LPS response, TLR9 signaling can also induce expression of IFN-beta and IFN-inducible genes, and up-regulation of CD40. However, all these effects were MyD88 dependent. Thus, in TLR4 signaling, IFN-beta expression can be induced either by the MyD88-dependent or -independent pathway, whereas, in TLR9 signaling, it is dependent on MyD88. In CpG DNA-stimulated DC, expression of IFN-inducible genes except IRG1 was dependent on type I IFN signaling as in LPS-stimulated DC. However, in contrast to TLR4 signaling, TLR9 signaling requires type I IFN signaling for CD40 up-regulation. Taken together, this study demonstrates differential involvement of type I IFN in TLR4- and TLR9-induced effects on DC.

The plasmid pcDNA3 differentially induces production of interferon-alpha and interleukin-6 in cultures of porcine leukocytes

An adjuvant effect of invertebrate DNA has been attributed to its relative high frequency of unmethylated CpG dinucleotides. Here we describe the interferon-alpha (IFN-alpha) and interleukin-6 (IL-6) inducing properties of a commonly used eukaryotic expression vector, pcDNA3, in porcine leukocytes. The magnitude of the cytokine response was compared to that induced by the synthetic ds RNA analogue poly(I):poly(C), inactivated preparations of Aujeszky's disease virus (ADV) and the Gram-negative bacteria Actinobacillus pleuropneumoniae. The plasmid, as well as poly(I):poly(C), required lipofectin to induce IFN-alpha production whereas both preparations induced IL-6 irrespective of preincubation with lipofectin. However, the nucleic acid-induced levels of IL-6 were low compared to those induced by A. pleuropneumoniae. The IFN-alpha response elicited by pcDNA3 in the presence of lipofectin was as high as, or higher than that induced by ADV. Interestingly, also A. pleuropneumoniae induced a substantial production of IFN-alpha when preincubated with lipofectin. Plasmid expression was not necessary for induction of IFN-alpha. Furthermore, the IFN-alpha inducing capacity of pcDNA3 was not reduced when the two predicted immunostimulatory sequences 5'AACGTT3' were deleted. Nor did the ability of the plasmid to induce IFN-alpha production decrease when the ampicillin resistance (ampR) gene was replaced with the kanamycin resistance (kanR) gene. However, methylation of all cytidines in CpG dinucleotides of pcDNA3 abolished the IFN-alpha inducing capacity. These in vitro results indicate an immunomodulatory role of bacterial DNA also in the pig. Unmethylated CpG dinucleotides are crucial for induction of IFN-alpha by the plasmid, but other CpG motifs than those within the 5'AACGTT3' sequences of the ampR gene contribute to this induction in porcine cells.

High mobility group 1 protein (HMG-1) stimulates proinflammatory cytokine synthesis in human monocytes

Lipopolysaccharide (LPS) is lethal to animals because it activates cytokine release, causing septic shock and tissue injury. Early proinflammatory cytokines (e.g., tumor necrosis factor [TNF] and interleukin [IL]-1) released within the first few hours of endotoxemia stimulate mediator cascades that persist for days and can lead to death. High mobility group 1 protein (HMG-1), a ubiquitous DNA-binding protein, was recently identified as a "late" mediator of endotoxin lethality. Anti-HMG-1 antibodies neutralized the delayed increase in serum HMG-1, and protected against endotoxin lethality, even when passive immunization was delayed until after the early cytokine response. Here we examined whether HMG-1 might stimulate cytokine synthesis in human peripheral blood mononuclear cell cultures. Addition of purified recombinant HMG-1 to human monocyte cultures significantly stimulated the release of TNF, IL-1alpha, IL-1beta, IL-1RA, IL-6, IL-8, macrophage inflammatory protein (MIP)-1alpha, and MIP-1beta; but not IL-10 or IL-12. HMG-1 concentrations that activated monocytes were within the pathological range previously observed in endotoxemic animals, and in serum obtained from septic patients. HMG-1 failed to stimulate cytokine release in lymphocytes, indicating that cellular stimulation was specific. Cytokine release after HMG-1 stimulation was delayed and biphasic compared with LPS stimulation. Computer-assisted image analysis demonstrated that peak intensity of HMG-1-induced cellular TNF staining was comparable to that observed after maximal stimulation with LPS. Administration of HMG-1 to Balb/c mice significantly increased serum TNF levels in vivo. Together, these results indicate that, like other cytokine mediators of endotoxin lethality (e.g., TNF and IL-1), extracellular HMG-1 is a regulator of monocyte proinflammatory cytokine synthesis.