Exposure to HIV-1-encoded Toll-like receptor 8 ligands enhances monocyte response to microbial encoded Toll-like receptor 2/4 ligands

Microbial RNA, the New PAMP of Many Faces

Traditionally, pathogen-associated molecular patterns (PAMPs) were described as structural molecular motifs shared by different classes of microorganisms. However, it was later discovered that the innate immune system is also capable of distinguishing metabolically active microbes through the detection of a special class of viability-associated PAMPs (vita-PAMPs). Indeed, recognition of vita-PAMPs triggers an extra warning sign not provoked by dead bacteria. Bacterial RNA is classified as a vita-PAMP since it stops being synthesized once the microbes are eliminated. Most of the studies in the literature have focused on the pro-inflammatory capacity of bacterial RNA on macrophages, neutrophils, endothelial cells, among others. However, we, and other authors, have shown that microbial RNA also has down-modulatory properties. More specifically, bacterial RNA can reduce the surface expression of MHC class I and MHC class II on monocytes/macrophages and help evade CD8+ and CD4+ T cell-mediated immune surveillance. This phenomenon has been described for several different bacteria and parasites, suggesting that microbial RNA plays a significant immunoregulatory role in the context of many infectious processes. Thus, beyond the pro-inflammatory capacity of microbial RNA, it seems to be a crucial component in the intricate collection of immune evasion strategies. This review focuses on the different facets of the immune modulating capacity of microbial RNA.

Antigen Presenting Cells Contribute to Persistent Immune Activation Despite Antiretroviral Therapy Initiation During Hyperacute HIV-1 Infection

Human immunodeficiency virus (HIV)-induced changes in immune cells during the acute phase of infection can cause irreversible immunological damage and predict the rate of disease progression. Antiretroviral therapy (ART) remains the most effective strategy for successful immune restoration in immunocompromised people living with HIV and the earlier ART is initiated after infection, the better the long-term clinical outcomes. Here we explored the effect of ART on peripheral antigen presenting cell (APC) phenotype and function in women with HIV-1 subtype C infection who initiated ART in the hyperacute phase (before peak viremia) or during chronic infection. Peripheral blood mononuclear cells obtained longitudinally from study participants were used for immunophenotyping and functional analysis of monocytes and dendritic cells (DCs) using multiparametric flow cytometry and matched plasma was used for measurement of inflammatory markers IL-6 and soluble CD14 (sCD14) by enzyme-linked immunosorbent assay. HIV infection was associated with expansion of monocyte and plasmacytoid DC (pDC) frequencies and perturbation of monocyte subsets compared to uninfected persons despite antiretroviral treatment during hyperacute infection. Expression of activation marker CD69 on monocytes and pDCs in early treated HIV was similar to uninfected individuals. However, despite early ART, HIV infection was associated with elevation of plasma IL-6 and sCD14 levels which correlated with monocyte activation. Furthermore, HIV infection with or without early ART was associated with downmodulation of the co-stimulatory molecule CD86. Notably, early ART was associated with preserved toll-like receptor (TLR)-induced IFN-α responses of pDCs. Overall, this data provides evidence of the beneficial impact of ART initiated in hyperacute infection in preservation of APC functional cytokine production activity; but also highlights persistent inflammation facilitated by monocyte activation even after prolonged viral suppression and suggests the need for therapeutic interventions that target residual immune activation.

The Role of Toll-Like Receptors in Retroviral Infection

Toll-like receptors (TLRs) are key pathogen sensing receptors that respond to diverse microbial ligands, and trigger both innate and adaptive immune responses to infection. Since their discovery, a growing body of evidence has pointed to an important role for TLRs in retroviral infection and pathogenesis. These data suggest that multiple TLRs contribute to the anti-retroviral response, and that TLR engagement by retroviruses can have complex and divergent outcomes for infection. Despite this progress, numerous questions remain about the role of TLRs in retroviral infection. In this review, I summarize existing evidence for TLR-retrovirus interactions and the functional roles these receptors play in immunity and pathogenesis, with particular focus on human immunodeficiency virus (HIV).

An assessment of toll-like receptor 7 and 8 gene polymorphisms with susceptibility to HIV-1 infection, AIDS development and response to antiretroviral therapy

Toll-like receptors (TLRs) play an important role in activating the innate immune response, inducing inflammation and initiating the adaptive immune response. In this study, we assess the influence of TLR7 and TLR8 gene polymorphisms on HIV-1 susceptibility, AIDS development, and treatment outcomes. The TLR7 and TLR8 single nucleotide polymorphisms (SNPs) were genotyped through real-time PCR in 222 patients living with HIV-1 and 141 healthy controls. Frequencies of the TLR7-IVS2-151 G/A and TLR7-IVS1 + 1817 G/T genotypes and alleles were not significantly increased in patients with HIV-1 infection compared to healthy controls both in males and females. Whereas, males carrying TLR8 Met allele were twice susceptible to HIV-1 infection compared to subjects with A allele (OR = 2.04, 95 % CI 1.10-3.76; p = 0.021). Interestingly, for TLR8-129 G/C, both males and females carrying G allele and GG genotype, respectively were significantly associated with HIV-1 infection (p < 0.0001). Moreover, the TLR7 IVS1 + 1817 G/T and the TLR8 rs3764880 were associated with protection to progress the AIDS stage in male and female, respectively (p < 0.05). Males carrying TLR7 IVS2-151-A allele showed a significant increased level of HIV-1 viral load pre-treatment, in comparison with individuals carrying the G allele (p-value = 0.036). Additionally, males carrying TLR8 Met allele showed statistically higher HIV viral load at baseline (p-value = 0.04) and after treatment (p-value = 0.013). Regarding CD4 + T cell counts, no significant association was found with TLR7 and TLR8 SNPs before and after antiretroviral treatment. This data demonstrates that TLR8 polymorphisms could affect HIV-1 infection. Moreover, an association between TLR7 IVS2-151-A and TLR8 Met alleles and plasma HIV viral load level was found.

Insights into the toll-like receptors in sexually transmitted infections

Toll-like receptors (TLRs) are like soldiers of an innate immune system, which protects vital biological processes against invading pathogens. TLR signalling pathways help in the removal of pathogens and mediate well-established inflammatory processes. However, these processes may also aid in the development or augmentation of an infection or an autoimmune disease. Recent studies have delineated TLR polymorphism's role in the loss of function, making hosts more resistant or vulnerable to the development of an infection. In this review, we have discussed the association of TLRs with sexually transmitted infections (STIs), especially to the pathogen-specific ligands. We have also assessed the impact on TLR downstream signalling and the maintenance of cellular homeostasis during immune responses. Besides, we have discussed the role of TLRs single nucleotide polymorphisms in various STIs. Since TLRs are known to play a part in defence mechanisms and in aiding infections therefore, a thorough understanding of TLRs structure and molecular mechanisms is required to explain how they can influence the outcome of an STI. Such a strategy may lead to the development of novel and useful immunotherapeutic approaches to control pathogen progression and prevent transmission.

Identification and validation of a novel dual small-molecule TLR2/8 antagonist

Toll-like receptor 2 (TLR2) and TLR8 are involved in the recognition of bacterial and viral components and are linked not only to protective antimicrobial immunity but also to inflammatory diseases. Recently, increasing attention has been paid to the receptor crosstalk between TLR2 and TLR8 to fine-tune innate immune responses. In this study, we report a novel dual TLR2/TLR8 antagonist, compound 24 that was developed by a modeling-guided synthesis approach. The modulator was optimized from the previously reported 1,3-benzothiazole derivative, compound 8. Compound 24 was pharmacologically characterized for the ability to inhibit TLR2- and TLR8-mediated responses in TLR-overexpressing reporter cells and THP-1 macrophages. The modulator showed high efficacy with IC₅₀ values in the low micromolar range for both TLRs, selectivity towards other TLRs and low cytotoxicity. At TLR2, a slight predominance for the TLR2/1 heterodimer was found in reporter cells selectively expressing TLR2/1 or TLR2/6 heterodimers. Concentration ratio analysis in the presence of Pam₃CSK₄ or Pam₂CSK₄ indicated non-competitive antagonist behavior at hTLR2. In computational docking studies, a plausible alternative binding mode of compound 24 was predicted for both TLR2 and TLR8. Our results provide evidence that it is feasible to simultaneously and selectively target endosomal- and surface-located TLRs. We identified a small-molecule dual TLR2/8 antagonist that may serve as a valuable pharmacological tool to decipher the role of TLR2/8 co-signaling in inflammation.

Bacterial RNA Contributes to the Down-Modulation of MHC-II Expression on Monocytes/Macrophages Diminishing CD4+ T Cell Responses

Brucella abortus, the causative agent of brucellosis, displays many resources to evade T cell responses conducive to persist inside the host. Our laboratory has previously showed that infection of human monocytes with B. abortus down-modulates the IFN-γ-induced MHC-II expression. Brucella outer membrane lipoproteins are structural components involved in this phenomenon. Moreover, IL-6 is the soluble factor that mediated MHC-II down-regulation. Yet, the MHC-II down-regulation exerted by lipoproteins was less marked than the one observed as consequence of infection. This led us to postulate that there should be other components associated with viable bacteria that may act together with lipoproteins in order to diminish MHC-II. Our group has recently demonstrated that B. abortus RNA (PAMP related to pathogens' viability or vita-PAMP) is involved in MHC-I down-regulation. Therefore, in this study we investigated if B. abortus RNA could be contributing to the down-regulation of MHC-II. This PAMP significantly down-modulated the IFN-γ-induced MHC-II surface expression on THP-1 cells as well as in primary human monocytes and murine bone marrow macrophages. The expression of other molecules up-regulated by IFN-γ (such as co-stimulatory molecules) was stimulated on monocytes treated with B. abortus RNA. This result shows that this PAMP does not alter all IFN-γ-induced molecules globally. We also showed that other bacterial and parasitic RNAs caused MHC-II surface expression down-modulation indicating that this phenomenon is not restricted to B. abortus. Moreover, completely degraded RNA was also able to reproduce the phenomenon. MHC-II down-regulation on monocytes treated with RNA and L-Omp19 (a prototypical lipoprotein of B. abortus) was more pronounced than in monocytes stimulated with both components separately. We also demonstrated that B. abortus RNA along with its lipoproteins decrease MHC-II surface expression predominantly by a mechanism of inhibition of MHC-II expression. Regarding the signaling pathway, we demonstrated that IL-6 is a soluble factor implicated in B. abortus RNA and lipoproteins-triggered MHC-II surface down-regulation. Finally, CD4⁺ T cells functionality was affected as macrophages treated with these components showed lower antigen presentation capacity. Therefore, B. abortus RNA and lipoproteins are two PAMPs that contribute to MHC-II down-regulation on monocytes/macrophages diminishing CD4⁺ T cell responses.

MicroRNA miR-126-5p Enhances the Inflammatory Responses of Monocytes to Lipopolysaccharide Stimulation by Suppressing Cylindromatosis in Chronic HIV-1 Infection

Persistent immune activation during chronic human immunodeficiency virus type 1 (HIV-1) infection facilitates immune dysfunction and thereby fuels disease progression. The translocation of bacterial derivatives into blood and the hyperinflammatory responsiveness of monocytes have been considered important causative factors for persistent immune activation. Whether microRNAs (miRNAs) are involved in regulating monocyte-mediated inflammatory responses during chronic HIV-1 infection remains elusive. In this study, we show that miR-126-5p functions as a positive regulator of monocyte-mediated inflammatory responses. Significantly increased miRNA miR-126-5p and decreased cylindromatosis (CYLD) were observed in primary monocytes from chronic HIV-1 patients. Inhibition of miR-126-5p in monocytes from chronic HIV-1 patients attenuated the responsiveness of these cells to lipopolysaccharide (LPS) stimulation. Gain-of-function assays confirmed that miR-126-5p could downregulate CYLD, which in turn caused an upregulation of phosphorylation of JNK protein (pJNK) and enhanced inflammatory responses of monocytes to LPS stimulation. Overall, miR-126-5p upregulates the responsiveness of monocytes to LPS stimulation in chronic HIV-1 infection, and the suppression of miR-126-5p and the promotion of CYLD expression in primary monocytes may represent a practical immune intervention strategy to contain persistent inflammation in chronic HIV-1 infection.IMPORTANCE Monocyte-mediated hyperinflammatory responses during chronic HIV-1 infection are important causative factors driving AIDS progression; however, the underlying mechanism has not been fully addressed. We demonstrated that miR-126-5p, one of the most upregulated miRNAs during chronic HIV-1 infection, could enhance the inflammatory responses of monocytes to LPS by suppressing the inhibitory protein CYLD and thereby unleashing the expression of pJNK in the LPS/Toll-like receptor 4/mitogen-activated protein kinase pathway. This observation reveals a new mechanism for HIV-1 pathogenesis, which could be targeted by immune intervention.

Stimulation of PBMC and Monocyte-Derived Macrophages via Toll-Like Receptor Activates Innate Immune Pathways in HIV-Infected Patients on Virally Suppressive Combination Antiretroviral Therapy

In HIV-infected, combination antiretroviral therapy (cART)-treated patients, immune activation and microbial translocation persist and associate with inadequate CD4 recovery and morbidity/mortality. We analyzed whether alterations in the toll-like receptor (TLR) pathway could be responsible for the immune hyperactivation seen in these patients. PBMC/monocyte-derived macrophages (MDMs) of 28 HIV+ untreated and 35 cART-treated patients with HIV-RNA < 40 cp/mL [20 Full Responders (FRs): CD4 ≥ 350; 15 Immunological Non-Responders (INRs): CD4 < 350], as well as of 16 healthy controls were stimulated with a panel of TLR agonists. We measured: CD4/CD8/CD14/CD38/HLA-DR/Ki67/AnnexinV/CD69/TLR4/8 (Flow Cytometry); PBMC expression of 84 TLR pathway genes (qPCR); PBMC/MDM cytokine release (Multiplex); and plasma lipopolysaccharide (LPS)/sCD14 (LAL/ELISA). PBMC/MDM from cART patients responded weakly to LPS stimulation but released high amounts of pro-inflammatory cytokines. MDM from these patients were characterized by a reduced expression of HLA-DR+ MDM and failed to expand activated HLA-DR+ CD38+ T-lymphocytes. PBMC/MDM from cART patients responded more robustly to ssRNA stimulation; this resulted in a significant expansion of activated CD38 + CD8 and the release of amounts of pro-inflammatory cytokines comparable to those seen in untreated viremic patients. Despite greater constitutive TLR pathway gene expression, PBMC from INRs seemed to upregulate only type I IFN genes following TLR stimulation, whereas PBMC from full responders showed a broader response. Systemic exposure to microbial antigens drives immune activation during cART by triggering TLRs. Bacterial stimulation modifies MDM function/pro-inflammatory profile in cART patients without affecting T-lymphocytes; this suggests translocating bacteria as selective stimulus to chronic innate activation during cART. High constitutive TLR activation is seen in patients lacking CD4 recovery, suggesting an exhausted immune milieu, anergic to further antigen encounters.

TLR8 regulation of LILRA3 in monocytes is abrogated in human immunodeficiency virus infection and correlates to CD4 counts and virus loads

BACKGROUND

LILRA3 is an immunostimulatory molecule which can conditionally induce the proliferation of cytotoxic cells. LILRA3 has a deletion genotype which is associated with multiple immune disorders. In this study, we wanted to analyze the regulation of LILRA3 and its significance in the context of HIV infection.

RESULTS

We analyzed a panel of TLR agonists and found that ssRNA40, a TLR8 agonist, is a potent inducer of LILRA3 in healthy individuals. However, this regulation is much diminished in HIV. Comparison of TLR8 to TLR4 induction of LILRA3 indicated that LPS induces less LILRA3 than ssRNA40 among healthy controls, but not HIV patients. Levels of LILRA3 induction correlated to virus load and CD4 counts in untreated patients. Recombinant LILRA3 can induce a host of proinflammatory genes which include IL-6 and IL-1α, as well as alter the expression of MHC and costimulatory molecules in monocytes and B-cells.

CONCLUSION

Our experiments point towards a beneficial role for LILRA3 in virus infections, especially in ssRNA viruses, like HIV, that engage TLR8. However, the potentially beneficial role of LILRA3 is abrogated during a HIV infection. We believe that more work has to be done to study the role of LILRA3 in infectious diseases and that there is a potential for exploring the use of LILRA3 in the treatment of virus infections.

Naturally Occurring Subclinical Endotoxemia in Humans Alters Adaptive and Innate Immune Functions through Reduced MAPK and Increased STAT1 Phosphorylation

Multiple studies have shown correlates of immune activation with microbial translocation and plasma LPS during HIV infection. It is unclear whether this activation is due to LPS, residual viral replication, or both. Few studies have addressed the effects of persistent in vivo levels of LPS on specific immune functions in humans in the absence of chronic viral infection or pathological settings such as sepsis. We previously reported on a cohort of HIV-negative men with subclinical endotoxemia linked to alterations in CD4/CD8 T cell ratio and plasma cytokine levels. This HIV-negative cohort allowed us to assess cellular immune functions in the context of different subclinical plasma LPS levels ex vivo without confounding viral effects. By comparing two samples of differing plasma LPS levels from each individual, we now show that subclinical levels of plasma LPS in vivo significantly alter T cell proliferative capacity, monocyte cytokine release, and HLA-DR expression, and induce TLR cross-tolerance by decreased phosphorylation of MAPK pathway components. Using this human in vivo model of subclinical endotoxemia, we furthermore show that plasma LPS leads to constitutive activation of STAT1 through autocrine cytokine signaling, suggesting that subclinical endotoxemia in healthy individuals might lead to significant changes in immune function that have thus far not been appreciated.

Aging of the human innate immune system in HIV infection

HIV infection is associated with a chronic inflammatory state arising from multiple factors, including innate immune recognition of HIV, increased microbial translocation, and release of endogenous ligands from damaged cells (such as CD4 T cells). In many respects, this heightened pro-inflammatory environment resembles that associated with aging in the absence of HIV infection, and evidence of dysregulated innate immune responses can be found in not only older HIV-negative adults, but also adults with HIV infection. While the study of innate immune aging in HIV infection is still in its early stages, it seems likely that at least additive, or potentially synergistic effects of aging and HIV infection will be found.

Immune activation and the role of TLRs and TLR agonists in the pathogenesis of HIV-1 infection in the humanized mouse model

Immune activation plays a critical role in HIV-1 pathogenesis, but the pathways that are responsible for HIV-1-associated immune activation are not well understood. Recent advances in the development of a humanized mouse model for HIV-1 infection might provide new approaches to study the mechanisms of HIV-1 associated immune activation, and to test interventions aimed at reducing HIV-1 pathogenesis.

Human TLR8 is activated upon recognition of Borrelia burgdorferi RNA in the phagosome of human monocytes

Phagocytosed Borrelia burgdorferi (Bb), the Lyme disease spirochete, induces a robust and complex innate immune response in human monocytes, in which TLR8 cooperates with TLR2 in the induction of NF-κB-mediated cytokine production, whereas TLR8 is solely responsible for transcription of IFN-β through IRF7. We now establish the role of Bb RNA in TLR8-mediated induction of IFN-β. First, using TLR2-transfected HEK.293 cells, which were unable to phagocytose intact Bb, we observed TLR2 activation by lipoprotein-rich borrelial lysates and TLR2 synthetic ligands but not in response to live spirochetes. Purified Bb RNA, but not borrelial DNA, triggered TLR8 activation. Neither of these 2 ligands induced activation of TLR7. Using purified human monocytes we then show that phagocytosed live Bb, as well as equivalent amounts of borrelial RNA delivered into the phagosome by polyethylenimine (PEI), induces transcription of IFN-β and secretion of TNF-α. The cytokine response to purified Bb RNA was markedly impaired in human monocytes naturally deficient in IRAK-4 and in cells with knockdown TLR8 expression by small interfering RNA. Using confocal microscopy we provide evidence that TLR8 colocalizes with internalized Bb RNA in both early (EEA1) and late endosomes (LAMP1). Live bacterial RNA staining indicates that spirochetal RNA does not transfer from the phagosome into the cytosol. Using fluorescent dextran particles we show that phagosomal integrity in Bb-infected monocytes is not affected. We demonstrate, for the first time, that Bb RNA is a TLR8 ligand in human monocytes and that transcription of IFN-β in response to the spirochete is induced from within the phagosomal vacuole through the TLR8-MyD88 pathway.

A role for TLR signaling during B cell activation in antiretroviral-treated HIV individuals

The mechanisms underlying B cell activation that persists during antiretroviral therapy (ART) are unknown. Toll-like receptor (TLR) signaling is a critical mediator of innate cell activation and though B cells express TLRs, few studies have investigated a role for TLR signaling in B cell activation during HIV infection. We addressed this question by assessing the activated phenotype and TLR expression/responsiveness of B cells from ART-treated HIV-infected subjects (HIVART(+)). We evaluated activation markers implicated in B cell-mediated T cell trans infection during HIV pathogenesis. We found no significant difference in TLR expression between B cells of HIVART(+) and HIV(-) subjects. However, B cells of HIVART(+) subjects exhibited heightened endogenous expression levels of IL-6 (p=0.0051), T cell cognate ligands CD40 (p=0.0475), CD54 (p=0.0229), and phosphorylated p38 (p<0.0001), a marker of TLR signaling. In vitro, B cells of HIVART(+) individuals were less responsive to TLR stimulation compared to B cells of HIV(-) subjects. The activated phenotype of in vitro TLR-stimulated B cells of HIV(-) subjects was similar to ex vivo B cells from HIVART(+) individuals. TLR2 stimulation was a potent mediator of B cell activation, whereas B cells were least responsive to TLR4 stimulation. Compared to HIV(-) subjects, the serum level of lipoteichoic acid (TLR2 ligand) in HIVART(+) subjects was significantly higher (p=0.0207), correlating positively with viral load (p=0.0127, r=0.6453). Our data suggest that during HIV infection TLR-activated B cells may exert a pathogenic role and B cells from HIVART(+) subjects respond to in vitro TLR stimulation, yet exhibit a TLR tolerant phenotype suggesting prior in vivo TLR stimulation.

Increased Escherichia coli-induced interleukin-23 production by CD16+ monocytes correlates with systemic immune activation in untreated HIV-1-infected individuals

The level of microbial translocation from the intestine is increased in HIV-1 infection. Proinflammatory cytokine production by peripheral antigen-presenting cells in response to translocated microbes or microbial products may contribute to systemic immune activation, a hallmark of HIV-1 infection. We investigated the cytokine responses of peripheral blood myeloid dendritic cells (mDCs) and monocytes to in vitro stimulation with commensal enteric Escherichia coli in peripheral blood mononuclear cells (PBMC) from untreated HIV-1-infected subjects and from uninfected controls. Levels of interleukin 23 (IL-23) produced by PBMC from HIV-1-infected subjects in response to E. coli stimulation were significantly higher than those produced by PBMC from uninfected subjects. IL-23 was produced primarily by CD16(+) monocytes. This subset of monocytes was increased in frequency and expressed higher levels of Toll-like receptor 4 (TLR4) in HIV-1-infected individuals than in controls. Blocking TLR4 on total CD14(+) monocytes reduced IL-23 production in response to E. coli stimulation. Levels of soluble CD27, an indicator of systemic immune activation, were elevated in HIV-1-infected subjects and were associated with the percentage of CD16(+) monocytes and the induction of IL-23 by E. coli, providing a link between these parameters and systemic inflammation. Taken together, these results suggest that IL-23 produced by CD16(+) monocytes in response to microbial stimulation may contribute to systemic immune activation in HIV-1-infected individuals.

Molecular characterization and expression analysis of TLR 7 and TLR 8 homologs in large yellow croaker (Pseudosciaena crocea)

The two toll-like receptor (TLR) genes, LycTLR7 and LycTLR8, were cloned from large yellow croaker (Pseudosciaena crocea), an economically important marine fish in China. The full-length cDNAs of LycTLR7 and LycTLR8 are 3544 and 3593 bp, with an open reading frame (ORF) of 3165 and 3093 bp, encoding 1053 and 1030 amino acids, respectively. The TLR family motifs, such as leucine rich repeat (LRR) and Toll/interleukin (IL)-1 receptor (TIR) domain, are conserved in the LycTLR7 and LycTLR8, with 17 and 14 LRRs, and with a TIR domain, respectively. It is also noted that an LRR N-terminal domain (LRR-NT, residues 24-60) is present in the LycTLR7 but not in the LycTLR8. Both LycTLR7 and LycTLR8 contain a conserved extracellular CxRCxxxxxPCxxC motif, which was found in TLR7/TLR8 of other species and required for stimulus-induced signal transduction. Homology comparison shows that LycTLR7 has 79%, 71.9%, 65.9% and 65.8% identity to fugu, rainbow trout, carp and catfish TLR7, while LycTLR8 has 67.1%, 60.7%, 60.6%, 52.4%, and 51.5% identity to fugu TLR8, rainbow trout TLR8a1, rainbow trout TLR8a2, catfish TLR8-2, and catfish TLR8-1, respectively. Subcellular localization analysis revealed that both LycTLR7 and LycTLR8 are located in the endoplasmic reticulum of epithelioma papulosum cyprini (EPC) cells, which is similar to TLR7/TLR8 in mammals. The two TLRs were constitutively expressed in all tissues tested, especially in immune-related tissues such as spleen, head kidney and gills. An increased expression of LycTLR7 and LycTLR8 was observed in head kidney and spleen of large yellow croakers stimulated by poly (I:C), a viral mimic. In head kidney, their mRNA expression was up-regulated more than 10 times compared to the controls at 12 h after poly (I:C) stimulation. These results suggested that LycTLR7 and LycTLR8 may play a role in the defense against viral infection like their mammalian homologs.

Learning from the messengers: innate sensing of viruses and cytokine regulation of immunity - clues for treatments and vaccines

Virus infections are a major global public health concern, and only via substantial knowledge of virus pathogenesis and antiviral immune responses can we develop and improve medical treatments, and preventive and therapeutic vaccines. Innate immunity and the shaping of efficient early immune responses are essential for control of viral infections. In order to trigger an efficient antiviral defense, the host senses the invading microbe via pattern recognition receptors (PRRs), recognizing distinct conserved pathogen-associated molecular patterns (PAMPs). The innate sensing of the invading virus results in intracellular signal transduction and subsequent production of interferons (IFNs) and proinflammatory cytokines. Cytokines, including IFNs and chemokines, are vital molecules of antiviral defense regulating cell activation, differentiation of cells, and, not least, exerting direct antiviral effects. Cytokines shape and modulate the immune response and IFNs are principle antiviral mediators initiating antiviral response through induction of antiviral proteins. In the present review, I describe and discuss the current knowledge on early virus–host interactions, focusing on early recognition of virus infection and the resulting expression of type I and type III IFNs, proinflammatory cytokines, and intracellular antiviral mediators. In addition, the review elucidates how targeted stimulation of innate sensors, such as toll-like receptors (TLRs) and intracellular RNA and DNA sensors, may be used therapeutically. Moreover, I present and discuss data showing how current antimicrobial therapies, including antibiotics and antiviral medication, may interfere with, or improve, immune response.

TLR8: the forgotten relative revindicated

The endosomal Toll-like receptors (TLRs) TLR3, TLR7, TLR8 and TLR9 are important in sensing foreign nucleic acids encountered by phagocytes. Because TLR8 was initially thought to be non-functional in mice, less is known about TLR8 than the genetically and functionally related TLR7. Originally associated with the recognition of single-stranded RNA of viral origin, there is now evidence that human TLR8 is also able to sense bacterial RNA released within phagosomal vacuoles, inducing the production of both nuclear factor (NF)-κB-dependent cytokines and type I interferons (IFNs), such as IFN-β. The functions of TLR8 extend beyond the recognition of foreign pathogens and include cross-talk with other endosomal TLRs, a process that may also have a role in the generation of autoimmunity.

Microbial translocation in HIV infection: causes, consequences and treatment opportunities

Systemic immune activation is increased in HIV-infected individuals, even in the setting of virus suppression with antiretroviral therapy. Although numerous factors may contribute, microbial products have recently emerged as potential drivers of this immune activation. In this Review, we describe the intestinal damage that occurs in HIV infection, the evidence for translocation of microbial products into the systemic circulation and the pathways by which these products activate the immune system. We also discuss novel therapies that disrupt the translocation of microbial products and the downstream effects of microbial translocation.

HIV Infection and TLR Signalling in the Liver

Despite the availability of effective combination antiretroviral therapy (cART), liver disease is one of the leading causes of morbidity and mortality in Human Immunodeficiency Virus (HIV)-infected individuals, specifically, in the presence of viral hepatitis coinfection. HIV, a single stranded RNA virus, can bind to and activate both Toll-like receptor (TLR)7 and TLR8 in circulating blood mononuclear cells, but little is known about the effect of HIV on TLRs expressed in the liver. HIV can directly infect cells of the liver and HIV-mediated depletion of CD4+ T-cells in the gastrointestinal tract (GI tract) results in increased circulating lipopolysaccharide (LPS), both of which may impact on TLR signaling in the liver and subsequent liver disease progression. The potential direct and indirect effects of HIV on TLR signaling in the liver will be explored in this paper.

HIV infection and aging of the innate immune system

The increased life expectancy of HIV-infected individuals due to improved treatment has revealed an unexpected increase in non-AIDS comorbidities that are typically associated with older age including cardiovascular disease, dementia and frailty. The majority of these diseases arise as the result of dysregulated systemic inflammation, and both the aged and HIV-infected individuals exhibit elevated basal levels of inflammation. In the elderly, increased inflammation and age-related diseases are associated with a state of impaired immunity called immunosenescence, which is thought to result from a lifetime of immune stimulation. It is now apparent that HIV induces premature immunosenescence within T-cells; however, the impact of HIV on aging of cells of the innate arm of the immune system is unknown. Innate immune cells play a central role in inflammation and are thus critical for the pathogenesis of inflammatory diseases. Limited evidence suggests HIV infection mimics age-related changes to innate immune cells; however, the extent of this effect and the mechanism underlying these changes remain to be defined. This review focuses on the impact of HIV infection on the function and aging of innate immune cells and discusses potential drivers of premature immunosenescence including chronic endotoxaemia, residual viraemia, telomere attrition and altered cellular signalling.

Inhibition of TLR8- and TLR4-induced Type I IFN induction by alcohol is different from its effects on inflammatory cytokine production in monocytes

BACKGROUND

Prolonged alcohol consumption is a significant co-factor in the progression of chronic viral infections including hepatitis C and HIV, which are both single-stranded RNA viruses. Toll like receptor 8 (TLR8), a pattern recognition receptor expressed in monocytes, senses viral single stranded RNA as a danger signal and leads to the induction of Type I interferon (IFN) as well as the pro-inflammatory cytokine, tumor necrosis factor alpha (TNF alpha). Lipopolysaccharide (LPS), a Toll like receptor 4 (TLR4) ligand, was shown to affect inflammatory cell activation after alcohol consumption and in HIV and HCV infections. Here we hypothesized that alcohol exposure modulates TLR8- and TLR4-ligand-induced monocyte activation and affects both type I IFN and inflammatory cytokine induction.

RESULTS

The TLR8 ligand, CL075, as well as the TLR4 ligand, LPS, resulted in a significant induction of TNF alpha both at the mRNA and protein levels in human monocytes. We found that both acute and prolonged alcohol treatment resulted in inhibition of type I IFN induction by either TLR8 or TLR4 ligands in human monocytes at the protein and mRNA levels. In contrast to Type I IFN production, the effects of acute and prolonged alcohol were different on inflammatory cytokine activation after TLR8 or TLR4 ligand stimulation. Acute alcohol inhibited TLR8- or TLR4-induced TNF alpha protein and mRNA induction while it augmented IL-10 production in monocytes. In contrast, prolonged alcohol treatment augmented TNF alpha without affecting IL-10 production significantly in response to either TLR8 or TLR4 ligand stimulation.

CONCLUSIONS

These novel results suggest first, that alcohol has a profound inhibitory effect on Type I IFN induction regardless of intracellular (TLR8) or cell surface-derived (TLR4) danger signals. Second, both acute and prolonged alcohol exposure can inhibit antiviral Type I IFN pathway activation. Third, the opposite effects of acute (inhibitory) and prolonged alcohol (augmentation) treatment on pro-inflammatory cytokine activation extend to TLR8-induced signals beyond the previously shown TLR4/LPS pathway.

Endotoxemia is associated with altered innate and adaptive immune responses in untreated HIV-1 infected individuals

Background

Microbial translocation may contribute to the immunopathogenesis in HIV infection. We investigated if microbial translocation and inflammation were associated with innate and adaptive immune responses in adults with HIV.

Methodology/Principal Findings

This was an observational cohort study. Sera from HIV-infected and HIV-uninfected individuals were analyzed for microbial translocation (soluble CD14, lipopolysaccharides [LPS], endotoxin core antibody, and anti-α-galactosyl antibodies) and inflammatory markers (high sensitivity C-reactive protein, IL-6, IL-1 receptor antagonist, soluble tumor necrosis factor receptor II, and IL-10) with enzyme-linked immunosorbent assays. Peripheral blood mononuclear cells (PBMC) from HIV-infected persons and healthy controls (primed with single-stranded HIV-1-derived RNA) were stimulated with LPS, and cytokine production was measured. Finally, HIV-infected patients were immunized with Prevnar 7vPnC±CpG 7909 followed by Pneumo Novum PPV-23. Effects of microbial translocation and inflammation on immunization were analyzed in a predictive regression model. We included 96 HIV-infected individuals, 76 on highly active antiretroviral therapy (HAART), 20 HAART-naive, and 50 healthy controls. Microbial translocation and inflammatory markers were higher among HIV-infected persons than controls. Cytokine levels following LPS stimulation were increased in PBMCs from HAART-naive compared to HAART-treated HIV-infected persons. Further, RNA-priming of PBMCs from controls acted synergistically with LPS to augment cytokine responses. Finally, high serum LPS levels predicted poor vaccine responses among HAART-naive, but not among HAART-treated HIV-infected individuals.

Conclusions/Significance

LPS acts synergistically with HIV RNA to stimulate innate immune responses in vitro and increasing serum LPS levels seem to predict poor antibody responses after vaccination among HAART-naive HIV-infected persons. Thus, our results suggest that microbial translocation may be associated with innate and adaptive immune dysfunction in untreated HIV infection.

Phagosomal signaling by Borrelia burgdorferi in human monocytes involves Toll-like receptor (TLR) 2 and TLR8 cooperativity and TLR8-mediated induction of IFN-beta

Phagocytosed Borrelia burgdorferi (Bb) induces inflammatory signals that differ both quantitatively and qualitatively from those generated by spirochetal lipoproteins interacting with Toll-like receptor (TLR) 1/2 on the surface of human monocytes. Of particular significance, and in contrast to lipoproteins, internalized spirochetes induce transcription of IFN-β. Using inhibitory immunoregulatory DNA sequences (IRSs) specific to TLR7, TLR8, and TLR9, we show that the TLR8 inhibitor IRS957 significantly diminishes production of TNF-α, IL-6, and IL-10 and completely abrogates transcription of IFN-β in Bb-stimulated monocytes. We demonstrate that live Bb induces transcription of TLR2 and TLR8, whereas IRS957 interferes with their transcriptional regulation. Using confocal and epifluorescence microscopy, we show that baseline TLR expression in unstimulated monocytes is greater for TLR2 than for TLR8, whereas expression of both TLRs increases significantly upon stimulation with live spirochetes. By confocal microscopy, we show that TLR2 colocalization with Bb coincides with binding, uptake, and formation of the phagosomal vacuole, whereas recruitment of both TLR2 and TLR8 overlaps with degradation of the spirochete. We provide evidence that IFN regulatory factor (IRF) 7 is translocated into the nucleus of Bb-infected monocytes, suggesting its activation through phosphorylation. Taken together, these findings indicate that the phagosome is an efficient platform for the recognition of diverse ligands; in the case of Bb, phagosomal signaling involves a cooperative interaction between TLR2 and TLR8 in pro- and antiinflammatory cytokine responses, whereas TLR8 is solely responsible for IRF7-mediated induction of IFN-β.

SIV infection in natural hosts: resolution of immune activation during the acute-to-chronic transition phase

SIV-infected natural hosts do not progress to clinical AIDS yet display high viral replication and an acute immunologic response similar to pathogenic SIV/HIV infections. During chronic SIV infection, natural hosts suppress their immune activation, whereas pathogenic hosts display a highly activated immune state. Here, we review natural host SIV infections with an emphasis on specific immune cells and their contribution to the transition from the acute-to-chronic phases of infection.