Cell Surface Expression of Endosomal Toll-Like Receptors-A Necessity or a Superfluous Duplication?

Timely and precise delivery of the endosomal Toll-like receptors (TLRs) to the ligand recognition site is a critical event in mounting an effective antimicrobial immune response, however, the same TLRs should maintain the delicate balance of avoiding recognition of self-nucleic acids. Such sensing is widely known to start from endosomal compartments, but recently enough evidence has accumulated supporting the idea that TLR-mediated signaling pathways originating in the cell membrane may be engaged in various cells due to differential expression and distribution of the endosomal TLRs. Therefore, the presence of endosomal TLRs on the cell surface could benefit the host responses in certain cell types and/or organs. Although not fully understood why, TLR3, TLR7, and TLR9 may occur both in the cell membrane and intracellularly, and it seems that activation of the immune response can be initiated concurrently from these two sites in the cell. Furthermore, various forms of endosomal TLRs may be transported to the cell membrane, indicating that this may be a normal process orchestrated by cysteine proteases-cathepsins. Among the endosomal TLRs, TLR3 belongs to the evolutionary distinct group and engages a different protein adapter in the signaling cascade. The differently glycosylated forms of TLR3 are transported by UNC93B1 to the cell membrane, unlike TLR7, TLR8, and TLR9. The aim of this review is to reconcile various views on the cell surface positioning of endosomal TLRs and add perspective to the implication of such receptor localization on their function, with special attention to TLR3. Cell membrane-localized TLR3, TLR7, and TLR9 may contribute to endosomal TLR-mediated inflammatory signaling pathways. Dissecting this signaling axis may serve to better understand mechanisms influencing endosomal TLR-mediated inflammation, thus determine whether it is a necessity for immune response or simply a circumstantial superfluous duplication, with other consequences on immune response.

Cathepsins in Bacteria-Macrophage Interaction: Defenders or Victims of Circumstance?

Macrophages are the first encounters of invading bacteria and are responsible for engulfing and digesting pathogens through phagocytosis leading to initiation of the innate inflammatory response. Intracellular digestion occurs through a close relationship between phagocytic/endocytic and lysosomal pathways, in which proteolytic enzymes, such as cathepsins, are involved. The presence of cathepsins in the endo-lysosomal compartment permits direct interaction with and killing of bacteria, and may contribute to processing of bacterial antigens for presentation, an event necessary for the induction of antibacterial adaptive immune response. Therefore, it is not surprising that bacteria can control the expression and proteolytic activity of cathepsins, including their inhibitors – cystatins, to favor their own intracellular survival in macrophages. In this review, we summarize recent developments in defining the role of cathepsins in bacteria-macrophage interaction and describe important strategies engaged by bacteria to manipulate cathepsin expression and activity in macrophages. Particularly, we focus on specific bacterial species due to their clinical relevance to humans and animal health, i.e., Mycobacterium, Mycoplasma, Staphylococcus, Streptococcus, Salmonella, Shigella, Francisella, Chlamydia, Listeria, Brucella, Helicobacter, Neisseria, and other genera.

Participation of Endosomes in Toll-Like Receptor 3 Transportation Pathway in Murine Astrocytes

TLR3 provides immediate type I IFN response following entry of stimulatory PAMPs into the CNS, as it is in HSV infection. The receptor plays a vital role in astrocytes, contributing to rapid infection sensing and suppression of viral replication, precluding the spread of virus beyond neurons. The route of TLR3 mobilization culminating in the receptor activation remains unexplained. In this research, we investigated the involvement of various types of endosomes in the regulation of the TLR3 mobility in C8-D1A murine astrocyte cell line. TLR3 was transported rapidly to early EEA1-positive endosomes as well as LAMP1-lysosomes following stimulation with the poly(I:C). Later, TLR3 largely associated with late Rab7-positive endosomes. Twenty-four hours after stimulation, TLR3 co-localized with LAMP1 abundantly in lysosomes of astrocytes. TLR3 interacted with poly(I:C) intracellularly from 1 min to 8 h following cell stimulation. We detected TLR3 on the surface of astrocytes indicating constitutive expression, which increased after poly(I:C) stimulation. Our findings contribute to the understanding of cellular modulation of TLR3 trafficking. Detailed analysis of the TLR3 transportation pathway is an important component in disclosing the fate of the receptor in HSV-infected CNS and may help in the search for rationale therapeutics to control the replication of neuropathic viruses.

First Insight into the Modulation of Noncanonical NF-κB Signaling Components by Poxviruses in Established Immune-Derived Cell Lines: An In Vitro Model of Ectromelia Virus Infection

Dendritic cells (DCs) and macrophages are the first line of antiviral immunity. Viral pathogens exploit these cell populations for their efficient replication and dissemination via the modulation of intracellular signaling pathways. Disruption of the noncanonical nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) signaling has frequently been observed in lymphoid cells upon infection with oncogenic viruses. However, several nononcogenic viruses have been shown to manipulate the noncanonical NF-κB signaling in different cell types. This study demonstrates the modulating effect of ectromelia virus (ECTV) on the components of the noncanonical NF-κB signaling pathway in established murine cell lines: JAWS II DCs and RAW 264.7 macrophages. ECTV affected the activation of TRAF2, cIAP1, RelB, and p100 upon cell treatment with both canonical and noncanonical NF-κB stimuli and thus impeded DNA binding by RelB and p52. ECTV also inhibited the expression of numerous genes related to the noncanonical NF-κB pathway and RelB-dependent gene expression in the cells treated with canonical and noncanonical NF-κB activators. Thus, our data strongly suggest that ECTV influenced the noncanonical NF-κB signaling components in the in vitro models. These findings provide new insights into the noncanonical NF-κB signaling components and their manipulation by poxviruses in vitro.

TLR3 co-localizes with LAMP1-labeled endosomes from the first minute following poly(I:C) stimulation of murine astrocytes

Toll-like receptor 3 (TLR3) senses dsRNA in the acidic environment of endosomes and plays a pivotal role during infection of the brain with viruses, including herpes simplex virus type 1 (HSV-1). Studies on various cell lines corroborate TLR3 presence in LAMP1-labeled endosomes following the receptor stimulation. However, co-localization times, as well as the subjection of co-localization on the TLR3 stimulation, remain divergent depending on the cell type. In this work, we examined TLR3 residence in the LAMP1-marked endosomes in nine different time intervals following poly(I:C) stimulation of C8-D1A murine astrocytes. Interestingly, TLR3 did not localize in the lysosomal compartment in resting cells, however, the presence of the receptor was observed in LAMP1-labeled endosomes from the first minute after the addition of poly(I:C). Furthermore, 24 h following stimulation, we observed numerous lysosomes co-localizing with TLR3, which were present in all astrocytes. Our results indicate LAMP1-labeled endosomes as important organelles which may serve as the TLR3 ligand recognition sites, however, further studies on the migration of dsRNA in astrocytes are essential.

Deficiency of Selected Cathepsins Does Not Affect the Inhibitory Action of ECTV on Immune Properties of Dendritic Cells

Ectromelia virus (ECTV), an orthopoxvirus, undergoes productive replication in conventional dendritic cells (cDCs), resulting in the inhibition of their innate and adaptive immune functions. ECTV replication rate in cDCs is increased due to downregulation of the expression of cathepsins - cystein proteases that orchestrate several steps during DC maturation. Therefore, this study was aimed to determine if downregulation of cathepsins, such as B, L or S, disrupts cDC capacity to induce activating signals in T cells or whether infection of cDCs with ECTV further weakens their functions as antigen-presenting cells. Our results showed that cDCs treated with siRNA against cathepsin B, L and S synthesize similar amounts of pro-inflammatory cytokines and exhibit comparable ability to mature and stimulate alloreactive CD4⁺ T cells, as untreated wild type (WT) cells. Moreover, ECTV inhibitory effect on cDC innate and adaptive immune functions, observed especially after LPS treatment, was comparable in both cathepsin-silenced and WT cells. Taken together, the absence of cathepsins B, L and S has minimal, if any, impact on the inhibitory effect of ECTV on cDC immune functions. We assume that the virus-mediated inhibition of cathepsin expression in cDCs represents more a survival mechanism than an immune evasion strategy.

Ectromelia virus suppresses expression of cathepsins and cystatins in conventional dendritic cells to efficiently execute the replication process

Background

Cathepsins are a group of endosomal proteases present in many cells including dendritic cells (DCs). The activity of cathepsins is regulated by their endogenous inhibitors – cystatins. Cathepsins are crucial to antigen processing during viral and bacterial infections, and as such are a prerequisite to antigen presentation in the context of major histocompatibility complex class I and II molecules. Due to the involvement of DCs in both innate and adaptive immune responses, and the quest to understand the impact of poxvirus infection on host cells, we investigated the influence of ectromelia virus (ECTV) infection on cathepsin and cystatin levels in murine conventional DCs (cDCs). ECTV is a poxvirus that has evolved many mechanisms to avoid host immune response and is able to replicate productively in DCs.

Results

Our results showed that ECTV-infection of JAWS II DCs and primary murine GM-CSF-derived bone marrow cells down-regulated both mRNA and protein of cathepsin B, L and S, and cystatin B and C, particularly during the later stages of infection. Moreover, the activity of cathepsin B, L and S was confirmed to be diminished especially at later stages of infection in JAWS II cells. Consequently, ECTV-infected DCs had diminished ability to endocytose and process a soluble antigen. Close examination of cellular protein distribution showed that beginning from early stages of infection, the remnants of cathepsin L and cystatin B co-localized and partially co-localized with viral replication centers (viral factories), respectively. Moreover, viral yield increased in cDCs treated with siRNA against cathepsin B, L or S and subsequently infected with ECTV.

Conclusions

Taken together, our results indicate that infection of cDCs with ECTV suppresses cathepsins and cystatins, and alters their cellular distribution which impairs the cDC function. We propose this as an additional viral strategy to escape immune responses, enabling the virus to replicate effectively in infected cells.

Electronic supplementary material

The online version of this article (10.1186/s12866-019-1471-1) contains supplementary material, which is available to authorized users.

Bacterial and viral pathogen-associated molecular patterns induce divergent early transcriptomic landscapes in a bovine macrophage cell line

Background

Pathogens stimulate immune functions of macrophages. Macrophages are a key sentinel cell regulating the response to pathogenic ligands and orchestrating the direction of the immune response. Our study aimed at investigating the early transcriptomic changes of bovine macrophages (Bomacs) in response to stimulation with CpG DNA or polyI:C, representing bacterial and viral ligands respectively, and performed transcriptomics by RNA sequencing (RNASeq). KEGG, GO and IPA analytical tools were used to reconstruct pathways, networks and to map out molecular and cellular functions of differentially expressed genes (DE) in stimulated cells.

Results

A one-way ANOVA analysis of RNASeq data revealed significant differences between the CpG DNA and polyI:C-stimulated Bomac. Of the 13,740 genes mapped to the bovine genome, 2245 had p-value ≤0.05, deemed as DE. At 6 h post stimulation of Bomac, poly(I:C) induced a very different transcriptomic profile from that induced by CpG DNA. Whereas, 347 genes were upregulated and 210 downregulated in response to CpG DNA, poly(I:C) upregulated 761 genes and downregulated 414 genes. The topmost DE genes in poly(I:C)-stimulated cells had thousand-fold changes with highly significant p-values, whereas in CpG DNA stimulated cells had 2–5-fold changes with less stringent p-values. The highest DE genes in both stimulations belonged to the TNF superfamily, TNFSF18 (CpG) and TNFSF10 (poly(I:C)) and in both cases the lowest downregulated gene was CYP1A1. CpG DNA highly induced canonical pathways that are unrelated to immune response in Bomac. CpG DNA influenced expression of genes involved in molecular and cellular functions in free radical scavenging. By contrast, poly(I:C) highly induced exclusively canonical pathways directly related to antiviral immune functions mediated by interferon signalling genes. The transcriptomic profile after poly(I:C)-stimulation was consistent with induction of TLR3 signalling.

Conclusion

CpG DNA and poly(I:C) induce different early transcriptional landscapes in Bomac, but each is suited to a specific function of macrophages during interaction with pathogens. Poly(I:C) influenced antiviral response genes, whereas CpG DNA influenced genes important for phagocytic processes. Poly(I:C) was more potent in setting the inflammatory landscape desirable for an efficient immune response against virus infection.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5411-5) contains supplementary material, which is available to authorized users.

Ectromelia virus suppresses cathepsins and cystatins expression at both mRNA and protein levels in dendritic cells

Cathepsins belong to cysteine proteases and play essential roles in processing of internalized peptides during antigen presentation in the context of the major histocompatibility complex (MHC) class II molecules. Function of cathepsins is regulated by cystatins – endogenous protein inhibitors. Viruses, such as poxviruses, have evolved many mechanisms to escape immune responses after infection. Our studies are focused on ectromelia virus (ECTV, a poxvirus closely related to variola virus, VARV – a causative agent of smallpox), and its influence on cathepsins or cystatins. Our results show that ECTV down-regulates gene and protein expression of selected cathepsins and cystatins in infected murine JAWS II dendritic cells (DCs) and GM-CSF-derived bone marrow cells (GM-BM), composed of conventional DCs and macrophages. Moreover, the ability to endocytose and process a soluble antigen is reduced in JAWS II and GM-BM cells during ECTV infection. After knockdown of cathepsins and cystatins using siRNA in JAWS II DCs, the virus titer increases when compared with control cells. The inhibition of cathepsins and cystatins together with elevated virus titers during the absence of these proteases may be a viral strategy to escape immune responses and simultaneously enable the virus to replicate effectively in infected cells. Importantly, defining the poxvirus-host interactions, including lysosomal proteases, may lead to development of potential therapeutic targets or vaccination strategies.

TLR3 of murine astrocytes is cleaved into several forms upon stimulation of cells with synthetic dsRNA

TLR3 (Toll-like receptor 3) belongs to the subfamily of TLRs that reside in the endosomal compartment of the cell. It recognizes dsRNA (double-stranded RNA) of viruses and initiates a signaling pathway leading to generation of antiviral molecules such as type I interferons. Precursor TLR3 present in mammalian cells has a size of approximately 110 kDa, however, recent findings show that the TLR3 may also occur in two progeny forms: C-terminal and N-terminal, appearing after cleavage of TLR3 in specific conditions by cysteine proteases – cathepsins. Furthermore, cleaved and/or associated TLR3 molecules may represent the primary form of the receptor and are capable of binding the TLR3 ligand. Our results show that in C8D1A murine astrocyte cell line, cleavage of TLR3 is time- and dose-dependent upon stimulation with poly(I:C) (polyinosinic:polycytidylic acid, synthetic dsRNA analogue). The cleavage process promotes and increases expression of the progeny receptor forms. Presentation of TLR3 in such manner may modulate the level of response to viral dsRNA, especially in central nervous system cells, however, this requires further investigation.

Bacterial and viral pathogen associated molecular patterns create different early transcriptomic landscapes in bovine macrophages

Pathogens stimulate immune functions of dendritic cells and macrophages (MΦ). There is no clear understanding of the nature of transcriptomic changes caused in cells upon stimulation through pattern recognition receptors by various pathogens. Our study investigated the early transcriptomic changes of bovine MΦ in response to stimulation with CpG or poly I:C. BMΦ were incubated with CpG or pI:C for 6 h and RNA isolated for transcriptomic analysis by RNAseq. Bioinformatics tools were used to analyze differentially expressed genes (DE). Results showed that, of the 13740 genes mapped to the bovine genome, 2245 had p-value ≤0.05, deemed as DE. At 6 h post stimulation, pI:C induced a different transcriptomic profile from that induced by CpG. CpG upregulated 347 and downregulated 210 genes. pI:C upregulated 761 and downregulated 414 genes. The highest expressed genes in both stimulations belonged to the TNF superfamily, TNFSF18 (CpG) and TNFSF10 (pI:C) and in both cases the lowest downregulated gene was CYP1A1. CpG induced canonical pathways that are unrelated to immune response in BMΦ in contrast to pI:C which induced canonical pathways directly related to antiviral immune functions dominated by IFN signalling genes. The transcriptomic profile after pI:C stimulation was consistent with TLR3 signalling. CpG influenced expression of genes involved in molecular and cellular functions in free radical scavenging. We conclude that CpG and pI:C induce different early transcriptional landscapes in BMΦ line, but each suited to a specific function of BMΦ during interaction with pathogens.

Cathepsins: innate immune proteases that regulate viral entry into host cells

Cathepsins are group of endolysosomal proteases that regulate the mechanisms of innate and adaptive immunity, including cell adhesion and migration, antigen processing and presentation and resistance to several viral infections. Some cathepsins are required for Toll-like receptor (TLR)3, TLR7 and TLR9 cleavage and the formation of functional receptors that participate in sensing viral nucleic acids. Moreover, cathepsins directly stimulate or inhibit cytokine secretion involved in the regulation of antiviral innate immune response. Recent findings underline the important role of cathepsins in the entry of filoviruses, reoviruses, retroviruses and other types of viruses into the host cell. Many enveloped viruses require the presence of cathepsins for efficient fusion with membranes of infected cells, and the inhibition of their activity results in a significant reduction of virus replication. In addition, many viruses utilize conserved cellular mechanisms, such as endocytosis or low pH within the endosome, for efficient penetration into the cell interior, disassembly of viral capsid, and other stages of productive viral replication cycle. Therefore, a better understanding of the functional role of cathepsin proteases in the pathogenesis of viral infections should lead to the development of novel therapeutics for a variety of particularly dangerous human pathogens.

Functional failure of TLR3 and its signaling components contribute to herpes simplex encephalitis

Herpes simplex encephalitis (HSE) is a severe neurological disease in children and adults caused by herpes simplex virus. This review discusses recent findings on the role of Toll-like receptor 3 (TLR3) deficiencies in the HSE development. Critical checkpoints in the TLR3 signaling that contribute to innate response are discussed, including the importance of TLR3 ligand recognition site and transportation in the cell. We also indicate unresolved issues in the TLR3 functioning that might lead to thorough understanding of immunity during HSE. Such a knowledge base will lead to discovery and design of a rationale therapeutic and preventive approach against HSE.

Functional paralysis of GM-CSF-derived bone marrow cells productively infected with ectromelia virus

Ectromelia virus (ECTV) is an orthopoxvirus responsible for mousepox, a lethal disease of certain strains of mice that is similar to smallpox in humans, caused by variola virus (VARV). ECTV, similar to VARV, exhibits a narrow host range and has co-evolved with its natural host. Consequently, ECTV employs sophisticated and host-specific strategies to control the immune cells that are important for induction of antiviral immune response. In the present study we investigated the influence of ECTV infection on immune functions of murine GM-CSF-derived bone marrow cells (GM-BM), comprised of conventional dendritic cells (cDCs) and macrophages. Our results showed for the first time that ECTV is able to replicate productively in GM-BM and severely impaired their innate and adaptive immune functions. Infected GM-BM exhibited dramatic changes in morphology and increased apoptosis during the late stages of infection. Moreover, GM-BM cells were unable to uptake and process antigen, reach full maturity and mount a proinflammatory response. Inhibition of cytokine/chemokine response may result from the alteration of nuclear translocation of NF-κB, IRF3 and IRF7 transcription factors and down-regulation of many genes involved in TLR, RLR, NLR and type I IFN signaling pathways. Consequently, GM-BM show inability to stimulate proliferation of purified allogeneic CD4+ T cells in a primary mixed leukocyte reaction (MLR). Taken together, our data clearly indicate that ECTV induces immunosuppressive mechanisms in GM-BM leading to their functional paralysis, thus compromising their ability to initiate downstream T-cell activation events.

Tyrosine kinase Syk interacts with Hrs after TLR3 stimulation in murine microglial cells

TLR3 (Toll-like receptor 3) belongs to the family of receptors involved in innate immune response. Present in endosomal compartment in cells, TLR3 recognizes the dsRNA (double-stranded RNA), viral nucleic acid or intermediate during viral replication and initiates signal transduction leading to inflammatory response. However, the first steps of signaling cascade occurring in cells immediately after TLR3 stimulation are still not well understood and require careful attention. In this paper we demonstrate that after poly(I:C) (dsRNA mimetic) stimulation of murine microglial cells (C8D1A cell line) tyrosine kinase Syk interacts with Hrs (hepatocyte growth factor regulated tyrosine kinase substrate), one of the ESCRT-0 (endosomal sorting complex required for transportation-0) components. This protein complex was recently implicated in trafficking of other endosomal TLRs, TLR7 and TLR9, and plays a possible role in the TLR3 signaling. Phosphorylation of Hrs, which is a very likely result of Hrs-Syk interaction, also takes place after poly(I:C) stimulation, strongly suggesting that an association between activated TLR3 and ESCRT-0 exists. Therefore, Hrs may influence TLR3 signaling pathways, but this requires further investigation.

Expression pattern of TRIM genes in bovine macrophages stimulated with PAMPs

In mammals innate immune mechanisms form the first line of defence against invading pathogens. Detection of viruses early in infection relies on intracellular receptors that sense microbial molecular patterns, subsequently leading to gene transcription that eventually produces IFN type I and II. Type I and II IFNs act to prevent replication of viruses. Tripartite Motif-containing (TRIM) proteins belong to a superfamily of RING-domain E3 ubiquitin ligases. They represent a novel class of antiviral molecules involved in innate immunity. These enzymes function in a wide variety of important cellular processes, particularly in innate antiviral response mechanisms. We studied the expression profile of 46 TRIM genes in a bovine macrophage cell line BoMac using RT2 PCR. PAMPs were used to imitate 2 important groups of pathogens. PolyI:C was used in place of viral dsRNA, LPS was used in place of Gram-negative bacteria, Pam3CSK4 was used in place of Gram-positive and negative bacteria, PolyI:C-LyoVec was used in place of viral sRNA and CpG was used in place of bacterial and viral DNA. Of the 46 TRIM genes, only 8 bovine TRIM genes were upregulated following stimulation of BoMac with individual PAMPs. PolyI:C induce upregulation of TRIM21, TRIM25 and TRIM56. All three TRIMs are known for their antiviral activity in human cell lines and mice. Pam3CSK4 and LPS, both upregulated TRIM10. PolyI:C-LyoVec upregulated TRIM9, TRIM40 and TRIM55. TRIM40 is an anti-inflammatory molecule. CpG upregulated TRIM40 and TRIM29. High expression of TRIM29 is regarded as a poor prognostic in many tumors, but no antimicrobial role has been described yet for TRIM29. Data will be discussed in the context of antiviral role of TRIMs in bovine viral infections.