Crosstalk between TLR8 and RIG-I-like receptors enhances antiviral immune responses

Background

Toll-like receptor (TLR) agonists have been investigated due to their potential dual effects as latency reverting agents and immune modulatory compounds in people living with HIV (PLWH). Here, we investigated whether co-stimulation of TLR7/8 agonists with RIG-I-like receptor (RLR) agonists enhances antiviral immunity.

Methods

Peripheral blood mononuclear cells (PBMCs) and monocyte-derived dendritic cells (DCs) were incubated with TLR and RLR-agonists for 24 h and innate and adaptive immune responses were determined (maturation markers, cytokines in supernatant, ISG expression).

Results

Both TLR7 and TLR8 agonists induced pro-inflammatory cytokines in DCs as well as PBMCs. TLR8 agonists were more potent in inducing cytokine responses and had a stronger effect on DC-induced immunity. Notably, while all compounds induced IL-12p70, co-stimulation with TLR8 agonists and RLR agonist polyI: C induced significantly higher levels of IL-12p70 in PBMCs. Moreover, crosstalk between TLR8 and RLR agonists induced a strong type I Interferon (IFN) response as different antiviral IFN-stimulated genes were upregulated by the combination compared to the agonists alone.

Conclusion

Our data strongly suggest that TLR crosstalk with RLRs leads to strong antiviral immunity as shown by induction of IL-12 and type I IFN responses in contrast to TLRs alone. Thus, co-stimulation of TLRs and RLRs might be a powerful strategy to induce reactivation of latent reservoir as well as antiviral immunity that eliminates the reactivated cells.

Fungal sensing by dectin-1 directs the non-pathogenic polarization of TH17 cells through balanced type I IFN responses in human DCs

The non-pathogenic T_H17 subset of helper T cells clears fungal infections, whereas pathogenic T_H17 cells cause inflammation and tissue damage; however, the mechanisms controlling these distinct responses remain unclear. Here we found that fungi sensing by the C-type lectin dectin-1 in human dendritic cells (DCs) directed the polarization of non-pathogenic T_H17 cells. Dectin-1 signaling triggered transient and intermediate expression of interferon (IFN)-β in DCs, which was mediated by the opposed activities of transcription factors IRF1 and IRF5. IFN-β-induced signaling led to integrin αvβ8 expression directly and to the release of the active form of the cytokine transforming growth factor (TGF)-β indirectly. Uncontrolled IFN-β responses as a result of IRF1 deficiency induced high expression of the IFN-stimulated gene BST2 in DCs and restrained TGF-β activation. Active TGF-β was required for polarization of non-pathogenic T_H17 cells, whereas pathogenic T_H17 cells developed in the absence of active TGF-β. Thus, dectin-1-mediated modulation of type I IFN responses allowed TGF-β activation and non-pathogenic T_H17 cell development during fungal infections in humans.

Infection and transmission of SARS-CoV-2 depend on heparan sulfate proteoglycans

The current pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and outbreaks of new variants highlight the need for preventive treatments. Here, we identified heparan sulfate proteoglycans as attachment receptors for SARS-CoV-2. Notably, neutralizing antibodies against SARS-CoV-2 isolated from COVID-19 patients interfered with SARS-CoV-2 binding to heparan sulfate proteoglycans, which might be an additional mechanism of antibodies to neutralize infection. SARS-CoV-2 binding to and infection of epithelial cells was blocked by low molecular weight heparins (LMWH). Although dendritic cells (DCs) and mucosal Langerhans cells (LCs) were not infected by SARS-CoV-2, both DC subsets efficiently captured SARS-CoV-2 via heparan sulfate proteoglycans and transmitted the virus to ACE2-positive cells. Notably, human primary nasal cells were infected by SARS-CoV-2, and infection was blocked by pre-treatment with LMWH. These data strongly suggest that heparan sulfate proteoglycans are important attachment receptors facilitating infection and transmission, and support the use of LMWH as prophylaxis against SARS-CoV-2 infection.

HIV-1 subverts the complement system in semen to enhance viral transmission

Semen is important in determining HIV-1 susceptibility but it is unclear how it affects virus transmission during sexual contact. Mucosal Langerhans cells (LCs) are the first immune cells to encounter HIV-1 during sexual contact and have a barrier function as LCs are restrictive to HIV-1. As semen from people living with HIV-1 contains complement-opsonized HIV-1, we investigated the effect of complement on HIV-1 dissemination by human LCs in vitro and ex vivo. Notably, pre-treatment of HIV-1 with semen enhanced LC infection compared to untreated HIV-1 in the ex vivo explant model. Infection of LCs and transmission to target cells by opsonized HIV-1 was efficiently inhibited by blocking complement receptors CR3 and CR4. Complement opsonization of HIV-1 enhanced uptake, fusion, and integration by LCs leading to an increased transmission of HIV-1 to target cells. However, in the absence of both CR3 and CR4, C-type lectin receptor langerin was able to restrict infection of complement-opsonized HIV-1. These data suggest that complement enhances HIV-1 infection of LCs by binding CR3 and CR4, thereby bypassing langerin and changing the restrictive nature of LCs into virus-disseminating cells. Targeting complement factors might be effective in preventing HIV-1 transmission.

Syndecan 4 Upregulation on Activated Langerhans Cells Counteracts Langerin Restriction to Facilitate Hepatitis C Virus Transmission

Sexually transmitted Hepatitis C virus (HCV) infections and high reinfections are a major concern amongst men who have sex with men (MSM) living with HIV-1 and HIV-negative MSM. Immune activation and/or HIV-1 coinfection enhance HCV susceptibility via sexual contact, suggesting that changes in immune cells or external factors are involved in increased susceptibility. Activation of anal mucosal Langerhans cells (LCs) has been implicated in increased HCV susceptibility as activated but not immature LCs efficiently retain and transmit HCV to other cells. However, the underlying molecular mechanism of transmission remains unclear. Here we identified the Heparan Sulfate Proteoglycan Syndecan 4 as the molecular switch, controlling HCV transmission by LCs. Syndecan 4 was highly upregulated upon activation of LCs and interference with Heparan Sulfate Proteoglycans or silencing of Syndecan 4 abrogated HCV transmission. These data strongly suggest that Syndecan 4 mediates HCV transmission by activated LCs. Notably, our data also identified the C-type lectin receptor langerin as a restriction factor for HCV infection and transmission. Langerin expression abrogated HCV infection in HCV permissive cells, whereas langerin expression on the Syndecan 4 expressing cell line strongly decreased HCV transmission to a target hepatoma cell line. These data suggest that the balanced interplay between langerin restriction and Syndecan 4 transmission determines HCV dissemination. Silencing of langerin enhanced HCV transmission whereas silencing Syndecan 4 on activated LCs decreased transmission. Blocking Heparan Sulfate Proteoglycans abrogated HCV transmission by LCs ex vivo identifying Heparan Sulfate Proteoglycans and Syndecan 4 as potential targets to prevent sexual transmission of HCV. Thus, our data strongly suggest that the interplay between receptors promotes or restricts transmission and further indicate that Syndecan 4 is the molecular switch controlling HCV susceptibility after sexual contact.

Synthetic Abortive HIV-1 RNAs Induce Potent Antiviral Immunity

Strong innate and adaptive immune responses are paramount in combating viral infections. Dendritic cells (DCs) detect viral infections via cytosolic RIG-I like receptors (RLRs) RIG-I and MDA5 leading to MAVS-induced immunity. The DEAD-box RNA helicase DDX3 senses abortive human immunodeficiency virus 1 (HIV-1) transcripts and induces MAVS-dependent type I interferon (IFN) responses, suggesting that abortive HIV-1 RNA transcripts induce antiviral immunity. Little is known about the induction of antiviral immunity by DDX3-ligand abortive HIV-1 RNA. Here we synthesized a 58 nucleotide-long capped RNA (HIV-1 Cap-RNA₅₈) that mimics abortive HIV-1 RNA transcripts. HIV-1 Cap-RNA₅₈ induced potent type I IFN responses in monocyte-derived DCs, monocytes, macrophages and primary CD1c⁺ DCs. Compared with RLR agonist poly-I:C, HIV-1 Cap-RNA₅₈ induced comparable levels of type I IFN responses, identifying HIV-1 Cap-RNA₅₈ as a potent trigger of antiviral immunity. In monocyte-derived DCs, HIV-1 Cap-RNA₅₈ activated the transcription factors IRF3 and NF-κB. Moreover, HIV-1 Cap-RNA₅₈ induced DC maturation and the expression of pro-inflammatory cytokines. HIV-1 Cap-RNA₅₈-stimulated DCs induced proliferation of CD4⁺ and CD8⁺ T cells and differentiated naïve T helper (T_H) cells toward a T_H2 phenotype. Importantly, treatment of DCs with HIV-1 Cap-RNA₅₈ resulted in an efficient antiviral innate immune response that reduced ongoing HIV-1 replication in DCs. Our data strongly suggest that HIV-1 Cap-RNA₅₈ induces potent innate and adaptive immune responses, making it an interesting addition in vaccine design strategies.

Sexually transmitted founder HIV-1 viruses are relatively resistant to Langerhans cell-mediated restriction

A single HIV-1 variant establishes infection of the host after sexual contact. Identifying the phenotypic characteristics of these Transmitted Founder (T/F) viruses is important to understand the restriction mechanisms during transmission. Langerhans cells (LCs) are the mucosal dendritic cell subset that has been shown to have a protective role in HIV-1 transmission. Immature LCs efficiently capture and degrade HIV-1 via langerin-mediated restriction. Here we have investigated the capacity of T/F HIV-1 strains to infect mucosal Langerhans cells (LCs). Notably, most T/F variants efficiently infected immature LCs derived from skin and vaginal tissue in contrast to chronic HIV-1 laboratory strains. Next we screened a panel of T/F viruses and their matched 6-month consensus sequence viruses. Interestingly most T/F variants infected immature LCs whereas donor-matched 6-month consensus sequence viruses had lost the ability to infect LCs. However, we also identified 6-month consensus sequence viruses that had retained an ability to infect LCs similar to that of the donor-matched T/F virus. Moreover, some T/F viruses and 6-month consensus sequence viruses were unable to infect immature LCs. Further analyses indicated that T/F viruses are less sensitive to langerin-mediated restriction. These data suggest that T/F HIV-1 variants have the ability to infect immature LCs, which will facilitate transmission.

Borrelia miyamotoi Activates Human Dendritic Cells and Elicits T Cell Responses

The spirochete Borrelia miyamotoi has recently been shown to cause relapsing fever. Like the Lyme disease agent, Borrelia burgdorferi, B. miyamotoi is transmitted through the bite of infected ticks; however, little is known about the response of the immune system upon infection. Dendritic cells (DCs) play a central role in the early immune response against B. burgdorferi We investigated the response of DCs to two different strains of B. miyamotoi using in vitro and ex vivo models and compared this to the response elicited by B. burgdorferi. Our findings show that B. miyamotoi is phagocytosed by monocyte-derived DCs, causing upregulation of activation markers and production of proinflammatory cytokines in a similar manner to B. burgdorferi. Recognition of B. miyamotoi was demonstrated to be partially mediated by TLR2. DCs migrated out of human skin explants upon inoculation of the skin with B. miyamotoi. Finally, we showed that B. miyamotoi-stimulated DCs induced proliferation of naive CD4⁺ and CD8⁺ T cells to a larger extent than B. burgdorferi. In conclusion, we show in this study that DCs respond to and mount an immune response against B. miyamotoi that is similar to the response to B. burgdorferi and is able to induce T cell proliferation.

RIG-I-like receptor activation by dengue virus drives follicular T helper cell formation and antibody production

Follicular T helper cells (TFH) are fundamental in orchestrating effective antibody-mediated responses critical for immunity against viral infections and effective vaccines. However, it is unclear how virus infection leads to TFH induction. We here show that dengue virus (DENV) infection of human dendritic cells (DCs) drives TFH formation via crosstalk of RIG-I-like receptor (RLR) RIG-I and MDA5 with type I Interferon (IFN) signaling. DENV infection leads to RLR-dependent IKKε activation, which phosphorylates IFNα/β receptor-induced STAT1 to drive IL-27 production via the transcriptional complex ISGF3. Inhibiting RLR activation as well as neutralizing antibodies against IL-27 prevented TFH formation. DENV-induced CXCR5+PD-1+Bcl-6+ TFH cells secreted IL-21 and activated B cells to produce IgM and IgG. Notably, RLR activation by synthetic ligands also induced IL-27 secretion and TFH polarization. These results identify an innate mechanism by which antibodies develop during viral disease and identify RLR ligands as potent adjuvants for TFH-promoting vaccination strategies.

DCs facilitate B cell responses against microbial DNA via DC-SIGN

Microbial DNA is highly immunostimulatory and is sensed by endosomal pattern recognition receptors after release from internalized microbes. It is unclear how extracellular DNA released from dead microbes is delivered to endosomal PRRs to induce immune responses. Here we have investigated the ability of DCs to bind and internalize extracellular E.coli DNA as well as synthetic DNA. DCs internalized E.coli and synthetic DNA, which was dependent on the C-type lectin receptor DC-SIGN. Notably, endosomal uptake of DNA by DCs enhanced TLR9-dependent responses of B cells against DNA. Hence, we have identified DC-SIGN as a cell surface receptor for DNA that facilitates immune responses directed against DNA.

RIG-I-like Receptor Triggering by Dengue Virus Drives Dendritic Cell Immune Activation and TH1 Differentiation

Dengue virus (DENV) causes 400 million infections annually and is one of several viruses that can cause viral hemorrhagic fever, which is characterized by uncontrolled immune activation resulting in high fever and internal bleeding. Although the underlying mechanisms are unknown, massive cytokine secretion is thought to be involved. Dendritic cells (DCs) are the main target cells of DENV, and we investigated their role in DENV-induced cytokine production and adaptive immune responses. DENV infection induced DC maturation and secretion of IL-1β, IL-6, and TNF. Inhibition of DENV RNA replication abrogated these responses. Notably, silencing of RNA sensors RIG-I or MDA5 abrogated DC maturation, as well as cytokine responses by DENV-infected DCs. DC maturation was induced by type I IFN responses because inhibition of IFN-α/β receptor signaling abrogated DENV-induced DC maturation. Moreover, DENV infection of DCs resulted in CCL2, CCL3, and CCL4 expression, which was abrogated after RIG-I and MDA5 silencing. DCs play an essential role in TH cell differentiation, and we show that RIG-I and MDA5 triggering by DENV leads to TH1 polarization, which is characterized by high levels of IFN-γ. Notably, cytokines IL-6, TNF, and IFN-γ and chemokines CCL2, CCL3, and CCL4 have been associated with disease severity, endothelial dysfunction, and vasodilation. Therefore, we identified RIG-I and MDA5 as critical players in innate and adaptive immune responses against DENV, and targeting these receptors has the potential to decrease hemorrhagic fever in patients.

HIV-1 blocks the signaling adaptor MAVS to evade antiviral host defense after sensing of abortive HIV-1 RNA by the host helicase DDX3

The mechanisms by which human immunodeficiency virus 1 (HIV-1) avoids immune surveillance by dendritic cells (DCs), and thereby prevents protective adaptive immune responses, remain poorly understood. Here we showed that HIV-1 actively arrested antiviral immune responses by DCs, which contributed to efficient HIV-1 replication in infected individuals. We identified the RNA helicase DDX3 as an HIV-1 sensor that bound abortive HIV-1 RNA after HIV-1 infection and induced DC maturation and type I interferon responses via the signaling adaptor MAVS. Notably, HIV-1 recognition by the C-type lectin receptor DC-SIGN activated the mitotic kinase PLK1, which suppressed signaling downstream of MAVS, thereby interfering with intrinsic host defense during HIV-1 infection. Finally, we showed that PLK1-mediated suppression of DDX3-MAVS signaling was a viral strategy that accelerated HIV-1 replication in infected individuals.

Herbal medicine IMOD suppresses LPS-induced production of proinflammatory cytokines in human dendritic cells

Traditional medicines that stimulate or modulate the immune system can be used as innovative approaches to treat immunological diseases. The herbal medicine IMOD has been shown to strongly modulate immune responses in several animal studies as well as in clinical trials. However, little is known about the mechanisms of IMOD to modulate immunity. Here we have investigated whether IMOD modulates the immunological function of human dendritic cells (DCs). IMOD alone did not induce DC maturation nor production of cytokines. Notably, IMOD decreased the production of pro-inflammatory cytokines IL-6, IL-12 p70, and TNFα by LPS-activated DCs at both mRNA and protein levels in a dose dependent manner. In contrast, treatment with IMOD did not affect LPS induced-production of the anti-inflammatory cytokine IL-10. Furthermore, IMOD inhibited T cell activation/proliferation by LPS-treated DCs and skewed T-cells responses toward the T helper type 2 polarization. These data strongly indicate that IMOD has a potent immunomodulatory ability that affects TLR signaling and thereby modulates DC function. Insight into the immunomodulatory effect of herbal medicine IMOD may provide innovative strategies to affect the immune system and to help combat various diseases.

Measles virus suppresses RIG-I-like receptor activation in dendritic cells via DC-SIGN-mediated inhibition of PP1 phosphatases

Dendritic cells (DCs) are targets of measles virus (MV) and play central roles in viral dissemination. However, DCs express the RIG-I-like receptors (RLRs) RIG-I and Mda5 that sense MV and induce type I interferon (IFN) production. Given the potency of this antiviral response, RLRs are tightly regulated at various steps, including dephosphorylation by PP1 phosphatases, which induces their activation. We demonstrate that MV suppresses RIG-I and Mda5 by activating the C-type lectin DC-SIGN and inducing signaling that prevents RLR dephosphorylation. MV binding to DC-SIGN leads to activation of the kinase Raf-1, which induces the association of PP1 inhibitor I-1 with GADD34-PP1 holoenzymes, thereby inhibiting phosphatase activity. Consequently, GADD34-PP1 holoenzymes are unable to dephosphorylate RIG-I and Mda5, hence suppressing type I IFN responses and enhancing MV replication. Blocking DC-SIGN signaling allows RLR activation and suppresses MV infection of DCs. Thus, MV subverts DC-SIGN to control RLR activation and escape antiviral responses.

Fucose-specific DC-SIGN signalling directs T helper cell type-2 responses via IKKε- and CYLD-dependent Bcl3 activation

Carbohydrate-specific signalling through DC-SIGN provides dendritic cells with plasticity to tailor immunity to the nature of invading microbes. Here we demonstrate that recognition of fucose-expressing extracellular pathogens like Schistosoma mansoni and Helicobacter pylori by DC-SIGN favors T helper cell type-2 (TH2) responses via activation of atypical NF-κB family member Bcl3. Crosstalk between TLR and DC-SIGN signalling results in TLR-induced MK2-mediated phosphorylation of LSP1, associated with DC-SIGN, upon fucose binding. Subsequently, IKKε and CYLD are recruited to phosphorylated LSP1. IKKε activation is pivotal for suppression of CYLD deubiquitinase activity and subsequent nuclear translocation of ubiquitinated Bcl3. Bcl3 activation represses TLR-induced proinflammatory cytokine expression, while enhancing interleukin-10 (IL-10) and TH2-attracting chemokine expression, shifting TH differentiation from TH1 to TH2 polarization. Thus, DC-SIGN directs adaptive TH2 immunity to fucose-expressing pathogens via an IKKε-CYLD-dependent signalling pathway leading to Bcl3 activation, which might be targeted in vaccination strategies or to prevent aberrant inflammation and allergy.

Selective C-Rel activation via Malt1 controls anti-fungal T(H)-17 immunity by dectin-1 and dectin-2

C-type lectins dectin-1 and dectin-2 on dendritic cells elicit protective immunity against fungal infections through induction of T_H1 and T_H-17 cellular responses. Fungal recognition by dectin-1 on human dendritic cells engages the CARD9-Bcl10-Malt1 module to activate NF-κB. Here we demonstrate that Malt1 recruitment is pivotal to T_H-17 immunity by selective activation of NF-κB subunit c-Rel, which induces expression of T_H-17-polarizing cytokines IL-1β and IL-23p19. Malt1 inhibition abrogates c-Rel activation and T_H-17 immunity to Candida species. We found that Malt1-mediated activation of c-Rel is similarly essential to induction of T_H-17-polarizing cytokines by dectin-2. Whereas dectin-1 activates all NF-κB subunits, dectin-2 selectively activates c-Rel, signifying a specialized T_H-17-enhancing function for dectin-2 in anti-fungal immunity by human dendritic cells. Thus, dectin-1 and dectin-2 control adaptive T_H-17 immunity to fungi via Malt1-dependent activation of c-Rel.

Fungal infections are a major health threat and the incidence is growing worldwide. There is a need for efficient antifungal vaccines. Adaptive immune responses and in particular T helper cell type 17 (T_H-17) responses are crucial in the defence against fungal infections. Human dendritic cells (DCs) induce T_H-17 responses after interaction with fungi. DCs express C-type lectins dectin-1 and dectin-2 that interact with the carbohydrate structures present in the cell-wall of fungi. It is unclear how signaling by these C-type lectins leads to specific T_H-17 responses. Here we demonstrate that the signaling molecule Malt1 present in the CARD9-Bcl10-Malt1 complex is responsible for T_H-17 induction by selectively activating the NF-κB transcription factor c-Rel, which drives transcription of the T_H-17-polarizing cytokines. Inhibition of either Malt1 or c-Rel prevents T_H-17 induction in response to fungi. Furthermore, we show that the C-type lectin dectin-2 selectively activates c-Rel, signifying a specialized T_H-17-enhancing function for this C-type lectin. Thus, novel vaccination strategies that target dectin-2 or activate Malt1 can induce predominant T_H-17 responses. Since aberrant T_H-17 responses underlie the pathology of atopic dermatitis and various autoimmune diseases, Malt1 is a rational therapeutic target to attenuate anomalous adaptive immune responses.