HIV and HCV Activate the Inflammasome in Monocytes and Macrophages via Endosomal Toll-Like Receptors without Induction of Type 1 Interferon

Abstract

Innate immune sensing of viral infection results in type I interferon (IFN) production and inflammasome activation. Type I IFNs, primarily IFN-α and IFN-β, are produced by all cell types upon virus infection and promote an antiviral state in surrounding cells by inducing the expression of IFN-stimulated genes. Type I IFN production is mediated by Toll-like receptor (TLR) 3 in HCV infected hepatocytes. Type I IFNs are also produced by plasmacytoid dendritic cells (pDC) after sensing of HIV and HCV through TLR7 in the absence of productive pDC infection. Inflammasomes are multi-protein cytosolic complexes that integrate several pathogen-triggered signaling cascades ultimately leading to caspase-1 activation and generation pro-inflammatory cytokines including interleukin (IL)-18 and IL-1β. Here, we demonstrate that HIV and HCV activate the inflammasome, but not Type I IFN production, in monocytes and macrophages in an infection-independent process that requires clathrin-mediated endocytosis and recognition of the virus by distinct endosomal TLRs. Knockdown of each endosomal TLR in primary monocytes by RNA interference reveals that inflammasome activation in these cells results from HIV sensing by TLR8 and HCV recognition by TLR7. Despite its critical role in type I IFN production by pDCs stimulated with HIV, TLR7 is not required for inflammasome activation by HIV. Similarly, HCV activation of the inflammasome in monocytes does not require TLR3 or its downstream signaling adaptor TICAM-1, while this pathway leads to type I IFN in infected hepatocytes. Monocytes and macrophages do not produce type I IFN upon TLR8 or TLR7 sensing of HIV or HCV, respectively. These findings reveal a novel infection-independent mechanism for chronic viral induction of key anti-viral programs and demonstrate distinct TLR utilization by different cell types for activation of the type I IFN vs. inflammasome pathways of inflammation.

HIV-1–Mediated BAFF Secretion in Macrophages Does Not Require Endosomal TLRs, Type-I IFN, and Nef, but Depends on the Cellular Phenotype Status

“…Moreover, the stimulation of TLR8 by HIV-1 single-stranded RNA has been reported to promote TNF release in human macrophages (57). Although we did not observe a type-I IFN response in virus-infected MDMs (data not shown), it has been shown that HIV-1 can activate the inflammasome in monocytes and macrophages via endosomal TLRs without induction of type-I IFN, a process leading to secretion of proinflammatory cytokines such as IL-1b and IL-18 (58). Therefore, we investigated whether endosomal TLRs could be involved in the HIV-1-mediated induction of BAFF secretion by MDMs.…”

“…A previous study reported that HIV-1 can activate the inflammasome in monocytes and macrophages via endosomal TLRs without the induction of type-I IFN (58). We postulated that HIV-1 could drive BAFF secretion in macrophages by exploiting this pathway.…”

“…Nevertheless, one study reported that HIV-1 RNA-mediated release of TNF by human alveolar macrophages is dependent of TLR8 signaling but silencing TLR7 had no effect on this response (57). Furthermore, TLR8 is the primary inflammasome sensor of HIV-1 in monocytes (58). Therefore, these findings suggest that TLR8 could play a more important role than TLR7 in sensing HIV-1 RNA in human macrophages.…”

DDX19A Senses Viral RNA and Mediates NLRP3-Dependent Inflammasome Activation

“…At present, five main classes of PRRs have been identified: TLRs, retinoic acid inducible gene-I (RIG-I)-like receptors, nucleotidebinding domain/leucine-rich repeat (LRR)-containing receptors (NLRs), C-type lectin receptors, and PYRIN-HIN domain containing family member (including absent in melanoma 2) (3). It has been described that DEAD/H-box helicase family members (e.g., RIG-I, MDA5, DHX36, and DHX33) (4-6), TLRs (e.g., TLR2, TLR3, TLR4, and TLR7) (7)(8)(9)(10), and NLRs (e.g., NLRX1, NLRP1, and NLPR3) (11)(12)(13) are involved in type I IFN production or inflammatory responses.…”

HIV-antibody complexes enhance production of type I interferon by plasmacytoid dendritic cells

“…HIV BaL and HIV IIIB were obtained from Joel Blankson and from Suzanne Gartner (University of Maryland, College Park, Maryland, USA), respectively, and expanded as previously described (18). In brief, 10 to 20 × 10 6 PHA-activated CD4 + T cells were spinoculated (1,200 g, room temperature, 2 hours) with 0.5 to 1 μg of HIV p24.…”