Toll-like receptor 7-adapter complex modulates interferon-α production in HIV-stimulated plasmacytoid dendritic cells

Abstract

Plasmacytoid dendritic cells (PDCs) and their production of interferon-alpha (IFN-α) are believed to play an important role in human immunodeficiency virus, type I (HIV-1) pathogenesis. PDCs produce IFN-α and other proinflammatory cytokines through stimulation of Toll-like receptor 7 (TLR7) and TLR9 present in endosomal compartments. TLR7 recognizes single-stranded viral RNA, while TLR9 recognizes unmethylated DNA. In this study, we examined the mechanisms that may underlie variations in IFN-α production in response to HIV, and the impact of these variations on HIV pathogenesis. In four distinct cohorts, we examined PDC production of IFN-α upon stimulation with inactivated HIV-1 particles and unmethylated DNA. The signaling cascade of TLR7 bifurcates at the myeloid differentiation protein 88 (MyD88) adaptor protein to induce expression of either IFN-α or TNF-α. To determine whether variations in IFN-α production are modulated at the level of the receptor complex or downstream of it, we correlated production of IFN-α and TNF-α following stimulation of TLR7 or TLR9 receptors. Flow cytometry detection of intracellular cytokines showed strong, direct correlations between IFN-α and TNF-α expression in all four cohorts, suggesting that variations in IFN-α production are not due to variations downstream of the receptor complex. We then investigated the events upstream of TLR binding by using lipid-like vesicles to deliver TLR ligands directly to the TLR receptors, bypassing the need for CD4 binding and endocytosis. Similar tight correlations were found in IFN-α and TNF-α production in response to the TLR ligands. Taken together, these results strongly suggest that differences in IFN-α production depend on the regulatory processes at the level of the TLR7 receptor complex. Additionally, we found no association between IFN-α production before HIV infection and disease progression.

IRF7 and RNH1 are modifying factors of HIV-1 reservoirs: a genome-wide association analysis

“…This is in contrast to a study that reported detectable IFN-α levels after PBMCs of healthy subjects were stimulated with the same imiquimod concentration [ 59 ]. Diminished IFN-α production in plasmacytoid dendritic cells after stimulation with a TLR7 agonist has been reported previously in PLHIV [ 60 , 61 ], which might explain the undetectable IFN-α levels in both groups. The importance of IRF7 and type 1 interferon signaling pathways was recently also shown in studies exploring the role of RNA deadenylase complex, CNOT, in HIV infection.…”

Toll‐like receptors in health and disease

“… 24 , 25 Primarily expressed in plasmacytoid DCs (pDCs), TLR7 identifies single‐stranded (ss) RNA found in viruses and acts as a small purine analog (imidazoquinoline); moreover, it identifies RNA from Streptococcus B bacteria in conventional DCs (cDCs), inducing type I IFN and proinflammatory cytokine production. 26 , 27 , 28 Human TLR8 is responsive to viral and bacterial RNA, and TLR8 is most similar phylogenetically to TLR7. TLR8 is expressed in a variety of tissues, is most highly expressed in monocytes, and is upregulated following bacterial or viral RNA stimulation, as well as inducing IFNs and inflammation‐related cytokine generation.…”

Recent Advances on Small-Molecule Antagonists Targeting TLR7

“…Endosomal TLRs might promote human immunodeficiency virus type 1 (HIV-1) replication and latency reversal via the stimulation of inflammatory responses [ 76 ]. TLR7 overexpression leads to the hypo-responsiveness of CD4+ T cells, and the production of IFN-α in HIV-1 replication [ 77 , 78 ]. TLR7 engagement in CD4+ T cells results in the dephosphorylation of transcription factor NFATc2 and then induces an anergic gene-expression program.…”