Toll-Like Receptor 7-Adapter Complex Modulates Interferon-Alpha Production In Hiv-Stimulated Plasmacytoid Dendritic Cells

Plasmacytoid dendritic cells (PDCs) and their production of interferon-alpha (IFN-alpha) are believed to play an important role in human immunodeficiency virus, type I (HIV-1) pathogenesis. PDCs produce IFN-alpha 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-alpha production in response to HIV, and the impact of these variations on HIV pathogenesis. In four distinct cohorts, we examined PDC production of IFN-alpha 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-alpha or TNF-alpha. To determine whether variations in IFN-alpha production are modulated at the level of the receptor complex or downstream of it, we correlated production of IFN-alpha and TNF-alpha following stimulation of TLR7 or TLR9 receptors. Flow cytometry detection of intracellular cytokines showed strong, direct correlations between IFN-alpha and TNF-alpha expression in all four cohorts, suggesting that variations in IFN-alpha 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-alpha and TNF-alpha production in response to the TLR ligands. Taken together, these results strongly suggest that differences in IFN-alpha production depend on the regulatory processes at the level of the TLR7 receptor complex. Additionally, we found no association between IFN-alpha production before HIV infection and disease progression.