Follicular regulatory T cells impair follicular T helper cells in HIV and SIV infection.

Human and simian immunodeficiency viruses (HIV and SIV) exploit follicular lymphoid regions by establishing high levels of viral replication and dysregulating humoral immunity. Follicular regulatory T cells (TFR) are a recently characterized subset of lymphocytes that influence the germinal centre response through interactions with follicular helper T cells (TFH). Here, utilizing both human and rhesus macaque models, we show the impact of HIV and SIV infection on TFR number and function. We find that TFR proportionately and numerically expand during infection through mechanisms involving viral entry and replication, TGF-ß signalling, low apoptosis rates and the presence of regulatory dendritic cells. Further, TFR exhibit elevated regulatory phenotypes and impair TFH functions during HIV infection. Thus, TFR contribute to inefficient germinal centre responses and inhibit HIV and SIV clearance.

Alpha-Synuclein Expression Restricts RNA Viral Infections in the Brain.

UNLABELLED: We have discovered that native, neuronal expression of alpha-synuclein (Asyn) inhibits viral infection, injury, and disease in the central nervous system (CNS). Enveloped RNA viruses, such as West Nile virus (WNV), invade the CNS and cause encephalitis, yet little is known about the innate neuron-specific inhibitors of viral infections in the CNS. Following WNV infection of primary neurons, we found that Asyn protein expression is increased. The infectious titer of WNV and Venezuelan equine encephalitis virus (VEEV) TC83 in the brains of Asyn-knockout mice exhibited a mean increase of 10(4.5) infectious viral particles compared to the titers in wild-type and heterozygote littermates. Asyn-knockout mice also exhibited significantly increased virus-induced mortality compared to Asyn heterozygote or homozygote control mice. Virus-induced Asyn localized to perinuclear, neuronal regions expressing viral envelope protein and the endoplasmic reticulum (ER)-associated trafficking protein Rab1. In Asyn-knockout primary neuronal cultures, the levels of expression of ER signaling pathways, known to support WNV replication, were significantly elevated before and during viral infection compared to those in Asyn-expressing primary neuronal cultures. We propose a model in which virus-induced Asyn localizes to ER-derived membranes, modulates virus-induced ER stress signaling, and inhibits viral replication, growth, and injury in the CNS. These data provide a novel and important functional role for the expression of native alpha-synuclein, a protein that is closely associated with the development of Parkinson's disease. IMPORTANCE: Neuroinvasive viruses such as West Nile virus are able to infect neurons and cause severe disease, such as encephalitis, or infection of brain tissue. Following viral infection in the central nervous system, only select neurons are infected, implying that neurons exhibit innate resistance to viral infections. We discovered that native neuronal expression of alpha-synuclein inhibited viral infection in the central nervous system. When the gene for alpha-synuclein was deleted, mice exhibited significantly decreased survival, markedly increased viral growth in the brain, and evidence of increased neuron injury. Virus-induced alpha-synuclein localized to intracellular neuron membranes, and in the absence of alpha-synuclein expression, specific endoplasmic reticulum stress signaling events were significantly increased. We describe a new neuron-specific inhibitor of viral infections in the central nervous system. Given the importance of alpha-synuclein as a cause of Parkinson's disease, these data also ascribe a novel functional role for the native expression of alpha-synuclein in the CNS.

Forced Complementation between Subgenomic RNAs: Does Human Immunodeficiency Type 1 Virus Reverse Transcription Occur in Viral Core, Cytoplasm, or Early Endosome?

Although the process of reverse transcription is well elucidated, it remains unclear if viral core disruption provides a more cellular or viral milieu for HIV-1 reverse transcription. We have devised a method to require mixing of viral cores or core constituents to produce infectious progeny virus by a bipartite subgenomic RNA (sgRNA) system, in which HIV-1 cplt_R/U5/gag/Δpol and nfl sgRNAs are complementary to each other and when together can complete viral reverse transcription. Only the heterodiploid virus containing both the nfl and cplt_R/U5/gag/Δpol sgRNAs can complete reverse transcription and propagate infectious virus upon de novo infection. Dual exposure of U87.CD4.CXCR4 cells with high titers of the homodimeric nfl and cplt_R/U5/gag/Δpol virus particles did not result in productive virus infection. On the other hand, in early endosomes, the HIV-1 sgRNAs released from viral cores can retain function and complete the reverse transcription and result in productive infection. These findings confirm the assumptions that, in natural infection, HIV-1 cores, and likely other retrovirus cores, remain largely intact and do not mix/fuse in the cytoplasm during the reverse transcription process, and circulating cytoplasmic HIV-1 sgRNA (produced through transfection) could not help the complementary sgRNA in the viral core to complement the reverse transcription process.

West nile virus-induced activation of mammalian target of rapamycin complex 1 supports viral growth and viral protein expression.

UNLABELLED: Since its introduction in New York City, NY, in 1999, West Nile virus (WNV) has spread to all 48 contiguous states of the United States and is now the leading cause of epidemic encephalitis in North America. As a member of the family Flaviviridae, WNV is part of a group of clinically important human pathogens, including dengue virus and Japanese encephalitis virus. The members of this family of positive-sense, single-stranded RNA viruses have limited coding capacity and are therefore obligated to co-opt a significant amount of cellular factors to translate their genomes effectively. Our previous work has shown that WNV growth was independent of macroautophagy activation, but the role of the evolutionarily conserved mammalian target of rapamycin (mTOR) pathway during WNV infection was not well understood. mTOR is a serine/threonine kinase that acts as a central cellular censor of nutrient status and exercises control of vital anabolic and catabolic cellular responses such as protein synthesis and autophagy, respectively. We now show that WNV activates mTOR and cognate downstream activators of cap-dependent protein synthesis at early time points postinfection and that pharmacologic inhibition of mTOR (KU0063794) significantly reduced WNV growth. We used an inducible Raptor and Rictor knockout mouse embryonic fibroblast (MEF) system to further define the role of mTOR complexes 1 and 2 in WNV growth and viral protein synthesis. Following inducible genetic knockout of the major mTOR cofactors raptor (TOR complex 1 [TORC1]) and rictor (TORC2), we now show that TORC1 supports flavivirus protein synthesis via cap-dependent protein synthesis pathways and supports subsequent WNV growth. IMPORTANCE: Since its introduction in New York City, NY, in 1999, West Nile virus (WNV) has spread to all 48 contiguous states in the United States and is now the leading cause of epidemic encephalitis in North America. Currently, the mechanism by which flaviviruses such as WNV translate their genomes in host cells is incompletely understood. Elucidation of the host mechanisms required to support WNV genome translation will provide broad understanding for the basic mechanisms required to translate capped viral RNAs. We now show that WNV activates mTOR and cognate downstream activators of cap-dependent protein synthesis at early time points postinfection. Following inducible genetic knockout of the major mTOR complex cofactors raptor (TORC1) and rictor (TORC2), we now show that TORC1 supports WNV growth and protein synthesis. This study demonstrates the requirement for TORC1 function in support of WNV RNA translation and provides insight into the mechanisms underlying flaviviral RNA translation in mammalian cells.

A cis-acting element in retroviral genomic RNA links Gag-Pol ribosomal frameshifting to selective viral RNA encapsidation.

During retroviral RNA encapsidation, two full-length genomic (g) RNAs are selectively incorporated into assembling virions. Packaging involves a cis-acting packaging element (Ψ) within the 5' untranslated region of unspliced HIV-1 RNA genome. However, the mechanism(s) that selects and limits gRNAs for packaging remains uncertain. Using a dual complementation system involving bipartite HIV-1 gRNA, we observed that gRNA packaging is additionally dependent on a cis-acting RNA element, the genomic RNA packaging enhancer (GRPE), found within the gag p1-p6 domain and overlapping the Gag-Pol ribosomal frameshift signal. Deleting or disrupting the two conserved GRPE stem loops diminished gRNA packaging and infectivity >50-fold, while deleting gag sequences between Ψ and GRPE had no effect. Downregulating the translation termination factor eRF1 produces defective virus particles containing 20 times more gRNA. Thus, only the HIV-1 RNAs employed for Gag-Pol translation may be specifically selected for encapsidation, possibly explaining the limitation of two gRNAs per virion.

Inhibition of both HIV-1 reverse transcription and gene expression by a cyclic peptide that binds the Tat-transactivating response element (TAR) RNA.

The RNA response element TAR plays a critical role in HIV replication by providing a binding site for the recruitment of the viral transactivator protein Tat. Using a structure-guided approach, we have developed a series of conformationally-constrained cyclic peptides that act as structural mimics of the Tat RNA binding region and block Tat-TAR interactions at nanomolar concentrations in vitro. Here we show that these compounds block Tat-dependent transcription in cell-free systems and in cell-based reporter assays. The compounds are also cell permeable, have low toxicity, and inhibit replication of diverse HIV-1 strains, including both CXCR4-tropic and CCR5-tropic primary HIV-1 isolates of the divergent subtypes A, B, C, D and CRF01_AE. In human peripheral blood mononuclear cells, the cyclic peptidomimetic L50 exhibited an IC(50) ∼250 nM. Surprisingly, inhibition of LTR-driven HIV-1 transcription could not account for the full antiviral activity. Timed drug-addition experiments revealed that L-50 has a bi-phasic inhibition curve with the first phase occurring after HIV-1 entry into the host cell and during the initiation of HIV-1 reverse transcription. The second phase coincides with inhibition of HIV-1 transcription. Reconstituted reverse transcription assays confirm that HIV-1 (-) strand strong stop DNA synthesis is blocked by L50-TAR RNA interactions in-vitro. These findings are consistent with genetic evidence that TAR plays critical roles both during reverse transcription and during HIV gene expression. Our results suggest that antiviral drugs targeting TAR RNA might be highly effective due to a dual inhibitory mechanism.