Nuclear factor (NF)-κB is a major survival pathway engaged by the Human T-Lymphotropic Virus type 1 (HTLV-1) Tax protein. Tax1 activation of NF-κB occurs predominantly in the cytoplasm, where Tax1 binds NF-κB Essential Modulator (NEMO/IKKγ) and triggers the activation of IκB kinases. Several independent studies have shown that Tax1-mediated NF-κB activation is dependent on Tax1 ubiquitination. Here, we identify by co-immunoprecipitation assays NEMO-Related Protein (NRP/Optineurin) as a binding partner for Tax1 in HTLV-1 infected and Tax1/NRP co-expressing cells. Immunofluorescence studies reveal that Tax1, NRP and NEMO colocalize in Golgi-associated structures. The interaction between Tax1 and NRP requires the ubiquitin-binding activity of NRP and the ubiquitination sites of Tax1. In addition, we observe that NRP increases the ubiquitination of Tax1 along with Tax1-dependent NF-κB signaling. Surprisingly, we find that in addition to Tax1, NRP interacts cooperatively with the Tax1 binding protein TAX1BP1, and that NRP and TAX1BP1 cooperate to modulate Tax1 ubiquitination and NF-κB activation. Our data strongly suggest for the first time that NRP is a critical adaptor that regulates the assembly of TAX1BP1 and post-translationally modified forms of Tax1, leading to sustained NF-κB activation. Oncogenic viruses (i.e., viruses that can induce cancer) have usually been found to deregulate several cellular signaling pathways controlling cell survival and proliferation. Among those, the NF-κB pathway is particularly important. In this study, we focus on the Human T-Lymphotropic Virus type 1 (HTLV-1), which infects immune T cells, and is associated with the development of a severe hematological disease, termed adult T cell leukemia. The viral Tax oncoprotein is known to activate the NF-κB pathway, but the precise mechanism is still under investigation. In cells, proteins can undergo modifications that can modulate their function. In the case of Tax, a modified form of the protein (ubiquitinated Tax) is able to activate the NF-κB pathway. Our aim was to identify cellular proteins that participate in the modification of Tax, and in turn in the regulation of its function. We show for the first time that the cellular protein NRP/Optineurin interacts with Tax and increases its ubiquitination, thus leading to an enhanced NF-κB activation. We further demonstrate that TAX1BP1, another cellular protein that had been previously identified as a partner of Tax, also participates in this regulation. Thus, this study uncovers new actors of the virally induced cell signaling.

Human T-cell leukemia virus type 1 (HTLV-1) and type 2 (HTLV-2) retroviruses infect T lymphocytes. The minus strand of the HTLV-1 genome encodes HBZ, a protein that could play a role in the development of leukemia in infected patients. Herein, we demonstrate that the complementary strand of the HTLV-2 genome also encodes a protein that we named APH-2 for “antisense protein of HTLV-2.” APH-2 mRNA is spliced, polyadenylated, and initiates in the 3′-long terminal repeat at different positions. This transcript was detected in all HTLV-2–infected cell lines and short-term culture of lymphocytes obtained from HTLV-2 African patients tested and in 4 of 15 HTLV-2–infected blood donors. The APH-2 protein is 183 amino acids long, is localized in the cell nucleus, and is detected in vivo. Despite the lack of a consensus basic leucine zipper domain, APH-2 interacts with cyclic adenosine monophosphate-response element binding protein (CREB) and represses Tax2-mediated transcription in Tax2-expressing cells and in cells transfected with an HTLV-2 molecular clone. Altogether, our results demonstrate the existence of an antisense strand–encoded protein in HTLV-2, which could represent an important player in the development of disorders, such as lymphocytosis, which is frequently observed in HTLV-2 patients.

ISG20 is a broad spectrum antiviral protein thought to directly degrade viral RNA. However, this mechanism of inhibition remains controversial. Using the Vesicular Stomatitis Virus (VSV) as a model RNA virus, we show here that ISG20 interferes with viral replication by decreasing protein synthesis in the absence of RNA degradation. Importantly, we demonstrate that ISG20 exerts a translational control over a large panel of non-self RNA substrates including those originating from transfected DNA, while sparing endogenous transcripts. This activity correlates with the protein’s ability to localize in cytoplasmic processing bodies. Finally, these functions are conserved in the ISG20 murine ortholog, whose genetic ablation results in mice with increased susceptibility to viral infection. Overall, our results posit ISG20 as an important defense factor able to discriminate the self/non-self origins of the RNA through translation modulation. The interferon-induced protein 20 (ISG20) is an RNA exonuclease endowed with broad antiviral properties. The prevailing mechanism of inhibition described for ISG20 indicates that this enzyme is capable of directly degrading viral RNA in the absence of apparent sequence specificity. This mode of action has been however challenged by recent studies that revealed that ISG20 could target specific structures on the hepatitis B virus, as well as by others that suggested inhibition in the absence of viral RNA degradation. We now demonstrate that ISG20 interferes with viral replication not by degrading viral RNA, but by impairing its translation. This mechanism of translational control targets all RNAs originated from ectopically introduced genetic material (through viral infection or transient transfection) that we define here collectively as non-self, independently from their viral/non-viral origins. However, ISG20 bears no effect on the translation of endogenous mRNAs transcripts, suggesting that ISG20 can discriminate between the cell’s own genetic material (self) and foreign one. By taking profit of their mode of replication through integration, or EBV-like episomal maintenance certain pathogens seemingly escape ISG20 by what can be defined as self-mimicry. Lastly, this mechanism of action is conserved in the ISG20 murine ortholog, whose genetic ablation results in mice with increased susceptibility to viral infection. Overall, our study reveals a novel role of ISG20 as a translational modulator of foreign genetic material playing important functions during viral infection in vivo.

HIV-1 and HTLV-1 are complex retroviruses mainly infecting CD4⁺ T lymphocytes. In addition, antigen-presenting cells such as dendritic cells (DCs) are targeted in vivo by both viruses, although to a lesser extend. Interaction of HIV-1 with DCs plays a key role in viral dissemination from the mucosa to CD4⁺ T lymphocytes present in lymphoid organs. While similar mechanisms may occur for HTLV-1 as well, most HTLV-1 data were obtained from T-cell studies, and little is known regarding the trafficking of this virus in DCs. We first compared the efficiency of cell-free versus cell-associated viral sources of both retroviruses at infecting DCs. We showed that both HIV-1 and HTLV-1 cell-free particles are poorly efficient at productively infecting DCs, except if DC-SIGN has been engaged. Furthermore, while SAMHD1 accounts for restriction of cell-free HIV-1 infection, it is not involved in HTLV-1 restriction. In addition, cell-free viruses mainly lead to a non-productive DC infection leading to trans-infection of T-cell a process important for HIV-1 spread but not for that of HTLV-1. Finally, we show that T:DC cell-to-cell transfer implies viral trafficking in vesicles that may both increase productive infection of DCs (“cis-infection”) and allow viral escape from immune surveillance. Altogether, these observations allowed us to draw a model on HTLV-1 and HIV-1 trafficking in DCs.

Innate immunity plays a critical role in the host response to a viral infection. The innate response has two main functions. First, it triggers effector mechanisms that restrict the infection. Second, it primes development of the adaptive response, which completes the elimination of the pathogen or of infected cells. In vivo, HTLV-1 infects T lymphocytes that participate in adaptive immunity but also monocytes and dendritic cells that are major players in innate immunity. Herein, we will review the interplay between HTLV-1 and innate immunity. Particular emphasis is put on HTLV-1-induced alteration of type-I interferon (IFN-I) function. In vitro, the viral Tax protein plays a significant role in the alteration of IFN synthesis and signaling. Despite this, IFN-I/AZT treatment of Adult T‑cell Leukemia/Lymphoma (ATLL) patients leads to complete remission. We will discuss a model in which exogenous IFN-I could act both on the microenvironment of the T-cells to protect them from infection, and also on infected cells when combined with other drugs that lead to Tax down-regulation/degradation.

Despite being discovered in animals in the early 20th century, the scientific interest in retroviruses was boosted with the discovery of human retroviruses (human T-leukemia/lymphoma virus (HTLV) and HIV), which are responsible for significant morbidity and mortality. HTLV was identified more than 25 years ago as the etiological agent of adult T-cell leukemia/lymphoma. It was then shown to be a complex retrovirus, given that it not only encodes the characteristic retroviral Gag, Pol and Env proteins, but also regulatory and accessory proteins. Since the first studies documenting the role of these proteins in viral expression, the picture has become increasingly more complex. Indeed, owing to the limited size of its genome that contains overlapping open-reading frames, HTLV has evolved unique ways to regulate its expression. Retroviral expression was originally thought to be mainly controlled through the regulation of transcription from the 5' long-terminal repeats, but we now know that the 3' long-terminal repeats also serve as promoters. Regulation of splicing and mRNA export, and post-translational modifications of viral protein also play a major role. This review discusses the latest insights gained into the field of HTLV gene expression.