Upregulation of Neuropilin-1 Inhibits HTLV-1 Infection.

Infection with human T-cell leukemia virus type 1 (HTLV-1) can produce a spectrum of pathological effects ranging from inflammatory disorders to leukemia. In vivo, HTLV-1 predominantly infects CD4+ T-cells. Infectious spread within this population involves the transfer of HTLV-1 virus particles from infected cells to target cells only upon cell-to-cell contact. The viral protein, HBZ, was found to enhance HTLV-1 infection through transcriptional activation of ICAM1 and MYOF, two genes that facilitate viral infection. In this study, we found that HBZ upregulates the transcription of COL4A1, GEM, and NRP1. COL4A1 and GEM are genes involved in viral infection, while NRP1, which encodes neuropilin 1 (Nrp1), serves as an HTLV-1 receptor on target cells but has no reported function on HTLV-1-infected cells. With a focus on Nrp1, cumulative results from chromatin immunoprecipitation assays and analyses of HBZ mutants support a model in which HBZ upregulates NRP1 transcription by augmenting recruitment of Jun proteins to an enhancer downstream of the gene. Results from in vitro infection assays demonstrate that Nrp1 expressed on HTLV-1-infected cells inhibits viral infection. Nrp1 was found to be incorporated into HTLV-1 virions, and deletion of its ectodomain removed the inhibitory effect. These results suggest that inhibition of HTLV-1 infection by Nrp1 is caused by the ectodomain of Nrp1 extended from virus particles, which may inhibit the binding of virus particles to target cells. While HBZ has been found to enhance HTLV-1 infection using cell-based models, there may be certain circumstances in which activation of Nrp1 expression negatively impacts viral infection, which is discussed.

HBZ upregulates myoferlin expression to facilitate HTLV-1 infection.

The complex retrovirus, human T-cell leukemia virus type 1 (HTLV-1), primarily infects CD4+ T-cells in vivo. Infectious spread within this cell population requires direct contact between virally-infected and target cells. The HTLV-1 accessory protein, HBZ, was recently shown to enhance HTLV-1 infection by activating intracellular adhesion molecule 1 (ICAM-1) expression, which promotes binding of infected cells to target cells and facilitates formation of a virological synapse. In this study we show that HBZ additionally enhances HTLV-1 infection by activating expression of myoferlin (MyoF), which functions in membrane fusion and repair and vesicle transport. Results from ChIP assays and quantitative reverse transcriptase PCR indicate that HBZ forms a complex with c-Jun or JunB at two enhancer sites within the MYOF gene and activates transcription through recruitment of the coactivator p300/CBP. In HTLV-1-infected T-cells, specific inhibition of MyoF using the drug, WJ460, or shRNA-mediated knockdown of MyoF reduced infection efficiency. This effect was associated with a decrease in cell adhesion and an intracellular reduction in the abundance of HTLV-1 envelope (Env) surface unit (SU) and transmembrane domain (TM). Lysosomal protease inhibitors partially restored SU levels in WJ460-treated cells, and SU localization to LAMP-2 sites was increased by MyoF knockdown, suggesting that MyoF restricts SU trafficking to lysosomes for degradation. Consistent with these effects, less SU was associated with cell-free virus particles. Together, these data suggest that MyoF contributes to HTLV-1 infection through modulation of Env trafficking and cell adhesion.

Human T-Cell Leukemia Virus Type 1 (HTLV-1) bZIP Factor Upregulates the Expression of ICAM-1 To Facilitate HTLV-1 Infection.

Human T-cell leukemia virus type 1 (HTLV-1) causes multiple pathological effects, ranging from a form of leukemia to a spectrum of inflammation-mediated diseases. These diseases arise from one or several infected CD4+ T cells among thousands acquiring proliferation and survival advantages and ultimately becoming pathogenic. Given the low incidence of HTLV-1-associated diseases among carriers, such cellular evolutionary processes appear to occur rarely. Therefore, infectious spread of HTLV-1 within the T-cell population may be one underlying factor influencing disease development. Free HTLV-1 virions are poorly infectious, so infection of T cells relies on direct contact between infected and target cells. Following contact, virions pass to target cells through a virological synapse or cellular conduits or are transferred to target cells within an extracellular matrix. Lymphocyte functioning antigen 1 (LFA-1) on the surface of the target cell engaging with its ligand, ICAM-1, on the surface of the infected cell (effector cell) initiates and stabilizes cell-cell contact for infection. We found that stable expression of an HTLV-1 accessory protein, HTLV-1 bZIP factor (HBZ), in Jurkat T cells increases homotypic aggregation. This phenotype was attributed to elevated ICAM-1 expression in the presence of HBZ. Using a single-cycle replication-dependent luciferase assay, we found that HBZ expression in Jurkat cells (used as effector cells) increases HTLV-1 infection. Despite this effect, HBZ could not replace the critical infection-related functions of the HTLV-1 regulatory protein Tax. However, in HTLV-1-infected T cells, knockdown of HBZ expression did lead to a decrease in infection efficiency. These overall results suggest that HBZ contributes to HTLV-1 infectivity.IMPORTANCE Human T-cell leukemia virus type 1 (HTLV-1) causes a variety of diseases, ranging from a fatal form of leukemia to immune-mediated inflammatory diseases. These diseases occur rarely, arising from one or a small subset of virally infected cells infrequently evolving into a pathogenic state. Thus, the process of HTLV-1 cell-to-cell transmission within the host helps influence the probability of disease development. HTLV-1 primarily infects T cells and initially spreads within this cell population when virally infected T cells dock to uninfected target T cells and then transfer HTLV-1 virus particles to the target cells. Here we found that the viral protein HTLV-1 bZIP factor (HBZ) promotes infectivity. HBZ accomplishes this task by increasing the surface abundance of a cellular adhesion protein known as intercellular adhesion molecule 1 (ICAM-1), which helps initiate and stabilize contact (docking) between infected and target T cells. These results define a novel and unexpected function of HBZ, diverging from its defined functions in cellular survival and proliferation.