Time-course of host cell transcription during the HTLV-1 transcriptional burst

Abstract

The human T-cell leukemia virus type 1 (HTLV-1) transactivator protein Tax has pleiotropic functions in the host cell affecting cell-cycle regulation, DNA damage response pathways and apoptosis. These actions of Tax have been implicated in the persistence and pathogenesis of HTLV-1-infected cells. It is now known that tax expression occurs in transcriptional bursts of the proviral plus-strand, but the effects of the burst on host transcription are not fully understood. We carried out RNA sequencing of two naturally-infected T-cell clones transduced with a Tax-responsive Timer protein, which undergoes a time-dependent shift in fluorescence emission, to study transcriptional changes during successive phases of the HTLV-1 plus-strand burst. We found that the transcriptional regulation of genes involved in the NF-κB pathway, cell-cycle regulation, DNA damage response and apoptosis inhibition were immediate effects accompanying the plus-strand burst, and are limited to the duration of the burst. The results distinguish between the immediate and delayed effects of HTLV-1 reactivation on host transcription, and between clone-specific effects and those observed in both clones. The major transcriptional changes in the infected host T-cells observed here, including NF-kB, are transient, suggesting that these pathways are not persistently activated at high levels in HTLV-1-infected cells. The two clones diverged strongly in their expression of genes regulating the cell cycle. Up-regulation of senescence markers was a delayed effect of the proviral plus-strand burst and the up-regulation of some pro-apoptotic genes outlasted the burst. We found that activation of the arylhydrocarbon receptor (AhR) pathway enhanced and prolonged the proviral burst, but did not increase the rate of reactivation. Our results also suggest that sustained plus-strand expression is detrimental to the survival of infected cells.

Dynamics and consequences of the HTLV-1 proviral plus-strand burst

“…Consistent with these previous observations, Tax expression in the two clones studied here was associated with slower progression through G2/M (Fig 2D). We recently reported the changes in expression levels of genes involved in DDR during successive phases of Tax expression [12]. ATM and Rad3-related (ATR) mRNA levels followed the pattern of Tax expression, whereas ataxia-telangiectasia mutated (ATM) mRNA levels fell during the burst.…”

“…We recently reported changes in host transcription during the successive stages of a Tax burst in naturally HTLV-1-infected T-cell clones [12]. These changes involved several cellular processes, including proliferation, cell-cycle, DDR and apoptosis.…”

“…Two naturally-infected T-cell clones harbouring a single copy of HTLV-1 provirus, a T-cell clone naturally-infected with a type 2 defective HTLV-1 provirus and an uninfected T-cell clone were isolated by Cook et al using limiting dilution of CD4 + CD25 + cells [54]. Two naturally-infected T-cell clones stably expressing the d2EGFP Tax reporter system were generated as described previously [12]. All clones were propagated in complete growth medium containing RPMI-1640 (Sigma-Aldrich) with 20% fetal calf serum (FCS), 50 IU/ml penicillin, 50 µg/ml streptomycin, 2 mM L-glutamine (all from ThermoFisher Scientific) and 100 IU/ml human interleukin 2 (IL-2, Miltenyi Biotec).…”

“…Recent in vitro studies showed that both plus-and minus-strands of HTLV-1 are expressed in intermittent bursts [8][9][10]. Tax protein binds or modulates the activity of many host proteins [11] and alters the transcription of many host genes [12]. However, the consequences of HTLV-1 proviral expression on the behaviour of the infected cell are less well understood.…”

The transcriptome of HTLV-1-infected primary cells following reactivation reveals changes to host gene expression central to the proviral life cycle

“…Investigations of HTLV-1-infected samples at the single-cell level have uncovered high levels of heterogeneity in proviral gene expression, suggesting transcriptional bursting of the proviral promoter and intermittent expression (11; 12). Live imaging of Tax expression using a shortlived GFP reporter revealed these transient expression episodes in MT-1 cells (21), a finding later confirmed by the use of a fluorescent timer reporter for Tax expression which allows identification of cells which have recently terminated Tax expression (22). Intermittent expression offers a 3 .…”