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Patchwork pattern of transcriptional reactivation in the lungs indicates sequential checkpoints in the transition
1Institute for Virology, Johannes Gutenberg-University, Mainz, Germany.
Abstract:
The lungs are a significant organ site of murine cytomegalovirus (mCMV) latency. We have shown that activity of the major immediate-early promoter (MIEP), which drives the transcription from the ie1-ie3 transcription unit, does not inevitably initiate the productive cycle (S. K. Kurz, M. Rapp, H.-P. Steffens, N. K. A. Grzimek, S. Schmalz, and M. J. Reddehase, J. Virol. 73:482-494, 1999). Thus, even though MIEP activity governed by the MIEP-enhancer is unquestionably the first condition for recurrence, regulation of the enhancer by transcription factors is not the only mechanism controlling latency. Specifically, during latency, focal and stochastic MIEP activity in lung tissue was found to selectively generate IE1 transcripts, while transactivator-specifying IE3 transcripts were missing. This suggested a control of mCMV latency that is effectual at IE1-IE3 precursor mRNA cotranscriptional processing. Here we have used this model for studying the kinetics of reactivation and recurrence in individual lung tissue pieces after hematoablative, genotoxic treatment. Notably, reactivation was triggered, but the number of transcriptionally active foci in the lungs did not increase over time. This result is not compatible with a model of spontaneous reactivations accumulating after withdrawal of immune control. Instead, the data support the idea that reactivation is an induced event. In some pieces, focal reactivation generated IE3 transcripts but not gB transcripts, while other pieces contained foci that had proceeded to gB transcription, and only a few foci actually reached the state of virus recurrence. This finding indicates the existence of several sequentially ordered control points in the transition from mCMV latency to recurrence.
Insights
Murine cytomegalovirus (mCMV) latency in lungs involves sequential control points. Reactivation is induced, not spontaneous, with distinct stages from IE1 transcript generation to full virus recurrence.
Area of Science:
- Virology
- Immunology
- Molecular Biology
Background:
- Murine cytomegalovirus (mCMV) establishes latency primarily in lung tissue.
- Major immediate-early promoter (MIEP) activity is crucial for viral reactivation but does not guarantee productive infection.
- Previous studies indicated that MIEP activity alone does not fully explain the control of mCMV latency.
Purpose of the Study:
- To investigate the kinetics of mCMV reactivation and recurrence in lung tissue following genotoxic treatment.
- To elucidate the regulatory mechanisms controlling the transition from mCMV latency to productive viral replication.
- To determine if reactivation is a spontaneous event or an induced process.
Main Methods:
- Utilized a model of mCMV latency in individual lung tissue pieces.
- Administered hematoablative, genotoxic treatment to trigger reactivation.
- Analyzed the kinetics of transcription, focusing on IE1, IE3, and gB transcripts, to track reactivation and recurrence stages.
Main Results:
- Reactivation was triggered by treatment, but the number of active viral transcription sites (foci) in the lungs did not increase over time.
- Data suggest reactivation is an induced event, not a cumulative process of spontaneous reactivations.
- Different foci exhibited sequential progression: some produced IE3 transcripts, others gB transcripts, and only a few led to full virus recurrence.
Conclusions:
- mCMV latency and recurrence are regulated by multiple, sequentially ordered control points.
- Reactivation of latent mCMV is an induced phenomenon.
- The transition from latency to recurrence involves distinct molecular stages, including differential gene expression and transcriptional processing.