Human cytomegalovirus major immediate early transcripts arise predominantly from the canonical major immediate early promoter in reactivating progenitor-derived dendritic cells.

Mark R Wills,Matthew B Reeves,Ian J Groves,Rebecca Mason,John H Sinclair

doi:10.1099/jgv.0.001419

Abstract

Human cytomegalovirus latency and reactivation is a major source of morbidity in immune-suppressed patient populations. Lifelong latent infections are established in CD34+progenitor cells in the bone marrow, which are hallmarked by a lack of major lytic gene expression, genome replication and virus production. A number of studies have shown that inhibition of the major immediate early promoter (MIEP) – the promoter that regulates immediate early (IE) gene expression – is important for the establishment of latency and that, by extension, reactivation requires reversal of this repression of the MIEP. The identification of novel promoters (termed ip1 and ip2) downstream of the MIEP that can drive IE gene expression has led to speculation over the precise role of the MIEP in reactivation. In this study we show that IE transcripts arise from both the MIEP and ip2 promoter in the THP1 cell macrophage cell line and also CD14+monocytes stimulated with phorbol ester. In contrast, we show that in in vitro generated dendritic cells or macrophages that support HCMV reactivation IE transcripts arise predominantly from the MIEP and not the intronic promoters. Furthermore, inhibition of histone modifying enzyme activity confirms the view that the MIEP is predominantly regulated by the activity of cellular chromatin. Finally, we observe that ip2-derived IE transcription is cycloheximide-sensitive in reactivating DCs, behaviour consistent with an early gene designation. Taken together, these data argue that MIEP activity is still important for HCMV reactivation but ip2 activity could play cell-type-specific roles in reactivation.

Highlights

A hallmark of human cytomegalovirus infection is the establishment of a lifelong latent infection of the host [1]
Robust major immediate early (IE) promoter (MIEP)-derived IE gene transcription was detected in the dendritic cells (DCs) suggesting that, in DCs at least, classical MIEP activity was required for IE gene expression (Fig. 1a, b)
This concept holds true for all members of the herpesvirus family and the mechanisms that control the initiation of lytic infection or the molecular switch from latency to reactivation have been widely studied

Summary

Introduction

A hallmark of human cytomegalovirus infection is the establishment of a lifelong latent infection of the host [1]. In healthy individuals these infections are asymptomatic but in immune- suppressed populations primary infection, re-infection, or reactivation from latency can be a major source of morbidity [2]. The establishment of HCMV latency is dependent on the eventual silencing of immediate early (IE) gene expression from the major IE promoter (MIEP). Given the central role IE gene expression plays in productive infection it has long been considered that an induction of previously silenced IE gene expression represents the first stage in the reactivation from latency. Studies of natural latency have shown that the MIEP is associated with host chromatin and, importantly, the post-translational repressive or activatory modifications associated with the histones bound to the MIEP

Methods

Results

Conclusion