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

Abstract

<jats:p>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 <jats:italic>in vitro</jats:italic> 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.</jats:p>

Control of Immediate Early Gene Expression for Human Cytomegalovirus Reactivation

“…However, these findings do not preclude the possibility that robust reactivation stimuli lead to re-expression from the MIEP to drive virus replication. In a recent study by Mason and colleagues, transcription from both the MIEP and iP2 was observed in the THP-1 model and in CD14 + monocytes treated with phorbol ester (Mason et al, 2020 ). Further, in dendritic cells derived from either CD34 + or CD14 + cells and in CD14 + -derived macrophages stimulated with IL-1b and M-CSF, MIE transcripts were predominantly derived from the MIEP.…”

In-Depth Profiling of T-Cell Responsiveness to Commonly Recognized CMV Antigens in Older People Reveals Important Sex Differences

“…IE protein expression will dominate immediately after reactivation and, in particular, if reactivation is abrogated and remains incomplete. The major immediate early promoter (MIEP) regulates IE protein expression and appears to be regulated itself by cellular chromatin activity, an increase of which is the common endpoint of a host of signaling pathways, particularly in inflammation (34). Memory subset composition was hence analyzed across all CMV proteins but also grouped by kinetic class, combining IE and E protein-specific responses in one group and E-L/L protein specific responses in the other.…”

Evasion of the Host Immune Response by Betaherpesviruses

“…However, a different group saw contrasting results in dendritic cells that indicated that MIEP is indeed important for HCMV reactivation. Mason et al, [ 198 ] repeated the experiments done by Collins-McMillen et al, [ 196 ] and observed high levels of both iP2- and MIEP-derived IE gene transcription in THP-1 cells rather than high levels of iP1- and iP2-derived expression and low levels of MIEP-derived expression. Mason et al, further found that MIEP-derived transcription predominated in DC reactivation and also noted diverging levels of MIEP- and iP2-derived transcription in CD14+ monocytes stimulated down different differentiation pathways.…”

“…iP2-derived transcription of IE genes was found to be cycloheximide dependent in DCs while MIEP transcription was not. This study concluded that control of HCMV reactivation may be cell type-specific and that MIEP still plays a key role in reactivation [ 198 ].…”