Latency, chromatin remodeling, and reactivation of human cytomegalovirus in the dendritic cells of healthy carriers

Abstract

Human cytomegalovirus (HCMV) persists as a subclinical, lifelong infection in the normal human host, but reactivation from latency in immunocompromised subjects results in serious disease. Latency and reactivation are defining characteristics of the herpesviruses and are key to understanding their biology; however, the precise cellular sites in which HCMV is carried and the mechanisms regulating its latency and reactivation during natural infection remain poorly understood. Here we present evidence, based entirely on direct analysis of material isolated from healthy virus carriers, to show that myeloid dendritic cell (DC) progenitors are sites of HCMV latency and that their ex vivo differentiation to a mature DC phenotype is linked with reactivation of infectious virus resulting from differentiation-dependent chromatin remodeling of the viral major immediate-early promoter. Thus, myeloid DC progenitors are a site of HCMV latency during natural persistence, and there is a critical linkage between their differentiation to DC and transcriptional reactivation of latent virus, which is likely to play an important role in the pathogenesis of HCMV infection. P rimary infection of healthy individuals with human cytomegalovirus (HCMV) is often asymptomatic and results in lifelong persistence in the host, a characteristic of all herpes viruses. However, primary infection and reactivation of latent HCMV causes serious disease in immunosuppressed transplant recipients and in advanced HIV infection (1-3). Transfusionmediated HCMV disease can be prevented by leukocyte depletion (4) of blood, but infectious virus cannot be detected in the blood of healthy carriers, suggesting that HCMV is transmitted as latent virus in the peripheral blood leukocyte population. Accumulating evidence has shown that HCMV is carried latently in mononuclear cells of the myeloid lineage during lifelong latency in naturally infected individuals (5-8). Differentiation of monocytes to macrophages in vitro is reported to induce immediate-early (IE) lytic gene expression from latent virus (9), and groups have intermittently reported that infectious virus can be recovered after differentiation of monocytes to macrophages through explant culture (10) and, more recently, by allogeneic T cell stimulation (11), suggesting that reactivation of latent virus is associated with both the differentiation and activation state of myeloid cells.The major IE genes of HCMV, driven by the viral major IE promoter͞enhancer (MIEP), are the two most abundantly transcribed genes at IE times of virus lytic infection (12), and their proteins play a critical role in control of viral early and late gene expression (for review, see ref. 12). Consistent with the differentiation-dependent induction of IE gene expression observed above, there is a clear correlation between the differentiation state of the cell and the regulation of HCMV IE gene expression in vitro. Thus, in transfection assays, the MIEP is transcriptionally repressed in undifferentiated but transcriptionally active ...

Host-viral effects of chromatin assembly factor 1 interaction with HCMV IE2

“…Although HCMV virion contains no histone proteins [55,56], a number of reports indicate that host histones with different modifications associate with HCMV DNA in infected cells cultured in labs or isolated from humans [49,[57][58][59][60][61][62]. Nitzsche et al [43] further indicate that HCMV DNA is chromatinized in infected cells; histones are localized with UL44 at replication foci and there is a robust increase of histone occupancy coupled with viral DNA replication.…”

Early reconstitution of effector memory CD4+ CMV-specific T cells protects against CMV reactivation following allogeneic SCT

“…Such extreme immunodominance of CMV in the post transplant period has been observed in other studies 28 and may reflect the observation of CMV latency and reactivation in myeloid DCs. 32 The CMV-specific CD4 þ T-cell immune response is unusual in that the great majority of the population is comprised of cytotoxic CD4 cells, a minority population within the CD4 þ T-cell repertoire. Cytotoxic CD4 þ T cells show very comparable phenotypic and functional features with CD8 þ cytotoxic T cells and this is reflected in their cytokine production.…”

The immunology of human cytomegalovirus latency: could latent infection be cleared by novel immunotherapeutic strategies?

“…Also, importantly, this latent transcription programme can be reactivated to lytic cycle by differentiation of latent CD34 1 cells or monocytes to macrophages or dendritic cells resulting in expression of the established lytic temporal cascade of viral gene expression, leading to viral DNA replication and de novo virus production. 36,[38][39][40] Crucially, this reactivation of the lytic cascade of gene expression is initiated by the expression of the major immediate early proteins (IE72 and IE86); IE gene expression, thus, acts as a master regulator to initiate lytic cycle [41][42][43][44] (Figure 4). It is worth noting that many of these analyses of naturally latently infected cells can also be, in general, recapitulated in experimental models of latency in vitro.…”