CD8 T Cells Control Cytomegalovirus Latency by Epitope-Specific Sensing of Transcriptional Reactivation

Abstract

During murine cytomegalovirus (mCMV) latency in the lungs, most of the viral genomes are transcriptionally silent at the major immediate-early locus, but rare and stochastic episodes of desilencing lead to the expression of IE1 transcripts. This low-frequency but perpetual expression is accompanied by an activation of lung-resident effector-memory CD8 T cells specific for the antigenic peptide 168-YPHFMPTNL-176, which is derived from the IE1 protein. These molecular and immunological findings were combined in the "silencing/ desilencing and immune sensing hypothesis" of cytomegalovirus latency and reactivation. This hypothesis proposes that IE1 gene expression proceeds to cell surface presentation of the IE1 peptide by the major histocompatibility complex (MHC) class I molecule L d and that its recognition by CD8 T cells terminates virus reactivation. Here we provide experimental evidence in support of this hypothesis. We generated mutant virus mCMV-IE1-L176A, in which the antigenic IE1 peptide is functionally deleted by a point mutation of the C-terminal MHC class I anchor residue Leu into Ala. Two revertant viruses, mCMV-IE1-A176L and the wobble nucleotide-marked mCMV-IE1-A176L*, in which Leu is restored by back-mutation of Ala codon GCA into Leu codons CTA and CTT, respectively, were constructed. Pulmonary latency of the mutant virus was found to be associated with an increased prevalence of IE1 transcription and with events of IE3 transactivator splicing. In conclusion, IE1-specific CD8 T cells recognize and terminate virus reactivation in vivo at the first opportunity in the reactivated gene expression program. The perpetual gene expression and antigen presentation might represent the driving molecular force in CMV-associated immunosenescence.After resolution of productive primary infection, in particular by CD8 T cells, cytomegaloviruses (CMVs) establish lifelong latent infections in their respective hosts (for reviews, see references 29, 31, 32, 52, 75, 83-85, and 87). Reactivation of latent human CMV (hCMV) to productive, cytopathogenic infection is still a health risk in immunocompromised patients (9, 57). Hematoablative therapy of leukemias, followed by bone marrow transplantation (BMT) or hematopoietic stem cell transplantation, is associated with a risk of CMV disease resulting from reactivation of latent donor and/or recipient CMV (15, 23). Among the manifestations of CMV disease in humans, interstitial pneumonia is the most dreaded because of its high fatality rate (79). Lungs were also identified as a major organ site of murine CMV (mCMV) disease, latency, and recurrence (4,43,70,78).Studies in the BALB/c mouse model of CMV infection in the BMT recipient have focused on the lungs for investigating mechanisms of immune control, latency, and reactivation (reviewed in references 25, 75, and 83). In this model, control of productive lung infection and prevention of disseminated viral pneumonia proved to be critically dependent upon the efficient reconstitution of CD8 T cells that infiltrated the lungs...

The (gradual) rise of memory inflation

“…(This is a little different from showing a role in viral control acutely or in the context of adoptive transfer during immunosuppression). One piece of evidence that the inflationary population is required to maintain latency comes from studies by the Reddehase group of a key early‐expressed epitope in IE1 (dominant in the BALB/c model) 5, 21, 66. It was previously noted that IE1 transcripts can be detected eg, in the lung, but IE3 was not found, suggesting a checkpoint restricting progression.…”

Differential role of IL‐2R signaling for CD8+ T cell responses in acute and chronic viral infections

“…While percentages of total IL-2Ra +/+ CD8 + T cells increased 7-to 8-fold in both test and control chimeras 8 days after infection, percentages of total IL-2Ra -/-CD8 + T cells increased only 2-fold (not shown). Expansion of LCMV gp [33][34][35][36][37][38][39][40][41] (gp33)-specific CD8 + T cells was assessed in blood, spleen, liver and lung by tetramer staining. Frequencies of specific T cells were comparable within the IL-2Ra -/-CD8 + T cell compartment and the control CD8 + T cell compartment: roughly 13% of Ly5.2 + (IL-2Ra -/-) as well as of wildtype Ly5.1 + CD8 + T cells were specific for the LCMVderived epitope gp33 (Fig.…”

“…Both are immunodominant epitopes, and whereas m45-specific CD8 + T cells expand and contract to constant memory levels, m38-specific CD8 + T cells show a continued accumulation over time [39]. It is believed that this "CD8 + T cell inflation" is caused by recurrent antigen encounter [39][40][41][42]. To investigate the IL-2 dependence of m45-and m38-specific CD8 + T cell dynamics, test and control chimeras were infected with 10 6 pfu MCMV, and frequencies of specific cells were analyzed over a period of 28 days (Fig.…”

“…Transgenic mice expressing a TCR specific for LCMV peptide gp [33][34][35][36][37][38][39][40][41] were described previously [56]. Ly5.2 + C57BL/6 mice were purchased from Janvier, and Ly5.1 + C57BL/6 and IL-2Ra -/-mice were purchased from Jackson and were maintained in a specific pathogen-free (SPF) facility.…”

Recent advances in CMV tropism, latency, and diagnosis during aging

“…Instead, in a murine model of mCMV infection after experimental hematopoietic cell transplantation, latency has been shown to be a highly dynamic state in which, at any given time, most of the viral gene loci are silenced while some hotspots are transiently in an Bon^state (Kurz et al 1999;Grzimek et al 2001;Simon et al 2006). This fact was recently coined with the term Bsleepless latency^ (Poole and Sinclair 2015).…”