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

Duringmurine cytomegalovirus (mCMV) latency in the lungs, most of the viralgenomes are transcriptionally silent at the major immediate-earlylocus, but rare and stochastic episodes of desilencing lead to theexpression of IE1 transcripts. This low-frequency but perpetualexpression is accompanied by an activation of lung-residenteffector-memory CD8 T cells specific for the antigenic peptide168-YPHFMPTNL-176, which is derivedfrom the IE1 protein. These molecular and immunological findings werecombined in the “silencing/desilencing and immune sensinghypothesis” of cytomegalovirus latency and reactivation. Thishypothesis proposes that IE1 gene expression proceeds to cell surfacepresentation of the IE1 peptide by the major histocompatibility complex(MHC) class I molecule Ld and that its recognition by CD8 Tcells terminates virus reactivation. Here we provide experimentalevidence in support of this hypothesis. We generated mutant virusmCMV-IE1-L176A, in which the antigenic IE1 peptide is functionallydeleted by a point mutation of the C-terminal MHC class I anchorresidue Leu into Ala. Two revertant viruses, mCMV-IE1-A176L and thewobble nucleotide-marked mCMV-IE1-A176L*, in which Leu is restored byback-mutation of Ala codon GCA into Leu codons CTA and CTT,respectively, were constructed. Pulmonary latency of the mutant viruswas found to be associated with an increased prevalence of IE1transcription and with events of IE3 transactivator splicing. Inconclusion, IE1-specific CD8 T cells recognize and terminate virusreactivation in vivo at the first opportunity in the reactivated geneexpression program. The perpetual gene expression and antigenpresentation might represent the driving molecular force inCMV-associatedimmunosenescence.