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...

. 74:7496-7507, 2000). It was proposed that these cells represent antiviral "standby" memory cells whose functional role might be to help prevent reactivation of latent virus. The pool of pulmonary CD8 T cells was composed of two subsets defined by the T-cell activation marker L-selectin (CD62L): a CD62Lhi subset of quiescent memory cells, and a CD62L lo subset of recently resensitized memory-effector cells. In this study, we have continued this line of investigation by quantitating CD8 T cells specific for the three currently published antigenic peptides of mCMV: peptide YPHFMPTNL processed from the immediate-early protein IE1 (pp89), and peptides YGPSLYRRF and AYAGLFTPL, derived from the early proteins m04 (gp34) and M84 (p65), respectively. IE1-specific CD8 T cells dominated in acute-phase pulmonary infiltrates and were selectively enriched in latently infected lungs. Notably, most IE1-specific CD8 T cells were found to belong to the CD62L lo subset representing memory-effector cells. This finding is in accordance with the interpretation that IE1-specific CD8 T cells are frequently resensitized during latent infection of the lungs and may thus be involved in the maintenance of mCMV latency.In human cytomegalovirus (hCMV) infection after bone marrow transplantation (BMT), recovery from CMV disease correlates with efficient reconstitution of CD8 T cells (50). Preemptive cytoimmunotherapy by adoptive transfer of hCMVspecific CD8 T-cell clones was found to be beneficial in that it reduced the incidence of CMV disease in BMT recipients (51, 56). Proof of principle for the protective effect of antiviral CD8 T cells was provided by the model of murine CMV (mCMV) infection of BALB/c mice subjected to hematoablative treatment. Early experiments performed in the absence of BMT documented an antiviral and protective function of adoptively transferred mCMV-specific CD8 T cells in the lungs as well as in other target organs of the disease (44, 46, 48; for a review, see reference 23). More recently, the course of mCMV infection was analyzed in the specific context of hematolymphopoietic reconstitution after either syngeneic BMT (18,38,39) or BMT performed across a single major histocompatibility complex (MHC) class I antigen disparity (1). Prevention of a disseminated and fulminant interstitial CMV pneumonia by the antiviral function of endogenously reconstituted CD8 T cells was inferred from the following observations: (i) CD8 T cells rather than CD4 T cells were recruited to infected lungs much more efficiently than to uninfected lungs (18); (ii) lung-infiltrating, blastoid CD62L lo CD8 T cells were not randomly distributed in lung tissue but were found to colocalize with infected lung cells in inflammatory foci, thereby secluding the infected cells from health tissue (18, 38); (iii) when isolated from the infiltrates, these activated CD8 T cells exerted ex vivo cytolytic activity against infected target cells (18) and secreted gamma interferon (IFN-␥) upon polyclonal triggering via CD3ε (38); (iv) the kinetics of infilt...

Activation of TNFR2 with a novel agonist expands T reg cells in vivo and protects allo-HCT recipients from acute GvHD while sparing antilymphoma and antiinfectious properties of transplanted donor T cells.

The importance of CD8 T cells for the control of cytomegalovirus (CMV) infection has raised interest inNotably, CD8 T-cell lines of both specificities protected against acute infection upon adoptive transfer. In contrast, the natural immune response to acute infection in draining lymph nodes and in the lungs indicated a somewhat broader specificity repertoire. We conclude that the low number of antigenic peptides identified so far for CMVs reflects a focused memory repertoire, and we predict that more antigenic peptides will be disclosed by analysis of the acute immune response.

؉ memory CD8 T cells were found to persist in lung tissue. One can thus operationally distinguish an early CMV-positive IP (phase 1) and a late CMV-negative IP (phase 2). According to the definition, phase 2 histopathology would not be diagnosed as a CMV-IP and could instead be misinterpreted as a CMV-induced immunopathology. We document here that phase 1 as well as phase 2 pulmonary CD8 T cells are capable of exerting effector functions and are effectual in protecting against productive infection. We propose that antiviral "stand-by" memory-effector T cells persist in the lungs to prevent virus recurrence from latency.