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...

Cytomegaloviruses express glycoproteins that interfere with antigen presentation to CD8 T cells. Although the molecular modes of action of these "immunoevasins" differ between cytomegalovirus species, the convergent biological outcome is an inhibition of the recognition of infected cells. In murine cytomegalovirus, m152/gp40 retains peptide-loaded major histocompatibility complex class I molecules in a cis-Golgi compartment, m06/gp48 mediates their vesicular sorting for lysosomal degradation, and m04/gp34, although not an immunoevasin in its own right, appears to assist in the concerted action of all three molecules. Using the L d -restricted IE1 epitope YPHFMPTNL in the BALB/c mouse model as a paradigm, we provide here an explanation for the paradox that immunoevasins enhance CD8 T-cell priming although they inhibit peptide presentation in infected cells. Adaptive immune responses are initiated in the regional lymph node (RLN) draining the site of pathogen exposure. In particular for antigens that are not virion components, the magnitude of viral gene expression providing the antigens is likely a critical parameter in priming efficacy. We have therefore focused on the events in the RLN and have related priming to intranodal viral gene expression. We show that immunoevasins enhance priming by downmodulating an early CD8 T-cell-mediated "negative feedback" control of the infection in the cortical region of the RLN, thus supporting the model that immunoevasins improve antigen supply for indirect priming by uninfected antigen-presenting cells. As an important consequence, these findings predict that deletion of immunoevasin genes in a replicative vaccine virus is not a favorable option but may, rather, be counterproductive.

The lungs are a noted predilection site of acute, latent, and reactivated cytomegalovirus (CMV) infections. Interstitial pneumonia is the most dreaded manifestation of CMV disease in the immunocompromised host, whereas in the immunocompetent host lung-infiltrating CD8 T cells confine the infection in nodular inflammatory foci and prevent viral pathology. By using murine CMV infection as a model, we provide evidence for a critical role of mast cells (MC) in the recruitment of protective CD8 T cells to the lungs. Systemic infection triggered degranulation selectively in infected MC. The viral activation of MC was associated with a wave of CC chemokine ligand 5 (CCL5) in the serum of C57BL/6 mice that was MC-derived as verified by infection of MC-deficient KitW-sh/W-sh “sash” mutants. In these mutants, CD8 T cells were recruited less efficiently to the lungs, correlating with enhanced viral replication and delayed virus clearance. A causative role for MC was verified by MC reconstitution of “sash” mice restoring both, efficient CD8 T-cell recruitment and infection control. These results reveal a novel crosstalk axis between innate and adaptive immune defense against CMV, and identify MC as a hitherto unconsidered player in the immune surveillance at a relevant site of CMV disease.

Cytomegalovirus (CMV) poses a threat to the therapy of hematopoietic malignancies by hematopoietic stem cell transplantation, but efficient reconstitution of antiviral immunity prevents CMV organ disease. Tumor relapse originating from a minimal residual leukemia poses another threat. Although a combination of risk factors was supposed to enhance the incidence and severity of transplantation-associated disease, a murine model of a liver-adapted B-cell lymphoma has previously shown a survival benefit and tumor growth inhibition by nonlethal subcutaneous infection with murine CMV. Here we have investigated the underlying antitumoral mechanism. Virus replication proved to be required, since inactivated virions or the highly attenuated enhancerless mutant mCMV-⌬MIEenh did not impact the lymphoma in the liver. Surprisingly, the dissemination-deficient mutant mCMV-⌬M36 inhibited tumor growth, even though this virus fails to infect the liver. On the other hand, various strains of herpes simplex viruses consistently failed to control the lymphoma, even though they infect the liver. A quantitative analysis of the tumor growth kinetics identified a transient tumor remission by apoptosis as the antitumoral effector mechanism. Tumor cell colonies with cells surviving the CMV-induced "apoptotic crisis" lead to tumor relapse even in the presence of full-blown tissue infection. Serial transfer of surviving tumor cells did not indicate a selection of apoptosis-resistant genetic variants. NK cell activity of CD49b-expressing cells failed to control the lymphoma upon adoptive transfer. We propose the existence of an innate antitumoral mechanism that is triggered by CMV infection and involves an apoptotic signal effective at a distant site of tumor growth.Complications of bone marrow transplantation (BMT) or hematopoietic stem cell transplantation include graft failure and graft-versus-host disease (46), infections, and minimal residual disease/leukemia giving rise to relapse of the initial tumor against which the hematoablative therapy was administered (12,16,21). Among infections in the immunocompromised BMT/hematopoietic stem cell transplantation recipients, human cytomegalovirus (hCMV) infection remains the most frequent and most severe (for recent reviews, see references 9, 17, and 33), despite antiviral therapy and although efficient reconstitution of antiviral immunity, of CD8 T cells in particular, can prevent lethal CMV organ disease in mouse models and in patients (for recent reviews, see references 22 and 34). While a wealth of literature separately deals either with tumor relapse or with CMV in clinical and experimental settings, information on an interference between CMV and tumors is scarce, except for a proposed role of hCMV genes in cell transformation (20, 57), a conspicuous association between hCMV and malignant gliomas (19), and an enhancement of tumor cell invasiveness by modulating the expression of adhesion molecules NCAM/CD56 and ␣5␤1 integrin VLA-5 in persistently infected neuroblastoma cells (8, 63). Tumor...

For recognition of infected cells by CD8 T cells, antigenic peptides are presented at the cell surface, bound to major histocompatibility complex class I (MHC-I) molecules. Downmodulation of cell surface MHC-I molecules is regarded as a hallmark function of cytomegalovirus-encoded immunoevasins. The molecular mechanisms by which immunoevasins interfere with the MHC-I pathway suggest, however, that this downmodulation may be secondary to an interruption of turnover replenishment and that hindrance of the vesicular transport of recently generated peptide-MHC (pMHC) complexes to the cell surface is the actual function of immunoevasins. Here we have used the model of murine cytomegalovirus (mCMV) infection to provide experimental evidence for this hypothesis. To quantitate pMHC complexes at the cell surface after infection in the presence and absence of immunoevasins, we generated the recombinant viruses mCMV-SIINFEKL and mCMV-⌬m06m152-SIINFEKL, respectively, expressing the K b -presented peptide SIINFEKL with early-phase kinetics in place of an immunodominant peptide of the viral carrier protein gp36.5/m164. The data revealed ϳ10,000 K b molecules presenting SIINFEKL in the absence of immunoevasins, which is an occupancy of ϳ10% of all cell surface K b molecules, whereas immunoevasins reduced this number to almost the detection limit. To selectively evaluate their effect on preexisting pMHC complexes, cells were exogenously loaded with SIINFEKL peptide shortly after infection with mCMV-SIINFEKA, in which endogenous presentation is prevented by an L174A mutation of the C-terminal MHC-I anchor residue. The data suggest that pMHC complexes present at the cell surface in advance of immunoevasin gene expression are downmodulated due to constitutive turnover in the absence of resupply. CD8 T cells recognize infected cells by interaction of theirT-cell receptor (TCR) with a cell surface presentation complex composed of a cognate antigenic peptide bound to a presenting allelic form of a major histocompatibility complex class I (MHC-I) glycoprotein (77,85,97,98). The number of such "peptide receptors" per cell has been estimated to be on the order of 10 5 to 10 6 for each MHC-I allomorph (for a review, see reference 82). Viral antigenic peptides are generated within infected cells by proteolytic processing of viral proteins, usually in the proteasome, and associate with nascent MHC-I proteins in the endoplasmic reticulum (ER) before the peptide-MHC (pMHC) complexes travel to the cell surface with the cellular vesicular flow (for reviews, see references 13, 87, 92, and 93). CD8 T cells have long been known to protect against cytomegalovirus (CMV) infection and disease in animal models (60, 72; reviewed in references 33 and 36) and in humans (9,61,67,75,76). As shown only recently in the murine CMV (mCMV) model of infection of immunocompromised mice by adoptive transfer of epitope-specific CD8 T cells, antiviral protection against CMV is indeed TCR mediated and epitope dependent. Specifically, memory cells purified by TCR-bas...