Late-phase expression of a murine cytomegalovirus immediate-early antigen recognized by cytolytic T lymphocytes

Murine Cytomegalovirus Infection of Melanoma Lesions Delays Tumor Growth by Recruiting and Repolarizing Monocytic Phagocytes in the Tumor

Cytomegalovirus (CMV) is a ubiquitous betaherpesvirus that infects many different cell types. Human CMV (HCMV) has been found in several solid tumors, and it has been hypothesized that it may promote cellular transformation or exacerbate tumor growth. Paradoxically, in some experimental situations, murine CMV (MCMV) infection delays tumor growth. We previously showed that wild-type MCMV delayed the growth of poorly immunogenic B16 melanomas via an undefined mechanism. Here, we show that MCMV delayed the growth of these immunologically "cold" tumors by recruiting and modulating tumor-associated macrophages. Depletion of monocytic phagocytes with clodronate completely prevented MCMV from delaying tumor growth. Mechanistically, our data suggest that MCMV recruits new macrophages to the tumor via the virus-encoded chemokine MCK2, and viruses lacking this chemokine were unable to delay tumor growth. Moreover, MCMV infection of macrophages drove them toward a proinflammatory (M1)-like state. Importantly, adaptive immune responses were also necessary for MCMV to delay tumor growth as the effect was substantially blunted in Rag-deficient animals. However, viral spread was not needed and a spread-defective MCMV strain was equally effective. In most mice, the antitumor effect of MCMV was transient. Although the recruited macrophages persisted, tumor regrowth correlated with a loss of viral activity in the tumor. However, an additional round of MCMV infection further delayed tumor growth, suggesting that tumor growth delay was dependent on active viral infection. Together, our results suggest that MCMV infection delayed the growth of an immunologically cold tumor by recruiting and modulating macrophages in order to promote anti-tumor immune responses.IMPORTANCE Cytomegalovirus (CMV) is an exciting new platform for vaccines and cancer therapy. Although CMV may delay tumor growth in some settings, there is also evidence that CMV may promote cancer development and progression. Thus, defining the impact of CMV on tumors is critical. Using a mouse model of melanoma, we previously found that murine CMV (MCMV) delayed tumor growth and activated tumor-specific immunity although the mechanism was unclear. We now show that MCMV delayed tumor growth through a mechanism that required monocytic phagocytes and a viral chemokine that recruited macrophages to the tumor. Furthermore, MCMV infection altered the functional state of macrophages. Although the effects of MCMV on tumor growth were transient, we found that repeated MCMV injections sustained the antitumor effect, suggesting that active viral infection was needed. Thus, MCMV altered tumor growth by actively recruiting macrophages to the tumor, where they were modulated to promote antitumor immunity.

Intratumoral Infection with Murine Cytomegalovirus Synergizes with PD-L1 Blockade to Clear Melanoma Lesions and Induce Long-term Immunity

Cytomegalovirus is an attractive cancer vaccine platform because it induces strong, functional CD8(+) T-cell responses that accumulate over time and migrate into most tissues. To explore this, we used murine cytomegalovirus expressing a modified gp100 melanoma antigen. Therapeutic vaccination by the intraperitoneal and intradermal routes induced tumor infiltrating gp100-specific CD8(+) T-cells, but provided minimal benefit for subcutaneous lesions. In contrast, intratumoral infection of established tumor nodules greatly inhibited tumor growth and improved overall survival in a CD8(+) T-cell-dependent manner, even in mice previously infected with murine cytomegalovirus. Although murine cytomegalovirus could infect and kill B16F0s in vitro, infection was restricted to tumor-associated macrophages in vivo. Surprisingly, the presence of a tumor antigen in the virus only slightly increased the efficacy of intratumoral infection and tumor-specific CD8(+) T-cells in the tumor remained dysfunctional. Importantly, combining intratumoral murine cytomegalovirus infection with anti-PD-L1 therapy was synergistic, resulting in tumor clearance from over half of the mice and subsequent protection against tumor challenge. Thus, while a murine cytomegalovirus-based vaccine was poorly effective against established subcutaneous tumors, direct infection of tumor nodules unexpectedly delayed tumor growth and synergized with immune checkpoint blockade to promote tumor clearance and long-term protection.

The genome of murine cytomegalovirus is shaped by purifying selection and extensive recombination

The herpesvirus lifestyle results in a long-term interaction between host and invading pathogen, resulting in exquisite adaptation of virus to host. We have sequenced the genomes of nine strains of murine cytomegalovirus (a betaherpesvirus), isolated from free-living mice trapped at locations separated geographically and temporally. Despite this separation these genomes were found to have low levels of nucleotide variation. Of the more than 160 open reading frames, almost 90% had a dN/dS ratio of amino acid substitutions of less than 0.6, indicating the level of purifying selection on the coding potential of MCMV. Examination of selection acting on individual genes at the codon level however indicates some level of positive selection, with 0.03% of codons showing strong evidence for positive selection. Conversely, 1.3% of codons show strong evidence of purifying selection. Alignments of both genome sequences and coding regions suggested that high levels of recombination have shaped the MCMV genome.

Murine cytomegalovirus immune evasion proteins operative in the MHC class I pathway of antigen processing and presentation: state of knowledge, revisions, and questions

Medical interest in cytomegalovirus (CMV) is based on lifelong neurological sequelae, such as sensorineural hearing loss and mental retardation, resulting from congenital infection of the fetus in utero, as well as on CMV disease with multiple organ manifestations and graft loss in recipients of hematopoietic cell transplantation or solid organ transplantation. CMV infection of transplantation recipients occurs consequent to reactivation of virus harbored in a latent state in the transplanted donor cells and tissues, or in the tissues of the transplantation recipient herself or himself. Hence, CMV infection is a paradigm for a viral infection that causes disease primarily in the immunocompromised host, while infection of the immunocompetent host is associated with only mild and nonspecific symptoms so that it usually goes unnoticed. Thus, CMV is kept under strict immune surveillance. These medical facts are in apparent conflict with the notion that CMVs in general, human CMV as well as animal CMVs, are masters of 'immune evasion', which during virus-host co-speciation have convergently evolved sophisticated mechanisms to avoid their recognition by innate and adaptive immunity of their respective host species, with viral genes apparently dedicated to serve just this purpose (Reddehase in Nat Rev Immunol 2:831-844, 2002). With focus on viral interference with antigen presentation to CD8 T cells in the preclinical model of murine CMV infection, we try here to shed some more light on the in vivo balance between host immune surveillance of CMV infection and viral 'immune evasion' strategies.

Antigen presentation under the influence of 'immune evasion' proteins and its modulation by interferon-gamma: implications for immunotherapy of cytomegalovirus infection with antiviral CD8 T cells

Cytomegalovirus (CMV) disease with multiple organ manifestations is the most feared viral complication limiting the success of hematopoietic cell transplantation as a therapy of hematopoietic malignancies. A timely endogenous reconstitution of CD8 T cells controls CMV infection, and adoptive transfer of antiviral CD8 T cells is a therapeutic option to prevent CMV disease by bridging the gap between an early CMV reactivation and delayed endogenous reconstitution of protective immunity. Preclinical research in murine models has provided 'proof of concept' for CD8 T-cell therapy of CMV disease. Protection by CD8 T cells appears to be in conflict with the finding that CMVs encode proteins that inhibit antigen presentation to CD8 T cells by interfering with the constitutive trafficking of peptide-loaded MHC class I molecules (pMHC-I complexes) to the cell surface. Here, we have systematically explored antigen presentation in the presence of the three currently noted immune evasion proteins of murine CMV in all possible combinations and its modulation by pre-treatment of cells with interferon-gamma (IFN-γ). The data reveal improvement in antigen processing by pre-treatment with IFN-γ can almost overrule the inhibitory function of immune evasion molecules in terms of pMHC-I expression levels capable of triggering most of the specific CD8 T cells, though the intensity of stimulation did not retrieve their full functional capacity. Notably, an in vivo conditioning of host tissue cells with IFN-γ in adoptive cell transfer recipients constitutively overexpressing IFN-γ (B6-SAP-IFN-γ mice) enhanced the antiviral efficiency of CD8 T cells in this transgenic cytoimmunotherapy model.

Human cytomegalovirus early protein pUL21a promotes efficient viral DNA synthesis and the late accumulation of immediate-early transcripts

We have previously reported that a newly annotated gene of human cytomegalovirus (HCMV), UL21a, encodes an early viral protein termed pUL21a. Most notably, the virions of a UL21a deletion virus had markedly reduced infectivity, indicating that UL21a is required to establish an efficient productive infection. In this study, we infected fibroblasts with equal numbers of DNA-containing viral particles and identified where in the viral life cycle pUL21a acted. The UL21a deletion virus entered cells and initiated viral gene expression efficiently; however, it synthesized viral DNA poorly and accumulated several immediate-early (IE) transcripts at reduced levels at late times of infection. The defect in viral DNA synthesis preceded that in gene expression, and inhibition of viral DNA synthesis reduced the late accumulation of IE transcripts in both wild-type and mutant virus-infected cells to equivalent levels. This suggests that reduced viral DNA synthesis is the cause of reduced IE gene expression in the absence of UL21a. The growth of UL21a deletion virus was similar to that of recombinant HCMV in which pUL21a expression was abrogated by stop codon mutations, and the defect was rescued in pUL21a-expressing fibroblasts. pUL21a expression in trans was sufficient to restore viral DNA synthesis and gene expression of mutant virus produced from normal fibroblasts, whereas mutant virus produced from complementing cells still exhibited the defect in normal fibroblasts. Thus, pUL21a does not promote the functionality of HCMV virions; rather, its de novo synthesis facilitates viral DNA synthesis, which is necessary for the late accumulation of IE transcripts and establishment of a productive infection.

Investigation of the impact of the common animal facility contaminant murine norovirus on experimental murine cytomegalovirus infection

Murine norovirus (MNV) is a recently discovered pathogen that has become a common contaminant of specific pathogen-free mouse colonies. MNV-1 induces a robust interferon-beta response and causes histopathology in some mouse strains, suggesting that it may impact other mouse models of infection. Despite many concerns about MNV-1 contamination, there is little information about its impact on immune responses to other infections. This study addresses whether MNV-1 infection has an effect on a model of murine cytomegalovirus (MCMV) infection. Exposure to MNV-1 resulted in a decreased CD8 T cell response to immunodominant MCMV epitopes in both BALB/c and C57BL/6 mice. However, MNV-1 did not impact MCMV titers in either mouse strain, nor did it stimulate reactivation of latent MCMV. These data suggest that while MNV-1 has a mild impact on the immune response to MCMV, it is not likely to affect most experimental outcomes in immunocompetent mice in the MCMV model.

MHC class I immune evasion in MCMV infection

Murine cytomegalovirus (MCMV) is a well-studied model of natural beta-herpesvirus infection. However, many questions remain regarding its control by and evasion of the immune response it generates. CD8 and CD4 T cells have both unique and redundant roles in control of the virus that differ based on the immunocompetence of the infected mice. MCMV encodes major histocompatibility complex (MHC) class I immune evasion genes that can have an impact in vitro, but their role in infection of immunocompetent mice has been difficult to identify. This review addresses the evidence for their in vivo function and suggests why they may be evolutionarily conserved.

Stalemating a clever opportunist: lessons from murine cytomegalovirus

Cytomegaloviruses and their specific hosts have come to an arrangement that avoids disease but allows the viruses to persist in the individual host and to spread in the host species. Recent work has uncovered some of the molecular details of this evolutionary "contract for mutual survival." Cytomegaloviruses encode proteins, referred to as "immunoevasins," which are specifically committed to subvert the immune defense of the host for evading virus elimination. In reply, the hosts have evolved countermeasures to overcome the viral immunoevasins and present antigenic peptides to an extent that is sufficient for confining virus replication to below a harmful level. Accordingly, cytomegalic inclusion disease is a disease only of the immunocompromised. Although the details of the contract differ between the various cytomegalovirus host pairs, the general principles are strikingly analogous. Paradigmatic findings were made in the murine model, which adds the advantage of providing proof of principle by in vivo studies. With the focus on CD8 T cells and the major histocompatibility complex class I pathway of antigen presentation, we will discuss our view on the immune surveillance of cytomegalovirus in the murine model.

Processing and presentation of murine cytomegalovirus pORFm164-derived peptide in fibroblasts in the face of all viral immunosubversive early gene functions

CD8 T cells are the principal effector cells in the resolution of acute murine cytomegalovirus (mCMV) infection in host organs. This undoubted antiviral and protective in vivo function of CD8 T cells appeared to be inconsistent with immunosubversive strategies of the virus effected by early (E)-phase genes m04, m06, and m152. The so-called immune evasion proteins gp34, gp48, and gp37/40, respectively, were found to interfere with peptide presentation at different steps in the major histocompatibility complex (MHC) class I pathway of antigen processing and presentation in fibroblasts. Accordingly, they were proposed to prevent recognition and lysis of infected fibroblasts by cytolytic T lymphocytes (CTL) during the E phase of viral gene expression. We document here that the previously identified MHC class I D(d)-restricted antigenic peptide (257)AGPPRYSRI(265) encoded by gene m164 is processed as well as presented for recognition by m164-specific CTL during the E and late phases of viral replication in the very same cells in which the immunosubversive viral proteins are effectual in preventing the presentation of processed immediate-early 1 (m123-exon 4) peptide (168)YPHFMPTNL(176). Thus, while immunosubversion is a reality, these mechanisms are apparently not as efficient as the term immune evasion implies. The pORFm164-derived peptide is the first noted peptide that constitutively escapes the immunosubversive viral functions. The most important consequence is that even the concerted action of all immunosubversive E-phase proteins eventually fails to prevent immune recognition in the E phase. The bottom-line message is that there exists no immune evasion of mCMV in fibroblasts.

Identification and characterization of novel murine cytomegalovirus M112-113 (e1) gene products

The human cytomegalovirus (HCMV) UL112-113 gene products play important roles in viral DNA replication and transcriptional regulation. In this report, we characterize two novel transcripts originating from the homologous M112-113 (e1) region of the murine cytomegalovirus (MCMV) genome. These transcripts of 2.0 and 2.4 kb represent alternatively spliced products of the e1 gene region. Analysis of the e1 proteins demonstrates the presence of a previously unidentified 87-kDa protein that is likely encoded by the 2.4-kb transcript. All four protein products derived from the e1 gene region are expressed with early kinetics, are coordinately regulated, and localize predominantly to the nucleus of MCMV-infected cells. The expression pattern and localization of the e1 proteins show significant similarity to those of the HCMV UL112-113 proteins, signifying that MCMV e1 will serve as a useful model for assessing the role of this early gene region during viral infection.

Early gene m18, a novel player in the immune response to murine cytomegalovirus

The identification of all antigenic peptides encoded by a pathogen, its T cell 'immunome', is a research aim for rational vaccine design. Screening of proteome-spanning peptide libraries or computational prediction is used to identify antigenic peptides recognized by CD8 T cells. Based on their high coding capacity, cytomegaloviruses (CMVs) could specify numerous antigenic peptides. Yet, current evidence indicates that the memory CD8 T cell response in a given haplotype is actually focused on a few viral proteins. CMVs actively interfere with antigen processing and presentation by the expression of immune evasion proteins. In the case of murine CMV (mCMV), these proteins are effectual in the early (E) phase of the virus replication cycle and should thus preclude the presentation of peptides derived from E proteins. Notably, the m18 gene is here added to a growing list of mCMV E genes that encode antigenic peptides in spite of the E phase immune evasion strategies of the virus.

Evasion of cytotoxic T lymphocytes by murine cytomegalovirus

Murine cytomegalovirus causes lifelong infections with little pathology in normal host animals. Control of viral replication and prevention of pathology depend on both innate and adaptive immune mechanisms, and cytolytic T lymphocytes play a key role in this process. The virus encodes a number of genes which alter the normal assembly of class I major histocompatability complex proteins, and thus interfere with the ability of infected cells to present antigen to CD8(+)T cells. This review will examine what is known about these viral genes, and present some unanswered questions regarding the role of CTL evasion in the viral infectious cycle.

The immunogenicity of human and murine cytomegaloviruses

Cytomegaloviruses are strictly host-species-specific. During an aeon of co-evolution, virus and host have found an arrangement: the productive and cytopathogenic cycle of viral gene expression is held in check by the host's immune response. As a consequence, cytomegalovirus disease is restricted to the immunocompromised host. The virus has evolved strategies to avoid its elimination and eventually hides itself in a silent state, referred to as 'viral latency'. Redundant molecular mechanisms have been identified by which cytomegaloviruses interfere with antigen presentation pathways to 'evade' immune control. In the annual period covered by this review, the IE1 protein was revisited as an immunodominant antigen of human cytomegalovirus and the identification of a first antigenic early-phase peptide of murine cytomegalovirus that escapes viral immunosubversive mechanisms may initiate a period of research on the immune control of cytomegaloviruses 'beyond immune evasion'.

The putative natural killer decoy early gene m04 (gp34) of murine cytomegalovirus encodes an antigenic peptide recognized by protective antiviral CD8 T cells

Several early genes of murine cytomegalovirus (MCMV) encode proteins that mediate immune evasion by interference with the major histocompatibility complex class I (MHC-I) pathway of antigen presentation to cytolytic T lymphocytes (CTL). Specifically, the m152 gene product gp37/40 causes retention of MHC-I molecules in the endoplasmic reticulum (ER)-Golgi intermediate compartment. Lack of MHC-I on the cell surface should activate natural killer (NK) cells recognizing the "missing self." The retention, however, is counteracted by the m04 early gene product gp34, which binds to folded MHC-I molecules in the ER and directs the complex to the cell surface. It was thus speculated that gp34 might serve to silence NK cells and thereby complete the immune evasion of MCMV. In light of these current views, we provide here results demonstrating an in vivo role for gp34 in protective antiviral immunity. We have identified an antigenic nonapeptide derived from gp34 and presented by the MHC-I molecule D(d). Besides the immunodominant immediate-early nonapeptide consisting of IE1 amino acids 168-176 (IE1(168-176)), the early nonapeptide m04(243-251) is the second antigenic peptide described for MCMV. The primary immune response to MCMV generates significant m04-specific CD8 T-cell memory. Upon adoptive transfer into immunodeficient recipients, an m04-specific CTL line controls MCMV infection with an efficacy comparable to that of an IE1-specific CTL line. Thus, gp34 is the first noted early protein of MCMV that escapes viral immune evasion mechanisms. These data document that MCMV is held in check by a redundance of protective CD8 T cells recognizing antigenic peptides in different phases of viral gene expression.

Enhancer requirement for murine cytomegalovirus growth and genetic complementation by the human cytomegalovirus enhancer

The cytomegalovirus (CMV) enhancer is a highly complex regulatory region containing multiple elements that interact with a variety of host-encoded transcription factors. Many of these sequence elements are conserved among the different species strains of CMV, although the arrangement of the various elements and overall sequence composition of the CMV enhancers differ remarkably. To delineate the importance of this region to a productive infection and to explore the possibility of generating a murine CMV (MCMV) under the control of human CMV (HCMV) genetic elements, the MCMV enhancer was resected and replaced either with nonregulatory sequences or with paralogous sequences from HCMV. The effects of these various deletions and substitutions on viral growth in transfected or infected tissue-culture cells were evaluated. We found that mutations in MCMV that eliminate or substitute for the enhancer with nonregulatory sequences showed a severe deficiency in virus synthesis. This growth defect is effectively complemented by the homologous MCMV enhancer as well as the HCMV enhancer. In the latter case, the chimeric viruses (hybrid MCMV strains) containing the molecularly shuffled human enhancer exhibit infectious kinetics similar to that of parental wild-type and wild-type revertant MCMV. These results also show that open reading frames m124, m124.1, and m125 located within the enhancer region are nonessential for growth of MCMV in cells. Most importantly, we conclude that the enhancer of MCMV is required for optimal infection and that its diverged human counterpart can advantageously replace its role in promoting viral infectivity.

Control of murine cytomegalovirus in the lungs: relative but not absolute immunodominance of the immediate-early 1 nonapeptide during the antiviral cytolytic T-lymphocyte response in pulmonary infiltrates

The lungs are a major organ site of cytomegalovirus (CMV) infection, pathogenesis, and latency. Interstitial CMV pneumonia represents a critical manifestation of CMV disease, in particular in recipients of bone marrow transplantation (BMT). We have employed a murine model for studying the immune response to CMV in the lungs in the specific scenario of immune reconstitution after syngeneic BMT. Control of pulmonary infection was associated with a vigorous infiltration of the lungs, which was characterized by a preferential recruitment and massive expansion of the CD8 subset of alpha/beta T cells. The infiltrate provided a microenvironment in which the CD8 T cells differentiated into mature effector cells, that is, into functionally active cytolytic T lymphocytes (CTL). This gave us the opportunity for an ex vivo testing of the antigen specificities of CTL present at a relevant organ site of viral pathogenesis. The contribution of the previously identified immediate-early 1 (IE1) nonapeptide of murine CMV was evaluated by comparison with the CD3epsilon-redirected cytolytic activity used as a measure of the overall CTL response in the lungs. The IE1 peptide was detected by pulmonary CTL, but it accounted for a minor part of the response. Interestingly, no additional viral or virus-induced antigenic peptides were detectable among naturally processed peptides derived from infected lungs, even though infected fibroblasts were recognized in a major histocompatibility complex-restricted manner. We conclude that the antiviral pulmonary immune response is a collaborative function that involves many antigenic peptides, among which the IE1 peptide is immunodominant in a relative sense.

Interference with antigen processing by viruses

Viruses that establish persistent infections in their host, such as herpesviruses, adenoviruses or HIV, express proteins designed to pre-empt or evade recognition and elimination by MHC class I restricted CD8+ T lymphocytes. Notable discoveries during the annual period of review have demonstrated that, in principle, each single step within the MHC class I pathway of antigen processing and presentation is fair game for manipulation by viral functions. The viral factors that are natural inhibitors of this pathway have been instrumental for the elucidation of the distinct molecular mechanisms that are exploited by viruses. The viral stealth strategies that downregulate MHC class I protein surface expression may lead, however, to a higher susceptibility of virus-infected cells to natural killer cell activity. Strikingly, there is evidence that some viruses counteract increased natural killer cell recognition by expressing viral MHC class I homologues that function as surrogate inhibitors of natural killer cell activity.

A mouse cytomegalovirus glycoprotein retains MHC class I complexes in the ERGIC/cis-Golgi compartments

The principle by which mouse cytomegalovirus blocks antigen presentation in the MHC class I pathway was investigated. The responsible gene m152, encoding a type I transmembrane glycoprotein of 40 kDa, is a member of a gene family located in the right-hand terminal region of the 230 kb virus genome. Expression of m152 in murine and human cells arrested the export of mouse class I complexes from the ER-Golgi intermediate compartment/cis-Golgi compartment and inhibited lysis by cytotoxic T cells. The plasma membrane transport of human MHC class I molecules was not affected. The deletion of the cytoplasmic tail of gp40 did not lift its effect on class I molecule export, indicating that this protein differs in its functions from known immunosubversive viral gene products and represents a novel principle by which a herpesvirus shuts off MHC class I function.

Mechanisms of interference with the MHC class I-restricted pathway of antigen presentation by herpesviruses

Herpesviruses are an ancient, ubiquitous family of DNA viruses, most of which share a lifestyle of latently or persistently infecting a young host, and spreading to infect a new host a generation later. Most herpesviruses interfere with antigen presentation via the MHC class I-restricted pathway of antigen presentation, suggesting that impairment of the cytotoxic T lymphocyte response is necessary for the maintenance of this lifestyle. The diverse molecular mechanisms that have so far been discovered employed by Epstein-Barr virus, herpes simplex viruses, and human and murine cytomegaloviruses are described in this review.

Lymphocytes induce enhanced expression of HLA class I antigens on cytomegalovirus-infected syngeneic human endothelial cells

Human cytomegalovirus (HCMV) infection has been associated with enhanced expression of HLA antigens on the endothelium and with cellular infiltrates within the graft following human organ transplantation. We investigated the interactions between human cytomegalovirus-infected cultured endothelial cells and cocultured syngeneic as well as allogeneic lymphocytes. Our objective was to find out whether cocultured lymphocytes elicit HCMV-mediated immune responses. In this report we focus on the modified expression of HLA antigens on the surface membrane of human umbilical vein endothelial cells (HUVECs). Endothelial expression of HLA class I and II antigens was measured by means of flow cytometry. Cocultures of HCMV-infected HUVECs with unprimed autologous PBLs led to virus-specific lymphocyte response, resulting in enhanced expression of HLA class I on HUVECs. This effect was only observed when lymphocytes were added to HUVECs during the very early phase after virus inoculation and was due to the stimulation of the CD8+ T-cell subpopulation. The modification of endothelial HLA expression was not observed in transwell cocultures, indicating the importance of cellular contact between endothelial cells and lymphocytes to elicit this effect. We conclude that HCMV-infected endothelial cells may induce virus-specific responses of unprimed syngeneic lymphocytes that lead to upregulated HLA class I expression on the endothelium. This pathway might be of important relevance for graft rejection crises after transplantation.

Identification of the mouse cytomegalovirus genomic region affecting major histocompatibility complex class I molecule transport

Mouse cytomegalovirus (MCMV) functions expressed at the beginning of the early phase of the viral replication cycle interfere with the major histocompatibility complex (MHC) class I-restricted pathway of antigen presentation (M. J. Reddehase, M. R. Fibi, G. M. Keil, and U. H. Koszinowski, J. Virol. 60:1125-1129, 1986; M. Del Val, K. Münch, M. J. Reddehase, and U. H. Koszinowski, Cell 58:305-315, 1989). Nascent MHC class I heavy chains associate with beta 2-microglobulin and peptide, but the assembled trimolecular complex is retained in the endoplasmatic reticulum/cis-Golgi compartment (M. Del Val, H. Hengel, H. Häcker, U. Hartlaub, T. Ruppert, P. Lucin, and U. H. Koszinowski, J. Exp. Med. 176:729-738, 1992). To locate the responsible genomic region, the cytoplasmic retention of MHC class I molecules after injection of MCMV DNA was tested. The function was mapped to the HindIII E fragment. A recombinant MCMV deletion mutant delta MS94.5 lacking 15.8 kb in HindIII-E was constructed. Restoration of MHC class I molecule maturation and recognition of antigenic peptides by cytolytic T lymphocytes during the first hours of the early phase in mutant virus-infected cells proved the correct location to a 6.8-kb region in the HindIII E fragment. At later stages of the early phase, membrane-resident MHC class I molecules and cytolytic T lymphocyte recognition disappeared in delta MS94.5 mutant virus-infected cells. These results demonstrate that more than one early-gene function of MCMV affects the MHC class I pathway of antigen presentation. The redundant MHC class I-reactive functions target the transport of MHC class I molecules at different steps.

Peripheral blood mononuclear phagocytes mediate dissemination of murine cytomegalovirus

Cytomegalovirus is transmitted with blood and organs from seropositive individuals, although the particular leukocyte population harboring latent or persistent virus remains poorly characterized. Murine cytomegalovirus, tagged with the Escherichia coli lacZ gene, was used to identify cells in which virus replicates during acute infection of immunocompetent mice. Recombinant murine cytomegaloviruses, RM461, RM460, and RM427, were constructed to express beta-galactosidase under control of the human cytomegalovirus ie1/ie2 promoter/enhancer. The lacZ gene was inserted between the ie2 and sgg1 genes in RM461 and RM460, disrupting a 0.85-kb late transcript that was found to be dispensable for replication in cultured cells as well as for infection of mice. In BALB/c mice, lacZ-tagged and wild-type viruses exhibited a similar 50% lethal dose and all had the capacity to latently infect the spleen. Peripheral blood mononuclear phagocytes were the major infected leukocyte cell type, as demonstrated by the ability of infected cells to adhere to glass and to phagocytize latex beads; however, these cells did not exhibit typical monocyte markers. Plaque assay for virus and 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside (X-Gal) staining of frozen sections of organs from infected mice revealed that the major target organs included the spleen, adrenal glands, liver, and salivary glands, although tissues as diverse as brown fat and lungs were also involved. Individual blue-staining cells were readily identified in all infected tissues. These studies identified a mononuclear phagocyte, possibly a macrophage or dendritic cell precursor, as the vehicle of virus dissemination during acute infection, and demonstrate the utility of using lacZ-tagged murine cytomegalovirus for tropism, pathogenesis, and latency studies.

Structure and expression of murine cytomegalovirus immediate-early gene 2

The immediate-early gene ie2 of murine cytomegalovirus was characterized. The 1.75-kb ie2 transcript is spliced from three exons, of 78, 124, and 1,283 nucleotides, which are separated by introns of 1,245 and 364 nucleotides. An ATG codon located in the third exon leads into an open reading frame of 391 codons. Immediate-early expression of the predicted polypeptide was confirmed by immunoprecipitation of a 43-kDa protein by using an antiserum raised against a bacterial fusion protein. The predicted IE2 amino acid sequence has regions with similarity to amino acid sequences of members of the human cytomegalovirus US22 family.

Enhancement of interleukin-1 activity by murine cytomegalovirus infection of a macrophage cell line

The effect of murine cytomegalovirus (MCMV) infection on interleukin 1 (IL-1) secretion was assessed using the macrophage cell lines P388D1 and J774A.1. The former proved to be nonpermissive for MCMV in that infectious virus and viral immediate early protein (pp89) were not expressed in these cells. MCMV infection of the P388D1 cells had no effect on release of biologically active IL-1. In contrast, J774A.1 cells, which were semipermissive for virus replication and pp89 expression, secreted enhanced levels of IL-1 activity following infection. The enhancement was evident when infection either preceded or followed lipopolysaccharide stimulation of the macrophages. The relative proportion of IL-1 alpha and beta secreted from MCMV-infected cells was similar to noninfected controls. In addition, the bioactivity of intracellular IL-1 alpha escaping membranes of fixed cells was unaffected by virus infection. From these findings, we conclude that limited MCMV expression in the J774A.1 macrophage cell line enhances secretion of IL-1 alpha and beta bioactivity.

Presentation of CMV immediate-early antigen to cytolytic T lymphocytes is selectively prevented by viral genes expressed in the early phase

The regulation of antigen processing and presentation to MHC class I-restricted cytolytic T lymphocytes was studied in cells infected with murine cytomegalovirus. Recognition by cytolytic T lymphocytes of the phosphoprotein pp89, the immunodominant viral antigen expressed in the immediate-early phase of infection, was selectively prevented during the subsequent expression of viral early genes. The surface expression of MHC class I glycoproteins and their capacity to present externally added pp89-derived antigenic peptides were not affected. Because recognition of several other antigens occurred during the early phase, a general failure in processing and presentation was excluded. Since neither rate of synthesis, amount, stability, nor nuclear transport of pp89 was modified, the failure in recognition indicates a selective interference with pp89 antigen processing and presentation.

Regulated expression of early and late RNAs and proteins from the human cytomegalovirus immediate-early gene region

Expression of RNA and protein from the human cytomegalovirus immediate-early (IE) gene region (map units 0.732 to 0.751) was analyzed at early and late times after infection. The level of RNA present at late times (48 to 72 h after infection) was significantly higher than that present at IE times (5 h after infection). The profile of IE RNA in the cytoplasm of infected cells was different from that previously reported on polysomes (R. M. Stenberg, P. R. Witte, and M. F. Stinski, J. Virol. 56:665-675, 1985). The data indicate that the 1.95-kilobase (kb) major IE region 1 mRNA, which codes for the 72-kilodalton (kDa) protein, and the 1.7-kb IE region 2 (IE2) spliced mRNA, which codes for the IE2 55-kDa protein, may be preferentially associated with polysomes. However, the IE2 2.2-kb unspliced mRNA, which codes for an 86-kDa protein, may be preferentially excluded. This RNA was abundant in the cytoplasm under IE conditions but was not present on polysomes in significant quantities. This indicates that IE gene products may be involved in translational control of cytomegalovirus RNA. At late times, new transcription takes place within region 2. A 1.5-kb RNA is transcribed from a late promoter in region 2 that apparently does not function in cells infected with DNA-negative mutant ts66. These results demonstrate that the IE gene region is transcribed throughout infection and that multiple levels of regulation exist.

Subcellular localization of the major immediate early protein (IE1) of human cytomegalovirus at early times after infection

The immediate early (IE) 1 protein of human cytomegalovirus was investigated by subcellular fractionation and immunocolloidal gold electron microscopy. The IE1 protein is phosphorylated and detected on intracytoplasmic membranes at early times after infection. An electron microscopy study confirmed that the IE1 antigen was an intracytoplasmic membrane-associated protein at early times after infection. The IE1 antigen as well as the IE2 antigen was also detected in the nucleus. However, the IE1 antigen was not detected on the plasma membrane surface of infected cells. The role of this viral protein in the immune response is discussed.

Sequence homology and immunologic cross-reactivity of human cytomegalovirus with HLA-DR beta chain: a means for graft rejection and immunosuppression

A peptide (Leu-Gly-Arg-Pro-Asp-Glu-Asp-Ser-Ser-Ser-Ser-Ser-Ser-Ser-Cys) that was identical to residues 82 through 96 of a predicted protein of 208 amino acids from the immediate-early region (IE-2) nucleic acid sequence of human cytomegalovirus was chemically synthesized. By computer analysis, the first five amino acids of this peptide showed sequence homology to the beta chain of the human histocompatibility complex HLA-DR. The homologous amino acids, 53 through 57, were located in a region that is conserved between the human DR beta chain and the beta chain of the H-2 class II histocompatibility antigen for mice. The shared region between the IE-2 protein and DR beta chain were similar in both hydrophilicity and predicted beta-turn potential. The IE-2 viral peptide induced antibodies that specifically recognized the human DR beta chain. These observations describe a protein encoded by the IE-2 region of human cytomegalovirus that contains sequence homology and shows immunologic cross-reactivity with a conserved domain of HLA-DR and suggest a mechanism to explain how human cytomegalovirus infection contributes to graft rejection after transplantation.

CD8-positive T lymphocytes specific for murine cytomegalovirus immediate-early antigens mediate protective immunity

We have shown in a murine model system for acute, lethal cytomegalovirus (CMV) disease in the immunocompromised natural host that control of virus multiplication in tissues, protection from virus-caused tissue destruction, and survival are mediated by virus-specific CD8+ CD4-T lymphocytes. Protection from a lethal course of disease did not result in a rapid establishment of virus latency, but led to a long-lasting, persistent state of infection. The CD8- CD4+ subset of T lymphocytes was not effective by itself in controlling murine CMV (MCMV) multiplication in tissue or essential for the protective function of the CD8+ CD4- effector cells. The antiviral efficacy of the purified CD8+ CD4- subset was not impaired by preincubation with fibroblasts that presented viral structural antigens, but was significantly reduced after depletion of effector cells specific for the nonstructural immediate-early antigens of MCMV, which are specified by the first among a multitude of viral genes expressed during MCMV replication in permissive cells. Thus, MCMV disease provides the first example of a role for nonstructural herpesvirus immediate-early antigens in protective immunity.

Cytolytic T lymphocyte recognition of the murine cytomegalovirus nonstructural immediate-early protein pp89 expressed by recombinant vaccinia virus

The murine immediate-early (IE) protein pp89 is a nonstructural virus-encoded phosphoprotein residing in the nucleus of infected cells, where it acts as transcriptional activator. Frequency analysis has shown that in BALB/c mice the majority of virus-specific CTL recognize IE antigens. The present study was performed to assess whether pp89 causes membrane antigen expression detected by IE-specific CTL. Site-directed mutagenesis has been used to delete the introns from gene ieI, encoding pp89, for subsequent integration of the continuous coding sequence into the vaccinia virus genome. After infection with the vaccinia recombinant, the authentic pp89 was expressed in cells that became susceptible to lysis by an IE-specific CTL clone. Priming of mice with the vaccinia recombinant sensitized polyclonal CTL that recognized MCMV-infected cells and transfected cells expressing pp89. Thus, a herpesviral IE polypeptide with essential function in viral transcriptional regulation can also serve as a dominant antigen for the specific CTL response of the host.

Stable expression of clonal specificity in murine cytomegalovirus-specific large granular lymphoblast lines propagated long-term in recombinant interleukin 2

The somatic stability of cloned long-term cytolytic T lymphocyte lines (CTLL) specific for antigens encoded by murine cytomegalovirus (MCMV) was tracked for more than two years of continuous in vitro propagation. Clone S1 retained its original specificity for a structural (S) antigen of MCMV for about eight months in the presence of antigen and interleukin 2 (IL 2), but not in IL 2 alone. In the following months, however, in spite of the continued presence of antigen, clonal variants developed that displayed distinct patterns of target cell recognition, including loss of the original specificity and acquisition of exclusive specificity for the natural killer target cell YAC-1. On the other hand, large granular lymphoblast (LGL) lines, line IE1-IL and a series of sublines thereof, could be established that stably expressed Ld-restricted specificity for a viral nonstructural immediate-early (IE) antigen more than two years after withdrawal of antigen and feeder cells when propagated in the presence of pure recombinant human IL 2. The finding that the presence of antigen was not essential for the stability of clone IE1-derived CTLL indicates that maintenance of specificity in LGL lines is not a result of antigen-mediated selection, but reflects an intrinsic property.

A nonstructural polypeptide encoded by immediate-early transcription unit 1 of murine cytomegalovirus is recognized by cytolytic T lymphocytes

We have constructed target cells by cotransfection of the MHC gene Ld and fragments of murine cytomegalovirus (MCMV) DNA coding for nonstructural immediate-early (IE) proteins. Transfectants were tested by using CTL clone IE1 with specificity for an IE epitope presented in association with Ld. Data show that clone IE1 recognizes a product of the ie1 transcription unit of MCMV, and that its specificity is shared by approximately 25% of polyclonal IE-specific CTL. The results provide the first definite evidence that expression of a herpes virus IE gene encoding a regulatory protein gives rise to antigen expression detectable by specific CTL.

Host immune response to cytomegalovirus: products of transfected viral immediate-early genes are recognized by cloned cytolytic T lymphocytes

To confirm that immediate-early (IE) genes of murine cytomegalovirus (MCMV) give rise to antigens recognized by specific cytolytic T lymphocytes (CTL), a 10.8-kilobase fragment of MCMV DNA which is abundantly transcribed at IE times was transfected into L cells expressing the Ld class I major histocompatibility glycoprotein. The viral genome fragment contains sequences of the three IE transcription units of MCMV: ie1, ie2, and ie3. In the transfected cell lines, only the predominant 2.75-kilobase transcript of ie1 and its translation product pp89 could be detected. The transfectants were analyzed for membrane expression of an IE antigen by employing clone IE1, an IE-specific CTL clone, as the probe. Only cells that expressed both the MCMV IE gene(s) and the Ld gene were recognized by the CTL clone.

Sequence and structural organization of murine cytomegalovirus immediate-early gene 1

In murine cytomegalovirus, abundant immediate-early transcription originates from 0.769 to 0.815 map units of the genome. This region contains the immediate-early gene (gene ieI) which encodes pp89, a phosphoprotein active in transcriptional regulation. In this paper we report on the precise location, structural organization, and sequence of gene ieI. The predominant ieI transcript, a 2.75-kilobase mRNA, is generated by splicing and composed of four exons. The precise termini of the 2.75-kilobase mRNA and the positions of the exons were determined by nuclease digestion experiments with either 5' or 3' end-labeled DNA fragments or in vitro transcribed cRNA probes. Exons of 300, 111, 191, and 1,703 nucleotides are separated by introns of 825, 95, and 122 nucleotides. The first AUG is located in the second exon of 111 nucleotides, and a single open reading frame of 1,785 nucleotides predicts a protein of 595 amino acids with a calculated molecular weight of 66,713. The N-terminal region of the protein contains sequences similar to a consensus sequence of histone 2B proteins. The regulatory function of pp89 and the role of this protein as an immunodominant antigen are discussed in relation to the amino acid sequence.