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

CXCR3-dependent recruitment of antigen-specific T lymphocytes to the liver during murine cytomegalovirus infection

Innate inflammatory events promoting antiviral defense in the liver against murine cytomegalovirus (MCMV) infection have been characterized. However, the mechanisms that regulate the selective recruitment of inflammatory T lymphocytes to the liver during MCMV infection have not been defined. The studies presented here demonstrate the expression of monokine induced by gamma interferon (IFN-gamma; Mig/CXCL9) and IFN-gamma-inducible protein 10 (IP-10/CXCL10) in liver leukocytes and correlate their production with the infiltration of MCMV-specific CD8 T cells into the liver. Antibody-mediated neutralization of CXCL9 and CXCL10 and studies using mice deficient in CXCR3, the primary known receptor for these chemokines, revealed that CXCR3-dependent mechanisms promote the infiltration of virus-specific CD8 T cells into the liver during acute infection with MCMV. Furthermore, CXCR3 functions augmented the hepatic accumulation of CD8 T-cell IFN-gamma responses to MCMV. Evaluation of protective functions demonstrated enhanced pathology that overlapped with transient increases in virus titers in CXCR3-deficient mice. However, ultimate viral clearance and survival were not compromised. Thus, CXCR3-mediated signals support the accumulation of MCMV-specific CD8 T cells that contribute to, but are not exclusively required for, protective responses in a virus-infected tissue site.

Genome-wide analysis reveals a highly diverse CD8 T cell response to murine cytomegalovirus

Human CMV establishes a lifelong latent infection in the majority of people worldwide. Although most infections are asymptomatic, immunocompetent hosts devote an extraordinary amount of immune resources to virus control. To increase our understanding of CMV immunobiology in an animal model, we used a genomic approach to comprehensively map the C57BL/6 CD8 T cell response to murine CMV (MCMV). Responses to 27 viral proteins were detectable directly ex vivo, the most diverse CD8 T cell response yet described within an individual animal. Twenty-four peptide epitopes were mapped from 18 Ags, which together account for most of the MCMV-specific response. Most Ags were from genes expressed at early times, after viral genes that interfere with Ag presentation are expressed, consistent with the hypothesis that the CD8 T cell response to MCMV is largely driven by cross-presented Ag. Titration of peptide epitopes in a direct ex vivo intracellular cytokine staining assay revealed a wide range of functional avidities, with no obvious correlation between functional avidity and the strength of the response. The immunodominance hierarchy varied only slightly between mice and between experiments. However, H-2(b)-expressing mice with different genetic backgrounds responded preferentially to different epitopes, indicating that non-MHC-encoded factors contribute to immunodominance in the CD8 T cell response to MCMV.

Development of a vaccine against murine cytomegalovirus (MCMV), consisting of plasmid DNA and formalin-inactivated MCMV, that provides long-term, complete protection against viral replication

We previously demonstrated that immunization of mice with plasmid DNAs (pDNAs) expressing the murine cytomegalovirus (MCMV) genes IE1-pp89 and M84 provided synergistic protection against sublethal viral challenge, while immunization with plasmids expressing putative virion proteins provided no or inconsistent protection. In this report, we sought to augment protection by increasing the breadth of the immune response. We identified another MCMV gene (m04 encoding gp34) that provided strong and consistent protection against viral replication in the spleen. We also found that immunization with a DNA pool containing 10 MCMV genes that individually were nonprotective elicited reproducible protection against low to intermediate doses of challenge virus. Moreover, inclusion of these plasmids into a mixture with gp34, pp89, and M84 DNAs provided even greater protection than did coimmunization with pp89 and M84. The highest level of protection was achieved by immunization of mice with the pool of 13 pDNAs, followed by formalin-inactivated MCMV (FI-MCMV). Immunization with FI-MCMV elicited neutralizing antibodies against salivary gland-derived MCMV, and of greatest importance, mice immunized with both the combined pDNA pool and FI-MCMV had undetectable levels of virus in the spleen and salivary glands after challenge. Intracellular cytokine staining of splenocytes from pDNA- and FI-MCMV-immunized mice showed that pDNA immunization elicited high levels of pp89- and M83-specific CD8(+) T cells, whereas both pDNA and FI-MCMV immunizations generated strong CD8(+)-T-cell responses against virion-associated antigens. Taken together, these results show that immunization with pDNA and inactivated virus provides strong antibody and cell-mediated immunity against CMV infection.

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 pathogenicity of cytomegalovirus

Human cytomegalovirus is ubiquitous, yet causes little illness in immunocompetent individuals. Disease is evident in immunodeficient groups such as neonates, transplant recipients and AIDS patients either following a primary infection or reactivation of a latent infection. Little is known of the mechanisms underlying the pathogenicity of the virus. The recent determination of the nucleotide sequence of both human cytomegalovirus (strain AD169) and murine cytomegalovirus (murine cytomegalovirus strain Smith) has allowed an analysis of the biological importance of several virus genes. Studies with human cytomegalovirus have indicated that many viral genes are non-essential for replication in vitro which are thus assumed to be important in the pathogenesis of the virus. This is being examined in the murine model where the role of the gene and its product in disease can be directly examined in vivo using viral mutants in which the relevant gene has been interrupted or deleted. Current information on the role of cytomegalovirus genes in tissue tropism, immune evasion, latency, reactivation from latency and damage is described.

Changing patterns of dominance in the CD8+ T cell response during acute and persistent murine gamma-herpesvirus infection

The murine gamma-herpesvirus MHV-68 causes an acute, transient pneumonitis, followed by an infectious mononucleosis (IM)-like illness with splenomegaly, widespread latent infection of B lymphocytes and an expansion of Vbeta4+ CD8+ T cells. CD8+ T cells specific for an H-2Db-restricted epitope were prominent during the acute respiratory infection, but their prevalence declined rapidly during the mononucleosis. In contrast, CD8+ T cells specific for an H-2Kb-restricted epitope, apparently expressed by virus-infected B lymphocytes, were most numerous during the mononucleosis illness and were maintained at relatively high frequencies thereafter. The prevalence of all peptide-specific CD8+ T cells decreased during the expansion of the Vbeta4+ CD8+ population, which did not recognize any peptide epitopes identified and was apparent also in an MHC class I-deficient environment. The CD8+ T cell population recognizing productively infected epithelial cells thus differed substantially from that responding during the IM illness.

Apoptosis mediated by Fas but not tumor necrosis factor receptor 1 prevents chronic disease in mice infected with murine cytomegalovirus

The role of Fas- and TNF-receptor 1 (TNF-R1)-mediated apoptosis in the clearance of virally infected cells and in the regulation of the immune response was analyzed after murine cytomegalovirus (MCMV) infection of C57BL/6 (B6)-+/+ mice, Fas-mutant B6-lpr/lpr mice, TNF-R1 knockout B6-tnfr0/0 mice, and double-deficient B6-tnfr0/0 lpr/lpr mice. There was approximately equivalent clearance of MCMV in B6-+/+, B6-tnfr0/0, and B6-lpr/lpr mice, and by day 28 no infectious virus could be detected in the liver, kidney, lung, or peritoneal exudate. However, delayed virus clearance was observed in B6-tnfr0/0 lpr/lpr mice. An acute inflammatory response occurred in the liver, lung, and kidney of all mice, which was most severe 7 d after MCMV infection, but resolved by day 28 in B6-+/+ and B6-tnfr0/0 mice, but not in B6-lpr/lpr or B6-tnfr0/0 lpr/lpr mice. These results indicate that apoptosis mediated by either Fas or TNF-R1 is sufficient for rapid clearance of the virus. However, apoptosis induced by Fas, but not TNF-R1, is required for the downmodulation of the immune response to the virus and prevention of a chronic inflammatory reaction.

Mucosal and parenteral vaccination against acute and latent murine cytomegalovirus (MCMV) infection by using an attenuated MCMV mutant

We used a live attenuated murine cytomegalovirus (MCMV) mutant to analyze mechanisms of vaccination against acute and latent CMV infection. We selected MCMV mutant RV7 as a vaccine candidate since this virus grows well in tissue culture but is profoundly attenuated for growth in normal and severe combined immunodeficient (SCID) mice (V. J. Cavanaugh et al., J. Virol. 70:1365-1374, 1996). BALB/c mice were immunized twice (0 and 14 days) subcutaneously (s.c.) with tissue culture-passaged RV7 and then challenged with salivary gland-passaged wild-type MCMV (sgMCMV) intraperitoneally (i.p.) on day 28. RV7 vaccination protected mice against challenge with 10(5) PFU of sgMCMV, a dose that killed 100% of mock-vaccinated mice. RV7 vaccination reduced MCMV replication 100- to 500-fold in the spleen between 1 and 8 days after challenge. We used the capacity to control replication of MCMV in the spleen 4 days after challenge as a surrogate for protection. Protection was antigen specific and required both live RV7 and antigen-specific lymphocytes. Interestingly, RV7 was effective when administered s.c., i.p., perorally, intranasally, and intragastrically, demonstrating that attenuated CMV applied to mucosal surfaces can elicit protection against parenteral virus challenge. B cells and immunoglobulin G were not essential for RV7-induced immunity since B-cell-deficient mice were effectively vaccinated by RV7. CD8 T cells, but not CD4 T cells, were critical for RV7-induced protection. Depletion of CD8 T cells by passive transfer of monoclonal anti-CD8 (but not anti-CD4) antibody abrogated RV7-mediated protection, and RV7 vaccination was less efficient in CD8 T-cell-deficient mice with a targeted mutation in the beta2-microglobulin gene. Although gamma interferon is important for innate resistance to MCMV, it was not essential for RV7 vaccination since gamma interferon receptor-deficient mice were protected by RV7 vaccination. Establishment of and/or reactivation from latency by sgMCMV was decreased by RV7 vaccination, as measured by diminished reactivation of MCMV from splenic explants. We found no evidence for establishment of splenic latency by RV7 after s.c. vaccination. We conclude that RV7 administered through both systemic and mucosal routes is an effective vaccine against MCMV infection. It may be possible to design human CMV vaccines with similar properties.

A mouse cytomegalovirus glycoprotein, gp34, forms a complex with folded class I MHC molecules in the ER which is not retained but is transported to the cell surface

Murine cytomegalovirus (MCMV) interferes with antigen presentation by means of retaining major histocompatibility complex (MHC) class I molecules in the endoplasmic reticulum (ER). Here we identify and characterize an MCMV-encoded glycoprotein, gp34, which tightly associates with properly conformed MHC class I molecules in the ER. Gp34 is synthesized in large quantities during MCMV infection and it leaves the ER only in association with MHC class I complexes. Many but not all class I molecules are retained in the ER during the early phase of MCMV infection, and we observe an inverse correlation between amounts of gp34 synthesized during the course of infection and class I retention. An MCMV deletion mutant lacking several genes, including the gene encoding gp34, shows increased class I retention. Thus, MCMV gp34 may counteract class I retention, perhaps to decrease susceptibility of infected cells to recognition by natural killer cells.

DNA immunization confers protection against murine cytomegalovirus infection

The murine cytomegalovirus (MCMV) immediate-early gene 1 (IE1) encodes an 89-kDa phosphoprotein (pp89) which plays a key role in protecting BALB/c mice against the lethal effects of the MCMV infection. In this report, we have addressed the question of whether "naked DNA" vaccination with a eukaryotic expression vector (pcDNA-89) that contains the MCMV IE1 gene driven by a strong enhancer/promoter can confer protection. BALB/c mice were immunized intradermally with pcDNA-89 or with the plasmid backbone pcDNAI/Amp (pcDNA) and then challenged 2 weeks later with either a lethal or a sublethal intraperitoneal dose of the K181 strain of MCMV. Variable results were obtained for the individual experiments in which mice received a lethal challenge. In four separate trials, an average of 63% of the mice immunized with pcDNA-89 survived, compared with 18% of the mice immunized with pcDNA. However, in two other trials there was no specific protection. The results of experiments in which mice were injected with a sublethal dose of MCMV were more consistent, and significant decreases in viral titer in the spleen and salivary glands of pcDNA-89-immunized mice were observed, relative to controls. At the time of peak viral replication, titers in the spleens of immunized mice were reduced 18- to >63-fold, while those in the salivary gland were reduced approximately 24- to 48-fold. Although DNA immunization elicited only a low level of seroconversion in these mice, by 7 weeks postimmunization the mice had generated a cytotoxic T-lymphocyte response against pp89. These results suggest that DNA vaccination with selected CMV genes may provide a safe and efficient means of immunizing against CMV disease.

Identification, analysis, and evolutionary relationships of the putative murine cytomegalovirus homologs of the human cytomegalovirus UL82 (pp71) and UL83 (pp65) matrix phosphoproteins

We have identified three open reading frames (ORFs) in murine cytomegalovirus (MCMV), designated M82, M83, and M84, which likely encode homologs of the human cytomegalovirus (HCMV) UL82 and UL83 matrix phosphoproteins. These ORFs, in the HindIII C fragment of MCMV, are colinear with the UL82, UL83, and UL84 ORFs of HCMV. M82 encodes a 598-amino-acid (aa) protein with homology to UL82, M83 encodes an 809-aa protein with homology to UL82 and UL83, and M84 encodes a 587-aa protein with homology to UL83 and UL84. Analysis of transcription by Northern (RNA) blotting indicated that the M82 and M83 ORFs are transcribed as 2.2- and 5-kb mRNAs, respectively, at 24 to 48 h postinfection (p.i.), while M84 is transcribed as a 6.9-kb mRNA only at 8 h p.i. All transcripts appear to terminate at the same position 3' of the M82 ORF. Of the products of the three ORFs, only M83 is strongly recognized by hyperimmune mouse serum. The M83 protein is a virion-associated phosphoprotein with an apparent molecular mass of 125 kDa. In MCMV-infected cells, it is detectable by Western blotting (immunoblotting) only at 48 h p.i. in the absence of phosphonoacetic acid, consistent with late gene expression. The M83 ORF is also expressed at high levels in cells infected by a recombinant vaccinia virus and yields a protein which is serologically cross-reactive and comigrates with the authentic MCMV protein in sodium dodecyl sulfate-polyacrylamide gel electrophoresis.

Fine specificity of cellular immune responses in humans to human cytomegalovirus immediate-early 1 protein

Cell-mediated immunity is important in maintaining the virus-host equilibrium in persistent human cytomegalovirus (HCMV) infection. The HCMV 72-kDa major immediate early 1 protein (IE1) is a target for CD8+ cytotoxic T cells in humans, as is the equivalent 89-kDa protein in mouse. Less is known about responses against this protein by CD4+ T cells, which may be important as direct effector cells or helper cells for antibody and CD8+ responses. Proliferative-T-cell responses to HCMV IE1 were studied in normal seropositive subjects. Peripheral blood mononuclear cells from 85% of seropositive subjects proliferated in response to HCMV from infected fibroblasts, and of these, 73% responded to recombinant baculovirus IE1. Responding cells were predominantly CD3+ CD4+. IE1 antigen preparations, including baculovirus recombinant protein, transfected rat cell nuclei, and synthetic peptides, induced IE1-specific T-cell lines which cross-reacted between the preparations. The fine specificity of these IE1-specific T-cell lines was studied by using overlapping synthetic peptides encompassing the entire sequence of the IE1 protein. The regions of the IE1 molecule recognized were identified and these varied between individuals, possibly reflecting differences in major histocompatibility complex (MHC) class II haplotype. In one subject, the peptide specificities of proliferative and MHC class I-restricted cytotoxic determinants on IE1 were spatially distinct. Thus, no single immunodominant T-cell determinant within HCMV IE1 was identified, suggesting that multiple peptides or a region of the 72-kDa IE1 protein would be required to induce specific T-cell responses in humans.

Negative and positive selection of antigen-specific cytotoxic T lymphocytes affected by the alpha 3 domain of MHC I molecules

The alpha 1 and alpha 2 domains of major histocompatibility complex (MHC) class I molecules function in the binding and presentation of foreign peptides to the T-cell antigen receptor and control both negative and positive selection of the T-cell repertoire. Although the alpha 3 domain of class I is not involved in peptide binding, it does interact with the T-cell accessory molecule, CD8. CD8 is important in the selection of T cells as anti-CD8 antibody injected into perinatal mice interferes with this process. We previously used a hybrid class I molecule with the alpha 1/alpha 2 domains from Ld and the alpha 3 domain from Q7b and showed that this molecule binds an Ld-restricted peptide but does not interact with CD8-dependent cytotoxic T lymphocytes. Expression of this molecule in transgenic mice fails to negatively select a subpopulation of anti-Ld cytotoxic T lymphocytes. In addition, positive selection of virus-specific Ld-restricted cytotoxic T lymphocytes does not occur. We conclude that besides the alpha 1/alpha 2 domains of class I, the alpha 3 domain plays an important part in both positive and negative selection of antigen-specific cells.

Human cytomegalovirus strain Towne pp65 gene: nucleotide sequence and expression in Escherichia coli

The human cytomegalovirus (HCMV) encodes a 65-kDa tegument protein (pp65), which has been reported to be a target of immune response during natural infection. We have cloned and sequenced the gene encoding pp65 of HCMV Towne strain (pp65Towne), and have expressed this gene in Escherichia coli in order to study certain antigenic and structural properties of this polypeptide. The pp65Towne gene had a 99% nucleotide similarity and 99.7% amino acid similarity to pp65 of HCMV AD169 strain (pp65AD169). However, unlike the pp65AD169 gene, the pp65Towne gene was found to be incapable of undergoing RNA splicing due to a base substitution in the critical 3' splice-acceptor site. Insertion of this protein coding sequence into the bacterial expression plasmids enabled synthesis in E. coli of an immunoreactive pp65-related polypeptide. The recombinant pp65 (rpp65) reacted strongly in immunoblot analysis with pp65-specific murine and human monoclonal antibodies as well as with anti-pp65 rabbit antiserum. In immunoblot analysis, the reactivity of rpp65 with a panel of human HCMV-immune sera indicated that some sera were reactive while other HCMV seropositive sera were nonreactive, a finding similar to that for native pp65.

Anti-adenovirus type 5 cytotoxic T lymphocytes: immunodominant epitopes are encoded by the E1A gene

Virus specific, major histocompatibility complex-restricted, cytotoxic T lymphocytes (CTL) generated in Fischer strain rats infected with human adenovirus type 5 (Ad5) were found to recognize antigenic determinants encoded within the Ad5 early region 1A (E1A) gene. Preliminary mapping studies suggest that the E1A CTL epitopes are encoded within the regions between bp 625 to 810 and 916 to 974 in the first exon of this gene. These epitope-coding regions occur within subregions of E1A that are conserved functionally, and to some extent structurally (approximately 50% sequence homology), among adenoviruses of different groups. Nevertheless, Ad5-specific CTL lysed only targets infected with adenoviruses of the same group (group C; e.g., Ad2) and not targets infected with adenoviruses of different groups (groups A, B, and E). These results suggest that virus-specific CTL may limit adenoviral dissemination by destroying virus-infected cells at an early stage in the viral replicative cycle, during E1A gene expression. Expression of other adenovirus genes does not appear to be required to target infected cells for elimination by CTL.

Efficacious control of cytomegalovirus infection after long-term depletion of CD8+ T lymphocytes

Although the relative contribution of different immune effector functions to clearing tissues of cytomegalovirus is controversial, the contribution of CD8+ T lymphocytes has generally been accepted as essential. In this report, we show that under certain conditions the CD8+ T-lymphocyte subset can be dispensable for clearance of cytomegalovirus. Mice depleted of the CD8+ T-lymphocyte subset eliminated infectious virus with a clearance kinetics similar to that of normal mice. Adoptive transfer studies revealed that the limitation of virus spread required the cooperation between the CD4+ subset and other cells. Comparison between protective functions generated in fully immunocompetent and in CD8- mice demonstrated that elimination of the CD8+ subset before infection altered the quality of the antiviral immune response. The compensatory protective activity gained by CD4+ cells in CD8- mice was absent in normal mice recovering from virus infection.

Structural analysis of a 64-kDa major structural protein of human cytomegalovirus (Towne): identification of a phosphorylation site and comparison to pp65 of HCMV (AD169)

The major 64-kDa structural protein of human cytomegalovirus (pp64) was isolated from a guanidinium chloride extract of the virions and dense bodies of HCMV (Towne) by reverse-phase HPLC. Purified pp64 was reduced and alkylated followed by digestion with trypsin. The molecular mass of each of the tryptic peptides was determined by fast atom bombardment/mass spectrometry and compared with the predicted molecular mass of the fragments deduced from the corresponding DNA-derived peptide sequence of pp65 from HCMV (AD169). Microsequence analysis was employed to confirm selected peptides. Results of protein sequence analysis of pp64 from HCMV (Towne) are in complete agreement with the DNA-derived protein sequence of pp65 predicted for HCMV (AD169) with the following exceptions and modifications. The protein isolated from HCMV (Towne) was found to contain an Ala at position 448 instead of Ser448 reported for the protein from HCMV (AD169). We also identified Ser472 as a site of phosphorylation in pp64 from HCMV (Towne). Finally, on the basis of the sequence of HCMV (AD169) DNA fragment encoding the matrix protein and on S1 nuclease protection analysis, it has been predicted that one version of the matrix protein (possibly the lower matrix protein, Mr 65K) is encoded by an mRNA that is formed through splicing of a short intron. However, we have obtained peptides that contain sequences spanning through the splice-junction region, suggesting that in HCMV (Towne), the matrix protein is encoded by an unspliced message.

Mechanism of escape of endogenous murine leukemia virus emv-14 from recognition by anti-AKR/Gross virus cytolytic T lymphocytes

It was previously shown that spleen cells from endogenous ecotropic murine leukemia virus emv-14+ AKXL-5 mice fail to stimulate an anti-AKR/Gross virus cytolytic T-lymphocyte (CTL) response in a mixed lymphocyte culture with primed C57BL/6 responder spleen cells, whereas spleen cells from AKXL strains carrying the very similar emv-11 provirus do stimulate a response (Green and Graziano, Immunogenetics 23:106-110, 1986). We wished to determine whether the lack of response with AKXL-5 spleen cells was at the level of recognition between effector cell and target cell and whether the relevant mutation was within the emv-14 provirus. It is shown here that EMV-negative SC-1 fibroblast cells transfected with the major histocompatibility complex class I Kb gene and infected with virus isolated from the AKXL-5 strain (SC.Kb/5 cells) were not lysed by H-2b-restricted anti-AKR/Gross virus CTL. SC.Kb cells infected with virus isolated from emv-11+ strains, however, were efficiently lysed by anti-AKR/Gross virus CTL, indicating that there is nothing intrinsic to EMV-infected SC.Kb cells that would prevent them from being recognized and lysed efficiently by anti-AKR/Gross virus CTL. Analysis of virus expression for the infected SC.Kb cells by XC plaque assay and by flow cytometry indicated that emv-14 virus expression for SC.Kb/5 cells was not significantly different from that for emv-11-containing SC.Kb/9 or SC.Kb/21 cells. These data show that the mutation responsible for the lack of CTL recognition and lysis is at the level of recognition between target cell and effector cell. Furthermore, these data strongly suggest that the mutation is within the emv-14 genome. Flow cytometry experiments with monoclonal antibodies against a number of viral determinants indicated that there was no gross mutation detectable in the viral determinants analyzed. The data suggest that the relevant mutation may be a point mutation or a small insertion or deletion within a coding sequence that is critical for CTL recognition.

C-terminal domain of the adenovirus E1A oncogene product is required for induction of cytotoxic T lymphocytes and tumor-specific transplantation immunity

Adenovirus genes required for the elicitation of adenovirus group C-specific cytolytic T lymphocytes (CTLs) and for the induction of adenovirus-specific transplantation antigen (TSTA) were identified by immunization with a library of adenovirus mutants. The group C Ad-specific CTL response was elicited by immunization with wild-type adenovirus type 5 (Ad5) or with recombinant adenoviruses containing Ad5 E1A gene. The specific CTL response was also elicited by Ad5 virus constructs which express only the 12 S or 13 S E1A early mRNA, but not with viruses unable to express E1A protein sequences normally encoded by the E1A early messages. The induction of transplantation immunity against tumorigenic Ad-transformed cells was studied next. The product encoded by either 13 S and 12 S E1A mRNA alone was sufficient for strong TSTA activity. A series of viruses with mutations within the first exon of the E1A message also induced strong TSTA, while Ad5 mutants with lesions within the second exon failed to induce syngraft immunity. These results provide strong evidence that amino acid sequence encoded by the second exon of the Ad5 E1A message is required, either directly or indirectly, for the induction of both Ad-specific CTL and Ad TSTA.

An antigenic peptide of the HIV-1 NEF protein recognized by cytotoxic T lymphocytes of seropositive individuals in association with different HLA-B molecules

By using target cells that expressed only the regulatory NEF protein of the human immunodeficiency virus (HIV), we were able to identify specific cytolytic T cells (CTL) from most of the seropositive patients tested. This NEF-specific cytolytic activity was mediated by CD8+ lymphocytes. In two different donors, we identified a unique peptide in the NEF protein that could be recognized in association with two different HLA class I molecules. The definition of such a peptide should prove useful in evaluating vaccine strategies.

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.

Brief report: killer cell defect and persistent immunological abnormalities in two patients with chronic active Epstein-Barr virus infection

Two members of a family have manifested a syndrome of chronic active Epstein-Barr virus (EBV) infection. A father and his daughter suffered prolonged or recurrent mononucleosis, with splenomegaly, anemia, and intermittent fever; persistent immunological abnormalities included defective natural killer (NK) cytotoxicity, inverted CD4/CD8 ratios, hyper IgG1, high EBV viral capsid antigen (VCA) and early antigen (EA) antibodies, and low or undetectable EBV nuclear antigen (EBNA) antibody titers. The EBV seronegative member of the family was free of these abnormalities. However, NK activity in the seronegative individual was low-normal and its EBV-specific antibody-dependent K-cell cytotoxicity (EBV-ADCC) was abnormally low, suggesting that this K-NK cell defect may be primary. The father, who suffered from the syndrome for more than 15 years, lacked (or lost) antibodies to EBV-envelope and infected cell membranes, such as antibody-dependent cellular cytotoxicity (ADCC), neutralizing (NT), and gp 350/220 antibodies. Slow improvement over a period of years was heralded by rising NK cytotoxicity.

Site-restricted persistent cytomegalovirus infection after selective long-term depletion of CD4+ T lymphocytes

We have established a murine model system for exploring the ability of a CD4 subset-deficient host to cope with cytomegalovirus infection, and reported three findings. First, an antiviral response of the CD8 subset of T lymphocytes could be not only initiated but also maintained for a long period of time despite a continued absence of the CD4 subset, whereas the production of antiviral antibody proved strictly dependent upon help provided by the CD4 subset. Second, no function in the defense against infection could be ascribed as yet to CD4-CD8- T lymphocytes, which were seen to accumulate to a new subset as a result of depletion of the CD4 subset. This newly arising subset did not substitute for CD4+ T lymphocytes in providing help to B lymphocytes, and was also not effective in controlling the spread of virus in host tissues. As long as a function of these cells in the generation and maintenance of a CD8 subset-mediated response is not disproved, caution is indicated with concern to an autonomy of the CD8 subset. Third, even though with delay, the CD8+ effector cells raised in the CD4 subset-deficient host were able of clear vital tissues from productive infection and to restrict asymptomatic, persistent infection to acinar glandular epithelial cells in salivary gland tissue.

A pentapeptide as minimal antigenic determinant for MHC class I-restricted T lymphocytes

Peptides that are antigenic for T lymphocytes are ligands for two receptors, the class I or II glycoproteins that are encoded by genes in the major histocompatibility complex, and the idiotypic alpha/beta chain T-cell antigen receptor. That a peptide must bind to an MHC molecule to interact with a T-cell antigen receptor is the molecular basis of the MHC restriction of antigen-recognition by T lymphocytes. In such a trimolecular interaction the amino-acid sequence of the peptide must specify the contact with both receptors: agretope residues bind to the MHC receptor and epitope residues bind to the T-cell antigen receptor. From a compilation of known antigenic peptides, two algorithms have been proposed to predict antigenic sites in proteins. One algorithm uses linear motifs in the sequence, whereas the other considers peptide conformation and predicts antigenicity for amphipathic alpha-helices. We report here that a systematic delimitation of an antigenic site precisely identifies a predicted pentapeptide motif as the minimal antigenic determinant presented by a class I MHC molecule and recognized by a cytolytic T lymphocyte clone.

Molecular basis for cytolytic T-lymphocyte recognition of the murine cytomegalovirus immediate-early protein pp89

The murine cytomegalovirus protein pp89, which is encoded by gene ieI, is a nonstructural regulatory protein expressed in the immediate-early phase of the viral replication cycle and located mainly in the nucleus of infected cells. Protection of BALB/c (H-2d) mice against a lethal murine cytomegalovirus challenge infection is achieved by vaccination with a recombinant vaccinia virus, MCMV-ieI-VAC, expressing pp89 as the only murine cytomegalovirus gene product. The protection is entirely mediated by T lymphocytes of the CD8+ subset. In the present report, we analyzed the molecular basis of the recognition of pp89 by BALB/c CD8+ cytolytic T lymphocytes. A series of internal and terminal deletion mutants of gene ieI was constructed and cloned in vaccinia virus, and the antigenicity and immunogenicity of the fragments of pp89 expressed by the recombinants were studied. A region of only one-sixth of the protein, from amino acids 154 to 249 and encoded by the fourth exon of gene ieI, was sufficient for both the recognition in vitro of the protein by pp89-specific cytotoxic T lymphocytes and the induction in vivo of pp89-specific cytotoxic T lymphocytes. By using synthetic peptides, the sequence between residues 161 and 179, which is located within the defined domain, was identified as an epitope presented to BALB/C cytotoxic T lymphocytes by the class I major histocompatibility antigen Ld.

The 89,000-Mr murine cytomegalovirus immediate-early protein stimulates c-fos expression and cellular DNA synthesis

The immediate-early (IE) genes of murine cytomegalovirus (MCMV) are expressed in the absence of prior viral protein synthesis and regulate the transcription of MCMV early genes. The effect of MCMV IE genes on growth induction was studied. Different plasmids containing MCMV IE genes were microinjected into arrested NIH 3T3 mouse fibroblasts. Plasmids containing the ieI gene coding for the 89,000-Mr major IE protein pp89 were found to stimulate the expression of the c-fos protooncogene. Synthesis of pp89 and its transport to the nucleus appeared to be required for c-fos expression. DNA synthesis occurred in cells that were injected with MCMV IE genes and in neighboring cells that were not injected. The results suggest that the phosphoprotein pp89 stimulates cells to enter the cell cycle.

Human cytomegalovirus-specific cytotoxic T cells. Relative frequency of stage-specific CTL recognizing the 72-kD immediate early protein and glycoprotein B expressed by recombinant vaccinia viruses

CTL are held to be an important host defense mechanism in persistent herpes-virus infections. We have therefore studied the nature and specificity of human cytomegalovirus (HCMV)-specific CTL in normal persistently infected individuals. This was achieved by using vaccinia recombinants encoding viral genes expressed at different stages of the virus replicative cycle, a structural glycoprotein gB (vac.gB) and the major 72-kD immediate early nonstructural protein (vac.IE) of HCMV, combined with limiting dilution analysis of the CTL response. In two subjects, 43 and 58% of HCMV CTL precursors (CTLp) lysed vac.IE-infected cells, in contrast to less than 6% lysing gB-infected cells. HCMV-specific CTL could also be generated by secondary in vitro stimulation with vac.gB- but not vac.IE-infected autologous fibroblasts. The high frequency of 72-kD IE protein-specific CTL suggests that this is at least a major recognition element for the HCMV-specific CTL response in asymptomatic persistently infected individuals, and CTL with this specificity may be important in maintaining the normal virus/host equilibrium.

Frequent detection of antibodies to hepatitis B virus x-protein in acute, chronic and resolved infections

Recombinant MS2- or beta gal fusion proteins containing parts of hepatitis B virus (HBV) HBx-, HBc-, and HBs-amino acid sequences were expressed in Escherichia coli and were used to screen 96 and 60 serum samples of HBV infected and uninfected patients, respectively, for the corresponding antibodies by immunoblotting. Antibodies against HBx were detected in 20 out of 65 sera of patients with previous resolved HBV-infection, in 3 out of 7 patients with persistent infection, and in 9 out of 24 sera of patients with acute HBV infection. The specificity of the immune reaction was confirmed by competition experiments with MS2- and beta gal-HBx fusion proteins, and by the lack of HBx antibodies in the sera of uninfected patients. Hbs and HBc antibodies were detected less frequently by immunoblotting with recombinant fusion proteins than by a commercial immunoassay. Our results indicate that HBx antibodies are induced early and frequently during HBV infection suggesting that the HBx protein is an early antigenic protein expressed in vivo.

Recognition of adenovirus E1A gene products on immortalized cell surfaces by cytotoxic T lymphocytes

The experiments described in this report were designed to examine whether target cells transfected with the adenovirus E1A gene and exhibiting increased susceptibility to lysis by natural killer cells and activated macrophages (J. L. Cook, T. A. Walker, A. M. Lewis, Jr., H. E. Ruley, F. L. Graham, and S. H. Pilder, Proc. Natl. Acad. Sci. USA 83:6965-6969, 1986) also express E1A proteins on their surfaces. MT1A, 12S, and 13S are strain Fischer baby rat kidney (BRK) cell lines immortalized by transfection with plasmids containing only the E1A gene of nononcogenic adenovirus. All of these cell lines were effective in stimulating the generation of cytotoxic T lymphocytes (CTL) in vitro, provided that the cultures were supplemented with an exogenous source of lymphokine and that the responding lymphocytes were from syngeneic Fischer rats previously immunized with a cell line containing the intact E1A gene. HrA2, a Fischer BRK cell line immortalized by transfection with a plasmid containing only exon 1 of the E1A gene, did not generate, nor was it lysed by, E1A-specific CTL. The cytolytic activity of E1A-specific CTL was blocked by antiserum from Fischer rats immunized with purified E1A proteins synthesized in Escherichia coli, supporting the conclusion that an epitope on E1A proteins encoded by the intact E1A gene constitutes part of the CTL target structure on adenovirus-transformed cells. These data suggest that in addition to their functions within host cells, E1A gene products are important immunogenic determinants on the surfaces of adenovirus-transformed cells.

A nonstructural viral protein expressed by a recombinant vaccinia virus protects against lethal cytomegalovirus infection

The nonstructural immediate-early protein pp89 of murine cytomegalovirus (MCMV) is the first viral protein synthesized after infection and has a regulatory function in viral gene expression. Despite its localization in the nucleus of infected cells, pp89 is also the dominant antigen recognized by MCMV-specific cytolytic T lymphocytes. The recombinant vaccinia virus MCMV-ieI-VAC, which expresses pp89, was used to study the capacity of this protein to induce protective immunity in BALB/c mice. Vaccination with MCMV-ieI-VAC induced a long-lasting immunity that protected mice against challenge with a lethal dose of MCMV but did not prevent infection and morbidity. In vivo depletion of CD8+ T lymphocytes before challenge completely abrogated the protective immunity. CD8+ T lymphocytes derived from MCMV-ieI-VAC-primed donors and adoptively transferred into sublethally irradiated and MCMV-infected recipients were found to limit viral replication in host tissues, whereas CD4+ T lymphocytes and pp89-specific antiserum had no protective effect. The data demonstrate for the first time that a single nonstructural viral protein can confer protection against a lethal cytolytic infection and that this immunity is entirely mediated by the CD8+ subpopulation of T lymphocytes.

Failure in generating hemopoietic stem cells is the primary cause of death from cytomegalovirus disease in the immunocompromised host

We have shown in a murine model system for cytomegalovirus (CMV) disease in the immunocompromised host that CMV infection interferes with the earliest detectable step in hemopoiesis, the generation of the stem cell CFU-S-I, and thereby prevents the autoreconstitution of bone marrow after sublethal irradiation. The antihemopoietic effect could not be ascribed to a direct infection of stem cells. The failure in hemopoiesis was prevented by adoptive transfer of antiviral CD8+ T lymphocytes and could be overcome by syngeneic bone marrow transplantation. CD8+ T lymphocytes and bone marrow cells both mediated survival, although only CD8+ T lymphocytes were able to limit virus multiplication in host tissues. We concluded that not the cytopathic effect of virus replication in host tissues, but the failure in hemopoiesis, is the primary cause of death in murine CMV disease.

Interaction of the 89K murine cytomegalovirus immediate-early protein with core histones

The conditions that permit the interaction of immediate-early proteins of murine cytomegalovirus (MCMV) with DNA were studied. Chromatography of extracts from infected cells on MCMV DNA cellulose and calf thymus DNA cellulose showed that pp89, the regulatory major immediate-early protein, interacts with DNA and dissociates at salt concentrations between 0.3 and 0.6 M NaCl. pp76, a cleavage product of pp89, and additional minor ie 1 proteins eluted already at low ionic strength. Cellular DNA-binding factors were required for association of pp89 with DNA. These factors were identified as core histones. Chromatography of IE proteins on histone-Sepharose in the absence of DNA revealed a high-binding affinity that was resistant to 2 M NaCl. These results suggest that pp89 has no direct DNA-binding activity. A role for an amino acid sequence homology in the N-terminal region of pp89 with histone H2B in the pp89-histone-DNA interaction is discussed.

Adoptive immunotherapy of murine cytomegalovirus adrenalitis in the immunocompromised host: CD4-helper-independent antiviral function of CD8-positive memory T lymphocytes derived from latently infected donors

The ability of memory T lymphocytes derived from latently infected mice to control murine cytomegalovirus disease in the immunocompromised host was studied by adoptive transfer experiments. At a stage of pathogenesis when virus had already colonized target tissues, a therapeutic antiviral function could be ascribed to the CD8+ subset. This in vivo function was not restricted to sites in which intravenously infused lymphocytes usually are trapped or home in, such as the lungs or the spleen, respectively, but was also evident in the adrenal glands, a site to which antiviral effector cells have to specifically migrate. Specific infiltration of adrenal gland cortical tissue by donor-derived CD8+ memory T lymphocytes was demonstrated. CD4+ memory T lymphocytes had no antiviral effect by themselves and also were not required for the function of the CD8+ effector cells in this short-term immunotherapy model. These findings should help settle the debate about which subset of T lymphocytes comprises the effector cells that can directly control cytomegalovirus infection in the murine model system.

Molecular analysis of herpesviral gene products recognized by protective cytolytic T lymphocytes

The infection of the mouse with murine cytomegalovirus (MCMV) served as a model system to understand the biology of human CMV infection. The contribution of cytolytic T lymphocytes (CTL) to the recovery from infection was studied. Protection against lethal MCMV disease could be conferred on immunodepleted hosts by adoptive transfer of lymphocytes. The antiviral effect was mediated by specifically sensitized T lymphocytes of the CD8+ subset. These cells limited viral spread, prevented tissue destruction by viral cytopathic effects, and protected from lethal disease. Transferred cells have protective therapeutic function even when the virus has already colonized host tissues. CD8+ cells do not require the contribution of CD4+ cells for in vivo function. Selective expression of immediate-early (IE) phase genes in target cells allowed the detection of the immunodominant IE antigen recognized by CTL. The major IE gene ieI encodes a non-structural viral phosphoprotein, pp89, which resides in the nucleus of infected cells where it acts as transcriptional regulator. Expression of gene ieI is under temporal control, and membrane presentation of the protein domain detected by CTL is down-regulated by MCMV early-phase products. A recombinant vaccinia virus expressing gene ieI induced immunity that protected mice against a subsequent challenge with a lethal dose of MCMV. The protective effect was entirely mediated by CD8+ T lymphocytes. Thus, an experimental vaccine expressing a single nonstructural herpesvirus protein can induce a protective cellular immune response.

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.