Characterization of a major virion envelope glycoprotein complex of murine cytomegalovirus and its immunological cross-reactivity with human cytomegalovirus

Murine cytomegalovirus m38.5 protein inhibits Bax-mediated cell death

Many viruses encode proteins that inhibit the induction of programmed cell death at the mitochondrial checkpoint. Murine cytomegalovirus (MCMV) encodes the m38.5 protein, which localizes to mitochondria and protects human HeLa cells and fibroblasts from apoptosis triggered by proteasome inhibitors but not from Fas-induced apoptosis. However, the ability of this protein to suppress the apoptosis of murine cells and its role during MCMV infection have not been investigated previously. Here we show that m38.5 is expressed at early time points during MCMV infection. Cells infected with MCMVs lacking m38.5 showed increased sensitivity to cell death induced by staurosporine, MG132, or the viral infection itself compared to the sensitivity of cells infected with wild-type MCMV. This defect was eliminated when an m38.5 or Bcl-X(L) gene was inserted into the genome of a deletion mutant. Using fibroblasts deficient in the proapoptotic Bcl-2 family proteins Bak and/or Bax, we further demonstrated that m38.5 protected from Bax- but not Bak-mediated apoptosis and interacted with Bax in infected cells. These results consolidate the role of m38.5 as a viral mitochondrion-localized inhibitor of apoptosis and its functional similarity to the human cytomegalovirus UL37x1 gene product. Although the m38.5 gene is not homologous to the UL37x1 gene at the sequence level, m38.5 is conserved among rodent cytomegaloviruses. Moreover, the fact that MCMV-infected cells are protected from both Bak- and Bax-mediated cell death suggests that MCMV possesses an additional, as-yet-unidentified mechanism to block Bak-mediated apoptosis.

The role of NKG2D signaling in inhibition of cytotoxic T-lymphocyte lysis by the Murine cytomegalovirus immunoevasin m152/gp40

Three proteins encoded by murine cytomegalovirus (MCMV) -- gp34, encoded by m04 (m04/gp34), gp48, encoded by m06 (m06/gp48), and gp40, encoded by m152 (m152/gp40) -- act together to powerfully impact the ability of primed cytotoxic CD8 T lymphocytes (CTL) to kill virus-infected cells. Of these three, the impact of m152/gp40 on CTL lysis appears greater than would be expected based on its impact on cell surface major histocompatibility complex (MHC) class I. In addition to MHC class I, m152/gp40 also downregulates the RAE-1 family of NKG2D ligands, which can provide costimulation for CD8 T cells. We hypothesized that m152/gp40 may impact CTL lysis so profoundly because it inhibits both antigen presentation and NKG2D-mediated costimulation. We therefore tested the extent to which m152/gp40's ability to inhibit CTL lysis of MCMV-infected cells could be accounted for by its inhibition of NKG2D signaling. As was predictable from the results reported in the literature, NKG2D ligands were not detected by NKG2D tetramer staining of cells infected with wild-type MCMV, whereas those infected with MCMV lacking m152/gp40 displayed measurable levels of the NKG2D ligand. To determine whether NKG2D signaling contributed to the ability of CTL to lyse these cells, we used a blocking anti-NKG2D antibody. Blocking NKG2D signaling did affect the killing of MCMV-infected cells for some epitopes. However, for all epitopes, the impact of m152/gp40 on CTL lysis was much greater than the impact of inhibition of NKG2D signaling. We conclude that the downregulation of NKG2D ligands by MCMV makes only a small contribution to the impact of m152/gp40 on CTL lysis and only for a small subset of CTL.

Coordinated function of murine cytomegalovirus genes completely inhibits CTL lysis

Murine CMV (MCMV) encodes three viral genes that interfere with Ag presentation (VIPRs) to CD8 T cells, m04, m06, and m152. Because the functional impact of these genes during normal infection of C57BL/6 mice is surprisingly modest, we wanted to determine whether the VIPRs are equally effective against the entire spectrum of H-2(b)-restricted CD8 T cell epitopes. We also wanted to understand how the VIPRs interact at a functional level. To address these questions, we used a panel of MCMV mutants lacking each VIPR in all possible combinations, and CTL specific for 15 H-2(b)-restricted MCMV epitopes. Only expression of all three MCMV VIPRs completely inhibited killing by CTL specific for all 15 epitopes, but removal of any one VIPR enabled lysis by at least some CTL. The dominant interaction between the VIPRs was cooperation: m06 increased the inhibition of lysis achieved by either m152 or m04. However, for 1 of 15 epitopes m04 functionally antagonized m152. There was little differential impact of any of the VIPRs on K(b) vs D(b), but a surprising degree of differential impact of the three VIPRs for different epitopes. These epitope-specific differences did not correlate with functional avidity, or with timing of VIPR expression in relation to Ag expression in the virus replication cycle. Although questions remain about the molecular mechanism and in vivo role of these genes, we conclude that the coordinated function of MCMV's three VIPRs results in a powerful inhibition of lysis of infected cells by CD8 T cells.

The murine cytomegalovirus M73.5 gene, a member of a 3' co-terminal alternatively spliced gene family, encodes the gp24 virion glycoprotein

We have identified a novel family of five 3' co-terminal transcripts in murine cytomegalovirus (MCMV) arranged in a tail-to-tail orientation with respect to the MCMV glycoprotein H (gH) gene M75. They share the same exon 2 sequence but possess different exon 1 sequences. Two of these spliced transcripts (M73) encode the MCMV homolog of glycoprotein N (gN) entirely within exon 1. Two other transcripts designated M73.5 encode a previously described virion glycoprotein gp24 that shares its first 20 amino acids with gN, but which has another 64 amino acids encoded within exon 2. The fifth transcript, designated m60, has an 80-bp exon 1 near the MCMV oriLyt region 10.8 kb upstream of exon 2. Both MCMV M73.5 and m60 encode type II glycoproteins expressed at the cell surface. Their shared exon 2 coding sequences likely represent the highly conserved region of an as yet unidentified betaherpesvirus-specific glycoprotein species.

Systemic priming-boosting immunization with a trivalent plasmid DNA and inactivated murine cytomegalovirus (MCMV) vaccine provides long-term protection against viral replication following systemic or mucosal MCMV challenge

We previously demonstrated that vaccination of BALB/c mice with a pool of 13 plasmid DNAs (pDNAs) expressing murine cytomegalovirus (MCMV) genes followed by formalin-inactivated MCMV (FI-MCMV) resulted in complete protection against viral replication in the spleen and salivary glands following sublethal intraperitoneal (i.p.) challenge. Here, we found that following intranasal (i.n.) challenge, titers of virus in the lungs of the immunized mice were reduced approximately 1,000-fold relative to those for mock-immunized controls. We next sought to extend these results and to determine whether similar protection levels could be achieved by priming with a pool of three pDNAs containing three key plasmids (IE1, M84, and gB). We found that the three-pDNA priming elicited IE1- and M84-p65-specific CD8+ T lymphocytes and, following FI-MCMV boost, high levels of virion-specific immunoglobulin G (IgG) and virus-neutralizing antibodies. When mice were i.n. challenged 4 months after the last boost, titers of virus in the lungs of immunized mice were reduced 1,000- to 2,000-fold from those for controls during the peak of viral replication. Additionally, titers of virus were either at or below the detection limits for the salivary glands, liver, and spleen of the majority of the immunized mice. Following sublethal i.p. challenge, virus was undetectable in all of the above target organs of the immunized mice. Virion-specific IgA in the lungs was consistently detected by day 6 post-i.n. challenge for the immunized mice and by day 14 for controls. These results demonstrate the immunity and high levels of protection of the priming-boosting vaccination against both systemic and mucosal challenge.

Characterization of the murine cytomegalovirus 38 kDa m137 gene product

Murine cytomegalovirus (MCMV) m137 null mutants, Deltam137A and Deltam137B, were generated by inserting a gpt cassette into a deleted region of the open reading frame. A polyclonal antiserum produced to an Escherichia coli expressed gst-m137 fusion protein was used to show that a 38 kDa polypeptide corresponding to the predicted m137 gene product was present in NIH 3T3 fibroblasts infected with wild-type MCMV but was not detected in Deltam137 infected cells. The protein did not fractionate with infected cell membranes and was not detectable in purified wild-type virions. Plaque size, plaque morphology, and viral yield did not differ significantly between Deltam137 and wild-type MCMV infected 3T3 fibroblasts. The results showed that deletion of the 38 kDa protein did not negatively effect viral growth in 3T3 fibroblasts indicating that the m137 gene product is not essential for replication in these cells. In vivo analysis revealed that two independently isolated m137 mutants showed a significant delay in time until death but ultimately killed 100% of the mice in a SCID mouse model of virulence.

Identification and expression of a murine cytomegalovirus early gene coding for an Fc receptor

Several herpesviruses, including cytomegalovirus, induce receptors for the Fc domain of murine immunoglobulin G (IgG) molecules. Viral genes coding for these receptors have been characterized only for alphaherpesviruses. In this report, we describe a new approach that led to the identification of an Fc receptor (FcR) of murine cytomegalovirus (MCMV). The Fc fragment of IgG precipitated glycoproteins (gp) of 86 to 88 and 105 kDa from MCMV-infected cells. Deglycosylation by endoglycosidase F resulted in a protein with a molecular mass of 64 kDa. Injection of complete MCMV DNA or of DNA fragments, and the subsequent testing of cytoplasmic binding of IgG by immunofluorescence microscopy, was used to search for the coding region in the MCMV genome. The gene was located in the HindIII J fragment, map units 0.838 to 0.846, where an open reading frame of 1,707 nucleotides predicts a gp of 569 amino acids with a calculated molecular mass of 65 kDa. The sequence of this gp is related to those of the gE proteins of herpes simplex virus type 1 and varicella-zoster virus. The defined length of the mRNA, 1,838 nucleotides, was in agreement with that of a 1.9-kb RNA expressed throughout the replication cycle, starting at the early stages of infection. Expression of the gene fcr1 by recombinant vaccinia virus resulted in the synthesis of gp86/88 and gp105, each with FcR properties, and the correct identification of the gene encoding the FcR was confirmed by the DNA injection method.

Identification of the murine cytomegalovirus glycoprotein B gene and its expression by recombinant vaccinia virus

The gene encoding glycoprotein B (gB) of murine cytomegalovirus (MCMV) strain Smith was identified, sequenced, and expressed by recombinant vaccinia virus. The gB gene was found adjacent to the polymerase gene, as it is in the genome of human cytomegalovirus (HCMV). The open reading frame consists of 2,784 nucleotides capable of encoding a protein of 928 amino acids. Comparison with gB homologs of other herpesviruses revealed a high degree of homology. The similarity between the MCMV gB and the HCMV gB is most prominent, since 45% of the amino acids are identical. In addition, all cysteine residues are at homologous positions, indicating a similar tertiary structure of the two proteins. In contrast to HCMV, the MCMV gB mRNA is a true late transcript. A recombinant vaccinia virus expressing the MCMV gB gene has been constructed (Vac-gB). Antibodies raised against the Vac-gB recombinant precipitated proteins of 130, 105, and 52 kDa from MCMV-infected cells. The identity of the MCMV gB with the major envelope glycoprotein of MCMV described by Loh et al. was shown (L. C. Loh, N. Balachandran, and L. F. Qualtiere, Virology 166:206-216, 1988). Immunization of mice with the Vac-gB recombinant gave rise to neutralizing antibodies.

Monoclonal IgM antibodies from cytomegalovirus-infected mice recognize the GlcNAc-containing receptor determinant of murine CMV as well as neutralizing anti-CMV IgG antibodies

This study examines monoclonal antibodies derived from mice at different time points after infection with attenuated murine cytomegalovirus (MCMV). The antibodies obtained from mice at 3 weeks p.i. were of IgG type (designated V-antibodies) and several could neutralize the virus. Those obtained at 5 weeks p.i. were of the IgM class (designated R-antibodies), bound to uninfected (MEF, mouse embryo fibroblast) cells, and thereby blocked MCMV plaque formation. In ELISA, the IgM monoclonals (R-antibodies) bound to GalB1-3GlcNAc and GalB1-4GlcNAc, the receptor determinants for MCMV. Similarly, these GlcNAc-containing residues blocked the binding of purified IgM monoclonal antibodies (MAbs) to MEF. The R- and V-series of antibodies showed mutual binding activities; for example, IgM MAb R-2D8 bound specifically to four (V-8C4, V-1C7, V-8C7, V-9C5) of six neutralizing IgG MAbs in ELISA. The same neutralizing IgG MAbs bound to antireceptor IgM antibodies in an immunoblot assay. This suggests that the IgM monoclonals directed against the known cell surface receptor determinant are anti-idiotypic antibodies against neutralizing antiviral IgG antibodies. The neutralizing antiviral IgG MAbs bound to 60- and 66-kDa MCMV polypeptides on Western blots, suggesting that these viral polypeptides may be important in MCMV binding to this receptor. The R-series might represent anti-idiotype antibodies capable of down-regulating antiviral V-antibodies and may also represent a mechanism for the induction of IgM autoantibodies directed at cell surface glycolipids present in autoimmune CMV-associated neuropathies.

Characterization of a membrane-associated phosphoprotein of murine cytomegalovirus (pp50) and its immunological cross-reactivity with a human cytomegalovirus protein

We have identified an abundant 50K phosphoprotein (pp50) in MCMV-infected 3T3-L1 cells and shown by immunofluorescence microscopy and surface-iodination experiments that pp50 is localized to the plasma membrane of the infected cell. Furthermore, the kinetics of its synthesis suggests that it belongs to the early-late class of herpesvirus proteins. Using monoclonal antibodies specific for pp50 to screen a lambda ZAP II expression library constructed from poly(A)+ mRNA of MCMV-infected cells, we have isolated a cDNA clone that synthesizes a truncated form of pp50 as a beta-galactosidase fusion protein. This allowed us to localize the partial pp50 transcript to a region between map coordinates 0.228 and 0.260 of the MCMV genome (Smith strain, Vancouver). Finally, we demonstrated that the MAb 5H10.21A recognizes an antigenic determinant that is conserved between pp50 and a 50K human cytomegalovirus (HCMV) nonstructural protein.

Synthesis and processing of the major envelope glycoprotein of murine cytomegalovirus

We have characterized the synthesis and processing pathway of the major envelope glycoprotein complex of murine cytomegalovirus (gp52/105/150). We have demonstrated that it belongs to the "late" kinetic class of MCMV proteins, and is initially synthesized as a 128K glycoprotein (gp128) which contains N-linked, high-mannose type oligosaccharide chains and is phosphorylated predominantly at serine residues. A fraction of the gp128 molecules also contains O-linked GalNAc residues. The majority of the gp128 molecules appears to be retained in the endoplasmic reticulum as evidenced by their sensitivity to endoglycosidase H digestion. The formation of disulfide linkages and dimerization allow the transport of gp128 to the Golgi compartments where modification of N-linked carbohydrate structures and extension of O-linked oligosaccharide chains take place, cumulating in the appearance of the mature gp150. The final processing step involves the cleavage of gp150 into gp52 and gp105. By blocking the transport of the glycoprotein precursor to the trans-Golgi compartments with the ionophore monensin, the cleavage process is inhibited, suggesting that the trans-Golgi compartment is the site where gp150 is cleaved. However, the cleavage process is incomplete, resulting in the formation of multiple disulfide-linked complexes made up of different combinations of gp52, gp105, and gp150. Therefore, the processing of the major envelope glycoprotein complexes of murine cytomegalovirus resembles that of the gcl/gp55-116 complex of human cytomegalovirus.

Synthesis and processing of a 22-26K murine cytomegalovirus glycoprotein recognized by a neutralizing monoclonal antibody

A 22-26K glycoprotein (gp24) of the murine cytomegalovirus (MCMV) virion was immunoprecipitated by a monoclonal antibody (MAb) 6A1.21A that neutralized MCMV infectivity only in the presence of complement. Pulse-chase experiments demonstrated that gp24, which contained only N-linked, complex-type oligosaccharides, was processed from an 18.4K high-mannose precursor (gp18.4). Analyses by two-dimensional (nonreducing/reducing) gel electrophoresis have shown that both gp18.4 and gp24 are present as disulfide-linked complexes, and rapid oligomerization of the 18.4K precursor is an early step in the processing pathway of gp24. Finally, we demonstrated that gp24 belongs to the "late" class of MCMV proteins.