Coronavirus translational regulation: leader affects mRNA efficiency

Cells infected with the murine coronavirus, mouse hepatitis virus (MHV), show decreased host protein synthesis concomitant with an increase in viral protein synthesis. We examined the in vitro translation property of the conserved MHV 5'-leader RNA sequence by constructing chimeric mRNAs in which the 72-nt 5'-leader of M protein mRNA (A59 strain) was positioned upstream of the human alpha-globin coding region in a T7 expression vector. Synthetic 5'-capped transcripts of these mRNA constructs were translated in cell-free extracts prepared from uninfected and MHV-infected murine DBT cells. Nonviral mRNAs translated readily in both uninfected and infected cell-free extracts. By contrast, replacement of the human alpha-globin 5'-untranslated region (UR) with the MHV 5'-leader increased translation ca. three- to fourfold in cell-free extracts from MHV-infected cells versus translation in extracts from uninfected cells. Chimeric globin mRNA containing the reverse complementary sequence of the viral leader RNA in the 5'-UR showed no such increase in translation, indicating sequence specificity for the effect. A 13-nt region (-UCUAAUCCAAACA-) immediately proximal to the start codon was found to be important for the increased translation of the MHV leader-containing mRNAs. These data indicate that the apparent down-regulation of host translation is not primarily due to an inhibition of host translation but also involves a significant stimulation of viral translation in cis by a structural feature of the MHV 5'-leader RNA sequence in conjunction with a virus-specified or virus-induced factor.

Location of antibody epitopes within the mouse hepatitis virus nucleocapsid protein

Thirteen monoclonal antibodies (Mab) specific for the nucleocapsid (N) protein of mouse hepatitis virus were mapped using a panel of carboxy-terminal N protein truncations expressed by recombinant vaccinia viruses. All of the Mab recognized both native protein and full-length N protein expressed in this vector by both Western blot and enzyme-linked immunoabsorbent assays (ELISA), indicating that they recognized linear epitopes. The results obtained by both Western blot and ELISA for binding to the truncated N proteins coincide for seven of the Mab tested. The linear epitopes recognized localize to four domains dispersed between amino acids 171 and 196, 231 and 277, and 374 and 455. The epitopes for six Mab were localized to domains comprising 29 amino acids or less as determined by ELISA. Seven Mab showed different reactivity patterns in Western blot versus ELISA, suggesting binding may be influenced by local conformation. Therefore, the fine specificity of these Mab could not be determined with certainty. These data represent the first determination of antibody binding domains within the mouse hepatitis virus N protein which forms the viral helical nucleocapsids and appears to perform a number of regulatory functions during virus replication.

Cytotoxic T cell repertoire selection. A single amino acid determines alternative class I restriction

CTL responses are governed by intracellular Ag processing, affinity of peptides for MHC class I molecules, and the T cell repertoire. In this report we demonstrate that a class I Dd-restricted 10-mer CTL epitope within the gp160 envelope glycoprotein of HIV-1 strain IIIB (residues 318-327) contains a 9-amino acid peptide (residues 319-327), which efficiently binds to both the Dd and Ld class I molecules in vitro. The potential for broadening the naturally limited CTL response to include presentation on the Ld class I molecules in vivo was examined using a minigene-based vaccine strategy to insure cytosolic expression of "preprocessed" forms of the gp160 epitope. Immunization with recombinant vaccinia viruses (vac) expressing either the gp160 10 mer or 9 mer, both including an initiation methionine (M318-327 and M319-327, respectively), induced predominantly Dd-restricted CTL specific for native gp160. By contrast, recombinant vac expressing eight gp160 amino acids (M320-327) generated predominantly Ld-restricted CTL which are specific for synthetic gp160 peptides but not native gp160. The ability to induce Ld-restricted CTL suggests that the absence of an Ld-restricted response to native gp160 cannot be attributed to a limited T cell repertoire, but to inefficient processing of gp160 for presentation on Ld. The switch in class I restriction, controlled by a single amino acid within one epitope, demonstrates that nonanchor residues have a profound effect on differential MHC restriction and CTL induction. Thus, minigene-based vaccines expressing minimal epitopes may be useful in inducing a more heterogeneous CTL response than previously appreciated.

Cytotoxic T cell repertoire selection. A single amino acid determines alternative class I restriction

CTL responses are governed by intracellular Ag processing, affinity of peptides for MHC class I molecules, and the T cell repertoire. In this report we demonstrate that a class I Dd-restricted 10-mer CTL epitope within the gp160 envelope glycoprotein of HIV-1 strain IIIB (residues 318-327) contains a 9-amino acid peptide (residues 319-327), which efficiently binds to both the Dd and Ld class I molecules in vitro. The potential for broadening the naturally limited CTL response to include presentation on the Ld class I molecules in vivo was examined using a minigene-based vaccine strategy to insure cytosolic expression of "preprocessed" forms of the gp160 epitope. Immunization with recombinant vaccinia viruses (vac) expressing either the gp160 10 mer or 9 mer, both including an initiation methionine (M318-327 and M319-327, respectively), induced predominantly Dd-restricted CTL specific for native gp160. By contrast, recombinant vac expressing eight gp160 amino acids (M320-327) generated predominantly Ld-restricted CTL which are specific for synthetic gp160 peptides but not native gp160. The ability to induce Ld-restricted CTL suggests that the absence of an Ld-restricted response to native gp160 cannot be attributed to a limited T cell repertoire, but to inefficient processing of gp160 for presentation on Ld. The switch in class I restriction, controlled by a single amino acid within one epitope, demonstrates that nonanchor residues have a profound effect on differential MHC restriction and CTL induction. Thus, minigene-based vaccines expressing minimal epitopes may be useful in inducing a more heterogeneous CTL response than previously appreciated.

TCR cross-linking induces CTL death via internal action of TNF

TCR-mediated stimulation can result in either activation or apoptosis, raising the question of what the mechanisms of these opposing effects might be. Here we demonstrate that microinjection of Ab specific for TNF inhibits activation-induced cell death, implicating TNF in anti-CD3-induced apoptosis. Induction of CTL in the presence of antisense TNF oligodeoxynucleotides inhibits TNF secretion and cell death after stimulation with anti-CD3. Incubation of cells with IL-2 for 48 h before stimulation protects from anti-CD3-induced death, whereas the presence of IL-2 during anti-CD3 treatment is without effect.

TCR cross-linking induces CTL death via internal action of TNF

TCR-mediated stimulation can result in either activation or apoptosis, raising the question of what the mechanisms of these opposing effects might be. Here we demonstrate that microinjection of Ab specific for TNF inhibits activation-induced cell death, implicating TNF in anti-CD3-induced apoptosis. Induction of CTL in the presence of antisense TNF oligodeoxynucleotides inhibits TNF secretion and cell death after stimulation with anti-CD3. Incubation of cells with IL-2 for 48 h before stimulation protects from anti-CD3-induced death, whereas the presence of IL-2 during anti-CD3 treatment is without effect.

Characterization of mouse hepatitis virus-specific cytotoxic T cells derived from the central nervous system of mice infected with the JHM strain

The cytotoxic T lymphocyte (CTL) activity of spleen cells from BALB/c (H-2d) mice immunized with the neurotropic JHM strain of mouse hepatitis virus (JHMV) was stimulated in vitro for 7 days. CTL were tested for recognition of target cells infected with either JHMV or vaccinia virus recombinants expressing the four virus structural proteins. Only target cells infected with either JHMV or the vaccinia virus recombinant expressing the JHMV nucleocapsid protein were recognized. Cytotoxic T cell lines were established by limiting dilution from the brains of mice undergoing acute demyelinating encephalomyelitis after infection with JHMV. Twenty of the 22 lines recognized JHMV-infected but not uninfected syngeneic target cells, indicating that they are specific for JHMV. All T-cell lines except one were CD8+. The specificity of the CTL lines was examined by using target cells infected with vaccinia virus recombinants expressing the JHMV nucleocapsid, spike, membrane, and hemagglutinin-esterase structural proteins. Seventeen lines recognized target cells expressing the nucleocapsid protein. Three of the JHMV-specific T-cell lines were unable to recognize target cells expressing any of the JHMV structural proteins, indicating that they are specific for an epitope of a nonstructural protein(s) of JHMV. These data indicate that the nucleocapsid protein induces an immunodominant CTL response. However, no CTL activity specific for the nucleocapsid protein could be detected in either the spleens or cervical lymph nodes of mice 4, 5, 6, or 7 days after intracranial infection, suggesting that the CTL response to JHMV infection within the central nervous system may be induced or expanded locally.

Encephalitogenic Th1 cells are inhibited by Th2 cells with related peptide specificity: relative roles of interleukin (IL)-4 and IL-10

Cytokines secreted by T-helper type 2 (Th2) cells inhibit the antigen-induced stimulation of type 1 (Th1) helper T cells. To study this form of regulation in an autoimmune disease model, the cytokines secreted by a Th2 clone specific for the encephalitogenic proteolipid protein (PLP) peptide 139-151 were tested for their ability to inhibit proliferation of an encephalitogenic Th1 clone specific for an epitope contained within the same peptide. Cytokines, produced by stimulation of the Th2 clone with CD3-specific monoclonal antibodies (mAbs), inhibited proliferation of the Th1 clone when stimulated by antigen and splenic antigen-presenting cells (APC). Inhibition was, however, not antigen-specific since cytokines released upon stimulation of an unrelated Th2 clone were also inhibitory. Inhibition was found to be caused by effects on either antigen presentation or co-stimulatory activity of the APC and not by direct effects on the Th1 cells. MAbs for the two major regulatory Th2 cytokines were used to identify the inhibitory component secreted by activated Th2 cells. Interleukin-10 (IL-10)-specific mAb abolished the inhibitory effect, while mAb specific for IL-4 had no effect on inhibition. The addition of recombinant IL-4 (rIL-4) and rIL-10 confirmed that inhibition of Th1 proliferation was due to secretion of IL-10 by the Th2 clone and its subsequent effects on APC. The studies described here demonstrate that PLP-specific Th2 cells which recognize peptide 139-151 inhibit encephalitogenic Th1 cells which respond to an epitope on the same peptide. This phenomenon may be important for local, antigen-specific regulation of inflammation in the central nervous system.

A spike protein-dependent cellular factor other than the viral receptor is required for mouse hepatitis virus entry

Previous studies have shown that some mouse strains are resistant to mouse hepatitis virus (MHV) infection despite the presence of functional viral receptors (K. Yokomori and M. M. C. Lai, J. Virol. 66, 6931-6938, 1992). To determine the molecular requirement for MHV infection, several cell lines derived from both susceptible and resistant mouse strains were tested for their ability to support infection by two different MHV strains, JHM and A59. Most of the cell lines tested, including ones from susceptible mouse strains, exhibited selective resistance to JHM, but were susceptible to A59, suggesting that there is an additional cellular factor(s) discriminating JHM from A59 infection. Both RNA and protein syntheses of JHM were inhibited in the resistant cells; however, transfection of JHM genomic RNA into these cells led to the production of infectious virus, suggesting that the restriction step(s) is during an early stage of viral replication cycle. The mRNA for the MHV receptor (the murine homolog of the carcinoembryonic antigen) is expressed in all cell lines, and expression in COS cells of the receptor isolated from the resistant murine cell lines rendered the COS cells susceptible to both A59 and JHM infections. Furthermore, the transfection of additional MHV receptors into the resistant cells did not overcome the resistance to JHM virus infection. These results suggested that the viral receptor is functional; nevertheless, the JHM infection is restricted at an early step of infection in these cells. The study of the growth properties of the various recombinant viruses between A59 and JHM revealed that one of the viral genes determining viral replication in these cell lines is the S protein gene; thus, the second factor required for viral infection may interact directly or indirectly with the S protein at an early step of infection. Taken together, these studies suggest that expression of a functional viral receptor is not sufficient to establish MHV infection, and that an additional factor(s) is required for an early step of viral infection, possibly during virus entry.

Localization of the RNA-binding domain of mouse hepatitis virus nucleocapsid protein

The 454-amino acid nucleocapsid (N) protein of mouse hepatitis virus (MHV) binds the leader RNA sequence located at the 5' ends of all plus-sense genomic and subgenomic viral mRNAs. Purified N protein was cleaved with formic acid to determine which domain interacts with the leader RNA sequence. Incubation at 42 degrees C resulted in partial cleavage into two fragments of M(r)s of approximately 32K and 37K and three fragments of 17K, 16K and 14K. Incubation at 56 degrees C resulted in complete cleavage yielding only the three lower molecular mass products. Both the 32K and 37K partial cleavage products and one of the complete cleavage products bind MHV leader RNA, suggesting that the central region of the N protein contains the RNA-binding domain. Monoclonal antibody mapping of the cleavage products confirmed that the MHV leader RNA binding domain is contained within the central 140-amino acid fragment, comprising amino acids 169 to 308. Analysis of the amino acids within this domain indicates no similarity to any previously described RNA-binding protein, suggesting that N protein may possess a unique RNA-binding motif.

Interaction of immune and central nervous systems: contribution of anti-viral Thy-1+ cells to demyelination induced by coronavirus JHM

The murine coronavirus JHM (JHMV or MHV-4) has been intensively studied as an experimental model of viral-induced demyelination; nonetheless, the degree to which demyelination results from direct viral cytolysis of oligodendroglia or immunological mechanisms remains controversial. To examine the contribution of immunity to the pathogenesis of JHMV in the central nervous system (CNS), mice were exposed to immunosuppressive doses of x-irradiation 3 days post infection and observed for clinical and pathological evidence of acute and subacute demyelination. Irradiated mice were found to have a nearly thousand-fold increase in central nervous system virus titer, as well as the presence of both abundant virus and viral antigen in white matter cells with the morphological characteristics of oligodendrocytes. Nonetheless, infected, irradiated mice had little or no evidence of demyelination or destruction of CNS cells. Adoptive transfers of spleen cells from syngeneic JHMV-immunized donors into irradiated JHMV-infected mice were carried out in order to determine the effect of immune reconstitution on pathogenesis. Splenocytes from JHMV-immune donors, but not naive donors or donors immunized with irrelevant antigen, completely restored demyelination in irradiated, JHMV-infected recipients. Depletion of Thy-1+ cells by treatment with monoclonal antibody and complement abolished the ability to transfer demyelination. We conclude that: 1) JHMV infection of the CNS does not result in acute or subacute demyelination in the absence of an intact immune response, and 2) viral-specific Thy-1+ cells are an essential element in the induction of demyelinating CNS lesions that result from JHMV infection.

The detection and characterization of multiple hemagglutinin-esterase (HE)-defective viruses in the mouse brain during subacute demyelination induced by mouse hepatitis virus

It has previously been shown that passive immunization with antibodies specific for the hemagglutinin-esterase (HE) protein of mouse hepatitis virus (MHV) prevents acute lethal encephalitis, resulting in a subacute and chronic demyelination in mice infected with JHM(2), an isolate of the neurotropic JHM strain of MHV. To determine possible genetic changes occurring during infection, viruses were isolated from the brain of infected mice at various time points after infection and examined for the patterns of their structural proteins. The results showed that the sizes and expression levels of the viral spike, membrane, and nucleocapsid proteins were constant among 161 virus isolates throughout the infection. In contrast, most of the viruses isolated later in infection did not synthesize HE protein. This finding suggests that the HE gene expression is extremely variable and is preferentially lost during prolonged viral infections. In contrast, when viruses were passaged in tissue culture, no significant accumulation of HE protein-defective mutants was observed, suggesting that the accumulation of HE protein-defective mutants in infected animals was most likely the result of the positive selection for these mutants during the subacute and chronic infection. The genetic defects of HE gene in these mutants were characterized by cloning, sequencing, and in vitro translation of HE genes. Most of the mutations in the HE protein-defective mutants consisted of deletions of various lengths at different sites within the HE-coding region, resulting in the change of the reading frame and early termination. However, most of the truncated HE proteins were not detected in the infected cells. Since viruses from different mice exhibited different types of defects, the HE mutations probably occurred de novo in the brain. These results demonstrated the exceptionally rapid selection of HE-defective mutants during viral infection of mice and suggest that their selection may be related to the neuropathogenicity or persistence of MHV.

Mouse hepatitis virus nucleocapsid protein-specific cytotoxic T lymphocytes are Ld restricted and specific for the carboxy terminus

Infection of mice with the JHM strain of mouse hepatitis virus (MHV) results in an acute encephalomyelitis associated with primary demyelination of the central nervous system. Efforts at understanding the components of the immune response in the development of chronic MHV-induced demyelination have implicated the antibody response and both the CD4+ and CD8+ T cell responses. In this report, we demonstrate that Balb/c (H-2d) mice immunized with the JHM (JHMV) strain of MHV develop a CD8+ cytotoxic T lymphocyte (CTL) response. One population of these virus-specific CTL recognize the nucleocapsid (N) protein. Recombinant vaccinia viruses expressing either the entire N protein or carboxy-terminal deletions were used to determine the number and location of the epitope(s) recognized. The CTLs were found to recognize a peptide contained within the carboxy-terminal 149 amino acids of the N protein. Analysis of infected cell lines expressing transfected major histocompatibility genes demonstrated that the anti-N protein CTLs were restricted exclusively to the Ld molecule. These data provide the first definition of a MHV-specific CTL response directed to a viral protein and suggest that the anti-N protein CTL response is one potential mechanism used by the host to clear JHMV from the central nervous system.

Hemagglutinin-esterase-specific monoclonal antibodies alter the neuropathogenicity of mouse hepatitis virus

Some of mouse hepatitis virus strains contain an optional envelope glycoprotein, hemagglutinin-esterase (HE) protein. To understand the functional significance of this protein, monoclonal antibodies (MAbs) specific for this protein were generated and used for passive immunization of mice. None of these MAbs showed any virus-neutralizing activity in vitro; however, mice passively immunized with the purified MAbs were protected from lethal infection by the JHM strain of mouse hepatitis virus. Passive immunization altered the pathogenicity such that the virus caused subacute and chronic demyelination instead of acute lethal encephalitis. Virus titers in the brains of the immunized mice were significantly lower than those for the nonimmunized control mice, suggesting that the virus replication or spread was inhibited. In addition, histopathological analysis indicated that the spread of virus in the brain and spinal cord was significantly inhibited in the immunized mice. Furthermore, the mononuclear cell infiltration in the immunized mice appeared earlier than in the nonimmunized mice, suggesting that the exogenous antibody might have activated host immune responses, and thus facilitated clearance of the virus or virus-infected cells. The same protective effects were observed for both JHM(2) and JHM(3) viruses, which expressed different amounts of the HE protein. In contrast, mice infected with At11f, a variant of JHM which does not express the HE protein, were not protected by these MAbs, suggesting that protection was mediated by the specific interaction between the MAb and the HE protein. Thus, the mechanism of protection by the exogenous HE-specific MAbs may represent the early activation of innate immune mechanisms in response to the interaction between the MAbs and the HE protein.

Characterization of brain-infiltrating mononuclear cells during infection with mouse hepatitis virus strain JHM

The eradication of infectious virus from the central nervous system (CNS) following infection with the neurotropic JHM strain of mouse hepatitis virus (JHMV) is thought to be immune-mediated. Furthermore, a significant decrease of infectious virus coincides with the appearance of prominent inflammatory infiltrates in the brain and spinal cord. In the present study, mononuclear cells infiltrating the brain during JHMV infection were isolated and characterized. While all subsets of immune cells were present, there appeared to be a temporal relationship between the peak incidence of CD8+ T cells (40% of total isolated cells) and reduction of virus at day 7 post-infection. Cells with the natural killer (NK) phenotype (at least 30%) were also present throughout infection. These data suggest that CD8+ T cells and NK cells are prominent among cells which infiltrate the brain during JHM virus infection and may have important roles in reduction of virus within the CNS.

Distinct subsets of accessory cells activate Thy-1+ triple negative (CD3-, CD4-, CD8-) cells and Th-1 delayed-type hypersensitivity effector T cells

The SJL strain of mice possess a unique developmental delay in the ability to exhibit delayed-type hypersensitivity (DTH) responses after immunization with a wide variety of Ag. Similar to other models of DTH, the adoptive transfer of syngeneic Ag-pulsed macrophages from DTH-responsive mice into these DTH-unresponsive mice results in the activation of Ag-specific, CD4+ DTH effector Th1 T cells. The absence of other defects in APC-dependent immune responses indicate that the macrophages is the sole APC required for the induction of DTH effector T cells in SJL mice. The defect occurs during the sensitization phase of the DTH response; however, it has not been determined whether a Th cell, which is required for the induction of CD4+ DTH effector T cells, was present in the DTH unresponsive SJL mice. In this study, we have determined that the Thy-1+ helper cell is induced upon Ag stimulation of nonresponder mice and present evidence for the existence of an accessory cell distinct from the macrophage that induces CD4+ DTH effector T cells. Our data indicate that CD4+ DTH effector T cells are induced in an Ag-specific and MHC-restricted manner by an adherent macrophage that expresses the Mac-1+, Mac-2-, Mac-3+, I-A+ phenotype. Adoptive transfer of as few as 100 of the Mac-1+, Mac-2-, or Mac-3+ subsets from DTH responsive donors to DTH unresponsive recipients is able to overcome the DTH deficit. The activation of CD4+ DTH effector T cells in the SJL mouse cells also requires a Thy-1+, Lyt-1+, CD3-, CD4-, CD8-, helper cell. In contrast to the Mac-1+, Mac-3+, I-A+ accessory cell, this helper cell requires an adherent, irradiation resistant, accessory cell that expresses the Mac-1+, Mac-2-, Mac-3-, I-A- surface phenotype for activation. Further, the interaction between this accessory cell and the Thy-1+ helper cell is neither Ag-specific nor MHC restricted. This is the first demonstration of an accessory cell requirement for the Thy-1+, Lyt-1+, B220-, CD4-, CD8-, CD3- DTH Th cell. These data indicate that the activation of the triple negative helper cells and subsequent activation of the CD4+ effector T cells are regulated by two distinct macrophage subpopulations.

Distinct subsets of accessory cells activate Thy-1+ triple negative (CD3-, CD4-, CD8-) cells and Th-1 delayed-type hypersensitivity effector T cells

The SJL strain of mice possess a unique developmental delay in the ability to exhibit delayed-type hypersensitivity (DTH) responses after immunization with a wide variety of Ag. Similar to other models of DTH, the adoptive transfer of syngeneic Ag-pulsed macrophages from DTH-responsive mice into these DTH-unresponsive mice results in the activation of Ag-specific, CD4+ DTH effector Th1 T cells. The absence of other defects in APC-dependent immune responses indicate that the macrophages is the sole APC required for the induction of DTH effector T cells in SJL mice. The defect occurs during the sensitization phase of the DTH response; however, it has not been determined whether a Th cell, which is required for the induction of CD4+ DTH effector T cells, was present in the DTH unresponsive SJL mice. In this study, we have determined that the Thy-1+ helper cell is induced upon Ag stimulation of nonresponder mice and present evidence for the existence of an accessory cell distinct from the macrophage that induces CD4+ DTH effector T cells. Our data indicate that CD4+ DTH effector T cells are induced in an Ag-specific and MHC-restricted manner by an adherent macrophage that expresses the Mac-1+, Mac-2-, Mac-3+, I-A+ phenotype. Adoptive transfer of as few as 100 of the Mac-1+, Mac-2-, or Mac-3+ subsets from DTH responsive donors to DTH unresponsive recipients is able to overcome the DTH deficit. The activation of CD4+ DTH effector T cells in the SJL mouse cells also requires a Thy-1+, Lyt-1+, CD3-, CD4-, CD8-, helper cell. In contrast to the Mac-1+, Mac-3+, I-A+ accessory cell, this helper cell requires an adherent, irradiation resistant, accessory cell that expresses the Mac-1+, Mac-2-, Mac-3-, I-A- surface phenotype for activation. Further, the interaction between this accessory cell and the Thy-1+ helper cell is neither Ag-specific nor MHC restricted. This is the first demonstration of an accessory cell requirement for the Thy-1+, Lyt-1+, B220-, CD4-, CD8-, CD3- DTH Th cell. These data indicate that the activation of the triple negative helper cells and subsequent activation of the CD4+ effector T cells are regulated by two distinct macrophage subpopulations.

Demyelination induced by murine hepatitis virus JHM strain (MHV-4) is immunologically mediated

The neurotropic mouse hepatitis viruses (MHV), in particular strain JHM (JHMV or MHV-4), cause experimental central nervous system demyelination that pathologically resembles multiple sclerosis, an important human demyelinating disease. The mechanism of JHMV-induced demyelination remains unclear, though its tropism for oligodendrocytes had led to the belief that JHMV causes demyelination by direct lysis of these myelin-producing cells. However, several studies have also implicated the involvement of immune responses in the demyelinating process. In this communication, we present evidence that generalized immunosuppression with gamma irradiation prevents JHMV-induced demyelination, a finding that was not limited to a particular strain of JHMV or to one strain of mouse. In addition, significant paralytic-demyelinating disease was restored to infected, irradiated mice after the adoptive transfer of nylon wool nonadherent splenic cells and appeared to be restricted by the major histocompatibility complex (MHC). These observations indicate that the principal mechanisms of JHMV-induced demyelination are most likely immunopathological.

Effective clearance of mouse hepatitis virus from the central nervous system requires both CD4+ and CD8+ T cells

Both CD4+ and CD8+ T cells are required for the clearance of virus from the central nervous system following infection with the JHM strain of mouse hepatitis virus. Development of antiviral antibodies requires the presence of CD4+ T cells but appears to play a minimal role in the reduction of virus. The data presented are consistent with the hypothesis that clearance of JHM virus is mediated by virus-specific CD8+ T cells, which appear to require the presence of CD4+ T cells.

Genetics of mouse hepatitis virus transcription: identification of cistrons which may function in positive and negative strand RNA synthesis

A panel of 26 temperature-sensitive mutants of MHV-A59 were selected by mutagenesis with either 5-fluorouracil or 5-azacytidine. Complementation analysis revealed the presence of one RNA+ and five RNA- complementation groups. None of the RNA- complementation groups transcribed detectable levels of positive- or negative-stranded RNA at the restrictive temperature. Temperature shift experiments after the onset of mRNA synthesis revealed at least two classes of RNA- mutants. RNA- complementation groups A, B, D, and E were blocked in the ability to release infectious virus and transcribe mRNA and genome, while group C mutants continued to release infectious virus and transcribe both mRNA and genome. Temperature shift experiments at different times postinfection suggest that the group C mutants encode a function required early in viral transcription which affects the overall rate of positive strand synthesis. Analysis of steady state levels of negative strand RNA after the shift indicate that the group C mutants were probably blocked in the ability to synthesize additional minus strand RNA under conditions in which the group E mutants continued low levels of minus strand synthesis. These data suggest that at least four cistrons may be required for positive strand synthesis while the group C cistron functions during minus strand synthesis.

Establishing a genetic recombination map for murine coronavirus strain A59 complementation groups

MHV-A59 temperature-sensitive mutants, representing one RNA+ and five RNA- complementation groups, were isolated and characterized by genetic recombination techniques. Maximum recombination frequencies occurred under multiplicities of infection greater than 10 each in which 99.99% of the cells were co-infected. Recombination frequencies between different ts mutants increased steadily during infection and peaked late in the virus growth cycle. These data suggest that recombination is a late event in the virus replication cycle. Recombination frequencies were also found to range from 63 to 20,000 times higher than the sum of the spontaneous reversion frequencies of each ts mutant used in the cross. Utilizing standard genetic recombination techniques, the five RNA- complementation groups of MHV-A59 were arranged into an additive, linear, genetic map located at the 5' end of the genome in the 23-kb polymerase region. These data indicate that at least five distinct functions are encoded in the MHV polymerase region which function in virus transcription. Moreover, using well-characterized ts mutants the recombination frequency for the entire 32-kb MHV genome was found to approach 25% or more. This is the highest recombination frequency described for a nonsegmented, linear, plus-polarity RNA virus.

Determinants of coronavirus MHV pathogenesis are localized to 3' portions of the genome as determined by ribonucleic acid-ribonucleic acid recombination

Recombinant viruses derived from coronaviruses mouse hepatitis virus strains JHM and A59, were used to map biologic properties of the virus to viral genes. The 3' portion (about 25%) of the viral genome, including the genes coding for all of the structural proteins, controls biologic properties such as organ tropism of the virus, pattern of the virus-induced central nervous system pathology in mice, plaque morphology, and virus yield in tissue culture.