T-cell-mediated clearance of mouse hepatitis virus strain JHM from the central nervous system

Clearance of the neurotropic JHM strain of mouse hepatitis virus from the central nervous system was examined by the transfer of spleen cells from immunized donors. A T cell with the surface phenotype of Thy1.2+ CD4+ CD8- asialo-GM1+ Mac-1- was found to be necessary for viral clearance. The surface phenotype and adherence to nylon wool suggest that these cells are activated helper-inducer T cells. Adoptive transfer to congenic histocompatibility strains demonstrated the necessity for compatibility at the D locus of the major histocompatibility complex. The expression of the CD4 surface marker and the requirement for major histocompatibility complex class I were further studied by the transfer of cells to recipients treated with anti-CD4 or anti-CD8 monoclonal antibodies. Treatment of recipients with either the anti-CD8 or the anti-CD4 antibodies inhibited virus clearance from the central nervous system. This suggests that the CD4+ cell acts as a helper and that virus is cleared from the central nervous system. This suggests that the CD4+ cell acts as a helper and that virus is cleared from the central nervous system by CD8+ cells that recognize viral antigen in the context of the H-2Db gene product.

Studies on the mechanism of protection from acute viral encephalomyelitis by delayed-type hypersensitivity inducer T cell clones

Previous studies have shown that mice can be protected from a lethal infection with the neurotropic JHM strain of mouse hepatitis virus (JHMV) by the adoptive transfer of delayed-type hypersensitivity (DTH)-inducer T cell clones specific for the virus. Protection does not involve the suppression of virus replication in the central nervous system (CNS) or via augmentation of the antiviral antibody response. In the present report we have compared the CNS lesions induced by JHMV in lethally infected and T cell clone protected mice. The presence of virus-specific T cell clones induced a transient increase in mononuclear cell infiltration into the parenchyma of the brains of protected mice, consistent with previous data suggesting that a DTH response was responsible for protection. Immunohistochemical studies suggested further that virus was not replicating in the ependyma or cellular infiltrate, but that the presence of the T cell clone prevented neuronal infection. While the mechanism of effectively altering the in vivo cellular tropism is unknown, survival is accompanied by increased specific destruction of target tissues with fulminant CNS demyelination and an increased incidence of persistent infection.

Evidence for a subpopulation of antigen-presenting cells specific for the induction of the delayed-type hypersensitivity response

Young adult SJL mice (8 weeks of age or younger) do not mount a delayed-type hypersensitivity (DTH) response due to the failure of a macrophage antigen-presenting cell (APC) to induce TDTH effector cells. SJL mice that are 10 weeks of age or older produce a normal DTH response. This genetic defect provides a model for the investigation of functional subpopulations of APC which interact with specific subpopulations of T cells. In this study, we used this model to examine whether macrophage APC impairment involves APC-dependent immune responses other than DTH. No age-dependent differences were found in the ability of spleen cells from SJL mice to proliferate and synthesize interleukin-2 in response to concanavalin A; nor was the proliferative response to a variety of antigenic stimuli affected. In addition, no differences were observed in the contact sensitivity response or in the in vitro generation of allogeneic cytotoxic T lymphocytes (CTL). In contrast, the in vivo generation of allogeneic CTL was significantly depressed in 6-week-old SJL and could not be restored to normal by the adoptive transfer of macrophages from DTH responsive 12-week-old SJL mice. Finally, examination of the humoral response of 6-week-old SJL indicated no impairment in IgM or IgG serum antibody levels or in the induction of splenic B cells. Thus, the macrophage APC regulating the induction of TDTH effector cells does not appear to participate in the induction of T helper cells for other cellular and humoral responses. These data support the hypothesis that distinct subpopulations of APC may regulate the induction of specific immune effector mechanisms.

Monoclonal antibodies to the matrix (E1) glycoprotein of mouse hepatitis virus protect mice from encephalitis

Monoclonal antibodies to the matrix or E1 glycoprotein of mouse hepatitis virus (MHV) were tested for their ability to protect mice from a normally lethal challenge of MHV-4. Four antibodies were tested, and two of these, J.1.3 and J.3.9, were protective. Protection did not correlate with virus neutralization in vitro, antibody isotype, recognition of a unique E1 antigenic site, or dependence on complement in vivo. Survival from acute encephalitis was followed by subacute demyelination, as has been shown with protection mediated by neutralizing monoclonal antibodies against the major glycoprotein, E2. These results demonstrate that antibodies which are specific for a viral matrix protein are able to alter the course of disease.

Specific interaction between coronavirus leader RNA and nucleocapsid protein

Northwestern blot analysis in the presence of competitor RNA was used to examine the interaction between the mouse hepatitis virus (MHV) nucleocapsid protein (N) and virus-specific RNAs. Our accompanying article demonstrates that anti-N monoclonal antibodies immunoprecipitated all seven MHV-specific RNAs as well as the small leader-containing RNAs from infected cells. In this article we report that a Northwestern blotting protocol using radiolabeled viral RNAs in the presence of host cell competitor RNA can be used to demonstrate a high-affinity interaction between the MHV N protein and the virus-specific RNAs. Further, RNA probes prepared by in vitro transcription were used to define the sequences that participate in such high-affinity binding. A specific interaction occurs between the N protein and sequences contained with the leader RNA which is conserved at the 5' end of all MHV RNAs. We have further defined the binding sites to the area of nucleotides 56 to 65 at the 3' end of the leader RNA and suggest that this interaction may play an important role in the discontinuous nonprocessive RNA transcriptional process unique to coronaviruses.

Interactions between coronavirus nucleocapsid protein and viral RNAs: implications for viral transcription

The interaction of the mouse hepatitis virus (MHV) nucleocapsid protein (N) and viral RNA was examined. Monoclonal antibody specific for N protein coimmunoprecipitated MHV genomic RNA as well as all six MHV subgenomic mRNAs found in MHV-infected cells. In contrast, monoclonal antibodies to the MHV E2 or E1 envelope glycoproteins, an anti-I-A monoclonal antibody, and serum samples from lupus patients did not immunoprecipitate the MHV mRNAs. Moreover, the anti-N monoclonal antibody did not coimmunoprecipitate vesicular stomatitis virus RNA or host cell RNA under conditions which immunoprecipitated all MHV RNAs. These data suggest a specific interaction between the N protein and the virus-specific mRNAs. Both the membrane-bound and cytosolic small MHV leader-specific RNAs of greater than 65 nucleotides long were immunoprecipitated only by anti-N monoclonal antibody. These data suggest that an N binding site is present within the leader RNA sequences at a site at least 65 nucleotides from the 5' end of genomic RNA and all six subgenomic mRNAs. The larger leader-containing RNAs originating from mRNA 1 and mRNA 6, as well as the MHV negative-stranded RNA, were also immunoprecipitated by the anti-N monoclonal antibody. These data indicate that the MHV N protein is associated with MHV-specific RNAs and RNA intermediates and may play an important functional role during MHV transcription and replication.

Maturation of the delayed-type hypersensitivity response in SJL mice: absence of effector cell induction

Immunization of young adult SJL mice (6 weeks of age) with a wide variety of particulate and soluble antigens does not elicit a delayed-type hypersensitivity (DTH) response. Young adult SJL lack a DTM response through 8 weeks of age but attain the mature adult level of responsiveness at 10 weeks of age. Nylon wool-enriched TDTH effectors and an antigen-specific T cell clone both elicited DTH responses when transferred into 6-week-old SJL. Thus, the cascade of events at the local site appears to be functionally intact in 6-week-old SJL. However, T cells from immunized 6-week-old SJL fail to transfer DTH responsiveness to naive 6-week-old SJL recipients unless macrophages from 12-week-old SJL supplemented the immunization. Thus, the unresponsiveness of 6-week-old SJL appears to be due to a lack of induction of TDTH effectors. In addition, SJL mice immunized at 6 weeks of age and challenged with antigen at the DTH responsive age of 12 weeks did not mount a DTH response. Immunized 6-week-old SJL, supplemented with macrophages from 12-week-old SJL, and 12-week-old SJL did respond to a second antigenic challenge when held for an equivalent 6-week period. Thus, 6-week-old SJL fail to induce TDTH effectors and to generate memory TDTH cells. Finally, antigen-pulsed macrophages from 12-week-old SJL transferred DTH responsiveness into naive 6-week and 12-week-old SJL recipients, while antigen-pulsed cells from 6-week donors were unable to transfer DTH responsiveness. These data indicate that the maturational deficit in the DTH responsiveness of 6-week-old SJL resides in the inability of macrophages to induce TDTH effectors.

Delayed-type hypersensitivity response in the central nervous system during JHM virus infection requires viral specificity for protection

The JHM strain of mouse hepatitis virus (JHMV) elicits an I-A-restricted delayed-type hypersensitivity (DTH) response mediated by a Thy-1+, Lyt-1+, and CD4+ T cell. Adoptive transfer of these polyclonal CD4+ T cells from immunized mice prevents death in lethally infected recipients without significantly reducing virus titer in the central nervous system (CNS). These observations raise the possibility that the recruitment of mononuclear cells into the CNS may play a critical role in survival from a lethal CNS infection. Transient DTH response to nonviral antigens induced an accumulation of monocytes in the CNS that was maximal at 48 h post-challenge and virtually resolved by 5 days post-challenge. By contrast the induction of prolonged DTH responses resulted in the accumulation of a large number of monocytes that persisted in the CNS for at least 5 days post-challenge. Neither type of DTH reaction suppressed virus replication or prevented death from concomitant lethal JHMV infection.

RNA recombination of murine coronaviruses: recombination between fusion-positive mouse hepatitis virus A59 and fusion-negative mouse hepatitis virus 2

It has previously been shown that the murine coronavirus mouse hepatitis virus (MHV) undergoes RNA recombination at a relatively high frequency in both tissue culture and infected animals. Thus far, all of the recombination sites had been localized at the 5' half of the RNA genome. We have now performed a cross between MHV-2, a fusion-negative murine coronavirus, and a temperature-sensitive mutant of the A59 strain of MHV, which is fusion positive at the permissive temperature. By selecting fusion-positive viruses at the nonpermissive temperature, we isolated several recombinants containing multiple crossovers in a single genome. Some of the recombinants became fusion negative during the plaque purification. The fusion ability of the recombinants parallels the presence or absence of the A59 genomic sequences encoding peplomers. Several of the recombinants have crossovers within 3' end genes which encode viral structural proteins, N and E1. These recombination sites were not specifically selected with the selection markers used. This finding, together with results of previous recombination studies, indicates that RNA recombination can occur almost anywhere from the 5' end to the 3' end along the entire genome. The data also show that the replacement of A59 genetic sequences at the 5' end of gene C, which encodes the peplomer protein, with the fusion-negative MHV-2 sequences do not affect the fusion ability of the recombinant viruses. Thus, the crucial determinant for the fusion-inducing capability appears to reside in the more carboxyl portion of the peplomer protein.

Antigenic assessment of coronaviruses isolated from patients with multiple sclerosis

Many studies have either supported or discounted the role of coronaviruses as etiologic agents in multiple sclerosis (MS). Two new approaches were applied to investigate this controversy. First, monoclonal antibodies specific for either murine coronaviruses (mouse hepatitis viruses) or human coronaviruses were used to characterize the antigenic features of MS-derived coronaviruses SK and SD. Both isolates were found to have a mouse hepatitis virus-type profile. Second, serum and cerebrospinal fluid antibodies to different coronaviruses, including SD, were measured in MS and control groups. No significant difference in antibody level to coronaviruses was found between MS and control samples. The results of these antigenic studies do not support a specific association between MS and coronaviruses.

In vivo RNA-RNA recombination of coronavirus in mouse brain

RNA-RNA recombination between different strains of the murine coronavirus mouse hepatitis virus (MHV) occurs at a very high frequency in tissue culture. To demonstrate that RNA recombination may play a role in the evolution and pathogenesis of coronaviruses, we sought to determine whether MHV recombination could occur during replication in the animal host of the virus. By using two selectable markers, i.e., temperature sensitivity and monoclonal antibody neutralization, we isolated several recombinant viruses from the brains of mice infected with two different strains of MHV. The recombination frequency was very high, and recombination occurred at multiple sites on the viral RNA genome. This finding suggests that RNA-RNA recombination may play a significant role in natural evolution and neuropathogenesis of coronaviruses.

RNA recombination of coronaviruses: localization of neutralizing epitopes and neuropathogenic determinants on the carboxyl terminus of peplomers

Murine coronaviruses undergo RNA recombination at a very high frequency. We have obtained a series of recombinant viruses using neutralizing monoclonal antibodies in conjunction with temperature-sensitive markers. All of the recombinants obtained have a crossover within gene C, which encodes the peplomer protein of the virus. The genetic structure of these recombinants suggests that the antigenic regions recognized by these neutralizing monoclonal antibodies are localized on the carboxyl-terminal one-third of the peplomer protein. Since the two monoclonal antibodies used are also associated with the critical determinants of virus neuropathogenicity, we conclude that both the neutralizing antibody binding sites and determinants of pathogenicity are localized at the carboxyl-terminal one-third of the peplomer. The variation of crossover sites in different recombinant viruses also allowed precise mapping of additional antigenic sites. RNA recombination thus presents a powerful genetic tool, and the carboxyl-terminal localization of the biological functions of peplomers suggests a distinct conformation of these viral membrane proteins.

Experimental neuropathology of chronic demyelination induced by a JHM virus variant (DS)

A small plaque variant of JHM virus has a markedly reduced ability to kill mice following intracerebral inoculation. Spinal cords of mice surviving 13 to 16 months following acute infection with this variant were examined ultrastructurally. Multiple subpial areas of demyelination in the anterior and lateral white matter were found in five of 13 mice. The lesions had more gliosis, fewer oligodendrocytes, and less remyelination than has been described following other infections with JHM virus. No conclusive evidence of active demyelination or viral-like particles was found. The pathogenesis of the lesions observed may be due to a persistent, attenuated infection of oligodendrocytes or to immunologic processes. These lesions were similar to chronic multiple sclerosis plaques. Therefore, this variant should prove to be a useful tool for studying the long-term effects of viral-induced demyelinating diseases.

Experimental demyelination induced by coronavirus JHM (MHV-4): molecular identification of a viral determinant of paralytic disease

The molecular basis for demyelination induced by the neurotropic murine coronavirus JHM (JHMV or MHV4) is unknown. We have attempted to explore this issue by using neutralizing monoclonal antibodies specific for the major JHMV glycoprotein (E2) to select sets of neutralization resistant (NR) antigenic variant viruses. Monoclonal antibodies J.7.2 and J.2.2 bind to topographically distinct sites on E2. NR variants selected with J.7.2, like parental JHMV, predominantly cause a fatal encephalitis when given intracerebrally to mice, while J.2.2-selected NR variants cause a subacute disease characterized by paralysis and severe demyelination. We report here that consecutive selection with both J.2.2 and J.7.2 monoclonal antibodies results in NR variants which are markedly attenuated in both encephalitic potential and ability to induce demyelination. Analysis of the different variants suggests that the subregion of E2 bound by monoclonal antibody J.7.2 may be a critical viral determinant of paralysis and demyelination in this model system.

Characterization of the structural proteins of the murine coronavirus strain A59 using monoclonal antibodies

Monoclonal antibodies reacting with the A59 strain of mouse hepatitis virus (MHV-A59) were characterized and those specific to the E2 major envelope glycoprotein were studied in detail. Antibodies were tested for their ability to neutralize viral infectivity (N+ characteristic) and prevent viral-induced cell-to-cell fusion (F+ characteristic). All four possible combinations of activities reflecting E2 functions were found, i.e., N+F+, N-F-, N+F-, and N-F+. In addition, competitive binding studies with these monoclonal antibodies revealed two nonoverlapping antigenic regions. The first region, designated A, was recognized by antibodies which included each of the four functional types. Region B was identified by a single monoclonal antibody with N-F- activities. The existence of antibodies which only neutralize virus or only block viral-induced fusion implies that the structures on E2 which serve as targets for neutralization and which induce fusion are not identical. The critical determinants for neutralization and fusion must be closely related topographically on E2 since both N+F- and N-F+ antibodies recognize the same antigenic region.

Analysis of intracellular small RNAs of mouse hepatitis virus: evidence for discontinuous transcription

We have previously shown the presence of multiple small leader-containing RNA species in mouse hepatitis virus (MHV)-infected cells. In this paper, we have analyzed the origin, structure, and mechanism of synthesis of these small RNAs. Using cDNA probes specific for leader RNA and genes A, D, and F, we demonstrate that subsets of these small RNAs were derived from the various viral genes. These subsets have discrete and reproducible sizes, varying with the gene from which they are derived. The size of each subset correlates with regions of secondary structure, whose free energy ranges from -1.6 to -77.1 kcal/mol, in each of the mRNAs examined. In addition, identical subsets were detected on the replicative intermediate (RI) RNA, suggesting that they represent functional transcriptional intermediates. The biological significance of these small RNAs is further supported by the detection of leader-containing RNAs of 47, 50, and 57 nucleotides in length, which correspond to the crossover sites in two MHV recombinant viruses. These data, coupled with the high frequency of RNA recombination during MHV infection, suggest that the viral polymerase may pause in or around regions of secondary structure, thereby generating pools of free leader-containing RNA intermediates which can reassociate with the template, acting as primers for the synthesis of full-length or recombinant RNAs. These data suggest that MHV transcription uses a discontinuous and nonprocessive mechanism in which RNA polymerase allows the partial RNA products to be dissociated from the template temporarily during the process of transcription.

The 5'-end sequence of the murine coronavirus genome: implications for multiple fusion sites in leader-primed transcription

The coronavirus leader-primed transcription model proposes that free leader RNA species derived from the 5'-end of the genomic RNA are utilized as a primer for the transcription of subgenomic mRNAs. To elucidate the precise mechanism of leader-priming, we cloned and sequenced the 5'-end of the mouse hepatitis virus genomic RNA. The 5'-terminal sequences are identical to the leader sequences present at the 5'-end of the subgenomic mRNAs. Two possible hairpin loop structures and an AU-rich region around the 3'-end of the leader sequence may provide the termination site for leader RNA synthesis. The comparison of 5'-end genomic sequences and the intergenic start sites for mRNA transcription revealed that there are homologous regions of 7-18 nucleotides at the putative leader/body junction sites. Some intergenic regions contain a mismatching nucleotide within this homologous region. We propose that free leader RNA binds to the intergenic region due to this homology and is cleaved at the mismatching nucleotide before serving as a primer. Thus, the free leader RNA species may be longer than the leader sequences in the subgenomic mRNAs and different mRNAs may have different leader/body junction sites.

Multiple recombination sites at the 5'-end of murine coronavirus RNA

Mouse hepatitis virus (MHV), a murine coronavirus, contains a nonsegmented RNA genome. We have previously shown that MHV could undergo RNA-RNA recombination in crosses between temperature-sensitive mutants and wild-type viruses at a very high frequency (S. Makino, J.G. Keck, S.A. Stohlman, and M.M.C. Lai (1986) J. Virol. 57, 729-737). To better define the mechanism of RNA recombination, we have performed additional crosses involving different sets of MHV strains. Three or possibly four classes of recombinants were isolated. Recombinants in the first class, which are similar to the ones previously reported, contain a single crossover in either gene A or B, which are the 5'-most genes. The second class of recombinants contain double crossovers in gene A. The third class of recombinants have crossovers within the leader sequence located at the 5'-end of the genome. The crossover sites of the third class have been located between 35 and 60 nucleotides from the 5'-end of the leader RNA. One of these recombinants has double crossovers within the short region comprising the leader sequences. Finally, we describe one recombinant which may contain a triple crossover. The presence of so many recombination sites within the 5'-end of the genome of murine coronaviruses confirms that RNA recombination is a frequent event during MHV replication and is consistent with our proposed model of "copy-choice" recombination in which RNA replication occurs in a discontinuous and nonprocessive manner.