Induction of protective immunity against coronavirus-induced encephalomyelitis: evidence for an important role of CD8+ T cells in vivo

Modulation of the immune response to the severe acute respiratory syndrome spike glycoprotein by gene-based and inactivated virus immunization

Although the initial isolates of the severe acute respiratory syndrome (SARS) coronavirus (CoV) are sensitive to neutralization by antibodies through their spike (S) glycoprotein, variants of S have since been identified that are resistant to such inhibition. Optimal vaccine strategies would therefore make use of additional determinants of immune recognition, either through cellular or expanded, cross-reactive humoral immunity. Here, the cellular and humoral immune responses elicited by different combinations of gene-based and inactivated viral particles with various adjuvants have been assessed. The T-cell response was altered by different prime-boost immunizations, with the optimal CD8 immunity induced by DNA priming and replication-defective adenoviral vector boosting. The humoral immune response was enhanced most effectively through the use of inactivated virus with adjuvants, either MF59 or alum, and was associated with stimulation of the CD4 but not the CD8 response. The use of inactivated SARS virus with MF59 enhanced the CD4 and antibody response even after gene-based vaccination. Because both cellular and humoral immune responses are generated by gene-based vaccination and inactivated viral boosting, this strategy may prove useful in the generation of SARS-CoV vaccines.

Communication between S1N330 and a region in S2 of murine coronavirus spike protein is important for virus entry into cells expressing CEACAM1b receptor

The soluble receptor-resistant (srr) mutants, srr7 and srr11, isolated from a murine coronavirus, mouse hepatitis virus (MHV) JHMV, have an amino acid mutation at positions 1114 (Leu to Phe) and 65 (Leu to His), respectively, in the spike (S) protein. These mutants failed to efficiently infect BHK cells expressing CEACAM1b (BHK-R2), due to their low entry into this cell line, although they infected cells expressing CEACAM1a (BHK-R1) in a manner similar to that of wild-type (wt) JHMV cl-2 (Matsuyama and Taguchi, Virology 273, 80-89, 2000). Following the repeated passage of these mutants through BHK-R2 cells, viruses were no longer isolated from srr11-infected cells, while two distinct mutants, srr7A and srr7B, were obtained from srr7-infected cells. Srr7A and srr7B grew 2 log10 higher than srr7 and induced fusion in BHK-R2 cells, being similar to wt virus. In addition to the amino acid change at position 1114 that stemmed from parental srr7, srr7A and srr7B had mutations around position 280, corresponding to the third region of the S1N330 receptor-binding site (S1N330-III) common to all MHV strains examined thus far. Srr7A and srr7B S proteins showed high fusogenicity in both BHK-R1 and BHK-R2 cells, like the wt virus, while srr7Aa and srr7Ba S proteins, which had mutations in S1N330-III but not at amino acid 1114, exhibited profoundly reduced fusion activity in these cell lines. These findings suggest that communication between S1N330-III and the amino acid at position 1114 is important for efficient fusion activity in BHK-R2 cells. S1N330-III is a possible region in the S1 involved in viral entry into cells.

Yellow fever virus encephalitis: properties of the brain-associated T-cell response during virus clearance in normal and gamma interferon-deficient mice and requirement for CD4+ lymphocytes

Viral encephalitis caused by neuroadapted yellow fever 17D virus (PYF) was studied in parental and gamma interferon (IFN-gamma)-deficient (IFN-gamma knockout [GKO]) C57BL/6 mice. The T-cell responses which enter the brain during acute fatal encephalitis of nonimmunized mice, as well as nonfatal encephalitis of immunized mice, were characterized for relative proportions of CD4+ and CD8+ cells, their proliferative responses, and antigen-specific expression of cytokines during stimulation in vitro. Unimmunized mice accumulated only low levels of T cells within the brain during fatal disease, whereas the brains of immunized mice contained higher levels of both T-cell subsets in response to challenge, with CD8+ cells increased relative to the CD4+ subset. The presence of T cells correlated with the time at which virus was cleared from the central nervous system in both parental and GKO mice. Lymphocytes isolated from the brains of challenged immunized mice failed to proliferate in vitro in response to T-cell mitogens or viral antigens; however, IFN-gamma, interleukin 4 (IL-4), and, to a lesser extent, IL-2 were detectable after stimulation. The levels of IFN-gamma, but not IL-2 or IL-4, were augmented in response to viral antigen, and this specificity was detectable in the CD4+ compartment. When tested for the ability to survive both immunization and challenge with PYF virus, GKO and CD8 knockout mice did not differ from parental mice (80 to 85% survival), although GKO mice exhibited a defect in virus clearance. In contrast, CD4 knockout and Igh-6 mice were unable to resist challenge. The data implicate antibody in conjunction with CD4+ lymphocytes bearing a Th1 phenotype as the critical factors involved in virus clearance in this model.

Impaired entry of soluble receptor-resistant mutants of mouse hepatitis virus into cells expressing MHVR2 receptor

Mouse hepatitis virus (MHV) JHMV and its soluble receptor-resistant (srr) mutants, srr7, srr11, and srr18, grew and induced syncytia equally well in BHK-R1 cells expressing the MHVR1 receptor derived from MHV-susceptible BALB/c mice. In contrast, srr growth and syncytia formations were drastically reduced relative to wild-type (wt) virus in BHK-R2 cells expressing the MHVR2 receptor from MHV-resistant SJL mice. Infections by these srr mutants in BHK-R2 cells were 0.7 to 1.5 log10 less efficient than those of wt virus. BHK cells expressing both MHVR1 and MHVR2 supported srr replication to the same extent as did BHK-R1 cells, suggesting that inefficient infection by srr mutants in BHK-R2 cells resulted from the absence of the effective receptor MHVR1. Virus-receptor binding tests failed to demonstrate a difference between the abilities of wt and srr18 to bind MHVR2. The binding of srr7 and srr11 to both MHVR1 and MHVR2 was revealed lower by two- to fourfold relative to the wt binding. The fusion activity of srr S proteins as examined by the expression with recombinant vaccinia virus was apparently lower than that of the wt S protein in BHK-R2 cells, while there was not such a remarkable difference in BHK-R1 cells. This suggests that the most likely reason for inefficient infection by mutants in BHK-R2 is impaired virus entry into cells. These observations suggest that inefficient infections in BHK-R2 cells by srr mutants occur in the absence of a functional receptor MHVR1, which plays an important role in srr entry into cells.

Isolation and characterization of murine coronavirus mutants resistant to neutralization by soluble receptors

Murine coronavirus mutants resistant to neutralization with soluble receptors were isolated to study the receptor-binding site on the S proteins since such mutants were expected to have mutations in an important site for receptor-binding. We have isolated five soluble receptor-resistant (srr) mutants which had mutations of a single amino acid at 3 different positions in S protein. Srr mutant 11 with an amino acid change at position 65 (Leu to His) in the S1 subunit showed an extremely reduced binding by virus overlay protein blot assay. However srr mutants with a mutation at 1114 (Leu to Phe) (srr mutants 3, 4 and 7) or 1163 (Cys to Phe) (srr mutant 18) in the S2 subunit had receptor-binding activity similar to that of wild type cl-2. These results suggest that an amino acid at position 62 located in a conserved region among MHV strains is in particular important for receptor binding. We also discuss why srr mutants with a mutation in S2 showed high resistance to neutralization by soluble receptor, irrespective of their binding to MHV receptors.

Isolation and recombinant expression of an MHV-JHM neutralising monoclonal antibody

The monoclonal antibody A1 (mab A1) efficiently neutralises the infection of susceptible cells by the murine hepatitis virus MHV-JHM in vitro and in vivo (Wege et al., 1984). The variable regions of mab A1 were amplified from mRNA of the respective hybridoma cell line by RT-PCR and integrated into different eukaryotic expression vectors. The biological function of the recombinant antibody constructs was verified by virus neutralisation assays. Whereas a complete recombinant antibody (mab A1rec.) expressed in transfected murine myeloma cells inhibited the MHV-JHM infection as well as the parental antibody, a single-chain Fv derived from mab A1 did not show any neutralising activity.

Antibody and cytokine responses in kittens during the development of feline infectious peritonitis (FIP)

Two recombinant FIPV spike proteins were assessed for their immunogenic properties in 8-week-old kittens, which were then challenged intranasally with FIPV 79-1146. Humoral responses were assessed by ELISA and serum neutralisation test. Changes in PBMC cytokine mRNA levels were detected by a reverse transcription, semiquantitative polymerase chain reaction assay (RT-sqPCR), assessing IL-2, IL-4, IL-6, IL-10, IL-12 and IFNgamma. All of the kittens developed clinical signs typical of FIP, which were confirmed on gross post mortem examination. The recombinant proteins induced little or no specific antibody response prior to challenge, and failed to alter the course of disease compared to controls. One week after virus challenge, the stimulated PBMCs showed small increases in the expression of IL-6 and IFNgamma mRNA, which correlated with a transient pyrexia. After this time expression of IL-6 mRNA remained unaltered but, as FIP developed, mRNA levels of IL-2, IL-4, IL-10, IL-12 and IFNgamma became markedly depressed.

Variability of persisting MHV RNA sequences constituting immune and replication-relevant domains

Survivors of acute infection with the neurotropic JHM strain of mouse hepatitis virus develop a persistent infection of the central nervous system associated with chronic ongoing demyelination. Persistence is characterized by viral RNA in the absence of infectious virus. To associate persistence with possible immune evasion and/or replication defects, viral RNA from brains of acutely and persistently infected mice was examined for mutations by reverse transcriptase-PCR. Sequences analyzed included the encapsidation sequence (ECS), the transmembrane domains of the matrix (M) protein, and a cytotoxic T cell (CTL) epitope within the nucleocapsid (N) protein. The ECS, present only on genomic RNA, revealed minimal variability and was detected out to 120 days postinfection, suggesting low levels of replication. The M gene sequence also remained stable during persistence despite random mutations during the acute phase. Although the N gene sequence exhibited the greatest diversity, mutations were random and not selected for during persistence. A single exception was detected comprising a prominent Pro to Ser substitution in a region of N not associated with any known regulatory or immune function. Of the N gene mutations found within the CTL epitope in responder mice (H-2d), one resulted in reduced CTL recognition with no evidence of antagonist activity. However, this mutation was also detected in nonresponder mice (H-2b), suggesting that escape variants arising from CTL pressure play no role in establishing persistence in immunocompetent hosts infected as adults.

Identification of spike protein residues of murine coronavirus responsible for receptor-binding activity by use of soluble receptor-resistant mutants

We previously demonstrated by site-directed mutagenesis analysis that the amino acid residues at positions 62 and 214 to 216 in the N-terminal region of mouse hepatitis virus (MHV) spike (S) protein are important for receptor-binding activity (H. Suzuki and F. Taguchi, J. Virol. 70:2632-2636, 1996). To further identify the residues responsible for the activity, we isolated the mutant viruses that were not neutralized with the soluble form of MHV receptor proteins, since such mutants were expected to have mutations in amino acids responsible for receptor-binding activity. Five soluble-receptor-resistant (srr) mutants isolated had mutations in a single amino acid at three different positions: one was at position 65 (Leu to His) (srr11) in the S1 subunit and three were at position 1114 (Leu to Phe) (srr3, srr4, and srr7) and one was at position 1163 (Cys to Phe) (srr18) in the S2 subunit. The receptor-binding activity examined by a virus overlay protein blot assay and by a coimmunoprecipitation assay showed that srr11 S protein had extremely reduced binding activity, while the srr7 and srr18 proteins had binding activity similar to that of wild-type cl-2 protein. However, when cell surface receptors were used for the binding assay, all srr mutants showed activity similar to that of the wild type or only slightly reduced activity. These results, together with our previous observations, suggest that amino acids located at positions 62 to 65 of S1, a region conserved among the MHV strains examined, are important for receptor-binding activity. We also discuss the mechanism by which srr mutants with a mutation in S2 showed high resistance to neutralization by a soluble receptor, despite their sufficient level of binding to soluble receptors.

Cooperation between transmissible gastroenteritis coronavirus (TGEV) structural proteins in the in vitro induction of virus-specific antibodies

Following infection of haplotype defined NIH-miniswine with virulent transmissible gastroenteritis coronavirus (TGEV), isolated mesenteric lymph node CD4+ T-cells mounted a specific proliferative response against infectious or inactivated purified virus in secondary in vitro stimulation. A specific, dose-dependent response to the three major recombinant viral proteins: spike (S), membrane (M), and nucleoprotein (N), purified by affinity chromatography, was characterized. Induction of in vitro antibody synthesis was analyzed. The purified recombinant viral proteins induced the in vitro synthesis of neutralizing TGEV-specific antibodies when porcine TGEV-immune cells were stimulated with each of the combinations made with two of the major structural proteins: S + N, S + M, and to a minor extent with M + N, but not by the individual proteins. S-protein was dissociated from purified virus using NP-40 detergent and then micellar S-protein oligomers (S-rosettes) were formed by removing the detergent. These occurred preferentially by the association of more than 10 S-protein trimmers. These S-rosettes in collaboration with either N or M-proteins elicited TGEV-specific antibodies with titers up to 84 and 60%, respectively, of those induced by the whole virus. N-protein could be partially substituted by a 15-mer peptide that represents a T helper epitope previously identified in N-protein (Antón et al. (1995)). These results indicate that the induction of high levels of TGEV-specific antibodies requires stimulation by at least two viral proteins, and that optimum responses are induced by a combination of S-rosettes and the nucleoprotein.

Genetic control of anti-idiotypic vaccination against coronavirus infection

The idiotypic network can be experimentally altered to induce protective immune responses against microbial pathogens. Both internal image and noninternal image anti-idiotypic (anti-Id) antibodies have been shown to trigger antigen (Ag)-specific immune responses. Therefore, mechanisms of anti-Id vaccination appear to go beyond structural mimicry of Ag, but remain undefined. Using the neurotropic murine coronavirus animal model, we have previously shown that a polyclonal noninternal image anti-Id (Ab2) could vaccinate BALB/c mice. To characterize its mode of action, we have examined the immune modulating capability of this Ab2 in vivo in strains of mice with different H-2 haplotypes. Even though only internal image anti-Id are expected to induce non-genetically restricted immunity, this noninternal image Ab2 induced protective immunity in four of eight genetically different strains of mice susceptible to coronavirus infection. These were BALB/c (H-2d), DBA/2 (H-2d), DBA/1 (H-2q), and SWR (H-2q) mice. Protection was generally correlated with the induction of specific antiviral Ab (Ab3) that showed biological properties, such as virus neutralization in vitro, similar to the initial Ab1. To evaluate the genetic implication of the H-2 haplotypes in protection, congenic mice were also tested. Vaccination profiles suggest that cooperation between background gene(s) of the BALB/c mouse with H-2d and H-2q loci is necessary for an optimal protective immune response, although the main genetic element(s) regulating the antiviral response to Ab2 inoculation appeared to be located outside the major histocompatibility complex. These results are consistent with the ability of Ab2 to induce protective antiviral antibodies in genetically different animals by biological mimicry.

Long-term humoral and cellular immunity induced by a single immunization with replication-defective adenovirus recombinant vector

This study examines the suitability of replication-defective adenovirus vectors for engineering recombinant vaccines. The immunological abilities and limitations of E1-deleted adenoviruses containing the lacZ gene (Ad-beta-gal) were investigated by examining the humoral and cellular immune responses to the beta-galactosidase protein. BALB/c mice (H-2d) were given in a single injection of recombinant adenovirus. The cytotoxic T lymphocyte (CTL) response of spleen cells was evaluated. Recognized target cells were H-2d-derived tumor cells transfected by the lac Z gene, or incubated with the 876-884 beta-galactosidase peptide known to be restricted by the Ld molecule of the major histocompatibility complex. A long-lasting beta-galactosidase-specific cytotoxic T cell response was obtained. By contrast, CTL from mice immunized with the Ld-restricted peptide were less specific for the endogenous epitope presented by the transfectants expressing beta-galactosidase. Ad-beta-gal-immunized mice were also protected against an intra-cerebral challenge with a recombinant vaccinia virus expressing the lac-Z gene. These results suggest that Ad-beta-gal-induced CTL have protective abilities in vivo. The induction of beta-galactosidase-specific T helper lymphocytes and humoral IgG responses were also examined. A proliferative response occurred only late after immunization and the primed T lymphocytes produced interleukin-2, but no interleukin-4. A humoral IgG response to the beta-galactosidase protein was detected 15-30 days after a single immunization and remained stable for 6 months without boosting. Lastly, we followed the evolution of the immune response over the course of successive immunizations. The magnitude and kinetics of the cellular and humoral responses were similar to those obtained after a single immunization. Consistent with these observations, an adenovirus-specific neutralizing antibody response was detected as early as the second immunization. Thus, a single immunization with a replication-defective adenovirus recombinant vector induces long-lasting humoral and cellular immune responses specific to the transgene product.

Mouse hepatitis virus-specific cytotoxic T lymphocytes protect from lethal infection without eliminating virus from the central nervous system

Acute infection of the central nervous system by the neurotropic JHM strain of mouse hepatitis virus (JHMV) induces nucleocapsid protein specific cytotoxic T lymphocytes (CTL) not found in the periphery (S. Stohlman, S. Kyuwa, J. Polo, D. Brady, M. Lai, and C. Bergmann, J. Virol. 67:7050-7059, 1993). Peripheral induction of CTL specific for the nucleocapsid protein of JHMV by vaccination with recombinant vaccinia viruses was unable to provide significant protection to a subsequent lethal virus challenge. By contrast, the transfer of nucleoprotein-specific CTL protected mice from a subsequent lethal challenge by reducing virus replication within the central nervous system, demonstrating the importance of the CTL response to this epitope in JHMV infection. Transfer of these CTL directly into the central nervous system was at least 10-fold more effective than peripheral transfer. Histological analysis indicated that the CTL reduced virus replication in ependymal cells, astrocytes, and microglia. Although the CTL were relatively ineffective at reducing virus replication in oligodendroglia, survivors showed minimal evidence of virus persistence within the central nervous system and no evidence of chronic ongoing demyelination.