Neutralizing antibody to Mengo virus, induced by synthetic peptides

Mapping B-cell epitopes in equine rhinitis B viruses and identification of a neutralising site in the VP1 C-terminus

Erbovirus is a genus of the family Picornaviridae and equine rhinitis B virus (ERBV) is the sole species. Erboviruses infect horses causing acute respiratory disease and sub-clinical and persistent infections. Despite the high seroprevalence and worldwide distribution of these viruses, the pathogenesis and antigenic structure of the three ERBV serotypes (ERBV1, 2 and 3) is poorly understood. To characterise linear epitopes on ERBV structural proteins, a set of fusion proteins were expressed in Escherichia coli. These proteins were tested in Western blot and ELISA and reactive proteins were also used to identify neutralisation epitopes. VP1 contained serotype specific epitopes whereas VP2 was highly cross-reactive across the serotypes. The C-terminus of VP1 accounted for most of the reactivity of full-length VP1 and was also the location of a neutralising site in each serotype.

Protection against lethal encephalomyocarditis virus infection in the absence of serum-neutralizing antibodies

Although the ability of serum-neutralizing antibodies to protect against picornavirus infection is well established, the contribution of cell-mediated immunity to protection is uncertain. Using major histocompatibility complex class II-deficient (RHAbeta-/-) mice, which are unable to mediate CD4(+) T-lymphocyte-dependent humoral responses, we demonstrated antibody-independent protection against lethal encephalomyocarditis virus (EMCV) infection in the natural host. The majority of RHAbeta-/- mice inoculated with 10(4) PFU of attenuated Mengo virus (vMC24) resolved infection and were resistant to lethal challenge with the highly virulent, serotypically identical cardiovirus, EMCV. Protection in these mice was in the absence of detectable serum-neutralizing antibodies. Depletion of CD8(+) T lymphocytes prior to lethal EMCV challenge ablated protection in vMC24-immunized RHAbeta-/- mice. The CD8(+) T-lymphocyte-dependent protection observed in vivo may, in part, be the result of cytotoxic T-lymphocyte (CTL) activity, as CD8(+) T splenocytes exhibited in vitro cytolysis of EMCV-infected targets. The existence of virus-specific CD8(+) T-lymphocyte memory in these mice was demonstrated by increased expression of cell surface activation markers CD25, CD69, CD71, and CTLA-4 following antigen-specific reactivation in vitro. Although recall response in vMC24-immunized RHAbeta-/- mice was intact and effectual shortly after immunization, protection abated over time, as only 3 of 10 vMC24-immunized RHAbeta-/- mice survived when rechallenged 90 days later. The present study demonstrating CD8(+) T-lymphocyte-dependent protection in the absence of serum-neutralizing antibodies, coupled with our previous results indicating that vMC24-specific CD4(+) T lymphocytes confer protection against lethal EMCV in the absence of prophylactic antibodies, suggests the existence of nonhumoral protective mechanisms against picornavirus infections.

Major linear antibody epitopes and capsid proteins differentially induce protective immunity against Theiler's virus-induced demyelinating disease

Theiler's murine encephalomyelitis virus-induced immunologically mediated demyelinating disease (TMEV-IDD) in susceptible mice provides a relevant infectious model for multiple sclerosis. Previously, we have identified six major linear antibody epitopes on the viral capsid proteins. In this study, we utilized fusion proteins containing individual capsid proteins and synthetic peptides containing the linear antibody epitopes to determine the potential role of antibody response in the course of virus-induced demyelination. Preimmunization of susceptible mice with VPI and VP2 fusion proteins, but not VP3, resulted in the protection from subsequent development of TMEV-IDD. Mice free of clinical symptoms following preimmunizations with fusion proteins displayed high levels of antibodies to the capsid proteins corresponding to the immunogens. In contrast, the level of antibodies to a particular linear epitope, A1C (VP1(262-276)), capable of efficiently neutralizing virus in vitro increased with the progression of disease. Further immunization with synthetic peptides containing individual antibody epitopes indicated that antibodies to the epitopes are differentially effective in protecting from virus-induced demyelination. Taken together, these results suggest that antibodies to only certain linear epitopes are protective and such protection may be restricted during the early stages of viral infection.

A multivalent minigene vaccine, containing B-cell, cytotoxic T-lymphocyte, and Th epitopes from several microbes, induces appropriate responses in vivo and confers protection against more than one pathogen

The development of safe and effective vaccines remains a major goal in the prevention, and perhaps treatment, of infectious diseases. Ideally, a single vaccine would confer protection against several pathogens and would induce both cellular and humoral arms of the immune response. We originally demonstrated that two virus-specific cytotoxic T-lymphocyte (CTL) epitopes, from the same virus but presented by different major histocompatibility complex alleles, when linked in tandem as minigenes in a recombinant vaccinia virus, could confer complete protection against subsequent viral challenge. In the study, we extended this approach, which we termed string of beads, expanding the immunogenic scope in two ways: first, by introduction of T helper (Th) and B-cell (antibody) epitopes alongside CTL epitopes and second, by including immunogenic sequences from a variety of infectious agents, five viruses and one bacterium. The vaccine (VV-sv) comprises CTL epitopes from Sendai virus, respiratory syncytial virus, and lymphocytic choriomeningitis virus (LCMV); Th epitopes from vesicular stomatitis virus and Mycobacterium tuberculosis; and an antibody epitope from mengovirus. The construct contains a single start codon, and the epitopes are linked directly, without intervening spacer amino acids. There was some concern that the combination of several normally immunodominant epitopes might result in a new hierarchy of dominance, in which certain epitopes predominated and others exhibited reduced immunogenicity. However we show that when analyzed in tissue culture and in vivo, all six epitopes are expressed. CTL and Th cells are induced in vivo, along with neutralizing antibody. The induced immunity is biologically relevant: after VV-sv immunization, the antimengovirus antibody confers protection against mengovirus challenge. Similarly, CTL induced by the LCMV epitope protected mice against challenge with this agent. Thus, a polyvalent, minigene-based vaccine can simultaneously induce several classes of immune response and thereby can confer protection against diverse pathogens.

Stimulation of a memory B cell response does not require primed helper T cells

The use of universally immunogenic T cell epitopes, such as those identified in tetanus toxin or malaria circumsporozoite protein, could represent a major improvement in the development of synthetic vaccines. However, one limitation of this approach is the lack of T cell cross-reactivity between the vaccine and the pathogen. To determine whether the memory B cell response elicited by immunization with a synthetic peptide containing a B cell epitope linked to a T cell epitope can be restimulated by the same B cell epitope linked to different T cell epitope(s), we used a synthetic peptide which contains non-overlapping B and T cell determinants from hepatitis B surface antigen (HBsAg) of hepatitis B virus (HBV). The results of this study clearly show that primed T cells can increase the antibody response against a B cell epitope linked to the priming T cell determinant. However, the antibody response obtained was weaker than that obtained after two injections of the peptide containing both B and T cell epitopes, showing the important role played by memory B cells in secondary antibody responses. Moreover, a strong antibody response against the B cell epitope was elicited by boosting mice with the B cell epitope linked to a heterologous carrier, thus demonstrating that a strong B cell memory response can be revealed in the absence of primed T cells. These results therefore provide new important information for the design of synthetic or recombinant vaccines.

Antibody recognition of picornaviruses and escape from neutralization: a structural view

Escape of picornaviruses from neutralization by monoclonal antibodies is mediated by substitutions of very few, defined amino acid residues of the capsid, generally located on the tip of some surface-exposed loops. Substitutions at the same positions are possibly of major relevance to antigenic variation of picornaviruses in the field. Such residues tend to cluster in discrete areas, termed antigenic sites. The structure of virus-antibody and peptide-antibody complexes, determined by cryoelectron microscopy and X-ray crystallography, combined with studies using site-directed mutagenesis, are beginning to reveal new features of picornavirus epitopes. This information complements and expands the view on picornavirus antigenicity previously provided by analyses of antibody-escape mutants. In addition to amino acids found replaced in escape mutants, other surface residues which remain invariant in spite of immune pressure also participate in contacts with the antibody molecule. Some invariant residues are even critical for the antigen-antibody interaction. Escape mutations occur at the subset of antigenically critical residues which are tolerant to change because they are not essentially involved in capsid structure or function. Restrictions to variation differ among epitopes; this may contribute to explain the different number of serotypes among picornaviruses, and the frequency at which antigenically highly divergent variants occur in the field.

Picornavirus-specific CD4+ T lymphocytes possessing cytolytic activity confer protection in the absence of prophylactic antibodies

Picornaviruses are a family of positive-strand RNA viruses that are responsible for a variety of devastating human and animal diseases. An attenuated strain of mengovirus (vMC24) is serologically indistinguishable from the lethal murine wild-type mengovirus and encephalomyocarditis virus (EMCV). Immunogen-specific stimulation of vMC24-immune splenocytes in vitro demonstrates preferential activation of CD4+ lymphocytes. vMC24-immune splenocytes adoptively transferred to naive recipients conferred protection against lethal EMCV challenge. Immune splenocytes, expanded in vitro, were > 92% CD4+ T lymphocytes. Interestingly, adoptive transfer of these expanded cells engendered protection against lethal challenge. In vivo depletion of CD4+ T lymphocytes prior to lethal challenge abrogated survival of transfer recipients, confirming that CD4+ T lymphocytes were essential for protection. Subsequent rechallenge of vMC24-immune splenocyte recipients with a greater EMCV dose elicited serum neutralizing antibody titers paralleling the high titers observed in vMC24-immunized mice. Unexpectedly, an augmented humoral response was absent in vMC24-specific CD4+ T-cell recipients after the secondary challenge. Moreover, comparably low serum neutralizing antibody titers failed to protect passive transfer recipients when correspondingly challenged. vMC24-immune splenocytes expanded in vitro (> 94% CD4+) lysed vMC24-infected A20.J target cells. The ability to transfer protection with primed CD4+ T cells, in the absence of primed B lymphocytes or immune sera, is novel for picornaviral infections. Consequently, mechanisms such as CD4+ cytolytic T-lymphocyte activity are implicated in mediating protection.

First peptide vaccine providing protection against viral infection in the target animal: studies of canine parvovirus in dogs

A synthetic peptide vaccine which protects dogs against challenge with virulent canine parvovirus is described. The amino acid sequence used was discovered in previous studies on the immunogenic properties of previously mapped antigenic sites and represents the amino-terminal region of viral protein VP2. As with marker vaccines, it is possible to discriminate between vaccinated dogs that have not been exposed to the virus and dogs that have been infected with the virus. The protective mechanism can be explained by a humoral response against the peptide aided by T-cell epitopes contained in the carrier protein used for peptide coupling. This is the first example of a synthetic peptide vaccine that induces protection in target animals.