A conserved coronavirus epitope, critical in virus neutralization, mimicked by internal-image monoclonal anti-idiotypic antibodies

Idiotype/anti-idiotype antibodies: as a glorious savior in COVID-19 pandemics

The idiotype network is experimentally modified to provide protective immunity against various microbial pathogens. Both internal and non-internal image-idiotype antibodies can trigger specific immune responses to antigens. The current outbreak of Severe Acute Respiratory Syndrome 2 (SARS-2) has provided a great opportunity to take advantage of idiotype / anti-idiotype antibodies as a protective regimen when no approved vaccine is available on earth. The current review identifies successful applications of idiotype/ anti-idiotype antibodies in various viral diseases and highlights their importance in COVID-19 pandemics. In the absence of vaccines and targeted therapies, polyclonal idiotype/ anti-idiotype antibodies against the viral structure may be a potential approach to the prevention and treatment of COVID-19 patients.

CD8+ T Cells Responding to the Middle East Respiratory Syndrome Coronavirus Nucleocapsid Protein Delivered by Vaccinia Virus MVA in Mice

Middle East respiratory syndrome coronavirus (MERS-CoV), a novel infectious agent causing severe respiratory disease and death in humans, was first described in 2012. Antibodies directed against the MERS-CoV spike (S) protein are thought to play a major role in controlling MERS-CoV infection and in mediating vaccine-induced protective immunity. In contrast, relatively little is known about the role of T cell responses and the antigenic targets of MERS-CoV that are recognized by CD8+ T cells. In this study, the highly conserved MERS-CoV nucleocapsid (N) protein served as a target immunogen to elicit MERS-CoV-specific cellular immune responses. Modified Vaccinia virus Ankara (MVA), a safety-tested strain of vaccinia virus for preclinical and clinical vaccine research, was used for generating MVA-MERS-N expressing recombinant N protein. Overlapping peptides spanning the whole MERS-CoV N polypeptide were used to identify major histocompatibility complex class I/II-restricted T cell responses in BALB/c mice immunized with MVA-MERS-N. We have identified a H2-d restricted decamer peptide epitope in the MERS-N protein with CD8+ T cell antigenicity. The identification of this epitope, and the availability of the MVA-MERS-N candidate vaccine, will help to evaluate MERS-N-specific immune responses and the potential immune correlates of vaccine-mediated protection in the appropriate murine models of MERS-CoV infection.

Monoclonal antibody against membrane protein of transmissible gastroenteritis virus

Transmissible gastroenteritis virus (TGEV) is a porcine coronavirus that can cause piglet diarrhea with high mortality rates. TGEV membrane (M) protein not only plays a vital role in the process of virus assembly and budding, but also induces the production of interferon-α during infection. In this study, a monoclonal antibody (MAb) designated 7G7, against the TGEV M protein was generated by inoculating BALB/c mice with TGEV followed by hybridoma technique. Immunofluorescence assays indicated that MAb 7G7 was capable of detecting cell infection by TGEV. Virus-based ELISA demonstrated that MAb 7G7 can be used as a highly specific diagnostic reagent for TGEV.

Absence of E protein arrests transmissible gastroenteritis coronavirus maturation in the secretory pathway

A recombinant transmissible gastroenteritis coronavirus (rTGEV) in which E gene was deleted (rTGEV-ΔE) has been engineered. This deletion mutant only grows in cells expressing E protein (E⁺ cells) indicating that E was an essential gene for TGEV replication. Electron microscopy studies of rTGEV-ΔE infected BHK-pAPN-E⁻ cells showed that only immature intracellular virions were assembled. These virions were non-infectious and not secreted to the extracellular medium in BHK-pAPN-E⁻ cells. RNA and protein composition analysis by RNase-gold and immunoelectron microscopy showed that rTGEV-ΔE virions contained RNA and also all the structural TGEV proteins, except the deleted E protein. Nevertheless, full virion maturation was blocked. Studies of the rTGEV-ΔE subcellular localization by confocal and immunoelectron microscopy in infected E⁻ cells showed that in the absence of E protein virus trafficking was arrested in the intermediate compartment. Therefore, the absence of E protein in TGEV resulted in two actions, a blockade of virus trafficking in the membranes of the secretory pathway, and prevention of full virus maturation.

Identification of a receptor-binding domain of the spike glycoprotein of human coronavirus HCoV-229E

Human coronavirus HCoV-229E uses human aminopeptidase N (hAPN) as its receptor (C. L. Yeager et al., Nature 357:420-422, 1992). To identify the receptor-binding domain of the viral spike glycoprotein (S), we expressed soluble truncated histidine-tagged S glycoproteins by using baculovirus expression vectors. Truncated S proteins purified by nickel affinity chromatography were shown to be glycosylated and to react with polyclonal anti-HCoV-229E antibodies and monoclonal antibodies to the viral S protein. A truncated protein (S(547)) that contains the N-terminal 547 amino acids bound to 3T3 mouse cells that express hAPN but not to mouse 3T3 cells transfected with empty vector. Binding of S(547) to hAPN was blocked by an anti-hAPN monoclonal antibody that inhibits binding of virus to hAPN and blocks virus infection of human cells and was also blocked by polyclonal anti-HCoV-229E antibody. S proteins that contain the N-terminal 268 or 417 amino acids did not bind to hAPN-3T3 cells. Antibody to the region from amino acid 417 to the C terminus of S blocked binding of S(547) to hAPN-3T3 cells. Thus, the data suggest that the domain of the spike protein between amino acids 417 and 547 is required for the binding of HCoV-229E to its hAPN receptor.

Induction of a protective immune response to murine coronavirus with non-internal image anti-idiotypic antibodies

Neurotropic murine coronaviruses (MHV) provide an excellent animal model to study experimental modulation of the immune response to a viral pathogen with anti-idiotypic antibodies. It is known that among the various types of anti-idiotypic antibodies (anti-Id), those designated beta (beta) or internal image can molecularly mimic the antigen and induce biological activities such as anti-viral protection and neutralization. We have recently shown that polyclonal non-internal image anti-idiotypic antibodies of the gamma-type could induce protective anti-coronavirus immunity. In the present study, a polyclonal anti-Id (Ab2) was induced against a neutralizing murine monoclonal antibody (MAb1), designated 5B170.11. Mice immunized with this affinity-purified rabbit Ab2 alpha, a non-internal image antibody, were partially protected against lethal infection by the JHM strain of MHV. However, other polyclonal and monoclonal non-internal image Ab2 induced against another neutralizing Mab1, designated 4-11G.6, were not able to protect mice against lethal infection with the A59 strain of MHV. These results demonstrate that anti-viral protection by altering the idiotypic network with non-internal image-bearing anti-idiotype reagents can be achieved even with some anti-Id of the alpha-type.

Moloney leukemia virus-induced cell surface antigen mimicry by monoclonal antibodies

We have investigated antigen-independent modulation of immune responses by monoclonal antibodies directed against both viral and nonviral antigens. BALB/c mice were immunized with monoclonal IgM (i.e. Ab1) specific for either Moloney murine leukemia virus-induced cell surface antigen (MCSA) or the hapten 2,4-dinitrophenyl (DNP). Injection with either Ab1 activated a functional idiotypic (Id) network as evidenced by production of both anti-Id (Ab2) antibodies and anti-anti-Id (Ab3) antibodies. A subset of induced Ab3 (designated Ab1'), exhibited specificity for antigen (virus or DNP). In mice immunized with anti-Id antibodies (Ab2), production of Ab3 and Ab1' was also observed. In the MCSA system, antibody-induced Ab1' responses were effective in protecting mice from tumor development upon subsequent challenge with live virus. Furthermore, antigen-independent modulation of immunity to both viral and nonviral antigens was found to be thymus-dependent. Similar findings in other viral systems suggest that antibody-induced activation of Id networks may prove a viable alternative vaccine strategy that can elicit antigen-specific responses, and in some cases protection, in the apparent absence of exposure to antigen.

A shared idiotope among antibodies against Semliki Forest virus

In the present study a shared idiotope was found among antibodies against a previously defined linear B-cell epitope of Semliki Forest virus (SFV). The synthetic B-cell epitope, located at amino acid positions 240 to 255 of the E2 membrane protein, was linked to an H-2d-restricted T-helper cell epitope of either SFV or influenza virus. Colinearly synthesized peptides of T-B polarity mixed with adjuvant were used to immunize BALB/c (H-2d) mice. After one booster immunization with either chimaeric peptide high serum antibody titers were measured against both synthetic peptide (240-255) and glutaraldehyde-fixed SFV-infected L cells. Against the synthetic peptide (240-255) a variety of monoclonal antibodies (MAbs) were produced that differed in reactivity with SFV, varied in heavy chain family, isotype, isoelectric point, and idiotype. Against one of the antipeptide MAbs (I02), that strongly reacted with SFV-infected L cells, an antiidiotypic MAb (ab2MAb), designated I02A3, was produced that could be inhibited in its binding to MAb I02 by the synthetic B-cell epitope. Therefore it was concluded that ab2 MAb I02A3 recognizes an idiotope closely associated with the antigen combining site of antipeptide MAb I02. This idiotope was definitively shared by two out of 15 antipeptide MAbs and by SFV-reactive antibodies present in both antipeptide sera and SFV-immune sera.

Antigen selection and presentation to protect against transmissible gastroenteritis coronavirus

The antigenic structure of the S glycoprotein of transmissible gastroenteritis virus (TGEV) and porcine respiratory coronavirus (PRCV) has been determined and correlated with the physical structure. Four antigenic sites have been defined (A, B, C, and D). The sites involved in the neutralization of TGEV are: A, D, and B, sites A and D being antigenically dominant for TGEV neutralization in vitro. These two sites have specific properties of interest: site A is highly conserved and is present in coronaviruses of three animal species, and site D can be represented by synthetic peptides. Both sites might be relevant in protection in vivo. PRCV does not have sites B and C, due to a genomic deletion. Complex antigenic sites, i.e., conformation and glycosylation dependent sites, have been represented by simple mimotopes selected from a library expressing recombinant peptides with random sequences, or by anti-idiotypic internal image monoclonal antibodies. An epidemiological tree relating the TGEVs and PRCVs has been proposed. The estimated mutation fixation rate of 7 +/- 2 x 10(-4) substitutions per nucleotide and year indicates that TGEV related coronaviruses show similar variability to other RNA viruses. In order to induce secretory immunity, different segments of the S gene have been expressed using a virulent forms of Salmonella typhimurium and adenovirus. These vectors, with a tropism for Peyer's patches may be ideal candidates in protection against TGEV.

Monoclonal anti-idiotypes induce neutralizing antibodies to enterovirus 70 conformational epitopes

Monoclonal antibodies (MAbs) directed against the prototype enterovirus 70 (EV-70) strain J670/71 were generated and characterized in order to produce anti-idiotypic MAbs (MAb2s) for use as surrogate immunogens. Western immunoblot and radioimmunoprecipitation assays suggested that all the MAbs recognize conformational epitopes on the virion surface. An EV-70-neutralizing antibody, MAb/ev-12 (MAb1), was selected for the production of MAb2s. Five MAb2s were selected for their capacities to inhibit the interaction of MAb/ev-12 with EV-70 in dot immunobinding inhibition and immunofluorescence assays. In addition, these five MAb2s inhibited virus neutralization mediated by MAb/ev-12, suggesting that they recognize paratope-associated idiotopes. In competition enzyme immunosorbent assays, none of the five MAb2s recognized other neutralizing and nonneutralizing EV-70-specific MAbs, demonstrating that the MAb2s were specific for private idiotopes. Immunization with each of the MAb2s was carried out for the production of anti-anti-idiotypic antibodies (Ab3). All five MAb2s induced an immune response. Moreover, results suggested that they share idiotopes, since MAb2-MAb/ev-12 binding could be inhibited by homologous as well as heterologous Ab3s. Ab3 sera were shown to possess antibodies capable of immunoprecipitating 35S-labeled viral proteins in the same manner as MAb/ev-12. Nine of 15 mice immunized with MAb2s demonstrated Ab3 neutralizing activity specific for the prototype EV-70 strain, J670/71. The potential application of MAb2s to serve as surrogate immunogens for conformational epitopes is substantiated by the results presented in this report.