Structure of viral B-cell epitopes

Recombinant helical plant virus-based nanoparticles for vaccination and immunotherapy

Plant virus-based nanoparticles (PVNs) are self-assembled capsid proteins of plant viruses, and can be virus-like nanoparticles (VLPs) or virus nanoparticles (VNPs). Plant viruses showing helical capsid symmetry are used as a versatile platform for the presentation of multiple copies of well-arrayed immunogenic antigens of various disease pathogens. Helical PVNs are non-infectious, biocompatible, and naturally immunogenic, and thus, they are suitable antigen carriers for vaccine production and can trigger humoral and/or cellular immune responses. Furthermore, recombinant PVNs as vaccines and adjuvants can be expressed in prokaryotic and eukaryotic systems, and plant expression systems can be used to produce cost-effective antigenic peptides on the surfaces of recombinant helical PVNs. This review discusses various recombinant helical PVNs based on different plant viral capsid shells that have been developed as prophylactic and/or therapeutic vaccines against bacterial, viral, and protozoal diseases, and cancer.

Antigenicity modulation upon peptide cyclization: application to the GH loop of foot-and-mouth disease virus strain C1-Barcelona

Foot-and-mouth disease virus (FMDV) isolate C(1)-Barcelona (or C-S30) includes four replacements within its immunodominant site (GH loop, residues 136-150 of capsid protein VP1, YTTSTRGDLAHVTAT), relative to reference strain C-S8c1 (YTASARGDLAHLTTT). Although one of the mutations in C-S30 (147Leu-->Val) is known to be detrimental for antibody recognition, reactivity of this isolate with the neutralizing monoclonal antibody (mAb) 4C4, raised against FMDV C1-Brescia (GH loop: YTASTRGDLAHLTAT), was indistinguishable from those of strains C-S8c1 or C1-Brescia. A structural interpretation for these somewhat striking findings is available, based on the observation that 15-residue peptides reproducing the C-S30 and C-S8c1 GH loops adopt very similar, quasi-circular, conformations in crystal complexes with 4C4. Nevertheless, surface plasmon resonance (SPR) kinetic analyses of the interactions between these peptides and three anti-GH loop mAbs have now revealed that the linear C-S30 peptides were less antigenic in solution than their C-S8c1 and C1-Brescia counterparts. We have, therefore, tried to modulate peptide antigenicity in solution by cyclization. Functional SPR and structural two dimensional proton nuclear magnetic resonance (2D-1H NMR) studies of both linear and cyclic peptide antigens are discussed here. Conformation seems to have an important role in peptide antigenicity, even when continuous (i.e. linear) antigenic sites are involved.

The antigenicity of tobacco mosaic virus

The antigenic properties of the tobacco mosaic virus (TMV) have been studied extensively for more than 50 years. Distinct antigenic determinants called neotopes and cryptotopes have been identified at the surface of intact virions and dissociated coat protein subunits, respectively, indicating that the quaternary structure of the virus influences the antigenic properties. A correlation has been found to exist between the location of seven to ten residue-long continuous epitopes in the TMV coat protein and the degree of segmental mobility along the polypeptide chain. Immunoelectron microscopy, using antibodies specific for the bottom surface of the protein subunit, showed that these antibodies reacted with both ends of the stacked-disk aggregates of viral protein. This finding indicates that the stacked disks are bipolar and cannot be converted directly into helical viral rods as has been previously assumed. TMV epitopes have been mapped at the surface of coat protein subunits using biosensor technology. The ability of certain monoclonal antibodies to block the cotranslational disassembly of virions during the infection process was found to be linked to the precise location of their complementary epitopes and not to their binding affinity. Such blocking antibodies, which act by sterically preventing the interaction between virions and ribosomes may, when expressed in plants, be useful for controlling virus infection.

Modulation of the antigenic reactivity of the citrus tristeza virus coat protein

Trapping properties of a panel of monoclonal antibodies (Mabs) raised against citrus tristeza virus (CTV) were analyzed in an indirect double-antibody sandwich ELISA (I-DAS-ELISA). These antibodies had been previously assigned by serological specificity into five groups (I to V). Mabs from group V, which are directed to conformational epitopes, trapped significant amounts of virus antigen from CTV-infected plant tissue at IgG concentration above 10 ng/ml. Mabs from groups I to IV, which are directed to linear, continuous epitopes, performed poorly as coating antibodies, even at a 1 microgram/ml concentration of the IgG's, indicating that the respective linear epitopes were inaccessible. However, when Mabs from groups I to IV were combined with a small amount of Mabs from group V, a substantial increase in trapping of the CTV antigen was recorded. In this 'two antibody-binding assay' previously cryptic, linear epitopes of the CTV CP apparently became accessible to the Mabs from groups I to IV. Modulation of the antigenic reactivity of the CTV CP was also recorded upon binding of the Mabs directed to the conformational epitopes in solution. Induced exposure of the linear epitopes of the CTV CP was revealed in 'two antibody-binding assays' with pairwise combinations of different mouse Mabs and several rabbit and chicken polyclonal antisera with different serological specificities, including antisera to bacterially expressed CP fragments. This mixed coating in I-DAS-ELISA resulted in substantially increased efficiency of the virus antigen trapping by antisera produced against bacterially expressed protein fragments and an increased sensitivity of the CTV detection after optimization of the ratio between conformational and linear antibodies.

Specificity of anti-P25 antibodies produced against whole HIV-1 particles or soluble forms of the protein

Specificity of anti-p25 antibodies produced against either whole Human Immunodeficiency Virus type 1 (HIV-1) particles in humans and chimpanzees, or against soluble forms of the protein in chimpanzees and rabbits was analyzed by ELISA using a panel of 37 long (> or = 30 residues) or shorter (9-21 residues) overlapping peptides covering the entire p25 sequence. Antibodies elicited by intact virions presented similar reactivity patterns in HIV-1-infected humans and in HIV-1-infected or immunized chimpanzees and recognized only a limited region mostly the C-terminus of the molecule. Moreover, 8 of the human sera (36%), which nonetheless reacted with high titers and avidity with native p25, did not bind to any long or short peptide. These results suggest that the majority of antibodies elicited by viral particles are presumably directed to conformational epitopes. In contrast, antibodies raised against soluble forms of p25 could react against all long peptides but one (residues 211-245) and against some short peptides, indicating that most of p25 sequence may be immunogenic under these conditions. These results suggest that the reactivity spectrum of anti p25 antibodies is rather different if they are produced against intact HIV-1 particles or the soluble protein. They also indicate that it may be possible to manipulate the specificity of the humoral immune response by using either intact virions or purified proteins.

Characterisation of monoclonal antibodies against a fimbrial structure of Salmonella enteritidis and certain other serogroup D salmonellae and their application as serotyping reagents

A panel of 13 monoclonal antibodies from different hybridomas was produced against a novel salmonella fimbrial antigen expressed predominantly by Salmonella enteritidis strains. The specificity of the monoclonal antibodies to this antigen (SEF14) was confirmed by enzyme-linked immunosorbent assay (ELISA) using purified SEF14, immune electron microscopy and, with 11 monoclonal antibodies, the identification of a repeating protein subunit (14,300kDa) on the antigen. Blocking-ELISA with the monoclonal antibodies identified epitopes in at least three, non-overlapping clusters which appeared evenly distributed on SEF14 in immune electron microscopy. The use of the monoclonal antibodies in direct-binding ELISA on a range of salmonella serotypes suggested that the epitopes on SEF14 are highly conserved and were expressed by all the S enteritidis strains examined; some strains of S dublin and the only strain of S moscow available were the only other serotypes that expressed SEF14. A latex agglutination reagent based on a monoclonal antibody was developed and used to test for SEF14 on 280 strains (representing 120 serotypes in 24 serogroups of salmonellae) that had been grown on Sensitest agar for 18 hours at 37 degrees C. All S enteritidis strains (64) and most S dublin strains (28 of 33) produced SEF14 as did the two strains representing S blegdam and S moscow. SEF14 was not detected in any other strains of serotypes from serogroup D or from any other serogroup examined.

Interaction between viruses and monoclonal antibodies studied by surface plasmon resonance

An automated biosensor system designed for measuring molecular interactions in real time and without any labelling of the reactants has been used to study the interaction of two animal viruses (vaccinia virus and poliovirus) and two plant viruses (cowpea mosaic virus and tobacco mosaic virus) with monoclonal antibodies. Using monoclonal antibodies specific for different conformational states of viral protein, it was found that the virus particles retained their conformational integrity when immobilized on the dextran matrix present on the sensor chip. Compared to conventional solid phase immunoassays, in which immobilized proteins are usually partly denatured, the biosensor system presents several advantages for studying virus-antibody interaction.