Immunochemical cross-reactivity between the dissociated capsid proteins of PVY group plant viruses

What is a B-cell epitope?

The antigenicity of proteins resides in different types of antigenic determinants known as continuous and discontinuous epitopes, cryptotopes, neotopes, and mimotopes. All epitopes have fuzzy boundaries and can be identified only by their ability to bind to certain antibodies. Antigenic cross-reactivity is a common phenomenon because antibodies are always able to recognize a considerable number of related epitopes. This places severe limits to the specificity of antibodies. Antigenicity, which is the ability of an epitope to react with an antibody, must be distinguished from its immunogenicity or ability to induce antibodies in a competent vertebrate host. Failure to make this distinction partly explains why no successful peptide-based vaccines have yet been developed. Methods for predicting the epitopes of proteins are discussed and the reasons for the low success rate of epitope prediction are analyzed.

Identificação e caracterização de um potyvírus isolado de Zinnia elegans

O presente trabalho teve como objetivo a identificação e caracterização de um potyvírus isolado de Zinnia elegans, na Região Noroeste do Estado de São Paulo. O potyvírus foi transmitido por inoculação mecânica e apresentou uma gama restrita de hospedeiras sendo que as espécies mais afetadas pertencem à família Asteraceae. Em SDS-PAGE, a massa molecular da proteína capsidial (CP) foi estimada em 33 kDa e, em "Western-blot", reagiu com anti-soro para o Bidens mosaic virus (BiMV). Um fragmento de aproximadamente 820 pb foi amplificado por RT/PCR, clonado e seqüenciado. O fragmento, que inclui o gene da proteína capsidial, mostrou similaridade de aminoácidos do "core" da CP variando de 55% (Tobacco vein mottling virus, TVMV) a 95% (Sunflower chlorotic mottle virus, SuCMoV) e da CP completa de 55% (TVMV) a 91% (SuCMoV). Na região N-terminal, o potyvírus de Zinnia tem uma deleção de quatro aminoácidos (posições 9 a 12 após o sítio de clivagem entre a proteína NIb e a CP) quando comparada com a seqüência do SuCMoV. A análise filogenética agrupou o potyvírus de Zinnia e o SuCMoV em um mesmo ramo em 100% das réplicas, mostrando uma relação de parentesco muito próxima entre esses dois vírus. Os resultados obtidos no presente trabalho demonstraram que o potyvírus de Zinnia e o SuCMoV são estirpes do mesmo vírus. Sugere-se o nome Sunflower chlorotic mottle virus, isolado Zinnia (SuCMoV-Zi), ao potyvírus encontrado em Z. elegans no Brasil.

Potyviruses, chaos or order?

At first potyviruses were easily distinguished by biological and serological properties because only a few were known and information on their host ranges was limited. The first evidence of serological cross reaction between two of these viruses was reported in 1951 and was further corroborated for three obviously distinct members of the group in 1960. In 1968 attention was drawn to the fact that some legume and non-legume potyviruses have much wider host ranges than previously known and that within the potyvirus group there is as much biological variation within viruses and overlap between viruses as there is in serology. The concept of continuity within the group was soon supported by others and became known as the "continuum hypothesis." Results with highly sensitive serological methods using polyclonal antisera were conflicting, and nucleic acid hybridization techniques did not unambiguously discriminate between potyviruses. Recent results, obtained with antibodies directed toward epitopes located in the N-termini of the coat proteins of potyviruses, suggest that there are ways to more definitely group strains of one potyvirus and distinguish them from other potyviruses. However, there are exceptions to this rule, as in the case of bean yellow mosaic virus and clover yellow vein virus which are clearly distinct in host range, inclusion bodies, and migration velocity of coat protein, but which still react with antibodies to the N-terminal epitopes of one virus. So the question remains of whether coat-protein properties, especially the serological reactivity of N-termini, which do not alter overall virus integrity when lost, sufficiently represent the genome of a pathogenic virus entity as a single criterion for classification.

Purification of plant viruses and virus coat proteins by high performance liquid chromatography

High performance liquid chromatography (HPLC) gel filtration has been successfully applied in the purification of elongated and isometric plant viruses. Two different approaches have been tested. In one approach, semi-purified virus particles were dissociated with lithium chloride and the released coat proteins purified by HPLC gel filtration. The purified coat protein was highly immunogenic and gave rise to very specific antisera reacting with intact virus particles as well as with SDS-denatured coat protein monomers. This method is generally applicable only for elongated viruses since many isometric viruses are not dissociated by the lithium chloride treatment. The second approach consisted in gel filtration of native, undissociated virus particles and could be used both with elongated and isometric viruses. Both methods were fast and simple to perform and removed all or most of the contaminating plant proteins as judged by sodium dodecylsulphate gel electrophoresis followed by silver staining or by immunoblotting with antiserum against healthy plant extracts. With both methods the recovery of virus coat protein was about 30% on average.

Identification and classification of potyviruses on the basis of coat protein sequence data and serology. Brief review

The identification and classification of potyviruses has been in a very unsatisfactory state due to the large size of the group, the apparent vast variation among the members and the lack satisfactory taxonomic parameters that will distinguish distinct viruses from strains. In the past, use of classical methods, such as host range and symptomatology, cross-protection, morphology of cytoplasmic inclusions and conventional serology, revealed a "continuum" implying that the "species" and "strain" concepts cannot be applied to potyviruses. In contrast nucleic acid and amino acid sequence data of coat proteins has clearly demonstrated that potyviruses can be divided into distinct members and strains. This sequence data in combination with information of the structure of the potyvirus particle has been used to develop simple techniques such as HPLC peptide profiling, serology (using polyclonal antibody probes obtained by cross-adsorption with core protein from trypsin treated particles) and cDNA hybridization. These findings, along with immunochemical analyses of overlapping synthetic peptides have established the molecular basis for potyvirus serology; explained many of the problems associated with the application of conventional serology; and provided a sound basis for the identification and classification of potyviruses. As a result, the virus/strain status of some potyviruses has been redefined, requiring a change in the potyvirus nomenclature. These new developments necessitate a re-evaluation of the earlier literature on symptomatology, cross-protection, cytoplasmic inclusion body morphology and serology.

Use of rabbit Fab'-peroxidase conjugates prepared by the maleimide method for detecting plant viruses by ELISA

In contrast to antibodies conjugated to enzyme with glutaraldehyde or by the periodate method, monomeric Fab' fragments conjugated to enzyme by means of a maleimide compound are not adversely affected by the conjugation procedure. We used such Fab'-enzyme conjugates prepared with antibody to tobacco mosaic virus (TMV), to TMV coat protein and to rabbit IgG for the detection of different tobamoviruses by direct and indirect double antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA). Compared to conjugates prepared by other methods, the sensitivity of TMV detection with Fab'-enzyme conjugates by direct DAS-ELISA was markedly increased. However, because of their monomeric nature, these FAb'-enzyme conjugates did not cross-react with serologically related tobamoviruses. Anti-globulin Fab'-enzyme conjugate was found to be the most efficient anti-globulin conjugate for detecting TMV by indirect DAS-ELISA. Because of their high sensitivity and serotype specificity, FAb'-enzyme conjugates are useful for detecting low amounts of contaminating viruses present in crude viral preparations.

Spurious cross-reactions between plant viruses and monoclonal antibodies can be overcome by saturating ELISA plates with milk proteins

It has been claimed recently [Dietzgen (1986) Arch Virol 91: 163-173] that a series of monoclonal antibodies (Mabs) produced against the nepovirus, arabis mosaic virus (ArMV) cross-reacted with the tobamovirus, tobacco mosaic virus (TMV). In the present report, this alleged cross-reactivity was re-examined by two ELISA procedures using Mabs produced against each of the two viruses. It was found that when highly concentrated preparations of Mabs were used, all antibodies reacted in a nonspecific manner with several plant viruses. However, when defatted milk instead of bovine serum albumin was used both as blocking agent and as diluent for the Mabs, the spurious cross-reactions between unrelated viruses were abolished. The use of milk as blocking agent did not prevent the detection of genuine cross-reactions between related nepoviruses.

Visualization by electron microscopy of the location of tobacco mosaic virus epitopes reacting with monoclonal antibodies in enzyme immunoassay

The binding of monoclonal antibodies obtained after immunization with tobacco mosaic protein was analyzed by electron microscopy. A method was developed for visualizing the viral antigen reacting in different ELISA procedures. It was found that the use of a pH 9.6 buffer during the coating of ELISA plates led to the dissociation of virions into subunits which bound preferentially to the solid phase. MAbs that reacted with both virions and subunits in ELISA were found to bind to one of the two extremities of viral rods. These MAbs also reacted with viral protein aggregated in the form of disks. In contrast, MAbs reacting only with virions in ELISA were found to bind over the entire surface of the virus.

A method for the quantitative analysis of ELISA data

A method is described for measuring relationships among virus serotypes. An indirect ELISA was used to obtain antibody-response curves for homologous and heterologous combinations of virus and dilutions of antibody. Data from the proportional response region of each curve were converted to logarithmic values and subjected to a form of regression analysis to derive relative titres for the various combinations of virus and antibody. SDI values were then obtained by calculating the mean difference in relative titres between homologous and reciprocal heterologous combinations. Virus models in two different groups were examined using this technique. Relative titres obtained were consistent and reproducible over several experiments indicating that the technique was capable of reliably discriminating among serotypes which differed by as little as 0.2 SDI.

Use of the biotin-avidin system for detecting a broad range of serologically related plant viruses by ELISA

The biotin-avidin detection system was used in direct and indirect ELISA for detecting a broad range of serologically related tobamoviruses. When compared to standard ELISA procedures that use antibodies labelled with alkaline phosphatase, the biotin-avidin system increased the assay sensitivity and allowed a wider range of related viral serotypes to be detected.

Use of ELISA for measuring the extent of serological cross-reactivity between plant viruses

The degree of antigenic relatedness between two plant viruses is commonly expressed by a serological differentiation index (SDI) which corresponds to the average number of two-fold dilution steps separating homologous from heterologous precipitin titers. Results obtained with several tobamo- and tombusviruses indicated that the indirect form of the enzyme-linked immunosorbent assay (ELISA) can also be used for calculating SDI values. This was achieved by comparing the antiserum dilutions that lead to the same absorbance measurements (for instance 1.0) when homologous and heterologous viruses are assayed by ELISA. SDI values calculated from ELISA were similar to those obtained from precipitin tests. Because of its greater sensitivity, ELISA is able to quantify weak cross-reactions that are not detectable by precipitin tests.

Enzyme-assisted immune detection of plant virus proteins electroblotted onto nitrocellulose paper

A technique for the detection of plant virus coat proteins in plant sap is described. The method entails the electroblotting of sodium dodecyl sulphate-polyacrylamide gel electrophoresis-fractionated plant extracts onto nitrocellulose paper, probing the paper with virus-specific rabbit antisera, and indirect detection of virus proteins with horseradish peroxidase-conjugated goat anti-rabbit globulins. The sensitivity and specificity of the technique were tested using brome mosaic and barley stripe mosaic viruses. As little as 1 ng per track of virus protein was detectable, either as pure virus or when mixed with plant sap. Distant serological relationships were detected amongst tobamoviruses, and amongst the bromoviruses, with single antisera. The uses of the technique in probing capsid configuration in a presumed aphid picornavirus, and in routine diagnostic practice, are described.