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

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.

Tobacco mosaic virus: a model antigen to study virus-antibody interactions

For more than 50 years, tobacco mosaic virus (TMV) has been used as a model system for studying various aspects of virus-antibody interactions. Distinct epitopes called neotopes and cryptotopes have been identified in intact TMV particles and dissociated viral protein respectively and a correlation has been found to exist between the location of continuous epitopes and the extent of segmental mobility along the viral polypeptide chain. The occurrence of bivalent antibody binding was shown to influence the observed affinity of TMV antibodies and kinetic measurements of antibody binding to viral peptides made it possible to analyze the mechanism of binding of monoclonal antibodies. It seems likely that the TMV model will continue to yield a rich harvest of immunochemical data relevant to many viral systems.

Some observations on the binding properties of alfalfa mosaic virus to polystyrene and its significance to indirect ELISA

The adsorption and retention properties of native (unfixed) and glutaraldehyde-fixed alfalfa mosaic virus (AMV) antigens to the polystyrene of ELISA plates were studied using [35S]-labelled virus preparations. It was shown that adsorption was a temperature-dependent, relatively slow process which varied between different AMV isolates. The amount of virus antigen adsorbed was dependent on the type and pH of the suspending buffer. Although native virus antigen adsorbed very efficiently at high pH when the particles had dissociated, significant amounts also adsorbed at pH 7.0, or lower. However, glutaraldehyde-fixed virus particles which retained their integrity even at pH as high as 9.6, adsorbed much more efficiently than native virus antigen above pH 9.0, but hardly at all around pH 7.0. The wide variation in adsorption of AMV antigen to microtitre plates under even slightly different conditions had significant influence on ELISA readings, which calls for extreme caution in interpreting serological results from indirect ELISA when antigen is used to coat the microtitre plates.

The use of serological differentiation indices for the phylogenetic analysis of plant virus relationships

The potential use of serological differentiation indices (SDIs) in the phylogenetic analysis of plant virus coat protein relationships is presented. Relationship dendrograms were constructed from SDI data for viruses in the tymovirus and Subgroup I geminivirus taxonomic groups, using distance or phenetic methods embodied in the computer programmes FITCH and NJTREE. Relationship dendrograms for geminiviruses agreed well with those constructed by others from sequence or other data; the dendrogram for tymoviruses was not in agreement with one calculated from amino acid composition data, or from other properties, but appears consistent with recently-shown sequence similarities between tymoviruses. Implications of these results for the classification and taxonomy of plant viruses are discussed.

Demonstration of antigenic variation among isolates of human cytomegalovirus using monoclonal antibodies and indirect ELISA

An indirect ELISA was developed for the detection of antigenic differences between isolates of human cytomegalovirus (HCMV) using a monoclonal antibody to an early (67 kDa) antigen. Antibody binding curves were analyzed using a microcomputer program (LISACRV) based on a nonlinear logistical model. The derivation of values for the average intrinsic association constant for seven isolates of HCMV and the prototype AD169 strain revealed significant differences between them. Because of the importance of HCMV as a pathogen, especially in immunosuppressed and AIDS patients, further investigation of the biological significance of differences between isolates of HCMV is clearly warranted.

Odontoglossum ringspot virus coat protein: sequence and antigenic comparisons with other tobamoviruses

Comparative immunochemical analysis of different tobamoviruses indicated that the previously reported coat protein sequence of Odontoglossum ringspot virus was likely erroneous. This sequence has been determined again by direct sequencing of the genomic RNA and was found to differ from the previously proposed sequence in 31 of the 157 amino acid residues. The extent of antigenic cross-reactivity between ORSV protein and other tobamovirus proteins was measured by ELISA and found to correlate satisfactorily with the degree of sequence homology.

Structurally distinct mammalian calmodulins

Rabbits immunized with dinitrophenylated calmodulin produced monospecific antibody against CaM. Using the purified antibody, enzyme-linked immunosorbent assays (ELISAs) were carried out for calmodulin (CaM). The immunological heterogeneity of human and bovine CaMs from erythrocytes was investigated by means of indirect and inhibition ELISAs. The cross-reactivity between the two CaMs was found to be 50% in the indirect ELISA. An eight times higher concentration of human CaM was necessary to produce 30% inhibition in an inhibition assay. The effect of anti-bovine CaM on the stimulation of the red cell membrane calcium pump by human and bovine CaM has also been studied. We have found that (a) the human and bovine CaMs showed indistinguishable activator activities; (b) the antibody partially inhibited the stimulating effects of CaMs; (c) the inhibition was much less effective in the case of stimulation by human CaM. These results suggest that there is a difference in the reactivity of the anti-bovine CaM antibody with bovine and human CaMs. This difference can be attributed to a slight deviation in the antibody binding structure of the two mammalian CaMs in or near the antigenic site of the CaMs.

Monoclonal antibodies to cyclosporin are representative of the major antibody populations present in antisera of immunized mice

Two series of mouse antisera raised against cyclosporin (Cs)-carrier conjugates exposing opposite sides of the Cs molecule and more than sixty monoclonal antibodies (mAbs) derived from the same animals were compared in terms of isotype and fine specificity for Cs. The predominant isotypes of the mAbs reflected the in situ distribution of the circulating anti Cs antibodies. The fine specificity of the antibodies was studied by determining their cross-reactivity for a series of Cs-derivatives and Cs-metabolites in competitive ELISA. The antisera raised by different immunizations showed very different cross-reactivity patterns for the Cs-derivatives. However, the in situ anti Cs antibody populations and the majority of mAbs derived from the corresponding animals showed a striking similarity in fine specificity for restricted clusters of residues on the Cs molecule. These results indicate that the mAbs produced against Cs are representative of the major antibody population present in the sera of the mice used for the fusion. By determining the characteristics of antibodies found in the serum of immunized mice it may thus be possible to select animals that are likely to give rise to mAbs of a certain isotype and specificity.

Quantitation of serological cross-reactivity between two geographical isolates of Oryctes baculovirus by a modified ELISA

An assay was developed for quantitation of the antigenic relationship between viruses, by modification of the indirect ELISA. The principle of this method is to estimate the epitopes not shared between the related viruses, after titration of the antibodies specific to the common epitopes as in a blocking ELISA. In practice, varying concentrations of purified virus are preincubated with a fixed dilution of heterologous or homologous antiserum and the unbound antibodies present in the mixture are back titrated with virus particles bound to microtitre plates. The antigenic relationship is described in terms of differentiation index (DI) and total antigenic reactivity (TAR). This method has been used to quantitate cross-reactivity between two geographically different isolates of Oryctes baculovirus.

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.

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.

Quantitative microcomplement fixation tests using chicken anti-viral antibody extracted from egg yolk

Yolk immunoglobulins obtained from hens immunized with human rotavirus and two plant virus antigens were used in quantitative microcomplement fixation tests. Difficulties inherent in the use of avian complement were overcome by utilizing a mixture of the C1 component of chicken complement and guinea-pig complement devoid of its own C1. The method is suitable for detecting small quantities of viral antigen and antibody and for detecting antigenic activity present on peptide fragments of viral proteins.