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

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.

Genome analysis and detection of a Chilean isolate of Grapevine leafroll associated virus-3

The complete genome of the Chilean isolate Cl-766 of Grapevine leafroll-associated virus-3 (GLRaV-3) has been sequenced. This is the first genome sequence obtained from a GLRaV-3 isolate of the Southern hemisphere. The genomic RNA of 17,919 nucleotides contains 13 open reading frames (ORFs) with 5' and 3' untranslated regions (UTR) of 158 and 277 nucleotides, respectively. Comparison with NY1, the only isolate with complete genomic sequence available today, shows 97.6% nucleotide identity between the two isolates. Examination of the genome variability shows that most of the genetic diversity is concentrated in ORF1a. Three additional isolates from different geographic regions of Chile were partially sequenced as well, one which showed sequence divergence with respect to the other local and foreign isolates, indicative of different evolutionary constrains. Immunodetection systems were developed using monoclonal and polyclonal antibodies produced against the recombinant major coat protein of GLRaV-3, providing sensitive and specific detection using a triple antibody sandwich-enzyme linked immunosorbent assay (TAS-ELISA) and an immunocapture-reverse transcription-polymerase chain reaction (IC-RT-PCR) assay.

Detection of a New and Unusual Isolate of Plum pox virus in Plum (Prunus domestica)

Plum pox virus (PPV) isolate 3174-01 was detected by triple-antibody sandwich enzyme-linked immunosorbent assay using the universal PPV monoclonal antibody (MAb) 5B as the secondary antibody, and by reverse-transcription polymerase chain reaction (RT-PCR) using primers that amplify a 243-bp fragment in the C-terminus of the coat protein coding region. The restriction sites RsaI and AluI were absent from this fragment, which is a feature unique to PPV-C isolates. The restriction sites in 3174-01 were replaced by GTAA/GTGA and GGCA, respectively. There was 95 to 99, 94, 91, and 92 to 94% identity of the 243-bp fragment of 3174-01 with the corresponding region of the strains C, D, EA, and M, respectively. Attempts to detect the virus by RT-PCR using strain C-specific primers in three different approaches were unsuccessful. All molecular techniques assessed in attempting to strain type isolate 3174-01 gave negative results, or results inconsistent for D or M in the case of P3-6K₁ restriction fragment length polymorphism analysis. Isolate 3174-01 reacted in Western blot assay with MAb 5B, with an estimated molecular mass of 32 kDa. No reaction was observed with D-, M-, EA-, or C-specific monoclonal antibodies in Western blot or enzyme-linked immunosorbent assay. The molecular and serological data seem to indicate that PPV isolate 3174-01 does not belong to any of the recognized strains of PPV.

Molecular Evidence of the Relationship Between a Virus Associated with Flat Apple Disease and Cherry rasp leaf virus as Determined by RT-PCR

Flat apple disease-associated virus (FAV) was mechanically transmitted to the propagation host Chenopodium quinoa and double-stranded (ds)RNA recovered using CFII chromatography. Purified dsRNA was used to generate cDNA clones which were sequenced and the information used to design oligonucleotide primers for reverse transcription-polymerase chain reaction (RT-PCR) and tube capture (TC)/RT-PCR analyses. Oligonucleotide primers for RT-PCR analysis and dot blot hybridization using digoxigenin-labeled cDNA clones were used for the detection of FAV and Cherry rasp leaf virus (CRLV) in C. quinoa, in leaf and bud wood tissue of apple, or both. Primers JQ3D33FF/FR amplified a virus-specific 429-bp fragment and reliably detected all isolates of FAV and CRLV tested by RT-PCR and TC/RT-PCR. Primers JQ2C1FF/FR amplified a 370-bp fragment and detected FAV and some isolates of CRLV. Comparison of amino acid residues derived from the 429-bp fragments of FAV and CRLV gave 95% identity. The RT-PCR assays provided strong evidence of a relationship between FAV and CRLV. These assays were also used to confirm virus elimination in apple plants after heat therapy. Western blot analysis of FAV revealed capsid protein subunits of approximately 22 and 24 kDa. Our data support biological and serological evidence that FAV and CRLV are isolates of the same virus. Searches of the database produced sequence matches only with RNA2 of Apple latent spherical virus (ALSV), a new member of the family Comoviridae. This suggests that both primer pairs presumably target regions on RNA2 of FAV/CRLV and that these viruses may be more closely related to ALSV than to other members of this family.

Isolation and Partial Characterization of a Filamentous Virus Associated with Peach Mosaic Disease

A filamentous virus, 752 nm long by 9.25 nm wide and with striations of 3.47 nm in pitch, was isolated from peach mosaic-affected peach. The peach mosaic-associated virus (PMV) has a coat protein subunit molecular mass of 20.5 kDa, and dsRNA bands of 7.0, 6.4, 4.2, and 1.9 × 10⁶ Da, with a number of bands of lower molecular mass ranging in size from 1.3 to 0.3 × 10⁶ Da. The herbaceous host range of PMV includes Chenopodium amaranticolor, C. quinoa, and Nicotiana occidentalis "37B." The virus was successfully bud-transmitted from infected peach to peach indicator plants, cv. Elberta, which displayed typical peach mosaic symptoms. Attempts to transmit PMV to Prunus avium cv. Bing and P. mahaleb were unsuccessful. In serological assays, PMV polyclonal antibody (PAb) cross-reacted with cherry mottle leaf virus (CMLV). CMLV PAb and CMLV monoclonal antibodies cross-reacted with all isolates of PMV tested. Blocking with skim milk had no effect on the cross-reactions observed. PMV was consistently associated with peach mosaic disease, and the bud transmission studies and serological data suggest that this virus is the causal agent of peach mosaic disease. Data indicating that there is no relationship between peach latent mosaic viroid and peach mosaic disease was also obtained. PMV and CMLV appear to be closely related viruses, with identical coat protein subunit size and dsRNA profiles.

Alleged common antigenic determinant of tobacco mosaic virus coat protein and the host protein ribulose-1,5-bisphosphate carboxylase is an artifact of indirect ELISA and western blotting

We have reported the detection of an antigenic determinant shared by the tobacco mosaic virus coat protein and the large subunit of ribulose-1,5-bisphosphate carboxylase, a host protein (R.G. Dietzgen and M. Zaitlin Virology 155, 262-266, 1986). This conclusion was questioned by D. Zimmermann and M.H.V. Van Regenmortel (Arch. Virol. 106, 15-22, 1989). Thus we have reinvestigated this unexpected serological cross-reaction in Western immunoblotting and indirect ELISA. We found that when skimmed milk instead of bovine serum albumin was used as a blocking agent and as a diluent for antibodies and alkaline phosphatase conjugates, the alleged cross-reaction was abolished. In light of these findings, we retract our previous conclusions.