A Strategy for Generating a Broad-Spectrum Monoclonal Antibody and Soluble Single-Chain Variable Fragments against Plant Potyviruses

Computer-aided profiling of a unique broad-specific antibody and its application to an ultrasensitive fluoroimmunoassay for five N-methyl carbamate pesticides

Pollution of N-methyl carbamate (NMC) pesticides is threatening the non-target organisms' survival. Thus, broad-specific antibodies and class-selective immunoassays are demanding for multiple NMCs determination. In this study, we employed a molecular docking-based virtual screening strategy to fast profile antibody spectrum, based on a designed chemical pool containing 17 compounds. A monoclonal antibody (mAb)-6G against carbofuran was used as the objective. The recombinant full-length IgG was successfully expressed to validate the antibody sequences for homology modeling. After docking, we manually categorized the antibody-chemical binding strength into three groups. Non-competitive surface plasmon resonance (SPR) demonstrated the mAb-6G affinitive binding toward five NMCs (carbofuran, isoprocarb, propoxur, carbaryl and carbosulfan), which were classified into strong and moderate binding categories. Antibody binding properties were confirmed again by ic-ELISA and lateral flow immunochromatographic strip. Subsequently, an ultrasensitive indirect competitive fluoromicrosphere-based immunoassay (ic-FMIA) was established with the IC₅₀ (half-maximal inhibitory concentration) values of 0.08-3.37 ng/mL. This portable assay presented a 30-230-fold improved sensitivity than traditional ic-ELISA and was applied in European surface water analysis. Overall, our work provides an efficient platform integrating in-silico and experimental methodologies to accelerate the characterization of hapten-specific antibody binding properties and the development of high-sensitive immunoassays for multi-pollutants monitoring.

Isolation of single-chain variable fragment (scFv) antibodies for detection of Chickpea chlorotic dwarf virus (CpCDV) by phage display

Chickpea chlorotic dwarf virus (CpCDV, genus Mastrevirus), has a wide host range and geographic distribution in many parts of the world, and it is one of the most important legume-infecting viruses. Detection of CpCDV-infected plants in the field and evaluation of viral resistance of plant cultivars are possible by conducting serological assays. Here, development and characterization of a specific recombinant monoclonal antibody for CpCDV as a diagnostic tool are described. For this purpose, the coat protein of CpCDV was expressed in Escherichia coli strain Rosetta (DE3) and used to screen a Tomlinson phage display antibody library to select a specific single-chain variable fragment (scFv). In each round of biopanning, the affinity of the phage for CpCDV-CP was tested by enzyme-linked immunosorbent assay (ELISA). The results showed that the specificity of the eluted phages increased after each round of panning. Testing of individual clones by ELISA showed that five clones of the monoclonal phage were more strongly reactive against CpCDV than the other clones. All selected positive clones contained the same sequence. The phage-displayed scFv antibody, which was named CpCDV-scFvB9, did not bind to other tested plant pathogens and showed high sensitivity in the detection of CpCDV. A Western blot assay demonstrated that CpCDV-scFvB9 reacted with the recombinant coat protein of CpCDV. Finally, the interaction CpCDV-scFvB9 and CpCDV-CP was analyzed in a molecular docking experiment. This is the first report on production of an scFv antibody against CpCDV, which could be useful for immunological detection of the virus.

Engineering partial resistance to cucumber mosaic virus in tobacco using intrabodies specific for the viral polymerase

A single-chain variable antibody fragment (scFv) library tested against the non-structural NSP5 protein of human rotavirus A was screened by a yeast two-hybrid system against three proteins derived from the RNA-dependent RNA polymerase (RdRp) of cucumber mosaic virus (CMV), with the aim of blocking their function and preventing viral infection once expressed in planta. The constructs tested were (i) '2a' consisting of the full-length 2a gene (839 amino acids, aa), (ii) 'Motifs' covering the conserved RdRp motifs (IV-VII) (132 aa) and (iii) 'GDD' located within the conserved RdRp motif VI (GDD, 22 aa). In yeast two-hybrid (Y2H) selection assays the '2a' and 'Motifs' constructs interacted with 96 and 25 library constructs, respectively, while the 'GDD' construct caused transactivation. Y2H-interacting scFvs were analyzed in vivo for their interaction with the 2a and Motifs proteins in a mammalian transient expression system. Eighteen tobacco lines stably transformed with four selected scFvs were produced and screened for resistance against two different CMV isolates. Different levels of resistance and rate of recovery were observed with CMV of both groups I and II, particularly in lines expressing intrabodies against the full-length 2a protein. This work describes for the first time the use of intrabodies against the RdRp of CMV to obtain plants that reduce infection of a pandemic virus, showing that the selected scFvs can modulate virus infection and induce premature recovery in tobacco plants.

Polyvalent Detection of Members of the Genus Potyvirus by Molecular Hybridization Using a Genus-Probe

The use of a unique riboprobe named polyprobe, carrying partial sequences of different plant viruses or viroids fused in tandem, has permitted the polyvalent detection of up to 10 different pathogens by using a nonradioactive molecular hybridization procedure. In the present analysis, we have developed a unique polyprobe with the capacity to detect all members of the genus Potyvirus, which we have named genus-probe. To do this, we have exploited the capacity of the molecular hybridization assay to cross-hybridize with related sequences by reducing the hybridization temperature. We observed that sequences showing a percentage similarity of 68% or higher could be detected with the same probe by hybridizing at 50 to 55°C, with a detection limit of picograms of viral RNA comparable to the specific individual probes. According to this, we developed several polyvalent polyprobes, containing three, five, or seven different 500-nucleotide fragments of a conserved region of the NIb gene. The polyprobe carrying seven different conserved regions was able to detect all the 32 potyviruses assayed in the present work with no signal in the healthy tissue, indicating the potential capacity of the polyprobe to detect all described, and probably uncharacterized, potyviruses being then considered as a genus-probe. The use of this technology in routine diagnosis not only for Potyvirus but also to other viral genera is discussed.

Characterization and application of a common epitope recognized by a broad-spectrum C4 monoclonal antibody against capsid proteins of plant potyviruses

A broad-spectrum monoclonal antibody (C4 MAb) against the capsid proteins (CPs) of plant potyviruses has been generated in previous studies. To clarify which epitope is recognized by this MAb, epitope mapping was performed via phage display library screening and amino acid substitution analysis. Subsequently, a 12-residue epitope in the core region of potyvirus CPs was identified and termed the C4 epitope (WxMMDGxxQxxY/F). This epitope contains tryptophan and tyrosine residues that are crucial for reacting with C4 MAb. The CP of Odontoglossum ringspot tobamovirus (ORSV) separately fused with the C4 epitope of Konjak mosaic potyvirus (KoMV), Zantedeschia mild mosaic potyvirus (ZaMMV), or Dasheen mosaic potyvirus (DsMV) was expressed in a bacterial system and purified. The results of indirect ELISA and Western blotting demonstrated that the C4 epitope of KoMV (Ko) fused to ORSV CP showed the strongest binding affinity to C4 MAb among the three viral epitope tags examined. The binding affinity between Ko tag (WTMMDGEEQIEY) and C4 MAb was determined. To examine the applicability of the Ko tag in planta, GFP and ORSV CP were transiently expressed in Nicotiana benthamiana, and both Ko-tagged proteins were specifically detected using C4 MAb. The Ko tag did not affect the silencing suppressor function of Tomato bushy stunt tombusvirus P19 in N. benthamiana. Furthermore, Ko-tagged EGFP could be successfully expressed, specifically detected and subsequently immunoprecipitated using C4 MAb in a mammalian cell system. Thus, the present study identified a common C4 epitope of potyviruses recognized by the broad-spectrum C4 and PTY 1 MAbs, and the results indicated that the newly designed Ko tag is suitable for application in bacterial, plant, and mammalian cell systems.