Mapping B-cell linear epitopes of NS3 protein of bovine viral diarrhea virus

Massively-multiplexed epitope mapping techniques for viral antigen discovery

Following viral infection, viral antigens bind specifically to receptors on the surface of lymphocytes thereby activating adaptive immunity in the host. An epitope, the smallest structural and functional unit of an antigen, binds specifically to an antibody or antigen receptor, to serve as key sites for the activation of adaptive immunity. The complexity and diverse range of epitopes are essential to study and map for the diagnosis of disease, the design of vaccines and for immunotherapy. Mapping the location of these specific epitopes has become a hot topic in immunology and immune therapy. Recently, epitope mapping techniques have evolved to become multiplexed, with the advent of high-throughput sequencing and techniques such as bacteriophage-display libraries and deep mutational scanning. Here, we briefly introduce the principles, advantages, and disadvantages of the latest epitope mapping techniques with examples for viral antigen discovery.

Highly Multiplexed Serology for Nonhuman Mammals

Emerging infectious diseases represent a serious and ongoing threat to humans. Most emerging viruses are maintained in stable relationships with other species of animals, and their emergence within the human population results from cross-species transmission. Therefore, if we want to be prepared for the next emerging virus, we need to broadly characterize the diversity and ecology of viruses currently infecting other animals (i.e., the animal virosphere). High-throughput metagenomic sequencing has accelerated the pace of virus discovery. However, molecular assays can detect only active infections and only if virus is present within the sampled fluid or tissue at the time of collection. In contrast, serological assays measure long-lived antibody responses to infections, which can be detected within the blood, regardless of the infected tissues. Therefore, serological assays can provide a complementary approach for understanding the circulation of viruses, and while serological assays have historically been limited in scope, recent advancements allow thousands to hundreds of thousands of antigens to be assessed simultaneously using <1 μL of blood (i.e., highly multiplexed serology). The application of highly multiplexed serology for the characterization of the animal virosphere is dependent on the availability of reagents that can be used to capture or label antibodies of interest. Here, we evaluate the utility of commercial immunoglobulin-binding proteins (protein A and protein G) to enable highly multiplexed serology in 25 species of nonhuman mammals, and we describe a competitive fluorescence-linked immunosorbent assay (FLISA) that can be used as an initial screen for choosing the most appropriate capture protein for a given host species. IMPORTANCE Antibodies are generated in response to infections with viruses and other pathogens, and they help protect against future exposures. Mature antibodies are long lived, are highly specific, and can bind to their protein targets with high affinity. Thus, antibodies can also provide information about an individual's history of viral exposures, which has important applications for understanding the epidemiology and etiology of disease. In recent years, there have been large advances in the available methods for broadly characterizing antibody-binding profiles, but thus far, these have been utilized primarily with human samples only. Here, we demonstrate that commercial antibody-binding reagents can facilitate modern antibody assays for a wide variety of mammalian species, and we describe an inexpensive and fast approach for choosing the best reagent for each animal species. By studying antibody-binding profiles in captive and wild animals, we can better understand the distribution and prevalence of viruses that could spill over into humans.

The identification of a B-cell epitope in bovine viral diarrhea virus (BVDV) core protein based on a mimotope obtained from a phage-displayed peptide library

Bovine pestivirus A and B, previously known as bovine viral diarrhea virus (BVDV)-1 and 2, respectively, are important pathogens of cattle worldwide, which causes significant economic losses. B-cell epitopes in BVDV glycoprotein E2 and nonstructural protein NS2/3 have been extensively identified. In this study, we screened a 12-mer phage display peptide library using commercial goat anti-BVDV serum, and identified a mimotope "LTPHKHHKHLHA" referred to as P3. With sequence alignment, a putative B-cell epitope "77ESRKKLEKALLA88" termed as P3-BVDV1/2 residing in BVDV core protein was identified. The synthesized peptides of both P3 and P3-BVDV1/2 show strong reactivity with BVDV serum in immune blot assay. Immunization of mice with these individual peptides leads to the production of antibody that cannot neutralize virus infectivity. Thus for the first time we identified a B-cell epitope, "77ESRKKLEKALLA88", in BVDV core protein. Interestingly, the epitope was highly conserved in Pestivirus A, B, C, D, as well as emerging Pestivirus E and I, but highly variable in Pestiviruses H, G, F, and J, as well as unclassified Pestivirus originated from non-ruminant animals. Whether this putative B-cell epitope is implicated in pestivirus pathogenesis or evolution needs further investigations once large numbers of isolates are available in the future.

Preparation of a novel monoclonal antibody against Avian leukosis virus subgroup J Gp85 protein and identification of its epitope

Avian leukosis virus subgroup J (ALV-J) is an avian oncogenic retrovirus that has caused huge economic losses in the poultry industry due to its great pathogenicity and transmission ability. However, the continuous emergence of new strains would bring challenges to diagnosis and control of ALV-J. .This study focuses on preparing the monoclonal antibody (MAb) against ALV-J Gp85 and identifying its epitope. The truncated ALV-J gp85 gene fragment was amplified and then cloned into expression vectors. Purified GST-Gp85 was used to immune mice and His-Gp85 was used to screen MAb. Finally, a hybridoma cell line named J16 that produced specific MAb against the ALV-J. Immunofluorescence assay showed that MAb J16 specifically recognized ALV-J rather than ALV-A or ALV-K infected DF-1 cells. To identify the epitope recognized by MAb J16, fourteen partially overlapping ALV-J Gp85 fragments were prepared and tested by Western blot. The results indicated that peptide 150-LIRPYVNQ-157 was the minimal epitope of ALV-J Gp85 recognized by MAb J16. Alignment analysis of Gp85 from different ALV subgroups showed that the epitope keep high conservation among 36 ALV-J strains, but significant different from that of ALV subgroup A, B, C, D, E and K. Overall, we prepared a MAb specific against ALV-J and identified peptide 150-LIRPYVNQ-157 as a novel specific epitope of ALV-J Gp85, which may assist in laying the foundation for specific ALV-J detection methods.

Identification of a novel epitope specific for Gp85 protein of avian leukosis virus subgroup K

During the past two decades, avian leukosis virus (ALV) caused tremendous economic losses to poultry industry in China. ALV-K as a newly found subgroup in recent years, which made the control and eradication of ALV more difficult as they were originated from the recombination of different subgroups. To date, specific rapid detection methods refer to ALV-K are still missing. Gp85 is the main structural protein of the virus, which mediates the invasion of host cells by the virus and determinates the classification of subgroups. In this study, we prepared a monoclonal antibody (Mab) named Km3 against Gp85 of ALV-K. Immunofluorescence assay showed that Km3 specifically recognized the strains of ALV-K rather than the strains of ALV-A or ALV-J. To explain the subgroups specificity of Km3, the epitope cognized by the Mab was identified by Western blotting using 15 overlapping fragments spanning the Gp85. Finally, the peptide ¹²⁹AFGPRSIDTLSDWSRPQ¹⁴⁵ was identified as the minimal linear epitope recognized by Km3. Alignment of Gp85 from different subgroups showed that the epitope was highly conserved among ALV-K strains, which was quite different from that of the strains from ALV -A, -B and -J. In conclusion, the Mab Km3 may serve as a useful reagent for ALV-K detection and diagnosis in the future.

Identification of a linear B-cell epitope on the avian leukosis virus P27 protein using monoclonal antibodies

Avian leukosis virus (ALV) is an avian oncogenic retrovirus that can induce various clinical tumors. The capsid protein P27 is the group-specific antigen of ALV and has many viral antigen sites that are easy to detect. In this study, we produced a monoclonal antibody (mAb), 3A9, that is specific for the P27 protein. A series of partially overlapping peptides were screened to define (181)PPSAR(185) as the minimal linear epitope recognized by mAb 3A9. The identified epitope could be recognized by chicken anti-ALV and mouse anti-ALV P27 sera. The epitope was highly conserved among a number of ALV-A, ALV-B and ALV-J strains. MAb 3A9 might be a valuable tool for the development of new immunodiagnostic approaches for ALV, and the defined linear epitope might help further our understanding of the antigenic structure of the P27 protein.

Identification of a novel B-cell epitope specific for avian leukosis virus subgroup J gp85 protein

Avian leukosis virus subgroup J (ALV-J) is an avian oncogenic retrovirus that has caused severe economic losses in China. Gp85 protein is the main envelope protein and the most variable structural protein of ALV-J. It is also involved in virus neutralization. In this study, a specific monoclonal antibody, 4A3, was produced against the ALV-J gp85 protein. Immunofluorescence assays showed that 4A3 could react with different strains of ALV-J, including the British prototype isolate HPRS103, the American strains, an early Chinese broiler isolate, and layer isolates. A linear epitope on the gp85 protein was identified using a series of partially overlapping fragments spanning the gp85-encoding gene and subjecting them to western blot analysis. The results indicated that (134)AEAELRDFI(142) was the minimal linear epitope that could be recognized by mAb 4A3. Enzyme-linked immunosorbent assay (ELISA) revealed that chicken anti-ALV-J sera and mouse anti-ALV-J gp85 sera could also recognize the minimal linear epitope. Alignment analysis of amino acid sequences indicated that the epitope was highly conserved among 34 ALV-J strains. Furthermore, the epitope was not conserved among subgroup A and B of avian leukosis virus (ALV). Taken together, the mAb and the identified epitope may provide valuable tools for the development of new diagnostic methods for ALV-J.

Unique epitopes recognized by monoclonal antibodies against HP-PRRSV: deep understanding of antigenic structure and virus-antibody interaction

Highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) is a member of the genus Arterivirus within the family Arteriviridae. N and GP3 proteins are the immunodominance regions of the PRRSV viral proteins. To identify the B-cell linear antigenic epitopes within HP-PRRSV N and GP3 proteins, two monoclonal antibodies (mAbs) against N and GP3 proteins were generated and characterized, designated as 3D7 and 1F10 respectively. The mAb 3D7 recognized only HuN4-F112 not the corresponding virulent strain (HuN4-F5). It also recognized two other commercial vaccines (JXA1-R and TJM-F92), but not two other HP-PRRSV strains (HNZJJ-F1 and HLJMZ-F2). The B-cell epitope recognized by the mAb 3D7 was localized to N protein amino acids 7–33. Western blot showed that the only difference amino acid between HuN4-F112-N and HuN4-F5-N did not change the mAb 3D7 recognization to N protein. The epitope targeted by the mAb 1F10 was mapped by truncated proteins. We found a new epitope (68-76aa) can be recognized by the mAb. However, the epitope could not be recognized by the positive sera, suggesting the epitope could not induce antibody in pigs. These results should extend our understanding of the antigenic structure of the N protein and antigen-antibody reactions of the GP3 protein in different species.

Antigenic analysis of monoclonal antibodies against different epitopes of σB protein of avian reovirus

BACKGROUND

Avian reovirus (ARV) causes arthritis, tenosynovitis, runting-stunting syndrome (RSS), malabsorption syndrome (MAS) and immunosuppression in chickens. σB is one of the major structural proteins of ARV, which is able to induce group-specific antibodies against the virus.

METHODS AND RESULTS

The present study described the identification of two linear B-cell epitopes in ARV σB through expressing a set of partially overlapping and consecutive truncated peptides spanning σB screened with two monoclonal antibodies (mAbs) 1F4 and 1H3-1.The data indicated that (21)KTPACW(26) (epitope A) and (32)WDTVTFH(38) (epitope B) were minimal determinants of the linear B cell epitopes. Antibodies present in the serum of ARV-positive chickens recognized the minimal linear epitopes in Western blot analyses. By sequence alignment analysis, we determined that the epitopes A and B were not conserved among ARV, duck reovirus (DRV) and turkey reovirus (TRV) strains. Western blot assays, confirmed that epitopes A and B were ARV-specific epitopes, and they could not react with the corresponding peptides of DRV and TRV.

CONCLUSIONS AND SIGNIFICANCE

We identified (21)KTPACW(26) and (32)WDTVTFH(38) as σB -specific epitopes recognized by mAbs 1F4 and 1H3-1, respectively. The results in this study may have potential applications in development of diagnostic techniques and epitope-based marker vaccines against ARV groups.

Identification of a linear B-Cell epitope on avian reovirus protein sigmaC

SigmaC (σC) protein, which mediates virus attachment to target cells, is the most variable proteins of avian reovirus (ARV). It is responsible for inducing protective antibody immune responses in animals. To understand the antigenic determinants of σC protein, a set of partially overlapping and consecutive peptides spanning σC were expressed and then screened with the monoclonal antibody (mAb) 2B5 directed against σC. The mAb 2B5 recognized peptides with the σC motif (45)ELLHRSISDISTTV(58). Further identification of the displayed B-cell epitope was conducted with a set of truncated peptides expressed as GST fusion proteins. The Western blot and ELISA results indicated that (45)ELLHRSISDI(54) was the minimal determinant of the linear B-cell epitope. Using sequences analysis, we found that this epitope was not a common motif shared among the other members of the ARV and DRV groups. Furthermore, cross reactivity analysis showed that the associated coding motif of other ARV and DRV groups was not recognized by 2B5. These data suggested that (45)ELLHRSISDI(54) was a type-specific linear B-cell epitope of avian reovirus. The results in this study may have potential applications in the development of diagnostic techniques and epitope-based marker vaccines against ARV, which is prevalent in China.