Single Chain Fragment Variable (scFv) Antibodies Targeting the Spike Protein of Porcine Epidemic Diarrhea Virus Provide Protection against Viral Infection in Piglets

Monovalent, bivalent and biparatopic nanobodies targeting S1 protein of porcine epidemic diarrhea virus efficiently neutralized the virus infectivity

BACKGROUND

Porcine epidemic diarrhea virus (PEDV) is a highly contagious coronavirus that causes severe diarrhea and death in neonatal piglets, which has brought huge economic losses to the pork industry worldwide since its first discovery in the early 1970s in Europe. Passive immunization with neutralizing antibodies against PEDV is an effective prevention measure. To date, there are no effective therapeutic drugs to treat the PEDV infection.

RESULTS

We conducted a screening of specific nanobodies against the S1 protein from a phage display library obtained from immunized alpacas. Through competitive binding to antigenic epitopes, we selected instead of chose nanobodies with high affinity and constructed a multivalent tandem. These nanobodies were shown to inhibit PEDV infectivity by the neutralization assay. The antiviral capacity of nanobody was found to display a dose-dependent pattern, as demonstrated by IFA, TCID50, and qRT-PCR analyses. Notably, biparatopic nanobody SF-B exhibited superior antiviral activity. Nanobodies exhibited low cytotoxicity and high stability even under harsh temperature and pH conditions, demonstrating their potential practical applicability to animals.

CONCLUSIONS

Nanobodies exhibit remarkable biological properties and antiviral effects, rendering them a promising candidate for the development of anti-PEDV drugs.

An RNA-hydrolyzing recombinant minibody prevents both influenza A virus and coronavirus in co-infection models

With the lifting of COVID-19 non-pharmaceutical interventions, the resurgence of common viral respiratory infections was recorded in several countries worldwide. It facilitates viral co-infection, further burdens the already over-stretched healthcare systems. Racing to find co-infection-associated efficacy therapeutic agents need to be rapidly established. However, it has encountered numerous challenges that necessitate careful investigation. Here, we introduce a potential recombinant minibody-associated treatment, 3D8 single chain variable fragment (scFv), which has been developed as a broad-spectrum antiviral drug that acts via its nucleic acid catalytic and cell penetration abilities. In this research, we demonstrated that 3D8 scFv exerted antiviral activity simultaneously against both influenza A viruses (IAVs) and coronaviruses in three established co-infection models comprising two types of coronaviruses [beta coronavirus-human coronavirus OC43 (hCoV-OC43) and alpha coronavirus-porcine epidemic diarrhea virus (PEDV)] in Vero E6 cells, two IAVs [A/Puerto Rico/8/1934 H1N1 (H1N1/PR8) and A/X-31 (H3N2/X-31)] in MDCK cells, and a combination of coronavirus and IAV (hCoV-OC43 and adapted-H1N1) in Vero E6 cells by a statistically significant reduction in viral gene expression, proteins level, and approximately around 85%, 65%, and 80% of the progeny of 'hCoV-OC43-PEDV', 'H1N1/PR8-H3N2/X-31', and 'hCoV-OC43-adapted-H1N1', respectively, were decimated in the presence of 3D8 scFv. Taken together, we propose that 3D8 scFv is a promising broad-spectrum drug for treatment against RNA viruses in co-infection.

Canine distemper virus (CDV)-neutralizing activities of an anti-CDV canine-derived single-chain variable antibody fragment 4-15 (scFv 4-15) screened by phage display technology

Canine distemper virus (CDV) is a highly contagious pathogen that causes severe diarrhea, fever and vomiting in domestic dogs, posing a serious threat to the dog breeding industry. Currently, there are no effective therapeutic agents for emergency treatment despite the availability of vaccines against CDV infection. Single-chain fragment variable (scFv) antibody has been demonstrated to effectively inhibit virus infections, suggesting a potential candidate as a therapeutic agent for canine distemper. In this study, a phage-displayed scFv library was constructed from the peripheral blood lymphocytes of dog immunized intramuscularly with live-attenuated CDV vaccine, and was subjected to four rounds of pannings against CDV. Subsequent indirect enzyme-linked immunosorbent assay screening revealed high-affinity scFv antibodies specific to CDV, and indirect immunofluorescence assay screening revealed CDV-neutralizing activity of scFv antibodies. Our results showed that a scFv antibody 4-15 (scFv 4-15) with high-affinity binding to CDV and neutralizing activity against CDV was obtained, which displayed effective therapeutic potential in vivo for dogs challenged with a lethal dose of CDV. Conclusively, the scFv 4-15 with high-affinity binding and neutralizing activity to CDV that was obtained by phage display technology provides a promising candidate for the therapeutic agents against CDV infection.

Generation of whole-porcine neutralizing antibodies of an alphacoronavirus by single B cell antibody technology

Porcine epidemic diarrhea virus (PEDV) is an alphacoronavirus that causes severe morbidity and mortality in piglets, resulting in substantial economic losses to the swine industry. While vaccination is currently the most effective preventive measure, existing vaccines fail to provide complete and reliable protection against PEDV infection. Consequently, there is a need to explore alternative or complementary strategies to address this issue. In this study, we utilized single B cell antibody technology to obtain a potent neutralizing antibody, C62, which specifically targets the receptor binding domain S1B of the PEDV-S1 protein. C62 exhibited potent neutralizing activity against PEDV and inhibited viral attachment to the cell surface in vitro. Furthermore, the effectiveness of C62 in mitigating PEDV infection was demonstrated in vivo, as evidenced by the delayed onset of diarrhea and reduced mortality rates observed in piglets following oral administration of C62. Our study provides an alternative approach for controlling PEDV infection. Meanwhile, C62 holds promise as a therapeutic biological agent to complement existing vaccines. More importantly, our study forms a solid foundation for the development of whole-porcine neutralizing antibodies against other swine coronaviruses, thus contributing to the overall improvement of swine health.

Antibody Phage Display Technology for Sensor-Based Virus Detection: Current Status and Future Prospects

Viruses are widespread in the environment, and many of them are major pathogens of serious plant, animal, and human diseases. The risk of pathogenicity, together with the capacity for constant mutation, emphasizes the need for measures to rapidly detect viruses. The need for highly sensitive bioanalytical methods to diagnose and monitor socially significant viral diseases has increased in the past few years. This is due, on the one hand, to the increased incidence of viral diseases in general (including the unprecedented spread of a new coronavirus infection, SARS-CoV-2), and, on the other hand, to the need to overcome the limitations of modern biomedical diagnostic methods. Phage display technology antibodies as nano-bio-engineered macromolecules can be used for sensor-based virus detection. This review analyzes the commonly used virus detection methods and approaches and shows the prospects for the use of antibodies prepared by phage display technology as sensing elements for sensor-based virus detection.

Recombinant antibodies by phage display for bioanalytical applications

Antibody phage display, aimed at preparing antibodies to defined antigens, is a useful replacement for hybridoma technology. The phage system replaces all work stages that follow animal immunization with simple procedures for manipulating DNA and bacteria. It enables the time needed to generate stable antibody-producing clones to be shortened considerably, making the process noticeably cheaper. Antibodies prepared by phage display undergo several affinity selection steps and can be used as selective receptors in biosensors. This article briefly describes the techniques used in the making of phage antibodies to various antigens. The possibilities and prospects are discussed of using phage antibodies as selective agents in analytical systems, including biosensors.

Adenovirus vector-mediated single chain variable fragments target the nucleocapsid protein of porcine epidemic diarrhea virus and protect against viral infection in piglets

Porcine epidemic diarrhea virus (PEDV) mainly infects the intestinal epithelial cells of pigs, causing porcine epidemic diarrhea (PED). In particular, the virus causes severe diarrhea, dehydration, and death in neonatal piglets. Maternal immunity effectively protects neonatal piglets from PEDV infection; however, maternal antibodies can only prevent PEDV attachment and entry into target cells, but have no effects on intracellular viruses. Intracellular antibodies targeting virus-encoded proteins are effective in preventing viral infection. We previously identified four single chain variable fragments (scFvs), ZW1-16, ZW3-21, ZW1-41, and ZW4-16, which specifically targeted the PEDV N protein and significantly inhibited PEDV replication and up-regulated interferon-λ1 (IFN-λ1) expression in host cells. In our current study, the four scFvs were subcloned into replication-defective adenovirus vectors to generate recombinant adenoviruses rAdV-ZW1-16, rAdV-ZW3-21, rAdV-ZW1-41, and rAdV-ZW4-16. ScFvs were successfully expressed in Human Embryonic Kidney 293 (HEK293) cells and intestinal porcine epithelial cell line J2 (IPEC-J2) and were biosafe for piglets as indicated by body temperature and weight, scFv excretion in feces, IFN-γ and interleukin-4 (IL-4) expression in jejunum, and pathological changes in porcine tissue after oral administration. Western blotting, immunofluorescence, and immunohistochemical analyses showed that scFvs were expressed in porcine jejunum. The prophylactic effects of rAdV-ZW, a cocktail of the four rAdV-scFvs, on piglet diarrhea caused by PEDV was investigated. Clinical symptoms in piglets orally challenged with PEDV, following a two-time treatment with rAdV-ZW, were significantly reduced when compared with PEDV-infected piglets treated with phosphate buffered saline (PBS) or rAdV-wild-type. Also, no death and jejunal lesions were observed. ScFv co-localization with the PEDV N protein in vivo was also observed. Next, the expression of pro-inflammatory serum cytokines such as tumor necrosis factor-α (TNF-α), IL-6, IL-8, IL-12, and IFN-λ was assessed by enzyme-linked immunosorbent assay (ELISA), which showed that scFvs significantly suppressed PEDV-induced pro-inflammatory cytokine expression and restored PEDV-inhibited IFN-λ expression. Therefore, our study supported a promising role for intracellular scFvs targeting the PEDV N protein to prevent and treat diarrhea in PEDV-infected piglets.

Development of a Single-Chain Variable Fragment of CR3022 for a Plasmonic-Based Biosensor Targeting the SARS-CoV-2 Spike Protein

Two years after SARS-CoV-2 caused the first case of COVID-19, we are now in the "new normal" period, where people's activity has bounced back, followed by the easing of travel policy restrictions. The lesson learned is that the wide availability of accurate and rapid testing procedures is crucial to overcome possible outbreaks in the future. Therefore, many laboratories worldwide have been racing to develop a new point-of-care diagnostic test. To aid continuous innovation, we developed a plasmonic-based biosensor designed explicitly for portable Surface Plasmon Resonance (SPR). In this study, we designed a single chain variable fragment (scFv) from the CR3022 antibody with a particular linker that inserted a cysteine residue at the second position. It caused the linker to have a strong affinity to the gold surface through thiol-coupling and possibly become a ready-to-use bioreceptor toward a portable SPR gold chip without purification steps. The theoretical affinity of this scFv on spike protein was -64.7 kcal/mol, computed using the Molecular Mechanics Generalized Born Surface Area (MM/GBSA) method from the 100 ns molecular dynamics trajectory. Furthermore, the scFv was produced in Escherichia coli BL21 (DE3) as a soluble protein. The binding activity toward Spike Receptor Binding Domain (RBD) SARS-CoV-2 was confirmed with a spot-test, and the experimental binding free energy of -10.82 kcal/mol was determined using portable SPR spectroscopy. We hope this study will be useful in designing specific and low-cost bioreceptors, particularly early in an outbreak when the information on antibody capture is still limited.

Angiotensin-Converting Enzyme 2-Based Biosensing Modalities and Devices for Coronavirus Detection

Rapid and cost-effective diagnostic tests for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are a critical and valuable weapon for the coronavirus disease 2019 (COVID-19) pandemic response. SARS-CoV-2 invasion is primarily mediated by human angiotensin-converting enzyme 2 (hACE2). Recent developments in ACE2-based SARS-CoV-2 detection modalities accentuate the potential of this natural host-virus interaction for developing point-of-care (POC) COVID-19 diagnostic systems. Although research on harnessing ACE2 for SARS-CoV-2 detection is in its infancy, some interesting biosensing devices have been developed, showing the commercial viability of this intriguing new approach. The exquisite performance of the reported ACE2-based COVID-19 biosensors provides opportunities for researchers to develop rapid detection tools suitable for virus detection at points of entry, workplaces, or congregate scenarios in order to effectively implement pandemic control and management plans. However, to be considered as an emerging approach, the rationale for ACE2-based biosensing needs to be critically and comprehensively surveyed and discussed. Herein, we review the recent status of ACE2-based detection methods, the signal transduction principles in ACE2 biosensors and the development trend in the future. We discuss the challenges to development of ACE2-biosensors and delineate prospects for their use, along with recommended solutions and suggestions.

A New Neutralization Epitope in the Spike Protein of Porcine Epidemic Diarrhea Virus

Porcine epidemic diarrhea virus (PEDV) infects piglets and causes serious diarrhea as well as vomiting, dehydration, and death. The trimeric S protein plays a crucial role in the induction of neutralizing antibodies, and many neutralizing monoclonal antibodies (mAbs) against PEDV S protein have been developed. However, these mAbs exclusively target the S1 protein. In this study, we obtained a neutralizing mAb, 5F7, against the S2 protein of PEDV, and this mAb could neutralize new variant genotype 2 PEDV strains (LNCT2), as well as a genotype 1 PEDV strain (CV777), in vitro. The core sequence of the epitope was found in amino acid sequence 1261 aa~1337 aa. These findings confirm that the S2 protein possessed neutralizing epitopes and provided knowledge to aid further research on this virus.

Novel Neutralizing Epitope of PEDV S1 Protein Identified by IgM Monoclonal Antibody

Porcine epidemic diarrhea virus (PEDV) causes devastating enteric disease that inflicts huge economic damage on the swine industry worldwide. A safe and highly effective PEDV vaccine that contains only the virus-neutralizing epitopes (not enhancing epitope), as well as a ready-to-use PEDV neutralizing antibody for the passive immunization of PEDV vulnerable piglets (during the first week of life) are needed, particularly for PEDV-endemic farms. In this study, we generated monoclonal antibodies (mAbs) to the recombinant S1 domain of PEDV spike (S) protein and tested their PEDV neutralizing activity by CPE-reduction assay. The mAb secreted by one hybrodoma clone (A3), that also bound to the native S1 counterpart from PEDV-infected cells (tested by combined co-immunoprecipitation and Western blotting), neutralized PEDV infectivity. Epitope of the neutralizing mAb (mAbA3) locates in the S1A subdomain of the spike protein, as identified by phage mimotope search and multiple sequence alignment, and peptide binding-ELISA. The newly identified epitope is shared by PEDV G1 and G2 strains and other alphacoronaviruses. In summary, mAbA3 may be useful as a ready-to-use antibody for passive immunization of PEDV-susceptible piglets, while the novel neutralizing epitope, together with other, previously known protective epitopes, have potential as an immunogenic cocktail for a safe, next-generation PEDV vaccine.

Single-Chain Fragment Variables Targeting Leukocidin ED Can Alleviate the Inflammation of Staphylococcus aureus-Induced Mastitis in Mice

Staphylococcus aureus is a vital bovine mastitis pathogen causing huge economic losses to the dairy industry worldwide. In our previous studies, leukotoxin ED (LukED) was detected in most S. aureus strains isolated from bovine mastitis. Here, four single-chain fragment variables (scFvs) (ZL8 and ZL42 targeting LukE, ZL22 and ZL23 targeting LukD) were obtained using purified LukE and LukD proteins as the antigens after five rounds of bio-panning. The complementarity-determining region 3 (CDR3) of the VH domain of these scFvs exhibited significant diversities. In vitro, the scFvs significantly decreased LukED-induced cell killing by inhibiting the binding of LukED to chemokine receptors (CCR5 and CXCR2) and reduced the death rates of bovine neutrophils and MAC-T cells caused by LukED and S. aureus (p < 0.05). In an S. aureus-induced mouse mastitis model, histopathology and MPO results revealed that scFvs ameliorated the histopathological damages and reduced the infiltration of inflammatory cells (p < 0.05). The ELISA and qPCR assays showed that scFvs reduced the transcription and expression levels of Tumor Necrosis Factor-alpha (TNF-α), interleukin-1β (IL-1β), IL-6, IL-8 and IL-18 (p < 0.05). The overall results demonstrated the protective anti-inflammatory effect of scFvs in vitro and in vivo, enlightening the potential role of scFvs in the prevention and treatment of S. aureus-induced mastitis.

A comprehensive comparison between camelid nanobodies and single chain variable fragments

By the emergence of recombinant DNA technology, many antibody fragments have been developed devoid of undesired properties of natural immunoglobulins. Among them, camelid heavy-chain variable domains (VHHs) and single-chain variable fragments (scFvs) are the most favored ones. While scFv is used widely in various applications, camelid antibodies (VHHs) can serve as an alternative because of their superior chemical and physical properties such as higher solubility, stability, smaller size, and lower production cost. Here, these two counterparts are compared in structure and properties to identify which one is more suitable for each of their various therapeutic, diagnosis, and research applications.

Anti-Staphylococcus aureus Single-Chain Fragment Variables Play a Protective Anti-Inflammatory Role In Vitro and In Vivo

Staphylococcus aureus is a causative agent of bovine mastitis, capable of causing significant economic losses to the dairy industry worldwide. This study focuses on obtaining single-chain fragment variables (scFvs) against the virulence factors of S. aureus and evaluates the protective effect of scFvs on bovine mammary epithelial (MAC-T) cells and mice mammary gland tissues infected by S. aureus. After five rounds of bio-panning, four scFvs targeting four virulence factors of S. aureus were obtained. The complementarity-determining regions (CDRs) of these scFvs exhibited significant diversities, especially CDR3 of the VH domain. In vitro, each of scFvs was capable of inhibiting S. aureus growth and reducing the damage of MAC-T cells infected by S. aureus. Preincubation of MAC-T cells with scFvs could significantly attenuate the effect of apoptosis and necrosis compared with the negative control group. In vivo, the qPCR and ELISA results demonstrated that scFvs reduced the transcription and expression of Tumor Necrosis Factor alpha (TNF-α), interleukin-1β (IL-1β), IL-6, IL-8, and IL-18. Histopathology and myeloperoxidase (MPO) results showed that scFvs ameliorated the histopathological damages and reduced the inflammatory cells infiltration. The overall results demonstrated the positive anti-inflammatory effect of scFvs, revealing the potential role of scFvs in the prevention and treatment of S. aureus infections.

Paper-based analytical devices for virus detection: Recent strategies for current and future pandemics

The importance of user-friendly, inexpensive, sensitive, and selective detection of viruses has been highlighted again due to the recent Coronavirus disease 2019 (COVID-19) pandemic. Among the analytical tools, paper-based devices (PADs) have become a leading alternative for point-of-care (POC) testing. In this review, we discuss the recent development strategies and applications in nucleic acid-based, antibody/antigen-based and other affinity-based PADs using optical and electrochemical detection methods for sensing viruses. In addition, advantages and drawbacks of presented PADs are identified. Current state and insights towards future perspectives are presented regarding developing POC diagnosis platform for COVID-19. This review considers state-of-the-art technologies for further development and improvement in PADs performance for virus detection.

Generation and Characterization of a Spike Glycoprotein Domain A-Specific Neutralizing Single-Chain Variable Fragment against Porcine Epidemic Diarrhea Virus

The emergence of the genotype (G) 2 and re-emergence of the G1 porcine epidemic diarrhea virus (PEDV) has caused severe economic impacts in the past decade. Developments of efficient vaccines against new variants of PEDV have been challenging, not least because of the difficulties in eliciting mucosal and lactogenic immunity. A single-chain fragment variable (scFv) capable of efficient antigen recognition is an alternative to vaccination and treatment of a viral infection. In the present study, the variable regions of the light chain and the heavy chain of a G2b PEDV spike domain A (S1^A)-specific neutralizing monoclonal antibody (mAb) were sequenced, constructed with a (G4S) x3 linker, and produced by a mammalian protein expression system. Our results demonstrated that the PEDV S1^A domain scFv was able to bind to S proteins of both G1 and G2b PEDVs. Nevertheless, the scFv was only capable of neutralizing the homologous G2b PEDV but not the G1 PEDV. The binding ability of the G2b-specific neutralizing scFv was not able to predict the neutralizing ability toward heterologous PEDV. The anti-PEDV S1^A scFv presented herein serves as a potential therapeutic candidate against the virulent G2b PEDV.

Developing Recombinant Antibodies by Phage Display Against Infectious Diseases and Toxins for Diagnostics and Therapy

Antibodies are essential molecules for diagnosis and treatment of diseases caused by pathogens and their toxins. Antibodies were integrated in our medical repertoire against infectious diseases more than hundred years ago by using animal sera to treat tetanus and diphtheria. In these days, most developed therapeutic antibodies target cancer or autoimmune diseases. The COVID-19 pandemic was a reminder about the importance of antibodies for therapy against infectious diseases. While monoclonal antibodies could be generated by hybridoma technology since the 70ies of the former century, nowadays antibody phage display, among other display technologies, is robustly established to discover new human monoclonal antibodies. Phage display is an in vitro technology which confers the potential for generating antibodies from universal libraries against any conceivable molecule of sufficient size and omits the limitations of the immune systems. If convalescent patients or immunized/infected animals are available, it is possible to construct immune phage display libraries to select in vivo affinity-matured antibodies. A further advantage is the availability of the DNA sequence encoding the phage displayed antibody fragment, which is packaged in the phage particles. Therefore, the selected antibody fragments can be rapidly further engineered in any needed antibody format according to the requirements of the final application. In this review, we present an overview of phage display derived recombinant antibodies against bacterial, viral and eukaryotic pathogens, as well as microbial toxins, intended for diagnostic and therapeutic applications.

Broad-Spectrum Antiviral Activity of 3D8, a Nucleic Acid-Hydrolyzing Single-Chain Variable Fragment (scFv), Targeting SARS-CoV-2 and Multiple Coronaviruses In Vitro

The virus behind the current pandemic, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is responsible for the etiology of novel coronavirus disease (COVID-19) and poses a critical public health threat worldwide. Effective therapeutics and vaccines against multiple coronaviruses remain unavailable. Single-chain variable fragment (scFv), a recombinant antibody, exhibits broad-spectrum antiviral activity against DNA and RNA viruses owing to its nucleic acid-hydrolyzing property. The antiviral activity of 3D8 scFv against SARS-CoV-2 and other coronaviruses was evaluated in Vero E6 cell cultures. Viral growth was quantified with quantitative RT-qPCR and plaque assay. The nucleic acid-hydrolyzing activity of 3D8 was assessed through abzyme assays of in vitro viral transcripts and cell viability was determined by MTT assay. We found that 3D8 inhibited the replication of SARS-CoV-2, human coronavirus OC43 (HCoV-OC43), and porcine epidemic diarrhea virus (PEDV). Our results revealed the prophylactic and therapeutic effects of 3D8 scFv against SARS-CoV-2 in Vero E6 cells. Immunoblot and plaque assays showed the reduction of coronavirus nucleoproteins and infectious particles, respectively, in 3D8 scFv-treated cells. These data demonstrate the broad-spectrum antiviral activity of 3D8 against SARS-CoV-2 and other coronaviruses. Thus, it could be considered a potential antiviral countermeasure against SARS-CoV-2 and zoonotic coronaviruses.

Significant Inhibition of Porcine Epidemic Diarrhea Virus In Vitro by Remdesivir, Its Parent Nucleoside and β-D-N4-hydroxycytidine

Porcine epidemic diarrhea (PED) caused by porcine epidemic diarrhea virus (PEDV) is widespread in the world. In recent years, the increased virulence of the virus due to viral variations, has caused great economic losses to the pig industry in many countries. It is always worthy to find effective therapeutic methods for PED. As an important class of antivirals, nucleoside drugs which target viral polymerases have been applied in treating human viral infections for half a century. Herein, we evaluated the anti-PEDV potential of three broad-spectrum antiviral nucleoside analogs, remdesivir (RDV), its parent nucleoside (RDV-N) and β-d-N⁴-hydroxycytidine (NHC). Among them, RDV-N was the most active agent in Vero E6 cells with EC₅₀ of 0.31 μmol/L, and more potent than RDV (EC₅₀ = 0.74 μmol/L) and NHC (EC₅₀ = 1.17 μmol/L). The activity of RDV-N was further confirmed using an indirect immuno-fluorescence assay. Moreover, RDV-N exhibited a good safety profile in cells and in mice. The high sequence similarity of the polymerase functional domains of PEDV with other five porcine coronaviruses indicated a broader antiviral spectrum for the three compounds. Generally, RDV-N is a promising broad-spectrum antiviral nucleoside, and it would be worthy to make some structural modifications to increase its oral bioavailability.

Phage Display Technique as a Tool for Diagnosis and Antibody Selection for Coronaviruses

Phage display is one of the important and effective molecular biology techniques and has remained indispensable for research community since its discovery in the year 1985. As a large number of nucleotide fragments may be cloned into the phage genome, a phage library may harbour millions or sometimes billions of unique and distinctive displayed peptide ligands. The ligand–receptor interactions forming the basis of phage display have been well utilized in epitope mapping and antigen presentation on the surface of bacteriophages for screening novel vaccine candidates by using affinity selection-based strategy called biopanning. This versatile technique has been modified tremendously over last three decades, leading to generation of different platforms for combinatorial peptide display. The translation of new diagnostic tools thus developed has been used in situations arising due to pathogenic microbes, including bacteria and deadly viruses, such as Zika, Ebola, Hendra, Nipah, Hanta, MERS and SARS. In the current situation of pandemic of Coronavirus disease (COVID-19), a search for neutralizing antibodies is motivating the researchers to find therapeutic candidates against novel SARS-CoV-2. As phage display is an important technique for antibody selection, this review presents a concise summary of the very recent applications of phage display technique with a special reference to progress in diagnostics and therapeutics for coronavirus diseases. Hopefully, this technique can complement studies on host–pathogen interactions and assist novel strategies of drug discovery for coronaviruses.

Drawing Comparisons between SARS-CoV-2 and the Animal Coronaviruses

The COVID-19 pandemic, caused by a novel zoonotic coronavirus (CoV), SARS-CoV-2, has infected 46,182 million people, resulting in 1,197,026 deaths (as of 1 November 2020), with devastating and far-reaching impacts on economies and societies worldwide. The complex origin, extended human-to-human transmission, pathogenesis, host immune responses, and various clinical presentations of SARS-CoV-2 have presented serious challenges in understanding and combating the pandemic situation. Human CoVs gained attention only after the SARS-CoV outbreak of 2002-2003. On the other hand, animal CoVs have been studied extensively for many decades, providing a plethora of important information on their genetic diversity, transmission, tissue tropism and pathology, host immunity, and therapeutic and prophylactic strategies, some of which have striking resemblance to those seen with SARS-CoV-2. Moreover, the evolution of human CoVs, including SARS-CoV-2, is intermingled with those of animal CoVs. In this comprehensive review, attempts have been made to compare the current knowledge on evolution, transmission, pathogenesis, immunopathology, therapeutics, and prophylaxis of SARS-CoV-2 with those of various animal CoVs. Information on animal CoVs might enhance our understanding of SARS-CoV-2, and accordingly, benefit the development of effective control and prevention strategies against COVID-19.

Selection of a Single Domain Antibody, Specific for an HLA-Bound Epitope of the Mycobacterial Ag85B Antigen

T cells recognizing epitopes on the surface of mycobacteria-infected macrophages can impart protection, but with associated risk for reactivation to lung pathology. We aimed to identify antibodies specific to such epitopes, which carry potentials for development toward novel therapeutic constructs. Since epitopes presented in the context of major histocompatibility complex alleles are rarely recognized by naturally produced antibodies, we used a phage display library for the identification of monoclonal human single domain antibody producing clones. The selected 2C clone displayed T cell receptor-like recognition of an HLA-A*0201 bound ₁₉₉KLVANNTRL₂₀₇ peptide from the Ag85B antigen, which is known to be an immunodominant epitope for human T cells. The specificity of the selected domain antibody was demonstrated by solid phase immunoassay and by immunofluorescent surface staining of peptide loaded cells of the T2 cell line. The antibody affinity binding was determined by biolayer interferometry. Our results validated the used technologies as suitable for the generation of antibodies against epitopes on the surface of Mycobacterium tuberculosis infected cells. The potential approaches forward the development of antibody in immunotherapy of tuberculosis have been outlined in the discussion.

Pyruvate dehydrogenase complex-enzyme 2, a new target for Listeria spp. detection identified using combined phage display technologies

The genus Listeria comprises ubiquitous bacteria, commonly present in foods and food production facilities. In this study, three different phage display technologies were employed to discover targets, and to generate and characterize novel antibodies against Listeria: antibody display for biomarker discovery and antibody generation; ORFeome display for target identification; and single-gene display for epitope characterization. With this approach, pyruvate dehydrogenase complex—enzyme 2 (PDC-E2) was defined as a new detection target for Listeria, as confirmed by immunomagnetic separation-mass spectrometry (IMS-MS). Immunoblot and fluorescence microscopy showed that this protein is accessible on the bacterial cell surface of living cells. Recombinant PDC-E2 was produced in E. coli and used to generate 16 additional antibodies. The resulting set of 20 monoclonal scFv-Fc was tested in indirect ELISA against 17 Listeria and 16 non-Listeria species. Two of them provided 100% sensitivity (CI 82.35–100.0%) and specificity (CI 78.20–100.0%), confirming PDC-E2 as a suitable target for the detection of Listeria. The binding region of 18 of these antibodies was analyzed, revealing that ≈ 90% (16/18) bind to the lipoyl domains (LD) of the target. The novel target PDC-E2 and highly specific antibodies against it offer new opportunities to improve the detection of Listeria.

An update on antiviral antibody-based biopharmaceuticals

Due to the vastness of the science virology, it is no longer an offshoot solely of the microbiology. Viruses have become as the causative agents of major epidemics throughout history. Many therapeutic strategies have been used for these microorganisms, and in this way the recognizing of potential targets of viruses is of particular importance for success. For decades, antibodies and antibody fragments have occupied a significant body of the treatment approaches against infectious diseases. Because of their high affinity, they can be designed and engineered against a variety of purposes, mainly since antibody fragments such as scFv, nanobody, diabody, and bispecific antibody have emerged owing to their small size and interesting properties. In this review, we have discussed the antibody discovery and molecular and biological design of antibody fragments as inspiring therapeutic and diagnostic agents against viral targets.

Construction of Antibody Phage Libraries and Their Application in Veterinary Immunovirology

Antibody phage display (APD) technology has revolutionized the field of immunovirology with its application in viral disease diagnostics and antiviral therapy. This robust and versatile technology allows the expression of an antibody fused to a phage coat protein on the surface of a filamentous phage. The DNA sequence coding for the antibody is packaged within the phage, linking the phenotype to genotype. Antibody phage display inherits the ability to rapidly generate and modify or improve high-affinity monoclonal antibodies, rendering it indispensable in immunology. In the last two decades, phage-display-derived antibodies have been extensively used in human medicine as diagnostic and therapeutic modalities. Recently, they are also gaining significant ground in veterinary medicine. Even though these advancements are mainly biased towards economically important animals such as chicken, cattle, and pigs, they are laying the foundation of fulfilling the unmet needs of veterinary medicine as antibody-based biologics in viral diagnostics, therapeutics, and immunoprophylaxis. This review provides a brief overview of the construction of antibody phage libraries and their application in diagnosis, prevention, and control of infectious viral diseases in veterinary medicine in detail.

Humoral immune responses in piglets experimentally infected with a field strain of porcine epidemic diarrhea virus

Porcine epidemic diarrhea virus (PEDV) causes severe clinical diarrhea in neonatal piglets, with reported mortality rates between 70-100%. The humoral immunity, especially the local intestinal IgA responses, plays an important role in the immune protection against PEDV infection. In this study, we evaluated the isotype antibody responses against the PEDV nucleocapsid (N) protein and the spike (S) protein subunits 1 (S1) and 2 (S2) in the serum and intestine of piglets. We also determined its serum neutralizing activity against the PEDV field strain HBMC2012 in 21-day-old piglets. Enzyme-linked immunosorbent assays (ELISA) revealed that the production of IgM against the N protein and S1 subunit was higher compared to the S2 subunit. The anti-S2 IgA antibodies were higher than the anti-N protein and anti-S1 IgA at 3 days post-infection (dpi). The specific IgA responses to the S2 subunit were higher than the responses observed in S1. The specific IgG responses against S1 and S2 subunits exceeded those of N protein. The serum neutralizing activities against PEDV were relatively low with a tendency to decline over time. No isotype-specific antibodies were found in the intestinal contents from infected pigs, except the one with weak IgA responses against N protein at 28 dpi. Immunohistochemical staining showed that a few IgM, IgA, and IgG antibody-secreting cells were mainly located in the mucosa of the duodenum and ileum of PEDV-infected pigs at 3 dpi. This study suggests poor systemic and intestinal isotype-specific antibody responses, especially those of IgA, and weak serum neutralizing activities against the field PEDV strain in piglets.

Soluble expression of single-chain variable fragment (scFv) in Escherichia coli using superfolder green fluorescent protein as fusion partner

Single-chain variable fragment (scFv) has great prospect in medical therapies and diagnostic applications due to its binding affinity and low immunogenicity. However, the application of scFv is limited by its heterologous expression facing challenges of insoluble aggregation. sfGFP has been developed as fusion tag to facilitate the solubility of fusion partner in Escherichia coli. We designed fusion protein of anti-influenza PB2 scFv at C-terminus of sfGFP and successfully obtained soluble expression of sfGFP-scFv-His in Escherichia coli. The expression level of sfGFP-scFv-His reached at 20 mg/L of bacterial culture when the culture was induced with 0.1 mM IPTG at 18 °C for 16 h. And 6 mg scFv-His was obtained from the cleavage of 10 mg pure sfGFP-scFv-His with TEV protease. In addition, we found that sfGFP-scFv-His was more stable than scFv-His in chicken serum, suggesting that sfGFP not only facilitated the solubility of scFv in Escherichia coli, but also promoted the stability of scFv. The immunologic activity of sfGFP-scFv-His was confirmed by Western blot and ELISA; the results showed that anti-PB2 sfGFP-scFv-His exhibited specific binding to PB2. Hemagglutination and comparative real-time RT-PCR analysis indicated that sfGFP-scFv-His and scFv-His inhibited the replication of H1N1 influenza virus in the infected A549 cells. These results further develop the application of scFv as an agent, such as anti-influenza. Furthermore, soluble expression of scFv using sfGFP as fusion partner provide a cost-effective preparation model for manufacturing scFv against pandemic disease.