Generation and utility of a single-chain fragment variable monoclonal antibody platform against a baculovirus expressed recombinant receptor binding domain of SARS-CoV-2 spike protein

Digestive system infection by SARS‑CoV‑2: Entry mechanism, clinical symptoms and expression of major receptors (Review)

Besides causing severe acute respiratory syndrome (SARS), SARS‑coronavirus 2 (SARS‑CoV‑2) also harms the digestive system. Given the appearance of numerous cases of SARS‑CoV‑2, it has been demonstrated that SARS‑CoV‑2 is able to harm target organs such as the gastrointestinal tract, liver and pancreas, and either worsen the condition of patients with basic digestive illnesses or make their prognosis poor. According to several previously published studies, angiotensin‑converting enzyme II (ACE2) and transmembrane serine protease II (TMPRSS2) are expressed either singly or in combination in the digestive system and in other regions of the human body. In order to change the viral conformation, create a fusion hole and release viral RNA into the host cell for replication and transcription, SARS‑CoV‑2 is capable of binding to these two proteins through the spike protein on its surface. As a result, the body experiences an immune reaction and an inflammatory reaction, which may lead to nausea, diarrhea, abdominal pain and even gastrointestinal bleeding, elevated levels of liver enzymes, acute liver injury, pancreatitis and other serious lesions. In order to provide possible strategies for the clinical diagnosis and treatment of digestive system diseases during the COVID‑19 pandemic, the molecular structure of SARS‑CoV‑2 and the mechanism via which SARS‑CoV‑2 enters the human body through ACE2 and TMPRSS2 were discussed in the present review, and the clinical manifestations of SARS‑CoV‑2 infection in the digestive system were also summarized. Finally, the expression characteristics of ACE2 and TMPRSS2 in the main target organs of the digestive system were described.

Stable production of recombinant SARS-CoV-2 receptor-binding domain in mammalian cells with co-expression of a fluorescent reporter and its validation as antigenic target for COVID-19 serology testing

SARS-CoV-2 receptor binding domain (RBD) recognizes the angiotensin converting enzyme 2 (ACE2) receptor in host cells that enables infection. Due to its antigenic specificity, RBD production is important for development of serological assays. Here we have established a system for stable RBD production in HEK 293T mammalian cells that simultaneously express the recombinant fluorescent protein dTomato, which enables kinetic monitoring of RBD expression by fluorescence microscopy. In addition, we have validated the use of this recombinant RBD in an ELISA assay for the detection of anti-RBD antibodies in serum samples of COVID-19 convalescent patients. Recombinant RBD generated using this approach can be useful for generation of antibody-based therapeutics against SARS-CoV-2, as well serological assays aimed to test antibody responses to this relevant virus.

An Escherichia coli Expressed Multi-Disulfide Bonded SARS-CoV-2 RBD Shows Native-like Biophysical Properties and Elicits Neutralizing Antisera in a Mouse Model

A large-scale Escherichia coli (E. coli) production of the receptor-binding domain (RBD) of the SARS-CoV-2 could yield a versatile and low-cost antigen for a subunit vaccine. Appropriately folded antigens can potentially elicit the production of neutralizing antisera providing immune protection against the virus. However, E. coli expression using a standard protocol produces RBDs with aberrant disulfide bonds among the RBD's eight cysteines resulting in the expression of insoluble and non-native RBDs. Here, we evaluate whether E. coli expressing RBD can be used as an antigen candidate for a subunit vaccine. The expressed RBD exhibited native-like structural and biophysical properties as demonstrated by analytical RP-HPLC, circular dichroism, fluorescence, and light scattering. In addition, our E. coli expressed RBD binds to hACE2, the host cell's receptor, with a binding constant of 7.9 × 10-9 M, as indicated by biolayer interferometry analysis. Our E. coli-produced RBD elicited a high IgG titer in Jcl:ICR mice, and the RBD antisera inhibited viral growth, as demonstrated by a pseudovirus-based neutralization assay. Moreover, the increased antibody level was sustained for over 15 weeks after immunization, and a high percentage of effector and central memory T cells were generated. Overall, these results show that E. coli-expressed RBDs can elicit the production of neutralizing antisera and could potentially serve as an antigen for developing an anti-SARS-CoV-2 subunit vaccine.

A gold nanoparticles-based lateral flow assay utilizing baculovirus expressed recombinant nucleocapsid and receptor binding domain proteins for serodetection of IgG and IgM against SARS-CoV-2

Serological assays for SARS-CoV-2 are being utilized at an exponential rate for surveillance programs. This enterprise was designed to develop and validate a qualitative immunochromatographic test, via the Lateral Flow Assay (LFA), for detection of immunoglobulins M and G (IgM and IgG) against both nucleocapsid (N) and the receptor-binding domain (RBD) of the spike protein of SARS-CoV-2. Both targeted proteins were cloned and expressed in baculovirus expression system utilizing insect cells Sf9. The recombinant RBD and N proteins were purified and conjugated with gold nanoparticles (AuNPs) to set up the coating antigens pad. Both anti-human IgG and IgM were dispensed on nitrocellulose membrane to capture human antibodies in serum samples. A home-made dispensing system was developed to draw identical test and control lines. The validity of the developed LFA was verified by testing serum samples from 103 convalescent COVID-19 patients who were PCR positive for SARS-CoV-2 along with 28 control serum samples. The developed strips showed distinctive bands for IgM and IgG of both proteins (RBD and N) in positive samples. The sensitivity of RBD-based LFA was 70.9% and 39.8% for IgG and IgM, respectively, with a specificity of 100% for both. The N-based LFA exhibited a sensitivity of 73.8% and 35.9% for IgG and IgM, respectively, while its specificity was 75% and 100% for IgG and IgM, respectively. Our developed LFA could afford a tool for surveillance programs in low-resource countries. Moreover, it might be functional for rapid and inexpensive monitoring of the anti-SARS-CoV-2 antibodies in the sera of vaccinated individuals.

Isolation of SARS-CoV-2-blocking recombinant antibody fragments and characterisation of their binding to variant spike proteins

COVID-19 is a severe acute respiratory disease caused by SARS-CoV-2. From its initial appearance in Wuhan, China in 2019, it developed rapidly into a global pandemic. In addition to vaccines, therapeutic antibodies play an important role in immediately treating susceptible individuals to lessen severity of the disease. In this study, phage display technology was utilised to isolate human scFv antibody fragments that bind the receptor-binding domain (RBD) of SARS-CoV-2 Wuhan-Hu-1 spike protein. Of eight RBD-binding scFvs isolated, two inhibited interaction of RBD with ACE2 protein on VeroE6 cells. Both scFvs also exhibited binding to SARS-CoV-2 Delta variant spike protein but not to Omicron variant spike protein in a Raman spectroscopy immunotest. The study demonstrates the potential of recombinant antibody approaches to rapidly isolate antibody moieties with virus neutralisation potential.

A Multi-Disulfide Receptor-Binding Domain (RBD) of the SARS-CoV-2 Spike Protein Expressed in E. coli Using a SEP-Tag Produces Antisera Interacting with the Mammalian Cell Expressed Spike (S1) Protein

An Escherichia coli (E. coli) production of the receptor-binding domain (RBD) of the SARS-CoV-2 (isolate Wuhan-Hu-1) spike protein would significantly accelerate the search for anti-COVID-19 therapeutics because of its versatility and low cost. However, RBD contains four disulfide bonds and its expression in E. coli is limited by the formation of aberrant disulfide bonds resulting in inclusion bodies. Here, we show that a solubility-enhancing peptide (SEP) tag containing nine arginine residues (RBD-C9R) attached at the C-terminus can overcome this problem. The SEP-tag increased the expression in the soluble fraction and the final yield by five times (2 mg/L). The folding properties of the E. coli expressed RBD-C9R were demonstrated with biophysical characterization using RP-HPLC, circular dichroism, thermal denaturation, fluorescence, and light scattering. A quartz crystal microbalance (QCM) analysis confirmed the binding activity of RBD-C9R with ACE2, the host cell’s receptor. In addition, RBD-C9R elicited a Th-2 immune response with a high IgG titer in Jcl: ICR mice. The RBD-C9R antisera interacted with both itself and the mammalian-cell expressed spike protein (S1), as demonstrated by ELISA, indicating that the E. coli expressed RBD-C9R harbors native-like epitopes. Overall, these results emphasize the potential of our SEP-tag for the E. coli production of active multi-disulfide-bonded RBD.