Development of Monoclonal Antibody 281-mG2a-f Against Golden Hamster Podoplanin.

Golden (Syrian) hamster (Mesocricetus auratus) is a small animal model of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections. Pathological analyses of the tissues are required to understand the pathogenesis of SARS-CoV-2 and the evaluation of therapeutic modalities, including neutralizing monoclonal antibodies (mAbs). However, mAbs that recognize the golden hamster-derived antigens and distinguish specific cell types, such as the pneumocytes, are limited. Podoplanin (PDPN) is an essential marker of lung type I alveolar epithelial cells, kidney podocytes, and lymphatic endothelial cells. In this study, an anti-Chinese hamster (Cricetulus griseus) PDPN mAb PMab-281 (IgG3, kappa) was established using the Cell-Based Immunization and Screening (CBIS) method. A defucosylated mouse IgG2a version of PMab-281 (281-mG2a-f) was also developed. The 281-mG2a-f strongly recognized both the Chinese hamster and the golden hamster PDPN using flow cytometry and could detect lung type I alveolar epithelial cells, lymphatic endothelial cells, and Bowman's capsules in the kidney from the golden hamster using immunohistochemistry. These results suggest the usefulness of 281-mG2a-f for analyzing the golden hamster-derived tissues and cells for SARS-CoV-2 research.

SARS-CoV-2 Spike Protein Mutation at Cysteine-488 Impairs Its Golgi Localization and Intracellular S1/S2 Processing.

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike protein binds to the cellular receptor-angiotensin-converting enzyme-2 (ACE2) as the first step in viral cell entry. SARS-CoV-2 spike protein expression in the ACE2-expressing cell surface induces cell-cell membrane fusion, thus forming syncytia. To exert its fusogenic activity, the spike protein is typically processed at a specific site (the S1/S2 site) by cellular proteases such as furin. The C488 residue, located at the spike-ACE2 interacting surface, is critical for the fusogenic and infectious roles of the SARS-CoV-2 spike protein. We have demonstrated that the C488 residue of the spike protein is involved in subcellular targeting and S1/S2 processing. C488 mutant spike localization to the Golgi apparatus and cell surface were impaired. Consequently, the S1/S2 processing of the spike protein, probed by anti-Ser-686-cleaved spike antibody, markedly decreased in C488 mutant spike proteins. Moreover, brefeldin-A-mediated endoplasmic-reticulum-to-Golgi traffic suppression also suppressed spike protein S1/S2 processing. As brefeldin A treatment and C488 mutation inhibited S1/S2 processing and syncytia formation, the C488 residue of spike protein is required for functional spike protein processing.

Development of novel monoclonal antibodies against nsp12 of SARS-CoV-2.

A novel coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has caused a global pandemic of coronavirus disease 19. Coronaviruses, including SARS-CoV-2, use RNA-dependent RNA polymerase (RdRP) for viral replication and transcription. Since RdRP is a promising therapeutic target for infection of SARS-CoV-2, it would be beneficial to develop new experimental tools for analysis of the RdRP reaction of SARS-CoV-2. Here, we succeeded to develop novel mouse monoclonal antibodies (mAbs) that recognize SARS-CoV-2 nsp12, catalytic subunit of the RdRP. These anti-nsp12 mAbs, RdMab-2, -13, and -20, specifically recognize SARS-CoV-2 nsp12 by western blotting analysis, while they exhibit less or no cross-reactivity to SARS-CoV nsp12. In addition, SARS-CoV-2 nsp12 was successfully immunoprecipitated using RdMab-2 from lysates of cells overexpressing SARS-CoV-2 nsp12. RdMab-2 was able to detect SARS-CoV-2 nsp12 transiently expressed in established culture cells such as HEK293T cells by indirect immunofluorescence technique. These novel mAbs against SARS-CoV-2 nsp12 are useful to elucidate the RdRP reaction of SARS-CoV-2 and biological cell response against it.

Development of an Anti-human CCR2 Monoclonal Antibody (C2Mab-9) by N-Terminal Peptide Immunization.

The CC chemokine receptor type-2 (CCR2) is one of the members of the G protein-coupled receptor superfamily, which are expressed on the cell surface of immune and tumor cells. CCR2 binds to the C-C motif chemokine ligand 2 (CCL2)/monocyte chemoattractant protein-1 (MCP-1), which is produced by various cells, including tumor and immune-related cells. Therefore, the development of sensitive monoclonal antibodies (mAbs) for CCR2 has been desired for treatment and diagnosis. In this study, we established a specific antihuman CCR2 (hCCR2) mAb, C2Mab-9 (mouse IgG1, kappa), using the synthetic peptide immunization method. Flow cytometric and immunocytochemical results showed that C2Mab-9 reacted with hCCR2-expressing U937 (human histiocytic lymphoma) and natural killer cells. Furthermore, C2Mab-9 showed the moderate binding affinity for both cells. Conclusively, C2Mab-9 can be a useful tool for analyzing hCCR2-related biological responses.

Epitope Mapping of an Anti-Chinese/Golden Hamster Podoplanin Monoclonal Antibody.

Chinese hamster (Cricetulus griseus) and golden hamster (Mesocricetus auratus) are important animal models of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections, which affect several organs, including respiratory tract, lung, and kidney. Podoplanin (PDPN) is a marker of lung type I alveolar cells, kidney podocytes, and lymphatic endothelial cells. The development of anti-PDPN monoclonal antibodies (mAbs) for these animals is essential to evaluate the pathogenesis by SARS-CoV-2 infections. Using the Cell-Based Immunization and Screening method, we previously developed an anti-Chinese hamster PDPN (ChamPDPN) mAb, PMab-281 (mouse IgG3, kappa), and further changed its subclass into IgG2a (281-mG2a-f), both of which can recognize not only ChamPDPN but also golden hamster PDPN (GhamPDPN) by flow cytometry and immunohistochemistry. In this study, we examined the critical epitope of 281-mG2a-f, using enzyme-linked immunosorbent assay (ELISA) with synthesized peptides. First, we performed ELISA with peptides derived from ChamPDPN and GhamPDPN extracellular domain, and found that 281-mG2a-f reacted with the peptides, which commonly possess the KIPFEELxT sequence. Next, we analyzed the reaction with the alanine-substituted mutants, and revealed that 281-mG2a-f did not recognize the alanine-substituted peptides of I75A, F77A, and E79A of ChamPDPN. Furthermore, these peptides could not inhibit the recognition of 281-mG2a-f to ChamPDPN-expressing cells by flow cytometry. The results indicate that the binding epitope of 281-mG2a-f includes Ile75, Phe77, and Glu79 of ChamPDPN, which are shared with GhamPDPN.

Development of a Monoclonal Antibody PMab-292 Against Ferret Podoplanin.

Ferrets (Mustela putorius furo) have been used as small animal models to investigate severe acute respiratory syndrome coronaviruses (SARS-CoV and SARS-CoV-2) infections. Pathological analyses of these tissue samples, including those of the lung, are, therefore, essential to understand the pathogenesis of SARS-CoVs and evaluate the action of therapeutic monoclonal antibodies (mAbs) against this disease. However, mAbs that recognize ferret-derived proteins and distinguish between specific cell types, such as lung epithelial cells, are limited. Podoplanin (PDPN) has been identified as an essential marker in lung type I alveolar epithelial cells, kidney podocytes, and lymphatic endothelial cells. In this study, an anti-ferret PDPN (ferPDPN) mAb PMab-292 (mouse IgG1, kappa) was established using the Cell-Based Immunization and Screening (CBIS) method. PMab-292 recognized ferPDPN-overexpressed Chinese hamster ovary-K1 (CHO/ferPDPN) cells by flow cytometry and Western blotting. The kinetic analysis using flow cytometry showed that the KD of PMab-292 for CHO/ferPDPN was 3.4 × 10-8 M. Furthermore, PMab-292 detected lung type I alveolar epithelial cells, lymphatic endothelial cells, and glomerular/Bowman's capsule in the kidney using immunohistochemistry. Hence, these results propose the usefulness of PMab-292 in analyzing ferret-derived tissues for SARS-CoV-2 research.