Critical Epitope of Anti-Rabbit Podoplanin Monoclonal Antibodies for Immunohistochemical Analysis

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 IgG_2a version of PMab-281 (281-mG_2a-f) was also developed. The 281-mG_2a-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-mG_2a-f for analyzing the golden hamster-derived tissues and cells for SARS-CoV-2 research.

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 K_D 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.

Epitope Mapping of the Anti-California Sea Lion Podoplanin Monoclonal Antibody PMab-269 Using Alanine-Scanning Mutagenesis and ELISA

Podoplanin (PDPN) plays a pivotal role in platelet aggregation, embryo development, and tumor progression. PDPN is universally expressed in many mammalian species, and is considered a typical lymphatic endothelial cell marker. We have previously developed the mouse anti-California sea lion (Zalophus californianus) PDPN (seaPDPN) monoclonal antibody (mAb), clone PMab-269, which is suitable for different experimental applications, including flow cytometry, Western blotting, and immunohistochemistry. In this study, we identified the PMab-269 epitope of the seaPDPN by enzyme-linked immunosorbent assay using deletion mutants and point mutants generated for seaPDPN. Our results demonstrated that PMab-269 recognized the peptide, corresponding to the amino acids 63-82 of seaPDPN. Furthermore, the reactions of PMab-269 to seven alanine-substituted peptides, such as P68A, D76A, F77A, H78A, L79A, E80A, and D81A, were abolished among 20 alanine-substituted peptides. We identified the seven amino acids (Pro68, Asp76, Phe77, His78, Leu79, Glu80, and Asp81) as the critical epitope targeted by PMab-269. The successful identification of the PMab-269 epitope might contribute to the pathophysiological investigations of seaPDPN.

The Epitope of PMab-210 Is Located in Platelet Aggregation-Stimulating Domain-3 of Pig Podoplanin

Podoplanin (PDPN)/T1alpha/Aggrus, a small mucin-type transmembrane glycoprotein, has been shown to be expressed on lymphatic endothelial cells and epithelial cells of many organs. PDPN is also upregulated in many cancers, and is involved in cancer metastasis and malignant progression. Human PDPN possesses three platelet aggregation-stimulating (PLAG) domains and the PLAG-like domain, which bind to C-type lectin-like receptor-2 (CLEC-2). Previously, we reported a novel antipig PDPN (pPDPN) monoclonal antibody (PMab-210) using Cell-Based Immunization and Screening (CBIS) method. PMab-210 specifically detected pPDPN-overexpressed Chinese hamster ovary (CHO)-K1 cells by flow cytometry and Western blot analysis. Immunohistochemical analyses demonstrated that PMab-210 stained pulmonary type I alveolar cells strongly and renal corpuscles weakly in pig or microminipig. However, the specific binding epitope of PMab-210 for pPDPN could not be determined by enzyme-linked immunosorbent assay using a series of pPDPN peptides. In this study, deletion mutants or point mutants of pPDPN were produced for analyzing the PMab-210 epitope using flow cytometry. The analysis of deletion mutants showed that N-terminus of PMab-210 epitope exists between 45th amino acid (aa) and 50th aa of pPDPN. In addition, the analysis of point mutants demonstrated that the critical epitope of PMab-210 could include Glu47, Asp48, Tyr49, Thr50, and Val51 of pPDPN, indicating that PMab-210 epitope is located in PLAG3 domain of pPDPN.

Establishment of Mouse Monoclonal Antibody LpMab-13 Against Human Podoplanin

Podoplanin (PDPN)/Aggrus is a type-I transmembrane sialoglycoprotein, which possesses a platelet aggregation-stimulating (PLAG) domain. The O-glycosylation on Thr52 of human PDPN (hPDPN) is critical for the interaction of hPDPN with C-type lectin-like receptor-2 (CLEC-2), resulting in platelet aggregation. Many anti-hPDPN monoclonal antibodies (MAbs) against PLAG domains and non-PLAG domains have been established; however, mouse anti-PLAG2/3 MAb, the epitope of which is consistent with rat anti-PLAG2/3 MAb NZ-1, has not been established. NZ-1 inhibits the hPDPN-CLEC-2 interaction and is also useful for anti-PA tag MAb. We recently established CasMab technology to produce MAbs against membranous proteins. Herein, we produced a novel anti-hPDPN MAb, LpMab-13, which binds to PLAG2/3 domains. LpMab-13 recognized endogenous hPDPN of cancer cells, including glioblastoma, oral cancer, lung cancer, and malignant mesothelioma, and normal cells such as lymphatic endothelial cells and podocytes of kidney in Western blot, flow cytometry, and immunohistochemistry. LpMab-13 recognized glycan-deficient hPDPN in flow cytometry, indicating that the interaction between LpMab-13 and hPDPN is independent of its glycosylation. The minimum epitope of LpMab-13 was identified as Ala42-Asp49 of hPDPN using Western blot and flow cytometry. The combination of different epitope-possessing MAbs could be advantageous for the hPDPN-targeting diagnosis and therapy.

LpMab-12 Established by CasMab Technology Specifically Detects Sialylated O-Glycan on Thr52 of Platelet Aggregation-Stimulating Domain of Human Podoplanin

Podoplanin (PDPN), also known as Aggrus, possesses three tandem repeat of platelet aggregation-stimulating (PLAG) domains in its N-terminus. Among the PLAG domains, sialylated O-glycan on Thr52 of PLAG3 is essential for the binding to C-type lectin-like receptor-2 (CLEC-2) and the platelet-aggregating activity of human PDPN (hPDPN). Although various anti-hPDPN monoclonal antibodies (mAbs) have been generated, no specific mAb has been reported to target the epitope containing glycosylated Thr52. We recently established CasMab technology to develop mAbs against glycosylated membrane proteins. Herein, we report the development of a novel anti-glycopeptide mAb (GpMab), LpMab-12. LpMab-12 detected endogenous hPDPN by flow cytometry. Immunohistochemical analyses also showed that hPDPN-expressing lymphatic endothelial and cancer cells were clearly labeled by LpMab-12. The minimal epitope of LpMab-12 was identified as Asp49-Pro53 of hPDPN. Furthermore, LpMab-12 reacted with the synthetic glycopeptide of hPDPN, corresponding to 38-54 amino acids (hpp3854: 38-EGGVAMPGAEDDVVTPG-54), which carries α2-6 sialylated N-acetyl-D-galactosamine (GalNAc) on Thr52. LpMab-12 did not recognize non-sialylated GalNAc-attached glycopeptide, indicating that sialylated GalNAc on Thr52 is necessary for the binding of LpMab-12 to hPDPN. Thus, LpMab-12 could serve as a new diagnostic tool for determining whether hPDPN possesses the sialylation on Thr52, a site-specific post-translational modification critical for the hPDPN association with CLEC-2.