1. Introduction
The Epidermal Growth Factor Receptor (EGFR) belongs to the ERBB family of receptor tyrosine kinases. EGFR is a type I transmembrane glycoprotein, which is composed of an extracellular ligand-binding domain and an intracellular tyrosine kinase domain [
1]. Upon ligand binding to the extracellular domain of EGFR, the downstream signaling pathways, such as the mitogen-activated protein kinases, the phosphoinositide 3-kinase/Akt, the Janus kinase/signal transducer and activator of transcription, and the phospholipase C-γ/protein kinase C pathways are activated [
2]. These pathways lead to the transcriptional activation of target genes, which are important for cell proliferation, survival, migration, invasion and cancer-initiating properties [
2]. In many carcinomas, EGFR overexpression and its mutation are involved in sustaining proliferative signaling, which is an important hallmark of cancer [
3,
4]. Therefore, EGFR has been considered as an important target for cancer therapy.
The EGFR-targeted therapies have been used in the clinic, including monoclonal antibodies (mAbs) [
5] and the tyrosine kinase inhibitors [
6]. Cetuximab is a mouse/human chimeric mAb, that binds to the extracellular domain (domain III) of EGFR, which is important for the neutralization activity [
7]. Cetuximab has been approved by the Food and Drug Administration for colorectal cancer [
8] and head and neck squamous cell carcinoma (HNSCC) [
9]. Although cetuximab has been used to treat patients with metastatic colorectal cancers, the use is limited to tumors harboring wild-type RAS [
10]. The cetuximab treatment did not exhibit the benefit in patients with colorectal cancer harboring RAS mutations [
10]. Moreover, the majority of HNSCC express EGFR; however, the benefit of therapy is limited to only 15-20% of HNSCC patients [
11].
Preclinical mouse models have been developed for the establishment of cancer therapy. The earliest models were built through the transplantation of murine tumors into immunocompetent host mice [
12]. Furthermore, tumors harvested from genetically modified mice can be transplanted and expanded into fully immunocompetent syngeneic hosts [
13]. These syngeneic models were used in preclinical studies to evaluate not only small molecule chemotherapeutic drugs but also immunotherapies including immune checkpoint inhibitors [
13]. Although several studies have shown that cetuximab can stimulate the innate or adaptive immune responses [
14,
15], the mechanisms have not been fully understood due to the lack of a preclinical model using anti-mouse EGFR (mEGFR) mAb. The model is also thought to be important for the evaluation of novel antibody-drug conjugates and prediction of the side effects.
The Cell-Based Immunization and Screening (CBIS) method includes the immunization of antigen-overexpressing cells and high-throughput hybridoma screening using flow cytometry. By the CBIS method, we have developed mAbs against human antigens, including human epidermal growth factor receptor 2 (HER2) [
16], human epidermal growth factor receptor 3 (HER3) [
17], epithelial cell adhesion molecule (EpCAM) [
18,
19], trophoblast cell surface antigen 2 (TROP2) [
20,
21], programmed cell death ligand 1 (PD-L1) [
22], podoplanin (PDPN) [
23,
24,
25,
26,
27,
28,
29,
30,
31,
32,
33,
34], the cluster of differentiation 19 (CD19) [
35], CD20 [
36,
37], CD44 [
38,
39,
40,
41], CD133 [
42], killer cell lectin-like receptor G1 (KLRG1) [
43], C-C motif chemokine receptor 9 (CCR9) [
44], and T cell immunoreceptor with Ig and ITIM domains (TIGIT) [
45] by immunization of antigen-overexpressing cells into mice. We also successfully developed mAbs against mouse antigens, including mouse CCR3 [
46] and mouse CCR8 [
47] by immunization them into rats. In this study, we developed novel anti-mEGFR mAbs by the CBIS method and evaluated its applications including flow cytometry.
2. Materials and methods
2.1. Preparation of plasmids
The expression plasmid of mEGFR (pCMV6-neo-mEGFR-Myc-DDK) was commercially available from OriGene Technologies, Inc. (Rockville, MD). The cDNA encoding mEGFR (NM_207655) was subcloned into pCAG-zeo_ssnPA and pCAG-zeo_MAP vectors, which are purchased from a pCAG-zeo vector (FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan), with N-terminal PA tag [
48,
49,
50] and MAP tag [
51,
52], respectively. The amino acid sequences of the tag system were as follows: PA tag, 12 amino acids (GVAMPGAEDDVV) and MAP tag, 12 amino acids (GDGMVPPGIEDK). The PA tag can be recognized by NZ-1 (an anti-human PDPN mAb) [
48,
49,
50,
53,
54,
55,
56,
57,
58,
59,
60,
61,
62,
63,
64,
65].
2.2. Antibodies
Alexa Fluor 488-conjugated anti-rat IgG was purchased from Cell Signaling Technology, Inc. (Danvers, MA).
2.3. Cell lines
P3X63Ag8U.1 (P3U1), Chinese hamster ovary (CHO)-K1, LN229, and NMuMG (a mouse mammary gland epithelial cell) were obtained from the American Type Culture Collection (ATCC; Manassas, VA). Lewis lung carcinoma was obtained from the Japanese Collection of Research Bioresources (JCRB; Osaka, Japan).
The pCAG-zeo_ssnPA-mEGFR and pCAG-zeo_MAP-mEGFR plasmids were transfected into LN229 and CHO-K1 cells, respectively. The stable transfectants were generated as described previously [
40,
41].
CHO-K1, mEGFR-overexpressed CHO-K1 (CHO/mEGFR), Lewis lung carcinoma, and P3U1 were cultured in an RPMI-1640 medium (Nacalai Tesque, Inc., Kyoto, Japan), with 10% fetal bovine serum (FBS; Thermo Fisher Scientific Inc.). A cocktail of 100 units/mL of penicillin, 100 μg/mL of streptomycin, and 0.25 μg/mL of amphotericin B (Nacalai Tesque, Inc.) was added to the medium. LN229, mEGFR-overexpressed LN229 (LN229/mEGFR), and NMuMG were cultured in DMEM (Nacalai Tesque, Inc.), supplemented as indicated above. For NMuMG cells, 10 μg/mL of insulin (Sigma-Aldrich Corp., St. Louis, MO, USA) was further added. All cells were cultured using a humidified incubator at 37°C, in an atmosphere of 5% CO2 and 95% air.
2.4. Development of hybridomas
A five-week-old Sprague–Dawley rat was purchased from CLEA Japan (Tokyo, Japan). To develop mAbs against mEGFR, we intraperitoneally immunized one rat with LN229/mEGFR (1 × 10
9 cells) plus Imject Alum (Thermo Fisher Scientific Inc.). After three additional injections in every week (1 × 10
9 cells/rat), a final booster injection (1 × 10
9 cells/rat) was performed two days before harvesting spleen cells. The hybridomas were produced, as described previously [
46]. The hybridoma supernatants were screened using flow cytometry using CHO/mEGFR, CHO-K1, and NMuMG.
2.5. Purification of EMab-300
The cultured supernatant of EMab-300 hybridomas was applied to 1 mL of Ab-Capcher (ProteNova, Kagawa, Japan). Ab-capcher is a gel carrier in which the alkali-resistant antibody-binding protein Protein A-R28, developed by Protenova’s patented technology, was immobilized at multiple points at high density. After washing with phosphate-buffered saline (PBS), the antibodies were eluted with an IgG elution buffer (Thermo Fisher Scientific Inc.). Finally, the eluates were concentrated, and the elution buffer was replaced with PBS using Amicon Ultra (Merck KGaA, Darmstadt, Germany).
2.6. Flow cytometric analysis
CHO/mEGFR, CHO-K1, NMuMG, and Lewis lung carcinoma were harvested after a brief exposure to 1 mM ethylenediaminetetraacetic acid (EDTA, Nacalai Tesque, Inc.). The cells were treated with EMab-300 or blocking buffer (control) (0.1% BSA in PBS) for 30 min at 4°C, followed by treatment with Alexa Fluor 488-conjugated anti-rat IgG. The data were analyzed using the SA3800 Cell Analyzer and SA3800 software ver. 2.05 (Sony Corp., Tokyo, Japan).
2.7. Determination of dissociation constant (KD) by flow cytometry
The serially diluted EMab-300 was suspended with CHO/mEGFR and NMuMG cells for 30 min at 4°C. The cells were treated with 50 μL of Alexa Fluor 488-conjugated anti-rat IgG (1:200). The fluorescence data were collected, using the SA3800 Cell Analyzer. The KD was subsequently calculated by GraphPad PRISM 8 (GraphPad Software, Inc., La Jolla, CA).
4. Discussion
Therapeutic strategies and modalities for cancer have been revolutionary developed. However, only about five percent of new cancer therapies are approved, and most fail due to the lack of efficacy [
13]. The failures cost enormous finance and lose the quality of patient life. The failures also indicate that current preclinical methods are not sufficient to predict successful outcomes. Although EGFR is one of the important targets for cancer therapy, anti-mEGFR mAbs for preclinical study have not been developed. In this study, we developed a novel anti-mEGFR mAbs (EMab-300) using the CBIS method and showed the application to flow cytometry (
Figure 2 and
Figure 3). EMab-300 could contribute to the preclinical study to evaluate the antitumor effects and predict the side effects of the EGFR targeting therapy.
EMab-300 could react with NMuMG and Lewis lung carcinoma cells (
Figure 2). NMuMG has been used in the study of epithelial-to-mesenchymal transition (EMT), which is induced by various cytokines, such as TGF-β [
66] and transcriptional factors [
67]. The activation of EMT program confers tumor cells the ability of migration, invasion, extravasation, and stemness [
68]. Once the EMT-induced tumor cells reach to distant organs, these mesenchymal properties revert to epithelial properties via mesenchymal–epithelial transition (MET) in order to form a secondary tumor in distant organs [
68]. The EMT-induced NMuMG cells can make spheres in the presence of EGF
in vitro, and exhibit the tumorigenic potential in vivo [
69]. Therefore, we can evaluate the neutralization activity of EMab-300 in the sphere formation assay in the future study.
Lewis lung carcinoma has been widely used as a syngenetic model [
70]. The model was a successful preclinical model for the evaluation of a chemotherapeutic agent, navelbine, prior to its implementation in clinical trials [
70]. Recently, syngenetic models are also important for the evaluation of combination therapy with immune checkpoint inhibitors [
71,
72]. Therefore, the conversion of EMab-300 (rat IgG
1, kappa) to mouse IgG is essential for evaluating antitumor effects and developing antibody-drug conjugates or chimeric antigen receptor T cell therapies. We previously produced recombinant antibodies, which were converted into mouse IgG
2a subclass from mouse IgG
1. Furthermore, we produced defucosylated IgG
2a mAbs using fucosyltransferase 8-deficient CHO-K1 cells to enhance the antibody-dependent cellular cytotoxicity activity [
73,
74,
75,
76,
77,
78,
79,
80]. The defucosylated mAbs showed an antitumor effect in xenograft models. Therefore, a class-switched and defucosylated type of EMab-300 could contribute to the treatment of syngenetic mouse tumors and spontaneous mice tumors in the future.
The identification of the epitope is also important to assess the property of mAbs. We have established anti-human EGFR mAbs using the CBIS method. Most of the mAbs recognized the conformational epitopes. Therefore, we have developed the RIEDL insertion for epitope mapping (REMAP) method [
81,
82,
83,
84] to identify the conformational epitope. We determined the conformational epitopes of anti-human EGFR mAbs (EMab-51 and EMab-134) [
82,
84] and anti-CD44 mAbs (C
44Mab-5 and C
44Mab-46) [
81,
83] using the REMAP method. Therefore, further investigations are required to determine the epitopes of EMab-300.