The spike (S) protein on the surface of SARS-CoV-2 mediates receptor binding and virus-host cell membrane fusion during virus entry. Many neutralizing antibodies (nAbs), which targeted the receptor binding domain (RBD) of S protein, lost the neutralizing activity against the newly emerging SARS-CoV-2 variants with sequence mutations at the RBD.

Many cases of avian influenza A(H7N9) virus infection in humans have been reported since its first emergence in 2013. The disease is of concern because most patients have become severely ill with roughly 30% mortality rate. Because the threat in public health caused by H7N9 virus remains high, advance preparedness is essentially needed. In this study, the recombinant H7N9 hemagglutinin (HA) was expressed in insect cells and purified for generation of two monoclonal antibodies, named F3-2 and 1C6B. F3-2 can only recognize the H7N9 HA without having cross-reactivity with HA proteins of H1N1, H3N2, H5N1, and H7N7. 1C6B has the similar specificity with F3-2, but 1C6B can also bind to H7N7 HA. The binding epitope of F3-2 is mainly located in the region of H7N9 HA(299-307). The binding epitope of 1C6B is located in the region of H7N9 HA(489-506). F3-2 and 1C6B could not effectively inhibit the hemagglutination activity of H7N9 HA. However, F3-2 can prevent H7N9 HA from trypsin cleavage and can bind to H7N9 HA which has undergone pH-induced conformational change. F3-2 also has the ability of binding to H7N9 viral particles and inhibiting H7N9 virus infection to MDCK cells with the IC50 value of 22.18 μg/mL. In addition, F3-2 and 1C6B were utilized for comprising a lateral flow immunochromatographic test strip for specific detection of H7N9 HA. Key points• Two mouse monoclonal antibodies, F3-2 and 1C6B, were generated for recognizing the novel binding epitopes in H7N9 HA.• F3-2 can prevent H7N9 HA from trypsin cleavage and inhibit H7N9 virus infection to MDCK cells.• F3-2 and 1C6B were developed as a lateral flow immunochromatographic test for specific detection of H7N9 HA.

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The nucleic acid test is still the standard assessment for the diagnosis of coronavirus disease 2019 (COVID-19), which is caused by human infection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). In addition to supporting the confirmation of disease cases, serological assays are used for the analysis of antibody status and epidemiological surveys. In this study, a single Western blot strip (WBS) coated with multiple Escherichia coli ( E. coli )-expressed SARS-CoV-2 antigens was developed for comprehensive studies of antibody profiles in COVID-19 patient sera. The levels of specific antibodies directed to SARS-CoV-2 spike (S), S2, and nucleocapsid (N) proteins were gradually increased with the same tendency as the disease progressed after hospitalization. The signal readouts of S, S2, and N revealed by the multi-antigen-coated WBS (mWBS)-based serological assay (mWBS assay) also demonstrated a positive correlation with the SARS-CoV-2 neutralizing potency of the sera measured by the plaque reduction neutralization test (PRNT) assays. Surprisingly, the detection signals against the unstructured receptor-binding domain (RBD) purified from E. coli inclusion bodies were not observed, although the COVID-19 patient sera exhibited strong neutralizing potency in the PRNT assays, suggesting that the RBD-specific antibodies in patient sera mostly recognize the conformational epitopes. Furthermore, the mWBS assay identified a unique and major antigenic epitope at the residues 1148, 1149, 1152, 1155, and 1156 located within the 1127–1167 fragment of the S2 subunit, which was specifically recognized by the COVID-19 patient serum. The mWBS assay can be finished within 14–16 min by using the automatic platform of Western blotting by thin-film direct coating with suction (TDCS WB). Collectively, the mWBS assay can be applied for the analysis of antibody responses, prediction of the protective antibody status, and identification of the specific epitope. Key points • A Western blot strip (WBS) coated with multiple SARS-CoV-2 antigens was developed for the serological assay. • The multi-antigen-coated WBS (mWBS) can be utilized for the simultaneous detection of antibody responses to multiple SARS-CoV-2 antigens. • The mWBS-based serological assay (mWBS assay) identified a unique epitope recognized by the COVID-19 patient serum.

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