Characterization of anti-MERS-CoV antibodies against various recombinant structural antigens of MERS-CoV in an imported case in China

Abstract

The first imported case of Middle East respiratory syndrome (MERS) in China recently occurred, allowing for the characterization of antibody titers in a series of the patient's sera using the following methods based on recombinant viral structural antigens: inactivated MERS coronavirus (MERS-CoV) enzyme-linked immunosorbent assay (ELISA), recombinant MERS-CoV spike (S, or fragments of S) ELISA, nucleoprotein (NP) ELISA and MERS S pseudovirus particle-based neutralization test (ppNT). A longitudinal profile of the infection showed that seroconversion detected by ELISAs based on the recombinant extracellular domain, S, S1 and receptor-binding domain (RBD) antigens occurred as early as neutralizing antibodies were detected by the ppNT and earlier than antibodies were detected by the inactivated MERS-CoV and N-terminal domain (NTD) ELISAs. Antibodies detected by the NP ELISA occurred last. Strong correlations were found between the S1, RBD and NP ELISAs and the inactivated MERS-CoV ELISA. The S and RBD ELISAs were highly correlated with the commercial S1 ELISA. The S ELISA strongly correlated with the ppNT, although the MERS-CoV, S1, NTD and RBD ELISAs were also significantly correlated with the ppNT (P<0.001).

Figure 1

Kinetics of the serological response in a MERS-CoV-infected patient as determined by ELISAs for various recombinant antigens and the ppNT. Sera sampled on days 2, 3, 4, 5, 6, 8, 9, 10, 12, 13, 15, 18, 20, 27 and 28 after admission to the hospital were retrospectively analyzed. (A) Longitudinal profiles of IgG antibodies against MERS-CoV, S, S1, NTD, RBD and NP in the patient who was the first to import MERS-CoV into China; sera were determined by 1: 80 dilution in all ELISAs except by 1:101 dilution in S1 ELISA. (B) Longitudinal profiles of IgM antibodies against MERS-CoV, S, NTD, RBD and NP; sera were determined by 1:80 dilutioin in all ELISAs. (C) Longitudinal profiles of NAb against MERS-CoV. enzyme-linked immunosorbent assay, ELISA; immunoglobulin G/M, IgG/M; Middle East respiratory syndrome coronavirus, MERS-CoV; neutralizing antibody, NAb; nucleoprotein, NP; N-terminal domain, NTD; optical density, OD; pseudovirus particle-based neutralization test, ppNT; receptor-binding domain, RBD; recombinant MERS-CoV full-length spike protein, S.

Figure 2

Sensitivities of ELISAs based on different viral antigens. ELISA plates were coated with inactivated MERS-CoV particles (A) or purified recombinant S (B), S1 (C), NTD (D), RBD (E) or NP (F) protein. Serum samples from the patient with MERS-CoV collected on days 2 (sample 1), 15 (sample 2) and 28 (sample 3) after admission to the hospital were serially diluted and dispensed into the wells of an ELISA plate. HRP-labeled goat anti-human IgG (left) and IgM (right) were used as the secondary antibody, with 3,3',5,5'-tetramethylbenzidine (TMB) as the substrate. The results are expressed as the absorbance reading at 450 nm. As the negative control had very poor ELISA responses, the results are not shown to avoid interference with the target profiles. enzyme-linked immunosorbent assay, ELISA; horseradish peroxidase, HRP; immunoglobulin G/M, IgG/M; Middle East respiratory syndrome coronavirus, MERS-CoV; nucleoprotein, NP; N-terminal domain, NTD; optical density, OD; receptor-binding domain, RBD; recombinant MERS-CoV full-length spike protein, S.

Figure 3

Correlations among the inactivated virus-based and other S or N protein-based ELISA results. The absorbance readings of the S (A), S1 (B), NTD (C), RBD (D) and NP (E) based IgG ELISAs (OD₄₅₀ values) were plotted against that of the virus-based IgG ELISA (OD₄₅₀ values) (data are those from Figure 1A). ***P<0.001. Enzyme-linked immunosorbent assay, ELISA; immunoglobulin G, IgG; Middle East respiratory syndrome coronavirus, MERS-CoV; nucleoprotein, NP; N-terminal domain, NTD; optical density, OD; receptor-binding domain, RBD; recombinant MERS-CoV full-length spike protein, S.

Figure 4

Correlation of the Euroimmune EIA Kit for detecting anti-S1 IgG antibody of MERS-CoV (OD₄₅₀ values) with other recombinant protein-based IgG ELISAs (OD₄₅₀ values) used in this study. The absorbance readings of the S (A), NTD (B) and RBD (C) based IgG ELISAs (OD₄₅₀ values) were plotted against those of the S1-based IgG ELISA (OD₄₅₀ values) (data are those from Figure 1A). ***P<0.001. Enzyme-linked immunosorbent assay, ELISA; immunoglobulin G, IgG; Middle East respiratory syndrome coronavirus, MERS-CoV; nucleoprotein, NP; N-terminal domain, NTD; optical density, OD; receptor-binding domain, RBD; recombinant MERS-CoV full-length spike protein, S.

Figure 5

Correlations among the ELISAs (IgG) (OD₄₅₀ values) and the MERS-CoV ppNT assay (neutralizing antibody titer) used in this study. The absorbance readings of the inactivated MERS-CoV (A), S (B), S1 (C), NTD (D) and RBD (E) based IgG ELISAs (OD₄₅₀ values) (data are those from Figure 1A) were plotted against neutralizing antibody titers (data are those from Figure 1C). *P<0.05; **P<0.01; ***P<0.001. Antibody, Ab; enzyme-linked immunosorbent assay, ELISA; immunoglobulin G, IgG; Middle East respiratory syndrome coronavirus, MERS-CoV; nucleoprotein, NP; N-terminal domain, NTD; optical density, OD; pseudovirus particle-based neutralization test, ppNT; receptor-binding domain, RBD; recombinant MERS-CoV full-length spike protein, S.