Abstract
Despite efforts to develop anti–severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antibody (Ab) immunoassays, reliable serological methods are still needed. We developed a multiplex addressable laser bead immunoassay (ALBIA) to detect and quantify anti-Spike S1 and nucleocapsid N Abs. Recombinant S1 and N proteins were bound to fluorescent beads (ALBIA-IgG-S1/N). Abs were revealed using class-specific anti-human Ig Abs. The performances of the test were analyzed on 575 serum samples including 192 from SARS-CoV-2 polymerase chain reaction–confirmed patients, 13 from seasonal coronaviruses, 70 from different inflammatory/autoimmune diseases, and 300 from healthy donors. Anti-S1 IgM were detected by monoplex ALBIA-IgM-S1. Comparison with chemiluminescent assays or enzyme-linked immunosorbent assays was performed using commercial tests. Multiplex ALBIA-IgG-S1/N was effective in detecting and quantifying anti–SARS-CoV-2 IgG Abs. Two weeks after first symptoms, sensitivity and specificity were 97.7 and 98.0% (anti-S1), and 100 and 98.7% (anti-N), respectively. Agreement with commercial tests was good to excellent, with a higher sensitivity of ALBIA. ALBIA-IgG-S1/N was positive in 53% of patients up to day 7, and in 75% between days 7 and 13. For ALBIA-IgM-S1, sensitivity and specificity were 74.4 and 98.7%, respectively. Patients in intensive care units had higher IgG Ab levels (Mann–Whitney test, p < 0.05). ALBIA provides a robust method for exploring humoral immunity to SARS-CoV-2. Serology should be performed after 2 weeks following first symptoms, when all COVID-19 (coronavirus disease 2019) patients had at least one anti-S1 or anti-N IgG Ab, illustrating the interest of a multiplex test.
Highlights
The emergence of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which caused the coronavirus disease 2019 (COVID-19) pandemic in December 2019, has led to the development of diagnostic molecular and serological tests
The identity and purity of these proteins were confirmed by Coomassie blue staining after SDS-PAGE, revealing a unique band (Supplementary Figure 1A) that was recognized by an anti–polyhistidine Ab in Western blot (Supplementary Figure 1B)
A calibration curve was obtained after serial dilutions of a highly anti–S1positive serum used as calibrator
Summary
The emergence of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which caused the coronavirus disease 2019 (COVID-19) pandemic in December 2019, has led to the development of diagnostic molecular and serological tests. The reference standard of molecular test for diagnosis of COVID-19 is reverse transcription–polymerase chain reaction (RT-PCR), which detects viral RNA using nasopharyngeal swabs or other upper respiratory tract specimens. Serological tests are essential complements to molecular tests because they can identify individuals with SARS-CoV-2 at a distance from infection, when RT-PCR has become negative or was inconclusive. Serological tests are useful for epidemiological purposes, vaccination research, and, possibly, for assessment of the level of protection toward reinfection. Serological assays evaluate the humoral immune response to nucleocapsid (N) or Spike (S) proteins as they have been shown to be the most immunogenic proteins among coronaviruses (Meyer et al, 2014)
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