Amplification-based quantitative polymerase chain reaction (qPCR) provides accurate and sensitive nucleic acid quantification. However, the requirement of temperature cycling and real-time monitoring limits its translation to many settings. Quantitative isothermal amplification methods alleviate the need for thermal cyclers; however, they still require continuous monitoring of the nucleic acid amplification on sophisticated readers. Here, we adapted an isothermal recombinase polymerase amplification (RPA) reaction to develop a semiquantitative method that relies on the final amplicon yield to estimate the initial target nucleic acid copy number. To achieve this, we developed a phenomenological model that captures the essential RPA dynamics. We identified reaction conditions that constrained the reaction yield corresponding to the starting DNA template concentration. We validated these predictions experimentally and showed that the amplicon yields at the end of the RPA reaction correlated well with the starting DNA concentration while reducing nonspecific amplification robustly. We demonstrate this approach, termed quantitative endpoint RPA (qeRPA), to detect DNA over five log orders with a detection limit of 100 molecules. Using a linear regression model of the normalized endpoint intensity (NEI) standard curve, we estimate the viral load from the serum of dengue virus-infected patients with comparable performance to qPCR. Unlike the conventional isothermal quantitative methods, qeRPA can be employed for robust and sensitive nucleic acid estimation at close to room temperature without real-time monitoring and can be beneficial for field deployment in resource-limited settings.
Background Spike protein domains are being used in various serology-based assays to detect prior exposure to SARS-CoV-2 virus. However, there has been limited comparison of antibody titers against various spike protein antigens among COVID-19 infected patients. Methods We compared four spike proteins (RBD, S1, S2 and a stabilized spike trimer (ST)) representing commonly used antigens for their reactivity to human IgG antibodies using indirect ELISA in serum from COVID-19 patients and pre-2020 samples. ST ELISA was also compared against the EUROIMMUN IgG ELISA test. Further, we estimated time appropriate IgG and IgA seropositivity rates in COVID-19 patients using a panel of sera samples collected longitudinally from the day of onset of symptoms (DOS). Results Among the four spike antigens tested, the ST demonstrated the highest sensitivity (86.2%; 95% CI: 77.8-91.7%), while all four antigens showed high specificity to COVID-19 sera (94.7-96.8%). 13.8% (13/94) of the samples did not show seroconversion in any of the four antigen-based assays. In a double-blinded head-to-head comparison, ST based IgG ELISA displayed a better sensitivity (87.5%, 95%CI: 76.4-93.8%) than the EUROIMMUN IgG ELISA (67.9%, 95% CI: 54.8-78.6%). Further, in ST-based assays, we found 48% and 50% seroconversion in the first six days (from DOS) for IgG and IgA antibodies, respectively, which increased to 84% (IgG) and 85% (IgA) for samples collected ≥22 days from DOS. Conclusions Comparison of spike antigens demonstrates that spike trimer protein is a superior option as an ELISA antigen for COVID-19 serology.
BackgroundSpike protein domains are being used in various serology-based assays to detect prior exposure to SARS-CoV-2 virus. However, there has been limited comparison of human antibody titers against various spike protein antigens among COVID-19 infected patients.MethodsWe compared four spike proteins (RBD, S1, S2 and a stabilized spike trimer (ST)) representing commonly used antigens for their reactivity to human IgG antibodies using indirect ELISA in serum from COVID-19 patients and pre-2020 samples. ST ELISA was also compared against the EUROIMMUN IgG ELISA test. Further, we estimated time appropriate IgG and IgA seropositivity rates in COVID-19 patients using a panel of sera samples collected longitudinally from the day ofonset of symptoms (DOS).ResultsAmong the four spike antigens tested, the ST demonstrated the highest sensitivity (86.2%; 95% CI: 77.8-91.7%), while all four antigens showed high specificity to COVID-19 sera (94.7-96.8%). 13.8% (13/94) of the samples did not show seroconversion in any of the four antigen-based assays. In a double-blinded head-to-head comparison, ST based IgG ELISA displayed a better sensitivity (87.5%, 95%CI: 76.4-93.8%) than the EUROIMMUN IgG ELISA (67.9%, 95% CI: 54.8-78.6%). Further, in ST-based assays, we found 48% and 50% seroconversion in the first six days (from DOS) for IgG and IgA antibodies, respectively, which increased to 84% (IgG) and 85% (IgA) for samples collected ≥22 days DOS.ConclusionsComparison of spike antigens demonstrates that spike trimer protein is a superior option as an ELISA antigen for COVID-19 serology.HighlightsSpike trimer displays the highest antibody titer in SARS-CoV-2 infections among spike protein antigens.Spike trimer IgG ELISA displays a sensitivity of 50% within six days and 86.2% after 14 days from onset of symptoms.IgA and IgG responses to spike trimer antigen were comparable and concomitant in time after infection.16% (IgG) and 15% (IgA) of COVID-19 RT-PCR positive patients did not seroconvert even after 21 days from onset of symptoms.
Amplification-based qPCR provides accurate and sensitive nucleic acid quantification. However, the requirement of temperature cycling and real-time monitoring limits its translation to different settings. Here, we adapted isothermal Recombinase Polymerase Amplification (RPA) reaction to develop a semi-quantitative method that relies on final amplicon yield to estimate initial target nucleic acid copy number. To achieve this, we developed a phenomenological model that captures the essential RPA dynamics. We identified reaction conditions that constrained the reaction yield corresponding to the starting DNA template concentration. We validated these predictions experimentally and show that the amplicon yields at the end of the RPA reaction correlates well to the starting DNA concentration while reducing non specific amplification robustly. We demonstrate this approach termed here as quantitative endpoint RPA (qeRPA) to detect DNA over five log orders with detection limit of 100 molecules. Using a linear regression model with normalized endpoint intensity (NEI) standard curve, we estimate viral load from the serum of dengue-infected patients with comparable performance to qPCR. Hence, qeRPA can be employed for robust and sensitive nucleic acid estimation at close to room temperature without real-time monitoring and can be beneficial for field-deployment in limited-resource settings.
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