This approach may provide a safe alternative to predict the presence of neutralizing antibodies after infection. Currently, a big challenge to control the pandemic is to rapidly and accurately identify asymptomatic infections at an early stage. primarily induce IgG antibody responses rather than IgA or IgM antibody responses. Detection of IgG against the S2 protein could supplement nucleic acid testing to identify asymptomatic patients. This study provides an antibody detection scheme for asymptomatic infections, which may contribute to epidemic prevention and control. Keywords:SARS-CoV-2, COVID-19, asymptomatic infections, IgG, antibody response, neutralizing antibody == Introduction == COVID-19, caused by SARS-CoV-2 infection, has caused more than 100 million laboratory-confirmed infections and more than 2 million deaths so far. The number of patients is increasing at an alarming rate of hundreds of thousands every day, resulting in a heavy medical and economic burden to the world (13). Since the outbreak of COVID-19, asymptomatic infection has been reported (4). Asymptomatic individuals do not show clinical symptoms, and are usually identified through mass-community screening or contact tracing (5), making them a likely population that contributes MK-8998 to the continuous community spread of SARS-CoV-2 virus. Some studies even believe that asymptomatic individuals are more infectious than those with symptoms (6). Recently, a systematic review showed that asymptomatic infections accounted for at least one-third of SARS-CoV-2 infections (7), further indicating that asymptomatic infections may play a pivotal role in the COVID-19 pandemic (8). Thereby, management of asymptomatic infections has become one of the key measures to control the COVID-19 pandemic (9). Detection of viral nucleic acid by RT-qPCR is considered the gold standard to diagnose SARS-CoV-2 infections (10). However, this approach is limited by the influence of sampling time, types of clinical specimens and method of inactivation, which can yield false-negative results and lead to misdiagnosis, especially in asymptomatic infections (1113). Detection of SARS-CoV-2-specific antibodies is an important complementary method for the diagnosis of COVID-19 (14). There is an urgent need for a more sensitive antibody detection scheme of asymptomatic infections. Here, we conducted a study to MK-8998 compare the antibody profile of asymptomatic infections to patients with different disease severity. We also compared the MK-8998 pros and cons of different antibody detection schemes for the diagnosis of MK-8998 asymptomatic infections. We found that asymptomatic infections elicit weaker antibody responses than symptomatic infections, and primarily induced IgG-based antibody responses throughout the disease course. Remarkably, detection of anti-SARS-CoV-2-S2 IgG is more sensitive than anti-RBD IgG in identifying asymptomatic COVID-19 patients. == Materials and Methods == == Patient Enrollment and Sample Collection == From January to April, 2020, 59 SARS-CoV-2 infected patients, confirmed by real-time PCR and hospitalized in the Guangzhou Eighth Peoples Hospital, were enrolled in this study. The patients were categorized into 3 groups based on their disease severity, including 18 asymptomatic patients, 33 mild-ill patients and 8 severe patients. Plasma samples were collected from each patient at multiple time points. Clinical data including patients demographic information and clinical outcome was retrieved from the medical records. Plasma samples from eight healthy donors collected in 2017-2018 were used as controls in this study. == Detection of Anti-SARS-CoV-2-RBD IgA, IgM and IgG by Electrochemiluminescence == The Kaeser 6600 automatic chemiluminescence immunoanalyzer and the matching reagents kit (Guangzhou Kangrun Biotech Co. Ltd, Guangzhou, China) was used to detect the SARS-CoV-2-specific IgM, IgA and IgG levels using a two-step indirect detection method as described previously (15). Briefly, the amino group on the RBD protein was coupled with the carboxyl group of the magnetic beads, and the antigen was fixed to form RBD-coating magnetic beads. The SARS-CoV-2 RBD-specific antibodies in the testing serum could bind to the RBD-coating magnetic beads, and acridine ester derivatives-coupled anti-human IgA, MK-8998 IgM and IgG antibodies were added. After the unbound substances were removed, a photomultiplier was used to detect light signals from acridine ester that were converted to obtain the corresponding signal value. The relative light signal values, expressed in relative light units (RLU), indicated the specific IgM, IgA and IgG levels. The relative light signal value is equivalent to the original signal value over the Rabbit polyclonal to TDGF1 specific antibody cut-off value. The cut-off values of IgM, IgA and IgG were 11 300, 56 492 and 42 213, respectively. A relative luminescence value (RLV) greater than or equal to 1.0 was considered positive for SARS-CoV-2 RBD specific IgM, IgA and IgG. == Comparison of Antibody Response Against Different SARS-CoV-2 Proteins == To assess the antibody response against the different SARS-CoV-2 proteins or different fragments of the spike protein, SARS-CoV-2 S (spike protein, 1203 aa), S1 (675 aa),.