3 ). Open in a separate window Fig. observed. In conclusion, although the SARS-CoV-2 immune response heterogeneity, the use of immunoassays can help in large-scale monitoring of COVID-19 samples, becoming a valid alternative to VNT test for diagnostic routine laboratories. Keywords: SARS-CoV-2, Live virus neutralization test, Anti-SARS-CoV-2 antibodies, Serological immunoassays, COVID-19 1.?Introduction The novel human coronavirus 2 associated to Severe Acute Respiratory Syndrome (SARS-CoV-2), discovered in Wuhan, China, as the causative agent of the 2019 Coronavirus Disease (COVID-19) [1], belongs to a distinct class of -coronaviruses and its genome shows a 79% gene sequence homology to SARS-CoV (Severe Acute Respiratory Syndrome CoronaVirus) and about 50% homology to MERS-CoV (Middle East Respiratory SAR-100842 Syndrome CoronaVirus) [2]. SARS-CoV-2 is an enveloped single stranded, positive sense RNA virus and its SAR-100842 genome encodes several nonstructural proteins (NSP) and 4 structural proteins: Spike protein (S); Membrane protein (M); Envelope protein (E) and Nucleocapsid protein (N) [3]. Among these 4 structural proteins, the Spike and Nucleocapsid proteins are the most immunogenic antigens, as previous studies for MERS-CoV and SARS CoV have shown [4]. The Spike protein is a very large transmembrane protein consisting of two subunits: N-terminal S1, responsible for virus binding to Angiotensin Converting Enzyme 2 (ACE2) receptor Runx2 of different cell types, and C-terminal S2, responsible for virus fusion to human cell membranes presenting the ACE2 receptor. The S1 subunit is in turn divided in two domains: NTD (N-Terminal Domain) and RBD (Receptor Binding Domain), who directly interacts with the ACE2 receptor of host cells [[5], [6], [7]]. Structural studies performed on the SARS-CoV-2 RBD/ACE2 complex showed that 6 amino acids are essential for ACE2 receptors binding and virus entry in human cells and the higher spread rate of SARS-CoV-2 in human population could be explained by the higher affinity of RBD domain to ACE2 receptor described in SARS-CoV-2 as compared to SARS-CoV [8,9]. After SARS-CoV-2 infection, the host usually develops an immune response with production of IgA and IgM antibodies (Abs) in 7C14?days from symptoms onset, followed by IgG response SAR-100842 after two weeks [10]. In this context, the RBD domain is considered the most antigenic protein and the primary specific target for active neutralizing antibodies (NAbs). The multiple conformational epitopes of the RBD are also responsible of the strong immune response and the anti-RBD Abs are considered the key player in viral response. Important differences in antibodies concentrations have been also reported in COVID-19 patients. [11]. Although SARS-CoV-2 antibodies level in human serum and/or plasma correlates with protective immune response and decline of viral load, high Abs titers and early seroconversion were associated to disease severity. In this line, Wu et al. have recently demonstrated the presence of a higher antibody titer in elderly than in young patients, hypothesizing a possible connection with the clinical status of these two patients’ categories [12]. Anti-RBD Abs are considered as the most clinically relevant antibodies against SARS-CoV-2 not only for their neutralizing activity, but also for the affinity to ACE2 receptors in human cells. These anti-RBD Abs, in fact, induce a competitive mechanism able to block the binding of the viral RBD to the ACE2 and the subsequent virus infection [13]. These observations demonstrate that SARS-CoV-2 RBD domain could be an important immunogenic target of antiviral drugs and COVID-19 vaccines. However, data on kinetics and duration of anti-RBD Abs responses of SARS-CoV-2 infected patients and vaccinated subjects are necessary to understand the mechanisms of protective immunity and the duration immunity against COVID-19, representing a key correlate of protection from possible reinfection. Many studies have reported that anti-RBD antibodies levels significantly decrease with time, remaining detectable in most individuals, but there are few data regarding virus sera neutralization tests in vitro [14]. The clinical utility of serological testing is controversial and a.
