Human Comp IgG/IgM Depleted Pooled (ml)

A newly identified coronavirus named SARS-CoV-2 emerged in Hubei Province, China, in December 2019 and quickly spread around the world; To date, it has caused more than 49.7 million cases of illness and 1.2 million deaths. The diagnosis of SARS-CoV-2 infection is currently based on the detection of viral RNA in nasopharyngeal swabs using molecular-based assays such as B. Real time RT-PCR. In addition, serological assays for the detection of different classes of antibodies represent an excellent surveillance strategy to obtain information about the humoral immune response to infection and thespread of the virus in the population. In addition, it can contribute to the evaluation of the immunogenicity of novel future vaccines and drugs for the treatment and prevention of COVID-19 disease.

The aim of this study was to determine SARS-CoV-2-specific antibodies in human serum samples using various commercial and in-house ELISA kits in order to first evaluate and compare their results with each other and then with those of functional assays using wild-type virus . It is important to determine the level of SARS-CoV-2 specific IgM, IgG and IgA antibodies to predict the immunity of the human population, possible cross-reactivity with other coronaviruses and to identify potentially infectious individuals.

In addition, a subtyping IgG ELISA was performed on a small subset of samples. Our results showed a remarkable statistical correlation between the neutralization titers and the IgG, IgM and IgA ELISA responses against the receptor-binding domain of the spike protein. Thus, it is confirmed that antibodies against this part of the viral spike protein are highly neutralizing and that the receptor-binding domain-based ELISA assay can be used as a valid substitute for the neutralization assay in laboratories that do not have Biosafety Level 3 facilities.

introduction

Coronaviruses (CoVs) are enveloped, positive, single-stranded RNA viruses belonging to the  Coronaviridae subfamily. The coronavirus subfamily includes 4 genera: alpha coronavirus, which includes human coronavirus (HCoV)-229E and HCoV-NL63; Coronavirus beta, the HCoV-OC43, human severe acute respiratory syndrome coronavirus (SARS-CoV-1), Middle East respiratory syndrome coronavirus (MERS-CoV) and emerging severe acute respiratory syndrome coronavirus 2 ( SARS-CoV-2) includes .

• Several members of this family, such as HCoV OC43, NL63, and 229E, cause common colds in the human population every year (Corman et al., 2019). 
• Three highly pathogenic novel CoVs have emerged in the last 18 years; The SARS-CoV-1 virus emerged in Guangdong Province in November 2002, causing more than 8,000 confirmed cases and 774 deaths (  de Wit et al., 2016; Gorbalenya et al., 2020). MERS-CoV virus was detected in June 2012 ( Zaki et al., 2012 ) caused 2494 laboratory-confirmed cases including 858 deaths , and SARS-CoV-2 virus emerged in December 2019 in Wuhan, Hubei Province, China. on; The latter was declared a pandemic by the World Health Organization (WHO) on March 11, 2020. 
• The global impact of the SARS-CoV-2 outbreak with over 49.7 million COVID-19 cases and 1.2 million deaths reported to WHO (as of November 10, 2020) ( WHO, nd-a ), are unprecedented.
• Multiple data have confirmed that the infection originally came from contact with animals at the Wuhan fish market. Human-to-human transmission subsequently occurred, leading to a very high rate of laboratory-confirmed infections in China (Chan et al., 2020; WHO, 2020). Accurate diagnosis of coronavirus disease (COVID-19) is essential to promptly identify infected individuals, limit the spread of the virus, and enable treatment of those infected in the early stages of infection.
• To date, real-time polymerase chain reaction (RT-PCR) is the most widely used method to diagnose COVID-19. However, rapid, large-scale testing has been hampered by high demand and a lack of mucosal sampling materials (Zou et al., 2020).

Standardized serological assays capable of measuring antibody responses can help overcome these problems and can support a significant number of relevant applications.

In fact, serological tests are the basis for determining the rate of infection (severe, mild and asymptomatic) in a given area, calculating the percentage of the population susceptible to the virus and determining the mortality rate of the disease. In a non-human primate model (Bao et al., 2020) it has been shown that reinfection and consequently virus shedding is unlikely once the antibody response has been established. Additionally, serological assays can help identify individuals with strong antibody responses who could serve as donors for the manufacture of monoclonal antibody therapeutics (Andreano et al., 2020).

The spike glycoprotein (S protein), a large transmembrane homotrimer of approximately 140 kDa, plays a central role in viral pathogenesis by mediating binding to target cells through the interaction between its receptor-binding domain (RBD) (Wrapp et al., 2020) and the human angiotensin converting enzyme 2 (ACE2) receptor. S protein has been found to be highly immunogenic and RBD may be considered the primary target in efforts to elicit potent neutralizing antibodies(Tay et al., 2020; Berry et al., 2010). Two subunits make up the S protein: S1, which mediates binding, and S2, which mediates membrane fusion. The CoV-S protein is a class I fusion protein and protease cleavage is required to activate the fusion process (Ou et al., 2016).

To date, the complexity of systemic immunoglobulin G (IgG) along with IgG subclasses and IgM and IgA in relation to responses against SARS-CoV-2 has not been elucidated. In addition, data comparing the differences between these responses and the neutralizing responses detected by functional assays such as the microneutralization test (MN) are still not well defined.

Undoubtedly, it is widely accepted that IgG levels play a crucial role in protection against viral diseases (Murin et al., 2019). In humans, the four IgG subclasses (IgG1, IgG2, IgG3, IgG4) differ in function (Schroeder and Cavacini, 2010), and IgG1 and IgG3 play key roles in many basic immunological functions, including viral neutralization, opsonization, and complement fixation (Frasca et al., 2013). Therefore, we performed a comparative study with two purposes: the first objective was to examine the sensitivity and specificity in terms of detection of different ELISA kits compared to MN results; The second objective was to examine the difference in spike RBD-specific IgG, IgM and IgA antibody responses in human serum samples.