Knowledge of the antibody-mediated immune response to SARS-CoV-2 is crucial to understand virus immunogenicity, establish seroprevalence, and determine whether subjects or recovered patients are at risk for infection/reinfection and would therefore benefit from vaccination. Here, we describe a novel and simple cell-ELISA specifically designed to measure viral spike S1-specific IgG produced in vitro by B cells in peripheral blood mononuclear cell (PBMC) cultures from a cohort of 45 asymptomatic (n = 24) and symptomatic (n = 21) individuals, with age ranging from 8 to 99 years. All subjects underwent ELISA serological screening twice, at the same time as the cell-ELISA (T2) as well as 35–60 days earlier (T1). Cryopreserved PBMCs of healthy donors obtained years before the COVID-19 pandemic were also included in the analysis. The preliminary results presented here show that out of 45 tested subjects, 16 individuals (35.5%) were positive to the cell-ELISA, 11 (24.5%) were concomitantly positive in the serological screening (T1 and/or T2), and only one person was exclusively positive in ELISA (T1) and negative in cell-ELISA, though values were close to the cutoff. Of note, five individuals (11.2%) tested negative in ELISA but positive in cell-ELISA and thus, they appear to have circulating B cells that produce antibodies against SARS-CoV-2, likely at levels that are undetectable in the serum, which challenges the negative results of the serological screening. The relative level of in vitro secreted IgG was measurable in positive subjects, ranging from 7 to 50 ng/well. Accordingly, all anti-SARS-CoV-2 antibody-positive subjects previously reported moderate to severe symptoms attributable to COVID-19, even though the RT-PCR data were rarely available to confirm viral infection. Overall, the described cell-ELISA might be an effective method for detecting subjects who encountered the virus in the past, and thus helpful to improve serological ELISA tests in the case of undetectable/equivocal circulating IgG levels, and a suitable and improved tool to better evaluate SARS-CoV-2-specific humoral immunity in the COVID-19 pandemic.

Coronavirus disease 2019 (COVID 19) is a systemic infection that exerts a significant impact on cell metabolism. In this study we performed metabolomic profiling of 41 in vitro cultures of peripheral blood mononuclear cells (PBMC), 17 of which displayed IgG memory for spike-S1 antigen 60–90 days after infection. By using mass spectrometry analysis, a significant up-regulation of S-adenosyl-Homocysteine, Sarcosine and Arginine was found in leukocytes showing IgG memory. These metabolites are known to be involved in physiological recovery from viral infections and immune activities, and our findings might represent a novel and easy measure that could be of help in understanding SARS-Cov-2 effects on leukocytes.

SARS-CoV-2 is the virus responsible for the COVID-19 pandemic, causing respiratory syndrome and other manifestations. The clinical consequences of the SARS-CoV-2 infection are highly heterogeneous, ranging from asymptomatic and mild to severe and fatal conditions, with the highest mortality rate reached among elderly people. Such heterogeneity appears strongly influenced by the host immune response, which in turn is profoundly affected by aging. In fact, the occurrence of a low-grade inflammation and a decline in specific immune defense is generally reported in older people. Although the low ability of B cells to provide primary and secondary specific responses with a consequent increase in susceptibility to and severity of virus infections is generally described in elderly people, we would like to present here the particular case of a 100-year-old woman, who recovered well from COVID-19 and developed a long-term memory against SARS-CoV-2. Following the infection, the patient’s blood was tested with both a classical ELISA and a specific Cell-ELISA addressed to measure the anti-spike S1 specific IgG released in plasma or produced in vitro by memory B cells, respectively. While showing negative on classical serological testing, the patient’s blood was positive in Cell-ELISA up to 1 year after the infection. Our observation highlights a potential mechanism of B cell-dependent, long-term protection in response to SARS-CoV-2 infection, suggesting that in a case of successful aging, the absence of specific antibodies in serum does not necessarily mean the absence of immune memory.

Knowledge of antibody-mediated immune response to SARS-CoV-2 is crucial to understand the virus immunogenicity, establish seroprevalence and determine possible risks for infection/reinfection. An antibody presence, indicative of an humoral immune response, can be evidenced at early/intermediated stages after infection and antibodies are mainly detected by ELISA-based platforms. However, a major drawback of ELISA systems is a decrease of serum antibody titers during time. On the other hand, circulating antigen-specific memory B cells (MBC) may be present for long time after immunization/infection. The spike S1 protein has been shown to be a serological marker for Covid-19, and we describe a novel, simple and robust Cell-ELISA assay specifically designed to measure viral spike S1 protein by detecting specific IgG produced in vitro by MBC in PBMC (doi:10.3390/v12111274). By applying the Cell-ELISA assay to a cohort of 150 asympto/symptomatic individuals, age ranging from 8–100 years, we detected individuals resulted negative in ELISA but positive in Cell-ELISA, thus showing the presence of MBC that produce in vitro antibodies against SARS-CoV-2 at levels that are undetectable in the serum. These data may challenge the negative results obtained from the serological screening and indicate a previous antigen exposure in the possible absence of serum IgG. We also detected a presence of MBC specific for S1 protein for >10 months after viral infection (doi:10.3390/v13091704), suggesting a potent antigenicity, and evaluated the relative amount of in vitro secreted IgG in positive subjects. The proposed assay could be a candidate tool to monitor the presence of an effective IgG antibody memory after SARS-CoV-2 infection and vaccination.