Comparison of the Humoral and Cellular Immunity in Covid-19 Convalescents

ABSTRACT

The SARS-CoV-2 virus caused the COVID-19 pandemic is related to the SARS-CoV-1 and MERS coronaviruses, which were resulted in 2003 and 2012 epidemics. Antibodies in patients with COVID-19 emerge 7-14 days after the onset of symptoms and gradually increase. Because the COVID-19 pandemic is still in progress, it is hard to say how long the immunological memory to the SARS-CoV-2 virus may be retained. The aim of this study was to study a ratio between humoral and cellular immunity against the SARS-CoV-2 S protein in COVID-19 convalescents. There were enrolled 60 adults with mild to moderate COVID-19 2 to 12 months prior to the examination. The control group consisted of 15 adults without COVID-19 or unvaccinated. Specific antibodies to the SARS-CoV-2 virus were determined by ELISA with the SARS-CoV-2-IgG-ELISA-BEST kit. To determine the specific IgG and IgA subclasses, the anti-IgG conjugate from the kit was replaced with a conjugate against the IgG subclasses and IgA. Additional incubation with or without denaturing urea solution was used to determine the avidity of antibodies. Peripheral blood mononuclear cells were isolated by gradient centrifugation, incubated with or without coronavirus S antigen for 20 hours, stained by fluorescently labeled antibodies, and the percentage of CD8(high)CD107a cells was assessed on flow cytometer BD FACSCanto II. In the control group, neither humoral nor cellular immunity against the SARS-CoV-2 S protein was found. In the group of convalescents, the level of IgG antibodies against the SARS-CoV-2 S protein varies greatly not being strictly associated with the disease duration, with 57% and 43% of COVID-19 patients having high vs. low level of humoral response, respectively. A correlation between level of specific IgG and IgA was r = 0.43. The avidity of antibodies increased over time in convalescents comprising 49.9% at 6-12 months afterwards. No virus-specific IgG2 and IgG4 subclasses were detected, and the percentage of IgG1 increased over time comprising 100% 6-12 months after recovery. 50% of the subjects examined had high cellular immunity, no correlations with the level of humoral immunity were found. We identified 4 combinations of humoral and cellular immunity against the SARS- CoV-2 S protein high humoral and cellular, low humoral and cellular, high humoral and low cellular, and vice versa, low humoral and high cellular immunity.