A highly specific assay for the detection of SARS-CoV-2-reactive CD4+ and CD8+ T cells in COVID-19 patients

Gaining detailed insights into the role of host immune responses in viral clearance is critical for understanding COVID-19 pathogenesis and future treatment strategies. While studies analyzing humoral immune responses against SARS-CoV-2 were available rather early during the pandemic, cellular immunity came into focus of investigations just recently. For the present work, we have adapted a protocol, designed for the detection of rare neoantigen-specific Memory T cells in cancer patients for studying cellular immune responses against SARS-CoV-2. Both, CD4+ and CD8+ T cells were detected after 6 days of in vitro expansion using overlapping peptide libraries representing the whole viral protein. The assay readout was an Intracellular cytokine staining and flow cytometric analysis detecting four functional markers simultaneously (CD154, TNF, IL-2, IFN-γ). We were able to detect SARS-CoV-2-specific T cells in 9 of 9 COVID-19 patients with mild symptoms. All patients had reactive T cells against at least one of 12 analyzed viral antigens and all patients had Spike-specific T cells. While some antigens were detected by CD4+ and CD8+ T cells, Membrane protein was mainly recognized by CD4+ T cells. Strikingly, we were not able to detect SARS-CoV-2-specific T cells in 9 unexposed healthy individuals. We are presenting a highly specific protocol for the detection of SARS-CoV-2-reactive T cells. Our data confirmed the important role of cellular immune responses in understanding SARS-CoV-2 clearance. We showed that Spike is the most immunogenic antigen. We have introduced Membrane protein as interesting target for studying humoral immune responses in convalescent COVID-19 patients.