Comparison between enzyme‐linked immunospot assay and intracellular cytokine flow cytometry assays for the evaluation of T cell response to SARS‐CoV‐2 after symptomatic COVID‐19

INTRODUCTION: Evaluation of different cell‐based assays for the study of adaptive immune responses against SARS‐CoV‐2 is crucial for studying long‐term and vaccine‐induced immunity. METHODS: Enzyme‐linked immunospot assay (ELISpot) and intracellular cytokine staining (ICS) using peptide pools spanning the spike protein and nucleoprotein of SARS‐CoV‐2 were performed in 25 patients who recovered from paucisymptomatic (n = 19) or severe COVID‐19 (n = 6). RESULTS: The proportion of paucisymptomatic patients with detectable SARS‐CoV‐2 T cells was low, as only 44% exhibit a positive T cell response with the ICS and 67% with the ELISpot. The magnitude of SARS‐CoV‐2 T cell responses was low, both with ICS (median at 0.12% among total T cells) and ELISpot (median at 61 SFCs/million peripheral blood mononuclear cells [PBMC]) assays. Moreover, T cell responses in paucisymptomatic patients seemed lower than among patients with severe disease. In the paucisymptomatic patients, the two assays were well correlated with 76% of concordant responses and a Cohen's kappa of 55. Furthermore, in four patients SARS‐CoV‐2 T cells were detected by ELISpot but not with ICS. Short‐term culture could improve the detection of specific T cells. CONCLUSIONS: In patients who recovered from paucisymptomatic COVID‐19, the proportion of detectable anti‐SARS‐CoV‐2 responses and their magnitude seemed lower than in patients with more severe symptoms. The ELISpot appeared to be more sensitive than the ICS assay. Short‐term culture revealed that paucisymptomatic patients had nonetheless few SARS‐CoV‐2 T cells at a very low rate in peripheral blood. These data indicate that various ex‐vivo assays may lead to different conclusions about the presence or absence of SARS‐CoV‐2 T cell immunity.