T-lymphocyte reactivity against Spike protein
heparinized venous blood
In all participants, the SARS-CoV-2-specific T-lymphocyte response was measured after stimulation of PBMCs isolated from heparinized venous blood obtained four weeks after the second vaccination, using SepMate tubes (STEMCELL). The number of Interferon-gamma (IFN-ɣ)-producing T-lymphocytes after stimulation with SARS-CoV-2 Spike overlapping peptide pools was assessed using an IFN-ɣ enzyme-linked immune adsorbent spot (ELISpot) assay. SARS-CoV-2 S1 and S2 peptide pools (JPT Peptide Technologies), consisting of 15-mer peptides overlapping 11 amino acids that cover the entire sequence of the viral proteins were used for overnight stimulation of the PBMCs in a concentration of 0.5 µg/mL. 0.4% dimethyl sulfoxide (DMSO, Sigma) was used as negative control and Phytohaemagglutinin (Remel Europe Ltd; 4 µg/mL) as a positive control. Spot forming cells (SFC) were quantified with the AID ELISpot/Fluorospot reader and calculated to SFCs/10