Cal lyse lysing solution

Cell surface markers were stained with these specific antibodies: anti-CD19-PE, anti-CD3-APC, anti-CD4-PE, anti-CD8-FITC, 7-AAD, and anti-annexin-V-FITC (eBioscience); anti-CD19-APC, anti-B220-Per-cy5.5, anti-IgM-FITC, anti-AA4.1-PE, anti-CD23-eFluor647, and anti-CD8-APC (Biolegend).
For ex vivo analysis, cells were stimulated with 25 ng/mL PMA (Sigma-Aldrich) and 1 g/mL ionomycin (Sigma-Aldrich) in the presence of 0.66 μL/mL Golgistop (BD PharMingen) for 6 h at 37 °C, 5 % CO₂. Intracellular staining of IFN-γ and IL-4 was performed using Transcription Factor Staining Buffer Set (eBioscience). Data was collected by FACS Calibur flow cytometer (BD Biosciences) and analyzed by FlowJo software (TreeStar, Ashland, OR).
For cell quantization, blood sample (100 μL) was stained with the specific antibodies: anti-CD19-FITC, anti-CD3-APC, anti-CD4-FITC, anti-CD8-FITC, anti-CD11c-APC, anti-F4⁄80-FITC, and anti-CD11b-APC. Erythrocytes were then lysed with Cal-lyse Lysing Solution (Invitrogen). After thoroughly mixing with 100 μL of Caltag Counting Beads (Invitrogen), 10,000 beads were acquired in the FACS Calibur flow cytometer for each sample.

As at the beginning of this pandemic, little information was available on the biosafety of blood samples obtained from severe SARS‐CoV‐2‐infected individuals, we decided to inactivate the samples with 0.5% PFA directly at the ICU premises. Blood samples were collected in BD Vacutainer^® Tubes containing sodium heparin, and 0.5% PFA was directly added to each sample. Blood was then processed in a BSL‐2 cabinet with Cal‐Lyse lysing solution (Invitrogen) following manufacturer's recommendations. Samples were finally resuspended in FBS +10% DMSO in the original blood volume and bio‐banked at −80°C.