Isolation and Cryopreservation of PBMCs

The isolation of peripheral blood mononuclear cells (PBMCs) was performed as described in Koskimaa et al., 2014 (link) and Paaso et al., 2015 (link)). Venous blood samples (74 ml) were collected in sodium‐heparin collection tubes. The isolation was performed by centrifugation for 3 h over a Ficoll‐Paque gradient (GE Healthcare Life Sciences, Uppsala, Sweden). Around ~10 × 10⁶ PBMCs were used for the lymphocyte stimulation test (LST). The leftover cells were cryopreserved in 80% Fetal Bovine Serum (FBS, Biowest, EU quality) and 20% DMSO (Merck, Darmstadt, Germany) at a density of 10 million PBMCs/vial. Autologous serum was used for the cell cultures in the short‐term T cell proliferation assay (Koskimaa et al., 2014 (link); Paaso et al., 2015 (link)).