As negative control, medium of uninfected MDCK cells was used (Mock).

As positive control, cells were stimulated with 2 μg/ml Con A (Sigma). All stimulations were performed in a total volume of 0.6 ml and were incubated for 20 h at 37 °C. After incubation, all (0.6 ml) supernatant of the PBMC was collected in cryotubes (Simport) and stored at −80 °C for determining cytokine concentrations. Remaining cells were lysed with 0.25 ml lysis buffer (150 mM NaCl, 15 mM Tris, 1% Triton X-100), transferred to Eppendorf tubes and stored at −80 °C for measuring granzyme B concentrations. Frozen cell lysates were subjected to three freeze/thaw cycles to enable release of granzyme B from the granules. PD-1/PD-L1 inhibitor Granzyme B standards were prepared in duplicate

in a 96-well plate (Corning) and the cell lysates (20 μl/well) were added in duplicate to the same plate. Subsequently, 80 μl enzyme reaction buffer containing 400 μM acetyl-Ile-Glu-Pro-Asp-paranitroanilide (Ac-IEPD-pNA, Calbiochem), 100 mM HEPES pH 7.5 (Sigma), 10% (w/v) sucrose (Sigma), 0.1% (w/v) CHAPS (Roche), and 10 mM DTT (Sigma) was added per sample. The plate was covered with thermo-plastic seal and Romidepsin mouse incubated in a dark humidified chamber at 37 °C for 20 h. After incubation, the plate was read at 405 nm. Granzyme B units in the cell lysates were calculated using a fourth-order polynomial curve (y = ax4 + bx3 + cx2 + dx + e) with a log (concentration) − log (absorbance) plot. The granzyme B units were corrected for protein content in the lysates (granzyme B units/mg protein). The protein concentration of the lysates was determined by the BCA protein assay (ThermoScientific). To prevent batch-to-batch variation of test kits, all laboratories used the same lot-number of the Th1/Th2 cytokine multiplex kit (Bio-Rad, Cat#171A11081, Batch#5008594), IL-17 singleplex kit (Bio-Rad, Cat#171B14076, Urease Batch#5008678), and Reagent kit (Bio-Rad, Cat#171304000, Batch#310002936). The multiplex assay was performed according to the protocol of the manufacturer with some modifications. The

plate was measured on the Bio-Plex suspension array system (Bio-Plex 100 or 200) under low photo multiplier tube settings. Calculation of cytokine concentrations in unknown samples was determined by 5-parameter fit regression analysis of the standard curve. The validation plan was designed to assess a range of parameters (Fig. 1) [33]. To determine linearity, range, detection limit, specificity, accuracy, and precision, PBMC were stimulated with mock, influenza H3N2 or concanavalin A (Con A, Sigma) and used for generating a bulk amount of cell lysate and culture supernatant for testing in the granzyme B and cytokine assay, respectively. Generation of this bulk amount of cell lysate and culture supernatant was performed by NVI, The Netherlands.