Raji cells

All cell lines used in the present study were cultured at 37 °C in a humidified atmosphere with 5% CO₂. Raji cells (German Collection of Microorganisms, DSMZ no.: ACC 319) and Mel JuSo cells (DSMZ no.: ACC 74) were cultured in RPMI (Gibco) supplemented with 10% tetracycline-negative fetal calf serum (FCS). Raji cells were grown in suspension culture. Cells were seeded at 0.2 × 10⁶ cells ml⁻¹, maintained at 1 × 10⁶ cells ml⁻¹, and split 1:10 every 72 h. Adherent Mel JuSo cells were seeded at 0.5 × 10⁶ cells in a T-75 cell culture flask and split 1:10 when confluency of 80% was reached.

Primary normal human epidermal keratinocytes (named primary human KCs in the paper) from the juvenile foreskin of a single donor were purchased from Promocell. KCs were cultured in serum-free medium (keratinocyte growth medium 2 (KGM-2), Promocell, Heidelberg). In some experiments, KCs were treated overnight with 100 ng/mL IFNγ (BioLegend, San Diego, CA) (as indicated in the respective figures).
Raji cells (ATCC, Manassas, CCL-86; Burkitt’s lymphoma cell line) were used as pAPCs. Raji cells were cultured in RPMI1640 medium (Gibco, Thermo Fisher Scientific, Waltham, MA) + 10 % FBS (PAN-BioTech, Aidenbach).
PBMCs were obtained from heparinized blood from healthy donors using Ficoll-Hypaque (Linaris, Dossenheim) density-gradient centrifugation. PBTs or naive CD4⁺ T cells were isolated from PBMCs using a Pan-T-cell isolation kit or naive CD4⁺ T-cell isolation kit (Miltenyi Biotec, Bergisch Gladbach). This study was approved by the Ethics Committee of Heidelberg University (S-089/2015).

HEK-293F, CHO-S, Raji, SK-OV-3 and Jurkat E6.1 cells were obtained from the American Type Culture Collection (ATCC). HEK-293F cells and CHO-S cells were cultured in FreeStyle™ 293 or FreeStyle™ CHO expression medium (Invitrogen, Carlsbad, CA, USA) respectively, supplemented with 1% Penicillin–Streptomycin (10,000 U/mL) (Invitrogen). Raji cells and SK-OV-3 cells highly expressing CD47 were grown in RPMI1640 containing 10% FBS (Gibco, GrandIsland, NY, USA) and 1% Penicillin–Streptomycin. Jurkat E6.1 cells were cultured in RPMI1640 supplemented with 10% heat-inactivated fetal calf serum (FCS) (Biological Industries, Kibbutz Beit Haemek, Israel), 1% Penicillin–Streptomycin and 2 mM L-glutamine (Gibco).