Transient Co-Transfection and Purification of Antibody and pMHC Proteins

Plasmids encoding the heavy chain and light chain of Abs were transiently co-transfected in pairs, at equivalent molar ratios, into cultured mammalian human embryonic kidney HEK293F cells in Freestyle 293F medium (Invitrogen, CA, USA, 12338018) following the standard protocol [34 (link),35 (link)]. Culture supernatants were collected after 6 days by centrifugation and filtration (0.22 μm, polyethersulfone; Corning). Abs were purified from the culture supernatants using a CaptivA™ Protein A-agarose chromatographic column (Repligen, MA, USA) and were extensively dialyzed to achieve the final composition of phosphate-buffered saline (PBS; pH 7.4). Likewise, the plasmid encoding the pMHC SCT protein was transfected into HEK293F cells. The pMHC protein was purified from the culture supernatant using Ni-NTA resin (GE Healthcare, IL, USA). Protein concentrations were determined using a bicinchoninic acid kit (Thermo Fisher Scientific, Waltham, MA, USA). To prepare an Ab-screening antigen, the purified pMHC SCT proteins were biotinylated using a BirA500 kit (Avidity LLC, Colorado, USA) following the manufacturer’s instructions [35 (link)].