Fusion constructs were cloned through overlap extension PCR (32 (link)) using wild type human CD137 (UniProt: Q07011; NCBI ref: NM_001561), human CD52 (Uniprot: P31358; NCBI ref: NM_001803.2) or the Rp3/Cp11 epitope (kind gift of Prof Martin Pule and Dr Brian Philip, UCL (33 (link), 34 (link))). Each fusion protein consisted of the CD52 leader sequence, Rp3/Cp11, CD137 (with domains removed based on the annotations provided with Uniprot reference sequence). These gene constructs were cloned into pcDNA3.1/- (neo) expression vector (Invitrogen).