Surrogate CD16-expressing effector cell lines for determining the bioactivity of therapeutic monoclonal antibodies
Traditional antibody dependent cellular cytotoxicity (ADCC) assays use donor derived natural killer (NK) or peripheral blood mononuclear cells, but donor genetic variability and the technically challenging nature of the assay means that alternative in vitro assay formats are required. We explored the utility of two reporter gene cell lines, the J2 and J9, as surrogate effector cells for ADCC assays. Both express the ADCC relevant Fcγ receptor CD16, crosslinking of which leads to firefly luciferase expression. For anti-CD20 rituximab and anti-HER2 trastuzumab (both IgG1 monoclonal antibodies, mAbs) a dose dependent firefly luciferase response was observed exclusively in the presence of their respective targets, representing the molecular interaction which potentiates ADCC activity. Importantly, both surrogate effector and NK cell based assays gave statistically similar values for rituximab ADCC activity. Increased engagement with target cell bound mAbs was determined to be cytotoxic for the J2 and J9 cell lines at the assay end point (at which luciferase expression is measured). However, use of the J9 cells containing the constitutively expressed renilla luciferase gene enabled data normalisation and corrected for fluctuations in both cell number and viability providing an advantage over currently available surrogate effector cell-lines. Abrogated ADCC activity with IgG4 mAbs, but enhanced activity with an IgG1 non-fucosylated mAb, was seen with the J9 cell line, as expected. Additionally, two rituximab products (biosimilars in development) with similar binding by flow cytometry, N-glycan profiles using HPLC and CD16 binding by surface plasmon resonance showed comparable ADCC activity to Mabthera. The ADCC activity of another anti-CD20 mAb, ofatumumab, reported only with primary cell based assays to date was also measured. This is the first report of a dual reporter gene based ADCC assay.