cells expressing CEA, but resistant to NKL

Hello
I'd like to test a cytotoxicity assay with cells expressing CEA and NKL (ADCC), so I need cells resistant to their natural cytotoxicity... I know I can use MC38-CEA (and MC38 as a control), but the person who must give us the permission to us it didn't answer... So I don't know what to do and I really need it.... Please help me...