IL2 and cancer: However, studies have shown that TILs can lose their ability to penetrate tumors after ex vivo expansion regardless of the technique used.45 One reason for this is that during the expansion process, TILs are often stimulated with high levels of IL2, which can cause them to differentiate into cells with effector phenotypes.46 These effector cells are better at killing cancer cells but may have reduced tumor-infiltrating ability.