- Leukapheresis (on two occasions). This is a method of collecting large numbers of white blood cells. The cells obtained in the first leukapheresis procedure are grown in the laboratory, and the TIL cells (called young TIL cells) are inserted into the cells using an inactivated (harmless) virus in a process called retroviral transduction. Cells collected in the second leukapheresis procedure are used to evaluate the effectiveness of the study treatment.