How did they remove / destroy the T-cell killing T-cells before the the 2nd marrow transplant? Otherwise, it would seem they (the engineered T-cells) would han

How did they remove / destroy the T-cell killing T-cells before the the 2nd marrow transplant? Otherwise, it would seem they (the engineered T-cells) would hang around, and destroy the T-cells created by the marrow transplant.