In general, during infections, the T-cell count spikes within your body. Your immune system is engineered to manufacture the ones successful at fighting a given infection. This is why white blood cell count is a good marker of latent infections. The technique applied by these researchers simulates that propagation- though artificially, driving production of T-cells that they have designed to be good fighters of your "cancer infection". Additionally, while multiplied by 1000, the amount originally injected into the patients is fairly low with respect to the total number of T-cells in your body. As the patients are surviving a year after treatment, and the levels of these particular modified T-cells are maintained, not exponentially accumulating out of control, I think they've shown this is a reasonably safe move.