From what I've seen in other places what was done is, they took some of the patients T-Cells, and altered them. They did this by taking the RNA out of some HIVs and replaced it instead with a sequence to look for a specific antigen on the cancer cells. Then they took the HIVs and introduced them to the T-Cells and let them get infected. Since the original nasty RNA wasn't present, it won't replicate, but it will make some T-Cells that target the antigen on the cancer cells. After reintroducing to the patient, their immune systems then pick up from other signals that we don't full understand that some T-Cells are attacking this antigen, so it should make more of them.