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 HIV

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.