T-cell therapy, a form of immunotherapy that uses a patient’s own immune cells to attack their cancer, has been making waves recently. The “living” therapy involves engineering the patient’s T cells in the laboratory to carry new proteins that guide the immune cells directly to tumor cells, allowing the engineered T cells to attack and kill the cancer.
While still experimental, the therapy currently takes between 2 and 3 weeks for researchers to harvest a patient’s T cells and turn them into tiny cancer assassins.
Now, a group of scientists led by Fred Hutchinson Cancer Research Center immunotherapy researcher Dr. Stanley Riddell has devised a new approach that could both speed up and bump up the effectiveness of this process, using a special, small protein tag that can be used to purify and track the T cells once they have been engineered in the laboratory.
The underpinnings of this work arose from a collaboration between Riddell and Dr. Dirk Busch of the Technical University of Munich to develop new cell processing technologies for immunotherapy and other applications.
Although not yet tested in humans, the researchers believe this new approach could improve on current T-cell therapy methods in several ways: by boosting the cells’ potency; by growing larger numbers of cancer-fighting T cells; by adding a potential “kill switch” to quickly deactivate the cells in patients’ bodies in the event of toxic side effects, and by cutting down the immune cell processing time from the current 14 to 20 days before reinfusion to 9 days or less.
Riddell and his team describe the approach, and its effect on human cancer cells in the laboratory and on a mouse model of lymphoma, in a study published Monday in the journal Nature Biotechnology.
“[The technique] allows us to generate a T-cell product that could be used for patient treatment in a very short time, perhaps only a few days,” said Dr. Lingfeng (Steven) Liu, Fred Hutch immunotherapy researcher and lead author of the study. “That will save a lot of time for the patient because for the patient, time is very important. Sometimes tumor cells grow much faster than we can imagine.”
T-cell therapy approaches are showing promise in early clinical trials for some patients with certain types of blood cancers at Fred Hutch and elsewhere, although the numbers of patients that have received the therapies are small and they will still need to be followed to determine whether the therapies’ effects are long-lasting.
Riddell hopes the new tag technology could be tested in humans in T-cell therapy clinical trials within the next one to two years — possibly in a clinical trial for patients with multiple myeloma the researchers are hoping to launch in the next year.
Riddell and Liu have patented their technology, known as Strep-tag. Juno Therapeutics, a biotechnology company initially formed on technology from researchers at Fred Hutch, Memorial Sloan-Kettering Cancer Center and Seattle Children's Research Institute, has an exclusive license to the tag technology for uses related to oncology (as well as a non-exclusive license for other purposes). The tag could also have future applications in other diseases such as infections or auto-immune diseases, Liu said.
The Strep-tag technique involves a special modification to the engineered molecules Riddell and his colleagues add to T cells. Riddell’s ongoing T-cell therapy clinical trials use cells engineered to house a synthetic protein known as a chimeric antigen receptor, or CAR, which is designed to recognize and bind to proteins present in large amounts on the surface of cancer cells — but not healthy cells — and then attack those diseased cells.
Different CARs have been engineered to recognize different cancer-specific molecules, but any of these CARs could hold a Strep-tag. Other T-cell therapies use a similar protein known as a T-cell receptor. The researchers showed that the Strep-tag can be incorporated into either type of receptor protein.
“There are so many things that you can do with this,” Riddell said.
CAR T-cell therapy is already showing promise as a potential cancer therapy, but there are still improvements to be made, Riddell said. For one, their current techniques to engineer T cells don’t work for every T cell extracted from the patient — but there’s no way to separate out the cancer-fighting T cells from the other cells, and although the mixed population still seems to work for many patients, he believes a more pure population could be even more potent.
The researchers used T cells engineered with the Strep-tag to sift out only those cells carrying a CAR protein. In their study, they found this sifting technique resulted in a nearly 95 percent pure collection of CAR T cells. Without sifting, only 43 percent were cancer-specific. Currently, researchers stimulate that mixed population of cells in the lab to encourage growth of the cancer-specific cells, but that stimulation process takes about 10 days (resulting in a total cell processing time of 14 to 20 days).
This sifting method would cut down the time patients wait for infusion of engineered T cells by a week or more. The researchers tested the tags after 8 days of growth in the laboratory in their mouse study, but Riddell projects it could be done in as few as 3 to 5 days, cutting down potentially precious waiting time for patients in need of therapy.
The Strep-tag would also allow the researchers to specifically track the engineered T cells using a fluorescent antibody specific for the tag itself.
For the majority of CARs, “there’s no way of measuring how much receptor you actually have on the cell,” Riddell said.
Tracking these cells can help scientists better understand how they work against tumors — or don’t work, as the case may be. If engineered T cells traffic to a tumor but aren’t working to shrink it, scientists could extract those cells and study their genes to better understand what went wrong in the therapy.
For patients with leukemia and lymphoma, the T-cell therapies currently being tested in clinical trials seem to work well even with a relatively small number of cells, Riddell said. But for patients with solid tumors such as breast, lung or pancreatic cancer, therapies will likely require multiple doses of potent cells to reach and effectively attack the tumors.
The pressing problem for those therapies is how to grow cells to large numbers in the lab. Liu, Riddell and their colleagues found that their tag can be used as a start button to stimulate the engineered cells, causing them to grow rapidly. The researchers found that, using a special antibody that binds the Strep-tag, they can expand the engineered cells by 200-fold, over and over.
“You start with even a million cells, then you have 200 million,” in one stimulation step, Riddell said. And it could go on past that.
“You can fill up Lake Union with T cells,” he said.
There are existing ways to stimulate CAR T-cells in the lab, but these methods stimulate all T cells, not just the cancer-specific ones, and the stimulation only seems to work once or twice, Riddell said. Stimulating the cells via the Strep-tag is specific, and the researchers saw that it worked three times in a row to trigger cell growth.
Finally, another type of antibody against the Strep-tag could in theory work as a “kill switch,” rapidly shutting off the cells in a patient’s body in the case of toxic side effects such as a cytokine storm (a potentially fatal immune reaction), although the researchers haven’t yet completed their tests of this application.
Riddell is hopeful that the technique will soon be put to translational use.
He and his team, in collaboration with Fred Hutch multiple myeloma expert Dr. Damian Green, have developed a CAR specific for multiple myeloma that incorporates the Strep-tag, and found that it works even better against cancer cells in the lab than the CAR without the tag. They haven’t yet worked out which receptor will be used in the clinical trial for patients with multiple myeloma.
“I’d really like to have [the multiple myeloma trial] in the clinic in a year, that would be my goal,” Riddell said. “We have so many myeloma patients here who could benefit.”
Rachel Tompa is a former staff writer at Fred Hutchinson Cancer Center. She has a Ph.D. in molecular biology from the University of California, San Francisco and a certificate in science writing from the University of California, Santa Cruz. Follow her on Twitter @Rachel_Tompa.