Kevin Mayer Senor Editor Genetic Engineering & Biotechnology News
Whether they involve screening for active cells or introducing new receptors, therapies that boost the immune system’s response to cancer are becoming more specific and widely available.
The idea has intrigued researchers for at least a couple of decades: Open a new front against cancer by enhancing the tumor-fighting abilities of T cells. Only recently, however, has the idea seemed anywhere near realization. T-cell ranks, so often full of live-and-let-live types, can now be populated with tumor-targeting recruits. T cells, however, cannot be so trigger-happy that they “destroy the village to save it.”
Approaches to the raising of spirited (but still disciplined) T-cell armies fall into three categories: tumor-infiltrating lymphocyte (TIL) therapies, T-cell receptor (TCR) therapies, and chimeric antigen receptor (CAR) therapies. In TIL therapies, T cells that show strong antitumor activity are harvested from the patient, multiplied in culture, and reintroduced the patient along with a promoter of T-cell growth. In TCR and CAR therapies, the patient’s T cells are genetically modified before they are returned.
The T cells in TCR therapies acquire genes for receptors that recognize specific antigens, provided the receptors are compatible with the patient’s immune type. (The receptor binds with antigens presented by the major histocompatibility complex.) The T cells in CAR therapies acquire genes that allow them to build hybrid constructs. The “business end” of each construct, the part that projects from the T cell, is an antibody. It binds to antigens that stud the surface of tumor cells. The indwelling part of the construct, similar to the indwelling parts of native or artificially incorporated receptors, consists of signaling machinery that activates the T cell once an antigen is bound.
Whichever approach is used—TIL, TCR, or CAR—the idea is to marshal T cells that are more likely to target tumorous than healthy tissues. In TIL, building an effective T-cell force is a matter of careful recruitment. In TCR and CAR, it is a matter of issuing the right gear.
Each approach has its limitations. With TIL, it is necessary to isolate effective T cells. Such T cells tend to congregate in certain tumors, especially melanomas; in many cancers, however, effective T cells are inconveniently dispersed.
With TCR, the need for receptors to match the patient’s immune type adds complexity. Regardless, there is an upside: When T cells accommodate the antigen-presentation mechanism, they can reach targets that originate within tumors, not just targets that reside on the surface of tumor cells. With CAR, immune type isn’t an issue, but T targets are limited to surface-dwelling antigens.
Although each approach has its limitations, each has shown promise in recent pilot studies and small trials. The results, in fact, are so encouraging that several academic centers and pharmaceutical companies are moving toward larger trials even as TIL, TCR, and CAR techniques undergo various refinements. In general, most of the larger trials are trying to generalize early successes, which have been concentrated in two areas: 1) TIL therapies against melanomas; 2) CAR therapies against B-cell leukemias and lymphomas.
TIL Therapy Studies
At the National Cancer Institute (NCI), researchers are working on ways to identify mutated tumor targets that can stimulate immune responses. Such targets, presumably, would be unique to tumors. T cells sensitive to them would not exert toxic, on-target, off-tumor effects.
In a study published July 1 in Clinical Cancer Research, NCI researchers led by Steven A. Rosenberg, M.D., Ph.D., described how they applied two screening approaches to tumor samples taken from two patients who had benefited from a melanoma immunotherapy. First, cDNA library screening, a conventional screening method, was used to identify nonmutated targets. Second, a novel method called tandem minigene library screening was used to identify mutated targets that are invisible to the conventional method.
For the second approach, the researchers used next-generation DNA sequencing to identify mutations in the coding regions of the DNA from the two patients’ tumors. Next, they generated a library of these mutations. Instead of synthesizing the entire mutated gene, they synthesized only a small region surrounding the mutation (hence the name “minigene” library). Finally, they screened the minigene library to identify those targets in the patients’ tumors that were recognized by their TILs.
The researchers identified three novel nonmutated tumor targets, and four previously known nonmutated tumor targets.
With the minigene library approach, Dr. Rosenberg and colleagues had previously reported another novel tumor target recognized by the activated T cells of a patient with bile duct cancer, who responded to a TIL therapy. The target turned out to be a mutation in a protein called ERBB2-interacting protein. In subsequent rounds of TIL therapy, 25% and then 95% of the transferred cells were mutation-reactive. Following these transfers, the patient experienced tumor regression.
This work, which was described in a paper that appeared May 9 in Science, indicates that the immune system can mount an effective response against mutant proteins produced by epithelial cell cancers. Found in the digestive tract, lung, pancreas, bladder, and other areas of the body, epithelial cell cancers account for more than 80% of all cancers.
In yet another NCI study, a TIL therapy was used against metastatic HPV-positive cervical cancer. Here, TILs were selected for HPV-E6 and HPV-EL reactivity. They were administered to nine patients, two of whom subsequently achieved complete remissions with no evidence of cancer at 15 and 22 months after a single infusion of T cells.
CAR Therapy Studies
To date, CAR therapies have focused on the CD19 protein, which is found on B cells. Speaking at a meeting of the American Society of Hematology last December, Stephan A. Grupp, M.D., Ph.D., a pediatric oncologist at the University of Pennsylvania, presented the results of a pilot study in which CAR therapy was used against high-risk acute lymphoblastic leukemia (ALL).
This study evaluated 27 patients (22 children and 5 adults) who had ALL that recurred after initial treatment or resisted treatment from the start. Within 28 days after receiving bioengineered T cells called CTL019 cells, 24 of 27 patients (89%) experienced complete responses. At a median follow-up 2.6 months after treatment, 18 of the 25 patients had ongoing complete responses.
The CTL019 cells did more than potently attack leukemia cells. They also stimulated an unwanted, toxic immune response called cytokine release syndrome, which can include severe flu-like symptoms (fever, muscle pain, low blood pressure, and difficulty breathing). The effects of the cytokine storm were managed, however, with two immunomodulating drugs. Because the CTL019 therapy eliminated healthy B cells along with cancerous B cells, patients had to receive infusions of immunoglobin, which then performed the immune function provided by normal B cells.
In a similar study, conducted by investigators from Memorial Sloan Kettering and reported February 19 in Science Translational Medicine, 14 of 16 ALL patients (88%) achieved complete remissions after treatment, and 7 of the 16 patients (44%) were able to successfully undergo bone marrow transplantation.
To extend CAR therapy to solid tumors, a research team at the University of Pennsylvania engineered T cells to target not C19, but mesothelin. (The team called their mesothelin-targeting T cells CARTmeso cells.)
C19-targeting CAR T cells had failed against solid tumors. These cells proved toxic because normal cells express C19, albeit at lower levels than cancer cells. Accordingly, the University of Pennsylania researchers, led by Carl H. June, M.D., took measures to deal with any toxicity that might accompany the use of their CARTmeso cells.
“So far, researchers have been permanently modifying T cells by using a variety of methods, including using viruses,” said Dr. June. “We engineered [mRNA-based T cells that transiently] express a CAR.” After about three days, “the mRNA is metabolized rapidly by the system, so the T cells basically revert to what they were before in the patient.”
The strategy was to give multiple infusions of CARTmeso cells while retaining the ability to quickly abort any signs of toxicity by simply stopping the infusions.
The results appeared December 20, 2013 in Cancer Immunology Research. The temporary CARs were safe, with no significant on-target, off-tumor toxicity. In addition, the CARTmeso cells showed antitumor activity in two patients, one with metastatic pancreatic cancer, and one with advanced mesothelioma.
“We found that these CARTmeso cells not only have antitumor activity, but also act like a vaccine, and trigger a response against the patient’s own tumor,” added Dr. June. “This new form of CAR therapy provides a new tool to evaluate CAR therapies for solid cancers.”
Following up on the success of proof-of-concept studies and small trials, several companies are participating in larger-scale trials, often in collaboration with academic medical centers. TIL trials are being organized with the participation of Lion Biotechnologies; TCR trials, by Adaptimmune and Kite Pharma; and CAR trials by Kite Pharma and Novartis.
In these efforts, the collaborators are attempting to scale-up the processes that make up the TIL, TCR, and CAR therapies, which are collectively known as adoptive cell transfer therapies. In addition, increasing emphasis is being placed on biomarkers, especially since it is still unclear why some patients respond to adoptive cell transfer, and some do not. Another emerging theme is combination therapy—using adoptive cell transfer therapy with other therapies, such as immune checkpoint blockade, which blocks receptors on the surface of T cells that tumors use to turn off immune attack.
While checkpoint blockade is the focus of a collaboration announced in March by MedImmune and MD Anderson Cancer Center, the principals are also exploring combination therapies that involve adoptive cell transfer. Collaborative efforts that more squarely address adoptive cell transfer include the following collaborations: Pfizer/Cellectis, Bluebird bio/Celgene/Baylor, and Novartis/University of Pennsylvania—the last of these, principals announced July 7, involves a CAR therapy against ALL that has achieved breakthrough therapy designation.
Kit Pharma, says company CMO and evp of R&D, David D. Chang, M.D., has a clear plan for developing its lead anti-CD19-CAR T cell therapy, KTE C-19, and is working toward initiating a Phase II study in patients with non-Hodgkin's lymphoma in early 2015. “We are also working closely with the NCI to generate clinical proof of concept with additional CAR (EGFRvIII CAR) and TCR (NY-ESO-1, MAGE) programs,” Dr. Chang added.
Refining Adoptive Cell Transfer
In addition to exploring potential biomarkers and combination therapies, researchers in academic and commercial institutions are considering various refinements. These include:
- Improving the assays used to identify cancer-targeting T cells.
- New generations of CAR-modified T cells that contain more elaborate signaling domains.
- Modifying T cells from healthy donors—widening the availability of adoptive cell transfer therapy by providing allogeneic and not just autologous T cell infusions.
- Engineering T cells to target cancer stem cells.
- Balancing T cell populations by introducing engineered T cell types of multiple types.
- Automation of cell culture systems.
With respect to this last item, a relevant advance was reported at the 2014 ASCO meeting. Kite Pharma and NCI described a closed system for T-cell separation, transformation, and expansion.
“This new process is amenable to cGMP operations,” commented Dr. Chang. “It has significantly simplified the ex vivo manipulation of the cells and has reduced the required production time to approximately seven days.” To date, separation and expansion procedures have lasted as long as six weeks.
This article was originally posted with the title “Cancer-Fighting T Cells Take ‘Know Your Enemy’ to the Next Level”.