CAR T-cell therapy, officially known as chimeric antigen receptor (CAR) T-cell therapy, is among the newest cancer treatments that harness our immune system— the powerful network of cells, tissues, and organs that fights off disease and infection. By synthetically engineering immune cells called T cells to precisely target malignant tumor cells, this method has the potential to revolutionize cancer treatment and more.
CAR T-cell therapy is showing great promise, and because of how powerful this approach is and how unlimited I believe its potential is, we will hopefully one day be able to target all tumor types.
Samuel Katz, MD, PhD
As this therapy continues to evolve, pathology labs will be a crucial player in supporting its success. Pathologists play an essential role in monitoring the modified T cells in patients to assess how the therapy is progressing. In an April 2024 review published in the American Journal of Pathology, Samuel Katz, MD, PhD, associate professor of pathology, and his team discussed existing technologies available to pathology labs and avenues for advancing this groundbreaking therapy.
“CAR T-cell therapy is showing great promise,” says Katz, who is a member of Yale Cancer Center. “And because of how powerful this approach is and how unlimited I believe its potential is, we will hopefully one day be able to target all tumor types with great degrees of specificity and sensitivity so that we can reduce the morbidity and mortality of cancers.”
The history of CAR T-cell therapy
T cells are immune cells that recognize foreign antigens [substances that trigger an immune response]. For example, when cells in the body are infected with a virus, T cell receptors identify the virus as a threat, and the immune cell in turn kills the infected cells. A CAR T cell is a modified version of a T cell that researchers have engineered to express a specialized receptor — the CAR — on its surface. Scientists designed these CAR T cells to recognize antigens that are specific to cancer.
Research on CAR T cells dates back to 1987, when a research group in Japan led by Yoshihisa Kuwana, PhD, first began experimenting with this technology by merging T cell receptors and antibodies. The development of what researchers consider to be “first- generation” CAR T cells was in 1989 by Israeli immunologist Zelig Eshhar, PhD. His group was one of the first to demonstrate the successful activation of a CAR receptor that triggered the modified T cell to kill its target.
The second generation of CAR T cells emerged in 1998, when researchers added costimulatory domains, molecular components that help enhance T cell function. In the late 2000s and 2010s, the first clinical trials of this generation of the therapy began reporting successes.
As a result, in 2014, the FDA designated CAR T cells as a “breakthrough” therapy. Three years later, the FDA approved the first CAR T-cell therapy to treat adults with certain types of large B-cell lymphoma.
There is also a third generation of CAR T cells designed with even more costimulatory domains, but a downside is that this generation undergoes more rapid T cell exhaustion—in which the engineered cells become less active. Thus, physicians don’t often use this generation clinically.
What are the different types of CAR T-cell therapies?
There are currently six FDA-approved CAR T-cell therapies. Four of these are designed to target an antigen known as CD19, which is found on the surface of B cells [another type of immune cell]. This therapy targets cancers derived from B cells such as B cell lymphomas. “It also does target normal B cells, but people can live without their B cells,” Katz explains. These therapies utilize different costimulatory domains that have a distinct impact on how the modified T cells behave.
The other two types of CAR T-cell therapies are for treating multiple myeloma or plasma cell myeloma. These versions target an antigen known as BCMA, a protein that is expressed on the surface of cancerous plasma cells. Both of these use the same costimulatory domain. However, the two differ in how the CAR binds to the antigen.
How do pathologists monitor CAR T cells in patients?
In addition to producing and administering the CAR T cells, it’s the pathologist’s role to monitor the engineered immune cells. “Now, we have to start to think about not just diagnosing the state of the disease, but also what the state is of that biological cell we’ve introduced into the patient,” says Katz.
One way to do this is to look at the DNA within the CAR, because the DNA that has been introduced contains the instructions for building functional proteins. Pathologists can do this by using polymerase chain reaction (PCR) tests. PCR is highly sensitive, accurate, and reproducible. However, a major limitation of this method is that it looks at CAR T cells circulating in the bloodstream, not the ones active in the cancerous tumors themselves. Furthermore, monitoring CAR T-cell therapy at the DNA level—not the protein level where the therapy actually functions—has its own limitations.
Another method, flow cytometry, allows pathologists to detect different proteins on the surface of cells. Pathologists currently use flow cytometry to diagnose blood cancers. “And much as we do for cancer, we could also do that for CAR T cells,” says Katz. The benefit of flow cytometry is that pathologists can monitor the therapy at the protein level. Its main drawback, much like PCR, is that it also measures cells in the blood rather than the tumor. It’s also not as sensitive as PCR.
Immunochemistry is a method in which pathologists obtain a piece of bodily tissue and process it in a way that they can view it on a microscope slide. “This allows us to see the disease directly,” says Katz. “We can see the cells in there in a morphological manner to understand what our battlefield looks like as opposed to what just the players are within the battlefield.” The limitations of immunochemistry are that it is not a quantitative test [doesn’t produce a numerical result], and there aren’t many antibodies—which researchers use to detect CAR T cells—that work for this method.
Finally, pathologists can use a method known as in situ hybridization, which uses a probe to measure RNA made from the DNA in the CAR. “This method is one step past the DNA level, but one step before the protein,” says Katz.
What are the challenges of CAR T-cell therapy?
While current and emerging CAR T therapies hold great potential for treating cancer, they are not without risk for patients. First, patients undergoing the therapy may experience cytokine release syndrome. As the engineered T cells attack the cancer, the tumors release chemicals known as cytokines. This can result in what doctors call a “cytokine storm,” creating inflammation in the body. Symptoms of cytokine release syndrome include fever, chills, tiredness, nausea, and vomiting. But, in the majority of cases, the condition is “mostly manageable to patients and it’s typically a good sign that the CAR is working,” says Katz. “There are enough therapies, and with close monitoring, we can prevent patients from facing serious consequences.”
Patients have also reported various neurological symptoms associated with CAR T therapy, such as dysgraphia [difficulty with written expression], tremors, confusion, and insomnia. While these symptoms are also typically manageable, says Katz, researchers are not fully sure why they occur.
Sometimes, patients develop secondary malignancies [new cancer that develops after treatment for another cancer] post-therapy. However, this happens in less than one percent of cases.
Most of the CAR T cells that researchers currently study deliver a patient’s own T cells. But researchers are investigating forms of the therapy in which they deliver cells from donors. This would allow scientists to create modified T cells at a larger scale and store them ahead of time, reducing wait times for patients receiving the therapy. However, this presents the risk of graft versus host disease, in which the T cells recognize the patient’s tissue as foreign as well as host versus graft disease, in which the patient’s own T cells reject the newly introduced cells.
Sometimes, the engineered T cells target a receptor on a tumor that is also present on other types of cells in the body. Patients receiving CAR T therapy targeting CD19, for example, also lose their healthy B cells. While, in this case, the loss can be managed, researchers will need to overcome this type of challenge as they study therapies that target other kinds of receptors.
Future avenues for CAR T-cell therapy research
There are several significant avenues of research that will be essential to continue the advancement of CAR T-cell therapy, says Katz. Most importantly, researchers will need to strive to find ideal targets that minimize the number of healthy cells that are killed by the therapy.
Furthermore, in about half of the incidences of CAR T-cell therapy that target CD19, the cancer cells end up losing that receptor. This diminishes the effectiveness of the therapy. Another important line of research will be figuring out how to overcome this challenge. Some researchers, for example, are developing strategies in which the therapy targets two receptors at the same time. “That way, if we lose one receptor, we can still target the other,” Katz explains.
Researchers are also trying to extend this work beyond bloodstream and lymph node-based cancers to solid cancers. “These are tumors that are more tissue-based as opposed to flowing through the bloodstream,” Katz explains. This includes cancers of the lung, breast, and brain. Solid cancers contain a very different microenvironment and are more difficult for engineered T cells to penetrate. Studying how to increase CAR T cell activity within these types of microenvironments will be essential to expand the therapy to more types of cancer.
CAR T even has applications beyond treating cancer, such as autoimmune diseases, for which the therapy may play a role in eliminating overactive immune cells.
“In general, all these avenues come down to getting better control of the CAR T cells through more specific engineering,” says Katz. “How can we add additional genes beyond the CAR to improve our ability to turn on and off the CARs in places we do or do not want them?”
At Yale School of Medicine, there are several labs studying CAR-T cell therapy. Katz’s lab is working on the development of a new CAR for targeting solid cancers and boosting CAR T-cell activity. Xiaolei Su, PhD, associate professor of cell biology, is investigating using new cell types and signaling modules to improve CAR T cell activity in solid tumors. Sidi Chen, PhD, associate professor of genetics and of neurosurgery, is studying new ways to engineer T cells and boost their activity.
As CAR T-cell therapy continues to evolve, Katz calls for pathologists to remain aware of ongoing research and stay informed on how to best serve patients. “As these therapies start getting approved, we need to start thinking about how we’re going to track and pay attention to these cells, which we haven’t been doing in typical practice,” he says. “All pathologists need to start thinking about how we can best evaluate patient biopsies to know how this therapy is working.”