The immune system is a complex physiological network, relying on specialized warriors such as T cells to fend off enemy attacks. Nature’s exquisite control over this process hardly requires help from scientists—or does it?
Researchers are now trying to rectify such malfunctions by engineering T cells for therapeutic purposes. “The immune system has much potential for the treatment of a range of diseases,” says Lance Kam, associate professor of biomedical engineering at Columbia University. “If properly tailored, this system promises a targeted, specific, and persistent response to pathogens and even pathological cells, including those associated with cancer.”
Arming T Cells with the Right Weapons
One of Kam’s collaborators is Carl June, a professor of immunotherapy at the Perelman School of Medicine at the University of Pennsylvania. June’s lab is giving T cells just the weapons they need—chimeric antigen receptors (CARs), which are designed to attack specific cell types targeted for elimination. June began working on T cells decades ago, trying to solve a problem with bone marrow transplants—they activated T cells damaged otherwise healthy tissues. Subsequently, they developed methods to make T cells active in vitro, and then attempted to replenish T cells in patients with AIDS. More recently, they also discovered that adding specific factors can keep T cells alive longer.
Today, June’s group uses an HIV-derived vector to deliver particular genes to activated T cells. Because the markers expressed by leukemia tumor cells were already well-characterized, the researchers began with designing CARs against those cells (1). “The gene we deliver to the T cells encodes the CAR that makes the T cell find the target, and when it finds the target, the CAR triggers the T cell to kill the target, yet stay alive and kill another target and another target, and so on,” says Marcela Maus, director of medical affairs for the Translational Research Program at the
University of Pennsylvania’s Abramson Cancer Center. “We also found that the T cells with CARs can stay alive in the patient as a memory immune cell for at least a decade. Natural T cells do this, of course, which is why vaccines work, but we were still surprised that T cells with CARs could do this.”
One of the challenges of this method is the task of finding the most specific markers on tumor cell membranes. “Some markers are expressed at high levels in the tumor, but are also expressed at lower levels in healthy tissues,” says Maus. “Whether these markers are suitable targets for CARs is an area of active investigation, and it will probably depend on both the tissue in question and the level of expression.”
For example, one must determine that the tumor marker is not present on vital organs such as the heart, brain, and lungs. It is also unclear whether the effectiveness of T cell therapies might differ for blood cancers versus solid tumors, which are surrounded by defensive layers of stroma. “But we have some early data to suggest that even though natural immune cells are blocked by the surrounding tissue in solid tumors, it is still possible that the engineered T cells will get in there and kill tumor,” says Maus.
Enhancing T Cell Therapy
Even though therapy with engineered T cells is relatively new, researchers are already working to make it as effective as possible, especially in the case of diseases that are unresponsive to defensive actions of our immune systems, and our pharmacological artillery. Kam’s lab is looking for better culture conditions for growing T cells with one purpose. Kam’s graduate student Keenan Bashour explains, “Culturing immune cells from a patient to be more effective, and subsequently transferring these cells to a patient, has great promise to treat diseases that include most forms of cancer, chronic disorders such as HIV, and autoimmunity.”
Growing T cells in bioreactors affords them the opportunity to control activation and proliferation according to what they add to the bioreactor. For example, they activate the T cells by adding solid beads with proteins attached to replicate antigen-presenting cells. These proteins activate two receptors on the surface of T cells: the T cell receptor complex and the co-stimulatory CD28 receptor.
A main goal of Kam’s group is to enhance T cell expansion in vitro. To do so, they are studying the interactions that occur at the interface between the T cell and the antigen-presenting cell. This interface—also known as the immune synapse—is a highly organized structure on a microscale level.
Kam, whose lab is part of the Nanomedicine Development Center of the National Institutes of Health, uses micro- and nanoscale surface engineering techniques to replicate the T cell–antigen-presenting cell interface. Using this system, they showed that “T cells can sense the microscale spatial organization of the stimulating signals, altering activation in response to different patterns of these proteins,” says Kam. “We have also demonstrated that T cells are able to respond to the mechanical rigidity of the substrate presenting the activating proteins, changing both their activation and proliferation profile.”
Kam’s group was surprised, however, to discover that mouse and human cells showed different responses to rigidity (2). “Mouse cells showed stronger activation on stiffer surfaces, while human cells show stronger responses to softer surfaces,” he says. “Understanding both why these cells exhibit different responses, and how this information can be used to improve immunotherapy, are major goals of my research group and the larger Nanomedicine Center.”
From Mouse to Human
Despite some differences from humans, transgenic mice are an invaluable tool for studying physiology before proceeding to clinical trials in humans. As a post-doctoral researcher, Nishimura had studied the genes used by T cells targeted to human melanoma cells. This work piqued his interest in using T cell receptor genes as markers of immune responses to cancer. It also pointed toward new possibilities for cancer therapy—taking advantage of the genes for T cell receptors.
Nishimura’s lab has since developed a line of transgenic mice that express the same T cell receptor that they use in patients during clinical trials. This system affords them the opportunity to learn more about the biology of the engineered T cells in both mouse studies and clinical trials. “In this way, things we learn that improve the biology of engineered T cells can be applied directly to the clinic,” says Nishimura.
One significant challenge facing researchers who are engineering T cells for clinical use is the price tag. “The cost of patient grade materials for clinical trials, and the clinical costs for treating patients are extremely expensive,” says Nishimura. “It can easily cost over $100,000 per patient to treat them with genetically modified T cells.” In addition, biosafety-compliant good manufacturing practice (GMP) labs are not only uncommon in most hospitals, but their construction and up-keep are also very expensive.
In addition, engineered T cells can have undesirable side effects. Maus believes that additional training for medical staff regarding T cell therapies could help. “Having your T cells go crazy feels a lot like a bad case of the flu, and can be quite severe. In the beginning, physicians giving this kind of therapy will need to be trained in how to manage these kinds of side effects, which are different than most of the medicines oncologists are familiar with,” says Maus.
Engineered T cells are steadily becoming a viable means of treatment for previously intractable conditions, despite some rocky beginnings. “It is no secret that the field of adoptive therapy has seen its share of unanticipated failures,” says Bashour. “It is our humble opinion that for success in this challenging but potentially rewarding therapeutic approach, having a strong network of collaborating labs is paramount.”
1. Scholler, J., T. L. Brady, G. Binder-Scholl, W.-T. Hwang, G. Plesa, K. M. Hege, A. N. Vogel, M. Kalos, J. L. Riley, S. G. Deeks, R. T. Mitsuyasu, W. B. Bernstein, N. E. Aronson, B. L. Levine, F. D. Bushman, and C. H. June. 2012. Decade-Long safety and function of Retroviral-Modified chimeric antigen receptor t cells. Science Translational Medicine 4(132):132ra53.
2. O'Connor, R. S., X. Hao, K. Shen, K. Bashour, T. Akimova, W. W. Hancock, L. C. Kam, and M. C. Milone. 2012. Substrate rigidity regulates human t cell activation and proliferation. Journal of immunology (Baltimore, Md. : 1950) 189(3):1330-1339.