Can we tip the immune system toward transplant tolerance?

First in a two-part series on transplant tolerance. Read part two.

Although organ transplant recipients take drugs to suppress the inflammatory immune response, almost all eventually lose their transplant. A new approach, perhaps added to standard immunosuppressant treatment, could greatly enhance people’s long-term transplant tolerance, report researchers at Boston Children’s Hospital.

The approach, which has only been tested in mice as of yet, works by maintaining a population of T cells that naturally temper immune responses. It does so by turning on a gene called DEPTOR, which itself acts as a genetic regulator. In a study published July 3 in the American Journal of Transplantation, boosting DEPTOR in T cells enabled heart transplants to survive in mice much longer than usual.

“We got prolonged graft survival in the mice, similar to what you would see with standard immunosuppressants,” says David Briscoe, MB, ChB, director of the Transplant Research Program at Boston Children’s and the study’s senior investigator.

When mice got no treatment at all, their heart transplants survived an average of 7 days. When DEPTOR was turned on in their T cells, survival increased to 35 days. But most exciting was when DEPTOR enhancement was combined with an immunosuppressive agent (in this case,  anti-CD154). These doubly-treated mice appeared to develop true transplant tolerance. As shown at right, their transplants survived almost indefinitely (100 days or longer), versus 25 to 30 days with anti-CD154 alone.

Curbing the inflammatory response

The immune system has two arms: An inflammatory arm, led by effector T-cells, and a regulatory arm that resolves or suppresses inflammation, led by regulatory T cells (Tregs). The immune system maintains these cells in different mixes, depending on the situation. After transplantation, the balance normally shifts toward effector cells. In fact, existing Tregs can morph into effector cells.

Standard immunosuppressive drugs suppress transplant rejection by curbing effector T-cell responses. But the study suggests a complementary approach: enhancing and stabilizing the regulatory T-cell response via DEPTOR.

“Normally, when a recipient’s immune system ‘sees’ a transplant, Tregs are overwhelmed by the effector T cell response and cannot shut down inflammation. They may even become effector cells,” says Johannes Wedel, MD, PhD, a postdoctoral fellow in the Briscoe lab and first author on the paper. “But when we turn on DEPTOR, they don’t do that – they remain stable and active, and potently inhibit rejection-causing effector cells.”

In multiple experiments, boosting DEPTOR activity made Tregs more long-lived and functional. While turning DEPTOR on in effector T cells didn’t inhibit organ rejection, doing so in Tregs was enough to tip the immune system toward long-term transplant survival.

Deconstructing DEPTOR

To better understand how DEPTOR functions in T cells, Briscoe collaborated with David Sabatini, MD, PhD, of the Whitehead Institute for Biomedical Research, and Mathieu Laplante, now at Université Laval (Quebec City), who originally identified DEPTOR in other cell types. Wedel worked with Peter Sage, PhD, of Harvard Medical School, to sequence Tregs’ messenger RNA to see what genes DEPTOR was turning on and off. Many of the genes that came up as “hits” are involved in cell metabolism.

“Metabolism of T cells is very important to their function,” says Wedel. “By boosting the activity of DEPTOR, we can shape metabolism in a direction that makes regulatory T cells much more stable and active.”

Drug treatment for transplant and autoimmune disease?

Briscoe thinks the study findings also have potential implications for treating autoimmune disease. “Classically, Tregs are not fully functional in autoimmune diseases,” he says. “So enhancing Treg stability or activity could help.”

Turning DEPTOR on in Tregs was enough to tip the immune system toward long-term transplant survival.

While the study used a special “knock-in” mouse that allowed Briscoe, Wedel and colleagues to turn on DEPTOR expression, Briscoe thinks it may be possible to boost DEPTOR with drugs. Existing compounds known as Cullin-RING E3 ligase inhibitors are known to increase DEPTOR activity by preventing its destruction.

“We are interested in identifying specific DEPTOR-enhancing agents and moving this area of research forward in pre-clinical models,” says Briscoe.

Briscoe envisions two possible methods of treatment. With some formulation work, drugs could be designed to be given to patients directly, along with immunosuppressants, to maintain a stable pool of Tregs. Alternatively, Tregs could be isolated from a patient’s blood, manipulated genetically or with a DEPTOR-enhancing drug to make them more stable, then given back to the patient.

In the meantime, Briscoe, Wedel and colleagues are trying to understand better how DEPTOR works. They are creating new models to track Tregs with different levels of DEPTOR expression and further investigate how they behave in different situations and disease models.

The study was supported by the National Institutes of Health (R21AI114223 and R01AI136503), an Advancing Research in Transplantation Science Investigator Initiated Grant from Pfizer Inc., the Casey Lee Ball Foundation, the German Research Foundation and Heidelberg University. Coauthors were Sarah Bruneau and Kaifeng Liu of the Transplant Research Program at Boston Children’s; Sek Won Kong of the Computational Health Informatics Program at Boston Children’s; Peter Sage of Harvard Medical School; David Sabatini of the Whitehead Institute for Biomedical Research (Cambridge, MA); and Mathiue Laplante of Université Laval (Quebec City).