Kill Switch for Activated T cells

To improve the safety of the Cartel cells, a kill switch can be implemented in order to specifically eliminate CAR T cells which produced side effects. This switch is controlled by a drug, that is not damaging unmodified cells An approach frequently used in clinical studies of transplanted lymphocytes is the virostatic drug ganciclovir (GVC) and its effector, the thymidine kinase commonly expressed by cells infected with herpesviruses like the herpes simplex virus 1 [HSV1].

Mechanism

Ganciclovir is commonly used as medication against human cytomegalovirus like Herpes Simplex Virus 1 as it acts as a virostatic. It has a high affinity to viral kinases, like the thymidine kinase expressed by infected cells. These kinases initiate the metabolization of Ganciclovir into a nucleoside analogue, which subsequently is integrated into the DNA by the DNA polymerase, resulting into disruption of DNA replication of proliferating cells. It is assumed, that the DNA instability leads to activation of caspase-9, which triggers processes associated with apoptosis.

Interview with Professor Cathomen

Toni Cathomen is Professor of Cell and Gene Therapy at the Center for Chronic Immunodeficiency and Director of the Institute for Transfusion Medicine and Gene Therapy in the University Medical Center Freiburg. He is leading the ’Cathomen research group’ which works with CAR T cells. Our team interviewed him to discuss different aspects of our project such as the design, the application costs and the possible bottlenecks but also the potential of our CARTelTM system in modern cancer therapy.

How are allogeneic CAR T cells used and what are the possible benefits and disadvantages?

Allogeneic CAR T cells are already in use for CAR therapy. The T cell receptor, which is responsible for the alloreactivity is knocked down because the used CAR T cells are not from the patient and would be seen as a threat. A strong chemotherapy is then used to destroy the immune system of the leukemic patient (like a classical leukemia treatment without T cells). Even the stem cells of the immune system have to be destroyed. A new immune system from a safe donator is inserted via bone marrow transplantation. Also allogeneic CAR T cells are then inserted. These allogeneic CAR T cells are active until the new immune system is up again and the CAR T cells are destroyed. This time span is about 8 to 12 weeks.
This allogene therapy is cheaper because the allogeneic CAR T cells are produced for multiple patients and are kept in stocks, but also come from healthy donors and not from a weak patient. The production itself is then controllable. The disadvantages are that the allogeneic cells are only available for a short period of time. One publication (Waseem Quasim, et al.) mentioned two boys with acute lymphoblastic lymphoma (ALL) who got the allogeneic CAR T cells therapy after all other therapies had not worked. And both boys healed. This method with allogeneic cells was a success.
If memory CAR T cells were produced by the body during the therapy, a possible relapse would be avoided. But all allogeneic CAR T cells are destroyed by the new immune system. Nevertheless, there is no known leukemia relapse after this allogenic CAR T cells therapy.

Our project focuses on solid tumors and not on cancer like leukemia. How would these allogeneic CAR T cells be used against a solid tumor?

Solid tumors have the microenvironment which reduces the T cell activity. Researchers try to counter it with the so called «checkpoint inhibitors». Whether the CAR T cells would actually work against solid tumors is not known and research may not be far enough to allow a reliable answer. But there are many studies going on about this precise topic, but none are published yet.
For allogeneic CAR T cells against solid tumors, the immune system is not destroyed in opposition to the leukemia therapy. The allogeneic cells are then instantly recognized as foreign. A possible solution would be to transform the allogeneic T cells to hide them from the immune system. This method would be more complicated and dangerous: cells which are not recognized by the immune system are also not controlled. This is a great risk for cancer. CAR T cells can be modified to be deactivated from the outside (with drugs) but it has been shown that CAR T cells get many mutations and they become unresponsive to this deactivation.

Do you think our project to activate T cells through the tumor microenvironment is a possible contribution to the current cancer research?

Your idea is very interesting and there is a chance that such a construct as yours will be used in the future. The next step would be to test the final CAR T cell construct within a mouse model with a solid tumor and its microenvironment. But then you should remember that mouse models react differently than humans because the T cells and the tumor have human origin.

Do you know other inputs for our AND gate which could be better activators?

Hypoxia, pH and growth factors like VEGF are well chosen and very tumor specific. Other inputs do not seem to be better. General inputs are very interesting because all tumors could be targeted and then the specific tumor could be targeted with the appropriate CAR. The general activation through the AND gate and the specific activation through the CAR is a good combination. This method promising.

Do you think that the CAR therapy will be an independent therapy one day?

I don’t think there will ever be a proper CAR therapy but rather a combination with other techniques like antibody therapy, of course chemotherapy, surgery, etc. Today, the CAR therapy is only seen as the last option when other methods did not work, for patients that have a low chance to survive their illness and are ready to take the risks of an unknown therapy. With a solid tumor the patient will probably not do the therapy, because like all new therapy, all other resources have to fail before they do the CAR therapy.

Will this change?

Many studies are running but still without safety feature. When switches are developed (like AND gate, kill switches, activation/deactivation switch…) and CAR can be deactivated, the CAR therapy will be applicated sooner and not be considered as a last possibility.

The CAR Therapy has often been used now against minimal residual disease after classical leukemia therapy, mostly with big success. Do you know if there is any long term impact?

With the CAR therapy, there is a co-production of memory T cells which can last decades. Treated patients who had leukemia then never have B cells again and have to take antibody mixes to keep the immune system stable. There is no other known long term impact.

If our AND gate T cells also produce memory cells, any new tumor relapse or growing residue with a tumor microenvironment would then be destroyed?

Exactly. In this case this would be advantageous.

The first CAR therapy was accepted by the FDA in the USA last August. Do you have an idea when we will see the CAR therapy in Germany or in Europe?

There is no infrastructure yet in Europe that allows the production of T cells the way Novartis is doing it in the USA. Novartis production is central and all T cell samples are gathered there, modified and then sent back. Nevertheless, there is a big interest to expand in Europe and Novartis is certainly waiting for an approval.
But in the future, the CAR T cells will be produced in the clinics itself and the production will be automated. There is already a machine which can modify T cells. The therapy will be cheaper and quicker. This leads me to believe that the CAR therapy will be present in Europe in the near future.

Waseem Quasim, et al. (2017): Molecular remission of infant B-ALL after infusion of universal TALEN gene-edited CAR T cells. Science Translational Medicine, 25 Jan, Vol. 9, Issue 374.