Following a single dose of CAR-T therapy, cancer patients for whom all other treatment options have failed have gone into complete remission. This remarkable feat is achieved by engineering immune cells to target and eliminate tumours. However this doesn’t come cheap. The patient’s cells must be genetically engineered in a certified laboratory before being re-infused into the patient, and the complex manufacturing processes required to achieve this are very costly. In a step-change, Pfeiffer et al. recently demonstrated that a targeted virus could be used to generate CAR-T cells inside the body potentially giving huge savings on cost and time, the latter being a crucial gain for terminally ill patients with no other treatment options.
Harnessing the immune system has shown great promise for providing treatments and potential cures for cancer. The Nobel Prize in Physiology or Medicine was awarded earlier this month to James Allison and Tasuku Honjo “for their discovery of cancer therapy by inhibition of negative immune regulation” (Press release: The Nobel Prize in Physiology or Medicine 2018). Their work focused on brakes in the immune system that help prevent autoimmune disease, demonstrating that cancer cells apply these brakes to avoid being recognised and eliminated by killer T-cells. A large proportion of cancer immunotherapies under investigation work to remove these brakes, aiming to give killer T cells unfettered access to cancer cells.
CAR-T therapies build on this premise, using engineered killer T-cells to more effectively recognise and eliminate cancer cells. Engineering immune cells is no easy feat. Current procedures involve taking blood from a patient, isolating their T-cells in a laboratory and then using a virus-based vector to insert genes encoding chimeric antigen receptors (CARs). CARs are designed to recognise specific marker proteins on the surface of cancer cells. As a result, when CAR-engineered T-cells are re-infused into the patient they home to and kill cells that express the specific marker proteins.
CAR-T therapies are very much personalised to the individual. The manufacturing process for CAR-T cells typically takes from 10-14 days, and during this time patients are in hospital waiting for their T-cells to be tailored to kill their cancer. However, this personalisation makes it difficult to scale these therapies up, which in turn makes them costly.
Numerous teams around the world are investigating other methods for generating CAR-T cells that would lower costs and improve accessibility to these highly effective treatments. In a recent study reported in EMBO Molecular Medicine (Pfeiffer et al. 2018), a team of scientists led by Professor Christian Buchholz in Heidelberg, Germany have used a targeted virus to enable T-cells to be engineered inside the patient’s body rather than in a laboratory.
As described above, viruses are already used to generate CAR-T cells in the laboratory setting. These viruses are coated in an envelope which enables them to enter a broad range of cell types very efficiently. However they cannot efficiently enter T-cells that are in a resting state, such as those that circulate in a patient’s blood (Amirache et al. 2014). To overcome this obstacle, T-cells are activated outside of the body before being treated with virus. An added obstacle for the viruses to overcome is their broad tropism: if infused directly into patients, they will enter the first cells they encounter in the body and so most will not make it to T-cells.
To get around these two obstacles, the team in Germany gave the viral vector a new envelope. This new coat made the virus blind to other cell-types but incorporated specific entities on the surface of the virus to enable it to recognise and therefore enter T-cells (Zhou et al. 2012, Bender et al. 2016). This new virus, when injected into the blood stream, would ignore all but T-cells. Taking this approach one step forward, the Heidelberg team have engineered the virus to deliver the genetic information necessary for T-cells to produce a CAR targeting CD19, a protein which is found on the surface of B-cells and B-cell derived blood cancers.
Injection of this new viral vector into mice resulted in engineering of killer T-cells in the blood, spleen and bone marrow. Crucially, even though only a small proportion of the killer T-cells were engineered, mice treated with the virus showed a depletion of B-cells that naturally express CD19 (the target for the CAR). An important component of the natural immune response is the expansion and multiplication of activated killer T-cells in response to ‘agressors’ such as pathogens. Importantly, the CD19 CAR engineered killer T-cells mimicked this natural process, expanding and multiplying in the presence of CD19-expressing B cells.
The next step was to test whether the virus was effective against human cancer cells. To do this, the new virus was injected into mice implanted with tumour cells derived from a patient with Burkitt’s lymphoma. Encouragingly, these mice showed signs of tumour cell killing. The team also showed evidence suggesting the engineered T-cells released cytokines that indicate cell killing and T-cell activation and expansion, further demonstrating their mechanism of action resembles that of CAR T-cells engineered outside of the body. Interestingly, some mice developed symptoms of a cytokine release syndrome resembling that observed in some patients treated with CAR-T cells engineered outside of the body.
In all, the data presented in the recent study by Pfeiffer et al., is extremely encouraging. However, by engineering T-cells in a laboratory you can conduct a battery of tests to demonstrate the safety and potency of the CAR T-cells before they go back into the patient. This is an important consideration as CAR-T cells have been shown to be very potent and at times lethal. Using the patient as the bioreactor for the manufacture of CAR-T cells significantly limits the amount of control you have on the manufacturing process and, by extension, the final clinical outcome. Before any clinical trials can begin, further studies are therefore needed to demonstrate the T-cells produced in the body using a targeted virus will have an acceptable safety profile, and that you will not see significant variation in the engineered T-cells in one patient versus another.
Despite these reservations, this exciting approach represents a significant step forward in CAR-T cell manufacture. Instead of a complex, lengthy and costly process where one batch of drug can only be used on one patient, this approach means that a single batch of CAR-T virus could potentially be used ‘off the shelf’ to treat thousands of patients.
Amirache, F., et al. (2014). “Mystery solved: VSV-G-LVs do not allow efficient gene transfer into unstimulated T cells, B cells, and HSCs because they lack the LDL receptor.” Blood 123(9): 1422-1424.
Bender, R. R., et al. (2016). “Receptor-Targeted Nipah Virus Glycoproteins Improve Cell-Type Selective Gene Delivery and Reveal a Preference for Membrane-Proximal Cell Attachment.” PLoS Pathog 12(6): e1005641.
Pfeiffer, A., et al. (2018). “In vivo generation of human CD19-CAR T cells results in B-cell depletion and signs of cytokine release syndrome.“ EMBO Mol Med.
Press release: The Nobel Prize in Physiology or Medicine 2018. NobelPrize.org. Nobel Media AB 2018. Tue. 23 Oct 2018. <https://www.nobelprize.org/prizes/medicine/2018/press-release/>
Zhou, Q., et al. (2012). “T-cell receptor gene transfer exclusively to human CD8(+) cells enhances tumor cell killing.“ Blood 120(22): 4334-4342.