"Armored" CAR T- Cells: A More Effective Form of Immunotherapy Against Solid Tumors?

Original Paper: “Targeted delivery of a PD-1-blocking scFv by CAR-T cells enhances anti-tumor efficacy in vivo”

Immunotherapy is a form of treatment that utilizes a patient's own immune system to fight diseases such as cancer [1]. This is accomplished by either stimulating the immune system to work "smarter" to attack cancer cells or by supplementing the immune system with components like man-made immune system proteins. Promising forms of immunotherapies include chimeric antigen receptor (CAR)-T cell therapy and immune checkpoint inhibitor treatments. 

CAR-T cell therapy manipulates T-cells in the lab to allow them to target and destroy cancer cells. More specifically, the cells are manipulated through the insertion of a gene that encodes for a special receptor called a chimeric antigen receptor, or CAR. The diagram below shows the general process of CAR-T cell therapy:

(www.cancer.gov/images/cdr/live/CDR774647-750.jpg)

Now let's consider checkpoint inhibitor treatments. A key role of a normally functioning immune system is the ability to discriminate between "self" and "non-self" cells. This ability allows for the immune system to attack foreign cells while leaving normal cells unbothered. The immune system accomplishes this through a series of "checkpoints", which are molecules on immune cells that need to be activated or inactivated to evoke an immune response. However, cancer cells are tricky: many evade these checkpoints to avoid being targeted by the immune system. Immune checkpoint inhibitor treatments focus on drugs that target these specific checkpoints. More particularly, PD-1 is a checkpoint protein that, upon binding to PD-L1, inhibits T cells from attacking other cells. Specific drugs can target either PD-1 or PD-L1 to block this binding and thus boost the immune response against cancer cells. The diagram below shows how blocking PD-L1/PD-1 binding with a checkpoint inhibitor allows T cells to kill tumor cells:

(https://www.cancer.gov/publications/dictionaries/cancer-terms/def/immune-checkpoint-inhibitor)

At Memorial Sloan Kettering (MSK), researchers are attempting to combine these immunotherapies. The current study explains the approach of combining CAR and PD-1 checkpoint inhibitor antibodies on the same engineered T-cell, ultimately resulting in an "armored" CAR. Brentjens et. al frame an important problem, accounting for why they are combining the approaches: CAR-T cells lack clinical efficiency for solid tumors, possibly because of an immunosuppressive tumor microenvironment (TME) [2]. Meanwhile, checkpoint inhibitors have been found effective in certain solid tumors, though they leave behind an array of immune-related side effects [3]. With this in mind, Brentjens et. al. sought to create a single therapy in which CAR-T cells secrete an immune checkpoint blockade scFv. 

First, the lab created mouse CAR constructs and modified them to secrete the mouse PD-1 blocking scFv. Two kinds of armored mouse CAR constructs were made, and each construct contained binding domains that recognized different proteins. More specifically, one construct recognized CD19, which is a molecule found on particular blood cancers. The other construct recognized MUC16, a molecule found on some ovarian and pancreatic cancers. Following this, they validated the presence of the PD-1 blocking scFvs using a variety of laboratory techniques. Next, the efficacy of the scFv-secreting CAR-T cells was evaluated in the mouse models, each model with different forms of cancer. They found that one form of the armored CAR construct enhanced survival as compared to both the other CAR/PD-1 construct and the unmodified CAR construct. From this, they found that mice treated with that particular construct were able to mount an anti-tumor response as compared to untreated mice. Essentially, in all conditions (including the solid tumor condition), armored CARs survived longer in the body than standard CAR constructs, and the mice lived significantly longer, too. Brentjens et. al also found the creation of a "bystander" effect: since the checkpoint molecules were injected at the tumor site, nearby T cells were also activated. This is a positive outcome, because these nearby T cells then aided the CAR-t cells in fighting the tumor.

Considering the promising results from the armored mouse CAR constructs, Brentjens et. al. decided to generate human armored CAR constructs. These human T cells expressed CAR on their surfaces and secreted E-27, which is a human PD-1 blocking scFv. The presence of E-27 was confirmed through multiple laboratory techniques. Following this, the in vivo anti-tumor efficacy of these armored constructs were considered. The lab found that mice treated with the armored CAR construct showed enhanced survival as compared to the mice treated with the CAR construct alone. With this in mind, Brentjens et. al sought to determine localized (i.e, around the tumor site) and systemic (i.e, around the body) CAR-secreted scFv levels in the tumor bearing mice. They found that E27 remained localized at the tumor site, suggesting that it remains in the tumor microenvironment. Meanwhile, commercial antibody was found to circulate outside the tumor area. These findings support their hypothesis that local checkpoint blockade using scFv may be a safer alternative to antibodies. That is, the lower levels of E27 circulating in the blood indicates that it does not spread too far from the tumor site.. this is of great importance, as it could imply less systemic side effects.

Brentjens et. al. ultimately found that the armored CAR construct enhanced survival of mice in a variety of solid tumor models. So, the armored CAR model is effective for the mouse model. These results are promising, especially since they utilized an experimental animal model in which they could manipulate several variables. This could create a whole new strain of immunotherapy, one that combines the two approaches. The combination of approaches alleviates the shortcomings of each: now there is potentially a way in which solid tumors could be destroyed in an immunosuppressive TME without the array of side-effects associated with immune checkpoint inhibitor treatments. Since this study was primarily concerned with PD-1 inhibitors, a next step could include trying this approach with inhibitors for immunosuppressive molecules like CTLA-4. Though immunotherapy "trains" your body to fight against cancers, could tumors acquire resistance to armored CAR-T cell therapy? How long after treatment can a beneficial response be maintained? These questions could be of great importance as clinical trials begin. 

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[1] https://www.cancer.org/treatment/treatments-and-side-effects/treatment-types/immunotherapy/what-is immunotherapy.html

[2] Kershaw, M.H. et al A phase I study on adoptive immunotherapy using gene-modified T cells for ovarian cancer. Clin Cancer Res. (2006)

[3] Brahmer, J.R. et al. Safety and activity of anti-PD-L1 antibody in patients with advanced cancer. Nature 515, 558-562 (2014).