T cell exhaustion poses a significant barrier for CAR T cell therapy in solid tumors. Therapeutic interventions aimed at metabolically reprogramming CAR T cells represents a promising avenue to overcome T cell exhaustion. Read on to learn more about T cell exhaustion and how IL-10 expressing CAR T cells are overcoming exhaustion, penetrating the immunosuppressive microenvironment, and destroying solid tumors.
CAR T cells are one of the most innovative forms of cancer therapy in existence today. They are made from a patient’s own T cells, supercharged with a custom receptor, called a chimeric antigen receptor (CAR), that makes them cancer-fighting super soldiers. CAR T cells are produced ex vivo, or outside the body – genetically manipulated to express a cancer antigen-specific CAR – and then injected back into a patient. Once injected, CAR T cells rapidly seek out and destroy any cell that presents the ligand specific to the CAR. A well-designed CAR T cell will only kill cancerous or other diseased cells while leaving healthy tissue untouched.
CAR T cells have proven particularly effective against hematological malignancies – leukemias and lymphomas – but the clinical effectiveness against solid tumors is still very poor. So bad in fact that there are currently no approved CAR T cell therapies for solid cancers.
So, what’s the problem? Why don’t CAR T cells work against solid tumors. Well, the answer you will get most often is: the highly immunosuppressive microenvironment associated with solid tumors completely destroys any killing potential of CAR T cells. And, yeah, this is true. Solid tumors tend to enclose themselves within a fortified structure called the tumor microenvironment that is composed of thick fibrous structures, packed full of immunosuppressive cytokines, toxic compounds such as lactate and reactive oxygen species, and other “junk” that extinguishes anti-tumor immune cell activity. So, in order to affect an anti-tumor response, CAR T cells must overcome this barrier, penetrate enemy lines, and withstand a continuous onslaught of attacks from the tumor cells.
So, yes, immunosuppression is a big problem, but it’s not the whole story – not even close.
T cell exhaustion suppresses CAR T cell efficacy
You might be shocked to learn – I know I was – that a big problem with CAR T cells is that they are TOO GOOD at killing cancer cells. That’s right. It turns out that these things are so good at finding and killing cancer cells, that they are prone to becoming exhausted; this is actually a technical term.
T cell exhaustion is a cellular state defined by reduced proliferative capacity and diminished cytotoxic effector function, and is usually the result of prolonged, persistent antigen stimulation. You see, when a T cell binds to a target, it gets super excited and starts dividing like crazy. This is in an attempt to make as much of itself as possible, improving the likelihood that it will completely eliminate the identified threat. The problem is that cell division is an energetically demanding processing and places a tremendous burden on the mitochondria.
In terms of cellular energetics, cell division is ridiculously expensive. DNA replication alone requires literally billions of ATP molecules, not to mention organelle duplication, microtubule organization, and the dozens of other processes associated with cell division, all requiring massive amounts of energy. Add to this the fact that an activated T cell will divide once every 6-8 hours and you get some serious problems for the mitochondria.
Normally, before the energetic demands of cell division become a problem for the mitochondria, the antigen-presenting target has been neutralized and the T cell discontinues it’s rapid division. However, in cases of chronic infections, or cancer, persistent antigen presentation results in an extremely prolonged period of T cell division. In these cases, the energetic output of the mitochondria is pushed to it’s limits and massive amounts of reactive oxygen species (ROS) are produced. When ROS become too abundant, they essentially rip apart the mitochondrial membrane, disrupt the proton gradient driving ATP synthase, and render the organelle completely dysfunctional. Without an abundance of healthy mitochondria, ATP levels plummet, cell division is halted, and any chance of eliciting a cytotoxic function is essentially eliminated.
Within solid tumors, highly concentrated antigen abundance results in a frenzy of CAR T cell activity which causes the initiation of this cascade. These cells are classified as terminally exhausted T cells and are a huge problem for solid tumor CAR T cells.
*** Preventing T cell exhaustion is, therefore, a therapeutic opportunity ***
IL-10 promotes mitochondrial fitness and reverses CAR T cell exhaustion
Okay, so a problem has been clearly identified: hyperactive T cells have dangerously high metabolic rates and tend to destroy their own mitochondria and become exhausted. How can we prevent this?
Cancer researchers from the Institute of Bioengineering in Lausanne Switzerland have recently published their findings that the immunostimulatory cytokine IL-10 is capable of rescuing terminally exhausted T cells, reigniting an anti-tumor immune response, and dramatically improving the effectiveness of CAR T cell therapy.
Here’s what they found.
Co-injecting purified IL-10 worked, kind of
It has previously been shown that treatment with the immunostimulatory cytokine IL-10 significantly enhanced the mitochondrial activity of macrophages. Because of this, the authors were determined to investigate the energetic effects of IL-10 on T cell function, specifically in the context of CAR T cell therapy. To do so, they established mouse tumor xenografts and treated the mice with tumor antigen-specific CAR T cells. They then injected purified IL-10 directly into the tumor – they called it “peritumoral injection of IL-10” – of one group of mice while the other received CAR T cells alone. The mice that received CAR T + IL-10 had significantly smaller tumors and the majority of mice were “cured” (tumors were not detected at the time of the termination of the study).
They then conducted a plethora of metabolic studies on the CAR T cells to determine how the IL-10 treatment affected their energetics. They were able to conclude that the injections of IL-10 were absolutely able to directly enhance the mitochondrial fitness of the CAR T cells and significantly reduce the number of exhausted T cells. Most notably, they were able to show that the reduction in exhausted cells was due to the revitalization of previously exhausted T cells; in other words, they observed a reversal of exhaustion. This was significant because it indicates the IL-10 may have a prolonged and stable effect of CAR T cell metabolic fitness and could therefore legitimately enhance their therapeutic potential.
However, there is an elephant in the room. Number one, they had to directly inject IL-10 into the tumor and, number two, they needed to inject it every other day. This is simply not feasible in most clinical settings. In order for their discovery to have any clinical applicability, they needed to figure out a way to get high levels of IL-10 into the tumor without having to inject it directly.
So, with the proof-of-concept under their belt, they headed back to the drawing board.
IL-10 expressing metabolically armored CAR T cells completely destroy solid tumors
A stroke of genius provided the team with an extremely elegant and inspiring solution: have the CAR T cell itself express and produce IL-10. This was, seemingly, their thinking:
When producing CAR T cells, the T cells are genetically manipulated – usually through viral infection – and made to express a custom gene, the chimeric antigen receptor (CAR). The gene for the CAR is on a DNA plasmid that becomes integrated into the T cells’ genome upon viral infection and is then expressed at high levels, using the T cell’s own molecular machinery. Well, what if the plasmid containing the gene for the CAR also harbored an IL-10 gene. Then, as the T cell expresses an exceptionally high amount of the CAR, it will simultaneously produce an exceptionally high volume of IL-10. This would result in a sort of “cloud” of IL-10 surrounding every CAR T cells, providing them with a rich source of IL-10 without the need for injection.
A simple and elegant solution. But does it work?
The team got to work designing the plasmid that would co-express both the CAR and IL-10. The final design included a CMV promoter, to promote high levels of transcription, the CAR gene, in this case a HER2-specific CAR, a linker that would automatically be cleaved upon translation, and finally, full-length IL-10.
The following diagram depicts the final design. Simple and elegant.
They called these CAR T cells “metabolically armored” which, I have to be honest, sounds so cool.
When they tested these cells against HER2-expressing solid tumor models, they showed tremendous efficacy, even outperforming CAR T cell + IL-10 injection therapy – and this was without the need for any intratumoral injections of any kind. Systemic administration of CAR-IL-10 T cells cured the majority of the mice in multiple experiments using various solid tumor models.
Metabolic reprogramming of T cells, like the ones described in this article, are becoming an increasingly promising area of adoptive cell therapy. I am very excited to see what innovations arise as a result of these kinds of breakthroughs.
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