Imagine a detective has cornered a criminal that has been on the run, but at the last second the fugitive dons a disguise and slips right past the detective. Although we see this ploy all the time in books and movies, this also a common occurrence with cancer cells. The ability to trick those that are trying to catch you is something cancer cells do quite well, and this is one reason why treating cancer is difficult. In the body, immune cells are our natural defense system, playing the role of detective to identify and detain rogue cancer cells. Harnessing this power of our immune systems for the treatment of cancer is known as cancer immunotherapy, and a major obstacle in this field has been ensuring that immune cells actually recognizing and destroy cancer cells. Tumors have methods of “shutting down” aspects of immune function, and it wasn’t until recently that some of these mechanisms were revealed by scientists. Researchers at the Houston Methodists Research Institute have found that cholesterol may be interfering in this process.
Your Immune System Can Seek Out And Destroy Cancer Cells, But Why Does It Often Fail?
One key player in the world of cancer immunotherapy is the CD8+ T cell, or cytotoxic T cell; these cells possess the tools required to recognize and kill cells that do not belong in the body such as virus-infected cells, bacterial cells, and cancer cells. Despite having this specialized, cancer-killing ability, CD8+ T cells often lose their function once they find a tumor. This is because the harsh environment within the tumor itself makes it difficult for the T cell to survive and function normally, meaning that they may lose their cytotoxic, or cell-killing, function at the time that they need it the most. This loss of killing capacity can be brought on by many factors, including the conditions within the tumor itself (high pH or low oxygen, for example) and the presence of other immune cells that directly inhibit T cell killing. Even the presence of an abundance of cancer cells may temper killing by CD8+ T cells, as these immune cells have a tendency to become overstimulated, tired, and non-functional in the face of a tumor full of foreign-looking cancer cells. This tired-out state is termed exhaustion.
When T-cells become exhausted, they often begin to display molecules on their surface that act as markers of this exhausted state. These exhaustion markers are proteins that festoon the cell’s outer membrane, where they contribute to the cell’s deactivated and exhausted state. The two most well-known examples of exhaustion markers are PD-1 and CTLA-4, for which the 2018 Nobel Prize in Physiology or Medicine was awarded (Figure 1). Clinicians and scientists have achieved some clinical success in targeting PD-1 and CTLA-4 with anti-cancer drugs, but much more remains to be learned about how these exhaustion markers are controlled and regulated. Specifically, what is it about tumors that causes CD8+ T cells to display these exhaustion markers and become ineffectual killers of cancer cells? In a recent study published in Cell Metabolism, Ma et al. answer one aspect of this question, demonstrating that CD8+ T cells absorb something once they are inside of tumors that induces their exhaustion – cholesterol.
Cholesterol, A Familiar Enemy
Although cholesterol has a negative reputation due to its connection to cardiovascular disease, cholesterol is a necessary component of cells and serves as a precursor for many hormones that allow us to function normally. The investigators out of Houston wanted to see if cholesterol was playing a role in the interaction between T cells and cancer cells. Using a mouse model of melanoma, they were able to determine that CD8+ T cells that did make it into the tumor (good) contained high levels of cholesterol and possessed markers of exhaustion on their membranes like PD-1 (bad). When they isolated these cells from the tumor and assessed their ability to kill cancer cells, they found that their ability to kill was decreased. These observations were initially made in mice, so they decided to check human cancer cells as well to see if the same process occurs in patients with cancer. In patient samples from both myeloma (a type of blood cancer) and colon cancer they found similar increases in cholesterol within cancer cells in conjunction with increased exhaustion markers on CD8+ T cells. This preliminary evidence provided support for the idea that CD8 T cells become exhausted, ineffectual killers after picking up cholesterol within tumors.
Next, the researchers wanted to ensure that the T cells were acquiring the cholesterol from the tumor itself, as opposed to somewhere else in the body. To investigate this, the authors then measured the cholesterol content of various tumors and compared it to levels normally found in healthy tissue – indeed, the tumors contained a much higher concentration of cholesterol. Again, they confirmed that CD8+ T cells within the tumor had decreases in the tools they need to effectively kill their targets. Interestingly, if they added a drug (ß-cyclodextrin) to reduce the cholesterol content of T cells in the laboratory, it protected them from the exhaustion effect. At this point they had discovered that cholesterol content is high in tumors, CD8+ T cells enter the tumor and absorb the cholesterol, and ultimately this exhausts the CD8+ T cells, rendering them useless. But the story doesn’t end there.
How Is Cholesterol Inducing These Negative Changes In Our Cancer-Killing T Cells?
The authors dug deeper into this connection between cholesterol and T cell exhaustion, and they did so by assessing T cell genes. Genes are stretches of our genetic code that can be read as directions to assemble structures the cell needs, much like a recipe. Fittingly, they observed an increase in genes associated with the assembly of exhaustion markers, but they also observed the largest increase in the assembly of a molecule called XBP1. This shed light on a completely different process that was occurring inside the T cell – endoplasmic reticulum (ER) stress. The ER is a large structure inside cells that is responsible for the processing of molecules to ensure that they fold and function properly. XBP1 is increased when the ER has more molecules to process than normal, meaning that increases in this molecule serve as a marker, or a warning signal, of a stressed-out, overworked ER. As such, the observation that cholesterol caused an increase in XBP1 indicates that cholesterol is causing ER stress, preventing it from doing its job properly.
To investigate further, the authors then tried to give the CD8+ T cells a drug that reduces ER stress, similar to how they used a drug to reduce cholesterol in a previous experiment. Even when grown in the presence of cholesterol, they observed that this drug stopped the increase of exhaustion markers on the T cells. Through genetic manipulation, the authors then eliminated XBP1 or caused cells to produce too much of XBP1; the results lined up with the previous findings – CD8+ T cells lacking XBP1 were protected from exhaustion, while CD8+ T cells with high amounts of XBP1 became increasingly exhausted. Together, these experiments indicate that excess cholesterol intake causes ER stress, leading to a compensatory increase in XBP1, which in turn causes T cell exhaustion (Figure 2). This was all very revealing, but would it be possible to prevent this T cell exhaustion inside a living creature?
But Would These Interventions Work In A Mouse?
Taking their findings one final step further, mice with established tumors were treated with the drug that stops ER stress. The result? The CD8+ T cells within their tumors had significantly fewer markers of exhaustion. These CD8+ T cells also had greater anti-tumor function and better controlled tumor growth. Treating the ER stress definitely helped, but what if you could stop the entire process from the beginning by reducing cholesterol? Using gene editing, they then created a tumor cell line that lacked the tools needed to create cholesterol and injected these cells into mice. This effectively reduced the amount of cholesterol within the tumor, decreasing the exhaustion markers on the T cells and allowing them to do their job and kill the cancer cells.
New Approaches In The Fight Against Cancer
In the fast-paced world of cancer immunotherapy, novel strategies to try and control tumor growth or increase immune cell function are constantly being investigated. Ma et al. bring two new strategies to the table in the fight against cancer: either reduce the amount of ER stress that our immune cells are experiencing or attempt to reduce the amount of cholesterol within a tumor to allow T cells to do their job properly. This is promising, as there are many cholesterol-reducing drugs on the market that are not only safe, but also affordable. The authors suggest that combining current immunotherapies with common cholesterol-reducing drugs (statins) may produce a synergistic (and safe) effect that improves outcomes for cancer patients.