In your lifetime you probably heard about someone who is or was suffering from some type of cancer. Cancer is a group of diseases that are united by the fact that for one reason or another the cells in the body multiply uncontrollably. Usually, our immune system is extremely efficient in recognizing and killing the suspiciously changed cells as soon as they appear. However, in cancer patients, the immune system somehow overlooks the cancer cells, allowing cancer to progress. For many years scientists around the world have been working on the question: is it possible to use our own immune system in the fight against cancer?
In an exciting new study published in June 2021, Stefanie Lesch and colleagues from Ludwig-Maximilians-Universität Munich tried to answer this question. By targeting chemokine-based immune cell communication, the researchers aimed to find a new type of treatment for pancreatic cancer.
How do immune cells communicate?
Immune cell communication is mediated by chemokines, which are small secreted proteins produced by many types of cell. Chemokines are recognized by binding to receptor proteins on immune cells, allowing communication between tissues and the immune system in health and disease. When damage occurs anywhere in the body, cells in the site of the injury send a chemokine ‘cry for help’, and immune cells quickly respond by migrating to the region in a targeted manner (Figure 1).
In order to establish themselves successfully, cancer cells can use chemokines to their advantage. By exploiting chemokines, a tumor can even disrupt immune cell recruitment and activation in distant lymphoid organs. One example of this is an increase in CXCL12 chemokine levels in the bone marrow caused by a distant tumor. This prevents immune cells from leaving the bone marrow into the blood circulation and coming to the tumor site. In addition, if immune cells do make it to the site of the cancer, they face another challenge: reaching the deeper layers of the tumor. For example, if a tumor is able to switch off chemokine secretion, this can leave immune cells clueless of where to go. In the cancer field this phenomenon is known as the ‘cold’ tumor.
Chemokines as the candidates for cancer immunotherapy
Due to their importance in directing immune cells precisely towards the tumor and within the tumor itself, chemokines represent a very important target for developing anti-cancer treatments. In an article published in Nature Biomedical Engineering, researchers from Munich used a specific chemokine-receptor pair, CXCL16-CXCR6, as a tool for the treatment of pancreatic cancer. This tumor type is especially hard to treat due to the dense fibrous tissue surrounding it and the low number of blood vessels within the tumor. The combination of these two tumor characteristics means that very few immune cells can infiltrate deep into the cancer tissue.
These factors represent one of the major obstacles for an otherwise promising cancer therapy known as chimeric antigen receptor T cell (CAR-T) cancer therapy. CAR is an engineered molecule, which consists of the recognition site for a particular tumor protein, as well as T cell components needed to initiate an immune response. CAR-T cells are highly efficient at killing tumor cells, but struggle to reach the deeper layers of solid tumors. That is where the researchers realized the chemokine system might come to help.
Using mouse cells, the scientists found that pancreatic tumor cells contained high levels of the CXCL16 chemokine. However, CXCR6 (CXCL16’s receptor) was mostly absent on cytotoxic T cells, the immune cells which attack and destroy cancer cells. The researchers hypothesized that adding CXCR6 to the cytotoxic T cells would guide them towards the CXCL16-expressing cancer cells, thereby increasing the ‘visibility’ of the tumor.
First of all, the authors tested these genetically engineered CXCR6-positive T cells in a petri dish system. They observed that the engineered T cells successfully recognized and migrated towards CXCL16, were more activated, and were very effective at killing tumor cells. The researcher then engineered T cells to have both CXCR6 and CAR, and injected them into mice with cancer. As predicted, these new ‘empowered’ T cells controlled tumor growth and led to rejection of tumors in a number of animals.
When looking closer at the cancer site using microscopy, the researchers detected an increased accumulation of CXCR6 T cells inside the tumor. In addition, CXCR6 T cells isolated from tumors showed higher levels of effector and activation molecules. This confirmed that these cells were now able to migrate deep into the tumor and attack it.
These results were extremely promising – but as we know, mice are quite different from humans. It was therefore important to check if these findings were valid in humans. In order to do so, the researchers looked for the presence of CXCL16 in several human pancreatic cell lines and selected those with the highest CXCL16 levels. Next, as with the mouse cells, the authors genetically engineered human T cells to have CXCR6. They were able to confirm that human CXCR6-T cells successfully infiltrated into cancer cell ‘spheroids’, a petri dish model often used to mimic real tumors.
Scientists then engineered human T cells to contain both CXCR6 and mesothelin CAR (MSLN-CAR). MSLN is a membrane protein which is highly abundant in tumors and considered to be a tumor-associated protein. When researchers added both CXCR6 and MSLN-CAR to human T cells, they observed that those cells were even better than MSLN-CAR-T cells at recognizing and killing cancer cells.
Next, the researchers wanted to know if CXCR6-MSLN-CAR-T cells would be able to attack human tumors. To do this, they induced the growth of human cancer cells in lab mice. Normally, mice with a strong immune system would quickly reject the human cancer cells, and the tumors would not be able to establish. To overcome this obstacle and induce the growth of human pancreatic tumors, the researchers used mice with a compromised immune system. When the tumors were fully established, scientists injected CXCR6-MSLN-CAR-T cells and were happy to see that out of 10 mice treated with these cells, nine rejected the tumors and stayed tumor-free for the 100-day period of observation. Similar results were obtained when tumor cells were injected into the pancreas of the mice, to recapitulate the microenvironment in which pancreatic cancer develops in patients. All 20 mice rejected the tumor. In addition, tumors derived from human patients and injected into the immunocompromised mice were smaller after the injection of CXCR6-MSLN-CAR-T cells.
All these findings indicate that the CXCL16-CXCR6 system, especially when combined with CAR therapy, represents a promising new strategy in the treatment of pancreatic cancer (Figure 2). The addition of the CXCR6 receptor to the T cells allows them to better recognize CXCL16-expressing cancer cells and guides them deep inside the tumor, where they can fight cancer in the right place. Importantly, in this study the pancreatic tumors derived from human patients showed greater patient-to-patient variability in CXCL16 levels. This means that only those patients with high levels of CXCL16, which can be measured in the tumor itself and in blood plasma, would benefit the most from CXCR6-CAR-T cell immunotherapy.
Interestingly, apart from pancreatic tumors, the authors and other research groups also observed elevated CXCL16 levels in ovarian, lung, and breast cancer, meaning that CXCL16-CXCR6 chemokine-receptor pair holds potential for several cancers. Hopefully this is only the beginning of the story, and chemokines and their receptors will soon bring us forward in the development of successful cancer treatments.
Journal Article: Lesch, S., Blumenberg, V., Stoiber, S. et al. T cells armed with C-X-C chemokine receptor type 6 enhance adoptive cell therapy for pancreatic tumors. Nat Biomed Eng 5, 1246–1260 (2021). https://doi.org/10.1038/s41551-021-00737-6
Cover Image: Cancer cell, CC BY