For glioblastoma, immunotherapy faces a particular challenge—the blood-Brain barrier blocks T cells from entering the brain to prevent brain inflammation that can be life-threatening. This “protective measure” is beneficial under normal conditions, but it prevents T cells from reaching the glioblastoma, leaving immunotherapy useless.

On September 5th, Nature published an article entitled “A homing system targets therapeutic T cells to brain cancer”, which reveals a new solution from a multi-agency international research team led by scientists at Baylor College of Medicine: Transform T cells to pass the blood-brain barrier and infiltrate brain tumor tissue, ultimately fighting cancer cells.

Nabil Ahmed, associate professor of pediatrics at the Cell and Gene Therapy Center at Baylor College of Medicine, said: “T-cell immunotherapy is an emerging field that has shown great potential in cancer and other diseases. However, effective localization of therapeutic T cells to targets remains a major limiting factor, particularly for Brain Tumors. ”

Find the “immune escape” mechanism of brain tumors

Encephalitis occurs when T cells that are supposed to be excluded from the brain pass through the blood-brain barrier. This process is a complex and synergistic migration process that requires active T cells circulating in the blood to attach to vascular Endothelial Cells. This attachment is dependent on the binding of ligand molecules on the surface of T cells to cell adhesion molecules such as ALCAM, ICAM-1, and VCAM-1 on endothelial cells.

The expression of these cell adhesion molecules in brain tissue is higher than normal. Binding of ALCAM to the T cell ligand CD6 blocks the progression of T cells through the blood vessels, allowing subsequent binding of ICAM-1 and VCAM-1. Once T cells reach a “critical threshold” by binding to cell adhesion molecules, T cells can migrate between endothelial cells, leaving the blood vessels into the brain.

However, in glioblastoma, the cerebral vasculature changes—overexpression of ALCAM, with little or no expression of ICAM-1 and VCAM-1. This “escape” change may be to help the tumor escape the identification and capture of T cells.

Scientists speculate that if we can increase the adhesion between T cells and endothelial cells in patients with glioblastoma, similar to the encephalitis process, it is possible to get T cells into the brain.

Let T cells “homing” into the brain

Heba Samaha, Principal Investigator of the Children’s Cancer Hospital of Egypt, and his team concluded that by transforming T cells, they can bind to ALCAM more firmly, thereby enhancing the anchoring of T cells in the endothelium, and finally overcoming the “deadlock” of the blood-brain barrier.

To validate the conjecture, the research team tried to engineer T cells and give them the necessary “molecular keys” to overcome the tumor’s escape mechanism and achieve anti-tumor effects.

They engineered CD6 ligands on T cells to synthesize “homing-system CD6 (HS-CD6), which allows individual ligands to interact to produce multi-molecular proteins.

Using retroviruses to introduce synthetic ligands into T cells, the researchers found that HS-CD6 on the surface of T cells enhances adhesion between T cells and endothelial cells expressing ALCAM, thereby promoting migration.

Specifically, when combined with ALCAM, HS–CD6 activates the SLP-76 protein on T cells, which causes the LFA-1 protein to move to the cell surface and bind to a small amount of ICAM-1 molecules on endothelial cells, further enhances binding between T cells and endothelial cells. These molecular changes activate FAK, a protein that regulates the T cell actin network, allowing T cells to squeeze between endothelial cells and cross the blood-brain barrier.

“The redesigned CD6 molecule is similar to a ‘homing system’, which enhances the binding of T cells on the endothelium to ALCAM and enhances the sensitivity of T cells to reduced levels of ICAM1 in cancer-associated blood vessels. Thus, T-cell-cancer endothelial cell interactions mediate the capture of circulating T cells, allowing them to cross the endothelium, effectively infiltrating brain tumors,” Nabil Ahmed explained.

Install “CAR” to accurately target brain tumors

How to ensure that T cells entering the brain can target tumor tissue? To this end, the researchers installed a “CAR” on the T cells, the chimeric antigen receptor (CAR), which is responsible for directing T cells against specific cancer cells.

First, they designed the antigen receptor to bind to human epidermal growth factor receptor 2 (HER2, an antigen produced by glioblastoma cells); subsequently, they modeled mice bearing human glioblastoma, injecting engineered T cells (bearing HS–CD6+ CAR) into the mouse brain and monitoring the survival of the mice.

The results were exciting: this combination strategy significantly reduced tumor volume in all treated mice. T cells expressing both HS-CD6 and HER2-specific antigen receptors were able to infiltrate glioblastoma, and eventually, the majority of mice were significantly relieved and survived for a long time.

In contrast, T cells lacking a homing system have a poor localization of tumors and therefore can only temporarily slow tumor growth. More importantly, the modified T cells can be strictly targeted to the tumor area and thus will not affect other normal brain and body tissues.

Difficulties have still remained for clinical validation

Next, the research team will attempt to validate this “homing” system in clinical trials while looking for next-generation homing molecules for targeted diagnosis or treatment of other diseases. They stressed that there are still many adjustments to realizing clinical success:

First, a variety of cell types express ALCAM, including bone marrow cells, so more research is needed to assess whether the function of these non-endothelial cells will be affected by treatment.

Second, if T cells directly or indirectly damage healthy brain tissue in the brain, it may cause toxicity problems. Scientists envisioned using gene “close switches” to limit the activity and longevity of T cells, thereby avoiding side effects. Researchers also need to determine the persistence and activity of HER2-targeted T cells in vivo and validate the feasibility of this strategy for brain tumors other than glioblastoma.

Finally, targeting T cells to brain tumors is only the first step in initiating an effective immune response against glioblastoma. T cells that enter brain tumors also face the poor tumor microenvironment caused by hypoxia, low pH, and immunosuppressive molecules. Although animal studies have shown that T cells entering mouse glioblastoma are not affected, these mice do not truly restore many of the key features of human brain tumors.

The researchers concluded that a successful glioblastoma immunotherapy strategy will ultimately be a combination therapy that allows enough specific T cells to enter the tumor tissue and survive in the tumor microenvironment.

Reference

Samaha H, Pignata A, Fousek K, et al. A homing system targets therapeutic T cells to brain cancer. Nature. 2018 Sep;561(7723):331-337. doi: 10.1038/s41586-018-0499-y.