Author: Idil Cazimoglu Edited by: Ines Barreiros
Researchers at Stanford University injected two substances into cancerous tumours in mice to eliminate the injected tumour and others of the same cancer type in different parts of the body. The injection also stops cancer from spreading in the body and prevents future tumours that might arise due to genetic conditions. The study, ' Eradication of spontaneous malignancy by local immunotherapy', was recently published in the Journal of Science Translational Medicine. In this review, the key concepts to understanding this novel cancer therapy are highlighted.
T cells play key role in fighting cancer
T cells are a class of white blood cells and form an important part of our immune system. They are produced in the bone marrow and mature in the thymus (hence the name ‘T cells’). The maturation process involves their multiplication and differentiation into specialised T cells.
Three main specialised T cell types are: helper, killer and regulatory. Helper T cells release molecules to activate other types of immune cells. Killer T cells attack infected or cancerous cells in the body. Regulatory T cells suppress the activity of other immune cells at the end of an immune response and prevent autoimmune diseases.
Cells, including the immune system cells, display a diverse range of receptors on their surfaces. These receptors are specialised proteins to which only certain molecules in the cell’s environment can bind. If an activating molecule binds, the receptor initiates a series of reactions inside the cell. This process allows membrane receptors to send signals about the external environment to the inside of the cell and can trigger an internal response. Killer T cells have specialised receptors that recognise specific antigens — small proteins on the surface of infected or cancerous cells. Generally speaking, each T cell is specialised to recognise only one antigen, but many T cells can recognise the same antigen. To recognise infected or cancerous cells, T cells must have a receptor bind to the specific antigen on this cell. Upon recognition, T cells release chemicals to kill the flagged cell.
When cancer cells start developing, T cells that recognise the antigens of this cancer type concentrate around the cancer cells and attack them. In some cases, cancer cells grow into lumps called tumours before the immune system can eliminate them. Even when infiltrated by immune cells, tumours can develop mechanisms such as inhibiting killer T cell activity or promoting regulatory T cell activity to evade the immune system. They can also spread from their initial location to different parts of the body through a process called metastasis.
In addition to those used by killer T cells to recognise tumour antigens, the study discussed in this article exploited two other receptors: Toll-like receptors (TLRs) and OX40 receptors. TLRs are naturally expressed on the surfaces of a variety of immune cells. When bacteria or viruses invade the body, TLRs recognise that the DNA in question is foreign and activate a range of immune cells. In this study, one of the substances injected into mouse tumours is a piece of DNA that mimics bacterial DNA and interacts with a TLR named TLR9. Then, TLR9 promotes the production of a receptor called OX40 on both killer and regulatory T cells. The second substance injected into the tumours is anti-OX40 antibody — a molecule that binds to OX40 receptors. By binding to their OX40 receptors, this molecule activates killer T cells and suppresses regulatory T cells, tipping the balance in favour of killer T cells. Since tumours are infiltrated by T cells that recognise the specific cancer type present, the injections activate the cells that have been ‘pre-screened’ by the body for their effectiveness.
Below is an animation and slide by slide graphic that summarizes the steps of the therapy.
1. Injection contains CpG which binds to TLR9 on some immune system cells. 2. Cells with bound TLR9 release chemicals to communicate with other immune system cells. 3. Killer and regulatory T cells sense these chemicals. 4. Killer and regulatory T cells produce more OX40 receptors. 5. Anti-OX40 antibody in injection binds to OX40 receptors on killer and regulatory T cells. 6. Having bound OX40 receptors activates killer T cells and suppresses regulatory T cells. 7. Activated killer T cells can now attack cancer cells!
The promising results
Tumours injected by two substances regressed in mice
B cell lymphoma tumours injected with two substances to activate TLR9 and OX40 receptors regressed in mice. The first substance was a CpG oligodeoxynucleotide (a special type of short DNA piece) called SD-101 that imitates bacterial DNA and binds to TLR9. The bound TLR9 then initiates an immune response producing high levels of OX40 on the surfaces of nearby T cells. The other substance in the injection was anti-OX40 antibody, a molecule that binds to OX40 receptors, and promotes killer T cells while suppressing regulatory T cells. Both of the injected substances are already being tested either on their own or in combination with other therapeutics in several clinical trials. This study shows that injecting a combination of the two substances into mouse tumours gives high rates of success not achieved with either substance alone. The treatment was also effective for breast, colon and skin cancers in mice.
Injection into one tumour also caused regression of another of the same type elsewhere in the mouse
The researchers also implanted B cell lymphoma tumours in two different parts in the body of each mouse and allowed them to grow. They then injected the two substances into only one of the tumours. Injection resulted in complete regression of not just the injected tumour, but also the non-injected one in a different part of the body. This effect was long-lasting and cured most of the mice.
They also tested the injections on tumours containing two different types of cancer cells by implanting three tumours in each mouse: one with B cell lymphoma cells, one with colon cancer cells, and one with both. When the tumour containing both cancers was injected, all three tumours regressed. The effectiveness of this therapy is promising as naturally occurring tumours can contain a mixture of cancer cells with different antigens.
Treatment protected mice against metastasis and cancer recurrence
Once mice genetically prone to developing breast cancer were treated with the injection, they did not develop future tumours. The treated mice also developed significantly fewer lung metastases.
Treatment holds promise for human cancers despite drawbacks
Addition of CpG also increased OX40 in T cells around human B cell lymphoma cells in the laboratory, so the researchers believe that the treatment will apply to human patients. For the treatment to work, the tumour must be accessible for injections, and enough immune cells must have infiltrated it. The researchers envision injecting the primary tumour before surgery in humans. This approach could prevent metastasis and recurrence of the cancer in patients with inherited genetic mutations such as those in BRCA genes which increase the risk of breast cancer. The treatment involves very low doses of the two substances and is regarded as safe. It is also practical because it does not require prior knowledge of the unique type of cancer; the injection activates the T cells in the tumour environment which already have the suitable receptors to deal with the specific cancer cells. A clinical trial will be conducted to test the efficacy of the treatment on non-Hodgkin lymphoma patients. If successful in humans, the treatment could serve as a rapid and inexpensive therapy with little side effect.
• Killer T cells: T cells that attack infected or cancerous cells in the body.
• Regulatory T cells: T cells that suppress killer T cell activity to prevent excessive immune response.
• OX40: A receptor that promotes killer T cells and suppresses regulatory T cells.
• TLR9: A receptor that recognises bacterial DNA and initiates a response across multiple immune cells, including promotion of OX40 receptors on T cells.
• CpG SD-101 (one of the two injected substances): A type of short DNA piece that imitates bacterial DNA to induce TLR9 activation.
• Anti-OX40 antibody (one of the two injected substances): A molecule that binds to and activates OX40 receptors.
• Antigen: Small proteins on the surfaces of infected or cancerous cells, used by immune cells to differentiate between harmful cells and body’s own healthy cells.
For more on immunology and immunotherapy:
BiteSized Immunology by British Society for Immunology https://www.immunology.org/public-information/bitesized-immunology
K. Murphy, C. Weaver. Janeway’s Immunobiology. 9th ed. New York: Garland Science, 2016.