Cancer immunotherapy is a hot topic in the news because of seemingly miraculous results in some patients with advanced cancers. Headlines tout the stories of people whose tumors shrank almost to nothing after immunotherapy, even after they were told they had “no hope” with conventional surgery, chemotherapy, or radiation. So what is cancer immunotherapy? How does it work? What types of cancers does it treat?
Cancer and the Human Immune System
Your body is made up of many organs and tissues, each of which consists of thousands of tiny cells. Cells contain DNA, which tells them how to grow, when to make new copies of themselves, and when to die to make room for new cells. Sometimes the DNA inside certain cells becomes damaged, by chemicals, radiation, viruses, or for reasons scientists do not yet understand. A cell with damaged DNA may begin to grow out of control. It may produce many copies of itself. It may not die to make room for new cells. Such a cell has become a cancer cell. It was once a normal body cell, but now it has become abnormal and cancerous because of damaged DNA.
Sometimes, the human immune system can recognize a cancer cell as abnormal, and destroy it. But this does not always happen. Cancer cells are not foreign invaders to the body, like viruses or bacteria. Cancer cells often look just like normal body cells to the immune system. By releasing certain substances, cancer cells can trick the immune system into passing them by. Other times, even if the immune system recognizes cancer cells as abnormal, it may not respond strongly enough to kill them. The immune system may respond to cancer cells only weakly because it must protect surrounding normal cells.
What is Immunotherapy?
Immunotherapy is a type of cancer treatment that helps the patient’s own immune system fight cancer more effectively. It is different than chemotherapy. Chemotherapy drugs kill cells that are forming new copies of themselves very quickly. Cancer cells usually form new copies of themselves much more quickly than normal cells, so chemotherapy drugs are meant to kill these fast-copiers. Unfortunately, some normal cells that copy themselves quickly (such as hair cells, or cells in the intestines) are damaged along with the cancer cells, and this causes chemotherapy side effects.
Immunotherapy works differently. Immunotherapy does not directly attack or kill cancer cells. It is designed to “jump start” the patient’s own immune system so that it can kill the cancer cells.
Different Types of Cancer Immunotherapy
There are five different categories of cancer immunotherapy in use to treat cancer today: immune checkpoint modulators, therapeutic antibodies, therapeutic vaccines, non-specific immunotherapies, and therapeutic viruses.
Immune checkpoint modulators: The immune system must strike a careful balance when it protects the body. It needs to react strongly enough to kill cells that have become abnormal. However, it must not react so strongly that it kills healthy, normal body cells. The immune system often allows cells that seem to be a little bit abnormal to continue to live. The cells of the immune system do this by scanning the surface of other cells they come into contact with for certain molecules. These molecules are known as “immune checkpoints”. When the immune cells see the immune checkpoint molecules, they will not attack. Cancer cells can trick immune cells by showing an immune checkpoint pattern on their surface that tells immune cells not to attack.
Immune checkpoint modulators are drugs that change the way immune cells react to immune checkpoint patterns. These drugs can make the immune system more likely to attack other cells that only seem a little bit abnormal, rather than attacking only those cells that are very abnormal. In this way, the immune system is more likely to attack and kill cancer cells even when those cancer cells are mimicking normal cells.
Cancer immunotherapy is being used today to treat some types of melanoma and lung cancers. It is being studied in some patients with other cancers as well.
Therapeutic antibodies: Antibodies are small proteins made by the immune system that circulate in the blood. Antibodies in the blood attach to foreign invaders (like viruses and bacteria) or abnormal cells (like cancer cells). Once attached, the antibodies prompt the immune system to kill the invader or the abnormal cell.
Therapeutic antibodies are antibodies made in the laboratory that are designed to recognize and attach to cancer cells. Sometimes, coating a cancer cell in therapeutic antibodies is enough to trigger the patient’s immune system to destroy it. In other cases, a therapeutic antibody is connected to a molecule of a drug that can kill cancer cells. When the antibody attaches to a cancer cell, the drug is carried alongside, killing the cell. These are known as antibody–drug conjugates. A third type of therapeutic antibody is actually made of two antibodies linked together. One antibody attaches to a cancer cell. The other antibody attaches to an immune cell. In this way, the cancer cell and immune cell are pulled close together, which often triggers the immune cell to destroy the cancer cell.
Currently, therapeutic antibodies are used to treat some types of breast cancer, stomach cancer, lymphoma, and leukemia. Clinical trials are underway to test therapeutic antibodies in other types of cancers.
Therapeutic vaccines: When most people think of vaccines, they think of injections that are given to prevent a bacterial or viral infection, such as whooping cough or influenza. These types of vaccines are made of parts of bacteria or viruses, or sometimes dead bacteria or viruses. The immune system is exposed to these substances and is triggered to attack them. If the vaccinated person is later exposed again to the live version of those viruses or bacteria, the immune system is already “primed” to attack. The immune system attacks very quickly. Therefore, the disease usually does not take hold and make the person sick.
Therapeutic vaccines for cancer work a little differently. They are designed to prompt an immune response to cancer cells that are already in the body. Some therapeutic vaccines are made of weakened cancer cells or parts of cancer cells. These are injected into the patient’s body to provoke the immune system to respond. Other times, some of the patient’s immune cells are removed and grown in the laboratory. After they have grown, they are exposed to therapeutic vaccines to prompt them to recognize and attack cancer cells. When they are “primed” to attack cancer cells, they are put back into the patient’s body.
The only therapeutic vaccine in use for cancer currently is used to treat advanced prostate cancer. Other therapeutic vaccines are in clinical trials for various types of cancers.
Immune modulator therapy: The immune system is complex. There are many different types of immune cells that work together to keep the body safe from invaders and to kill abnormal cells. Immune cells communicate with each other using small proteins that can be released into the bloodstream. These proteins are known as immune modulators. The body normally makes its own immune modulators, but scientists can also make them in the laboratory. These proteins can be given to patients with cancer. Although immune modulators do not target cancer cells specifically, they can be used to increase the activity of the immune system as a whole. A more active immune system is more likely to attack and kill cancer cells. Currently, immunomodulatory therapy is being used to treat kidney cancer, bladder cancer, melanoma, lymphoma, leukemia, and multiple myeloma.
Therapeutic viruses: In this relatively new technique, scientists take a virus that normally infects human cells and alter it to infect cancer cells instead of healthy cells. Once cancer cells are infected with this man-made virus, they are more likely to be killed by the immune system. The only cancer that therapeutic viruses are used to treat today is melanoma. The virus that is used is a modified version of the herpes simplex virus that causes cold sores and fever blisters.
The Future of Cancer Immunotherapy
There are many exciting developments happening in cancer immunotherapy. Many clinical trials are underway to test new techniques. One of the most exciting new techniques is based on taking immune cells from the patient’s own body, and then treating them in the laboratory to make them more reactive against cancer cells. These highly active cells are then are injected back into the patient, where it is hoped that they will seek out and kill cancer cells. In addition to studying new techniques for immunotherapy, further work is being done to improve upon already existing immunotherapies and to test them in different types of cancer.
American Cancer Society. Cancer Immunotherapy
Cancer.Net. Understanding Immunotherapy
National Cancer Institute. Immunotherapy: Using the Immune System to Treat Cancer.