Vaccine Therapy for Cancer
How does the immune system work?
The immune system helps keep our bodies healthy. It attacks foreign invaders. The immune system fights against things like bacteria, viruses, parasites, and fungus that can enter your body. The immune system also fights against cells already in the body that have changed due to illness, like when there is cancer in the body. The immune system is made up of organs, cells, and proteins.
White blood cells, also called leukocytes, look for these foreign invaders and destroy them. There are two types of leukocytes:
- B lymphocytes: Find the pathogens. B lymphocytes also make antibodies. Antibodies are proteins in the body that fight infections that your body has already been exposed to.
- T lymphocytes: Destroy the pathogens.
What is a cancer vaccine?
Most of us have gotten vaccines for infectious diseases, like measles and hepatitis. These vaccines use weak or dead forms of viruses, bacteria, or other germs to start an immune response in the body. Cancer vaccines teach the immune system to find, attack and destroy cancer cells.
When foreign cells enter the body (like with an infection), the immune system responds and clears the body of the foreign cells. However, cancer cells are not seen as foreign by the body. The immune system thinks the cancer cells are part of the normal body and do not start an immune response against the cancer. Cancer vaccines help the immune system recognize cancer cells as foreign. Your immune system can then attack the cancer cells. Cancer vaccines are one way to use the body’s immune system to fight cancer.
How do non-cancer vaccines work?
Vaccines are often used to prevent infections. By putting a vaccine in your body before an infection actually happens, the immune system is "primed" to recognize future infections. Your body has a “memory” of fighting the same infection from the vaccine. Your body makes antibodies that are specific for the vaccine. Your body will then respond quickly to potential infections by the virus or bacteria that the vaccine was made to target. In this way, actual infections can be quickly recognized by the immune system before a serious infection can happen.
How do cancer vaccines work?
A cancer vaccine has cancer cells, parts of cells, or pure antigens in them. The vaccine helps with the immune response against cancer cells that are already in the body. This is different from other vaccines that are used to prevent infection. Cancer vaccines help the immune system recognize tumor cells as foreign invaders so that they may be destroyed.
Tumor cells often have distinct antigens (called tumor-associated antigens, or TAAs). Most TAAs are also found in normal cells. Because the immune system sees these antigens as “normal,” no immune response is started. If the immune system can be taught to see the TAAs as foreign, an immune response can be started against the tumor. Several TAAs have been found that are in some types of cancers, but not in normal cells. By targeting these TAAs with cancer vaccines, cancer vaccines can cause the immune system to attack cancer cells while leaving normal, healthy cells mostly safe. Currently, cancer vaccines targeting cancers of the breast, prostate, liver, kidney, pancreas, lung, melanoma, and certain types of leukemias and lymphomas are in clinical trials.
Are there any vaccines that prevent cancer?
Some cancers are known to be caused by viruses.
- Infection with human papilloma virus (HPV) can be a cause of cervical cancer, as well as some head and neck, penile, anal, vulvar, and vaginal cancers.
- Hepatitis B and C viruses are known to cause liver cancer.
Vaccines that prevent infection from these viruses would help prevent these cancers. While these vaccines may prevent cancer, these are not cancer vaccines. These vaccines are directed against viruses, rather than cancer itself.
Do cancer vaccines work?
There have been many clinical trials testing cancer vaccines. There is one cancer vaccine found to improve overall survival. Sipuleucel-T (Provenge®) is approved for use in some men with metastatic prostate cancer. It starts an immune response to prostatic acid phosphatase (PAP), an antigen present on most prostate cancers. This vaccine is customized for each patient. It is not known exactly how sipuleucel-T works, but it is likely that the APCs that have taken up PAP-GM-CSF help your T cells to kill tumor cells that express PAP.
Another viral therapy approved for use is called talimogene laherparepvec and it is used to treat melanoma that can't be removed with surgery. The drug is a weakened form of Herpes Simplex Virus Type 1. The medication is injected directly into the melanoma tumor on the skin or in a lymph node. This makes the cancer cells burst and die.
A viral therapy called Bacillus Calmette-Guerin (BCG) is a weakened strain of Mycobacterium bovis. It works against cancer as a biologic response modifier. This means that it can trigger the immune system to attack tumors. BCG is thought to work by starting an immune response and causing inflammation of the bladder wall that destroys cancer cells within the bladder.
Cancer vaccines remain an important and growing area of cancer research. Many trials are being done to test these vaccines. A few key questions and areas of research remain. We are still trying to research cancer vaccines so that one day they may be used in the routine care of cancer patients. The possibility of using the body's own immune system to destroy cancer cells remains an appealing possibility and results of early trials are promising. Research into new ways of treating cancer, such as cancer vaccines, is an important part of the search for a cure.
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