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Can HPV-associated cancers be treated by a vaccine? Researchers hope to provide this new treatment soon.
What is HPV?
HPV – human papillomavirus – is a group of viruses that commonly infect men and women. About 75% of men and women will have at least one HPV infection in their lifetime.
HPV infection can lead to many cancers. HPV causes nearly all cases of cervical cancer, as well as 5 other types of cancer.
HPV vaccination programs have been very successful in preventing HPV infections and associated disease in Canada, but HPV cancers are a problem worldwide. Developing countries have many more cases of HPV-associated cancers because they have limited access to screening programs or vaccines, and treatments for existing HPV infections and diseases are urgently needed.
What do HPV vaccines do?
The current HPV vaccines are called prophylactic vaccines. They prevent healthy people from getting HPV infections and prevent previously infected people from getting re-infected.
But these vaccines are not able to clear existing HPV infections in people or treat any tissue changes caused by persistent HPV infection. These tissue changes are the first thing we see before cancer develops.
A different type of vaccine, called a therapeutic vaccine, is being tested to treat existing HPV infections and associated diseases.
How do HPV vaccines work?
The prophylactic HPV vaccine prevents infection. It causes the immune system to create antibodies that neutralize HPV viral particles in the body before they can cause infection.
Therapeutic vaccines target cells and tissues that are already infected. These vaccines train the immune system to find and kill cells infected with HPV.
We need different vaccines to prevent and treat HPV infections because the virus is different before and after it has infected a cell. When a virus infects a cell and is not cleared by the immune system, the virus integrates its DNA into the host cell’s DNA. Some of the genes the virus no longer needs are deleted during this process.
Unfortunately, these deleted genes create the markers that the vaccine targets to prevent infection, making prophylactic vaccines useless in treating HPV-associated cancers and diseases.
How are researchers improving HPV vaccines for cancer treatment?
When some of these genes are deleted, there are changes in other genes that are expressed, or turned on. Researchers think increased expression of other HPV genes, called E6 and E7, contributes to cancer development. They have shown that HPV-associated cancers need these gene products, called oncoproteins E6 and E7, to grow.
Researchers are developing therapeutic HPV vaccines to target the E6 and E7 gene products. These gene products are specific to the virus, so they are “foreign” to our immune system and can be more easily recognized and destroyed.
Researchers have developed several types of therapeutic vaccines that are being tested in preclinical and clinical trials. Researchers are trying to prove in these clinical trials whether a therapeutic HPV vaccine is able to control HPV infections in humans. They will need to do more clinical studies to further prove how effective these therapeutic HPV vaccines are in treating cancer.
Therapeutic HPV vaccines, with continued research efforts, could be clinically available in the near future and could be used along with other available therapies to treat a variety of HPV-associated cancers.
Katherine Wright, PhD