Cancer
vaccines are intended either to treat existing
cancers (therapeutic vaccines) or to prevent the development of cancer (prophylactic vaccines). Both types of vaccines have the potential to reduce the burden of cancer. Treatment or therapeutic vaccines are administered to cancer patients and are designed to strengthen the body''s natural defenses against cancers that have already developed. These types of vaccines may prevent the further growth of existing cancers, prevent the recurrence of treated cancers, or eliminate cancer cells not killed by prior treatments. Prevention or prophylactic vaccines, on the other hand, are administered to healthy individuals and are designed to target cancer-causing viruses and prevent viral infection. At this time, two vaccines have been licensed by the U.S. Food and Drug Administration to prevent virus infections that can lead to cancer: the hepatitis B vaccine, which prevents infection with the hepatitis B virus, an infectious agent associated with some forms of liver cancer; and GardasilTM, which prevents infection with the two types of human papillomavirus (HPV) - HPV 16 and 18 -- that together cause 70 percent of cervical cancer cases worldwide. Gardasil also protects against infection with HPV types 6 and 11, which account for 90 percent of cases of genital warts.There are no licensed therapeutic vaccines to date. However, several treatment vaccines are in large-scale testing in humans. Vaccines used to treat cancers take advantage of the fact that certain
molecules on the surface of cancer cells are either unique or more abundant than those found on normal or non-cancerous cells. These molecules, either proteins or carbohydrates, act as
antigens, meaning that they can stimulate the
immune system to make a specific immune response. Researchers hope that when a vaccine containing
Cancer-specific antigens is injected into a patient, these antigens will stimulate the immune system to attack cancer cells without harming normal cells. The immune system generally doesn''t see tumors as dangerous or foreign, and doesn''t mount a strong attack against them. One reason
tumor molecules do not stimulate an effective immune response may be that tumor cells are derived from normal cells. Therefore, even though there are many molecular differences between normal cells and tumor cells, cancer antigens are not truly foreign to the body, but are normal molecules, either altered in subtle ways or more abundant.Another reason tumors may not stimulate an immune response is that cancer cells have developed ways to escape from the immune system. Scientists now understand some of these modes of escape, which include shedding tumor antigens, and reducing the number of molecules and receptors that the body normally relies on to activate T cells (specific immune cells) and other immune responses. Reducing these molecules makes the immune system less responsive to the cancer cells; the tumor becomes less visible to the immune cells. Scientists hope that this knowledge can be used by researchers to design more effective vaccines.
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