Cancer Vaccines

Vaccines boost the immune system’s natural ability to defend the body against infection and to protect it from dangers posed by certain types of damaged or abnormal cells, including cancer cells.

Some cancer vaccines, known as cancer preventive vaccines, are designed to prevent cancer from developing in healthy people. Other cancer vaccines, known as cancer treatment vaccines, are intended to treat cancers that have already occurred.

The U.S. Food and Drug Administration (FDA) has approved two types of cancer preventive vaccines: A vaccine against the hepatitis B virus, which can cause liver cancer in chronically infected people, and a vaccine against human papillomavirus types 16 and 18, which are responsible for about 70 percent of all cases of cervical cancer

Cancer treatment vaccines are designed to treat cancer by stimulating the immune system to recognize and attack cancer cells.

Effective cancer treatment vaccines are difficult to develop because some cancers can escape detection by the immune system or weaken natural immune responses against cancer cells

The side effects of cancer vaccines vary from patient to patient and according to the type of vaccine being used. Most of the side effects reported thus far have been mild and limited to inflammation at the site of the vaccine injection.

Vaccines are medicines that boost the immune system’s natural ability to protect the body against “foreign invaders” that may cause disease. These invaders are primarily microbes, which can be seen only under a microscope. Microbes include bacteria, viruses, parasites, and fungi.

The immune system is a complex network of organs, tissues, and specialized cells that act collectively to defend the body. When a particular type of microbe invades the body, the immune system recognizes it as foreign, destroys it, and “remembers” it to prevent another infection. Vaccines take advantage of this response.

Traditional vaccines usually contain harmless versions of microbes—killed or weakened microbes, or parts of microbes—that do not cause disease but are able to stimulate an immune response. When the immune system encounters these substances through vaccination, it responds to them, eliminates them from the body, and develops a memory of them. This vaccine-induced memory enables the immune system to act quickly to protect the body if it becomes infected by the same microbe in the future.

The immune system’s role in defending against disease-causing microbes has long been recognized. Scientists have also discovered that the immune system can protect the body against threats posed by certain types of damaged, diseased, or abnormal cells, including cancer cells.

Vaccines stimulate the immune system
White blood cells, or leukocytes, play the main role in immune responses. These cells carry out the many tasks required to protect the body against disease-causing microbes and abnormal cells.

Some types of leukocytes patrol the body, seeking foreign invaders and diseased, damaged, or dead cells. These white blood cells provide a general—or nonspecific—level of immune protection.

Other types of leukocytes, known as lymphocytes, provide targeted protection against specific threats, whether from a specific microbe or a diseased or abnormal cell. The most important groups of lymphocytes responsible for carrying out immune responses against such threats are B cells and cytotoxic (cell-killing) T cells.

B cells make antibodies, which are large proteins secreted by B cells that bind to, inactivate, and help destroy foreign invaders or abnormal cells. Most preventive vaccines, including those aimed at hepatitis B virus (HBV) and human papillomavirus (HPV), stimulate the production of antibodies that bind to specific, targeted microbes and block their ability to cause infection. Cytotoxic T cells, which are also known as killer T cells, kill infected or abnormal cells by releasing toxic chemicals or by prompting the cells to self-destruct (apoptosis).

Other types of lymphocytes and leukocytes play supporting roles to ensure that B cells and killer T cells do their jobs effectively. Cells that help fine-tune the activities of B cells and killer T cells include helper T cells and dendritic cells, which help activate killer T cells and enable them to recognize specific threats.

Cancer treatment vaccines work by activating B cells and killer T cells and directing them to recognize and act against specific types of cancer. They do this by introducing one or more molecules known as antigens into the body, usually by injection. An antigen is a substance that stimulates a specific immune response. An antigen can be a protein or another type of molecule found on the surface of or inside a cell.

Microbes carry antigens that “tell” the immune system they are foreign—or “non-self”—and, therefore, represent a potential threat that should be destroyed. In contrast, normal cells in the body have antigens that identify them as “self.” Self antigens tell the immune system that normal cells are not a threat and should be ignored.

Cancer cells can carry both types of antigens. They have self antigens, which they share in common with normal cells, but they may also have antigens that are unique to cancer cells. These cancer-associated antigens mark cancer cells as abnormal, or non-self, and can cause B cells and killer T cells to mount an attack against the cancer.

Cancer cells may also make much larger than normal amounts of certain self antigens. These overly abundant self antigens may be viewed by the immune system as being foreign and, therefore, may trigger an immune response against the cancer.

Cancer vaccines
Cancer vaccines are medicines that belong to a class of substances known as biological response modifiers. Biological response modifiers work by stimulating or restoring the immune system’s ability to fight infections and disease. There are two broad types of cancer vaccines:

Preventive (or prophylactic) vaccines, which are intended to prevent cancer from developing in healthy people; and

Treatment (or therapeutic) vaccines, which are intended to treat already existing cancers by strengthening the body's natural defenses against cancer.

Cancer preventive vaccines in Action 
Cancer preventive vaccines target infectious agents that cause or contribute to the development of cancer (8). They are similar to traditional vaccines, which help prevent infectious diseases such as measles or polio by protecting the body against infection. Both cancer preventive vaccines and traditional vaccines are based on antigens that are carried by the infectious agents and that are relatively easy for the immune system to recognize as foreign.

Other microbes associated with cancer
Microbes cause or contribute to between 15 percent and 25 percent of all cancers diagnosed worldwide each year, with the percentages being lower in developed countries than in developing countries (4, 8, 13, 14). The International Agency for Research on Cancer (IARC) has classified several microbes as carcinogenic (causing or contributing to the development of cancer in people), including HPV and HBV (15). These infectious agents—bacteria, viruses, and parasites—and the cancer types with which they are most strongly associated are listed in the table below.

Cancer treatment vaccines
Cancer treatment vaccines are designed to treat cancers that have already occurred. They are intended to delay or stop cancer cell growth; cause tumor shrinkage; prevent cancer from coming back; or eliminate cancer cells that are not killed by other forms of treatment, such as surgery, radiation therapy, or chemotherapy.

Developing effective cancer treatment vaccines requires a detailed understanding of how immune system cells and cancer cells interact. The immune system often does not “see” cancer cells as dangerous or foreign, as it generally does with microbes. Therefore, the immune system does not mount a strong attack against the cancer cells.

There are many reasons the immune system does not easily recognize the threat posed by an already growing cancer. Most important is the fact that cancer cells carry normal self antigens in addition to any cancer-associated antigens. Furthermore, cancer cells sometimes undergo genetic changes that lead to the loss of cancer-associated antigens. Finally, cancer cells can produce chemical messages that suppress specific anticancer immune responses by killer T cells. As a result, even when the immune system recognizes a growing cancer as a threat, the cancer may still escape a strong attack by the immune system.

Side effects seen in cancer vaccines
Vaccines intended to prevent or treat cancer appear to have safety profiles comparable to those of traditional vaccines . However, the side effects of cancer vaccines can vary widely from one vaccine formulation to another and from one person to another.

The most commonly reported side effect of cancer vaccines is inflammation at the site where the vaccine is injected into the body. Reported symptoms include redness, pain, swelling, heightened temperature (the skin surrounding the injection site feels hot to the touch), itchiness, and occasionally a rash.

People sometimes experience flulike symptoms after receiving a cancer vaccine, including fever, chills, weakness, dizziness, nausea or vomiting, muscle ache, fatigue, headache, and occasional breathing difficulties. Blood pressure may also be affected.

Other, more serious health problems have been reported in smaller numbers of people after receiving a cancer vaccine. These problems may or may not have been caused by the vaccine. The reported problems have included asthma, appendicitis, pelvic inflammatory disease, and certain autoimmune diseases, including arthritis and systemic lupus erythematosus.

Vaccines, like any other medication affecting the immune system, can cause adverse effects that may prove life threatening. For example, severe hypersensitivity (allergic) reactions to specific vaccine ingredients have occurred following vaccination. However, such severe reactions are quite rare.

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Source: NCI (National Cancer Institute)