The immune system protects our body from infection. Everyone's had a cold – which are caused by viruses – and most of us have had an infection for which we took antibiotics. You never get the same cold twice because your immune system remembers how it fought off the virus the last time around – this is how vaccines work, too. Most vaccines give you a weak form of a virus or parts of the virus to teach your body to recognize and fight an infection before it becomes a problem. This is called immunity.
Cancer Vaccines and Immunotherapy 101
The Immune System Basics
Our immune system protects our body from infection, and can already recognize some cancers. It's composed of specialized cells that recognize any threatening attack from foreign invaders such as bacteria or viruses. Some hereditary immune deficiencies and certain medical conditions can weaken a normal immune system, leading to an increased risk of certain infections. HIV (AIDS) is an example of this – it attacks the immune system. Typically, when someone becomes sick or dies with AIDS, it's the result of an infection that their immune system was unable to control. Also, certain medications (such as those used following an organ transplant) may cause chronic suppression of the immune system, increasing the risk of infection.
There are two main types of immunity: innate, and adaptive. Innate immunity is the general protection we get from having intact skin and gut lining, as well as the protection provided by certain white blood cells called "phagocytes" that destroy bacterial microbes in a non-specific fashion.
Adaptive immunity, provides more specific immunity against foreign invaders. It's very specific for distinct molecules and is able to generate memory for those molecules. The effector cells of the adaptive immune response are called lymphocytes.
Lymphocytes provide important protection against potentially pathogenic organisms. There are two types of lymphocytes: B lymphocytes (B cells) and T lymphocytes (T cells). The B cells are responsible for antibody production. Antibody molecules may bind to antigens and cause destruction of bacteria, fungus, and viruses.
T cells work in different ways. There are two types of T cells: helper T cells and cytolytic (killer) T cells (also known as CTL). Helper T cells release cytokines when they confront fragments of immunogenic proteins. These cytokines naturally help boost the immune response.
T cells help B cells make antibodies, or they can directly kill cells that they recognize to be infected. Infected cells may be identified by the immune system if they contain proteins that can act as antigens. Antigens are foreign substances that induce specific immune responses.
Proteins are important building blocks in all living cells and may sometimes act as antigens. Cells contain many types of proteins, though not all proteins are recognized by the immune system. Each encounter with a foreign antigen stimulates increasingly effective defense mechanisms called "memory immunity" and is the basis for booster immunization.
The way that T cells recognize and process antigen usually depends on a person's "human leukocyte antigen" (HLA) type. HLA proteins are present on the surface of most cells of the body and serve as identifiers that help lymphocytes distinguish between normal and foreign cells. HLA type can be determined through a blood test and is similar to the blood type testing that a person might have before receiving a blood transfusion.
The Immune System Recognizes Cancer
Although much has been learned about interactions between the immune system and cancer, many questions are still unanswered. We know that in order to fight off colds or flu, our immune system must recognize the foreign proteins (antigens) produced by these infections. The same is true for cancer. Some cancers are capable of inducing a measurable immune response, but this response may not be sufficient to prevent tumor growth. Cancers may grow in spite of a normal immune system for several reasons. One potential reason is that cancers are not very immunogenic, meaning they don’t stimulate immune system recognition despite being composed of abnormal cells. Cancers may also suppress the immune system directly.
We do have clues that vigorous immune responses may be protective against some types of cancer. Evidence comes from studies in both mice and humans using different immune-stimulating chemicals. For example, if antibodies are made to bind to cancer antigens, the tumor may become more recognizable for immune destruction. An example of this is the monoclonal antibody Herceptin(r) (trastuzumab). This is an antibody against HER-2/neu, an antigen present in overabundance in some breast cancers and other tumors. Trastuzumab has been found to be an effective treatment for this kind of breast cancer when used either with chemotherapy or alone. There are several ways in which trastuzumab is thought to work, one of which is by increasing immune recognition. Similarly, T cells can directly recognize an antigen on a tumor cell and may assist in destruction of that cell.
We also know that tumor antigens exist. Some cancer cells make abnormal or excessive amounts of protein that can serve as a tumor antigen. One example of tumor antigen used in our studies is HER-2/neu (also known as c-erbB-2).
Immune-Based Treatments for Cancer
There are few cancer treatments that effectively use our immune systems. Cancer is very clever at evading immune recognition and it is difficult for the body to mount an immune response to cancer. Researchers at many institutions continue to work on designing immune-based treatments for cancer in an effort to uncover its vulnerable side.
Cancer immunotherapy is a type of treatment designed to use immune system cells to fight cancer. Immunotherapy can be used alone or in conjunction with other conventional treatments such as surgery, chemotherapy, and radiation. Conventional treatments have had substantial impact in the war on cancer. Scientists searching to improve on these gains have begun focusing on the immune system for other useful ways to treat cancer. Using the immune system’s cells to fight cancer is mostly experimental. However, there are a few immune-based therapies that are "standard of care" in the treatment of cancer such as using antibodies to fight breast cancer and lymphoma. Those drugs are called Herceptin(r) and Rituxan(r), respectively.
Immune-Based Treatments for Cancer: Cytokine Therapy
Cytokines are proteins that are produced naturally in our bodies. In general, cytokines help our immune system operate more effectively. One of the most studied cytokines, for example, is interleukin-2 (IL-2). IL-2 helps stimulate the immune system by increasing the number of lymphocytes and activating them to make them more effective. Groups worldwide are investigating the use of different cytokines as cancer therapies. Cytokines are also being studied as agents to assist vaccines in being recognized by the immune system (vaccine adjuvants).