CancerQuest | Antigen Based Vaccines

Antigen based tumor vaccines do not involve the insertion of modified tumor cell or immune cell into the body. Instead, purified tumor proteins (antigens) are injected to stimulate the patient's antigen presenting cells (APCs) to take up the antigen and present it to T cells. The challenge to this type of therapy is finding a tumor-specific antigen for any particular type of cancer. It is important that the vaccine therapy does not stimulate an immune response against antigens found extensively on normal body cells, or else the T cells would destroy normal cells as well as cancer cells. Researchers have had some success in identifying tumor-specific antigens for melanomas, but antigens for other cancer types have proven to be more difficult to pinpoint. These tumor vaccine strategies are only effective in cancer types that have well defined tumor antigens.⁽¹⁾

Antigen therapy vaccines may be grouped to four general categories:

Peptide-based vaccines- Peptide-based vaccines entail the injection of purified tumor proteins into the patient. These tumor-derived protein fragments have modified segments that make them easily presented by APCs. Since the peptides are tumor-specific, they generate a highly tumor-specific immune response. The challenge is, once again, identifying a tumor-specific protein for a particular cancer type. Due to differences in our genes, the way that the immune systems of different people 'see' antigens is not necessarily the same. That means that a peptide which is very effective in generating an immune response in one individual may not work for someone else. The advantage of this strategy, however, is that these vaccines are relatively easy to manufacture and store.
Heat shock protein vaccines- Heat shock proteins are a group of proteins that are normally found in cells. They function to help proteins fold up into their correct shape and prevent misfolding of proteins in times of stress. The proteins actually get their name from the fact that they are found at increased levels in cells that are kept at a higher than normal temperature (hence-, heat shock). High temperature causes proteins to misfold, much like the very high heat of a frying pan causes the clear portion of an egg to turn white and harden. In addition to this role, heat shock proteins can act as carriers for tumor protein antigens. Highly specialized APCs known as dendritic cels have special receptors on their surface for heat shock proteins (so that they can identify sick cells). If a combination of tumor protein and heat shock protein is injected into a patient, dendritic cells will bind to the heat shock portion of the joined proteins and then take up and present the tumor proteins to T cells.
DNA vaccines- DNA vaccines involve the injection of DNA containing the genes for tumor-specific proteins. The DNA is injected into a patient's muscle. The hope is that normal body cells will take up the DNA and produce a tumor protein from it via transcription and translation. The tumor protein will be taken up by APCs, which will then stimulate tumor-specific T cells. More on transcription and translation
Viral and bacterial vector vaccines- In this strategy, instead of directly injecting naked DNA into a patient, the DNA will be transported into the body in a viral or bacterial 'vector'. Vectors are organisms, like bacteria or viruses, or DNA constructs that are able to take genetic information from one organism and put it into another. (A more common use of the word vector is in the spread of disease; birds are a vector for bird-flu and ticks are a vector for lyme disease). Viral or bacterial vectors can also be used as a second injection after an initial injection of a DNA vaccine. The second dose increased the immune response and may be more effective than either strategy alone.⁽¹⁾

No matter which strategy from the above list is used, the general strategy is the same: Expose the patient's immune system to tumor proteins so that they will be able to better respond to those same proteins on the surface of the tumor cells themselves. The differences lie only in HOW the the target proteins are delivered to the immune system for recognition.