Peptides are short protein sequences. Infectious or disease-causing organisms use protein for structure or for virulence. Peptide vaccines use only part of an organism to spark an immune response. This type of vaccine has applications in immunization against infectious organisms, allergens or even tumors.
The immune system generally recognizes the proteins that are associated with a specific disease-causing organism to target it for destruction. These recognizable proteins are called antigens. Traditional vaccines use live or killed organisms to spark the immune system response so the body can recognize the foreign substance in the future.
Peptides are made up of a sequence of amino acids, which are the building blocks of proteins. A scientist first identifies parts of an organism, such as influenza, that evoke the immune system, and then he or she figures out the sequence of the antigen. Then the scientist can build an identical peptide to the section of that antigen that evokes the best immune response.
Vaccines evoke an immune response when the body's immune system cells bind and react to them. Peptide vaccines do not stimulate these cells in exactly the same way as a traditional vaccine. For example, a peptide vaccine does not cause a thymus cell, or T-cell, to react as much as other vaccines. To combat this, the peptide vaccine can be bound to a carrier protein to improve cell interaction. A scientist can look at the way receptors in cells attach to the antigen and synthetically create a collection of peptides with slightly different sequences to bind to as many cells as possible, increasing the strength of the immune response.
Peptide vaccines have several advantages over traditional vaccines. The vaccine is only part of an infectious organism, so there is no risk from other virulence factors and reactions to other parts of the organism. Peptides are easily and cheaply produced synthetically, and they do not break down easily. A problem with peptide vaccines is that sometimes the antigenicity of the targeted organism is down to the three-dimensional structure of the antigen, which is difficult to replicate in the laboratory.
Many infectious organisms, such as influenza, are highly variable, so a traditional vaccine might need to be changed regularly to combat the current strain. A peptide vaccine can be made from stable areas of a hypervariable virus to have an effect on a variety of mutated strains. Peptide vaccines also can contain an array of antigenic peptides to cover the widest range of antigens possible.
A peptide vaccine also has potential application in regulating an immune response to allergens and autoimmune diseases in which the body mistakenly attacks its own cells. Peptide vaccines for human immunodeficiency virus (HIV) also have been studied. This type of vaccine might also have application in cancer treatment, because the peptides could be engineered to enter tumor cells so that the immune system will recognize them and destroy them.