Types of Immunogen Carriers

Synthetic haptens mimicking some critical epitopic structures on larger macromolecules are often conjugated to carriers to create an immune response to the larger parent molecule. For instance, short peptide segments can be synthesized from the known sequence of a viral coat protein and coupled to a carrier to induce immunogenicity toward the native virus. This type of synthetic approach to immunogen production has become the basis of much of the current research into the creation of vaccines. 

Some synthetic carriers actually are designed to have low immunogenicity on their own to minimize the potential for antibody production against them. When a hapten is coupled to these molecules, the immune response is directed principally toward the modification, not at the carrier. This design approach guides most of the immune response toward the desired target and minimizes the production of carrier-specific antibodies. 

The most common carrier proteins in use today are keyhole limpet hemocyanin (KLH MW 4.5 X 105 to 1.3 X 107), BSA (MW 67,000), aminoethylated (or cationized) BSA (cBSA), thyroglobulin (MW 660,000), ovalbumin (OVA MW 43,000), and various toxoid proteins, including tetanus toxoid and diphtheria toxoid. Other proteins occasionally used include myoglobin, rabbit serum albumin, immunoglobulin molecules (particularly IgG) from bovine or mouse sera, tuberculin purified protein derivative, and synthetic polypeptides such as poly-L-lysine and poly-L-glutamic acid. 

Native, multi-subunit KLH also should not be frozen. Freeze-thaw effects cause extensive denaturation and result in considerable amounts of insoluble material. Commercial preparations of native KLH are typically freeze-dried solids that no longer fully dissolve in aqueous buffers and do not display the protein s typical blue color due to loss of chelated copper. The partial denatured state of these products often makes conjugation reactions difficult. 

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Keyhole limpet hemocyanin (KLH) is an immune stimulant and a hapten carrier, derived from a circulating glycoprotein of the marine mollusk Megathura crenulata. KLH has significant antiproliferative effects in vitro against several types of cancers (23). It has also been conjugated to a variety of immunogens and used as a vaccine adjuvant (24). 

Simply put, antigens are minute substances that evoke an immune response and are generally classihed as immunogens, tolerogens and allergens, each of which stimulates antibody production. While antigens are usually proteins or polysaccharides, they can be any type of molecule, including haptens (small molecules) covalently bound to carrier proteins (which facilitate transport in blood and into tissues or cells). 

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