Vaccine adjuvants and Freund.
Vaccination is currently the best method of preventing infectious disease. A vaccine elicits a protective immune response to the desired virus or bacteria, so that subsequent exposure to the pathogen prevents disease. Probably the most successful vaccination effort was the global eradication of smallpox, while polio virus may soon be defeated through the recent efforts of the World Health Organisation. For a vaccine to be safe for human use it must not be infectious itself. Therefore, most vaccines are produced by killing the desired pathogen or by attenuating its virulence. In so doing, although the vaccine is rendered safe it becomes less immunogenic; meaning that it becomes poor at stimulating an immune response. To counteract this effect vaccines are delivered with an adjuvant, which enhances the immune response to the vaccine. The greatest pioneer of adjuvant design was the Hungarian born Jules Freund, who provided extensive insights into the best methods for vaccine delivery.
In 1942 Freund and Katherine McDermott discovered that paraffin oil greatly boosted antibody production to a co-administered antigen. For their adjuvant they used a preparation of heat-killed Mycobacterium bacilli suspended in paraffin oil, which was co-administered with horse serum. Guinea pigs were injected with this mixture and the subsequent antibody response was shown to be significantly enhanced. Although paraffin oil by itself still improved antibody production, the inclusion of killed Mycobacterial bacilli further boosted the antibody titre and was essential for hypersensitivity reactions to the horse serum. The adjuvant preparation was the forerunner of what is now known as Freund’s complete adjuvant (FCA), which is the adjuvant of choice for the experimental production of antibodies. FCA is a slightly modified version of the original preparation, as it includes mannide monooleate, which acts as an emulsifier, paraffin oil and killed Mycobacterium tuberculosis (the infectious agent of tuberculosis). However, due to certain side effects such as granuloma/lesions at the site of injection and a high level of toxicity, FCA is not a safe adjuvant for human use.
How does FCA enhance antibody production? It is thought that the paraffin oil in FCA acts as a depot for the co-delivered antigen. This depot effect extends the longevity of the antigen, reduces degradation and therefore gives the immune system more time to react. Paraffin oil is also likely to enhance the uptake of antigen by specialised cells such as macrophages, which in turn provide stimulatory signals for other immune cells. The Mycobacterial component is comprised of dead bacteria, which will consist of lipopolysaccharide (LPS), lipoproteins and bacterial DNA. These molecules are known to enhance immunity by directly interacting with various cells of the immune system. For example, LPS is the molecule responsible for endotoxic shock and is therefore a powerful immune stimulator. These multiple effects act to accentuate the immune response to the co-delivered antigen, one outcome being elevated antibody production.
Most vaccines generate good antibody responses, due mainly for historical reasons as antibody production was the only reliable immunological endpoint. As a result the most widely used human adjuvant is aluminium sulphate (alum) because it enhances antibody production. Other adjuvants are now on the market that not only stimulate antibody production but other aspects of immunity as well, including cell-mediated immunity. Freund’s work, which began in the 1940’s, provided the basis for the successful delivery of many vaccines and hence the prevention of a myriad infectious diseases.