Aluminum compounds as vaccine adjuvants

Aluminum compounds are the only adjuvants used widely with routine human vaccines and are the most common adjuvants in veterinary vaccines also. Though there has been a search for alternate adjuvants, aluminum adjuvants will continue to be used for many years due to their good track record of safety, low cost and adjuvanticity with a variety of antigens. For infections that can be prevented by induction of serum antibodies, aluminum adjuvants formulated under optimal conditions are the adjuvants of choice. It is important to select carefully the type of aluminum adjuvant and optimize the conditions of adsorption for every antigen since this process is dependent upon the physico-chemical characteristics of both the antigens and aluminum adjuvants. Adsorption of antigens onto aluminum compounds depends heavily on electrostatic forces between adjuvant and antigen. Two commonly used aluminum adjuvants, aluminum hydroxide and aluminum phosphate have opposite charge at a neutral pH. The mechanism of adjuvanticity of aluminum compounds includes formation of a depot; efficient uptake of aluminum adsorbed antigen particles by antigen presenting cells due their particulate nature and optimal size (<10 µm); and stimulation of immune competent cells of the body through activation of complement, induction of eosinophilia and activation of macrophages. Limitations of aluminum adjuvants include local reactions, augmentation of IgE antibody responses, ineffectiveness for some antigens and inability to augment cell-mediated immune responses, especially cytotoxic T-cell responses.