Aluminum hydroxide has proven to be a safe adjuvant in a number of vaccines destined for humans and animals. It can act alone or synergistically with other adjuvants (co-adjuvants). Characteristic is the case of ginseng (the dry extract prepared from the Panax ginseng C.A. Meyer-root) known to contain ginseno-cides, which demonstrably exhibit adjuvant properties. In this sense, it has been shown that T-ginseng and Al(OH)3 act synergistically and improve potently antibody response to various immune antigens (e.g. porcine parvovirus-antigen).6 The mechanisms of interactions between Al(OH)3 and ginseng are not known. In general, it is thought that the existence of a co-adjuvant in an Al(OH)3-adjuvanted vaccine may imply interactions with Al(OH)3 at the site of injection and concomitant stimulation of antigen-presenting cells in a more efficient manner as a whole. Alternatively, the two adjuvants may stimulate a broader spectrum of immune cells than each adjuvant alone, thus providing for an enhanced antibody titer of the vaccine. It appears that co-adjuvants such as ginseng may (a) enhance the potency of the aluminum-adjuvanted vaccine in a simple, safe and cheap manner, (b) offer stability in the antibody titer induced by the vaccines, and (c) help reduce the total amount of aluminum for the establishment of specific bioactivity in the pharmaceutical preparation of the employed vaccines.
It is worth noting that Al(OH)3-adjuvanted vaccines induce higher levels of IgG1 antibodies than IgG2. Interestingly, the presence of ginseng as a co-adjuvant results in a shift of the IgG profile, favoring high levels of IgG2 over IgG1. Thus, a preferential induction of IgG factors through the concerted action of Al(OH)3 and ginseng over that observed for the Al(OH)3 alone indicates the importance of the co-adjuvant in the clinical profile of the vaccine. Active ingredients in ginseng that might contribute to the enhanced immuno-genic activity of the employed vaccines with two co-adjuvants include the potent saponins, shown to (a) promote killer-cell activity and interferon production and (b) stimulate the phagocytic activity of macrophages and poly-morphonuclear cells.
Significant insight into the interactions between the major molecular components of human immune vaccines and aluminum-containing adjuvants was provided in the case of the recombinant anthrax vaccine containing Al(OH)3 as an adjuvant.7 The study focused on the major component of the second-generation vaccine, the recombinant protective antigen (rPA) protein, and compared the effect of the presence of Al(OH)3 (zero charge point (zcp) = 11.5) versus that of AlPO4 (zcp = 5.4). The rPA binds rapidly and efficiently to Al(OH)3 under physiological conditions, in contrast to poor binding of the same molecule to AlPO4. Taking into consideration the pI of rPA (5.6 at pH = 7.4) and the zcp of the employed adjuvants, it was shown how important the surface charge is in the adsorption of rPA to aluminum-containing adjuvants. It is noted that the effectiveness of the adjuvant in a vaccine, preparation is dependent on the complete adsorption of the antigen on the adjuvant surface. To that end, the type of interactions between the primary molecular target and the aluminum adjuvant is a determining factor of the overall biological activity of the vaccine. These interactions lead to the formation of complexes, with electrostatics often dictating the nature of the molecular assembly in such complexes. The significance of electrostatic interactions has previously been studied3,8 and reported using model proteins, thus lending credence to the observations made on the recombinant vaccine protein. It appears that as the pI/zcp of the aluminum adjuvant approaches the corresponding pI of the protein (rPA), the decrease of the charge difference between the two species in the assembled complex is reduced with a concomitant release of the antigen in the vaccine preparation. In this sense, phosphate ions contribute to the desorption of the antigen from the aluminum adjuvant, resulting in an antigen comparable in biological activity to unadsorbed control material.
Past studies have shown that addition of phosphate anion to an antigen-aluminum-hydroxide adjuvant complex lowers significantly the zcp of the adjuvant.
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