Adjuvants

The term adjuvant designates substances that enhance the immune response without affecting the specificity of recognition. “Adjuvanticity” was first described in the 1920 by Ramon, who made the observation that mineral substances (such as metal salts and aluminum) or crude materials (such as tapioca) considerably augment the immune response to various vaccines. He then invented the name of “substances stimulantes et adjuvantes de l'immunité.” When an immune response is monitored by the kinetics of occurrence of circulating antibodies, it can easily be shown that, in the presence of adjuvants, the antibody titer is considerably higher and is maintained for a much longer period of time. Although first discovered over 70 years ago, very little progress has been made in this area, and the mode of action of adjuvants remains somewhat elusive. It is generally accepted that they act essentially in two ways: (1) retain the antigen in emulsion or aggregates, depending on the nature of the adjuvant, ensuring a slow but relatively constant release of antigen that may continuously restimulate the immune system; and (2) behave as a nonspecific activator of some partners of the immune response, like mobilizing macrophages that are acting as antigen-presenting cells or by exerting a polyclonal activation of lymphocytes, which is the case for the bacterial lipopolysaccharide (LPS), a potent polyclonal activator of B cells.

To date, the best and universally used substance is the so-called Freund's complete adjuvant, which is an emulsion prepared with a suspension of killed mycobacteria in mineral oil. Unfortunately, it cannot be used for human purposes and must be strictly confined to laboratory animals. Attempts have been made to isolate active molecules from mycobacteria (and also from many other microorganisms). This resulted in a long list of molecules, from which the muramyl dipeptide (N-acetylmuramyl-L-alanyl-D-isoglutamine, or MDP) was the most extensively studied. Endowed with good adjuvant properties, it is still too toxic for human use; and many attempts have been made, and are still being made, to define nontoxic homologues. For vaccination purposes in humans thus far, the old recipes, among which are aluminum hydroxide, aluminum phosphate, or alum precipitate, are still the most widely used.

An interesting advance was made with ISCOM (which stand for immuno-stimulating complex), proposed by Morein et al. (1) ISCOM is a cage-like matrix made up of cholesterol and Quil A, which is a substance extracted from the bark of a tree, Quillaja saponaria, that contains, after partial purification, five main components with triterpenoid structures. The matrix spontaneously organizes in the presence of antigen, most often protein isolated from a viral coat envelope. The construction ensures high immunogenicity and is used in a number of animal vaccines.

It is likely that one key event played by adjuvants is at the steps of antigen uptake and processing by antigen-presenting cells. Supporting this idea is the fact that bacterial, or more generally particulate antigens, are far better immunogens than the purified molecules isolated from cells. This constitutes an obvious difficulty for designing sophisticated “pure” vaccines, which would at first sight appear advantageous over using complete bacteria that usually contain toxic components, but that also serve as carrier with an adjuvant effect for the desired antigen. To date, an ideal vaccine would have to reconcile the purity of a recombinant protein with a well-characterized potent adjuvant. We know much about making recombinant proteins, but have at present no convincing pure adjuvant. One interesting approach is to use a living vector, such as vaccinia virus, that has been genetically modified to express a given antigen at the surface of the microorganism. One may also couple a gene expressing the antigen with a gene encoding a cytokine that would specifically favor the amplification of a helper T cell compartment. Another possibility is to embed the antigen in liposomes that could be eventually targeted to well-defined cells of the immune system.

All these approaches have been attempted with variable success. To date, one must unfortunately admit that, despite the fantastic progress made in recent years in the understanding of the basic mechanisms that operate in the immune system, no really significant advances have been made in defining more efficient and more rational vaccines.

References

1. B. Morein, B. Sundquist, S. Höglund, K. Dalsgaard, and A. Osterhaus (1984) Iscom, a novel structure for antigen presentation of membrane proteins from envelopped viruses. Nature, 308, 457–460.