Immunological adjuvants and their mode of action

Evaluation of novel aggregate structures as adjuvants: composition, toxicity studies and humoral responses

Adjuvants are compounds that, when combined with an antigen, potentiate an immune response in an immunized species. There are numerous pathogens for which there are no protective vaccines and since alum is the only adjuvant licensed for use in humans, there is a clear need for more effective adjuvant preparations. In this study we describe the immunopotentiating properties of three novel molecular aggregate formulations based on tomatine (RAM1), a glycosylamide lipid (RAM2) and a fifth generation dendrimeric polymer (RAM3) respectively. These formulations were evaluated for their ability to augment antigen-specific antibody responses when administered with a soluble protein antigen. All three adjuvants were shown to be nontoxic to mice and elicited antigen-specific antibody responses. Of the three formulations, RAM1 was found to induce the highest titers of antibody; these were substantially higher than those induced by reference control adjuvants. RAM1 elicited antibodies of the IgG1 and IgG2a subclasses indicating, indirectly, that this adjuvant can stimulate Th2 and Th1 type immunity.

Approaches to new vaccines

The explosive technological advances in the fields of immunology and molecular biology in the last 5 years had an enormous impact on the identification of candidate vaccines against diseases, which until a few years ago seemed uncontrollable. Increased knowledge of the immune system has helped to define the mechanisms that underlie successful immunization and is now being exploited to develop improved versions of existing vaccines and new vaccines against emerging pathogens, tumors, or autoimmune diseases. An understanding of the mechanisms of action of novel adjuvants and the development of new vector and delivery systems will have a major impact on vaccine strategies. The use of DNA encoding antigens from pathogenic viruses, bacteria, and parasites as vaccines is a new approach that is receiving considerable attention. This and other innovative approaches, including vaccine production in plants, are appraised in this review. The successful eradication of smallpox and the imminent eradication of poliomyelitis by worldwide immunization campaigns provide positive examples of how the vaccine-mediated approach can lead to disease elimination; with the advent of new vaccines and improved delivery systems, there is no scientific reason why these successes cannot be repeated.

Preservation of surface-dependent properties of viral antigens following immobilization on particulate ceramic delivery vehicles

B-cell stimulation for the purpose of evoking an effective neutralizing humoral immune response is a surface phenomenon that is exquisitely specific to antigen conformation. Consequently, successful delivery of antigen, such as would be desired in a vaccine, entails preservation of an antigen's apparent native surface (conformational) properties. Prior to testing the actual vaccinating efficacy of delivered antigens, the surface properties could be assessed through a variety of in vitro and in vivo assays in which the measurement standard would be the properties of the antigens in their native state (whole virus). Using surface modified nanocrystalline carbon and calcium-phosphate ceramic particulates (carbon ceramics and brushite), we evaluated the surface activity of immobilized non-nuclear material extracted from HIV-1. Physical characterization showed that the particles with immobilized antigen ("HIV decoys") measured 50 nm in diameter (HIV = 50-100 nm) and exhibited the same zeta potentials as whole (live) HIV. In vitro testing showed that the HIV decoys were recognized by both conformationally nonspecific and specific monoclonal antibodies, were recognized by human IgG from HIV antibody-positive patients, and could promote surface agglomeration among malignant T-cells similar to live HIV. Last, in vivo testing in three vaccinated animal species showed that the HIV decoys elicited humoral and cellular immune responses similar to that evoked by whole (live) HIV.

Water-soluble phosphazene polymers for parenteral and mucosal vaccine delivery

PCPP can be used in two different ways to potentiate an immune response. The soluble form of the polymer has been found to have immunoadjuvant activity. A single subcutaneous injection of polymer/influenza dramatically increases the ELISA, neutralizing, and HI antibodies to influenza virus compared to CFA. The polymer has also succeeded in dramatically increasing the amount of ELISA antibodies to TT. The antibody response elicited was predominantly of the IgG1 isotype. PCPP has also been used to generate micron-sized hydrogel microspheres through a process of divalent ion cross-linking of the polymer strands. These microspheres can induce significantly higher anti-TT serum IgG titers after a single intranasal immunization than TT alone.

Combination vaccines

The availability of new vaccines for pediatric use provides an unprecedented opportunity to prevent serious infectious diseases and their associated morbidity and mortality. However, each new product entails the need for additional injections that could become so numerous as to discourage the administration of new vaccines. This challenge can be overcome by mixing individual vaccines before delivery such that multiple vaccines are administered as single combination vaccines. Technical, clinical, manufacturing, regulatory and marketing issues relative to combination vaccines in general are the major challenges for ongoing and future development.