Leading Article Biologicals & Immunologicals: Immunological adjuvants: Mechanisms of action and clinical applications

Recent advances in immunological adjuvants: the development of particulate antigen delivery systems

New generation vaccines, including those based on recombinant proteins, are safer than traditional vaccines, but are less immunogenic. Therefore, there is an urgent need for the development of new and improved vaccine adjuvants. A number of potent immunostimulatory molecules obtained from bacterial cells or plants have been extensively evaluated as adjuvants. However, a number of these molecules have displayed significant toxicity, both in preclinical animal models and in human clinical trials. An alternative approach to the development of novel adjuvants involves the preparation of particulate antigen delivery systems of similar dimensions to natural pathogens. In the absence of additional immunostimulatory molecules, emulsion droplets and microparticles have been shown to be potent adjuvants for the induction of both humoral and cell-mediated immune responses following systemic administration. Moreover, particulate delivery systems have been shown to display an acceptable toxicity profile in a number of clinical trials. Particulate antigen delivery systems also have the potential to function as potent adjuvants following administration by mucosal routes, including oral and intranasal. An alternative approach to the mucosal delivery of vaccines involves the use of genetically detoxified mutant toxins, e.g., LT-K63, as mucosal adjuvants. The use of novel adjuvants and antigen delivery systems is likely to extend the use of vaccines into the area of therapeutics, involving the eradication of infectious diseases and cancers, or the amelioration of autoimmune disorders.

Immunobiology of the Tomatine adjuvant

Soluble or sub-unit protein vaccines alone are incapable of generating antigen-specific cellular immune responses. This failure can be attributed to the manner in which the immune system processes antigen; endogenous antigens are cycled through the MHC class I pathway to stimulate CD8+ restricted responses and exogenous antigens are processed through the MHC class II pathway to generate humoral immunity. Traditionally sub-unit vaccines have been formulated with adjuvants to enhance immunogenicity, however in the last decade a number of adjuvants have been developed that effectively stimulate the generation of both humoral and cellular immune responses, although the manner in which they exert their effects has not been investigated. Here we describe Tomatine, a glycoalkaloid based adjuvant, capable of stimulating potent antigen-specific humoral and cellular immune responses that contribute to protection against malaria, Francisella tularensis and regression of experimental tumors. Using in vivo models we investigated the manner in which cellular immune responses were generated by Tomatine. We established that Tomatine did not require either lymph node or splenic macrophages to generate cytotoxic T lymphocytes (CTL) and delivered soluble protein into a pathway not dependant on the machinery of the classical MHC class I pathway. We also observed that at the molecular level Tomatine required both CD80 and CD86 costimulation to engender antigen-specific cellular immunity.

Differential requirements for CTL generation by novel immunostimulants: APC tropism, use of the TAP-independent processing pathway, and dependency on CD80/CD86 costimulation

A major drawback of subunit vaccines is their inability to generate cytolytic T lymphocytes (CTL), a deficit attributed to segregation of the class I and class II antigen-processing pathways. We sought to understand processes involved in CTL induction by three proprietary adjuvants: Tomatine, PROVAX, and a synthesized glycolipid (Glc-N-(8/16), Glycolipid). We used in vivo models to investigate antigen uptake, macrophage involvement, TAP-independent processing, and costimulatory molecule dependencies. Glycolipid required splenic and lymph node macrophages, whereas Tomatine generated CTL independently of either macrophage population. In contrast, PROVAX showed partial macrophage requirements. Immunized TAP knockout mice revealed that ovalbumin (OVA)-Tomatine and OVA-PROVAX, but not OVA-Glycolipid, generate class I-peptide complexes. All three immunostimulants also elicited CD86-dependent TH1 cytokine responses.

Cancer vaccine design: a novel bacterial adjuvant for peptide-specific CTL induction

The recent identification of tumor Ags as potential vaccines has prompted the search for efficient adjuvants and delivery systems, especially in the case of peptide-based vaccination protocols. Here, we investigated the adjuvant potential of the recombinant 40-kDa outer membrane protein of Klebsiella pneumoniae (P40) for specific CTL induction. We studied the CTL response induced in HLA-A*0201/K(b) transgenic mice immunized with peptides derived from two melanoma-associated differentiation Ags, the HLA-A*0201-restricted decapeptide Melan-A(26--35) substituted at position 2 and the K(b)-restricted tyrosinase-related protein 2(181--188) T cell epitope. We found that both peptides are able to generate a specific CTL response when mixed with the protein in the absence of conventional adjuvant. This CTL response is a function of the amount of P40 used for immunization. Moreover, the CTL response generated against the tyrosinase-related protein 2(181-188) peptide in presence of P40 is associated with tumor protection in two different experimental models and is independent of the presence of CD4(+) T lymphocytes. Thus, the recombinant bacterial protein P40 functions as a potent immunological adjuvant for specific CTL induction.

Nucleic acid immunization: concepts and techniques associated with third generation vaccines

A radical change in vaccine methodology arrived nine years ago with the advent of nucleic acid immunization. Aspects such as plasmid design, gene selection, the use of immunostimulatory complexes and clinical trials are discussed in this review. Furthermore, concepts and protocols involved in the construction, evaluation and immunization of a DNA vaccine have been examined as new strategies to enhance this technology continues to grow.

Novel aggregate structure adjuvants modulate lymphocyte proliferation and Th1 and Th2 cytokine profiles in ovalbumin immunized mice

Cytokines are important mediators of effector lymphoid cell function during an immune response. The principal cytokine producers are the T helper (Th) cells and macrophages. Vaccine strategies need to take into account the balance of Th (Th1/Th2) cytokines they induce. Adjuvants are compounds that, when combined with an antigen, potentiate an immune response in an immunized species. The use of adjuvants has been shown to activate differentially Th1 and Th2 subsets. 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 Th1 or Th2 cytokine responses when administered with a soluble protein antigen. Of the three formulations, RAM1 was found to induce predominantly Th1 cytokines; the levels of which were substantially higher than those induced by reference control adjuvants. It was also found that at a late post-vaccinated period, RAM1 can stimulate Th2 responses. In contrast, RAM2 and RAM3 induced cytokine profiles typically associated with Th2 responses.

Generation of antigen specific CD8+ cytotoxic T cells following immunization with soluble protein formulated with novel glycoside adjuvants

Presentation of peptide on MHC class I molecules is essential to elicit cytolytic T cell (CTL) activity. Such peptides are a result of the cytosolic, or class I, antigen processing pathway. Due to the segregation of the class I and the exogenous processing pathway, soluble protein cannot enter the class I pathway and is thus incapable of inducing CTL. However careful formulation with adjuvants can overcome this obstacle. In this study we evaluated the capacity of two novel amphiphilic adjuvants, better termed delivery vehicles, to elicit CTL activity in a C57Bl/6 murine model with ovalbumin (OVA) as an antigen. Incomplete Freund's adjuvant and aluminium hydroxide (Alhydrogel) were used as reference adjuvants. In addition the oil-in-water emulsion Provax was used throughout as a positive control adjuvant. Both amphiphile preparations were capable of eliciting potent CTL activity after administration of one immunizing dose of ovalbumin. CTL were CD8+ restricted as assessed by in vitro depletion of CD8+ and CD4+ T cells. CTL activity was also MHC-restricted as well as specific for the H-2Kb OVA motif SIINFEKL.

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.